Abstract
Microdamage in layers of CF Thornel® T650 8-harness satin woven composite with thermosetting polyimide NEXIMID® MHT-R resin was analysed. After cooling to room temperature multiple intra-bundle cracking due to tensile transverse thermal stresses was observed in the studied [(+45/-45)/(90/0)]2s composite. The composite was subjected to thermal cycling quantifying the increase of crack density in layers. Comparison of two ramps with the same lowest temperature shows that the highest temperature in the cycle has a significant detrimental effect. Exposure for 40 days to 288°C caused many new cracks after cooling down to room temperature. Both aged and not aged specimens were tested in uniaxial quasi-static tension. Cracking was analysed using fracture mechanics and probabilistic approaches. Cracking in off-axis layers was predicted based on Weibull analysis of the 90- layer. The thermal treatment degraded the cracking resistance of the surface layer and of the next layer.
Highlights
An increasing interest of aerospace industry in composite materials for applications in harsh environments has been driving improvement and development of new composites
According to the Classical Laminate Theory (CLT) thermal transverse stresses in a quasi-isotropic laminate is the same in all layers and the in-plane shear stress is zero
In all cases much higher rate of crack density growth was found in surface layers because of the free surface effect on initiation and on the propagation of intralaminar cracks
Summary
An increasing interest of aerospace industry in composite materials for applications in harsh environments has been driving improvement and development of new composites. Composites designated for aero-engine applications considered in the current paper are subjected to high temperatures. Long exposure to elevated temperatures can cause irreversible changes in the morphology (termed aging in this paper) reducing strength and stiffness [1]. Cyclic variations between high and low temperatures can lead to fatigue resulting in microdamage accumulation [2]. The elevated temperature level during thermal cycling with a fixed lowest temperature has an accelerating effect on the microdamage development [4] which currently does not have solid explanation. Irreversible phenomena take place at the highest temperature and microdamage evolves when the lowest temperature in the cycle is reached
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