Acoustic emission standards and technology update ASTM special technical publication 1353

Sach tiếng Anh: Acoustic Emission: Standards and Technology Update, Acoustic Emission (AE) has been commercially available for more than thirty (30) years. Has any progress been made? The purpose of the Symposium held in January 1998 in Plantation, Florida was to discuss the evolution of the technology of AE over the years in instrumentation, applications, standards and codes and its overall worldwide acceptance. Authors have made comparisons between AE and other Nondestructive Testing (NDT) technologies as to their suitability in solving practical industrial problems worldwide.

Library of Congress Cataloging-in-Publication Data

Acoustic emission : standards and technology update / Sotirios J

1 Acoustic emission testing I Vahaviolos, Sotirios J

II Series: ASTM special technical publication : 1353

TA418.84 A263 1999

CIP Copyright 9 1999 AMERICAN SOCIETY FOR TESTING AND MATERIALS, West Conshohocken,

PA All rights reserved This material may not be reproduced or copied, in whole or in part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent

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Each paper published in this volume was evaluated by two peer reviewers and at least one editor 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 Philadelphia, PA October 1999

Foreword

This publication, Acoustic Emission: Standards and Technology Update, contains papers pre- sented at the symposium of the same name held in Plantation, Florida, on 22-23 January 1998 The symposium was sponsored by ASTM Committee E7 on Nondestructive Testing The symposium chairman was Sotirios J Vahaviolos, Physical Acoustics Corporation

Contents

A E SOURCES: CHARACTERIZATION

Use of Acoustic Emission to Characterize Focal and Diffuse Microdamage in

B o n e m R M RAJACHAR, D L CHOW, C E CURTIS, N A WEISSMAN, AND D H KOHN

CONCRETE APPLICATIONS

A Proposed Standard for Evaluating Structural Integrity of Reinforced Concrete

Beams by Acoustic E m i s s l o n - - s , YUYAMA, T OKAMOTO, M SHIGEISH], M OHTSU,

AND T KISI-H

On the Necessity of a New Standard for the Acoustic Emission Characterization of

Concrete and Reinforced Concrete Structures E o NESWJSKI

AE Evaluation of Fatigue Damage in Traffic Signal Poles H R HAMILTON, HI,

T J FOWLER, AND J A PUCKETI"

25

41

50

INTmGRrrY AND LEAK DETECTION/LOCATION METHODS

T h e D e v e l o p m e n t of Acoustic Emission for Leak Detection and Location in Liqnid-

Filled, B u r i e d Pipelines R ~ MILLER, A A P O L L O C K , P FINKEL, D J WATI'S,

J M CARLYLE, A N TAFURI, AND J J y1~7.71 JR

Acoustic Emission and Ultrasonic Testing for Mechanical Integrity s J TERNOWCHEK,

T J GANDY, M V CALVA, AND T S PATIT~SON

67

79

AE SENSORS, STANDAm)S, AND QUANTITATIVE A E

Calibration of Acoustic Emission Transducers by a Reciprocity M e t h o d - - H HATANO 93

Acoustic Emission Applied to Detect Workpiece Burn During G r i n d i n g - -

P R DE AGUIAR, P WILLETI', AND J WEBSTER

Analysis of Fracture Scale and Material Quality Monitoring with the Help of

Acoustic Emission Measurementsms A NIKULIN, M A SHTREMEL, V G KnANZH~,

E Y KURIANOVA, AND A P MARKELOV

Characterization of Micro and Macro Cracks in Rocks by Acoustic E m i s s i o n - -

G M NAGARAJA RAO, C R L MURTHY, AND N M RAJU

Prediction of Slope Failure Based on AE Activity T SHIOTANI AND M OHTSU

A E SOURCES: RESEARCH T o P I c S

Identification of AE Sources by Using SiGMA-2D Moment Tensor Analysism

M SHIGEISHI AND M OHTSU

TRANSPORTATION APPLICATIONS, STANDARDS, AND METHODOLOGy

P r a c t i c a l A E Methodology f o r U s e o n A i r c r a f t m J M CARLYLE, H L BODINE, S S HFa'qLEY,

COMPRESSED G A S APPLICATIONS AND STANDARDS

Periodic AE Re-Tests of Seamless Steel Gas Cylindersmp R BLACKBURN

Field Data on Testing of Natural Gas Vehicle (NGV) Containers Using Proposed

ASTM Standard Test Method for Examination of Gas-Filled Filament-Wound Pressure Vessels Using Acoustic Emission (ASTM E070403-95/1)

R D FULTINEER, JR AND J R MITCHELL

Acoustic Emission Testing of Steel-Lined FRP Hoop-Wrapped NGV C y l i n d e r s - -

A AKHTAR AND D KUNG

Overview

Acoustic Emission (AE) has been commercially available for more than thirty (30) years Has any progress been made? The purpose of the Symposium held in January 1998 in Plantation, Florida was to discuss the evolution of the technology of AE over the years in instrumentation, applications, standards and codes and its overall worldwide acceptance Authors have made comparisons between

AE and other Nondestructive Testing (NDT) technologies as to their suitability in solving practical industrial problems worldwide

As the newcomer in the Nondestructive Evaluation (NDE) industry, AE was first tried on applica- tions where other NDT technologies had previously failed or was used where wild financial cost savings were promised The issue of suitability of AE for an application was never considered until the very late 70's and early 80's, when a new breed of industrial and university researchers entered the field in USA, Europe and Japan AE "noise counting" was replaced with basic work on source characterization, wave propagation, mode conversion, the study of the inverse problem using a number of Green's functions, pattern recognition and, most importantly, they considered AE as a science, using all available tools at their disposal While the university academics worked hard to identify certain AE waveform features with source and failure mechanisms, a number of industrial researchers explored a myriad of "Pseudo-sources" of AE and their statistical nature Instead of absolute one-on-one correlations and exact location of defects, practitioners developed zonal location and data bases based on case studies that enabled them to relate AE to fracture mechanics, corrosion phenomena, and overall part integrity assessment, especially in composite structures first and then

in pressurized systems and individual components The introduction of artificial intelligence, coupled with existent data bases, led to the development of ready-to-use knowledge-based systems based

on very complex structures that are found in power utilities, refineries, chemical plants, complex pipelines, wind tunnels, aircraft structures, etc The hard work of the late 70's and early 80's by CARP (Committee on AE for Reinforced Plastics) and the wide application of AE in testing of Fiberglass (FRP/GRP) vessels and pipes rejuvenated the technology! Eventually they became ASTM Standards now widely in use

