Astm stp 1380 2000

The papers are organized into four sections by topic: Background of Impact Testing; Reference Energies, Machine Stability and Calibration; Impact Test Procedures; and Fracture Toughness

Pendulum Impact Testing:

A Century of Progress

Thomas A Siewert and Michael P Manahan, editors

ASTM Stock Number: STPI380

Library of Congress Cataloging-in-Publication Data

Pendulum impact testing : a century of progress / Thomas A Siewert and Michael R

Manahan, editors

p cm. (STP; 1380)

ASTM Stock Number: STP1380

ISBN 0-8031-2864-9

1 Materials Dynamic testing 2 Impact 3 Notched bar testing 4 Testing-machines

I Siewert, T.A I1 Manahan, Michael P., 1953- II1 ASTM special technical publication;

1380

TA418.34 P463 2000

620.1' 125 dc21

00-038123

Copyright 9 2000 AMERICAN SOCIETY FOR TESTING AND MATERIALS, West Conshohocken,

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Peer Review Policy

Each paper published in this volume was evaluated by two peer reviewers and at least one edi- tor The authors addressed all of the reviewers' comments to the satisfaction of both the technical editor(s) and the ASTM Committee on Publications

To make technical information available as quickly as possible, the peer-reviewed papers in this publication were prepared "camera-ready" as submitted by the authors

The quality of the papers in this publication reflects not only the obvious efforts of the authors and the technical editor(s), but also the work of the peer reviewers In keeping with long-standing publication practices, ASTM maintains the anonymity of the peer reviewers The ASTM Committee

on Publications acknowledges with appreciation their dedication and contribution of time and effort

on behalf of ASTM

Printed in Baltimore, MD May 2000

Foreword

This publication primarily consists of papers presented at the Symposium on Pendulum Impact Testing: A Century of Progress, sponsored by ASTM Committee E28 on Mechanical Testing and its Subcommittee E28.07 on Impact Testing The Symposium was held on May

19 and 20, 1999 in Seattle, Washington, in conjunction with the standards development meetings of Committee E28 The Symposium marks the 100 year anniversary of the inven- tion of the pendulum impact test by an American civil engineer named S Bent Russell, and the research and standardization efforts of G Charpy during the early part of the 20 th century This book includes 21 papers that were presented at the Symposium and two others submitted only for the proceedings (one with lead author Yamaguchi and the other with lead author Hughes) The papers are organized into four sections by topic: Background of Impact Testing; Reference Energies, Machine Stability and Calibration; Impact Test Procedures; and Fracture Toughness Assessment from Impact Test Data In addition, the background section includes reprints of two landmark papers, one published in 1898 and one in 1901, that describe significant achievements in the development of the test equipment and procedures The symposium was chaired jointly by Tom Siewert, of the National Institute of Standards and Technology, and Dr Michael P Manahan, Sr., of MPM Technologies, Inc

Experiments with a New Machine for Testing Materials by I m p a c t -

s BENT RUSSELL, Transactions of the American Society of Civil Engineers,

Indirect Verification of Pendulum Impact Test Machines: The French

Subsidiary from Its Origins to the Present, Changes in Indirect

Verification Methods, Effects on Dispersion, and PerspectivesmG GALBAN,

G REVISE, D M O U G I N , S L A P O R T E , A N D S L E F R A N ~ O I S

Maintaining the Accuracy of Charpy Impact Machines D e VIGLIOTTI,

T A S I E W E R T , A N D C N M c C O W A N

Characterizing Material Properties by the Use of Full-Size and Subsize

Charpy Tests: An Overview of Different Correlation Proceduresm

S T A K A G I , A N D H N A K A N O

The Difference Between Total Absorbed Energy Measured Using an

Instrumented Striker and That Obtained Using an Optical E n c o d e r - -

M P M A N A H A N , SR AND R B STONESIFER

On the Accuracy of Measurement and Calibration of Load Signal in the

Instrumented Charpy Impact Test T KOBAYASHI, N INOUE, S MORITA,

European Activity on Instrumented Impact Testing of Subsize Charpy

V-Notch Specimens (ESIS TC5) E LUCON

Dynamic Force Calibration for Measuring Impact Fracture Toughness using

the Charpy Testing Maehine K K I S H I M O T O , H INOUE, A N D T S H I B U Y A

Low Striking Velocity Testing of Precracked Charpy-type Specimens

T V A R G A A N D F L O I B N E G G E R

In-Situ Heating and Cooling of Charpy Test SpeeimensnM P MANAHAN, SR

The Effects of OD Curvature and Sample Flattening on Transverse Charpy

V-Notch Impact Toughness of High Strength Steel Tubular Products m

Application of Single-Specimen Methods on Instrumented Charpy Tests:

Results of DVM Round-Robin Exercises w BOHME AND H.-J SCHINDLER

Relation Between Fracture Toughness and Charpy Fracture Energy:

An Analytical Approach H.-J SCHINDLER

Use of Instrumented Charpy Test for Determination of Crack Initiation

Toughness H.-W VIEHRIG, J BOEHMERT, H RICHTER, AND M VALO

327

337

354

On the Determination of Dynamic Fracture Toughness Properties by

Estimation of N D T a n d C r a c k - A r r e s t Toughness f r o m C h a r p y Force-

D i s p l a c e m e n t Traces -M SOKOLOV AND J G MERrO-E 382

Overview

Overview

ASTM Subcommittee E28.07 (and its predecessor E01.7) has sponsored six symposia on impact testing, published in Proceedings of the Twenty-Fifth Annual Meeting (1922), Pro-

1072 (1990), and STP 1248 (1995) These symposia covered a broad range of topics and occurred rather infrequently, at least until 1990 The period before 1990 might be charac- terized as one in which the Charpy test procedure became broadly accepted and then changed very slowly However, the last two symposia, "Charpy Impact Test: Factors and Variables" and "Pendulum Impact Machines: Procedures and Specimens for Verification," were driven

by new forces; a recognition within ISO Technical Committee 164-Subcommittee four (Pen- dulum Impact) of some shortcomings in the procedure; and a growing interest in instru- mented impact testing These STPs, 1072 and 1248, proved to be of interest to many general users of the test, but were of particular interest to the members of ASTM Subcommittee E28.07 (the subcommittee responsible for Standard E23 on the Charpy test) During the past ten years, the data presented at those Symposia have been the single most important factor

in determining whether to change various requirements in Standard E23 The data have also been useful in supporting tolerances and procedural details during the reballoting of ISO Standard 442 on Charpy testing, and in the refinement of instrumented impact test procedures

