Effects of Low Level Temperature Cycle on the Delamination Progress in Quasi-isotropic Laminates

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Effects of temperature on the damage propagation and strength of quasi-isotropic laminates [0/45/-45/90]s were investigated. Non-linearity is found to be due to the increase of crack density both in -45 and 90 degree layrs at low temperature of -100 in centigrade and due to the delamination at high temperature of 150 in centigrade. The knee point of stress-strain relation corresponds to the coalescence of delaminations in each interface and the their propagation to the depth direction at both temperatures. Other characteristic phenomena are as follows such as (1) the direction of matrix crack propagation in -45 degree layr, (2) the multi cracks generated along the depth-direction in -45 degree layr, (3) the increase of delamination density between 45/-45 layrs, (4) the progress of the gap between side edges of each layr at high temperature and (5) fiber break cross to the edge surface in 0 degree layr at high temperature.Thermo-mechanical constants were obtained in each temperature and the numerical results based on these data explain the above (3). The followings were also found that the longitudinal Young's modulus at 150 degree in centigrade is 10% less than that of room temperature and that the shear modulus between longitudinal and normal increases at -100 degree in centigrade much more than the other moduli.At high temperature a new carbon fiber reinforced polyimide without toxicity was studied to obtain its delamination characteristics. A new toughness parameter is proposed for the crack with time-dependent behavior near the crack tip under high temperature.