J. D. Achenbach

Effect of interfacial zone on mechanical behavior and failure of fiber-reinforced composites

Abstract The mechanical behavior of the interphase between fibers and matrix is modeled by continuity of tractions, and a linear relation between displacement differences across the interphase and the conjugate tractions. The proportionality constants characterize the stiffness and the strength of the interphase. By the use of this model, and for transverse loading of a rectangular-array fiber-reinforced composite, numerical results have been obtained for the stresses in the matrix and in the fibers. Variation of the interphase parameters causes pronounced changes in these stress distributions. The initiation, propagation and arrest of interphase cracks have been analysed based on a criterion of critical strain energy density in the interphase. The results have also been used to display the effect of the interphase parameters on the overall elastic moduli of the composite, prior as well as subsequent to the development of interphase cracks.

Computation of the Weight Function from a Stress Intensity Factor

A simple representation for the crack-face displacement is employed to compute a weight function solely from stress intensity factors for a reference loading configuration. Crack face displacements given by the representation are shown to be in good agreement with analytical results for cracked tensile strips, and stress intensity factors computed from the weight function agree well with those for edge cracks in half planes, radial cracks from circular holes, and radially cracked rings. The technique involves only simple quadrature and its efficacy is demonstrated by the example computations. The weight function for a corner crack in an LMFBR hexagonal sub-assembly duct is constructed from stress-intensity-factor results for the uniformly over-pressurized case, and it is shown how this may be used to determine the stress intensity factors.

Elastic constants of single‐crystal transition‐metal nitride films measured by line‐focus acoustic microscopy

The elastic constants of single‐crystal NbN, VN, and TiN films were determined from surface acoustic wave (SAW) dispersion curves obtained by the use of an acoustic microscope with a line‐focus beam. Measurements were carried out for single‐crystal nitride films grown on the (001) plane of single‐crystal cubic‐symmetric MgO substrates. The phase velocities measured as functions of the angle of propagation display the expected anisotropy. Dispersion curves of SAWs propagating along the symmetry axes were obtained by measuring the wave velocities for various film thicknesses and frequencies. Using a modified simplex method, an inversion of the SAW dispersion data yielded the elastic constants of cubic symmetry, namely c11, c12, and c44. The Rayleigh surface wave velocities calculated from the determined elastic constants and known mass densities agree well with a result measured by Brillouin scattering spectroscopy reported elsewhere.

Quantitative nondestructive evaluation

Abstract Quantitative Nondestructive Evaluation (QNDE) provides techniques to assess deterioration of a material or a structure, and to detect and characterize discrete flaws. It plays, therefore, an important role in the prevention of failure. QNDE techniques are used in processing, manufacturing and for in-service inspection. QNDE is particularly important for the in-service inspection of high-cost and critical load-bearing structures whose failure could have tragic consequences. In this paper, we briefly review the most important techniques, and then we focus the discussion on quantitative ultrasonics, particularly for crack detection and for the determination of elastic constants. The important role of measurement models is emphasized. New techniques in quantitative ultrasonics are discussed, including laser-based ultrasonics and acoustic microscopy. The possible applications of neural networks are indicated. Attention is also devoted to the probability of detection concept and its relation to probabilistic fatigue methods and fatigue reliability.

Scattering of elastic waves by a surface-breaking crack

Abstract Scattering of incident surface waves and incident body waves by a surface-breaking crack is investigated in a two-dimensional geometry. By decomposing the scattered fields into symmetric and antisymmetric fields with respect to the plane of the crack, two boundary value problems for a quarter-plane have been obtained. The formulation of each boundary-value problem has been reduced to a singular integral equation which has been solved numerically. For incident surface waves the back-scattered and forward-scattered surface waves have been plotted versus the dimensionless frequency. Curves are also presented for the scattered displacement fields in the interior of the body generated by incident body waves, both versus the angle of incidence and versus the dimensionless frequency.

