Applied Mechanics and Materials Vol. 225

Abstract: A laser ultrasonic based nondestructive evaluation (NDE) technique has been widely used in aerospace industries for inspecting parts and structures made of composite materials. The thermoelastic regime is used for the ultrasonic generation, so no plasma is formed on the surface of composite structure. Generally, the service lifetime for an aircraft could be more than 25 years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy of a laser ultrasonic generator in the long-term periodic maintenance inspection. In this paper, the effect of laser pulse fatigue on the mechanical characteristics of a carbon-fiber-reinforced polymer (CFRP) plate (USN175BX Carbon UD preprag) with the stacking sequence of [0/45/-45/90]s is investigated to verify the reliability of the use of a laser ultrasonic based NDE technique on the CFRP plate specimen inspection. A high-speed laser ultrasonic scanning system (400mm/s at the intervals of 0.4 mm) was setup to perform repeat scanning of 1300 times on a CFRP plate specimen with the scanning area of 70 mm x 60 mm. These repeat scanning times were set in consideration of the periodic maintenance inspection scheduled to be 1 time/week x 52 weeks/year x 25 years. A 532nm Q-switched continuous wave laser (QL) was used and set at the laser pulse energy levels of 0.6 mJ and 1.2 mJ. Lamb wave assessment based on pitch-catch method was proposed in this paper to monitor the mechanical characteristics of a composite specimen. In each completion of 100 times repeat scanning, the Young’s modulus of the scanning area was evaluated based on the group velocity of S0 Lamb wave mode. In addition, the surface condition of the scanning area was investigated by using a microscope.

Abstract: This paper presents a research progress on the laser cutting characteristic of the carbon fiber reinforced plastic (CFRP) composites material for the aerospace structure panel. The CFRP is a high performance material and become one of the most important materials in the aerospace industry. The current machining techniques used by the aerospace composites manufacturing for cutting and trimming process have created some quality issues such as fiber pulled out and delamination. The use of laser cutting technology has shows promising advantages. However, the Laser cutting represents the interaction between laser beam and the CFRP composites, that produce heat affected zone (HAZ), kerf width and taper angle. Thermal damage is a direct consequence of the large difference in thermal properties of the carbon fiber and the polymer matrix. Therefore, the effect of laser cutting parameters such as pulse energy, pulse repetition rate, cutting speed and pulse duration need to be taken into consideration. A 300W Pulsed Nd:YAG laser machine was used in the experiment and successfully cut or trim the 1.5mm thickness of the CFRP component. The results also showed that the pulse energy and pulse repetition rate gave the most significant effect on the cutting characteristic in particular of kerf width, HAZ and taper angle.

Abstract: The use of laminated composites in aircraft structures is not totally new. However, the idea of using woven fiber glass as reinforcement in primary structural members is not widely addressed as compared to unidirectional fibers. In an effort to characterize the dynamic behavior of a woven laminated composite subject to dynamic loads, modal testing is performed experimentally on a cantilevered laminated woven glass fiber/epoxy composite flat plate which resembles an aircraft wing with aspect ratio of 5. To that end, the effect of stacking sequence and fiber orientation of the laminated composite plate on the modal properties is assessed. 6-layer laminated composite configurations with various stacking sequence and fiber orientation are fabricated so as to generate variable stiffness plates. The modal test employs the single roving hammer technique to obtain the frequency response of the plate and the results of the first five modes against the fiber orientation and stacking sequence are analyzed.

Abstract: There are two main factors that need to be considered as important parameters that affect the response of a structure: kinetic energy (E=1/2mv^2 ) and potential energy (E=mgh). For instance, if one has a large mass but with lower height, the amount of damage produced on the structure may not be the same as if one has a smaller mass with a higher dropping height although the potential energies will be the same. Therefore, before performing tests on the structures, the selection for the appropriate test apparatus and test procedures must be made carefully to ensure that the test conditions are similar to the actual impact conditions. In this present work, a study was conducted to fully understand the damage progression and growth, not only should the impacted surface be evaluated, but also the cross sectional defects on the impacted area must be accurately identified and examined. In this current work, the impacted test specimens will be observed at different magnifications to distinguish the types of failure mechanisms using Scanning Electron Microscopy (SEM). To perform this, the impacted specimens will be examined by two different approaches: surface defects and cross-sectional defects. This allows the failure mechanism to be observed more precisely.

