A one-dimensional finite difference analysis describing the transient ther mal response of fiber-reinforced organic matrix composte plates subjected to intense surface heating is presented. The effects of fiber ablation, matrix decomposition, and radiation and convective heat losses are included in the formulation. Numerical results are in good agreement with mass loss and thermocouple mesurements obtained from laser irradiation tests on AS/3501-6 graphite epoxy coupons. A steady state analytic solution is also given which provides a reasonable estimate of the surface recession rate over much of the surface irradiation period.
An analytical procedure is presented for predicting the loss in integrity of composite structures subjected to simultaneous intense heating and applied mechanical loads. An in tegral part of the method is a nonlinear, two-dimensional, finite difference thermal analysis which considers the effects of surface ablation, re-irradiation losses, and temperature-dependent thermophysical properties. Another important feature of the structural survivability model is a flat-plate finite element code, based on the Mindlin theory, which is coupled to a maximum stress failure criterion. Predictions from the analysis methodology are compared with experimental results obtained on 24, 48, and 96 ply graphite epoxy tension specimens which were spot-irradiated at various intensity levels.
Three-point bend tests were conducted to examine the geometry dependence of JIc in two intermediate-strength aluminum alloys, 2024-T351 and 7005-T6351. The amounts of crack growth at selected points on the load-versus-deflection curves were delineated using a heat tinting technique. Resistance curves of J-versus-crack extension were constructed to facilitate determination of JIc at incipient crack movement. The through-thickness variation in fracture toughness of a 7.6-cm-thick plate of 7005 aluminum was determined and correlated with fractographic and metallographic observations. The current JIc measurements from subsized specimens of 7005 aluminum are compared with previous KIc characterizations using linear-elastic, full-thickness specimens.
This study reveals that degradation in load-carrying capacity owing to sustained-load cracking (SLC) can occur in alloys of the Ti-6Al-4V family. Each of eight alloys tested with fatigue-precracked specimens in ambient room air exhibited the phenomenon, with degradations ranging from 11 to 35 percent. The degree of susceptibility to SLC was seen to be dependent upon crack orientation in rolled plate materials. The threshold stress-intensity factor below which SLC failures will not occur in these alloys, KIt, appears to represent a more conservative fracture-safe design parameter than the plane-strain fracture toughness KIc, which cannot account for time-delayed failures owing to SLC.
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