Stephen R. Swanson

Failure of Carbon/Epoxy Lamina Under Combined Stress:

It is well known that matrix failure in carbon/epoxy composites is influenced by multiaxial states of stress. However most of the experimental work to measure this inter action has not focused exclusively on matrix failure, and a general multiaxial stress criterion for matrix failure has not been established. To examine this question experimen tally we have carried out a series of tests involving torsional shear combined with axial tension or compression of unidirectional hoop wound cylinders, using AS4/55A carbon/ epoxy lamina. The matrix failure stresses are seen to be well correlated with the Tsai-Wu quadratic polynomial. There is also a strong interaction between the strains at failure, with small amounts of transverse tension giving a significant reduction in the shear strain at fail ure. The effect of σ2 on the nonlinear shear stress-strain curves is also presented.

Analysis of impact response in composite plates

Abstract The problem of impact is of considerable interest in laminated composite materials. Although important contributions have been made in understanding the impact problem through numerical solutions, an analytical solution has not been available for the problem through numerical solutions, an analytical solution has not been available for the problem of impact of laminated plates. The present work gives an analytical solution to this problem, based on the usual Fourier series expansion for simply-supported plates, combined with Laplace transform techiques for the impact problem solution.

An Experimental Study of Scaling Rules for Impact Damage in Fiber Composites

The importance and complexity of impact in composite structures suggest that experiments will be needed to understand the mechanics involved. Scaling of results from subscale structures necessitates an understanding of scaling rules governing impact. An experimental study was performed in which composite plates ranging in size by a fac tor of five were geometrically scaled, and subjected to impact. The results show that analytically derived scaling rules could accurately describe the undamaged response to impact. An analysis based on the dynamic plate equations showed excellent agreement with the experiments. The formation of damage is complex, showing an apparent de pendence of delamination on absolute size as suggested by fracture mechanics.

Design of sandwich structures for concentrated loading

While sandwich construction offers well-known advantages for high stiffness with light weight, the problem of designing the sandwich structure to withstand localized loading, such as from accidental impact, remains an important problem. This problem is more difficult with lower stiffness cores, such as expanded foam. In the present study, experiments have been carried out on foam core sandwich beams with carbon/epoxy faces, under conditions of concentrated loading. The variables considered were the density of the foam and the relative thickness of the core. The common failure modes of sandwich structures were observed, including core failure in compression and shear, delamination, and fiber failure in the faces. These failure modes were systematically related to the test variables by means of a detailed stress analysis of the specimen, and a consideration of the failure properties of the constituent materials. The loading is characterized by localized high stress and strain concentrations that are not predicted in first-order shear deformation sandwich beam theory. The three-dimensional elasticity solution of Pagano was used to obtain the stress distributions. The strength prediction requires a detailed consideration of the localized nature of the loading, including the effects of strain gradients in the faces. The results show that failure modes and load levels can be predicted for sandwich structures under concentrated loading, but that accurate predictions require a consideration of the details of the concentrated loading. The results have a direct application in predicting the ability of sandwich structures to withstand localized loading such as from accidental impact.

Limits of quasi-static solutions in impact of composite structures

Abstract Transverse impact is an important service condition for composite structures because of the potential for material damage. A problem that often arises in the study of transverse impact is whether a quasi-static analysis if sufficient, or whether a dynamic solution is required. A rule is developed that is intended to provide a convenient method of classifying impact problems in this regard. The rule is based on the calculation of an equivalent structure mass that can be considered to be lumped at the impact site, and then uses the ratio of impact mass to equivalent lumped structure mass to establish the limits of applicability of the quasi-static solution procedure. This rule is closely related to others based on natural frequencies, but only requires knowledge of the static deformation response. Comparisons with dynamic calculations demonstrate the general applicability of this rule for the impact response of composite plates and cylinders.

A comparison of solution techniques for impact response of composite plates

Abstract Calculations of impact response in composite material structures are important in damage-tolerant design. In general, impact of composite structures is a complicated event, and it is important to assess the accuracy of the calculation. The present work attempts to address this point by comparing the results of different solution techniques. One of the techniques is based on a Rayleigh-Ritz approach with numerical integration in time, and another is an analytical approach using Laplace transformation of the governing differential equations, but requiring a linearization of the contact deformation. The results are also compared with finite element calculations and experimental measurements. The results show the range of numerical parameters required to give good accuracy of solution.

Response of Quasi-Isotropic Carbon/Epoxy Laminates to Biaxial Stress

The results of biaxial tension tests on AS4/3501-6 carbon/epoxy are presented for a quasi-isotropic [90, ±45,0]s laminate. These tests were performed using a tubular spec imen subjected to internal pressure and axial tension. The specimen design appears to minimize stress concentrations in the gage section. The measured stress-strain response shows a small but definite reduction in stiffness associated with progressive matrix failure. The failure stresses and strains are consistent with a maximum fiber strain failure criterion, and are most accurately modeled with a progressive failure model that incor porates ply stiffness changes.

Strength of quasi-isotropic laminates under off-axis loading

Abstract An experimental investigation was carried out to determine the failure properties of quasi-isotropic AS4/3501-6 carbon/epoxy laminates under conditions where the loading axis did not coincide with the fiber axis. A tubular specimen subjected to combinations of pressure and axial tension was used to determine the response under various tension-tension biaxial stress states. The results showed no loss of strength with rotation of the laminate orientation with respect to the loading axes at angles including 22·5°,the maximum possible for the laminate used. Maximum fiber direction strains at failure were essentially independent of the rotation of the laminate as well as the biaxial stress state.

An examination of a higher order theory for sandwich beams

Sandwich construction is known to provide high stiffness with light weight. Lower modulus cores, such as characteristic of the expanded polymers used in commercial applications, require a more refined analytical treatment. A higher order theory for sandwich beams available in the literature is investigated in the present paper, and applied to a number of sandwich beam problems of interest. The application of this theory reveals certain features of the solution process that must be addressed. The results show significant stress concentrations in both the core and the faces at load application and support points. These stress-concentrations are revealed by the higher order theory, and are important in design.

Multiaxial characterization of T800/3900-2 carbon/epoxy composites

Abstract The individual plies of fiber composites are typically used under conditions of multiaxial stress. However, relatively little is known about the response of fiber composites to multiaxial stress, and this is particularly true with respect to strength properties. This paper presents the results of an investigation into the multiaxial strength and stiffness properties of Toray T800/3900-2, which is a high-strain-capability carbon fiber with a high-toughness epoxy-matrix system. Tests were carried out on tubular specimens loaded under combinations of internal pressure, axial tension or compression, and torsion. The tests involved both unidirectional lamina specimens, to determine matrix-dominated properties, and laminate specimens involving three different types of layup. The laminate test results showed that ultimate failure could be satisfactorily correlated by using a maximum-fiber-direction-strain failure criterion.