The well-publicized early failures of AE in several metal vessels tests, especially in Europe

by INEXPERIENCED personnel, were now reconsidered Unknown to most AE Researchers/ Practitioners a behind the scenes branch of CARP known as CAM (Committee for Acoustic Emission for Metal) start looking carefully utilizing vast experience in Fracture Mechanics, Civil Engineering, NDT and, most importantly, vessel construction maintenance and use, realized early on that the same inexperience that prevented the use of AE in FRP in the early 70's has prevented users to do Metal Vessel Testing by AE

With the help of t h e ' 'core members" of CARP, metal vessel testing was reconsidered, especially after the successes of MONPAC ~ (a commercially available knowledge-based expert system that formed the basis of acceptance of AE by American Society of Mechanical Engineers (ASME) and Department of Transportation (DOT) and, thus, gave credence to the newcomer NDE technology)

In addition, the more than ten AE ASTM Standards and AE's acceptance by American Society for Nondestructive Testing (ASNT) as another major NDT technique and the establishment of Level III in AE were major steps forward for the technology worldwide

vii

viii ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

In this Symposium basic important work is being presented that constitutes the basis for Natural Gas Vehicle (NGV) Cylinder Testing with AE, no matter how controversially some people might view their work When properly applied, AE can save NGV assets for customers as the ASTM FRP vessel has done for the past 10-plus years

It is interesting to note that infrastructure and slope stability applications worldwide and especially

in Japan are now to the point of standardization of existing working procedures We were very much encouraged by the continuing success of the Reciprocity Method for Calibrating AE Sensors and hope that it eventually will become another ASTM Standard As for the other applications, I can only comment on their existing uniqueness from micro damage in bones to burning of grinding tools in high speed manufacturing

We hope this publication will prove interesting to a wide spectrum of readers, especailly those who look for new AE Standards and are interested to explore the future directions for the application

of the Acoustic Emission Technology

Sotirios J Vahavidos, Ph.D

Physical Acoustics Corporation Princeton Junction, NJ 08550 Symposium Chairman and Editor

AE Sources: Characterization

Rupak M Rajachar, 1 Dann L Chow, l Christopher E Curtis, 2 Neil A Weissman, 2 and David H Kohn 3

A B S T R A C T : Fatigue of cortical bone results in the initiation, accumulation, and propagation of microdamage AE techniques were adopted to monitor damage generated during ex-vivo tension-tension fatigue testing of cortical bone The primary objectives were to determine the sensitivity of AE in detecting microdamage in cortical bone and to elucidate mechanisms guiding the onset of microdamage Fatigue cycle data and histological data show that AE techniques are more sensitive than modulus reduction techniques in detecting incipient damage in cortical bone Confocal microscopy revealed the ability of AE to detect crack lengths and damage zone dimensions as small as 25 gm Furthermore, measured signal parameters such as AE events, event amplitude, duration, and energy suggest that AE techniques can detect and distinguish microdamage

mechanisms spatially and temporally in bone As fatigue processes continue, AE increases in terms of number of events, event intensities and spatial distribution Diffuse damage appears to be a precursor to the development of linear microcracks The spatial and temporal sequence of AE events enables differentiation between linear microcracks and more diffuse damage

K E Y W O R D S : acoustic emission, bone, microdamage, confocal microscopy

l Graduate Student, Department of Biomedical Engineering, College of Engineering,

University of Michigan, Ann Arbor, MI 48109-2125

2 Undergraduate Student, Department of Biologic and Materials Sciences, School of

Dentistry, University of Michigan, Ann Arbor, MI 48109-1078

3 Associate Professor, Departments of Biologic and Materials Sciences, School of

Dentistry and Biomedical Engineering, College of Engineering, University of

Michigan, Ann Arbor, MI 48109-1078

Copyright9 by ASTM International

3

www.astm.org

4 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

Age-related skeletal fragility is a significant medical and economic problem In the United States, an estimated 250,000 hip fractures occur each year among those age 50 and over [t] The cost involved in the treatment of these fractures is believed to be over 7 billion dollars [2,3] In younger people, who have a greater physiologic capacity to withstand the functional demands placed on the musculosketetal system, stress fractures and impact/trauma-related fractures are also of significance However, with age and pathologic conditions, the compensatory mechanisms needed to maintain the mechanical stability of skeletal tissue become significantly more impaired [3] As a result, diffuse distributions of sub-threshold microcracks, which have been observed even in normal bone, increase exponentially in number with age These cracks range in length up to 300

~tm and may ultimately contribute to age-related property degradation and fatigue fracture in skeletally mature cortical bone (e.g the dense bone of the mid-femur, or outer shell of the proximal femur and spine) and trabecular bone (e.g the porous, spongy bone inside the cortical shell) [4] Consequently, the processes guiding the location,

orientation, size, and accumulation of microdamage are important in assessing the competence of skeletal tissues and in developing a basic understanding of structure- function relationships

