Structural integrity of reinforced polymer matrix composites subjected to hygrothermal conditioning
Date of Issue2015
School of Mechanical and Aerospace Engineering
The mechanical properties of polymer matrix composites (PMCs) are prone to the attack of hygrothermal environment. Much effort has been devoted to the investigation of the behavior of laminated composites under combined hygrothermal environments. In most of the published research work on the durability of PMCs under hygrothermal environments, the various properties of composite structures were reported to degrade after hygorthermal conditioning. However, positive effects of hygrothermal ageing were also reported. Therefore, more and rigorous investigation is crucial to solve the contradictions existing in the literature. Hygrothermal environments attack composite materials through two principal mechanisms, microstructure degradation and moisture uptake. Structural defects including matrix cracking, delamination, weakening of the fiber/matrix interface may be triggered by hygrothermal conditioning. The absorbed moisture in PMCs is reported to act as plasticizers and reduce the glass transition temperature of the epoxy resin, which consequently affects the mechanical properties of the material at elevated temperatures. However, in many cases hygrothermal ageing induced structural defects and the effects of moisture may coexist and have different influences on specific properties of composite materials. This fact was frequently ignored in the literature. In recognition of this phenomenon, this Ph.D. thesis is devoted to the durability of materials made from PMCs under hygrothermal environments with full consideration of both hygrothermal ageing induced damages and the effects brought about by the moisture absorbed during hygrothermal ageing. Experimental work began with the characterization of the response of hygrothermally aged CFRP (carbon fiber reinforced polymer) unidirectional (UD)-prepreg and woven-prepreg laminates to low-velocity impact. The hygrothermal conditionings designed included immersion in water at constant temperatures, including 60°C and 80°, and exposure to hygrothermal cycles. Each hygrothermal cycle consisted of 12 hours in water at 60°C or 80°C and 12h in a freezer at -30°C. It was found that both CFRP UD-prepreg and woven-prepreg laminates did not have any visible damages formed during conditoning. Instead, their impact resistance was improved by the absorbed moisture. The moisture residing in the CFRP laminates delayed the impact induced damages and preserved their strength up to larger projectile deflections. Consequently, CFRP laminates with higher moisture contents exhibited higher impact resistance and experienced lighter impact damage. Hygrothermal conditioning of this category was defined as short-term hygrothermal conditioning, which did not cause damages but mainly incurred moisture uptake in composite materials. Long-term conditioning on CFRP UD-prepreg laminates was therefore conducted to identify the durability of the CFRP laminates under prolonged hygrothermal conditioning. It was found that as the sample was continuously immersed in water, individual plies of a UD-prepreg laminate were squeezed out under the combined action of ply swelling and interlaminar shear stress, causing the formation of delamination between plies of different fiber orientations. Delamination or wrinkles began to take place at the end of Stage II of the moisture absorption process. These damages in turn provided extra free spaces for moisture absorption. Consequently, after the saturation level was reached the laminates continued to absorb moisture, resulting in the Stage III moisture uptake state. A three-stage moisture absorption behavior of CFRP UD-prepreg laminates exposed to long-term conditioning was therefore identified. Hygrothermal conditioning induced damages caused the reduction in the strength and modulus of the laminates. Consequently, the impact resistance of CFRP UD-prepreg laminates also declined. The mechanical properties of 7781 E-glass fabric reinforced epoxy composite laminates after hygrothermal ageing were also evaluated. It was found that the tensile elongation and strength of the GFRP (glass fiber reinforced polymer) laminates decreased remarkably after hygrothermal drying. Upon the removal of the absorbed moisture, only a small percentage (less than 5%) of recovery in the tensile strength was obtained. It was proposed that the strength of the E-glass fiber was negatively affected by hygrothermal ageing, leading to the degradation in the tensile properties of the GFRP composite materials. Due to the reduction in the tensile strength, the impact resistance of the GFRP laminates decreased after hygrothermal ageing. The absorbed moisture did not alleviate the impact damage in GFRP laminates but rather reduced their contact stiffness under low-velocity impact. Core-shell polymer (CSP) particles have been proved to be effective impact resistance modifier. However, it was found that the advantage brought about by the CSP particles diminished after hygrothermal ageing since these particles absorbed moisture and experienced evident property degradation under hygrothermal environments. Research thrust was then focused on the durability of fiber metal laminates. Water immersion conditioning was conducted on GLARE (glass laminate aluminum reinforced epoxy) 4A laminates, which had S2 glass fiber as the reinforcement in the composite layer. The decrease in the tensile strain and strength of GLARE 4A laminates after hygrothermal conditioning was remarkable. Upon removal of the absorbed moisture, only minor recovery in the tensile strain and strength was observed. It was proposed that the ultimate tensile strength of the S2 glass fiber in the GLARE 4A laminates might decrease upon exposure to hygrothermal ageing, which consequently led to the decrease in the tensile strength of the composite layer in the GLARE 4A laminates. Due to the degradation in the tensile strength, a pronounced decrease in the fatigue life of GLARE 4A laminates happened.