Page Foreword...3 1 Scope ...4 2 Normative references ...4 3 Principles of characterization of discontinuities...4 3.1 General...4 3.2 Requirements for surface condition ...5 4 Pulse ech
General
Characterization of a discontinuity involves the determination of those features which are necessary for its evaluation with respect to known acceptance criteria.
Characterizing a discontinuity involves several key steps: first, determining essential ultrasonic parameters such as echo height and time of flight; second, assessing its basic shape and orientation; third, sizing the discontinuity, which can include measuring dimensions or using parameters like echo height to represent physical size; fourth, locating the discontinuity in relation to the surface or other discontinuities; fifth, identifying any additional parameters necessary for a comprehensive evaluation; and finally, assessing the probable nature of the discontinuity, such as whether it is a crack or inclusion, based on knowledge of the test object and its manufacturing history.
According to EN 583-1:1998, if the examination of a test object provides adequate data on the discontinuity for evaluation against the relevant acceptance criteria, additional characterization is not required.
The techniques used for characterisation shall be specified in conjunction with the applicable acceptance criteria.
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Requirements for surface condition
The surface finish and profile must allow for precise sizing of discontinuities, with smoother and flatter surfaces yielding more accurate results.
For most practical purposes a surface finish of R a = 6,3 m for machined surfaces and 12,5 m for shotblasted surfaces are recommended The gap between the probe and the surface should not exceed 0,5 mm.
The above surface requirements should normally be limited to those areas from which sizing is to be carried out as, in general, they are unnecessary for discontinuity detection.
The method of surface preparation shall not produce a surface that gives rise to a high level of surface noise.
General
The principal ultrasonic characteristics/parameters of a discontinuity that are most commonly used for evaluation by the pulse echo techniques are described in 4.2 to 4.7 inclusive.
The characteristics/parameters to be determined shall be defined in the applicable standard or any relevant contractual document, and shall meet the requirements of 10.1 of EN 583-1:1998.
Location of discontinuity
The location of a discontinuity is defined as its position within a test object with respect to an agreed system of reference co-ordinates.
The measurement will be based on specific datum points, considering the index point and beam angle of the probe, as well as the probe's position and the beam path length where the maximum echo height is detected.
To accurately identify the location of a discontinuity in a test object, it may be essential to verify it from different angles or directions This approach helps ensure that the detected echo is not mistakenly attributed to a change in wave mode due to the object's geometric features.
Orientation of discontinuity
The orientation of a discontinuity is defined as the direction or plane along which the discontinuity has its major axis (axes) with respect to a datum reference on the test object.
Orientation can be established through a geometrical reconstruction similar to that used for location However, it typically requires additional beam angles and/or scanning directions compared to basic location methods.
The orientation may also be determined from observation of the scanning direction at which the maximum echo height is obtained.
In various applications, it is sufficient to identify the projection of a discontinuity onto designated planes or sections within the test object, rather than needing to determine its exact orientation in space.
Assessment of multiple indications
The method for distinguishing between single and multiple discontinuities may be based on either qualitative assessment or quantitative criteria.
Qualitative determination involves observing variations in ultrasonic indications to identify the presence of one or more distinct discontinuities Typical examples of signals from grouped discontinuities in forgings or castings are illustrated in Figure 1.
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Preliminary quantitative measurements must be conducted to assess whether separate discontinuities should be evaluated individually or collectively, based on acceptance criteria defined by maximum allowable dimensions and established evaluation rules for the group.
Rules governing discontinuities within a group can be determined by the total lengths, areas, or volumes of these discontinuities in relation to the group's overall dimensions Additionally, these rules may establish a minimum distance between individual discontinuities, typically expressed as a ratio of the sizes of the neighboring discontinuities.
To achieve a more precise characterization of a set of indications, it is essential to investigate whether the echoes originate from multiple closely spaced discontinuities or from a single continuous discontinuity with several distinct reflecting facets, utilizing the methods outlined in annex A.
Shape of discontinuity
Ultrasonic testing identifies a limited number of basic reflector shapes, and often, evaluating these shapes against the relevant acceptance criteria necessitates a straightforward classification This classification, detailed in annex B, subclause B.1, categorizes the discontinuity into specific types.
1) point, i.e having no significant extent in any direction;
2) elongated, i.e having a significant extent in one direction only;
3) complex, i.e having a significant extent in more than one direction.
When required, this classification may be sub-divided into: a) planar, i.e having a significant extent in 2 directions only, and b) volumetric, i.e., having a significant extent in 3 directions.
Acceptance standards may require either distinct criteria for each classification or a unified approach where any discontinuity, regardless of its characteristics, is projected onto predefined sections and treated conservatively as a crack-like planar discontinuity.
Simple classification will normally be limited to the use of those probes and techniques specified in the examination procedure Additional probes or techniques shall only be used where agreed.