Cortical bone can be modeled as a fiber-reinforced composite [5] Osteons or Haversian canals are cylindrical layers of bone around blood vessels The inter-osteonal bone, which is arranged in lamellae, is called interstitial bone Thus, osteons in bone, which are primarily oriented in a longitudinal direction, are analogous to unidirectional fibers in a composite As in many engineering composites, a comparatively high

strength/low toughness interface exists between each osteon or "fiber" and its

surrounding interstitial bone or "matrix" This region is referred to as the cement line and

is thought to be a site of relatively easy crack nucleation [6] Also of importance to the overall structure of bone are embedded cellular compartments and their connecting network, lacunae and canaliculi, respectively [6] These regions represent heterogeneous sites of porosity, akin to processing defects in man-made structural materials

Accordingly, these pores may serve as additional sites of crack nucleation and

accumulation [7,8]

Bone is therefore a hierarchical composite structure subject to internal

microdamage Many of the difficulties which exist in characterizing the mechanics of structural composites also exist with bone The long-term objectives of this research program are to model bone as a composite material and: 1) determine the effects of mechanical history on microdamage initiation and growth; 2) provide insight into mechanisms of damage initiation and accumulation; and 3) determine how damage phenomena are modulated by changes in tissue hierarchy

Previous investigations have attempted to use AE to monitor integrity of bone in- vitro [9-12], and as a non-invasive diagnostic in-vivo [13-15] These studies made inferences that the AE detected was due to specific failure mechanism(s), but no direct correlations between AE and damage were established Using more rigorous analyses,

we have shown that AE techniques can be effectively used to distinguish mechanisms of crack nucleation, slow crack propagation, and rapid crack propagation, spatially and temporally, in biomaterials, microstructured materials and inhomogeneous materials

[16,17] The critical resolution of AE for detecting crack nucleation was shown to be on the order of 10 ~tm [16] Because of the complex inhomogeneous nature of cortical bone,

RAJACHAR ET AL, ON MtCRODAMAGE IN BONE 5

application of these more detailed AE techniques may provide similar insight into the mechanical processes involved in the nucleation and growth of damage in bone

Moreover, the sensitive and non-destructive nature of AE testing may allow multiple

crack sites to be distinguished spatially and temporally

In support of our long-term objectives, the specific aims of this project were to: 1) detect and characterize incipient microdamage in cortical bone via AE, 2) verify and

quantify the microdamage histologically, 3) compare the sensitivity of AE and modulus reduction (AE) techniques, 4) compare microdmnage in bone of different initial stress intensities, and 5) associate AE signals with microstructural failure mechanisms

Materials and M e t h o d s

Cortical Bone Specimens

The flow chart in Figure 1 provides an overview of the experimental design used

in preparing and testing cortical bone ex-vivo Cortical bone specimens were prepared from mature bovine femoral and tibial central diaphyseal sections Each diaphysis was sectioned on a band saw into paratlelepipeds, such that the longitudinal axis of each

parallelepiped was aligned with the long axis of the bone These rough-cuts were then machined into smooth parallelepiped blanks (L = 120 mm, W = 12 mm, T = 4.5 mm) and gage sections were machined using a precision milling machine Buffered saline

irrigation was used during all machining steps to avoid heating the bone and to maintain tissue saturation Two gage section geometries were created: V-notched specimens

(p = 200 ktm, K t = 2.5), which provided a localized region of strain, and C-notched

specimens (K t = 1), which provided a distributed strain region Specimens not tested

immediately after machining were wrapped in moist towels and stored at -65~

successive uniaxial tensile ramp loading and unloading cycles in the elastic range until a steady state value was reached Peak stresses were approximately one-quarter of the

yield stress for bovine cortical bone (~y - 140 MPa), and a steady state modulus value was typically reached in 5 cycles The initial elastic modulus was then used to define a fatigue loading regime [18] Fatigue loading was performed in uniaxial tension under load control Specimens were loaded parallel to the longitudinal axis of the bone,

following a sinusoidal waveform, over an effective strain range (AEef f = A(Y]E0) of 0 to

3000 ge, at a frequency of 1 Hz The maximum strain was chosen such that it was in the upper range of physiologic strain experienced by cortical bone during normal loading

[19]

6 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

I Mature Bovine Bone (femur) I

1

I Diaphyseal Longitudinal Cut I

J End Milled Paralleleplped J

I [ localized damage zone I v-Notch I I , diffuse damage zone I C-Notch I

I Fatigue Loading Conditions I

3000 petrain, 1 Hz sine wave

I AE Parameteis Measured

events, Ioc., ampL, (mergy, etc

I 10 mm notched section Histology

bulk staining and embedding

RELATE AE TO FAILURE MECHANISMS

RAJACHAR ET AL ON MICRODAMAGE IN BONE 7

Acoustic Emission Analysis

AE was recorded using a planar array of four Physical Acoustics Corp (PAC, Princeton, N J) nano-30 piezoelectric transducers having a broad-band frequency range of 125-750 kHz Two sensors were placed on the shoulders o f the 120 mm x 12 m m face, for longitudinal location, and the other two sensors were on opposite sides of the width of the specimen, above and below the notched region, for transverse location The sensors were coupled to the specimens with an acoustic couplant and fixed in place using water- resistant surgical grade adhesive tape AE data were collected, stored and analyzed with PAC LOCAN-320 data acquisition and analysis software The pertinent operating

parameters were: variable gain/total gain = 42 dB/80 dB; peak definition time = 500

gtsec; hit definition time = 2 msec; dead time = 1 msec; sample time = 100 msec;

threshold = 1 V A threshold value of 42 dB was used to eliminate background noise

produced by specimen irrigation For the tests stopped at the onset of AE, the first AE signals above 42 dB simultaneously detected at all 4 sensors were taken to signify the onset of microdamage