Several years ago, the E28 Subcommittee on Symposia suggested that it was time to schedule another symposium on Charpy impact testing that would bring together impact test researchers from around the world to share their latest discoveries and to provide input for further improvements in the test standards The test was also near its Centenary, and a symposium to mark this anniversary seemed appropriate Of course, this fact led to our very striking title However, the choice of the date for the symposium was complicated by the fact that the inventory of the pendulum impact test is S Bent Russell, while the test bears the name of G Charpy Details concerning the history of the test are reported in the first paper of this STP While G Charpy did publish a landmark paper in 1901 (translated and reprinted in this volume) and later led the international committee that proved the value of pendulum impact testing, an 1898 paper by Russell (also reprinted in this volume) was the first to both describe the mechanics of the pendulum impact machine design and report impact data obtained using such a machine The 1898 Russell paper also offers an excellent tutorial on the contemporary knowledge of the effect of loading rate on impact resistance (then known as resilience), important variables in machine calibration, and representative data on common construction materials The date of the symposium was chosen to honor the contributions of both Russell and Charpy As can be seen from a review of the early papers in this field, it seems as though the turn of the last century marked the time of the most rapid development and use of impact testing

As was the previous symposium, the 1999 symposium was successful in attracting con- tributions from many countries In fact, the majority (thirty-seven) of the fifty authors and coauthors are from outside the U.S., a broader international participation than previous symposia

ix

X PENDULUM IMPACT TESTING

The future of pendulum impact testing appears bright, as it continues to be specified in many construction codes and standards Additional details on the economic importance of pendulum impact testing were included in an earlier version of our review of the history and importance of impact testing (the first paper in this STP) This earlier paper can be found

on page 30 of the February 1999 issue of Standardization News, where itwas recognized

as winning third place in the ASTM Impact of Standards Competition The early history of impact testing which led to the recognition of Russell as the inventory of the Charpy impact test was reported in October 1996 issue of Standardization News

Even after 100 years of use, new aspects of the test continue to be discovered, and of course, any test can be improved as technology reveals new ways to reduce the scatter in the test variables The symposium also reflects the beginning of a new research thrust to obtain fracture toughness from the Charpy test It is expected that fracture toughness research, particularly in relation to the Charpy test, will continue over the next 100 years We anticipate many more symposia on impact testing in the future

Acknowledgments

We appreciate the assistance of Subcommittee E28.07, its Chairman, Chris McCowan, and its members, many of whom helped by chairing the sessions and by reviewing the manu- scripts We also appreciate the assistance of E Ruth (U.S Delegate to ISO Committee 164- TC4 for a number of years) and J Millane (Secretary of ISO Committee 164-TC4) who helped to encourage international participation We would also like to thank the ASTM staff who helped with symposium arrangements and the other myriad of details that are necessary for a successful symposium

Tom A Siewert

NIST, Boulder, CO;

symposium co-chairman and editor

Michael P Manahan, Sr

MPM Technologies, Inc

State College, PA;

symposium co-chairman and editor

Background of Impact Testing

T A Siewert, 1 M P Manahan, 2 C N McCowan, 3 1 M Holt, 4 F 1 Marsh, 5 and E A Ruth 6

The History and Importance of Impact Testing*

Reference: Siewert, T A., Manahan, M R, McCowan, C N., Holt, J M., Marsh,

E J., and Ruth, E A., " T h e History and Importance of Impact Testing,"

Manahan, Sr., Eds., American Society for Testing and Materials, West

Conshohocken, PA, 2000

Abstract: Charpy impact testing is a low-cost and reliable test method which is commonly required by the construction codes for fracture-critical structures such as bridges and pressure vessels Yet, it took from about 1900 to 1960 for impact-test technology and procedures to reach levels of accuracy and reproducibility such that the procedures could

be broadly applied as standard test methods This paper recounts the early history of the impact test and reports some of the improvements in the procedures (standard specimen shape, introduction of a notch, correlation to structural performance in service, and introduction of shrouds) that led to this broad acceptance

Keywords" absorbed energy, Charpy impact testing, history, impact testing, pendulum impact

Without uniformity of test results from day to day and from laboratory to laboratory, the impact test has little meaning Over the years, researchers have learned that the results obtained from an impact test can depend strongly upon the specimen size and the

geometry of the notch, anvils, and striker: To a lesser degree, impact test results

also depend upon other variables such as impact velocity, energy lost to the test machine, and friction The goal of those who have written and modified ASTM Standard Test

1 Supervisory Metallurgist, Materials Reliability Division, National Institute of

Standards and Technology, Boulder, CO 80303

z President, MPM Technologies, Inc., 2161 Sandy Drive, State College, PA 16803

3 Materials Research Engr., Materials Reliability Division, NIST, Boulder, CO 80303

4 Alpha Consultants & Engineers, Pittsburgh, PA

5 Retired (Bethlehem Steel), San Marcos, CA

Tinius Olsen Test Machine Co., Willow Grove, PA

* Contribution of NIST; not subject to copyright Further details on the economic impact of Charpy impact testing are included in a previous version of this

Copyright9 by ASTM International

3

www.astm.org

Methods for Notched Bar Impact Testing of Metallic Materials (E 23) has over the years been to standardize and control the variables associated with impact testing This report looks at the history of impact testing, with emphasis on the key advances in understanding and application of the impact test, as reflected in the evolution of the test standard

Impact Testing: 1824 to 1895

The earliest publication that we could find on the effects of impact loading on materials was a theoretical discussion by Tredgold in 1824 on the ability of cast iron to resist impulsive forces [1] In 1849, the British formed a commission to study the use of iron in the railroad industry, which began by considering practical approaches to impact testing [2] Apparently, failures of structures in the field were leading some researchers to speculate that impact loads affected materials far differently than static loads, so tensile- strength data (from slowly applied loads) was a poor predictor of performance under dynamic loads

In 1857, Rodman devised a drop-weight machine for characterization of gun steels, and over the subsequent 30-year period, his machine was widely used to test railroad steels and for qualification of steel products [2] Many of the early experiments with impact tests were performed on final product forms, such as pipes or axles Thus they served as proof tests for a batch of material, or yielded comparative data for a new product design, or basic reference data on the impact resistance of different construction materials (such as the comparison of wrought iron to ductile iron) Instrumentation was poor for the early impact tests, so the data is often only as break or no-break for a mass dropped through a certain distance These early drop weight tests were conducted using smooth (no notch or crack starter) rectangular bars While the test worked well for brittle materials, where crack initiation is easy, specimens of ductile materials often just bent LeChatalier introduced the use of notched specimens while conducting drop-weight tests

in 1892 [3] He found that some steels that showed ductile behavior (bending without fracture) in a smooth rectangular bar, would exhibit fragile behavior when the test specimen was notched While the addition of a notch was a major improvement in the test method, a test procedure was needed that would provide a continuous, quantitative measure of the fracture resistance of materials Also, substantial work was needed to develop test procedures that produced consistent data, and to answer the objections of those who doubted the value of impact testing

