ASTM E751-96
(Practice)Standard Practice for Acoustic Emission Monitoring During Resistance Spot-Welding
Standard Practice for Acoustic Emission Monitoring During Resistance Spot-Welding
SCOPE
1.1 This practice describes procedures for the measurement, processing, and interpretation of the acoustic emission (AE) response associated with selected stages of the resistance spot-welding process.
1.2 This practice also provides guidelines for feedback control by utilizing the measured AE response signals during the spot-welding process.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 751 – 96 An American National Standard
Standard Practice for
Acoustic Emission Monitoring During Resistance Spot-
Welding
This standard is issued under the fixed designation E 751; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope and size of the nugget, the amount of expulsion, and the
amount of cracking. Therefore, in-process AE monitoring can
1.1 This practice describes procedures for the measurement,
be used both as an examination method, and as a means for
processing, and interpretation of the acoustic emission (AE)
providing feedback control.
response associated with selected stages of the resistance
spot-welding process.
5. Basis of Application
1.2 This practice also provides guidelines for feedback
5.1 Personnel Qualification—Nondestructive Testing
control by utilizing the measured AE response signals during
(NDT) personnel shall be qualified in accordance with a
the spot-welding process.
nationally recognized NDT personnel qualification practice or
1.3 This standard does not purport to address all of the
standard such as ANSI/ASNT CP-189, SNT-TC-1A, MIL-
safety concerns, if any, associated with its use. It is the
STD-410, or similar document. The practice or standard used
responsibility of the user of this standard to establish appro-
and its applicable revision shall be specified in the contractual
priate safety and health practices and determine the applica-
agreement between the using parties.
bility of regulatory limitations prior to use.
5.2 Qualification of Nondestructive Agencies—If specified
2. Referenced Documents in the contractual agreement, NDT agencies shall be qualified
and evaluated in accordance with Practice E 543. The appli-
2.1 ASTM Standards:
cable edition of Practice E 543 shall be specified in the
E 543 Practice for Evaluating Agencies that Perform Non-
contractual agreement.
destructive Testing
2 5.3 Procedures and Techniques—The procedures and tech-
E 1316 Terminology for Nondestructive Examinations
niques to be used shall be as described in this practice unless
2.2 ASNT Standards:
otherwise specified. Specific techniques may be specified in the
SNT-TC-1A Recommended Practice for Nondestructive
contractual agreement.
Testing Personnel Qualification and Certification
ANSI/ASNT CP-189 Standard for Qualification and Certi-
6. Ordering Information
fication of Nondestructive Testing Personnel
6.1 If the spot-weld monitoring or process control methods
2.3 Military Standard:
described in this practice are performed as a service, the
MIL-STD-410 Nondestructive Testing Personnel Qualifica-
following items should be addressed in the purchase specifi-
tion and Certification
cation, and are subject to agreement between the purchaser and
3. Terminology the supplier:
6.1.1 Description of the welded parts in terms of geometry,
3.1 Definitions—For definitions of terms relating to acoustic
dimensions, number and position of welds, and materials.
emission testing, see Section B of Terminology E 1316.
6.1.2 Description of the welding machine, type and dimen-
4. Significance and Use
sions of the electrodes, type of weld controller, welding
schedule, and distance between the welding head and the
4.1 The AE produced during the production of a spot-weld
controller.
can be related to weld quality parameters such as the strength
6.1.3 Location and mounting method for the acoustic emis-
sion sensors, and design of the mounting fixture, as appropri-
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde-
ate.
structive Testing and is the direct responsibility of Subcommittee E07.04 on
6.1.4 In the event that the process is actually controlled by
Acoustic Emission.
Current edition approved May 10, 1996. Published July 1996. Originally
acoustic emission, the circuit requirements associated with the
e1
published as E 751 – 80. Last previous edition E 751 – 80 (1991) .
electronic interface to the weld controller to ensure synchro-
Annual Book of ASTM Standards, Vol 03.03.
3 nous operation.
Available from American Society for Nondestructive Testing, 1711 Arlingate
6.1.5 The performance and limiting AE parameters which
Plaza, P.O. Box 28518, Columbus, OH 43228-0518.
Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700
were predetermined.
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 751
6.1.6 Method of recording or reporting (that is, form and which can be related to nugget size and inclusions. As the
content of the report), if applicable. nugget cools during the hold period, AE can result from
6.1.7 Technical qualifications of the personnel performing solid-solid phase transformations and cracking.
the examination. These should be based on a documented 7.2.3 During the lift-off stage, separation of the electrode
program that certifies personnel for conducting AE examina- from the part produces signals that can be related to the
tions. condition of the electrode as well as the cosmetic condition of
the weld.
7. Principles of Application
7.3 Using time, and amplitude or energy discrimination, or
both, the AE response corresponding to each stage can be
7.1 The resistance spot-welding process consists of several
separately detected and analyzed. Although the AE associated
stages. These are the set-down of the electrodes, squeeze,
with each stage of the spot-welding process can be relevant to
current flow, forging, hold time, and lift-off. Various types of
weld quality, this practice only gives detailed consideration to
acoustic emission signals are produced during each of these
the AE generated by nugget formation and expansion, expul-
stages. Often, these signals can be identified with respect to the
sion, and cracking.
nature of their source. The individual signal elements may be
greatly different, or totally absent, in various materials, thick-
8. Apparatus
nesses, and so forth. Fig. 1 is a schematic representation
showing typical signal elements which may be present in the 8.1 Acoustic Emission System:
AE signature from a given spot-weld. 8.1.1 The AE sensor should be a contacting type having an
7.2 Most of the depicted AE signal features can be related to appropriate frequency response within the range from 0.1 to
factors of weld quality. The AE occurring during set-down and 1.0 MHz. Free resonances associated with electrode vibrations
squeeze can often be related to the condition of the electrodes may necessitate the use of sensors with a frequency response in
and the surface of the parts. The large, often brief, signal at the range from 0.30 to 1.0 MHz.
current initiation can be related to the initial resistance, and the 8.1.2 The electronic instrument should contain adjustable
cleanliness of the part. For example, burning through of certain amplification over the range from 40 to 100 dB, or an
oxide layers contributes to the acoustic emission response equivalent amplification and adjustable threshold. The instru-
during this time. ment should be capable of performing time and amplitude or
7.2.1 During current flow, plastic deformation, nugget ex- energy discrimination. Using some timing reference, it is
pansion, friction, melting, and expulsions produce AE signals. necessary to detect the AE contained within a certain time
The signals caused by expulsion (spitting or flashing, or both) interval and within a certain signal or energy amplitude range.
generally have large amplitudes and can be distinguished from This is required for each characteristic stage of the AE signal
the rest of the acoustic emission associated with nugget that is to be separately measured. Thus, the instrument should
formation. Fig. 2 shows typical AE response signals during contain one or more signal amplitude or energy level detectors,
current flow for both d-c and a-c welding. timing gates, and counters. It should also contain a comparator
7.2.2 Following termination of the welding current, some and signaling output if it is used for on-line monitoring.
materials exhibit appreciable AE noise during solidification 8.1.3 If feedback control is to be used, the instrument should
FIG. 1 Typical AE Response Signals During Resistance Spot Welding
E 751
FIG. 2 Typical AE Response Signals During Current Flow
facilitate the selection of an optimum AE level, and it should apparatus should be equipped with an automatic marking
generate an appropriate control signal whenever this level is attachment. With the markings and the records, the acceptabil-
exceeded. This control signal should terminate the welding ity of the welded part can be based on the percentage of
synchronously with the zero-crossing points of the weld unacceptable welds and their location distribution.
current.