The following AE parameters were recorded and analyzed: AE source location, number of AE events, and intensities of AE events Event intensities are a collective term for event amplitude, counts per event, event duration, event energy counts and event rise time Subsets of events were also created, based on event location, fatigue cycle number, and stress range at which events were generated Subsets of event intensities generated within different ranges of location, fatigue cycle number and stress level were also

analyzed Subsets of events were then analyzed by evaluating location distribution

histograms (LDH) and intensity distribution histograms (IDH) o f events LDH and IDH are general terms for the distribution of events and event intensities as functions of

location, fatigue cycle number or stress

Initial analysis of the spectral components of waves was also carried out Digital transient capture of waveforms was performed using an F4000 Fracture Wave Detector (Digital Wave Corp., Englewood, CO) A maximum digitization sampling rate of 12.5 MHz was used and 1024 points of digitized data were collected from each waveform Characteristic extensional and flexural waves were generated by breaking a lead pencil at multiple sites on selected bone specimens Actual AE waves generated during testing o f bone were recorded and digitized, and mode shapes and dominant frequency contents were determined and compared to the waveforms generated by the pencil breaks [20,21]

8 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

Histological Analysis

Following fatigue loading, 10 mm blocks from the region near the dominant AE sources were cut using a diamond wafering blade, and stored at -65~ in buffered saline solution until histological processing Specimens were stained en-bloc using a graded series of 1% basic fuchsin solutions in ethanol [22] Basic fuchsin is a fluorescent stain that preferentially marks exposed external and internal defects in biological structures Bulk staining with basic fuchsin prior to histological sectioning enables visualization of microdamage and differentiation between cracking due to mechanical factors and artifactual cracks induced during histological preparation

Following staining each specimen was embedded in poly-(methyl methacrylate) and serially sectioned (-150 p.m thick) parallel to the 4.5 mm thickness Each serial section was analyzed using laser scanning confocal microscopy (LSCM) to assess damage at the notch tip and throughout the bulk of the specimen Three-dimensional data

on crack morphology were obtained by using a z-axis reconstruction [23] A l-ram square grid system was imposed on each histological section Each grid space was analyzed for number of cracks, crack length, crack density, and crack angle

R e s u l t s

In support of Specific Aims 1 and 2, we were able to detect and characterize AE generated during fatigue of cortical bone and validate, via histological comparison to unloaded controls, that the sources of this AE were fatigue-induced microdamage An LDH of AE events generated during fatigue of a V-notched specimen is shown in Figure

2 AE events are localized at the notch-tip (location 0) Representative LSCM photos of the notch-tips of fatigue and control specimens are shown in Figure 3 The highly luminous regions at the notch-tip of the fatigue specimen represent damage nucleation More well defined linear (Mode I) microcracks are observed ahead of the diffusely damaged region Quantitative histology revealed crack lengths as small as 25 p.m

RAJACHAR ET AL ON MICRODAMAGE IN BONE 9

specimen, and b) specimen fatigued until the onset of AE In the fatigue specimen,

damage accumulation is seen at the notch-tip Diffuse damage is observed at the notch- elp, with well-defined linear microcracks observed ahead of the diffuse damage zone (Both I OOX)

10 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

The average number of fatigue cycles to the onset of damage as determined by the two techniques, for V- and C-notched specimens, is presented in Table 1 (Specific Aim 3) For both specimen groups, incipient damage was detected at a significantly lower number of fatigue cycles with AE than AE (p < 0.01, p < 0.02, respectively, via Student's t-tests)

TABLE 1 - Number of cycles to the onset of fatigue as determined by acoustic emission

(AE) and modulus reduction (AE) techniques

Technique Specimen Type

commensurate with the longer fatigue regime of this specimen Evaluating IDHs (Fig 5), shows that peak and maximum values of signal intensities generated in bone that was fatigued until a 1% AE are greater than those generated in bone fatigued until just the onset of AE Average intensities were also greater in the AE specimens Comparison of confocal micrographs from AE and AE specimens (Fig 6) suggests the presence of diffuse microdamage followed by development of linear microcracks as fatigue processes continue Diffuse damage is characterized by relatively larger zones of staining opacity and minimally resolvable crack dimensions While many histological sections exhibited Mode I microcracks, several sections revealed the presence of linear microcracks, adjacent to regions of diffuse damage, which were oriented parallel to the direction of loading (Fig 6c)

In support of Specific Aims 4 and 5, we were able to compare microdamage in bone of different initial stress intensities and associate AE signals with microstructural failure mechanisms The mechanisms, observed histologically, were characterized based upon differences in specific AE signal parameters, as shown in the LDH and IDH in Figures 7 and 8 Specimens with a lower initial stress concentration generated a greater number of events, a greater spatial distribution of events and more high intensity events

RAJACHAR ET AL ON MICRODAMAGE IN BONE 1 1

notched specimens fatigued until the onset of AE and until a 1% modulus loss The greater number of events generated in the AlE specimen is commensurate with the longer fatigue process in this specimen

12 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

notched specimens fatigued until tile onset o f A E and m~til a 1% modulus loss A E signal hm, nsities in the A E specimens are lower than those o f the AE group, since the tests were stopped at an earlier stage o f fittigue

RAJACHAR ET AL ON MICRODAMAGE IN BONE 13

microcracks (I OOX), and c) combination of diffuse damage and Mode H linear

microcracks (60X)

14 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

notched and C-notched specimens fatigued until the onset of AE The C-notched specimens exhibited a greater spatial distribution of AE events

RAJACHAR ET AL ON MICRODAMAGE IN BONE 1 5

and C-notched specimens fatigued until the onset of AE AE signal intensities are

different in the two specimens, indicating different damage mechanisms

16 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

Discussion

The objectives of this research were to determine the sensitivity of AE in

detecting microdamage in cortical bone and to use the early warning capabilities of AE to help elucidate the underlying mechanisms guiding the nucleation and growth of

microdamage in bone

LDHs (Fig 2) and histology (Fig 3) provided evidence that damage in cortical bone can be detected and characterized using AE This is apparent by the prominent AE that was detected and microcracking that was observed at the notch-tips Quantitative histology revealed that the AE was first generated at crack lengths and damage zone dimensions as small as 25 ~m