1895 to 1922

This period saw the establishment of a number of national and international standards bodies, which took up the causes of developing robust test procedures and developing consensus standards for many technologies, including impact testing One of these standards bodies was The American Society for Testing and Materials, established in

1898 Another was the International Association for Testing Materials, officially

established in 1901, but this association grew out of the good response to two previous International Congresses that had been held a number of years before These two

standards bodies seem to have had a good working relationship, and the President of

SIEWERT ET AL ON IMPACT TESTING 5

ASTM, Prof H M Howe, also served on the Board oflATM during this time [4]

In 1902, only four years after the founding of ASTM, the ASTM "Committee on the Present State of Knowledge Concerning Impact Tests" published a bibliography on impact tests and impact testing machines in the second volume of the Proceedings of ASTM [5] This bibliography listed more than 100 contemporary papers on impact testing published in the U.S., France, and Germany Many of these papers contained information that was also known to the members of IATM In fact, some of the papers had been presented and discussed at the IATM Congresses

Among the references is a report by Russell (published in 1898 and reprinted in this STP) that shows remarkable insight into the needs of the design engineers of the time and introduces quantitative measurement to the test [6] He pointed out that none of the machines of the time, typically of a drop-weight design, had the ability to determine any data beyond whether the specimen broke or remained intact Therefore, he designed and built a pendulum machine which "would measure the energy actually absorbed in breaking the test bar" His report shows a test machine that is based on the same swinging

pendulum coficept as those in common use today and mentions his careful analysis of the mechanics of the test, including corrections for friction losses and calculation and

comparison of the centers of gravity and percussion Since this was before the time of compact, standardized test specimens, the machine was vary large and massive, and was capable of breaking many full-size products Besides showing a prototype of the machines used today, this report is valuable in that it includes data on over 700 tests of typical construction materials, and emphasizes the effect of the rate of loading in evaluating materials for different service conditions Russell's pendulum impact machine finally provided a means for quantifying the energy absorbed in fracturing a test specimen for a wide range &materials and conditions His paper nicely summarizes the test-machine technology and knowledge for material performance at the end of the past century, and so served as a benchmark for future research To the best of our knowledge, Russell was the first to develop and demonstrate the advantages of the pendulum design for impact testing machines

The members of IATM Commission 22 (On Uniform Methods of Testing Materials) continued to conduct research that addressed the shortcomings in the impact testing techniques, until they had developed a knowledge of most of the important factors in the test procedure Even though many of these early machines and reports are simplistic by today's standards, they provided previously unknown data on the impact behavior of materials France seems to have been an early adopter of impact testing for infrastructure construction standards, and so French researchers provided much data on the effects of procedure variables and were the most prolific contributors to the IATM Proceedings between 1901 and 1912 Incidentally, it was a representative from France, G Charpy, who became the chair of the impact testing activity after the 1906 IATM Congress in Brussels, and presided over some very lively discussions on whether impact testing procedures would ever be sufficiently reproducible to serve as a standard test method [7] Charpy's name seems to have become associated with the test because of his dynamic efforts to improve and standardize it, both through his role as Chairman of the IATM Commission and through his personal research [8] He seems to have had a real skill for recognizing and combining key advances (both his and those of other researchers) into

continually better machine designs and consensus procedures For example, Charpy acknowledges the benefits of Russell's pendulum design in his 1901 paper [8] by stating:

"Russell described in a paper presented in 1897 at the American Society of Civil Engineers some 'experiments with a new machine for testing materials by impact.' The machine he is using is designed to determine the work absorbed by the rupture of a bar, for this, the ram used appears in the form of a pendulum arranged in such a way so that when it is released from its equilibrium position, it meets the test bar in passing through the vertical position, breaks it and afterward rises freely under the influence of the acquired speed The difference between the starting height and the finishing height of the pendulum allows evaluation of the work absorbed by the rupture of the bar."

By 1905, Charpy had proposed a machine design that is remarkably similar to present designs and the literature contains the first references to "the Charpy test" and "the Charpy

other standard machine designs and procedures were also under consideration at this time, and in 1907 the German Association for Testing Materials adopted one developed by

impact machine designers and manufacturers offered three major types; Drop Weight (Fremont, Hart-Turner, and Olsen), Pendulum Impact (Amsler, Charpy, Dow, Izod, Olsen, and Russell), and Flywheel (Guillery)

This was a period during which the configuration and size of specimens closely approached what we use today [7] Originally, two standard specimen sizes were most popular The smaller had a cross section of 10 by 10 mm, a length of about 53 mm (for a distance of 40 mm between the points of support), a notch 2 to 5 mm deep, and a notch tip radius near 1 mm The larger and initially more popular of these specimen sizes was scaled up by a factor of three in all these dimensions The group favoring the larger specimen pointed out the advantage of sampling a larger cross section of the material (for reduced scatter in the data) and the difficulty of producing the small notch radius on the smaller specimen However, the group favoring the smaller specimen eventually won because a more compact and lower-cost machine could be used, and not all structures were thick enough to produce the larger specimen Besides specimen dimensions that are very similar to what we use today, the Commission proposed features for a standard impact procedure that included:

the edges of a wide striker [7]

One report at the 1912 meeting [7] included the testimonial from a steel producer of how the improved impact test procedures had allowed them to tailor the refining processes to produce less brittle steel The report describes a reduction by a factor of 20 in the number

o f production parts that were rejected for brittle performance

SIEWERT E f AL ON IMPACT TESTING 7

1922 to 1933: The Beginning of ASTM Method E 23

ASTM Committee E-1 on Methods for Testing sponsored a Symposium in 1922 on Impact Testing of Materials as a part of the 25th Annual Meeting of the Society, in Atlantic City, New Jersey The Symposium included a history of the developments in this area, a review of work done by the British Engineering Standards Association, several technical presentations, and the results of a survey sent to 64 U.S testing laboratories

[11] Twenty-three respondents to the survey offered detailed information on topics such

as the types of machines in use, the specimen dimensions, and procedures In addition, many responded positively to a question about their willingness to develop an ASTM standard for impact testing

Based on the information in this survey, an ASTM subcommittee began to prepare a standard test method for pendulum impact testing in 1923 This effort took until 1933, when ASTM published "Tentative Methods of Impact Testing of Metallic Materials," ASTM designation E 23-33T (An ASTM specification of"Tentative" indicated that it was subject to annual review and was a work in progress The tentative designation is no longer used by ASTM.) (Other countries also developed their own standards; however,

we found it difficult to find their records and to track their developments.)

