Acoustic emission from a growing crack

An analytical method is being developed to determine the signature of an acoustic emission waveform from a growing crack and the results of this analysis are compared to experimentally obtained values. Within the assumptions of linear elastic fracture mechanics, a two dimensional model is developed to examine a semi-infinite crack that, after propagating with a constant velocity, suddenly stops. The analytical model employs an integral equation method for the analysis of problems of dynamic fracture mechanics. The experimental procedure uses an interferometric apparatus that makes very localized absolute measurements with very high fidelity and without acoustically loading the specimen

Dynamic Pressure Measurements

Issued as final repor

Characterization of Rayleigh Wave Propagation in Concrete Using Laser Ultrasonics

Ultrasonic nondestructive evaluation of non-traditional civil engineering materials such as aluminum are fairly well advanced. However the advancement of ultrasonic methodologies for the nondestructive evaluation of concrete lag behind primarily because of the lack of quantitative understanding of the propagation characteristics of ultrasonic waves in concrete due to the highly attenuating nature of concrete

Nonlinear Resonance Ultrasonic Spectroscopy of Precipitation in 17-4PH Stainless Steel

Nonlinear resonance ultrasonic spectroscopy (NRUS) has been widely used to determine the hysteretic nonlinearity parameter, α for mesoscopic elastic materials. The current research adopts the NRUS to characterize microscopic material damage in metallic specimens with small dimensions. The geometric limitations in these small samples greatly restrict applications of other nonlinear ultrasonic techniques such as the higher harmonic generation technique. Two opposite edges of a rectangular plate specimen (10x10x2mm) are clamped by a specially designed holder and an excitation piezoelectric transducer is attached to one of the other edges. The excitation transducer provides a frequency sweep around a few of the natural vibration modes of the specimen while the resonant response of the specimen is detected by a laser vibrometer. To simulate the copper-rich precipitate (CRP) formation and the associated irradiation damage in reactor pressure vessel steels, thermally aged 17-4 PH stainless steel is used to study the effects of CRPs on the measured hysteretic nonlinearity parameter, α. These results are compared to previous research [1] and it is shown that the NRUS method is quite sensitive to the microstructure changes due to copper precipitates in this material

Calibration of Air-Coupled Transducers for Absolute Nonlinear Ultrasonic Measurements

We describe a novel calibration technique of air-coupled transducers for nonlinear ultrasonic measurements through homogenous, isotropic media. Our calibration technique combines laser interferometrywith a model-based approach to derive a relationship between received force at the transducer face and the measured output voltage. Conventional nonlinear ultrasonic measurement techniques have relied upon contact receiving transducers that are heavily influenced by contact conditions (e.g. inconsistent coupling) and laser interferometers that are prohibitively expensive and rely on a mirror-finished surface or complicated optics. Air-coupled transducers are significantly less expensive than laser interferometers and are robust relative to surface conditions, but current calibration techniques such as self-reciprocitymethods pose fundamentally challenging problems. We describe a method to experimentally deduce a transfer function |H(ω)| that can be used to predict surface displacements of fundamental and second harmonic wave components with the aid of proper acoustic field modelling in the pursuit of measuring the absolute nonlinearity parameter, β

A Non-Collinear Mixing Technique to Measure the Acoustic Nonlinearity Parameter of Adhesive Bond from Only One Side of the Sample

The acoustic nonlinearity parameter (ANLP) of a material is often positively correlated with the damage in the material. Therefore, the ability to nondestructively measure the ANLP may enable the nondestructive characterization of the material’s remaining strength. In this work, we developed a non-collinear mixing technique to measure the ANLP of adhesive bonds. One of the most significant features of the new method is that it requires only one-side access to the adhesive bond being measured, which significantly increases it utility in field measurements. Specifically, the test sample considered in this study consists of two aluminum plates adhesively joined together through a commercial adhesive tape. The non-collinear wave mixing technique consists of generating a longitudinal wave and a shear wave by piezoelectric transmitters attached to the same surface of the sample under test. These waves are introduced into the sample in such an angle that they will mix at the adhesive bond region. Mixing of these two waves generates a shear wave that propagates back towards the surface where the two waves were generated. This mixing wave is then recorded by a shear wave receiver placed on the same surface where the longitudinal and shear wave transmitters are located. It was shown that amplitude of this mixing wave is proportional to the ANLP of the adhesive bond. To demonstrate the effectiveness of the newly developed technique, a freshly made adhesive sample was first measured using the non-collinear mixing technique to obtain the ANLP of the adhesive bond. This sample is then placed inside a thermal chamber for aging to change its ANLP. The sample was taken out the thermal chamber periodically to measure its ANLP. The measured results clearly show that the ANLP varies with aging time. Initially, the ANLP decreases with aging time, possibly due to further curing. Afterward, the ANLP begins to increase with aging time, likely due to aging induced damage in the polymer adhesive. To verify that the signals received from the shear wave receiver are indeed the mixing wave, the finite element method was used to simulate the wave motion in the test sample. The simulation results clearly show that the signals recorded by the shear wave receiver are indeed the desired mixing wave, whose amplitude is proportional to the ANLP of the adhesive bond

Simulative Evaluation of Intragrain Precipitate Influence on the Material Nonlinearity using Nonlinear Ultrasound

Nondestructive evaluation using ultrasonic waves is commonly used to experimentally probe for the presence of defects (i.e. dislocations, precipitates, cracks) in complex metallic microstructures. Such defects and abnormalities are evidenced by monitoring the acoustic nonlinearity parameter β. However, from the mathematical standpoint, the correlation between the microstructural behavior and the measured acoustic nonlinearity parameter is not explicit yet. The present work aims to assess the existence of statistical correlations between microstructural defects and material nonlinearity. The effects of defect geometry, density, and geometrical arrangements (i.e. relative position) on material nonlinearity are studied. To do so, the acoustic response of Fe-Cu single crystals containing 1 % Cu precipitates with radii on the order of 10 nm is simulated by means of finite element analysis. Several thousand initial microstructures with random arrangement of precipitates are virtually tested using statistical methods, such as principal component analysis. Therefore, it is expected that a causal link can be made between the acoustic nonlinearity parameter and the precipitates-induced microstructural behavior via the proposed numerical analysis

Determination of Absolute Material Nonlinearity in Aluminum and Fused Silica with Air-Coupled Ultrasonic Receivers

Knowledge of the absolute material nonlinearity parameter, β, of a specimen allows for quantitative evaluation of its current microstructural state, making it a powerful tool in the NDE toolbox. However, the various methods used in the past to measure β each suffer from significant limitations. Piezoelectric contact transducers, while easy to use in many ways, are hindered by the unreliability of the interfacial contact between transducer and specimen surface, which offsets their high sensitivity to nonlinear signals. Laser interferometry provides non-contact detection, but requires carefully prepared specimens or expensive and complicated optics to maximize sensitivity to the nonlinear components of a received waveform, and additionally is expensive and relatively difficult to use in the field. Air-coupled piezoelectric transducers offer the strengths of both of these technologies and the weaknesses of neither, but are notoriously difficult to calibrate for use in nonlinear measurements. This work proposes a hybrid modeling and experimental approach to air-coupled transducer calibration and the use of this calibration in a model-based optimization to determine the β parameter of the material under investigation. This approach is applied to aluminum and fused silica, which are both well-documented materials and provide a strong reference for comparison of experimental and modeling results