Advanced Materials Research Vols. 118-120

Abstract: One of the problems preventing the industrial application of composites is the lack of an efficient method to detect and discriminate among types of damage occurring during service. To solve this problem, low velocity impact experiments are carried out on T300/QY8911 composite laminates. And synchronously, the acoustic emission (AE) technique and impact monitoring systems were used to record the AE signals and the impact force. The damage evolution, damage modes and acoustic emission (AE) activity were easily detected and evaluated by the analysis of both AE waveform and impact load. In this way, the damage development process containing matrix cracking, delamination and fibers breakage is investigated. The energy release of damage are theoretically approximated and correlated with the AE energy. By the theory, the “high energy damage zone” is defined in the scatter diagrams of amplitude-frequency. It is easily to prove that the primary damage mode of “high energy damage zone” is delamination.

Abstract: The purpose of this work was to predict the fatigue life of pre-corroded LC4 aluminum alloy by applying artificial neural network (ANN). Specimens were exposed to the same corrosive environment for 24h, 48h, and 72h. Fatigue tests were conducted under different stress levels. The existing experimental data sets were used for training and testing the construction of proposed network. A suitable network architecture (2-15-1) was proposed with good performance in this study. For evaluating the method efficiency, the experimental results have been compared to values predicted by ANN. The maximum absolute relative error for predicted values does not exceed 5%. Therefore it can be concluded that using neural networks to predict the fatigue life of LC4 is feasible and reliable.

Abstract: An experimental study of low-velocity impact characteristics and strength after impact was carried out on both woven fiber-reinforced resin matrix composites and woven fiber-reinforced ceramic matrix composites. The test specimens were impacted using a dropped-weight impact test apparatus with an instrumented spherical tip. Ultrasonic C-scan was used in nondestructive testing to characterize and quantify the impact damage. Much more damage of ceramic matrix composites than that of resin matrix composites occur and process in loading stage. The peak load of resin matrix composites is higher than that of ceramic matrix composites. According to the results of observing optical photographs and C-scan images, the damage area of ceramic matrix composites is greater than that of resin matrix composites and the difference increases as the energy increases. Damage resistance of ceramic matrix composites is lower than that of resin matrix composites, but damage tolerance of ceramic matrix composites is higher than that of resin matrix composites.

Abstract: The electrical potential technique was applied to monitor the crack initiation and propagation in LY12-CZ plates. The feasibility of this method was validated by a simulation experiment firstly. Then fatigue crack monitoring experiment based on LY12-CZ aluminum alloy center-hole plate specimen was carried out. After a special coating sensor was prepared on the monitoring area of the specimen by surface technology, a set of fatigue crack monitoring experiments were carried out. The changes in the potential values of the coating sensor resulting from crack damage were documented. Since the property of the coating sensor and the introduced current were constant, the measured voltage values would only relate to the crack’s length and position. Based on the experimental results, the relation curves between voltage values and crack lengths were obtained, and a set of empirical formulae were deduced.

Abstract: It is very complex problem that the reliability of aircraft wing box structural system with many random variables (such as area, thickness, material modulus, load etc.) is analyzed. In the paper, the reliability analysis method of aircraft wing box structural system is proposed based on the theory of structural reliability and stochastic finite element. The explicit expression of safe margin and the sensitivity of safe margin to each random variable are given, which improves the accuracy and efficiency of reliability calculation. The relationship of the level of failure path with structural system failure probability is discussion. The failure probability of element is calculated by using first order second moment method, and the main failure path of structural system is identified by using the advanced branch and bound method, and the failure probability of structural system is evaluated by using probabilistic network evaluation technique method. Numerical examples show that the method is of efficient and accurate, and the 3 or 4 level of failure path is acceptable for wing box structural system considering efficiency and accuracy.

Abstract: Based on progressive damage theory, a 3D laminated model with an orthotropic property in plane was established to simulate the response of plain weave carbon fiber reinforced silicon carbide(C/SiC) ceramic matrix composites(CMC) under low velocity impact(LVI). Intra-layer damage and inter-layer damage were taken into account, respectively. Three scalar damage variables, associated with the degradation of warp modulus, weft modulus and shear modulus, respectively, were proposed to characterize intra-layer damage evolutions. The intra-layer constitutive model was implemented into MSC.Dytran, via its user subroutine EXFAIL1. The potential delamination region was considered as a discrete cohesive zone. Three vector spring elements were placed into every two adjacent nodes to simulate the inter-layer joints. A scalar damage variables, associated with the degradation of the three vector spring elements, were brought forward to characterize the inter-layer damage evolutions. The inter-layer constitutive model was implemented into MSC.Dytran, via its user subroutine EXELAS. Damage area, indentation depth of C/SiC composite plates and time history of impact force were obtained to compare with experimental results. The numerical results show overall good agreement with experimental results.

Abstract: Retractable roof becomes popular in recent years because it can make the roof close, open or partly open to meet different purpose. The roof status can be changed according to climate condition outside. It can make people inside feel more comfortable and save energy sometimes. Mechanical system of retractable roof plays a very important role in system safety and reliability. With the experience of retractable roof of Kunshan indoor stadium, design of mechanical driving and supporting system is introduced. Calculation method for key parts and limitation for architecture structure are provided. This paper provides helpful reference for design of mechanical system on retractable roof.

Abstract: Plain plate specimens of 2D plain woven C/SiC composites were performed on Instron8801. Infrared (IR) thermography was recorded using an infrared camera. Acoustic emission (AE) signal was detected by two AE wide band sensors attached on specimen. They were measured synchronously and real-timely. Thermal dissipation Q was deduced based on the first law of thermodynamics. When the applied stress was lower than fatigue endurance limit, Q rose in the early cyclic loading stage and then the rate of Q accumulation gradually approached a steady value as the proceeding cycles, conversely, Q rose quickly until led to failure of the composites. AE accumulated energy was discussed based on the AE data. Higher applied stress would cause more damage within the composites, and more AE signals were detected. Compared with damage calculated from modulus, Q and AE accumulated energy had fairly well agreement with the damage. It can be concluded that it is possible to employ these non-destructive evaluation methods as in-situ damage evolution indicators for 2D C/SiC composites.

Abstract: Acoustic emission (AE) technique was utilized for real-time monitoring the damage evolutions of 2-D and 3-D C/SiC ceramic matrix composites (CMC) under mechanical loading. AE signals for damage initiation and propagation were captured by AE equipment during the entire loading process. Different damage mechanisms of the two kinds of C/SiC composites were revealed on the basis of multi-parameter analysis. The experimental results validate the availability of AE technique on damage monitoring of C/SiC composites. And the AE technique can be used to distinguish the slight differences in damage mechanisms during damage evolution owing to different woven structures.

Abstract: Tower vacuum dryer room is the key device for continuous vacuum dryer. In order to lower the cost of manufacture, the wall of dryer should be thinner. But as the pressure container, it must have the well intensity while operating. Because its complex structure, the simple calculation is not enough, the finite element analysis in intensity control was carried out. The model of vacuum dryer was built, the intensity of general primary membrane stress and the intensity of primary bending stress was picked-up on the max stress intensity point through the path of wall thickness. Accord with JB4732-1995, the structural strength was fulfilled the requirement.