ASTM E 23-33T specified that a pendulum-type machine was to be used in testing and

"recognized two methods of holding and striking the specimen", that is, the Charpy test and the Izod test (where the specimen is held vertically by a clamp at one end) It did not specify the geometry of the striking edge (also known at the time as the "tup") for either test It stated that "the Charpy type test may be made on unnotched specimens if indicated

by the characteristics of the material being tested, but the Izod type test is not suitable for other than notched specimens" Only a V-notch was shown for the Charpy test Although the dimensions for both types of specimens were identical with those currently specified, many tolerances were more restrictive The units were shown as English preferred, metric optional The committee pointed out many details that influence the test results, but because they did not have the knowledge and database needed to specify values and/or tolerances for these details, the document was issued as a tentative The original

document contains an appendix with general discussions of applications, the relation to service conditions, and comparisons between materials As our understanding of the variables in Charpy testing has grown, ASTM E 23 has been revised repeatedly to incorporate the new knowledge

1934 to 1940

The first revision of E 23 was issued in 1934 and it added a dimension for the radii of the anvil and specifically stated that "these specimens (both the Charpy and the Izod) are not considered suitable for tests of cast iron" referencing a report of ASTM Committee A3 on Cast Iron The method retained the "tentative" designation

The geometry of the Charpy striking tup, specifically the radius of the tup that

contacted the specimen, was not specified in the 1934 revision However, the minutes of the 1939 and 1940 meetings for the Impact Subcommittee of E1 state that this item was discussed and a survey was made of the geometries used in the United Kingdom and in

France Those countries had been using radii of 0.57 mm and 2 mm, respectively For reasons that were not recorded, the members of the Subcommittee agreed to a radius of 8

mm at the 1940 meeting and ASTM E 23 was revised and reissued as E 23-41T Two other changes that occurred with this revision were that metric units became the preferred units, and keyhole and U notches were added for Charpy-test specimens

1940 to 1948

Impact testing seems to have been a useful technique for evaluating materials, but was not a common requirement in purchase specifications and construction standards until the recognition of its ability to detect the ductile-to-brittle transition in steel Probably the greatest single impetus toward implementation of impact testing in fabrication standards and material specifications came as a result of the large number of ship failures that occurred during World War II These problems were so severe that the Secretary of the U.S Navy convened a Board of Investigation to determine the causes and to make recommendations to correct them The final report of this Board stated that of 4694 welded-steel merchant ships studied from February 1942 to March 1946, 970 (over 20%)

from minor fractures that could be repaired during the next stop in port, to 8 fractures that were sufficiently severe to force abandonment of these ships at sea Remedies included changes to the design, changes in the fabrication procedures and retrofits, as well as impact requirements on the materials of construction The time pressures of the war effort did not permit thorough documentation of the effect of these remedies in technical reports

at that time; however, assurance that these remedies were successful is documented by the record of ship fractures that showed a consistent reduction in fracture events from over

130 per month in March 1944 to less than five per month in March 1946, even though the total number of these ships in the fleet increased from 2600 to 4400 during this same

After the war, the National Bureau of Standards released its report on an investigation

of fractured plates removed from some &the ships that exhibited these structural failures

study included chemical analysis, tensile tests, microscopic examination, Charpy impact tests, and reduction in thickness at the actual ship fracture plane A notable conclusion of the report was that the plates in which the fracture arrested had consistently higher impact energies and lower transition temperatures than those in which the fractures originated This was particularly important because there was no similar correlation with chemical composition, static tensile properties (all steels met the ABS strength requirements), or microstructure In addition, the report established 15 ft-lb (often rounded to 20 J for metric requirements) as a minimum toughness requirement, and recommended that "some criterion of notch sensitivity should be included in the specification requirements for the procurement of steels for use where structural notches, restraint, low temperatures, or shock loading might be involved", leading to a much wider inclusion of Charpy

requirements in structural standards

SIEWERT ET AL ON IMPACT TESTING 9

1948 to Present

By 1948, many users thought that the scatter in the test results between individual machines could be reduced further, so additional work was started to more carefully specify the test method and the primary test parameters By 1964, when the ASTM E 23 standard was revised to require indirect verification testing, the primary variables

summarized the most significant causes of erroneous impact values as follows: (1)

improper installation of the machine, (2) incorrect dimensions of the anvil supports and striking edge, (3) excessive friction in moving parts, (4) looseness of mating parts, (5) insufficient clearance between the ends of the test specimen and the side supports, (6) poorly machined test specimens, and (7) improper cooling and testing techniques While the machine tolerances and test techniques in ASTM E 23 addressed these variables, it was becoming apparent that the only sure method of determining the performance of a Charpy impact machine was to test it with standardized specimens (verification

in service could perform well enough to meet the indirect verification requirements, but that most impact machines could meet the proposed requirements if the test was

conducted carefully and the machine was in good working condition With the adoption

of verification testing, it could no longer be convincingly argued that the impact test had too much inherent scatter to be used as an acceptance test

Early results of verification testing showed that 44% of the machines tested for the first time failed to meet the prescribed limits, and it was thought that as many as 50% of all the machines in use might fail [16] However, the early testing also showed that the failure

machines were retired, and more attention was paid to testing procedures It was

estimated that approximately 90% of the machines in use could meet the prescribed limits

Figures 3 through 5, confirm these predictions Failure rates for verification tests at low, high, and super-high energy ranges are currently estimated to be 12, 7, and 10%,

originally published by D.E Driscoll, Reproducibility of Charpy Impact Test, AS134 STP

176, 1955

of maintenance can vary by more than 100% at this energy level In effect, the limits imposed by ASTM E 23 have produced a population of impact machines that are arguably the best impact machines for acceptance testing in the world

While ASTM E 23 is used around the word, there are other forums for the

development of global standards One of these, the International Organization for Standardization, ISO, allows qualified representatives from all over the world to come

together as equal partners in the resolution of global standardization problems [18] ISO

Committee TC 164 handles the topic of Mechanical Testing, and its Subcommittee SC 4 handles toughness testing While this subcommittee has developed and maintains ten

standards on toughness testing, perhaps the most pertinent is 1S0 StandardR 442:1965

Metallic Materials -lmpact Testing - Verification of Pendulum Impact Machines This standard covers the Charpy test and is presently undergoing balloting for revision An important feature of this document is that it recognizes Charpy testing with both the 2-mm and 8-mm radius striker There are other regional and national standards that specify

impact testing procedures, such as the Japanese standard, JIS Z2242, Method for Impact

Test for Metallic Materials

SIEWERT ET AL ON IMPACT TESTING 1 1

Typical Applications Today

$500,000 per day I f Charpy data can be used to extend the life o f a plant one year beyond the initial design life, a plant owner could realize revenues as large as

