PROGRESS in ACOU5nC EMISSION IXCopyright@1998AFNG&AEGroup

CHARACTERIZATION OF THE LAMB WAVES PRODUCED BY LOCALIMPACT FRACTURE IN BRITTLE THIN PLATES

YOSHIHffiO MIZUTANI, MIKIO TAKEMOTOFaculty of Science and Engineering

Aoyama Gakuin University6-16-1, Chitosedai, Setagaya, Tokyo 157, JAPAN

HIDED CHOFaculty of Engineering

Tohoku UniversityAobaku, Sendai, 980-77 JAPAN

KANJIONODepartment of Materials Science and Engineering

UCLA .

Los Angeles, CA 90095-1595 U.S.A.

ABSTRACT

Acoustic emission (AE) signal analysis was used to detect local fracture of a PMMA platesubjected to a steel ball impact. Main AE signal types in a 3-mm-thick PMMA plate are foundto be Lamb waves. Next we examined the Lamb waves produced by impact of flying steel ballat low velocities. Waveform and relative peak amplitudes of the Lamb waves agreed well withthose predicted from laser- and PZT-based simulation. Major peak amplitudes of the Lambwaves were quantified as a function of ball velocities. The fracture-produced Lamb waves wereextracted by the time-shift subtraction method from the waves detected at higher ball veloc­ities. This study demonstrates the feasibility of sepa.rating impact- and fracture-induced AEsignals.

KEYWORDS

Lamb Wave; Impact-induced AE; Fracture-induced AE; Time shift subtraction; Brittle thinplate

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INTRODUCTION

Brittle thin plates such as glass, polymethlymethacrylate (PMMA) and CFRPs sufferlocal fracture when they are impacted by a flying object. Cross-ply and quasi-isotropic CFRPplates used for transportation equipment sustain complicated internal damages such as fiberfracture, delamination and transverse cracks from the collision with particulate and ice. Al­though the visualization of such internal damages in CFRP members is possible by ultrasonicmicroscopes, our final goal is to monitor the onset of internal damages in impacted CFRPmembers by acoustic emission (AE) methods.

The critical condition to cause damages depends on the physical and mechanical proper­ties of flying objects and structural members. AE monitoring for such damages also needs toaccount for special conditions. For instance, high velocity and massive flying objects producelarge shock waves, which cause serious damage to AE sensors. Special mounting methods toprotect them from impact/shock are required. However, the damage of sensor and membersmay be small when tough members are impacted by low velocity light objects. In such cases,AE signals from the damage have small amplitudes, and are embedded in large amplitudeimpact waves. A special signal processing method is needed to extract the fracture-inducedsignals from the impact waves.

The AEs produced by impact and/or internal fracture were detected as Lamb wavesin thin plates. Extraction and classification of the fracture-produced Lamb waves from thoseby impact appears to be difficult because of their multi-mode dispersive nature. No such re­search has been reported so far.

We are exploring the fundamental approach for characterizing the Lamb waves producedby complicated internal damages in cross-ply CFRP coupons subjected to static point loading.The Lamb waves produced by different fracture modes were classified by the modal analysisvia wavelet contour maps[I].

In this paper, we characterized first the Lamb waves produced by impact with a flyingsteel ball at low velocities in PMMA plates, followed by those due to impact and local fractureby higher velocity impact. The Lamb waves produced by impact can be simulated by using anexperimental transfer function including the impulse excitation of a Q-switched laser in com­bination with bell-shaped impact source functions. Finally, we extracted the fracture-inducedLamb waves from impact-generated large amplitude Lanlb waves by the time-shift subtractionmethod. The waveform and frequency components of the extracted waves are analyzed in termsof local fracture.

EXPERIMENTAL METHODS AND LAMB-WAVE GENERATION

Figure 1 shows an impact test method. A rectangular PMMA plate of 3 mm thicknesswith 90 nUll width and 180 mm long was axed by circular steel flanges with an inner diameterof 50 mm. A 7-11ull-diameter steel ball was accelerated by a high-pressure nitrogen gas gunand hit the center of the plate. The ball velocity and crack behavior were monitored by a high

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Fig. 1 AE monitoring from PMMA plate impacted by flying steel ball.

speed camera with 27,000 flames/so Initially, we used PMMA plates without slit, and oftenobserved radial cracks hitting AE sensors, damaging them. Crack behavior was unpredictable,making it difficult to monitor AE signals. Therefore, we made a shallow straight slit (6 mIll

length and 0.3 mm depth by cutting with a razor blade) on the opposite surface of an impactpoint such that the slit direction is normal to the AE sensor. The AE waves propagated alongthe length of the plate. As the length is twice the width, the wave reflection from plate edgeswas separated from the segment of interest.

Sensor mounting needed special attention, since the sensors tend to fly out upon im­pact. A small AE sensor (PAC PICO) was affixed on the impact plane at 20 mm from the slit.It was pressed to the plate via a screw on a slender aluminum beam, whose ends were firmlyattached to the flange. Sensor's outputs were attenuated by 20 dB using a high impedanceattenuator and digitized by an A/D converter at 100 ns sampling interval with 4096 samplingpoints at 10 bit. Digitized data were analyzed using·a signal processing system we have devel­oped previously [2],[3).

