Stresses around fiber ends at free and embedded ply edges

Multi-scale modeling of high-temperature polymer matrix composites for aerospace applications

“…Micromechanical damage evolution parameters used in FEA modeling for IM-7/BMI are given in Table 6.1 for pristine and isothermally aged specimens. Elastic properties of IM-7/BMI unidirectional lamina for 0.6 fiber volume fraction were taken from work published by Andrews and Garnich (2008). Significant (w40%) degradation was observed in the elastic material properties of IM-7/BMI after 1000 h of thermo-oxidative aging at 260 C in air.…”

Effects of moisture on matrix cracking in a cryo-cycled cross-ply laminate

“…As described later, sufficient accuracy in stress predictions was obtained by assuming the diffusion was the same both parallel and transverse to the fibers. The diffusivity was estimated to be 3.0x10 -7 mm 2 /s at room temperature based on data reported by Bao and Yee 15 for Cytec™ 5250-4. The graphite fibers were assumed impermeable.…”

“…In this section, finite element micromechanical models are described that predict local stresses around a fiber end assuming a uniform hexagonal packed fiber distribution. The modeling details were described previously by Andrews and Garnich 2 who were predicting the effects local to the fiber ends for a uniform temperature decrease combined with transverse tension associated with a balanced cross-ply laminate. They showed that cryogenic temperatures would create peel and shear stresses at the fiber matrix interface near the free surface at the fiber end.…”

“…This led to simplified models that isolate the effect of moisture on the microstresses found previously. 2…”

“…These flat polished surfaces create idealized geometry at the fiber ends that can be analyzed using micromechanics local to the ply free edges. 2 Fiber-matrix interface failure at the free edge creates initiation sites for through ply cracks in the presence of ply level transverse tensile stresses. Through-thickness cracks form in the presence of this damage and propagate through the matrix, parallel to the fibers as a result of cyclic thermal stress (thermal fatigue).…”