No abstract

The acoustoelastic technique is a nondestructive technique for analyzing the stress state in materials using ultrasonic waves. The velocities of the ultrasonic waves propagating through Polymer bonded explosive (PBX) change depending on the applied stress (compression or tension). In this paper, PBX specimens at close range of densities and ultrasonic wave velocities under similar initial conditions are applied in the compressive loading tests. Ultrasonic longitudinal wave propagation direction is parallel and perpendicular to applied stress direction respectively. Longitudinal wave velocities are accurately and efficiently achieved by real-time Ultrasonic time differences (velocity) measurement unit. In the range of stress evaluated under compression between 0 and 15MPa, these relations between the relative variation of the longitudinal wave velocity and the applied stress vary linearly. The value of acoustoelastic constant (K) can be determined by the compression tests. These founds suggest the potential of applying the acoustoelastic effect to determine the compressive stress condition of PBX in a certain stress range.

The measurement of acoustic nonlinear response is known as a promising technique to characterize material micro-damages. In this paper, nonlinear ultrasonic approach is used to characterize the evolution of fatigue induced micro-cracks in polymer bonded explosives. The variations of acoustic nonlinearity with respect to fatigue cycles in the specimens are obtained in this investigation. The present results show a significant increase of acoustic nonlinearity with respect to fatigue cycles. The experimental observation of the correlation between the acoustic nonlinearity and fatigue cycles in carbon/epoxy laminates, verifies that an acoustic nonlinear response can be used to evaluate the progressive fatigue damage in the granular polymer bonded explosives. The sensitivity comparison of nonlinear and linear parameters of ultrasonic waves in the specimens shows that nonlinear acoustic parameters are more promising indicators to fatigue induced micro-damage than linear ones. The feasibility study of the micro-damage assessment of polymer bonded explosives by nonlinear ultrasonic technique in this work can be applied to damage identification, material degradation monitoring, and lifetime prediction of the explosive parts.

Magnetic Incremental Permeability (MIP) method practiced as a NDT technique to evaluate the material degradation is based on the eddy current testing in addition to a low frequency bias magnetic field generated by an electromagnetic magnet. To clarify the mechanism of MIP and to find optimal probe design, a numerical method to simulate MIP profile is important. In this paper, a simulation scheme and a numerical code are developed for MIP of ferromagnetic material based on the simulated polarization strategy and the reduced vector potential formulae. Experiments are also conducted for test-pieces of carbon steel to demonstrate the validity of the proposed scheme.

Aluminum foam is a functional material which is highly porous with cells of stochastic geometry. In distinction to polymer foam, aluminum foam is electrically conductive and has typical applications in many engineering areas. Optimal design and manufacture of foam structure usually require detailed understanding of the electrical property of the aluminum foam. In this study, a three-dimensional finite element numerical model based on the statistic characteristics of the geometrical structure of the closed-cell aluminum foam was proposed. The proposed numerical method was applied to study the property of the current conduction and to clarify the dependence of the electrical conductivity on the porosity as well as the cell size. A shape factor defined based on the numerical simulation results is introduced to the theoretical model of the electrical conductivity regarding porosity, which can describe the relationship between the electrical conductivity and the porosity of the closed-cell aluminum foam properly. It was found that the porosity has a negative effect on the electrical conductivity in a power law approximately, while the cell size has a slight effect on the electrical conductivity of the closed-cell aluminum foam. Finally, the simulation results were compared to the experimental ones and their good agreement demonstrated the feasibility and accuracy of the proposed numerical model of the closed-cell metallic foam.