$150,000,000 Further, the cost avoidance from a vessel related fracture is expected to be

in the billion-dollar range To date, the NRC has shut down one U.S plant as a result o f Charpy data trends It is important to note that this plant's pressure vessel was

constructed from a one-of-a-kind steel and is not representative o f the U.S reactor fleet

, , , , , ' -I0.16

10"14

] 0.08 = [0o,

SIEWERT ET AL ON IMPACT TESTING 13

0.1 -a

Figure 5 -Distribution of the super-high energy verification data

Data for 1995-1997 Approximately 650 tests Each test is an

average of five specimens The vertical lines at • represent the

acceptance criteria

Nonetheless, with decisions like this based on the Charpy test, the importance of ASTM E

23 and the restraints it applies cannot be overemphasized

Steel

The Charpy V-notch (CVN) test specimen and associated test procedure is an effective cost-saving tool for the steel industry The specimen is relatively easy to prepare, many specimens can be prepared at one time, various specimen orientations can be tested, and relatively low-cost equipment is used to test the specimen In many structural steel applications, the CVN test can be used: (1) as a quality control tool to compare different heats of the same type of steel, (2) to check conformance with impact requirements in standards, and (3) to predict service performance of components Also, CVN test

information can be correlated with fracture toughness data for a class of steels so that the results of fracture-mechanics analyses can be compared with the material toughness CVN data have many uses, such as during the design and construction of a bridge or an offshore oil platform Before full-scale production of the steel order can begin, the supplier needs to demonstrate to the buyer that the steel plate is capable of meeting certain design criteria The process begins by making the steel grade and then testing a portion of the plate to determine if all required criteria are met Also, steel mill equipment imposes limitations on plate size; therefore, individual steel plates need to be welded together in the field to produce lengths which can reach deep into ocean waters Small sections of the sample plate are welded together, and fracture mechanics tests are conducted to determine

the crack tip opening displacement (CTOD) toughness in the heat affected zone ffIAZ) and in areas along the fusion line where the weld metal meets the base metal Then, a steel supplier might correlate the CTOD test results with CVN 50% ductile-brittle transition temperature (DBTT) By agreement between the customer and supplier, this correlation can allow the steel supplier to use the Charpy test instead of the more

expensive and time-consuming CTOD testing

Continuing Standardization Efforts

Even after 100 years, the Charpy impact test procedures still have room for

improvement The ASTM E 23 standard has recently been redrafted to provide better organization and to include new methods such as in-situ heating and cooling of the test specimens Two new related standards are also under development through ASTM Task Group E 28.07.08, "Miniature and Instrumented Notched Bar Testing", which was formed a little more than two years ago The first standard covers miniature notched bar impact testing and relies on many of the existing practices related to test machine

requirements and verification as specified in existing standard E 23 The second standard

is focused on instrumented testing, where strain gages attached to the striker provide a force-deflection curve of the fracture process for each specimen Research is focused on using these data to obtain plane strain fracture toughness as well as other key test

parameters Upon acceptance of the standard by ASTM, both the existing E 23 standard and the new miniature notched bar standards would reference the instrumented impact standard

The state of the art in impact testing continues to advance in other parts of the world also ISO is balloting a standard (14556) on instrumented impact testing, there is work in Europe on miniature Charpy specimens, and ESIS is investigating the use ofpre-cracked Charpy specimens for determining fracture toughness It can be expected that

harmonization efforts will bring some of this work into E 23 in the future

Conclusion

The ASTM E 23 standard is a document that continues to improve as our technical knowledge increases Several years ago, at the ASTM Symposium on "The Charpy

there is a Symposium on the Charpy Test; what can be new there?" Since then, the document has been updated twice and is currently being revised to reflect new

developments and to make it more "user friendly." Although ASTM E 23 has been a useful standard for many years, it continues to be a "work in progress," a work used extensively to help evaluate existing and new materials for products and structures a test

to ensure safety as well as to reduce the initial and lifetime costs for structures

Knowledge which will help make the test more accurate and reliable is continually being gained New technologies such as miniaturization of the test, instrumenting the striker to obtain additional data, and developing mechanics models to enable extraction of plane strain fracture toughness will be areas of development over the next 100 years We

SIEWERT ET AL ON IMPACT TESTING 15

anticipate that the benefits from the application of E 23 during the next 100 years will overshadow the benefits from those in the past 100 years

References

Engineering Research, University of Michigan, 1925

Testing Commission, Volume 3, 1892

Brussels Congress, 1906

Transactions ASCE, Vol 39, June 1898, p 237

[7] Proceedings of the Sixth Congress of the International Association for Testing Materials, New York, 1912

249-250

[10] Whittemore, H L., "Resume of Impact Testing of Materials, with Bibliography,"

ProceedingsASTlv[, Vol 22, Part 2, 1922, p 7

ASTAI, Vol 22, Part 2, 1922, p 78

[12] The Design and Methods of Construction of Welded Steel Merchant Vessels: Final

July 1947, p 569

Removed from Welded Ships, National Bureau of Standards Report, December 9,

1948

Fahey, N H., "The Charpy Impact Test - Its Accuracy and Factors Affecting Test

McCowan, C N., Wang, C M., and Vigliotti, D P., "Summary of Charpy Impact

More information is on the ISO World Wide Web site, at http://www.iso.ch

ASTM, 1990

S B e n t R u s s e l l I

Experiments with a New Machine for Testing Materials by Impact (Reprint from 1898)*

REFERENCE: Russell, S B., "Experiments with a New Machine for Testing Materials

by Impact (Reprint from 1898)," Pendulum Impact Testing: A Century of Progress, STP

1380, T A Siewert and M P Manahan, Sr., Eds., American Society for Testing and Materials,

West Conshohocken, PA 2000

When stress is applied to a solid body, the material is distorted and a certain amount of work or energy is absorbed The work thus absorbed in the deformation of the material is called resilience If the stress changes from zero up to the elastic limit of the material, the energy absorbed during the change is the "elastic resilience" of the material If the stress changes from zero up to the ultimate strength of the body, the energy absorbed is the "ul- timate resilience" of the body 2

In the study of this subject it must be borne in mind that resilience is work, and hence depends upon two essential factors, force and distance acted through The latter is fully as important as the former The word toughness, as used by engineers, is synonymous with resilience In fact, the latter may be defined by saying that resilience is toughness reduced

to measurement

Having defined resilience, it is next found that, as it depends upon change of stress, different results may be looked for when the stress is applied suddenly, from those obtained when it is applied slowly The resilience under impact may not be the same as the resilience under gradual load In this connection impact should not be confused with sudden load The effect on resilience of rapidity of change in stress can only be determined by actual exper- iment This is especially true in the case of material not perfectly elastic, or where the stress has passed the elastic limit of the material