The fatigue data shows that AE techniques are more sensitive than modulus reduction techniques for detecting damage in cortical bone for each specimen geometry (Table 1) A greater number of AE events was also detected in specimens which were allowed to fatigue until a 1% AE was achieved (Fig 3) The IDHs (Fig 4) reflect the greater number of events, greater peak amplitude and greater number of high intensity events for the AE specimens The damage inferred by the AE and AE techniques was subsequently verified histologically All of these data support that conclusion that AE is more sensitive than modulus reduction techniques in discriminating incipient damage in bone Currently, modulus reduction techniques represent the accepted real-time means of detecting damage in bone [24]

Since the sensitivity of AE is greater than that of AE, it is expected that there should also be a difference in the type and extent of microstmctural damage observed in the two groups It is therefore hypothesized that different mechanisms of damage initiation and propagation in bone will express distinguishable AE signal profiles The IDHs suggest these differences

Based on the relative amounts of diffuse and linear damage observed in control,

AE and AE specimens, there, qualitatively, appears to be a progression of diffuse damage accumulation which ultimately coalesces into more distinct linear microcracks There appears to be a greater damage density (i.e # of cracks or # of diffusely stained

regions/unit area) in AE samples compared to AE samples, and a threshold in damage density above which AE is generated This threshold corresponds to an accumulation of plastic strain energy sufficient to generate detectable AE

Coupled with histological data, AE analysis suggests an initial diffuse matrix damage mechanism in the fatigue failure of cortical bone, analogous to microvoiding and craze formation in fiber reinforced composites Further fatigue results in the formation of linear microcracks LDH data allows the differentiation of diffuse matrix damage away from the notch-tip and damage at the primary fracture process zone

Linear microcracks were most often associated with cement lines, an observation made by others as well [7,25,26] The specific nature of the interactions between

microcracks and cement lines is unclear Microcracks were observed to propagate both along and across cement lines Combined with the fact that linear microcracks were observed to be oriented in both Mode I and Mode II directions, the overlapping of AE signal parameters in the different test groups is likely due to the combination of

ultrastructural failure mechanisms observed

RAJACHAR ET AL ON MICRODAMAGE IN BONE 1 7

The observation of longitudinal and shear oriented cracks motivated an analysis

of AE waveforms Based on the characteristic extensional and flexural waves generated during pencil breaks (Fig 9), analysis of spectral components of AE signals generated during fatigue of bone revealed the following At the notch-tip, longitudinal waves, with

a dominant frequency range of 250-400 kHz, were primarily detected, whereas in the bone matrix, flexural waves, with a dominant frequency range of 100-180 kHz, were

primarily detected These characteristics are consistent with those of fiber reinforced

plastics [27] While it is acknowledged that modal analyses are limited to plate specimens and may not be applicable to in-vivo diagnosis of a whole femur, the modes of micro- failure observed in this study are consistent with those observed in-vivo Idealized testing

on plate-specimens, which are amenable to modal analysis, may yield significant insight into damage mechanisms

Whether diffuse damage and modes I and II linear microcracking processes can

be linked to specific AE signals is a continuing focus of this research It may ultimately

be possible to link damage location with particular initiation and propagation

mechanisms, which may provide necessary information to the detection and prevention of catastrophic damage associated with many degenerative orthopaedic conditions If a

continuum can be established between diffuse damage and linear microcrack initiation and propagation mechanisms, it may also be possible to relate mechanical and biological components affecting the overall structure of cortical bone under different loading

conditions Thus, future work will not only attempt to quantify the relationship of the strain history of cortical bone to damage initiation and propagation mechanisms, but also

to define local biologic conditions favoring damage mechanisms

18 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

frequencies- 300-500kHz, b) flexural wave, dominant frequencies ~ 100-200 kHz

RAJACHAR ET AL ON MICRODAMAGE IN BONE 19

Summary and Conclusions

AE techniques were employed in association with histological analysis to

determine the effects of strain history on the initiation and accumulation of fatigue

microdamage in cortical bone Microstructural damage mechanisms were identified and related to AE parameters spatially and temporally Specifically, the following conclusions are drawn:

(1) AE techniques are effective in detecting incipient microdamage generated during fatigue of cortical bone

(2) Histological comparison between loaded and unloaded specimens validated the

AE technique and revealed that AE can detect microcracks in bone as small as

25 ~tm Multiple microcracks and diffuse regions of damage may also be detected (3) Incipient fatigue damage is detected at a significantly lower of fatigue cycles with

AE techniques, as compared to modulus loss techniques and, as a result, less AE

Acknowledgements

Supported by NSF BES-9410303, the Whitaker Foundation and the Natural Sciences and Engineering Research Council of Canada We gratefully acknowledge Mitch Schaffler, Ph.D of Henry Ford Hospital for his help in the histological processing and analysis, and Gordon Schneider of Digital Wave Corp for his help performing the modal analyses

Melton, L.J., Eddy, D.M., and Johnston, C.C Jr., 1990, "Screening for

Osteoporosis," Ann Int Med Vol 112, pp 516-528

Holbrook, T.L., Grazier, K., Kelsey, J.L., and Stauffer, R.N., 1984, American

Academy of Orthopedic Surgery, Rosemont, IL

Birdwood, G., 1996, Understanding Osteoporosis and its Treatment A Guide for Physicians and their Patients, Pearl River, New York

Schaffler, M.B., Choi, K., and Milgrom, C., 1995, "Aging and Matrix

Microdamage Accumulation in Human Compact Bone," Bone, Vol 17, pp 521-

525

20 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

[51

[61

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Martin, R.B., Burr, D.B., 1989, Structure, Function, and Adaptation of Compact Bone, Raven Press, New York

[7] Frost, H.M., 1960, "Presence of Microscopic Cracks In Vivo in Bone," Henry Ford Hospital Bulletin, Vol 8, pp 25-35

