Carl T. Herakovich

Edge effects in angle-ply composite laminates

This paper presents the results of a zeroth-order solution for edge effects in angle-ply composite laminates obtained using perturbation tech niques and a limiting free body approach. The general solution for [±θ] laminates is applied to the special case of a [±45]s graphite/epoxy lamin ate. Interlaminar stress distributions are obtained as a function of the laminate thickness-to-width ratio h/b and compared to finite difference results. The solution predicts stable, continuous stress distributions, determines finite maximum tensile interlaminar normal stress σz, and provides mathe matical evidence for singular interlaminar shear stresses τ xz and τyz in [±45] graphite/epoxy laminates.

Composite Laminates with Negative Through-the-Thickness Poisson's Ratios

A simple analysis using two-dimensional lamination theory combined with the ap propriate three-dimensional anisotropic constitutive equation is presented to show some rather surprising results for the range of values of the through-the-thickness ef fective poissons ratio v xz for angle-ply laminates. Results for graphite-epoxy show that the through-the-thickness effective poissons ratio can range from a high of 0.49 for a [90] laminate to a low of -0.21 for a [±25]S laminate. It is shown that negative values of v xz are also possible for other laminates.

Shear characterization of unidirectional composites with the off-axis tension test

The influence of end constraints on accurate determination of the intralaminar shear modulusG12 from an off-axis tension test is examined both analytically and experimentally. The Pagano-Halpin model is employed to illustrate that, when the effect of end constraints is properly considered, the exact expression forG12 is obtained. When the effect of end constraints is neglected, expressions for the apparent shear modulusG12* and apparent Youngs modulusExx* are obtained. Numerical comparison for various off-axis configurations and aspect ratios is carried out using typical material properties for graphite/polyimide unidirectional composites. It is demonstrated that the end-constraint effect influences accurate determination ofG12 more adversely than it affects the laminate Youngs modulusExx in the low off-axis range. Experimental results obtained from off-axis tests on unidirectional Gr/Pi specimens confirm the above. Based on the presented analytical and experimental evidence, the 45-deg off-axis coupon is proposed for the determination of the intralaminar shear modulusG12.

On the Relationship between Engineering Properties and Delamination of Composite Materials

The influence of the coefficient of mutual influence, poissons ratio and coefficients of thermal and moisture expansion on delamination is studied. Engineering theories are compared to finite element and experimental results. It is shown that the mismatch in coefficient of mutual influence can have a strong influence on delamination with fiber angles in the 10°—15° range being critical for adjacent (±θ) layer combinations. The mismatch in coefficient of mutual influence is reduced by a factor of two and the interlaminar shear stress τzx is reduced significantly when the ±θ layers are interspersed between 0° and 90° layers. It is shown how the results can be used for design of composite laminates.

A Methodology for Accurate Shear Characterization of Unidirectional Composites

A combined experimental-analytical methodology is presented for accurate determina tion of the intralaminar shear modulus G 12 of unidirectional composites using the off-axis tension test and/or the Iosipescu test. It is demonstrated that consistent values of G 12 can be obtained with the two methods provided that: (1) specimen geometry is optimized for the off-axis test, (2) correction factors are employed to account for the shear stress nonuni formity in the test section of Iosipescu specimens. The problem of measuring the shear strength with these specimens is discussed. The 45-deg off-axis tensile coupon is recom mended for determination of the shear modulus. The 0-deg Iosipescu specimen is recom mended for determining an upper lower bound on the shear strength.

A normal stress criterion for crack extension direction in orthotropic composite materials

A criterion for predicting the direction of crack extension in orthotropic composite materials is presented. The criterion is based upon the normal stress and the anisotropic tensile strength on arbitrary planes about the tip of a crack. Results are ob tained, via finite element solutions, for: (a.) isotropic mixed mode fracture, (b.) cracks in unidirectional off-axis slotted composite tensile coupons and (c.) cracks in cross plied laminates. Comparisons are made with other theories and experimental results.

Influence of layer thickness on the strength of angle-ply laminates

Experimental results are presented showing that the strength and toughness of finite-width angle-ply laminates can be increased significantly by using an alternating layer stacking sequence as opposed to a clustered con figuration. The ultimate tensile stress of an alternating plus/minus θ laminate of the form [(±θ)2] s can be as much as 1.5 times that a clustered configura tion of the form [θ2/—θ2] s . Further, the toughness of the alternating layer configuration can be as much as 2.7 times that of the clustered configuration. These differences are explained analytically through consideration of the in fluence of layer thickness on the magnitude of the interlaminare shear stress and by examination of failed specimens. It is shown that the two laminate configurations exhibit distinctly different failure modes for some fiber angles. Both laminate configurations exhibit catastrophic failure with the damage limited essentially to a small region defined by the length of a single crack across the face of the specimen, parallel to the fiber direction. Results are presented for T300/5208 graphite-epoxy for fiber orientations of 10°, 30 °, and 45 °.

Thermally Induced Transverse Cracking in Graphite-Epoxy Cross-Ply Laminates:

Thermally induced transverse cracking in T300/5208 graphite-epoxy cross-ply laminates was investigated experimentally and theoretically. The six laminate configura tions studied were: [0/903] s, [02/902]s, [03/90] s, [90/03]s, [902/02] s, and [903/0]s. The thermal load required to initiate transverse cracking was determined experimentally and compared to theoretical prediction. Experimental results for the accumulation of transverse cracks, under cyclic thermal loading between —250°F and 250°F for up to 500 thermal cycles, are presented. The calculated in situ transverse lamina strength was deter mined to be at least 1.9 times the unidirectional lamina transverse tensile strength. All laminate configurations exhibited an increase in crack density with increasing thermal cycles.

Damage-induced property changes in composites subjected to cyclic thermal loading

Abstract Damage in the form of transverse cracks resulting from thermal loading is studied as it relates to the dimensional stability of flat laminates and stiffness changes in cylindrical tubes. Graphite-epoxy specimens were subjected to cyclic thermal loading in the temperature range −250 to +250°F. It is shown that transverse cracking is the dominant damage mechanism in both types of structural elements. Fiber splitting is also quite common at the low test temperatures. Experimental results indicate that damage significantly reduces the inplane coefficients of thermal expansion of flat laminates and the torsional stiffness of the tubes. Theoretical predictions for coefficients of thermal expansion as a function of crack spacing in flat laminates followed the same trend as experimental results.

Three-Dimensional Inelastic Finite Element Analysis of Laminated Composites

A three dimensional theory based on Hills orthotropic yield criterion and the incremental flow theory of plasticity is proposed for the inelastic response prediction of laminated composites. Techniques for including nonlinear hardening of orthotropic materials via unidirectional Ramberg-Osgood coef ficients as well as temperature dependent plasticity and first and second order nonlinear thermal expansions are presented. Results confirm the importance of including such effects in composite materials.