To accurately identify the types of discontinuities outlined in the acceptance criteria and conduct a proper fitness-for-purpose evaluation, a detailed assessment of the discontinuity's shape may be required.
For a more detailed classification, refer to annex B, subclause B.2, which outlines methods that may necessitate additional probes and scanning directions beyond those specified in the examination procedure for detecting discontinuities The process can also be enhanced by utilizing special techniques found in annexes E, F, and G.
The classification of discontinuity shapes is essential for accurately assessing them against acceptance criteria and other requirements It is crucial to validate this classification for specific applications, taking into account factors such as the materials and configuration of the examination object, the examination procedure, and the types of instrumentation and probes used.
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Maximum echo height of indication
The maximum echo height from a discontinuity depends on its size, shape, and orientation, and is measured against a specified reference level as outlined in prEN 583-2:2000.
The maximum echo height can be evaluated based on the application and acceptance criteria in several ways: it can be directly compared to a reference level that serves as the acceptance standard; it can help determine the equivalent size of a discontinuity by comparing it to the echo from a reference reflector at the same sound path range in the material or a reference block with similar acoustic properties; and it can be utilized in probe movement sizing techniques that rely on a specified echo drop, such as 6 dB, below the maximum.
Size of discontinuity
The sizing of a discontinuity consists in determining one or more projected dimensions/areas of the discontinuity onto pre-established directions and/or sections.
A short description of these techniques is found in annex F and further details are given in prEN 583-2:2000.
These techniques are based on a comparison of the maximum echo height from a discontinuity with the echo height from a reference reflector at the same sound path range.
For effective evaluation of discontinuities, it is essential that their shape and orientation are conducive to reflection, necessitating echo height measurements from multiple angles unless these parameters are already established Additionally, the dimensions of the discontinuity, measured perpendicular to the beam axis, must be smaller than the beam width in one or both directions Lastly, the reference target's basic shape and orientation should closely resemble those of the discontinuity being assessed.
The reference target may be either a disc shaped reflector, e.g flat-bottomed hole or an elongated reflector, e.g a side drilled hole or notch.
Discontinuities subject to sizing can be categorized into two types: first, those with reflective areas smaller than the beam width in all directions; and second, those with reflective areas that are narrow and elongated, characterized by a length exceeding the beam width while having a transverse dimension smaller than the beam width.
For the discontinuities mentioned, the projected area onto a section normal to the ultrasonic beam axis is considered equivalent to that of a disc-shaped reflector This reflector, positioned perpendicular to the beam axis, generates a maximum echo of equal height at the same sound path range.
Discontinuities related to the second category typically involve elongated reference reflectors that are oriented transversely to the ultrasonic beam axis and possess a defined transverse profile These reflectors can take various forms, including notches with rectangular, U-shaped, or V-shaped profiles, as well as cylindrical holes.
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When using an angle beam probe, the dimensions generally determined are: i) dimension, l, parallel to the lateral scanning direction, determined by lateral movement of the probe
(see Figure 2); ii) dimension, h, normal to the transverse scanning direction, determined by transverse movement of the probe (see Figure 2).
When using a straight beam probe the dimensions generally determined are 11 and 12, in directions parallel to the scanning surface, by probe movement in two mutually perpendicular directions (see Figure 3).
The techniques are classified into three categories, as follows:
1) fixed amplitude level techniques where the ends of a discontinuity are taken to correspond to the plotted positions at which the echo height falls below an agreed assessment level;
Techniques involve plotting the edges of discontinuities at positions where the maximum echo height decreases by a specified number of dB These edges can be represented along the beam axis or along a predetermined beam edge.
3) techniques which aim to position the individual echoes from the tips of the discontinuity, or from reflecting facets immediately adjacent to the edges.
The principal probe movement sizing techniques are described in annex D.
The selection of sizing technique(s) depends upon the specific application and product type, and on the size and nature of the discontinuity.
When measuring dimensions using ultrasonic techniques, several key rules must be followed: Maximum echo height techniques are applicable only if the dimension is less than the 6 dB beam width of the probe Fixed amplitude level techniques can be used for any discontinuity size, but should only be employed when specified in the acceptance standard due to their dependence on the chosen amplitude level Techniques that involve probe movement at a specified dB drop from the maximum echo height are valid only if the measured dimension exceeds the beam width at that dB drop; otherwise, the discontinuity's dimension is assumed to equal the beam width Additionally, techniques that position the edges of a discontinuity require the ultrasonic indication to display two or more resolvable echo maxima Finally, if multiple techniques are used to measure the same dimension, the value from the technique with the highest demonstrated reliability and accuracy should be considered correct.
Alternatively, the highest measured value shall be assumed.
