Applications of Laser Light Probes to Quantitative Sensing Stress Waves

The recent development of laser light probes for stress wave measurements has aided our understanding of acoustic emission and ultrasonic signals by allowing quantitative measurements of stress waveforms. This paper reports on applications of the laser interferometer probe for surface detection of stress waves and the laser transmission probe for sensing of stress waves inside transparent materials. Laser light probes were used to characterize ultrasonic and acoustic emission transducers' response in a realistic configuration, with transducers in actual contact with a solid. Laser light probes also were applied in directly detecting acoustic emission due to stress corrosion cracking in 7039 aluminum and crazing of Plexiglass. The results of the laser light probe measurements indicate that conventional piezoelectric transducers, although adequate for many ultrasonic pulse inspection tests, are severely limited as stress pulse sensors for acoustic emission measurements. The acoustic emission signals measured by the laser light probe showed a pulse-like waveform which has not been previously recorded by conventional piezoelectric acoustic emission sensors. INTRODUCTION In almost all industrial applications, conventional piezoelectric transducers are used to detect acoustic emission and ultrasonic transients in solid materials. As the demand for accuracy and reproducibility of nondestructive tests and inspection methods increases, the limitations of electromechanical devices such as piezoelectric transducers are becoming more noticeable and must be specified. Industrial piezoelectric ultrasonic transducers (see Fig. 3) are complex structures of layered active and passive elements, packaged into a small unit which is coupled to the workpiece by a variety of impedance matching fluids. Because of the contact with the test piece, such devices inadvertently mechanically load the contact surface and thus change the test piece behavior.· Also, the need for coupling agents between transducer and contact surface introduces the problem of reliable test repeatibility. However, these problems are only secondary to the strong need for industry-wide calibration and standardized comparison of different ultrasonic and acoustic emission transducer units. To be able to address some of the abovementioned problems, an experimental system was developed that utilizes laser light probes to measure stress waves in solids. The combination of laser probes with digital signal capture and computer data processing enabled accurate evaluation and analysis of the piezoelectric transducers' response. Figure 1 is a schematic representation of a whole test system that allowed ultrasonic or acoustic emission signals to be measured simultaneously by laser probe and piezoelectric sensor. Two types of laser light probes were used to measure a physically identifiable parameter associated with stress wave propagation. A laser transmission probe sensitive to material 520 density changes was used to measure bulk ultrasonic waves inside transparent test specimens and a laser interferometer probe was used to measure the normal surface displacement produced. by propagating ultrasonic waves. The performance of these laser light probes was described in other publications(l,2). However, it is important to note that a laser probe sensor does not alter the stress waves, produce no change in mechanical boundary conditions, and has no intrinsic frequency-response limitations. Thus, the simultaneous use of optical probes and conventional piezoelectric transducers enabled direct signal comparison and critical evaluation of the performance limitations exhibited by piezoelectric sensing devices. ULTRASONIC TRANSDUCER CALIBRATION AND CHARACTERIZATION Ultrasonic transducers mounted on the face of a transparent cube test block (such as Plexiglass) were evaluated using a laser transmission probe. By scanning the YZ plane, this arrangement allowed the pointwise measurements of the stress pulse waveform, relative pulse intensity, and arrival time at all test block locations. These measurements were then used to indicate the transducers' beam profile (near field [NF] and far field [FF]) as well as any regions of anomalous nodal lines or high intensities. The three dark photographs in Fig. 2 are indicative of the signals measured at the three different test block locations. The top trace is the signal from the piezoelectric inspection transducer and the bottom trace is due to the signal sensed by the laser transmission probe. Note that the laser probe centered on the block shows the first-signal-arrival-time delay of l/4 of the total time necessary for the pulse reflection to return to the transmitting transducer. Thus, the centered laser probe sees stress waves twice as often as the surface mounted transducer and nicely illustrates a phase shift between a transmitted and a reflected ultrasonic stress wave packet. The off-center signal sample by the laser transmission probe is used to demonstrate a spread of the stress wave packet. The signal observed by the laser probe became significant only after a time delay; a few reflections of the ultrasonic pulse allowed significant widening of the original stress wave packet. Beam intensity assymetry of an ultrasonic inspection transducer is demonstrated by a fourwaveform sample taken across the transducer main beam. Transducers with such assymetric beam intensities are not desirable and can be very troublesome when used for ultrasonic inspection. These characteristics sometimes develop only when a transducer is in contact with a solid and cannot be seen with conventional liquid-immersion transducer-beam profile measuring methods. In general, surface laser interferometer probe measurements agree with transmission probe indications. The transducer and laser interferometer probe signals shown in Fig. 2 are compared in frequency domain. The power spectra of the two sensors simultaneously measuring the same stress wave are similar except for an unavoidable harmonic peak around 3 MHz for the piezoelectric transducers. ACOUSTIC EMISSION TRANSDUCER CHARACTERIZATION The response and accuracy of waveform measurements by conventional piezoelectric transducers degrades when complicated waveforms, such as acoustic emissions, are encountered. Figure 3 illustrates the test block arrangement used to evaluate acoustic emission transducers. As reported in other publications, this design (1 ,2,3) utilizes a glass capillary fracture as a repeatable stress wave pulse. The stress wave signals generated by this arrangement have been analyzed theoretically and confirmed experimentally. For this purpose, the test block was used as a stress signal source and measured by a laser interferometer probe. The response of a typical piezoelectric acoustic emission transducer to the same stimulus is also shown. The difference in the signals observed are more easily explained by a power spectra plot of the signals from the two measurements. The transducer power spectrum peaks at 175 kHz, which was the resonant frequency of the piezoelectric element. Thus, the complexity of the acoustic emission transducer signal output mostly is characteristic of the piezoelectric element and sensor design, thereby masking the true stress waveform signal present in the test block. ACOUSTIC EMISSION MEASUREMENTS USING LASER INTERFEROMETER PROBES The laser interferometer probe can be used to sense and characterize stress waves directly. It is especially useful for complex signals such as those encountered in acoustic emissions due to cracking in metals or crazing of polymers. Acoustic emissions were generated by initiating stress corrosion cracking using dilute 521 sodium chloride solution on a self-loaded, doublecantilever beam specimen made from 7039 aluminum alloy. The observed, fast rise time, pulse-like waveforms suggested that acoustic emission from stress corrosion cracking is due to catastrophic, microscopic fracture events inside the growing crack. It should be noted that these signals have not been observed by conventional acoustic emission methods. Similar in form, but much slower in time (on the order of milliseconds), are acoustic emissions due to polymer crazing. The crazing of Plexiglass, induced on the surface of a bendloaded test bar, generated a complex acoustic emission signal lasting a few milliseconds. The complexity of this signal(4) was identified to have been generated by reflections of stress waves at a boundary of the specimen and subsequent constructive or destructive interference of the waves at the location of the laser interferometer probe. The experimental work on polymer crazing is still in progress and should aid our understanding of the dynamics of the failure mechanisms in polymer materials under load. SUMMARY By utilizing laser light probes, it is now possible to accurately sense the true ultrasonic signal waveform in solid materials. Therefore, laser probes can be used to critically evaluate piezoelectric transducer response. In multi-frequency, complex, stress wave signal measurements such as acoustic emissions, laser light probes can measure stress waveforms not observable by piezoelectric devices, which exhibit resonance-like behavior.