| Popis: |
The effect of temperature on the material elastic properties, acoustically-induced strains, damage initiation, damage propagation and residual thermal strains of composite materials has been investigated. An experimental rig, using the free-free beam technique, was built to attain accurate measurements of Young's modulus and loss factor of CFRP beams in the temperature range -40oC to 150oC. These results were then compared with measurements taken from a commercially available Dynamic Mechanical Thermal Analyser. Using the finite element method a study was undertaken to determine the effect of temperature on the free vibration of clamped (but no in-plane constraints) CFRP plates of various layups. Predictions of natural frequencies of two CFRP plates were then compared with experimentally determined values. CFRP plates subjected to broadband acoustic excitation (20-600 Hz) of OSPL up to 160 dB showed no significant changes in the strain response with increasing temperature. Also predictions of RMS strains using the simple single mode formulae agreed reasonably well with measured values for most OSPL and temperatures studied. A flexural fatigue apparatus, using a half-sine-clamped cantilevered arrangement, was modified to allow flexural cyclic loading, when placed in an environmental chamber or oven, of CFRP coupons at various temperatures (-40oC to 120oC). Wet and dry XAS/914C coupons of layup [0,45,0]_s were subjected to cyclic surface strain reversals at temperatures -40^oC, 20^oC and 120^oC. Flexural fatigue results showed a considerable decrease in flexural fatigue reistance as temperatures were increased to 120^oC. An optical microscopic analysis showed damage in CFRP appears to be in the form of translaminar cracking and delamination. Also an SEM analysis showed an increased propensity of fibre/matrix debonding under adverse conditions. A finite element analysis of the cantilevered coupon, under flexural and thermal loads, showed that large laminate residual thermal strains, as well as flexural strains, were present. In fact surface strains can be altered considerably when residual thermal strains are considered. Summarizing, it is expected that CFRP materials will suffer reduced acoustic fatigue reistance when exposed to elevated temperature conditions. The situation will worsen for hot/wet conditions. |
