Effects of hydrostatic pressure on mechanical behavior of graphite/epoxy composites and thermal behavior of elastomers

A new temperature controlled high pressure torsion testing apparatus with internal sensors and furnace, that is capable of containing pressures up to 7 kbar and controlling temperature from {dollar}-{dollar}100{dollar}sp{lcub}rm circ{rcub}{dollar}C to 200{dollar}sp{lcub}rm circ{rcub}{dollar}C, was designed and built. The torsional shear stress-strain behavior of various unidirectional, 60% volume fraction, graphite fiber-epoxy composite laminates, viz., (0{dollar}sp{lcub}rm circ{rcub}{dollar}), (90{dollar}sp{lcub}rm circ{rcub}{dollar}), (+45{dollar}sp{lcub}rm circ{rcub}{dollar}) and ({dollar}-{dollar}45{dollar}sp{lcub}rm circ{rcub}{dollar}) composite samples, has been investigated at various hydrostatic pressures. The (0{dollar}sp{lcub}rm circ{rcub}{dollar}) samples were machined from a press-molded, 1/2 inch thick panel. For the other composite samples, hollow circular cylinders were fabricated using mandrel-wrapped technique and also machined into a particular shape of the torsion specimen. All samples were then polished and coated with rubber latex in order to prevent possible penetration of high pressure fluid.; The shear properties that were determined include the in-plane shear modulus, shear yield strength and fracture strength (maximum stress) and the strain to fracture in terms of applied pressure and fiber orientation. These results were then compared with those for unreinforced epoxy resin matrix molded in vacuum oven tested at the same pressure. In all cases, the shear properties increased with increasing pressure but in different rates depending on the fiber orientation. The increase in the strengths and the toughness was manifested in the change in the modes of fracture. Visual and SEM examination were made to study fracture surface morphology and possible mechanisms.; The pressure dependence of the glass transition temperature of two elastomers, 3,3-bis(azidomethyl)oxetane/tetrahydrofuran and Solithane 113 has been determined, employing high-pressure differential thermal analysis (HP-DTA) up to 8.5 kbar. The glasses of the elastomers were named the specific or the general glass depending on how the glasses were formed. The specific glass formed by lowering temperature under a constant pressure. The general glass consists of a series of specific glasses and the {dollar}Tsb g{dollar} for the glass is determined only at the pressures under which glass is formed. The {dollar}Tsb g{dollar} for both glasses increased with increasing pressure. However, the {dollar}Tsb g{dollar} for the specific glass appears to level off at very high pressure while the {dollar}Tsb g{dollar} of general glass increased monotonically.