Papers by Keyword: Matrix Cracking

Abstract: The overall purpose of the research is to investigate the effect of thickness on fiberglass reinforced epoxy laminates. In this study, simplified coupon specimens made from Epoxy/C-glass (200 g/m²) and Epoxy/C-glass (600 g/m²) with different thicknesses were used. To perform the high velocity impact tests (250 m/s), an instrumented single stage gas gun was used. The impacted specimens were examined to determine the extent of damages induced around the impacted point. For the projectiles velocity of 250 m/s, it was found that Epoxy/C-glass 200 g/m² was able to absorb 21.5 J of energy at the thickness of 12 mm, while Epoxy/C-glass 600 g/m² was able to absorb 96.1 J of energy at 10 mm thickness. Both the fibreglasses compute damage in terms of slight matrix breakage and cracking. A general trend was observed on the overall ballistic test results, which indicated that, as the plate thickness continue to increase, the damage at the lower skin decreases and could not be seen.

Abstract: Microcracking is often observed in the resin pockets of as-cured 3D woven composites with significant through-the-thickness fiber constraint. The resin is subjected to a triaxial tensile stress during cooling due to thermal expansion mismatch and shrinkage during curing. However, the temperature dependence of the failure surface for resins subjected to triaxial tensile stress is not known and there is no standard measurement method for applying a triaxial tensile stress. We have developed a novel method for measuring the triaxial tensile failure stress surface by confining the shrinkage of the resin to tubes of different thicknesses and made of materials with different thermal expansion coefficient. The difference in thermal contraction and shrinkage between the resin and the confining tube subjects the resin to a triaxial tensile stress during cooling and curing. We vary the stress-temperature state by selecting tubes with different coefficient of thermal expansion. We infer the stress in the resin from the deflections of the tube measured by a high resolution dilatometer assuming that the tube is a linear elastic, thick-walled pressure vessel.

Abstract: Composite laminates meanwhile are of common use, especially in aerospace engineering. Inter-fiber cracks within a laminate ply are often accepted in practice to be still within failure tolerance, although the structural mechanics of this situation is not fully understood. The situation gets even more complex when the interaction of inter-fiber cracks in neighboring plies is considered. In this work, such three-dimensional crack configurations in composite laminates involving inter-fiber cracks and the influence of the laminate free-edge effect are studied by means of the Scaled Boundary Finite Element Method (SBFEM). The SBFEM is an efficient semi-analytical method that permits the analysis of linear elasticity problems including stress singularities or infinite domains. It is shown that in crack configurations in composite laminates so-called hypersingularities (or supersingularities) can occur, i.e. stress singularities which are of higher order than the classical crack singularity. This indicates that the laminate failure risk induced by certain considered crack configurations is not to be underestimated.

Abstract: The most important concern in design of filament-wound composite pressure vessels reflects on the determination of the optimum shape and optimum laminate stacking sequence of composite vessels based on the matrix cracking pressure and burst pressure of composite laminates. In this study the Imperialist Competitive Algorithm (ICA) is used to find the optimum laminate stacking sequence of composite vessels that the design considerations are stability and strength constraints. the matrix cracking pressure of filament-wound composite pressure vessels made of different number of helical layers and different layers of Circumferential layers was calculated by using orthotropic material formulae and then, the burst pressure of composite vessels was calculated by using netting analysis. The optimum laminate stacking sequence of filament winding composite was found to maximize the matrix cracking pressure and the burst pressure by using Imperialist Competitive algorithm.

Abstract: Signal processing is an important element used for identifying damage in any SHM-related application. The method here is used to extract features from the use of different types of sensors, of which there are many. The responses from the sensors are also interpreted to classify the location and severity of the damage. This paper describes the signal processing approaches used for detecting the impact locations and monitoring the responses of impact damage. Further explanations are also given on the most widely-used software tools for damage detection and identification implemented throughout this research work. A brief introduction to these signal processing tools, together with some previous work related to impact damage detection, are presented and discussed in this paper.

Abstract: Various aspects of the effect of microstructural randomness exhibited by carbon fibre-reinforced cross-ply laminates on the delamination damage mechanism is investigated in this paper. In the first part, the matrix cracks with different spacings measured in experiments are simulated using finite elements in order to obtain the levels of degradation and effective properties for a composite beam loaded in bending. The results show significant levels of degradation of obtained effective properties depicting the importance of accounting for the inherent stochasticity in these laminates. In the second part of the paper, initiation of delamination at an interface between 0° and 90° layers due to stress concentrations at tips matrix cracks is simulated for a beam under tension. Stochastic cohesive zone elements with fracture parameters presented as random fields are used to model this interface in a composite. Different values of the axial stress are obtained for initiation of damage for a number of realisations based on this approach. The results emphasize the need to take into consideration the microstructural randomness in fibre-reinforced laminates for adequate predictions of damage and load carrying capacities.

Abstract: The laminated composite structures applied to the wing and the speed brake of an aircraft or the turbine blade of a compressor. These structures may be impacted by birds and hails during operation. They may also be impacted by drop of a tool during manufacture or repair. Unlike high velocity impact damage, which can be easily found by the naked eye, the damage due to low velocity impact may be difficult to detect. Damage which is not detected may cause failure of a structure and result in damage propagation. Growth of damage means reduction of stiffness on the structure. So, exact prediction of damage caused by a low velocity impact is very important in order to guard against sudden failure of the structure. In this study, modified delamination failure criterion has suggested in order to predict the failure behavior of a composite plate subjected to low-velocity impact. The criterion includes the assumption which is matrix cracking mode causes delamination failure. Predicted damage using supposed delamination criterion is similar to experiment results.

Abstract: The impedance-based structural health monitoring method is used to successfully detect different damage mechanisms in composites and to correlate the changes in impedance measurements with the changes in the structure. Specifically, graphite/epoxy composite samples are manufactured and tested. Piezoceramic (PZT) patches are attached to the composite coupons to actuate the structure with high-frequency excitations. Bonding the patches to the sample allows changes in the structural mechanical impedance to be monitored with the electrical impedance of the PZT. Samples are tested using quasi-static tensile loading to introduce damage. To determine the extent of damage incurred, impedance signatures are acquired before and after the tensile load is applied. A change in impedance from the baseline shows the presence of damage. The amount of damage is characterized using a damage metric. Radiography is used to verify the extent of damage.