Acoustic emission studies on metallic specimen under tensile loading

Materials and Design 24(2003) 471–481

0261-3069/03/$ - see front matter� 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0261-3069(03)00067-0

Technical report

Acoustic emission studies on metallic specimen under tensile loading

Sanjay Kumar Singh, K. Srinivasan, D. Chakraborty*

Department of Mechanical Engineering Indian Institute of Technology, Guwahati, North Guwahati 781039, India

Received 9 January 2003; accepted 7 April 2003

Abstract

Flat mild-steel specimen with central and off-center holes were subjected to tensile loading, while simultaneously monitoringtheir acoustic emissions(AEs). The effect of hole diameter and location were studied. The results indicate that AE signals havea strong relationship with load–deflection characteristics, and particularly the yield and ultimate points. Also, the effect ofpositions of holes on AE signals has been presented.� 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Acoustic emission; Crack initiation; Mild steel

1. Introduction

Structural components may fail due to a variety ofreasons. The fact that is common to most of the failuresis the existence of a defect in the component, whichbeing the weakest link, acts as the nucleating point forthe crack initiation, leading to the ultimate failure of thecomponent. Therefore, predicting the presence of suchdefects, as well as identifying the crack initiation is ofsignificant importance from the point of view of failureprevention in structural components. Since phenomenasuch as crack initiation and propagation emit highfrequency acoustic waves, acoustic emission(AE)meas-urement has been acknowledged as an appropriate tech-nique to monitor such micro-scale events. Even thoughthere is a lot of work available in the literature on AEsof loaded structural components(see for example, ref-erencesw1,2x), the authors are aware of very few studiesdirectly relating AEs with zones in the load deflectioncurve. This methodology of relating AE with distinctzones of load deflection curves(e.g. onset of yielding,or post-yielding) could be effectively used to warnagainst imminent failure of structural components.Huang et al.w3x have correlated AE events such as

*Corresponding author. Tel.:q91-0361-2690321-383x2093; fax:q91-370-261-7018.

E-mail address: [email protected](D. Chakraborty).

cumulative AE count and AE count rate with the stressstrain curve, and they observed that AE activity is veryquiet in the initial portion of the linear elastic regionand it reaches its peak in the second stage just beforeyielding occurs.Therefore in the present study, AE from mild steel

specimen subjected to tensile loading has been used torelate AE events with the various zones on the load-deflection curve of the material. The study ultimatelyaims at identifying the AE features of a material whichact as precursors to failure.

2. Experimental setup and procedure

This section describes the geometry of the test spec-imen, test procedure, sensors and instrumentation.

2.1. Details of test specimen

Flat samples of size 300-mm length, 30-mm widthand 3-mm thickness made of mild steel have beenchosen for the experiments. The following types ofspecimen have been tested:

● Specimen without hole.● Specimen with a hole at the center(hole diameters

1, 1.5 and 2 mm).

472 S.K. Singh et al. / Materials and Design 24 (2003) 471–481

Fig. 1. Experimental setup.

● Specimen with 1-mm diameter hole, offset from thecenter(offset distances 20, 30 and 40 mm).The photographs showing the various components of

the experimental setup used are presented in Fig. 1.

2.2. Sensor

A wideband AE sensor(100 kHz–1 MHz) has beenused to capture the AE from the specimen. The sensorwas attached to the specimen at the center in all thesamples. In order to obtain proper acoustic contact,vacuum grease was used at the interface between thesensor and the specimen surface. The sensor wasattached to the specimen by means of a C-clamp asshown in Fig. 2.

2.3. Experimental procedure

A Mixed Signal Oscilloscope manufactured by Agi-

lent (54622D) connected to the sensor, was used toanalyze the signals captured by the AE sensor. Theoscilloscope was configured for the AE measurement asfollows:

● Low frequency rejection at 50 kHz.● Trigger mode with a trigger threshold of 10 mV(no

preamplifier was used with the sensor).● Peak-to-peak, r.m.s., selected for measurement in time

domain mode.

Upon loading the specimen on the UTM, the oscil-loscope starts receiving data captured by the AE sensor.When trigger occurs, the signal is saved, as well as theload and deflection values are recorded from the UTM.The oscilloscope is again set to receive the next trigger.This process is repeated till the specimen breaks. Afterfailure of the specimen, the load vs. displacement plot

473S.K. Singh et al. / Materials and Design 24 (2003) 471–481

Fig. 2. Close-up view of the sensor attached to the test specimen.

is obtained from the UTM instrumentation. The pointson the load vs. displacement curve at which AE eventsare triggered on the oscilloscope are marked.At the beginning of loading on UTM there are very

few AE signals above the minimum(trigger) level, andafter yielding occurs the rate of AE signals triggeredwas very high, and so it was not possible to record allAE data after yield point.