Again, the resilience of solids may be studied under the four principal kinds of stress, viz., tension, compression, torsion and bending The relative resilience under these different forms o f stress can only be determined by experiment A knowledge of the resilience of materials of construction is of the greatest importance to the engineer It is the great resilience

of the battle ship's steel armor that enables it to withstand the impact of heavy projectiles without destruction It is the low resilience of cast iron that makes it so inferior for railway bridges It is on account of the high resilience of wood that it cannot, in many cases, be supplanted by masonry, glass or other decay-proof material A concrete railroad tie cannot take the place of the oak tie because it lacks resilience

* Reprinted with the permission of the American Society of Civil Engineers from Transactions, Amer-

ican Society of Civil Engineers, Vol 39, No 826, 1898, pp 237-250

1 Member of the American Society of Civil Engineers

2 This use of the word resilience will be objected to by some as not being in conformity with the original meaning of the word It is sanctioned, however, by some authorities (see Thurston's "Materials

of Engineering"), and, for want of a good substitute, may be considered as a technical tenn

Copyright9 by ASTM International

17 www.astm.org

Admitting the importance of a knowledge of resilience, a brief consideration of the dif- ficulties to be overcome in obtaining such knowledge is naturally next in order It is at once found that they are of considerable proportions To find the strength of a beam under given conditions it is only necessary to find its weakest section and study that To find the resilience

of the beam all sections must be taken into account I f the beam is irregular in form, the problem becomes quite a difficult one If the final stress exceeds the elastic strength of the material, the difficulties are increased

The actual measurement of the resilience of a beam has been found quite difficult The load must be increased gradually and the deflection measured and recorded with its corre- sponding load As the breaking point is neared the difficulties of accurate work become important, especially in the more ductile materials If the determination of the resilience by impact or drop test is attempted, other complications arise The mass or weight o f the beam itself now becomes a factor in the test The work absorbed by the anvil and hammer and that are taken up in abrasion, etc., are difficult to estimate

To one who has a proper understanding o f these difficulties in measuring resilience, it is not surprising that the subject is somewhat neglected in the studies of practical men At present it may be said that the knowledge of comparative resilience of materials is "appre- ciable, but not describable." It is known that a cubic in of oak has more resilience than a cubic in o f pine, but the value of either cannot be expressed in inch-pounds or foot-pounds What is known about resilience, and the modern methods of determining its values, will be briefly considered

A n interesting series of experiments on the resilience of beams under impact was made

by Mr Hodgkinson The following quotations from a book well known to engineers 3 will show the more important results of these experiments:

"The power of a beam to resist impact is the same at whatever part of the length it is struck; this remarkable result has been confirmed by experiment."

"In rectangular beams of unequal dimensions the resistance 4 is the same, whether the bar

is struck on the narrow or broad dimension."

"With rectangular beams the resistance to impact R is simply proportional to the weight

o f the beam between supports, irrespective o f the particular dimensions."

The above laws exclude the effect of inertia

"Mr Hodgkinson has shown by his experiments that in resisting impact, the power of a heavy beam is to that of a light one as the inertia of the beam, plus the falling weight, is to the falling weight alone, or as (I + W ) / W "

" I is the inertia of the beam and the load upon it."

"The inertia of a beam, uniform in cross-section from end to end, supported at the ends and struck in the center, may be taken at half the weight between supports To this has

to be added the whole central load, if any."

In the second column of Table 20 will be found some values for the resilience of certain materials, which were obtained from the b o o k above referred to.s In modern practice, the testing of materials by impact is by no means uncommon Such tests, however, axe generally made on the finished shape, as in the case of railway axles In a code for testing materials, recommended by a committee to the American Society of Mechanical Engineers, 6 it was

3 "Strength of Materials" by Thos Box

4 Resilience?

s Interesting matter on the subject of impact, resilience, etc., will be found in Engineering News,

August 2, 1894 See also "A Photographic Impact Testing Machine" with discussion, Journal of the Franklin Institute, November, 1897, and January, 1898

6 See Engineering News, March 7, 1891

RUSSELL ON TESTING MATERIALS BY IMPACT 19

prescribed that drop tests should be made with a steel ball, weighing 1 000 to 2 000 lbs., having a clear fall of 20 ft The anvil, block, frame, etc., should weigh not less than ten times as much as the ball Drop tests were recommended for rails, fires and axles Again, the Master Car Builders' Committee, 7 have recommended drop tests for railway axles These tests were to be made with a tup, weighing 1 640 lbs The anvil should weigh 17 500 lbs., and should rest on springs The axle should rest on supports 3 ft apart Cast-steel drawbars are now regularly furnished by contract; under specifications which call for drop tests of sample drawbars, specifying weight of tup, height of drop and number of blows Drop tests

of steel rails have been in practical use for many years

Besides the above tests of finished shapes, the following methods, which are used in commercial practice, may be noted These tests, while they do not measure the resilience so directly, are, nevertheless, intended to prove the toughness of the material

In testing cast-iron water pipe by hydraulic pressure, it is customary to strike the pipe smartly with a hand hammer while the pressure is on In inspecting steel where a sample bar is nicked and then bent with the hammer, the behavior of the bar indicates the degree

of toughness which the material will have under impact A high percentage of phosphorus

in steel is believed to reduce its ability to withstand shocks, while its strength and percentage

of elongation remain unchanged 8 So that it may be said that the specified chemical deter- minations of phosphorus in structural steel, which are now in use, are really indirect tests

of resilience under impact

Users of structural steel will readily see the necessity which now exists for a definite physical test for the ultimate resilience of steel under impact It was this special necessity which led the author into the study of the subject, and suggested the experiments described

in this paper

If, instead of limiting the percentage of phosphorus in the steel, a certain ultimate resilience per cubic inch of the metal, when tested by impact, could be called for, a step would be made in advance If a definite resilience under impact could be specified, just as a definite strength and ductility are now called for, the proper inspection of steel would be much more simple and satisfactory

The difficulties of making impact tests have already been suggested Some machines which have been used for making such tests are of a type greatly open to criticism For example:

In some machines the supporting parts are either so light or so yielding that an important part of the energy of the blow is absorbed by them, and the test piece appears to sustain a much heavier blow than it would in fact on the proper rigid supports

Two general forms of testing machine have been used in recorded tests In Mr Hodgkin- son's experiments the hammer used was in the form of a pendulum striking with a horizontal blow The weight of the hammer was concentrated in the head or ball, and the effect of the rod or radius arm was probably neglected The most common form of impact testing machine

is doubtless the heavy weight falling vertically, somewhat after the fashion of the common pile-driver In none of these machines is there any means for measuring how much energy

is left in the hammer after breaking the piece

The Impact Testing Machine

The machine used in making the experiments given herewith was devised by the author and has some special features

