H.T. Hahn

Process Modeling of Composite Materials: Residual Stress Development during Cure. Part I. Model Formulation

In the production of composite parts the pertinent processing parameters are time, temperature, and pressure. A judicious choice of these three parameters pro duces composites which are fully cured, compacted, and of high quality. Slight deviations from the recommended processing conditions can result in unacceptable quality. One of the most significant problems in the processing of composites is residual stresses. Processing-induced residual stresses can be high enough to cause cracking within the matrix even before mechanical loading. This microcracking of the matrix can expose the fibers to degradation by chemical attack. Strength is adversely affected by residual stresses since a pre-loading has been introduced. The topics considered and discussion presented in this paper have been chosen to ad dress the issue of understanding how residual stresses develop during processing and how they can be predicted. A process model has been developed which can be used to predict the residual stress history during the curing of composite laminates. This model includes the effects of chemical and thermal strains and assumes the material to exhibit linear, vis coelastic behavior. A phenomenological model is used to predict the degree of cure history during the cure cycle. Mechanical properties are allowed to develop based on a functional dependence on the cure state (degree of cure) and the transverse compliance is taken as the only time-dependent compliance. Simultaneous application of the cure kinetics and a vis coelastic stress analysis yields the residual moments and curvatures for unsymmetric cross-ply laminates. An experimental correlation is provided in an accompanying paper.

Fatigue Behavior of Composite Laminate

The paper discusses a characterization of fatigue behavior of [0/±45/90] s glass/epoxy laminate in terms of the following parameters: static properties; S-N relationship; reliability; effect of thickness variation; damage initiation and growth; temperature increase; secant modulus change; effect of preloading on residual modulus and strength; and effect of ply failure on compression buckling strength. Some of the findings are the following. The primary failure process responsible for up to about 106 cycles of fatigue life is the wear-out followed by the chance failure. Change of secant modulus can be used as a measure of damage extent. Preloading to a high level has negligible effect on the residual tensile strength when the fatigue stress is low. However, ply failure and partial delamination result in a moderate loss of compression buckling strength.

Process Modeling of Composite Materials: Residual Stress Development during Cure. Part II. Experimental Validation

In a companion paper [1] a process model was developed for investigation of residual stress development during autoclave or hot press processing of thermosetting polymer matrix composites. Several material property characterization studies are re quired as input to this model. The present paper summarizes the results of the character ization studies required for input to the model and validation of the model is accomplished by the intermittent cure of unsymmetric cross-ply laminates in which the processing- induced residual curvatures are measured. An IM6/3100 graphite/bismaleimide composite system was chosen for the study. Longi tudinal and transverse mechanical properties were shown to increase during the cure cycle due to increase in matrix strength and stiffness and development of the fiber/matrix bond. Thermal strains were shown to remain relatively constant during cure. Chemical strains occur early in the cure cycle and are completed before cure is fully developed. The vis coelastic mechanical response of BMI is strongly dependent on the cure state. At low cure states the creep response is quite significant. As full cure is approached, the material becomes predominantly elastic. Based on the measured input data, the viscoelastic analy sis showed that the contribution of chemical strains to residual stress was less than 4% for a typical cure cycle. A good correlation was obtained between model predictions and ex perimental warpage data for the cure cycle investigated.

Proof Testing of Composite Materials

The paper presents a concept of proof testing for composite materials. A unique relationship between the static strength and time to rupture is demonstrated for a unidirectional glass/epoxy composite subjected to static fatigue. The maximum significance level at which a Weibull distribution is applicable to represent scatter is 35% for the static strength and 61% for the fatigue life. Limitations and variations of the strength degradation model for the life prediction are discussed.