4.7.5 Sizing techniques with focussing ultrasonic probes
When using focusing probes for sizing, the methods outlined in sections 4.7.2 and 4.7.3 are applicable, as long as the discontinuity is located within the beam's focal zone Additionally, the guidelines specified in section 4.7.4 are generally relevant to focusing probes.
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Where a higher accuracy of sizing is requested, an alternative technique can be used that is based on the construction of a series of C-scan images of the discontinuity.
The plots are generated through an iterative process involving 6 dB drop steps, beginning with an initial plot that reflects a 6 dB reduction from the maximum echo of discontinuity This process continues until the plot corresponding to a 6 dB drop step is equal to or less than the 6 dB half-width of the ultrasonic beam.
This iterative technique is applicable to both focused and unfocused ultrasonic beams; however, it is especially effective with focused beams when high accuracy is essential For a detailed illustration of this technique, refer to Annex E.
4.7.6 Use of mathematical algorithms for sizing
The sizing techniques outlined in sections 4.7.2 and 4.7.3 aim to evaluate the measured size of discontinuities against established acceptance criteria based on maximum allowable dimensions When greater accuracy is necessary for estimating the true size of a discontinuity, mathematical algorithms can assist, especially when only the data from these techniques are accessible.
Annex F provides a comprehensive overview of algorithms designed to estimate the actual size of discontinuities, whether they exceed or fall short of the ultrasonic beam's diameter.
Special sizing techniques are additional to those described in 4.7.2 to 4.7.6 and may be used in particular applications where higher levels of reliability and accuracy are called for.
To ensure compliance with specified acceptance criteria, the reliability and accuracy of a specialized technique must be validated using the same configuration, material type, examination procedure, and instrumentation, including probes.
The list of special techniques presented here is not exhaustive, as there are numerous methods available that are continually evolving However, the techniques described are among the most widely used and have a well-established application in the field, including tip diffraction techniques.
These techniques are essential for confirming the planar nature of a discontinuity and for measuring its transverse dimension, denoted as "h" in Figure 2 They rely on detecting and locating echoes that are diffracted by the edges of the discontinuity, as well as employing mode conversion techniques.
These techniques are effective for detecting and characterizing planar discontinuities by utilizing mode conversion to produce an additional ultrasonic beam at a different angle and velocity when the discontinuity's plane is aligned correctly with the incident beam While they can also assist in sizing, this requires special reference blocks that represent the test object and include planar reflectors of various sizes.
General
The general principles and requirements of the transmission technique are given in EN 583-3.
The subsequent clauses outline various ultrasonic parameters and characteristics of transmitted signals that can be utilized, individually or in combination, to assess discontinuities using this technique.
Location of discontinuity
In normal beam probes, the position of a discontinuity is identified on the surface of the test object based on a two-dimensional coordinate system, where the maximum decrease in transmitted signal amplitude occurs.
To accurately locate discontinuities, it is effective to direct ultrasonic beams in two different directions using pairs of angle probes, as shown in Figure 4 This method enables three-directional assessment of the area under investigation.
Evaluation of multiple discontinuities
Whether a discontinuity is continuous or intermittent should first be determined qualitatively by observing variations in signal amplitude as the probe is scanned over the discontinuity.
If the signal amplitude remains relatively constant the discontinuity can be classified as continuous and evaluated as such against the acceptance criteria.
If the signal amplitude exhibits significant peaks and troughs, the discontinuity can be categorized as intermittent It is essential to quantitatively assess whether the density of discrete discontinuities in the affected region meets the size and area limitations set by the acceptance criteria.
The concentration of discontinuities in a specific area can be quantified by examining the ratio of individual discontinuity dimensions to the distance separating them, the total length of discontinuities to a specified overall length, and the total area of individual discontinuities to a defined overall area.
Reduction of signal amplitude
This parameter is taken into account whenever the signal amplitude falls below the specified evaluation level.
If the signal is lost completely, the limits of the zone on the scanning surface over which this occurs should be determined.
In cases of partial signal loss, it is essential to identify the location on the scanning surface where the maximum amplitude reduction occurs, along with the corresponding dB value of the reduction compared to the signal measured in an area without discontinuities.
When the area impacted by signal reduction on the scanning surface is smaller than the ultrasonic beam's cross-sectional area, the size of the discontinuity perpendicular to the beam can be estimated This estimation is achieved by comparing the amplitude reduction to that caused by a known reference reflector, such as a flat-bottomed hole, within a representative sample of material that is free of discontinuities.
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A consistent partial reduction in signal amplitude over a significantly larger area than the ultrasonic beam suggests the presence of a discontinuity This discontinuity may manifest as a band of small inclusions, an abnormal grain structure, a layer of semi-transparent material, or a large discontinuity subjected to high compressive stress.
Sizing of discontinuity
The sizing of a discontinuity involves measuring its dimensions or the area of its projection on the scanning surface These measurements are then compared to relevant acceptance standards, particularly when these standards specify maximum allowable dimensions or areas, to evaluate whether the discontinuity is acceptable or unacceptable.