9. Procedure
8.2 Support Equipment—An analog or digital waveform
recorder is normally used for performing measurements. A 9.1 Sensor and Preamplifier Attachment—The sensor
means for detecting current initiation independent from the AE should be mounted to the lower (grounded) electrode or
signals should be available. electrode holder. If the measurements are to be made only as a
8.3 Data-Recording Devices (optional)—If it is desired to periodic sampling of weld quality, a liquid couplant may be
record processed AE data permanently, a digital printer, tape used provided that it is periodically replenished and calibration
recorder, or similar device must be interfaced with the AE of the system response is maintained. For sustained monitor-
instrument. ing, such as on-line AE examination or control of each nugget,
8.4 Audio or Visual Alarm—An alarm can be used in the sensor should be permanently mounted using an epoxy
applications where the acceptability of individual spot welds is adhesive or a similar material. A preamplifier is usually
to be determined in real-time, and where no record of rejected positioned near the sensor. However, when the instrumentation
welds is necessary. is located less than 1 m from the sensor, the gain otherwise
8.5 Print-out Device—A print-out device may be used to supplied by the preamplifier may be incorporated into the main
provide a permanent record, and it is usually employed as amplifier of the instrument.
follows: 9.2 Preliminary Measurements—The AE signal from a
8.5.1 Whenever a permanent record is necessary to docu- single spot-weld should be displayed on a waveform recorder.
A wire coil or Hall effect sensor positioned near an electrode
ment the quality of individual welds, the printer should print
out such information as is necessary to segregate and identify can be used as a current sensor, thus providing a timing
reference and trigger signal for viewing and measuring the AE
rejectable welds.
8.5.2 When the joined parts contain a large number of signal. This reference signal can be also obtained through an
spot-welds, and the integrity of the product does not depend on appropriate interconnection to the weld controller. Having
the quality of individual welds but rather on the number of established a typical AE trace, characteristic stages should be
unacceptable welds expressed as a percentage of the total identified and one or more selected as an AE examination
number of welds. The print-out should consist of a weld parameter. For example, weld quality indicators may be
sequence number and a running percentage of unacceptable obtained from the AE response to nugget formation, expulsion,
welds when the individual spot-welds are identifiable by or cracking.
sequence number. 9.2.1 New Applications—If the instrumentation was not
8.5.3 If weld identification is not possible, then the welding previously applied to a specific welding problem, preliminary
E 751
measurements must be made to determine the instrument 10.1.5 A storage scope or other device to record AE
settings and the conditions for monitoring. The weld controller response should be used to verify the several stages of AE
settings are determined from normal welding considerations. generation and detection shown for d-c and a-c spot welding in
First, the complete AE response should be observed on the Fig. 2.
oscilloscope. Next, the gain of the instrumentation should be 10.2 AE from Expulsion:
set to the maximum value where the AE signals, representing 10.2.1 Expulsion occurs after a sufficient weld is formed.
the selected examination parameter, do not saturate the ampli- Within the weld period, it will occur sooner if the electrodes
fier. This step will ensure that the measurement will be made are clean or thinner material is welded. It will occur later if the
with the best obtainable signal-to-noise ratio. Next, the detec- welding conditions have deteriorated or thicker stock is welded
tion threshold level should be established at a value that is with the same controller setting.
slightly above (or below) the peaks of the AE signals that are 10.2.2 Expulsion, however, is not desirable. It removes
to be excluded from the measurement. The timing control is material from the weld nugget area. It may automatically be
referenced to the onset of the weld current and consists of a kept to a minimum when the expulsion signals are used to
delay and a time interval. These time intervals should be generate a feedback signal to control the welding process (see
selected so that the monitoring is restricted to the time interval Section 11).
when relevant signals are present. Finally, the count multiplier 10.2.3 Where a feedback control arrangement is not used,
should be set to a value that allows utilization of the maximum expulsion may be knowingly tolerated from production-
number of significant digits in the readout. The finalized oriented considerations. In this case, several test coupons
settings of the weld controller and the AE instrumentation should be welded and the resulting weld strength or other
should be recorded along with a photograph of the total quality parameter should be determined through destructive
acoustic emission signal. The counts obtained from individual testing. Weld strength may be correlated with a suitable
welds should also be recorded. These records should be kept on measure of the expulsion AE. In this way, a maximum
file for future reference. Special considerations associated with acceptable level of expulsion AE can be determined and used
each of the various examination parameters are discussed in to segregate unacceptable welds or we
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.