S.B. Palmer

Quantitative studies of thermally generated elastic waves in laser‐irradiated metals

Quantitative experimental measurements have been made in the study of thermoelastic generation of elastic waves in a metal by unfocused laser radiation. A calibrated wide‐band detection system, incorporating a capacitance transducer, has enabled acoustic waveforms to be recorded with a minimum of distortion. From these measurements, a theoretical model has been developed. The transfer function of the metal block has been deconvoluted to give the acoustic source function, which was modeled as a rapidly expanding point volume of material. The thermoelastic source generated longitudinal (L) and (S) waves, but the latter predominated at the epicenter, where, in experiments presented here, both wave amplitudes L and S were proportional to the total absorbed energy in the laser pulse.

Quantitative measurements of laser‐generated acoustic waveforms

The generation of acoustic waves in metals by pulsed laser irradiation over a wide range of material conditions has been studied. Capacitance transducers have been used to obtain quantitative measurements of the amplitude of bulk acoustic waveforms where the laser beam was directed onto free metal surfaces in the presence and absence of surface plasmas, and onto modified metal surfaces. The application of acoustic wave propagation theory has allowed theoretical waveforms to be determined. By combining data for thermoelastic and normal force sources, waveforms have been produced that follow closely those measured experimentally.

Estimation of the thickness of thin metal sheet using laser generated ultrasound

A noncontacting technique has been developed to measure the thickness of thin metal sheet by using a pulsed laser to generate both symmetric and antisymmetric Lamb waves. These have been detected with a laser interferometer. Analysis of the waveforms allows an estimation of the sheet thickness and accuracies to within 2% are attainable on sheets as thin as 27 μm.

Laser generated ultrasound at modified metal surfaces

Abstract A study has been made of the ultrasonic pulses generated at modified metal surfaces by laser irradiation. Both longitudinal and shear acoustic modes have been investigated, generation at modified and free surfaces being compared. The use of a Q -switched Nd: YAG laser enabled experiments to be carried out at optical power densities above and below the threshold for plasma formation. Qualitative models of the acoustic sources existing in each case are proposed, which explain the experimental effects of surface modification; these include the observed changes in generation efficiency and longitudinal directivity.

Laser generation as a standard acoustic source in metals

Absolute acoustic waveforms, generated by laser irradiation of aluminium and mild steel samples, have been detected at epicenter with a wide‐band capacitance transducer. Comparison has been made with theoretical waveforms, utilizing theory for wave propagation within a plate and assuming certain source characteristics. Results have shown that laser generation is able to produce three types of standard acoustic source, namely a horizontal force dipole and a normal force monopole, each with step‐function time dependence, and a normal force monopole with δ‐function time dependence. Underlying generation mechanisms are discussed.

A remote laser system for ultrasonic velocity measurement at high temperatures

The temperature dependence of the longitudinal ultrasonic velocity in iron and Dural has been obtained using a totally remote laser technique. The ultrasound is generated by irradiation of one face of a sample with a Q‐switched Nd:YAG laser pulse and is detected on the opposite face using a modified Michelson laser interferometer. The system has proved capable of measurements up to temperatures in excess of 1000 °C, with an absolute accuracy in velocity of ±1%, and relative accuracies of better than 0.1%. Anomalies in the data for iron have been discerned at the Curie temperature of ∼768 °C, due to the ferromagnetic to the paramagnetic phase transition, and at 910 °C due to the crystallographic phase transition from ferrite to austenite.

Comparative study of wide-band ultrasonic transducers

Abstract The relative performance of several types of ultrasonic transducer are assessed using reproducible acoustic transients generated from Q-switched Nd:YAG laser pulses incident on an aluminium alloy sample. A laser interferometer, a capacitance transducer, two types of electromagnetic acoustic transducers (EMATs), and a broad-band piezoelectric transducer are examined as detectors. The comparison includes a study of their rise-times, and typical signal-to-noise ratios. In the case of the interferometer and capacitance transducer, displacement measurements are shown to be highly consistent with theory.

The elastic constants of chromium

Abstract Measurements have been made of the three single-crystal elastic stiffness constants of chromium from 4·2°K to 345°K using an ultrasonic pulse-echo technique. The compressional constants C 11 and C 12 show large anomalies at the antiferromagnetic Neel temperature T N=311°K, and much smaller anomalies at the spin-flip temperature, T SF=123°K. The shear constants C 11–C 12 and C 44 show rather small anomalies at T N but there are large changes at T SF. Between T SF and T N all the elastic constants are magnetic field dependent and the anomalous behaviour may be understood, in principle, in terms of a stress-induced movement of antiferromagnetic domain boundaries.

Antiferromagnetic domains in rare earth metals and alloys

Anomalies in the c-axis elastic properties of antiferromagnetic Dy 50% Tb-Ho and 60% Gd-Y are reported. The anomalies are only present when the sample is cycled from the ferromagnetic to the antiferromagnetic state and are attributed to domains in the helical regime.

LASER-BASED NON-DESTRUCTIVE TESTING TECHNIQUES FOR THE ULTRASONIC CHARACTERIZATION OF SUBSURFACE FLAWS

Abstract Laser generation and detection of ultrasound has been used for remote characterization of flat-bottomed holes, acting as subsurface flaws, drilled into an aluminium test sample. A pulsed Nd:YAG laser system was used to generate the ultrasonic source whilst a HeNe laser interferometer detected the subsequent surface displacements. The technique allows both the width and depth of flawa to be assessed. Moreover, an analysis of the data shows that the method is particularly suited to detection of flaws ≤ 4 mm below the surface of the sample, a region which is difficult to inspect by conventional contact ultrasonic techniques.