Sizing techniques can be categorized into two main types The first type involves comparing the maximum amplitude reduction of a signal to that of an equivalent reflector These techniques are applicable only when the size of the area on the scanning surface, where the signal amplitude reduction falls below the evaluation level, is smaller than the probe's projected size on that surface.
The maximum amplitude reduction of the signal is assessed in relation to the amplitude in a discontinuity-free zone This is achieved using a reflector, typically a flat-bottomed hole positioned perpendicularly to the beam axis at a specific depth, such as half the thickness, which results in a consistent maximum reduction of the transmitted signal amplitude.
The dimension of the discontinuity projected on a plane perpendicular to the beam axis is assumed to match that of the flat-bottomed hole Techniques that utilize signal amplitude reduction alongside probe movement involve identifying areas on the scanning surface where there is either a loss of signal or a decrease in amplitude, typically compared to a reference value of 6 dB, relative to the signal amplitude from a region without discontinuities.
Values other than 6 dB may be used when specified by the referencing documents, particularly when evaluating discontinuities which are partially transparent to ultrasound.
The extent of the zone so determined is assumed to be the extent of the discontinuity projection on the scanning surface.
The transmission technique is primarily utilized for identifying larger discontinuities, where high sizing accuracy is not critical Consequently, the methods outlined previously are sufficient for most applications In this regard, the data obtained from the earlier techniques serve as a reference to ensure examination reproducibility, rather than as a foundation for directly sizing discontinuities.
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II Time of flight a) Resolvable grouped discontinuities b) Unresolvable grouped discontinuities
Figure 1 — Examples of A-scan signals from grouped discontinuities in a forging or casting
Figure 2 — Projected parameters l and h for the conventional sizing of a discontinuity by an angle beam probe
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Figure 3 — Parameters l 1 and l 2 for the conventional sizing of a discontinuity by a straight beam probe
Figure 4 — Location of discontinuities by transmission technique using angle probes
Discontinuity lies at the intersection of the two beam paths A 1 A 2 and B 1 B 2, at which the maximum reduction in transmitted signal amplitude is observed
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Some of the techniques which may be used to distinguish between intermittent and continuous discontinuities are described below.
Techniques A.1 and A.2 are specifically designed for weld inspections but can also be modified for other scenarios where angle probe examination is feasible In contrast, Technique A.3 has broader applicability, although it is restricted by the minimum size of discontinuities that can be assessed.
To effectively detect discontinuities with a distinct A-scan indication, it is essential to choose the scanning direction, beam angle, probe size, and frequency that yield the narrowest beam width at the discontinuity's location Additionally, a thorough lateral scan should be performed while maintaining consistent coupling conditions.
Marked dips in the echo height envelope indicate that the discontinuity is intermittent To confirm this, swivel and orbital scans should be conducted near the apparent breaks, observing that the echo height decreases sharply around the normal and that no significant secondary echoes are present Any alternative response may imply that the apparent break results from a change in lateral orientation.
Transverse scans must be conducted meticulously across the discontinuity from at least two different directions at short sound path ranges, while also observing the characteristics of the echo envelope.
Significant dips, or complete breaks, in the echo envelope suggest that the discontinuity may be intermittent.
To effectively analyze a discontinuity, it is advisable to create a comprehensive through-thickness image by plotting echoes from various angles and directions This technique requires smooth, flat scanning surfaces on both sides of the discontinuity and high accuracy in plotting to ensure its effectiveness.
This technique is useful when the dimensions of the discontinuity, or group of discontinuities, are approximately equal to the beam width.
Angle probes, as shown in Figure A.1, are also relevant for normal beam probes, whether utilizing distinct transmitting and receiving probes or observing changes in the back wall echo height.
A robust transmitted signal in the affected area indicates that there is no significant discontinuity present The strength of this transmitted signal is directly related to the ratio of the discontinuity area to the beam area.
The resolution of all the above techniques may be improved by the use of focussing probes having a focal length close to the sound path range of the discontinuity.
The through-thickness dimension of a discontinuity is crucial and should be considered continuous unless there is definitive proof indicating it is intermittent in that direction.
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Figure A.1 — Shadow technique for distinguishing between major and intermittent discontinuities
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Techniques for the classification of discontinuity shape
Discontinuities can be classified based on their significant extent in one or more directions In this context, "significant extent" refers to a dimension that exceeds the minimum measurable size, considering the beam width and the resolution of the probe along the discontinuity's beam path.
According to this standard, the main types of discontinuity shapes are categorized as follows: a point, which has no significant extent in any direction; an elongated shape, which extends significantly in only one direction; and a large shape, which has a significant extent in either two perpendicular directions (planar) or three perpendicular directions (volumetric).