University of Twente Student Theses


Derivation of crack growth law from high cycle fatigue vibration testing in thermoplastic-based composites

Simanjuntak, Denny Hariadi (2020) Derivation of crack growth law from high cycle fatigue vibration testing in thermoplastic-based composites.

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Abstract:The global demand for carbon fibre-reinforced composite is steadily increasing over the years and will continue through the foreseeable future. The market for thermoplastics alone is increasing by almost 5%. The interests of thermoplastic composites are growing due to its ability to reshape and reuse. However, in primary aircraft structures, it raises the importance of damage tolerance of this material. The most common failure mechanism in laminated composites is delamination. The ability to predict delamination behaviour is important, not only for static load but also for establishing dynamic damage tolerance criteria. Ply-drop is one of the challenges in tapered composites due to it is the area of high-stress concentration. Hence, it is likely the point from which the crack and the delamination could initiate. The study on the influence of ply-drop under fatigue condition, especially with thermoplastic-based composites laminates, still received little attention. Moreover, there is no standard specimen for high cycle fatigue testing. ASTM D5228 and ASTM D6115 only specify specimen for a quasi-static load test. A novel ply-drop design has been proposed and tested to better understand the crack growth rate under fatigue tests. This thesis started with designing and manufacturing the fit ply-drops specimen to be studied. The discovered specimens, specimen UD and specimen 0-90 are brought to vibration fatigue test. The specimen themselves are the product of iteration from the manufacturing of composite and vibration testing. The effect of high cycle fatigue is evaluated through its stiffness degradation, involving dynamics parameters, and micrograph to understand its structural performance. The results give an understanding of structural integrity change of thermoplastic composite, which is reflected by stiffness degradation, response phase shift, and base acceleration, and resonance frequency decay caused by the crack propagation. Furthermore, the characterization of fatigue of both specimens is determined. 3D Finite Element (FE) model is carried out with the use of ANSYS-Software to simulate the real experiment to understand the involving stress, which is used to determine the strain energy release rate. Delamination growth per fatigue cycle, da/dN, which is from the experiment is related to strain energy release rate, G from the model. At last, the ultimate aim of derivation crack growth law, which is an expression that relates delamination growth per cycle with strain energy release rate, G, is determined. The exponents of the power-law relationship can be used as a crack growth prediction tool in the case of vibration fatigue loading.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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