[8] Martin, R., 1982, "A Hypothetical Mechanism for the Stimulation of Osteonal Remodelling by Fatigue Damage," J Biomech., Vol 15, pp 137-139

[9] Knet-s, I.V., Krauya, U.E., and Vilks, Y.K., 1975, "Acoustic Emission in Human Bone Tissue Subjected to Longitudinal Extension," Mekh Polim., Vol 4, pp 685-

Wright, T.M., Vosburgh, F., and Burstein, A.H., 1981, "Permanent Deformation

of Compact Bone Monitored by Acoustic Emission," J Biomech., Vol 14, pp 405-409

Fischer, R.A., Arms, S.W., Pope, M.H., and Seligson, D., 1986, "Analysis of the Effect of Using Two Different Strain Rates on the Acoustic Emission in Bone,"

J Biomech., Vol 19, pp 119-127

Zioupos, P., Currey, J.D., and Sedman, A.J., 1994, "An Examination of the Micromechanics of Failure of Bone and Antler by Acoustic Emission Tests and Laser Scanning Confocal Microscopy," Med Eng Phys., Vol 16, pp 203-212 Wright, T.M., Arnoczky, S.P., and Burstein, A.H., 1978, "In-situ monitoring of ligament damage in the canine knee by acoustic emission," Mater Eval., Vol 37 Wright, T.M., Hood, R.W., and Flynn, W.J., 1981, "Acoustic emission

monitoring in the diagnosis of loosening in total knee arthroplasty," 1981

Biomechanics Symposium, Van Buskirk, W.C and Woo, S.L.-Y., Eds., American Society of Mechanical Engineers, pp 203-212

Poliakoff, S.J., Miller, R.K., Jones, C.B., and Bright, R.W., 1989, "Acoustic emission monitoring of physeal separation: an experimental study," Trans Orthop Res Soc., Vol 14, p 483

Kohn, D.H., Ducheyne, P and Awerbuch, J., 1992, "Acoustic Emission During Fatigue of Ti-6A1-4V: Incipient Fatigue Crack Detection Limits and Generalized Data Analysis Methodology," J Mater Sci., Vol 27, pp 3131-3142

RAJACHAR ET AL ON MICRODAMAGE IN BONE 21

Kannatey-Asibu, E and Emel, E., 1987, "Linear Discriminant Function Analysis

of Acoustic Emission Signals for Tool Condition Monitoring," J Mech Sys

Signal Proc., Vol 1, pp 333-347

Kohn, D.H., 1995, "Acoustic Emission and Non-Destructive Evaluation of

Biomaterials and Tissues," Crit Rev Biomed Eng., Vol 22, pp 221-306

Burr, D.B., 1995, "Alterations to the En Bloc Fuchsin Staining Protocol for the Determination of Microdamage Produced In Vivo," Bone, Vol 17, pp.431-433

Ross, M., 1995, Histology A Text andAtlas, Williams and Wilkins, New York

Schaffier, M.B., Radin, E.L and Burr, D.B., 1989, "Mechanical and

Morphological Effects of Strain Rate on Fatigue of Compact Bone," Bone, Vol

10, pp 207-214

Carter, D.R., and Hayes, W.C., 1977, "Compact Bone Fatigue Damage - I

Residual Strength and Stiffness," J Biomech., Vol 10, pp 325-337

Schaffier, M.B., Pitchford, W.C., Choi, K., and Riddle, J.M., 1994, "Examination

of Compact Bone Microdamage Using Back-Scattered Electron Microscopy,"

Bone, Vol 15, pp 483-488

Prosser, W.H., Jackson, K.E., Kellas, S., Smith, B.T., McKeon, J., and

Friedman, A., 1995, "Advanced Waveform-Based Acoustic Emission Detection

of Matrix Cracking in Composites," Mater Eval., Sept., pp 1052-1058

Concrete Applications

Shigenori Yuyama, ~ Takahisa Okamoto, ~ Mitsuhiro Shigeishi, 3 Masayasu Ohtsu, aand Teruo Kishi 4

A PROPOSED STANDARD FOR EVALUATING STRUCTURAL INTEGRITY

OF REINFORCED CONCRETE BEAMS BY ACOUSTIC EMISSION

REFERENCE: Yuyama, S., Okamoto, T., Shigeishi, M., Ohtsu, M., and Kishi, T., " A Proposed Standard for Evaluating Structural Integrity of Reinforced Concrete

Beams by Acoustic Emission," Acoustic Emission: Standards and Technology

Update, ASTM STP 1353, S J Vahaviolos, Ed., American Society for Testing and Materials, West Conshohocken, PA, 1999

ABSTRACT: A series of studies has been performed to evaluate the structural

integrity of reinforced concrete (RC) beams by acoustic emission (AE) Cyclic loadings were applied to RC beams with a single reinforcing bar, large repaired beams, beams deteriorated due to corrosion of reinforcement, and two beams with different damage levels in an aging dock The test results demonstrated that the Kaiser effect starts to break down when shear cracking starts to play a primary role It has been also shown that high AE activity is observed during unloadings after serious damage (slips between the concrete and the reinforcement or those between the original

concrete and the repaired part) has occurred A standard for evaluating structural integrity of RC beams by AE is proposed, based on these results

KEYWORDS: acoustic emission, cyclic loading test, evaluation criteria, Kaiser effect, reinforced concrete, structural integrity

President, Nippon Physical Acoustics Ltd., 8F Okamoto LK Bldg., 2-17-10, Higashi, Shibuya-ku, Tokyo 150, Japan

2 Chief researcher, Central Research Laboratory, Nihon Cement Co., Ltd., 1-2-23, Kiyosumi, Koto-ku, Tokyo 135, Japan

3 Associate professor and professor, ,'espectively, Depamnent of Civil and Environmental Engineering, Faculty of Engineering, Kumamoto University, 2-39-1, Kurokami,