Cure Cycle Optimization for the Reduction of Processing-Induced Residual Stresses in Composite Materials

The control and reduction of processing-induced residual stresses has been investigated by modifying processing conditions for a graphite/BMI composite material. The effects of dwell temperature, dwell time, cool-down rate, cool-down pres sure, and postcure on residual stresses were investigated using unsymmetric cross-ply laminates. The effects on transverse mechanical properties were also measured. Experi mental results have shown that residual stresses can be reduced by as much as 25-30% while retaining or enhancing transverse mechanical properties by curing at lower tempera tures for longer times or utilizing an intermediate low-temperature dwell in three-step cure cycles. Overall process cycle times are not lengthened for three-step curing.

A simplified analysis of transverse ply cracking in cross-ply laminates

Abstract This paper presents a method of analyzing transverse crack initiation and multiplication in symmetric cross-ply laminates. The method is based on the concept of a through-the-thickness inherent flaw and the energy balance principle. With a second-order polynomial assumed for the crack opening displacement, the perturbed stress field due to the presence of ply cracks is determined from the equilibrium conditions. The energy released as a result of ply cracking is then calculated and used to predict the increase in crack density. Based on an experimental correlation of the analytical result, a resistance curve is proposed to be used as a measure of the resistance to crack multiplication. The resistance to crack multiplication is shown to increase with the increasing crack density.

Residual stress development during processing of graphite/epoxy composites

Abstract Residual stresses induced during processing can have deleterious effects on the structural integrity and dimensional stability of composite structures. While the residual stresses in fully cured laminate have been well characterized, the manner in which these stresses develop during processing is still not fully understood. In the present work, intermittent curing of an unsymmetric laminate was carried out to monitor the residual stress development. The resulting warpage was measured in order to assess the extent of residual stresses. The warpage and transverse modulus changed during cure in the same way as did the degree of cure, and they increased rapidly after the gel point. It is shown that a simple elastic analysis predicts reasonably well the change of warpage as a function of cure time when the change of elastic moduli during cure was accounted for. Postcure, especially of undercured laminates, was found to increase the warpage significantly while resulting in a substantial weight loss.

Nonlinear Behavior of Laminated Composites

This paper extends the nonlinear behavior of unidirectional laminae of Reference [1] to that of laminated composites. A comparison between the theory and experimental results is shown through the analysis of uniaxial tension behavior of various laminates. The effect on buckling stress owing to nonlinearity is illustrated for a ±45° angle-ply laminate with simply- supported edges.

Ply cracking and property degradations of symmetric balanced laminates under general in-plane loading

Abstract The authors have previously proposed a method of using a resistance curve to characterize transverse crack multiplication in balanced symmetric laminates. The method was based on the concept of a through-the-thickness inherent flaw and energy balance principle. In the present paper, this model is further extended to shear and general in-plane loading conditions. The corresponding mechanical property degradations due to transverse ply cracking are also investigated. The effective longitudinal modulus and Poissons ratio depend not only on the crack density but also on the thermal residual and applied stresses. The reason is that the cracks do not close tightly even upon unloading due to the residual stress. Experimental results for the mechanical property degradations correlate well with the analysis except for the shear modulus. The nonlinear behavior of the shear modulus should be considered for better correlation.

Compression failure mechanisms in unidirectional composites

The present paper examines compression failure mechanisms in unidirectional composites. Possible failure modes of constituent materials are summarized and analytical models for fiber microbuckling are reviewed from a unified viewpoint. Due to deficiencies in available models, a failure model based on nonlinear material properties and initial fiber curvature is proposed. The effect of constituent properties on composite compression behavior was experimentally investigated using two different graphite fibers and four different epoxy resins. The predominant macroscopic-scale failure mode was found to be shear crippling. In a soft resin, shear crippling was in the form of buckling of fibers on a microscopic scale. However, for stiff resins, failure was characterized by the formation of a kink band. For unidirectional laminates, compressive strength, and compressive modulus to a lesser extent, were found to increase with increasing magnitude of resin modulus. The change in compressive strength with resin modulus was predicted using the proposed nonlinear model.