Combinations of failure mechanisms are frequently encountered in the life prediction of composite materials. A life prediction methodology is developed and applied to one such failure mechanism combination. This method uses experimental data and analytical tools to predict the long-term behavior of a composite under service conditions. The prediction scheme is based on the assumption that damage accumulation progressively reduces the remaining strength of a composite. An overview of the fundamental concepts of the life prediction method is presented. The method is used to model the elevated temperature fatigue behavior of a unidirectional AS-4 carbon fiber/PolyPhenylene Sulfide (PPS) matrix composite material. The nonlinear combined effects of time at elevated temperature and fatigue are taken into account by considering elevated temperature tensile rupture and room temperature fatigue behavior. The life prediction for the combined loading is compared to 90°C tensile-tensile fatigue data. This comparison shows good correlation between the prediction and data and demonstrates the method's effectiveness in life prediction modeling.

The shaft-loaded blister test (SLBT) was used to investigate the adhesion between a model epoxy coating and a silicon oxide surface as a function of relative humidity. Critical and subcritical strain energy release rates were measured using specimens that incorporate reinforcing layers of Kapton 1 film. A simplified procedure that eliminates the need for video imaging to measure the blister radius and fracture energy was developed. A critical relative humidity level for adhesion loss was observed, in agreement with measurements that have been made previously in a number of polymeric systems. The SLBT was confirmed to be particularly attractive for fracture energy measurements on thin, strongly adhered coatings and films which otherwise tend to be problematic.

A key need for the commercial acceptance and viability of composite structural material systems is a methodology for reliability and lifetime predictions. In this paper, we present a philosophy in which the residual strength of the composite material is calculated as a function of applied loading conditions, environment, and resulting failure mode. Two specific example applications are considered: the assessed and verified performance of fiber wound composite tubes and pultruded shapes. We attempt to demonstrate the understanding of the controlling damage mechanisms and the degradation processes that contribute to the loss in stiffness and strength under simulated service conditions.

The investigation presented examines the compressive strengths, Xc, of thermoplastic-toughened epoxy composites with fiber surface chemistry altered by various levels of surface treatments and two distinctly different sizings. The thermoplastic, polyvinylpyrrolidone (PVP) size improved the IITRI Xc by 51% over an unreacted bisphenol-A epoxy size for a 100% surface-treated fiber. It is important to point out that effects were produced with the sizings making up less than 1% (by weight) of the composite. Thus, appropriate selection of sizings may present an effective and economical means for tailoring composite material performance. It is suggested that properties other than interfacial shear strength may be important in the description of composite compression strength, e.g., interphase modulus and fracture resistance.

In this paper, we utilize a concentric cylinder model to evaluate the stress state in a continuous fiber reinforced composite subjected to transverse loading. We present a method by which this strain to failure may be maximized. In addition, we present experimental results for two different composite systems which are in good agreement with the theory.