09:00   Session 10: Structural integrity of composite laminates Chair: Xiasheng SUN 09:00 20 mins A numerical scheme for fatigue simulation of laminated composites using CZM-XFEM Rong-Can Hong, Ryo Higuchi, Tomohiro Yokozeki, Takahira Aoki Abstract: A numerical method for fatigue damage accumulation in laminated composites is developed in this paper. Extended finite element method (XFEM) and cohesive element are integrated into a numerical program for fatigue fracture. In this work, XFEM and cohesive element are applied for modelling matrix cracking and delamination in CFRP laminates respectively. A fatigue model based on cohesive zone model (CZM) is also introduced into the numerical scheme. Standard DCB and ENF fatigue tests are used as the identification of parameters of the fatigue model, and simulation models of the standard tests are established for calibrating the fatigue model of pure mode. With the parameters calibrated by the benchmarks of pure mode tests, the numerical analysis of open hole tensile (OHT) test is conducted to investigate the fracture behaviors of composite laminates under cyclic loading. The delamination, splitting in the 0° plies and transverse cracking in the 90° plies are all predicted in the numerical results. The progression of fatigue damage in the simulation model is consistent with previous experimental work. This study demonstrates that the proposed numerical method can correctly predict the initiation and evolution of fatigue damage under mixed mode loading. By XFEM, in-ply matrix cracking can be modelled as multiple mesh-independent path in FE model. This paper provides a convenient approach utilizing the features of XFEM for simulating fatigue cracking in composite laminates. 09:20 20 mins Experimental investigation of planar delamination behavior of composite laminates under out-of –plane loading Wenjie Tu, John-Alan Pascoe, René Alderliesten Abstract: Delamination growth is a key damage mode threatening the structural integrity of fibre reinforced polymer composite structures. To guide design and damage management of composite structures, research efforts have been made to understand delamination behaviours and establish standardized evaluation methods based on mainly one-dimensional delamination tests. However, as most delamination growth in real structures will be planar, the question arises whether these approaches are adequate enough to evaluate planar delamination behaviour. Thus, the objective of this study was to better comprehend the physical mechanisms underlying the planar delamination behaviour and to inspire the development of a more reliable damage evaluation method for composite structures. A novel experimental method was developed to investigate the planar delamination behaviour under quasi-static out-of-plane loading. The planar central loaded split (PCLS) specimen was designed to investigate the planar delamination behaviour under mode II loading condition. By analysing digital image correlation (DIC) and C-scan data, the delamination progress was monitored. Acoustic emission (AE) system was used to capture the initiation of damage and to identify different damage types. In the oral presentation, planar delamination patterns of carbon fibre reinforced polymer (CFRP) composite panels and the underlying mechanisms will be illustrated. The planar delamination growth was found to be dependent on the stacking sequence and interface properties. Additionally, it was found that positioning a rubber mat between the indenter and the specimen prevented the occurrence of delaminations at undesired interfaces. The artificially embedded delamination propagated in the direction to which the fibre orientation of the layer above the crack interface was parallel, but migrated initially to an upper interface at the place where the fibre was perpendicular. Although the delamination propagated as the loading increased, a constant stiffening process due to in-plane stretching was discovered. Only when there were substantial matrix and surface cracks did significant stiffness degradation occur. The research results provide a clearer understanding of the mechanisms of planar delamination under out-of-plane loading. Now that a reliable test method has been established, qualitative and quantitative analysis based on fracture mechanics will be employed to establish a prediction model for planar delamination growth. 09:40 20 mins Giga-cycle fatigue properties of transverse crack initiation in cross PLY CFRP laminates using ultrasonic fatigue testing Atsushi Hosoi, Shinji Ito, Tsuyoshi Miyakoshi, Momoka Shima, Yuki Nishi, Hikaru Saito, Hiroyuki Kawada Abstract: An application of carbon fiber reinforced plastic (CFRP) laminates is being expanded to rotating components such as jet engine fan blades. Since the blade members are subjected to cyclic loading exceeding 10^9 cycles during their design life, it is important to clarify the gigacycle fatigue properties of CFRP laminates and their fracture mechanisms. Thus, the objective of this study was to evaluate the gigacycle fatigue properties of CFRP laminates in transverse crack initiation. A conventional hydraulic fatigue test is too time-consuming, and accelerated tests must be conducted to evaluate gigacycle fatigue properties. In this study, accelerated fatigue tests were conducted at a test frequency of 20 kHz by ultrasonic fatigue testing. The temperature rise of the specimens due to self-heating was prevented by air cooling with dry air and intermittent operation. Therefore, the apparent test frequency was approximately 1.8 kHz. So far, no definite fatigue limit of CFRP laminates has been confirmed. It is known that the initial damage in fatigue of CFRP laminates is transverse cracks, and that its growth and increase can lead to major damage such as delamination and fiber breakage. It is possible to obtain the initial fatigue properties before macroscopic damage occurs by evaluating the initiation life of transverse cracks. In addition, a mechanical model based on a variational approach was proposed for a cross-ply laminates with transverse cracks resonated in the first-order mode by ultrasonic vibration. In this study, [0/906]s cross-ply CFRP laminates were used, and the fatigue test was conducted at a stress ratio of R = -1 by the hydraulic fatigue and the ultrasonic fatigue testing. The experimental results showed that no transverse crack initiation occurred in the giga-cycle region, suggesting the existence of the fatigue limit. The stress analysis results of the proposed model showed good agreement with the results of the finite element analysis.

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