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Title: The effect of temperature on mixed-mode fracture toughness and fatigue delamination growth of fibre-reinforced composites
Author: Charalambous, Georgia
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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A significant part of composite structures in service are made of Fibre-Reinforced Polymers (FRPs). These are exposed to complex static and time-dependent loading, often in aggressive environments. In pristine condition, FRPs can have very high fatigue strength. However FRP components that have undergone prior damage (e.g. impact) are particularly susceptible to fatigue and fail primarily under delamination. The latter is the progressive disbond of contiguous plies driven by a combination of fracture modes. As preventing delamination initiation is somewhat unavoidable, a damage tolerance (DT) design philosophy is commonly adopted for mitigating the impact of delamination on the long-term durability of composite structural elements. In the literature, characterisation data for Fatigue Delamination Growth (FDG) in composite materials is sparse, particularly for mixed-mode regimes. This is partially due to the fact that there are no standardised methodologies for performing fatigue characterisation tests on composites. Since most of primary composite structures are exposed to variable temperature conditions, it is vital that the characterisation tests are carried-out in representative environments. However, practical challenges in testing make the investigation of environmentally assisted fatigue extremely difficult. From limited references in literature, there is no general consensus whether an increasing temperature delays or accelerates FDG. This study proposes a novel methodology for mixed-mode FDG characterisation under variable temperature conditions. A new four-point bending test fixture, which can also be installed into an environmental chamber, has been designed and built. Asymmetric Cut-Ply specimens have been employed to characterise the fracture toughness and FDG in a second-generation toughened carbon/epoxy, namely IM7/8552. Tests were performed at -50°C, Room Temperature (RT), 50°C and 80°C and a constant mode-mixity of 0.43 has been applied throughout the testing campaign. In Asymmetric cut-ply coupons, the mode mixity is controlled solely by the geometry of the specimen, i.e. the ratio of number of cut plies to continuous layers in the specimens. The proposed testing protocol has been validated against other mixed-mode testing techniques, using RT data. It is demonstrated that the testing method proposed here does not require a compliance calibration and yield 'results in agreement with the widely employed mixed-mode bending coupons. Quasi-static results highlight that at 80°C the fracture toughness increases with respect to that at RT. On the contrary, an increase in temperature accelerates the FDG at relatively low severities, i.e. in the near-threshold regime. A semi-empirical equation has been introduced in order to represent the effect of temperature on the Fatigue Delamination Growth Rate (FDGR) as a function of the Strain Energy Release Rate (SERR) at the crack tip. The equation is in the Power law form and it comprises temperature-dependent pre-factor and exponent. The semi-empirical equation has been implemented in a cohesive zone formulation for finite element analysis (FEA) purposes. The results point out the importance of considering representative temperature conditions in obtaining fundamental material properties for DT design. Experimental and numerical investigations have been also performed to study the influence of temperature on delamination growth at a sub-element scale, by employing tapered specimens.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available