'Wave types in a PMMA plate were examined using the laser/AE system shown in Fig. 2.vVe launched broad-band AE signals due to the break-down of a silicon grease film by a point­focused Q-s"'itched YAG laser at the plate center, and monitored the transmitted waves by twoPICO sensors on both surfaces of the plate at 20 mm from the source. Two typical waveformsare shown in Fig. 2, which are triggered by the laser pulse detected by a photo-diode. The firstarriving wave with the positive polarity was at 7.8 J.lS (corresponds to the P-wave velocity of2.74mm/J.ls), and was followed by So-mode Lamb waves at 8.36 J.lS (with 2.39 nun/J.ls velocity)

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and Ao-mode Lamb at around 13 ps (note the opposite phases of the Ao-waves). Arrival timesagreed well with those calculated from the velocity dispersion data except the Ao-arrival time(faster than the calculated one of 14.5 J..lS). Signal characterization in the following sections wasmainly performed for the So-mode Lamb waves because the trailing So·waves were overlappingwith the Ao-mode waves.

Point focusedVAG laser

S'I' ./I Icongrease

PicaSensor

20mm

3mm PMM

, Ao

Q)

i IIo 10 20 30 40 50

Time, ~s

Fig. 2 Characterization of AE signals in 3 mm thick PMMA plate

RESULTS AND DISCUSSION

AE Signals by Ball Impact

We first examined AE signals produced by a steel-ball impact at the ball velocities below13 mis, during which no fracture occurred. Six waveforms detected at ball velocities from 7 to13 mls are compared in Fig. 3(a).

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6Ball velocity

4(a)

A,-1/(b)

13m1s

> 2 12m.s >,P 11m/s.,;10mls ~

::l 0 :::l.e- 9m1s Co-::l 7rn1s :::l0-2 0

-4

-60 50 0

Fig. 3 Comparison of waveforms produced by ball impact (a) and simulated onefor a bell-shaped impact source function of 16JLs long duration (b).

The time a.xis was shifted so that P-wave arrivals were matched. We observed weak noise beforethe P-wave arrival, possibly due to the high-pressure gas blast since its amplitude increasedwith the gas pressure (or ball velocity). Waveforms by impact agree well from 10 to 20 JLSwhere P- and So-components arrived, but developed velocity-dependent differences at longertimes when Ao-waves arrived. It is noted that the frequency components of the Lamb wavesare unchanged over the velocity range studied. As shown in (b), the waveform simulated tobell-shaped impact source fuction of 16JLs long duration agrees well with the measured onesin (a). The squares of major peak amplitude of the Lamb waves (indicated by So, Ao-1 andAo-2) shown in Fig. 4 increased almost linearly with the ball velocity. However, the slope forSo peaks is much smaller than for Ao-waves. These relations can be utilized for predicting thewave amplitudes at higher ball velocities. Above 14 mls ball velocity, local fracture occurs.Thus, we can obtain fracture-induced AE signals by subtracting the predicted impact waveformfrom the actual measured wave.

(6.6)2 at 21 rnIs:> 45.-----.....---.....--­a) 40-g 35~ 30~ 25co 20-o 15~ 10co5- 5 Soen ~::::....~~II="*::dE*~:J:;;:;==::::;:~( 1.2)2

5 10 15 20Ball velocity I m/s

Fig. 4 Relationship between the ball velocities and the squares of major peak amplitudes.

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Signals Produced by Local Fracuture in Impacted Plate

'Ve extracted the fracture-produced wave components from the signals monitored at ballvelocities of 14 and 21 mls by the time-shift subtraction method. Figure 5(a) represents thesignal detected at ball velocity of 14 m/s. This contains two overlapping signals from impactand local fracture. The signal shown in (b) is the impact signal at ball velocity of 13 m/s.Ignoring the peak amplitude changes of impact-based Lamb waves, we matched the P-wave ar­rivals and subtracted the wave (b) from (a). The subtracted wave (c) is expected to representthe fracture-produced wave component. In this case, a 15-mm-Iong crack was produced by theimpact.

rack length (a) 14m1s15mm

.: \ao-6-2-4

-6

6 (b) 13m1s4

>~.:aO-6-2

-4

-6

6 (c)=(aHb)4

>

10 20 30Time,lJs

40 50

Fig. 5 Extraction of fracture-produced waveform at ball velocity 14m/s by time-shiftsubtraction method.(a) Signal detected by ball impact at 14m/s(b) Signal detected by ball impact at 13m/s(c) Extracted waveform due to local fracture

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Figure 6 shows another example of detected waves at. ball velocity of 21 m/s. Here, a32-mm-Iong crack was formed upon impact. The impact waveform (a) to be subtracted wasconstructed by using the peak amplitudes extrapolated by the data of Fig. 4. As a comparison,the 13m/s-impact wave is shown by broken line. The wave in (c) is the result of subtractionrepresenting the waves generated by local fracture.

