Hisao Fukunaga

Probabilistic Failure Strength Analyses of Graphite/Epoxy Cross-Ply Laminates

This paper treats the failure characteristics of [0/90/0] and [90/0/90] cross-ply laminates based upon the statistical strength analysis. The stress redistributions at the failure of the 90 ° ply are analyzed using a shear-lag model, taking the thermal residual stresses and Poisson effect into consideration. The formulae for determining first cracking, subsequent multiple cracking and ultimate fracture are derived. The present analysis is compared with the existing experimental results for graphite/epoxy cross-ply laminates, and reasonable agreements have been obtained.

Simplified Design Techniques for Laminated Cylindrical Pressure Vessels under Stiffness and Strength Constraints

This paper treats the optimum design of graphite/epoxy laminated composite cylindrical pressure vessels under stiffness and strength constraints based upon the membrane theory. Stiffness constraints are specified as strain conditions both in the axial and circumferential directions. Tsai-Wu failure criterion is used for determining a first-ply-failure strength. The stiffness and strength characteristics of the laminate are discussed based upon the concept of lamination parameters. For small values of allowable axial and circumferential strains, the strain (or stiffness) condition is critical, while for large values of allowable strains the strength condition is critical. The optimal laminate configurations are determined for the various kinds of strain and strength constraints.

Strength of Intermingled Hybrid Composites

This paper first treats the stress concentrations in an intermingled hybrid composite. An eigenvector expansion method is used to solve the governing equations of shear-lag analyses. Two effects on the stress concentrations are examined, namely, the effect of the low elongation fiber volume fraction and the effect of the bundle sizes of the low elongation and high elongation fibers. Next, the strength characteristics of hybrid com posites with and without the scattering in the strength of the individual fibers are discussed on the basis of the stress concentrations. Finally, the occurrence of hybrid effects is examined.

On laminate configurations for simultaneous failure

A particular class of laminated composites is presented where all layers fail simul taneously. Lamiante configurations and loading conditions for simultaneous failure are ob tained for laminated composites under in-plane loads. The present paper also shows the optimal laminate configurations to maximize the in-plane strength for graphite/epoxy composites. Tsai-Wu criterion is used as a first ply failure strength criterion. The particular failure strength characteristics are shown for graphite/epoxy composites. The strength characteristics of the laminate configurations for simultaneous failure are compared with those of the optimal ones. It is shown that the laminate configurations for simultaneous failure are optimal only for a special loading condition.

On Isotropic Laminate Configurations

A particular class of laminated composites is presented where the stiffness characteristics are isotropic with respect to both in-plane and out-of-plane stiffnesses. The paper presents a determining method for isotropic and orthotropic laminate configurations and shows examples of isotropic laminates consisting of 40 plies. Experimental verifica tion by a tension test and a four-point flexure test has been made on a 40-ply isotropic lam inate. Experimental results of elastic properties show almost isotropic properties.

Probabilistic strength analyses of interlaminated hybrid composites

Abstract This paper deals with the ultimate failure strength of four-layered hybrid laminated composites. A shear-lag model is first applied to obtain the stress redistributions after the breakage of layers. On the basis of the knowledge of these stress redistributions, the probabilistic ultimate failure strength is evaluated by applying the approach of Harlow and Phoenix. The effects on the ultimate strength of hybrid laminates, due to the scatter of lamina strengths, relative fiber volume fractions, composite size and laminate stacking sequence, are identified.

Probabilistic initial failure strength of hybrid and non-hybrid laminates

This paper deals with the initial failure of unidirectional hybrid laminates and [±θ/90]s non-hybrid laminates. The initial failure strength or strain at the failure of low elongation layers is analysed by the statistical approach based upon the weakest link model. First ply failure is adopted as a criterion for the initial failure of the laminate. An expression for determining the first ply failure strength has been derived and this expression can be reduced to a volumetric relation as a special case. It is also shown that the initial failure strength or strain is greater in composites composed of low elongation and high elongation materials than in the pure low elongation composite. This is the result of the “size effect”, that is the failure probability is lower in the composite with the smaller size of low elongation material. A good agreement between the theoretical and experimental results is achieved.

Graphite-epoxy laminates with almost null coefficient of thermal expansion under a wide range of temperature

A lamination tailoring technique is proposed in order to control a coefficient of thermal expansion of graphite-epoxy composites in a principal direction. This technique consists of two concepts of the thermoelastic invariants and the lamination parameters. The expansion free condition yields to a parabola in the feasible region of the lamination parameters. The calculated curves for a wide range of temperatures intersect almost at a point. A laminate with the lay-up construction corresponding to this point will exhibit an approximately null coefficient of thermal expansion in one direction in that temperature range. Some preliminary experimental results indicate that the present procedure is possible and promising. The tailored material will be appropriate for the space station structure.

A solution procedure for determining laminate configurations from lamination parameters

The present paper shows a solution procedure for determining laminate configurations corresponding to in-plane and/or out-of-plane lamination parameters for symmetric and balanced laminates. The solution procedure is shown for the laminate with an infinite number of plies as well as that with a finite number of plies. As a numerical example of the present method, the laminate configurations with isotropic stiffness characteristics are obtained. A particular class of laminate configurations is also presented where the stiffness characteristics are isotropic with respect to both in-plane and out-of-plane stiffnesses.

CFRP-laminates with a controlled coefficient of thermal expansion nearly to zero in one principal direction under a wide range of temperature.

A lamination tailoring technique is proposed in order to control a coefficient of thermal expansion of graphite/epoxy composites in a principal direction. This technique consists of two concepts of the thermoelastic invariants and the lamination parameters. The expansion free condition yields to a parabola in the feasible region of the lamination parameters. The calculated curves for a wide range of temperature intersect almost at a point. A laminate with the lay-up construction corresponding to this point exhibits an approximately null coefficient of thermal expansion in one direction in that temperature range. Some experimental results indicate that the present procedure is possible and promising.