Abstract: The orientation of fibers in the layers is an important factor that must be obtained in order to predict how well the finished composite product will perform under real-world working conditions. In this research, a five-layer glass-epoxy composite truncated cone structure under buckling load was considered. The simulation of the structure was done utilizing finite element method and was confirmed comparing with the published experimental results. Then the effect of different orientation of fibers on the buckling load was considered. For this, a computer programing was developed to compute the buckling load for different orientations of fibers in each layer. These orientations were produced randomly with the delicacy of 15 degrees. Finally, neural network and genetic algorithm methods were utilized to obtain the optimum orientations of fibers in each layer using the training data obtained from finite element simulation. There are many parameters such as the number of hidden layers, the number of neurons in each hidden layer, the training algorithm, the activation function and so on which must be specified properly in development of a neural network model. The number of hidden layers and number of neurons in each layer was obtained by try and error method. In this study, multilayer back-propagation (BP) neural network with the Levenberg-Marquardt training algorithm (trainlm) was used. Finally, the results showed that the truncated cone with optimum layers withstand considerably more buckling load.

Abstract: Impact resilient structures are of great interest in many engineering applications varying from civil, land vehicle, aircraft and space structures, to mention a few examples. To design such structure, one has to resort fundamental principles and take into account progress in analytical and computational approaches as well as in material science and technology. With such perspective, the first objective of this work is to develop a computational algorithm to analyze flat plate as a generic structure subjected to impact loading for numerical simulation and parametric study without considering the surface impact effect. The analysis is carried out from first principles for static and dynamic analysis; the latter is based on dynamic response analysis in the elastic region. The second objective is to utilize the computational algorithm for direct numerical simulation, and as a parallel scheme, commercial off-the shelf numerical code is utilized for parametric study, optimization and synthesis. Through such analysis and numerical simulation, effort is devoted to arrive at optimum configuration in terms of loading, structural dimensions, and material properties, among others. The codes developed are validated for generic cases. Further simulations are carried out using commercial codes for some sample applications to explore impact resilient structural characteristics in the elastic region.

Abstract: Fatigue problems become an important topic in the maintenance of aircraft structures. Efficient repair technique, called composite patch repair, was used to reinforce the damaged structures and extend the service life. In this paper, using empirical approach, the effect of composite patch repair on fatigue crack growth was investigated on 7050 aluminum alloy. In additional, loading parameters associated with patch repair (Graphite/Epoxy) was studied in order to show their influence on fatigue life and fatigue crack growth rate, namely stress ratio. Results provide an increasing in fatigue life and fatigue crack growth rate in increased stress ratio. A delayed in fatigue life and diminution of FCGRs is the results of composite patch repair. Effect of patch repair was shown highly at high stress intensity factor when maximum applied loading is kept constant.

Abstract: A finite strip method for geometric non-linear static analysis based on the tangential stiffness matrix has been developed using the new concept of polynomial finite strip elements, with Reissner (higher order shear deformable element) plate-bending theory for composite plates. A finite strip analysis programming package, which is capable of performing non-linear analysis for composite flat panels, has also been developed with Reissner plate bending element. Good agreement with the finite element results has been observed through various test cases, confirming the accuracy and reliability of the new developed finite strip method.

Abstract: It is well known that the structural performance of thin-walled compression members is subject to the effects of local buckling and material yielding. Due to these effects, the compressive carrying capability of short strut members can be significantly reduced. This paper employs finite element simulation to examine the post-buckled response of thin-walled box-sections that covers complete loading history of the compression struts from the onset of elastic local buckling through the nonlinear elastic and elasto-plastic post-buckling phases of behaviour up to final collapse and unloading. A detailed account of the growth and redistribution of stresses on the surfaces is given in the paper. The results from finite element simulations are shown to compare well with the analytical method of analysis.

Abstract: This paper works on the curvature effect of wing leading edge structure subjected to impact loading. At first stage, rigid spherical projectile and semi-elliptical panel were used. The impact testing has been carried out by varying the radius of curvature and the thickness of the panel. The experimental results show the trend of specific energy absorption capability of structure in function of the radius and thickness of panel. From experimental observation, it shows that the failure of structure subjected to impact loading can be distinguished in two types of failure; the projectile went through the structure and large displacement of curved panel. The two failure criteria are used later on to determine the energy absorption capability using Finite Element Analysis method. A FEA model is proposed to simulate the behavior of curved composite structure and validated by the experimental results in order, as final goal, to propose the simulation as a tool of designing the leading edge of wing with an optimum radius curvature and thickness of wing leading edge panel to absorb a specific magnitude of impact energy.