Kumamoto 860, Japan

4 Professor, Research Center for Advanced Science and Technology, The University of Tokyo,

4-6-1, Komaba, Meguro-ku Tokyo 153, Japan

Copyright9 by ASTM International

25

www.astm.org

26 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

Introduction

In recent years the deterioration and cracking of concrete structures such as bridges and buildings has been a significant problem Proper techniques for the inspection of damaged structures are important in making rational decisions regarding rehabilitation, repair or replacement Thus, the development of techniques to evaluate degradation of concrete structures in long-term service has been one of the most important issues for an effective maintenance program

A series of studies has been performed to evaluate the structural integrity of rcinforced concrete (RC) beams by acoustic emission (AE) Cyclic loadings were applied to RC beams with a single reinforcing bar [/], large repaired beams [2], beams deteriorated due to corrosion of reinforcement [3l, and two beams with different damage levels in an aging dock [4] The test results demonstrated that the Kaiser effect starts to break down when shear cracking starts to play a primary role It has been also shown that high AE activity is observed during unloadings after serious damage (slips between the concrete and the reinforcement or those between the original concrete and the repaired part) has occurred A concrete beam integrity (CBI) ratio, the ratio of the load at onset of AE and the maximum prior load, has been proposed as

an effective criterion to measure the severity of the damage induced in the beams The high AE activities during unloadings have been shown to be an effective index to estimate the level of deterioration

This paper proposes an AE test method for RC structures, demonstrating four case studies conducted for different types of RC beams Test procedure and evaluation criteria are presented as guidelines for practical AE tests of RC beams

Case Study 1: RC Beams with a Single Reinforcing Bar

Shown in Fig 1 is a configuration of the specimen used for the cyclic bending test A single reinforcing bar of 19 mm dia with lateral lugs is encased eccentrically

in the rectangular concrete beam Concrete cover (depth of reinforcing bar) is 30 mm Compressive and tensile strengths of the concrete were 36.2 and 3.5 MPa, respectively Six PAC R15 (150 kHz resonant) sensors were attached on the specimen to perform both a moment tensor analysis using the SIGMA code [.5] and parameter analysis The specimens were subjected to repeated four-point bending loadings by a strain-control type machine The maximum load of each loading cycle was increased gradually in order to investigate the relationship between the cracking process and AE behavior Figure 2 presents the relationship between the number of AE hits and the applied load AE signals are detected at a lower load than the maximum prior load (49kN) during the second loading Accordingly, the Kaiser effect breaks down during the second loading It was shown that the Kaiser effect starts to break down when the

YUYAMA ET AL ON REINFORCED CONCRETE BEAMS 2 7

FIG.2-Relationship between number o f A E hits and the applied load

crack width exceeds 0.12 mm The breakdown of the effect becomes clearer as the cracking progresses in the third, fourth and fifth loadings High AE activities are observed during the third, fourth and fifth unloadings

The moment tensor analysis revealed that the contribution of shear cracks

increases as the breakdown of the Kaiser effect becomes clearer with the progress of the fracture It was also indicated that high AE activity is observed during the third, fourth and fifth unloadings after the maximum width of the surface cracks has

exceeded about 0.25 mm The moment tensor analysis found that the shear cracks

28 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

generated near the reinforcing bar is responsible fl)r this activity "lhe origin ,,i t h e ~ emissions was attributed to rubbings between the faces of the existing cracks or ttiction between the reinforcement and concrete

Case Study 2: Repaired RC Beams

The configuration of the specimen and the locations of displacement transducers and AE sensors are shown in Fig 3 Six PAC R15 sensors were placed lineally on the top plane of the specimen The repaired part is in the tensile side of the specimen The depth and length of the repaired part are 100 mm and 2200 mm, respectively In addition to steel bars as reinforcement, stirrups were embedded in the specimen to prevent beams shear failure

The specimens were subjected to repeated four point bending loadings by a strain-control type machine During each loading, measurements of AE, crack width, slip length between the repaired part and the original part, and strain of concrete and reinforcement were made by using AE sensors and two different types of displacement transducers

These measurements indicated that the initiation of the early tensile

microcracks, main tensile cracks, local slips, and large-scale slips are clearly detected

by AE hit measurement It was shown that once large-scale slips have occurred at the interface between the original concrete and the repaired part, A E starts to emanate at

m m AE Sensor FIG 3-Configuration o f the repaired RC beams and the locations o f displacement lransduce~ and A E sensors

YUYAMA ET AL ON REINFORCED CONCRETE BEAMS 2 9

FIG 4-Amplitude and displacement histories

much lower load than the previous maximum load, that is, the Kaiser effect no longer holds for the next loading and high AE activity can be seen even during unloading

Thus, the breakdown of the Kaiser effect and the high AE activity during unloading can be a good indicator for the occurrence of large-scale slips in repaired RC beam Amplitudes of all hits are plotted versus time together with the displacement in Fig 4 It is obvious that the initiation of the early tensile microcracks or the local

slips and the mechanical rubbings of the interlocked faces due to large-scale slips gave amplitude levels between 40 and 60 dB, while the initiation of the main tensile crack at 38.2 kN produced very high amplitudes that reached nearly 80 dB Thus, the different

AE sources could be clearly distinguished by comparing the amplitude data with the results of the visual observation and the measurement by displacement transducers

A concrete beam integrity (CBI) ratio, given below, was proposed as a criterion

to measure the severity of damage induced in repaired concrete beams

CBI ratio = load at onset of AE / maximum prior load

In the field of fiber-reinforced plastic (FRP) structures, AE tests have been

widely used to evaluate structural integrity and testing has been standardized by ASME Code, Section V, Article 11 In this code, the Felicity ratio obtained from the ratio of the load at onset of AE and the maximum prior load gives the criterion to measure the severity of previously induced damage It has been shown that the Felicity effect,

