Vibrothermography and Ultrasonic Pulse-Echo Methods Applied to the Detection of Damage in Composite Lamintates

It has recently been shown in our laboratories that quasi-isotropic, graphite-epoxy, composite laminates develop a typical damage state that eventually leads to final failure. This damage state cannot be represented by a single through crack that propagates in a self-similar manner in the fashion ordained by fracture mechanics. To the contrary, the damage state is a complex one which begins by transverse cracking in the weakest lamina, continues by an increase in transverse crack density until a stable equilibrium spacing is achieved, proceeds by growth into the adjacent laminae.and ends by final, catastrophic failure. In certain stacking sequences, the damage state is further complicated by delamination. Several NDE methods are being developed in our laboratories specifically to identify and quantitatively describe this damage state. The vibrothermography technique uses low amplitude vibrations as a steady state energy source in the composite laminate. The mechanical energy is preferentially absorbed in the region of damage and converted to heat, which can then be detected by thermography. This technique is especially applicable to detecting delamination. An ultrasonic pulse-echo method utilizing a straightforward diffraction analysis is. being developed to detect the transverse cracks which, as they approach and attain an equilibrium spacing, present the appearance of a changing diffraction grating to the ultrasonic beam

Multiple Wave Scattering And Calculated Effective Stiffness And Wave Properties In Unidirectional Fiber-Reinforced Composites

) Analytic methods of elastic wave scattering in fiber-reinforced composite materials are investigated in this study to calculate the effective static stiffness (axial shear modulus, ) and wave properties (axially shear wave speed, B and attenuation, Y) in composites. For simplicity only out-of-plane shear waves are modeled propagating in a plane transverse to the fiber axis. Statistical averaging of a spatially random distribution of fibers is performed and a simultaneous system of linear equations are obtained from which the effective global wave numbers are numerically calculated. The wave numbers, K=Re(K)+iIm(K), are complex numbers where the real parts are used to compute the effective axial shear static stiffness and wave speed; the imaginary parts are used to compute the effective axial shear wave attenuation in composites. Three major parts of this study are presented. The first part is the discussion of multiple scattering phenomena in a successive-events scattering approach. ..

Tensile and Flexure Strength of Unidirectional

A Local Load Sharing (LLS) model recently developed by Curtin and co-workers for the numerical simulation of tensile stress-strain behavior in fiber-reinforced composites is used to predict the tensile strength of metal matrix composites consisting of a Titanium matrix and unidirectionally aligned SiC fibers. This model is extended to include the e#ects of free boundary conditions and non-constant load gradients and then used to predict the strength of a Ti-6Al-4V matrix reinforced with Sigma SiC fibers under 4point flexure testing. The predicted tensile and flexure strengths agree very well with the values measured by Gundel and Wawner and Ramamurty et al. The composite strength of disordered spatial fiber distributions is investigated and is shown to have a distribution similar to the corresponding ordered composite, but with a mean strength that decreases (as compared to the ordered composite) with increasing Weibull modulus. A modified Batdorf-type analytic model is developed and similarly extended to the case of non-uniform loading to predict the strength of composites under tension and flexure. The flexure model is found to be inappropriate for application to the experimental materials, but the tensile model yields predictions similar to the Local Load Sharing models for the experimental materials. The ideas and predictions of the Batdorf-type model, which is essentially an approximation to the simulation model, are then compared in more detail to a simulationbased model developed by Ibnabdeljalil and Curtin to more generally assess the accuracy of the Batdorf model in predicting tensile strength and notch strength versus composite size and fiber Weibull modulus. The study shows the Batdorf model to be accurate for tensile strength at high Weibull moduli and to ca..