1(a)

>...::::JQ.-:::J0

-11

>.::::JCo-:::J0

-11

>...::::JCo-:::J0

13m1s ­irnpactwave

1

Detected waveCrack length: 32mm

Extracted wave dueto local fracture

10 20 30 40 50Time, J.ls

Fig. 6 Extraction of fracture-produced waveform at ball velocity 21m/s bytime-shift subtraction method.

The present results indicate that we ·can separate the wave components from twodifferent sources. The waves generated by local fracture are delayed by 6-7 p,s following theimpact.

Simulation of Impact Waves

In order to simulate a crack opening, a dipole source is needed. This is provided bya PZT element, excited by a ramp function and sandwiched between a larger plate and thesubject plate, as shown in Fig. 7.

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5

(b)

(a)

80 100

Detected wave

40 60 80 100

.JIi~mm ~mPMMA plate PZT Element

Cross section25mm

Fig. 7 Overall transfer function for dipole source.

The source displacement was determined by a laser interferometer. The transfer functioninclusive of the PICO sensor is shown in Fig. 7(c), while the detected wave and source vol­umetric displacement (accounting for the contact size of 2 x 6 mm) are given in (a) and (b).With this transfer function and a ramp input of 12 J.LS rise time and crack volume of 2.0 x10-10 m3, we obtain a simulated waveform illustrated in Fig. 8. The broken curve also shownin Fig. 8 is that of the extracted fracture wave given in Fig. 6(c).

, \. i \'\ '~ /\ .J.."... '-'v" \

Extracted

SQ. 0I--1'"Ili.os0_

·4

·6

8r-----~r=:::::======="ATr=12J.ls

6 Simulated A V=3mmX32mmX2.11J\ =2.0Xl 0

10m3

20 40 60Time,IJS

80 100

Fig. 8 Comparison of simulated waveform due to crack generatiionand extracted one (Fig. 6(c)).

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The initial part matches very well and the peak positions at around 18, 21 and 28 p,s agreewith experiment. However, the experimentally observed peak amplitudes at the third peaks arelower than the simulated waveform. In spite of some differences, the present simulation proce­dure provides an excellent result, giving the quantitative description of the underlying fractureprocess of the fracture wa,:,e extracted. Note that the crack length was 32 mm in this case.The average crack propagation speed is 1.3 mm/J1.s, assuming that the crack extends in twodirections over the rise time of 12J1.s. This speed is about one-half of the P-wave velocity and iscomparable to the Ao- Qr shear wave velocity. The observed velocity is two to three times fasterthan those found in PMMA by Schardin [4]. Our high-speed camera results also indicated thecrack velocity of 0.4-0.8 mm/J.ls in the above experiment. Thus, more detailed analysis of crackpropagation and AE generation is clearly needed to resolve the observed discrepancy.

Disucussion

The procedures developed here with the aid of laser instrumentation and the use of apiezoelectric element allow one to characterize fracture-induced AE signals hidden in over­lapping impact waves. This method will be useful for analyzing more complicated fracturephenomena expected in CFRPs subjected to impact. Such a study has been initiated.

We have also examined AE signals due to quasi-static fracture of similar PMMA plates.In these cases, the fracture induced plate vibration and we were unable to separate fracturewaves. Further work is needed to ascertain why fracture-induced AE cannot be singled out inthe quasi-static cases.

CONCLUSION

Fracture in 3-mm-thick PMMA plate impacted by a flying steel ball was detected andanalyzed using quantitative AE methods. The AE signals due to impact and local fracture inimpacted plates were separated by the time-shift subtraction method following the characteri­zation of impact-only signals. AE signals produced only by ball impact are dominated with lowfrequency Lanlb waves and can be simulated by using a bell-shaped source function of 16 J.lSduration. Fracture-induced waves due to local fracture in an impacted PMMA plate correspondto the ramp-type source function of 12 J1.S rise time, according to the simulation analysis withthe aid of laser interferometer. The estimated crack velocity of 1.3 mm/J.ls was faster thanthose in other experiments and points up the need of detailed crack radiation modeling. Thepresent method can be applied to the fracture analysis of various composite materials subjectedto impact.

ACKNOWLEDGMENTS

A part of this research was supported by the fellowship from the Japan Society for Promo­tion of Science and the Japan Science Society to Y. Mizutani. K. Ono gratefully acknowledgesthe support of .Japan Soc. For Promotion of Sciences for an extended stay at Aoyama GakuinUniversity during his sabbatical leave from UCLA.

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REFERENCES

(1] Mizutani,Y., Nagashima,K., Takemoto,M., Ollo,IC, Proceeding of AECM, Texas, ASNT.to be published.

[2] Suzuki,H., Kinjo,T., Takemoto,M., Ono,K., Proceeding of PROGRESS in ACOUSTICEMISSION VII, The Japanese Society for NDI, Tokyo, 1996, pp.47-52.

[3J Suzuki,H., Kinjo,T., Hayashi,Y., Takemoto,M. and Ono,K., J.Acoustic Emission, Vol. 14No.2, 1996, pp.69-84

{41 Schardin,H., Velocity Effects in Fracture" , in Fracture, eds. By B.L. Averbach et aI.,MIT Press, Cambridge, MA, 1959, pp. 297-330

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