Ultrasonic investigation of concrete with distributed damage

Bridge decks deteriorate due to many causes including low level fatigue cycling, thermal loading, chemical attack, and reinforcing steel corrosion. This deterioration takes the form of distributed microscopic damage that may evolve into large defects such as cracks, delaminations, spalling, and scaling. An experimental program was conducted to evaluate ultrasonic techniques for measuring distributed cracking in concrete structures. Distributed cracking refers primarily to microcracking and other high porosity regions that generally precede large cracks. An investigation of distributed cracking yields information on weaknesses in the materials that may ultimately lead to major cracking and failure, but also can be used to evaluate distress mechanisms that do not necessarily result in large cracks. Distributed cracking in concrete was induced by freeze-thaw cycling and salt-scaling. Ultrasonic tests were used to measure changes in attenuation, pulse velocity, and peak frequency of the ultrasonic waves due to the distributed damage. The ultrasonic measurements were correlated with damage observed using optical microscopy. It was found that ultrasonic pulse velocity was not very sensitive to changes caused by distributed microcracking. The change in signal amplitude (a measure of ultrasonic attenuation) was quite sensitive to changes caused by microcracking, although the measurements showed considerable scatter. The peak frequency of the ultrasonic signal was also quite sensitive to the condition of the concrete. These results must be considered in the development field tests for evaluation of concrete structures.

Detection of thermal fatigue in composites by second harmonic Lamb waves

Composite materials which are widely used in the aerospace industry, are usually subjected to frequent variation of temperature. Thermal cyclic loading may induce material degradation. Considering the long-term service of aircraft composites and the importance of safety in the aircraft industry, even a little damage that may be accumulative via thermal fatigue is often of great concern. Therefore, there is a demand to develop non-destructive approaches to evaluate thermal fatigue damage in an early stage. Due to the sensitivity of acoustic nonlinearity to micro-damage, the nonlinear ultrasonic technique has been explored as a promising tool for early detection of micro-damage. This paper investigates an experimental scheme for characterizing thermal fatigue damage in composite laminates using second harmonic Lamb waves. The present results show a monotonic increase of acoustic nonlinearity with respect to thermal fatigue cycles. The experimental observation of the correlation between the acoustic nonlinearity and thermal fatigue cycles in carbon/epoxy laminates verifies that nonlinear Lamb waves can be used to assess thermal fatigue damage rendering improved sensitivity over conventional linear feature based non-destructive evaluation techniques. Velocity and attenuation based ultrasonic studies are carried out for comparison with the nonlinear ultrasonic approach and it is found that nonlinear acoustic parameters are more promising indicators of thermal fatigue damage than linear ones.

Elastodynamic Near-Tip Stress and Displacement Fields for Rapidly Propagating Cracks in Orthotropic Materials

The near-tip angular variations of elastodynamic stress and displacement fields are investigated for rapid transient crack propagation in isotropic and orthotropic materials. The 2-dimensional near-tip displacement fields are assumed in the general form r/sup P/ T(t, c) K(theta, c), where c is a time-varying velocity of crack propagation, and it is shown that p = 0.5. For isotropic materials, K(theta, c) is determined explicitly by analytical considerations. A numerical procedure is employed to determine K(theta, c) for orthotropic materials. The tendency of the maximum stresses to move out of the plane of crack propagation as the speed of crack propagation increases is more pronounced for orthotropic materials, for the case that the crack propagates in the direction of the larger elastic modulus. The angular variations of the near-tip fields are the same for steady-state and transient crack propagation, and for propagation along straight and curved paths, provided that the direction of crack propagation and the speed of the crack tip vary continuously.

The influence of heat conduction on propagating stress jumps

Abstract T he propagation of discontinuities of the stresses and the temperatures is studied in a one-dimensional medium, in which the displacement and the temperature fields are coupled, and in which the transport of heat takes place at a finite velocity. It is shown that the application of a thermal or a mechanical disturbance gives rise to two wave fronts, whose speeds are expressed in terms of the material constants. The temperatures at the wave fronts are discontinuous unless Fouriers classical law of heat conduction is valid. The jumps at the wave fronts decay exponentially, and expressions for the exponents in terms of the material constants are presented. All results are obtained through application of the theory of propagating surfaces of discontinuity.

Laser generation of narrow‐band surface waves

Tone‐burst‐like narrow‐band surface waves are generated in the thermoelastic regime by illuminating the surface of a solid with an array of laser‐generated line sources. The laser line array is formed by a system of lenses and an optical diffraction grating that provide for flexible and easy control of the line‐array parameters. It is shown experimentally, consistent with theory, that the generation of narrow‐band surface waves can be controlled by adjusting the line‐array parameters such as the number of line sources in the array, the width of each line source, and the separation distance between them. Certain optimum generation conditions are experimentally determined whereby the amplitude of the narrow‐band surface waves can be increased by a factor of N (the effective number of lines in the array) over the corresponding broadband signals that would be generated using only a single line source. A laser‐interferometric system is used to detect the generated surface waves. By suitably bandpass filtering ...