7 See Railroad Gazette, June 26, 1896

8 See Johnson's "Materials of Construction," pages 166 and 167

In designing it the main idea was to make a machine which would measure the energy actually absorbed in breaking the test bar This was to be done by using a hammer in the form of a pendulum, and so arranged that it would strike a horizontal blow, breaking clear through the bar and swinging freely up to the height due to the velocity after the impact The difference between the height through which the hammer fell before striking, and the height to which it rose after striking, would measure the energy absorbed in breaking the bar The test piece would rest against two vertical knife-edges and be struck in the middle

by the falling pendulum, thus giving the ultimate resilience of the bar under transverse stress

In developing this idea it was found best to make the pendulum or hammer of the very simplest form, so that the center of percussion and center of gravity could be definitely computed 9 The hammer adopted was a rectangular steel bar pierced by a shaft at the upper end and provided with a suitable striking edge near the lower end

Figs 1, 2 and 3 show the form and dimensions of the machine used in the experiments Plate XII is from a photograph which shows somewhat imperfectly the general appearance

of the apparatus The hammer used weighed 103 lbs The fixed knife-edges were designed

so as to allow the broken bar to swing out of the way of the moving hammer, and were secured in a manner which allowed them to be adjusted for spans of 8, 12, 16, 20 and 24 ins The heavy anvil plates behind them were bolted to a large anvil block of concrete which was sunk in the earth Adjustable supports were provided to hold the test bar in position with the axis of the bar opposite the center of percussion The pivot blocks which support the hammer shaft are adjustable to allow for test bars of different depths Attached to the hammer shaft is a registering device on which the swing of the hammer is read The pivot blocks, etc., are supported by a strong wooden frame Attachments are provided for raising and releasing the hammer The plans for this machine were made in May, 1896 In making the design, the author was assisted by Mr William E Schaefer and Mr Vernon Baker Figs 4, 5 and 6 show the plans and Fig 12 the details of a later design which it is thought embodies some improvements in detail, although the essential features are the same In this design the frame will be of iron and the operator will have more room in which to work while setting the test bars in place

In using the testing machine the first point that comes up is the loss due to friction of the hammer in its bearings In practice it was found best to determine the friction anew for each set of experiments If the bar was to be given a blow of 6 ins., the friction loss was deter- mined for a fall of 6 ins If the hammer rose 2 ins after breaking the bar, the friction loss for a fall of 2 ins was determined by trial The average of the two values was called the correction for friction

To test the rigidity o f the knife-edges and their supports, a nickel 5-cent piece was placed

on edge on the top end of one of the knife-edges A cast-iron test bar 2 ins by 1 in was then broken by a single blow This experiment was repeated a number of times, and, in the majority of cases, the coin was not overturned by the shock

An effort was then made to measure the movement of the knife-edge under a heavy blow The movement was found to be so small that in the case of a cast-iron test bar, the energy absorbed by the yielding of the knife-edges would be quite inconsiderable Every impact testing machine should be tested in this way, to see if any considerable percentage of the energy is absorbed by the yielding of parts that support the test piece

In this method o f testing materials some energy is absorbed in overcoming the inertia of the bar itself The proportionate amount of this energy is probably dependent on the weight

9 The formula for finding the center of percussion will be found in Rankine's Applied Mechanics,

Article 581

RUSSELL ON TESTING MATERIALS BY IMPACT 21

PLAT;: Xi|

TRANS AM SOC CIV ENGRS

VOL XXXIX, No, 826

RUSSELL ON IMPACT TESTING EXPERIMENTS,

RUSSELL ON TESTING MATERIALS BY IMPACT 23

of the test bar compared with the weight of the hammer, and also upon the velocity of the hammer

Owing to the difficulties of ascertaining how much energy is absorbed in this way, it is best to use a test-bar whose weight is small in comparison to that of the hammer In this way the error due to inertia of the test piece can be reduced, if not eliminated

In Table 5 will be found the results of tests made to determine the effect of changing the initial fall of the hammer The results are somewhat contradictory, but, in a general way, it may be said that the experiments indicate that a small change in the initial fall of the hammer will not change the amount of energy absorbed, to any great degree This conclusion may

be regarded as important, as upon it depends somewhat the interpretation of all the experi- ments Table 5 will be referred to again

The machine having been described, it only remains to present the experiments themselves Over 700 specimens have been broken, up to the present writing These tests are not all recorded here; only those which were thought to be most instructive are given In order to learn the possibilities of the testing machine, the study of each material was continued only until it was thought that the principal difficulties peculiar to such material had been over- come It is obvious that the resilience values obtained for different materials cannot be taken

as final, and should only be used by the designer in the absence of more accurate determi- nations All the experiments were made by the author, with the assistance of Mr William E Schaefer

Tests of Brittle Materials

The first tests were made with cast iron Table 1 shows the resilience of cast-iron bars tested both by impact and by gradual load Each value given is the average of several tests

In making the impact tests, the following values are obtained by observations:

F = the initial fall of the hammer in inches

S = the rise after the blow in inches

C1 = the correction for friction

L = the distance between supports

h = the depth of beam

TABLE 1 Resilience by impact and by gradual load Cast-iron bars 1 in by 2 ins., broken flatwise

Length Resilence per Resilence per Lot or Number between cubic inch, in Number cubic inch, in Melt Experiment of Supports inch-pounds of inch-pounds

RUSSELL ON TESTING MATERIALS BY IMPACT 25

b = the width of beam

All dimensions are in inches

Then, by computation, when 103 is the weight of the hammer in pounds, the resilience

in inch-pounds per cubic inch of the material, or

as obtained by a strain diagram, would be slightly greater than this, but the error is not important as the strain diagram for cast-iron is nearly straight to the point of rupture Returning to Table 1 and comparing the resilience by impact and by gradual load, it will

be seen that the former exceeds the latter more than 40% This difference is so great that it can hardly be accounted for by losses due to inertia of bar, indentation, or movement in supports The bar is light compared with the hammer, so that not more than 7% could be lost by inertia according to Mr Hodgkinson's rule The supports are so rigid that not more than 1% could be lost by their movement The indentation is so slight as to be inconsiderable when compared with the deflection of the bar, hence there can be no great loss in this way The logical conclusion is that more energy is absorbed in the sudden rupture of a bar than

is the case with rupture under a gradual increase of load

It has occurred to the author, that perhaps the causes of this difference may be traced back

to the heat which is liberated under change of stress Under gradual increase of stress the heat liberated has time to be conducted away from the distorted fibers In the case of sudden rupture, the heat has no time to escape and must produce a rise in temperature If this be admitted, it seems not impossible that the resilience may be affected by the rise in temper- ature of the distorted particles This suggestion should be taken for what it may prove to be worth

