Showing papers by "Ajit Mal published in 1995"

Crack initiation in plates with multiple defects

Abstract: Defects are commonly found on aging aircraft skins and panels and are either caused by localized corrosion or other conditions. Even at low levels of cyclic loading, these localized defects can create a crack, which can lead to catastrophic failure if the crack is not found within a critical period. It has also been found that rivet holes can sometimes be attacked by corrosive environment and then serve as initiation sites under fatigue loading. Fracture experiments were carried out on thin 2024 aluminum plates with different sizes and orientations of small holes and pits to mechanically simulate localized corroded plates and to examine the strength degradation of this material during the corrosion process under static uniaxial tensile loading conditions. Different sizes of holes and pits in varying arrangements were used in fracture experiments to evaluate the diversity of pit topology in corroded plates. Crack initiation locations and stress distribution were evaluated by using boundary integral equation methods. The results showed some interesting variations in material strength depending on the spatial distribution and size of the small defects. This research seeks to analyze the pitting corrosion fatigue problem from a mechanical point of view.

Characterization of Fiber-Matrix Interface Degradation in a Metal Matrix Composite

Abstract: The fracture and failure of fiber-reinforced composites are known to be strongly influenced by the properties of the fiber-matrix interface zone. Since these properties may be altered during processing, or may suffer degradation during service, the quality control of interface zones through nondestructive evaluation techniques is highly desirable. The overall properties of fiber-reinforced composites are related to their microstructure. In particular, the velocity of the average waves that can be transmitted in these materials depends on the elastic moduli and the densities of the constituents as well as the properties of the fiber-matrix interface. Thus, the interface conditions can, in principle, be determined from the measurement and analysis of wavespeeds in these materials. However, in practice, this is not a straightforward exercise due to the fact that the relationship between the overall and microstructural properties of a composite are in general nonlinear and thus may lead to an amplification of measurement errors. It is nevertheless extremely important to develop the capability to monitor the integrity of the fiber-matrix interface, especially in high temperature applications where the composites must operate in harsh environments.

Ultrasonic Characterization of Defects in Lap Joints

Abstract: This paper is concerned with the detection and characterization of material loss within lap joints. One of the most commonly used nondestructive methods to inspect lap joints is the eddy current method [1]. In this technique, the percentage of material loss is determined by comparing the signal from the defective joint with a reference signal. Ultrasonic tests based on pulse-echo technique can also be used to detect thickness reduction in lap joints. However both of these methods require that the sensor be directly above the defects, thus making their practical implementation extremely time consuming. Improving the efficiency of lap joint inspection is one of the most critical tasks currently facing the NDE community. In this paper we discuss the feasibility of using an ultrasonic technique based on guided waves launched across the lap joint. It is well known that the characteristics of guided waves can be used to detect defects in plates [2]. The geometry of the lap joint makes it much more difficult to extend the guided wave based method to lap joints. There is no closed form solution to the problem of wave propagation across lap joints. Due to the change of thickness and the existence of vertical stress free boundaries, nonpropagating modes may play an important role inside and near the overlapped region. With material loss inside the lap joint, the geometry becomes even more complicated. Thus the problem of wave propagation in the lap joint can only be solved by numerical and experimental methods.

Scattering of Elastic Waves by Multiple Inclusions and Cracks

Abstract: A new volume integral equation method (VIEM) is used to study problems involving wave scattering by multiple inclusions and cracks. The major objective of this study is to determine the characteristics of the scattered field in metal matrix composites with large fibers in presence of microcracks and other defects. Since composites consist of a large number of closely packed fibers, the elastic field in these materials is strongly affected by the interaction between microcracks and the fibers. Thus, to analyze the effects of microcracks on the scattered field in composites is of particular interest in ultrasonic NDE for crack detection and characterization. In this paper we consider the scattering of elastic waves by a distribution of long parallel fibers in presence or absence of interface debonding. We use the VIEM to calculate the scattering cross section from the fibers and discuss the possible application of the results in the nondestructive characterization of the fibermatrix interface degradation in structural composites.

Elastic Waves from a Distributed Surface Source in a Unidirectional Composite

Abstract: The response of a unidirectional composite plate of infirritc lateral dimensions to localized dynamic surface sourws is investigated through theoretical modeling and laboratory tests. In the theoretical sitnulations, the material ofthc plate is assumed to be dissipative and transversely isotropic with its symmetry axis parallel to the fibers. The source is assumed to hsrve an arbitrary spatial and time dependence. The associated clastodynamic boundary value problem is solved by means of an integral transform technique followed by numerical evaluation of the inversion intc~als. The laboratory tests are carried out on unidirectional graphite epoxy plates of thicknesses ranging from 1-25 mm and large Iateml dimensions (> 30 cm? excited by means of broadband transducers attached to its surface. The calculated surface response of the plate at different distances and directions from the source is shown to agree very well with the recorded response in the ultrasonic range. INTRODUCTION It is well known that Iaminatcd tibcr reinforced composites often suffer significant internal damage when they arc subjected to Iocalizcd dynamic surface loads. The damage may involve frbcr breakage and debonding as well as delamination bctwccn the individual laminae. Such damage has been observed to occur even at relatively low impact speeds resulting in a severe loss in the load carrying capacity of the larninac. Although the damage is clearly caused by the stresses which develop within the material, the prccisc nature of these stresses and their relationship to the degree and mode ofthc damage are not clearly understood at present. This is particularly true in the dynamic case where the stresses are caused by waves whose propagation characteristics are strongly influenced by the inherent an isotropy and heterogeneity of the composite material. Dynamic response of plates has been studied theoretically by many authors through past decades. The linear elastic solutions of the isotropic or anisotropic plates have been investigated by, c. While nurncrous analytical investigation of wave propagation in plate have been executed, concurrent experimental studies of such wave processes in the laboratory have been far Icss prevalent. Most clXort in this direction were devoted to a cmnpwiwn of pred Icted phase and group velocities of surface waves.