3 0 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

which is referred to as the breakdown of the Kaiser effect, is an indication of defects The felicity ratio has been well accepted to examine structural integrity of chemical plant equipment such as pressure vessels, tanks and piping However, beams, pillars, columns, and slabs are inspected in concrete structures In the case of chemical plant equipment, structures are loaded by pressurization In contrast, concrete structures are subjected to tensile, shear and bending loadings by jacking or running a heavy vehicle

As shown in the test results, failure mechanisms vary with the progress of damage in

RC beams It is obvious that the decrease of the CBI ratio is related to the generation and propagation of shear cracks Thus, the ratio can be a practical index for

evaluating structural integrity of RC beams Taking these points into consideration, the CBI ratio was introduced [6]

Listed in Table 1 are CBI ratios for each loading cycle, based on A E hit rate activity The ratios obtained by AE energy rate activity for all channels are also given

in the last column As shown in Table 1, the CBI ratio decreases from the fifth loading after large-scale slips have occurred due to the fourth loading along the interface between the original concrete and the repaired part It continues to decrease

as the damaged areas grow It is known that the occurrence of large-scale slips is an essential feature for damage that may result in a serious disaster in repaired concrete structures As shown above, the CBI ratio has very good correlation with the onset of large-scale slips and the growth of damaged areas Thus, the CBI ratio can be a very useful and effective criterion to measure the severity of damage induced in repaired RC beams

It was also revealed that high AE activity is observed during unloadings once large-scale slips have initiated between the original concrete and the repaired part The source of these emissions was ascribed to mechanical rubbings between the interlocked faces introduced by the large-scale slips

TABLE 1-Concrete Beam Integrity (CBI) ratios during the repeated loading tests o f

YUYAMA ET AL ON REINFORCED CONCRETE BEAMS 31

Case Study 3: RC Beams Deteriorated Due to Corrosion of Reinforcement

Shown in Fig 5 are dimensions (cm) of the specimen and sensor locations

Six PAC R6 (60 kHz resonant) sensors were attached on the specimen to perform the moment tensor analysis as well as AE parameter analysis The lower quarter part of the specimen was immersed in a 3% sodium chloride solution and an anodic current

was galvano-statically charged to the main steel bars until the maximum width of

surface cracks due to corrosion of the bars reached 1 mm or 4 mm Thus, three

different types of specimens i.e specimen with no corrosion damage and those with the surface cracks determined as above were subjected to repeated four-point bending

3 2 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

10000 -

ll.-,* 8000

m l 6000-

YUYAMA ET AL ON REINFORCED CONCRETE BEAMS 3 3

TABLE 2-Concrete Beam Integrity (CBI) ratios for the second and the third loadings

Loading cycle I I III

Crack width lmm 0.71 0.49

Crack width 4mm 0.28 0.10

CBI ratios for the second and the third loadings are summarized in Table 2 It

is obvious that the CBI ratios exhibit smaller values than Case 1 because of the

breakdown of the Kaiser effect during the second loading in the deteriorated specimens

It is also seen that the ratio becomes smaller as the deterioration due to corrosion of the reinforcement becomes greater During the third loading, the CBI ratios are

smaller than 1 for all the specimens Again the ratios exhibit smaller values in the specimens with the greater deterioration induced by the corrosion Thus, it has been confirmed that the CBI ratio can be an effective criterion to measure the severity of the damage due to corrosion of the reinforcement in RC beams

It is also observed in Fig 6 that different levels of AE activity are detected during unloadings, depending on the different damage levels In the specimen with no corrosion damage, relatively high AE activity is first observed during the 2nd

unloading, as shown in Fig 6(a) However, some AE activity is already detected

during the 1st unloading in the case of the deteriorated specimen (crack width lmm) High activity is seen during the 2nd unloading (Fig 6(b)) Quite high AE activity is observed during the 1st and the 2nd unloadings in the heavily deteriorated specimen (crack width 4 mm), as seen in Fig 6(c) Thus the levels of AE activity during

unloadings reflect the damage levels induced in the specimens Since high AE activity corresponds to the occurrence of serious damage, it can be an effective index to

estimate the level of deterioration

Case Study 4: An Aging Dock

Structural integrity of RC beams was evaluated in an aging dock Shown in Fig 7 is a cross section of the tested pier A repaired beam and a damaged one

without repair were subjected to three loadings by running a dump truck with three different load levels i.e empty (ll3kN), half load (142kN), and full load (171kN)

Figure 8 schematically illustrates how the tests were performed Two R15 and R6 sensors were attached on the beams However, only the data collected by R6

sensors could be analyzed since there were no significant AE signals detected by R15 sensors due to high attenuation at higher frequencies A strain gage was attached on the main reinforcing bar to measure strain changes during the loadings Two cracks

3 4 ACOUSTIC EMISSION: STANDARDS AND TECHNOLOGY

FIG 8-Schematic illustration o f the loading test

the maximum opening width of which reached 0.8 mm were visually observed on the surface of the unrepaired beam and measurements of corrosion potential confirmed that serious damages due to corrosion of the reinforcement existed Strain gage

measurements showed that the strain change is much larger in the unrepaired beam than the repaired one for the same loadings

AE hit rate, strain and amplitude histories for the damaged beam are given in

YUYAMA ET AL ON REINFORCED CONCRETE BEAMS 35

@ ~ ~

FIG 9-AE hit rate, strain and amplitude histories for the damaged (unrepaired) beam

Fig 9 Although the repaired beam (no damage) was very quiet, high AE activity was observed since the first loading in the unrepaired one The Kaiser effect breaks down during the third loading and high AE activities are seen during the second and third unloading The AE source during the third unloading was thought to be frictions due

to slips between the reinforcement and the concrete The amplitudes from this source are smaller than 60 dB, as shown in Fig 9 The CBI ratio is smaller than 0.6 during the third loading Thus, it has been shown that the CBI ratio and the AE activities during unloadings are very good indicators for extensive deterioration in RC beams