Table 2 needs no explanation Bars of the same melt, but o f different spans, are compared

A bar of 12-in span has twice the strength and one-quarter the deflection of a b a r 24 ins

in span With the former, then, a greater loss of energy by movement of the knife-edges and

TABLE 2 Resilience for different spans Cast-iron bars, 1 in by 2 ins., broken flatwise

Lot

Number

by indentation might be expected Theoretically, the error from these sources would be about eight times as great for the shorter span On the other hand, the error from inertia should be about twice as great in the longer span as in the shorter one It will be seen by the table that the difference in the resilience per cubic inch ranges in value from nothing up to about 10%, and that the shorter span shows the higher average resilience It is fair to conclude from these experiments, as far as they go, that the ultimate resilience of a bar of cast-iron

is proportional to its volume and is independent of the span

Table 3 shows that a flat bar has about the same resilience whether broken flatwise or edgewise All these bars were cast from the same melt In the case o f a bar 2 ins wide and

1 in thick, it should have, when broken edgewise, twice the strength and half the maximum deflection that it would have flatwise The error from yielding supports and from indentation should be about four times as great in the former position The error from inertia of bar should be the same in both cases It would be expected that the bars would show greater resilience when broken edgewise The observed resilience was, however, somewhat greater

in the average, with the bars broken flatwise

As in testing bars in this manner, it is possible for the experimenter to raise the hammer considerably higher than is necessary to break the bar, the question naturally comes up: Will the height to which the hammer is raised affect the results obtained? A number of experi- ments were made to decide this point, and the results are recorded in Table 5 The experi- ments were made in this manner: Twelve to sixteen bars were taken from the same melt of cast-iron Four of these bars would be broken with the hammer falling 5 ins., which would barely break them The resilience would be measured The next four bars would be tested with the hammer falling 6 ins.; the next with a fall of 7 ins., etc The results obtained will

be seen in the last column of the table It is evident that more experiments would have to

be made to find the true relation between the height through which the hammer falls and the energy absorbed in the rupture It is f a i l however, to conclude in a general way, as has been stated, that a slight increase in the height will not materially affect the results obtained There seems to be a tendency for the resilience to increase as the height is increased; but this tendency is all but concealed by variations from other causes

TABLE 3 Resilience o f cast-iron bars Cross-section, 1 in by 2 ins Span, 24 ins Melt No 2

Resilience per cubic inch in

TABLE 4 Resilience o f cast-iron bars Effect o f planing Melt number 4

Weight of Resilience per Span in Depth of Width of Bar in cubic inch in Experiment Number of inches Beam in Beam in pounds inch-pounds

NOTE For effect o f span, see Table 2 All bars were rectangular

Coming back to the regular order: Table 4 shows the effect of planing on the resilience

of a cast-iron bar The results shown are somewhat remarkable The bar, after planing off the surface on all four sides, is much tougher than it was before This difference cannot be due to any fault in the method of testing, as may be seen from a comparison of this table with Tables 2 and 3 The superiority of the planed bar is probably due to the lessening of the shrinkage strains when the surface of the rough casting is removed It is possible that the same gain might be made by annealing the rough bar The discovery of the great increase

in resilience after planing might have been prophesied, perhaps, from studies heretofore made

of the loss of strength due to shrinkage strains This fact, however, has never before been demonstrated by actual impact tests, to the author's knowledge The great advantage of finishing castings exposed to shocks should be taken into account by designers of machinery Table 6 gives the results of tests o f paving brick The first tests of brick, made with the hammer, were unsuccessful on account of the great thickness of a brick compared with its length The broken brick would wedge between the hammer and the opposing knife-edge,

so that the hammer could not swing through To remedy this, the author devised a knife- edge which would be immovable when struck squarely, but which would move freely by a side pressure The form and dimensions of this device are shown in Fig 13 As soon as the brick is broken, the knife edges are thrown outward and the hammer swings freely through With the aid of these "free knife-edges" bricks were tested with good results

Owing to the low resilience of a brick compared with its weight, it was found advisable

to raise the hammer no higher than was necessary to break the brick A higher drop usually showed a higher resilience It is probable that the values given in Table 6 are higher than would be obtained could the error due to inertia be entirely eliminated It is hardly safe to accept these results in comparing bricks, unless they be of the same dimensions

Table 7 shows the results of a few tests of red brick The comparative values obtained from soft and hard bricks are as might be expected The familiar test of striking two bricks together in the hands is a crude impact test, and, in experienced hands, probably determines the comparative toughness of the brick with some accuracy

Table 17 gives a comparison of the values obtained with different materials, tested in the manner described They are classed as brittle materials because they can be tested in the same way as cast iron, and do not require special treatment, as do wrought iron and steel The table gives a good rough idea o f the comparative value of these materials under impact The values given in the last column are the mean of several tests in each case They should not be taken as typical, as the samples were taken from materials at hand and may not be truly representative

Tests of Tough Materials

Having now dealt more or less effectively with the brittle materials, a class that presents greater difficulties must be considered How, for example, shall the ultimate resilience of a sample of wrought iron be determined? If an attempt is made to break a rectangular bar of soft iron, it will only be bent To break such a bar successfully, it must first be nicked A nicked bar can be broken, and the resilience to be overcome is but little more than that of the metal lying close to the nick

For want of some better method, the author adopted t h e plan of using a nicked bar for testing soft iron and steel, and determining the ultimate resilience per square inch of cross- section at the nick If the nick is deep enough to cause the bar to break off short, and is always of the same form, it would seem that the resilience should be in some degree pro- portional to the area of the reduced section If, furthermore, the reduced section be always

of the same depth, the resilience should be directly proportional to the area

TABLE 7 Resilience o f red brick All broken on a span o f 7 ins

Kind of Brick

Dimensions

or metal Fig 11 is the same as Fig 10, but with the section reduced as in Fig 9 In the last two forms, the hammer strikes the bar at the side of the smaller nick

Table 8 shows the results of nicked tests made with cast-iron The values given in the last column show that the metal was all of equal toughness The observed values, given in the column next to the last, indicate that the resilience per square inch of section is not constant for varying depths of section

Table 9 shows the results of tests with different kinds of wood The resilience values shown by this table are probably somewhat high on account of loss by denting the wood

Fig 10

Fig 11

RUSSELL ON TESTING MATERIALS BY IMPACT 31

~b

at nick, at nick, inch of bars (rough)

Fig inches inches Number of nick melt

* Figure No giving shape of nick (see Figs 7 to 11)

N B. All bars 2 ins x 1 in All nicked bars broken edgewise, on 12-in span Weight of each bar about 6.4 lbs