3. Results and discussion

The load–deflection curves along with the pointscorresponding to AE events triggered are shown in Figs.3–9. Fig. 3a shows the load deflection curve for aspecimen without hole. It is observed that there are notriggered events in the beginning portion(linear elasticregion) of the curve, and the AE activity starts after theonset of yielding. This is in agreement with the obser-vations of Huang et al.w3x. Fig. 3b shows the magnifiedview of the load–deflection curve in the acousticallyactive region, along with the points of occurrence ofAE triggers and their magnitudes(peak-to-peak). In thisfigure, it may be observed that there is an AE event,which is significantly larger in magnitude than all otherevents. Fig. 4a shows the load deflection curve for aspecimen with a 1 mm diameter central hole, and Fig.4b shows the corresponding magnified view. Similarobservations could be made for this specimen, except

that one prominent AE event in the previous case isreplaced with two successive AE events having magni-tude (peak-to-peak) much higher than that of other AEevents in this zone. Same trend has been observed inthe case of specimen with 1.5- and 2-mm diameter holesat the center(Figs. 5 and 6). In all specimens with holeat the center, the amplitude of the peak AE eventexceeded the average by more than 65%, as shown inTable 1. Figs. 7–9 show similar plots for holes offsetfrom the center. For all these cases, the sensor wasplaced at the center. It may be observed from thesefigures that as the hole location is moved away fromthe sensor location, the number of AE events decreases.There is no significant change in the magnitude of theAE events. However, the amount by which the peak AEevent surpassed the average AE magnitude reduces,especially at higher offsets.

4. Conclusions

From the above results, the following conclusionsmay be drawn:

1. AE events are more frequent between yield andultimate points.

2. Specimens with holes start emitting AE earlier thanthose without holes.

3. Generally, largest amplitude AE event and the secondlargest, occur in succession.


Fig. 3. (a) Load deflection curve for specimen without hole. Bullets on the curve represent points of occurrence of AE events.(b) Magnifiedview of the acoustically active region of the load deflection curve.


Fig. 4. (a) Load deflection curve for specimen with 1-mm central hole. Bullets on the curve represent points of occurrence of AE events.(b)Magnified view of the acoustically active region of the load deflection curve.


Fig. 5. (a) Load deflection curve for specimen with 1.5-mm central hole. Bullets on the curve represent points of occurrence of AE events.(b)Magnified view of the acoustically active region of the load deflection curve.


Fig. 6. (a) Load deflection curve for specimen with 2-mm central hole. Bullets on the curve represent points of occurrence of AE events.(b)Magnified view of the acoustically active region of the load deflection curve.


Fig. 7. (a) Load deflection curve for specimen with 1-mm hole at an offset of 20-mm from the center. Bullets on the curve represent points ofoccurrence of AE events.(b) Magnified view of the acoustically active region of the load deflection curve.






Table 1Comparison of amplitudes of AE events

Configuration Maximum Average Difference % Riseamplitude amplitude (mV) above(mV) (mV) average

Without hole 52.6 29.6 22.9 77.361-mm hole 49.4 25.9 23.5 90.731.5-mm hole 44.4 26.9 17.5 65.052-mm hole 53.8 28.2 25.6 90.7820-mm offset 36.9 20.8 16.1 77.4030-mm offset 41.3 28.1 13.2 46.9740-mm offset 43.8 29.6 14.2 47.97

4. The difference between the maximum and averageamplitudes was above 15 mV for all the samplestested.

5. There seems to be no significant effect of diameterof hole on the AE emission.

6. AE amplitudes are strongest when the sensor isdirectly above the hole.

References

w1x Gorman MR. Plate wave acoustic emission. J Acoust Soc Am1990;90:358–364.

w2x Hamstad MA, McColskey JD. Wideband and narrow bandacoustic emission waveforms from extraneous sources duringfatigue of steel samples. J Acoust Emission 1997;15(1–4):1–18.

w3x Huang M, Jiang L, Liaw PK, Brooks CR, Seeley R, KlarstromDL. Using Acoustic Emission in Fatigue and Fracture MaterialsResearch. J Mater 1998; 50(11) wweb publicationx.

Documents

Acoustic emission studies on metallic specimen under tensile loading