Interfiber/interlaminar failure of composites under multi-axial states of stress
Introduction
The design and analysis of thick composite sections require understanding of the behavior of these materials under general three-dimensional states of stress and the availability of reliable failure criteria in three dimensions. Three-dimensional characterization of composites is essential for the reliable and robust design and analysis of thick structural composite sections where a three-dimensional state of stress exists, as in right angle brackets and flanges, joints, as well as in composites with 3D reinforcement like stitching and pinning. Both unidirectional tape based laminates and textile composites are used for such structural applications.
In the macromechanical analysis of these composites, the material is treated as a homogeneous orthotropic one with three mutually perpendicular principal material axes (Fig. 1). In the case of textile composites, the warp, fill and the normal through-thickness directions are treated as the principal material axes (Fig. 1). Macromechanical failure criteria are expressed in terms of nine lamina strength parameters, tensile and compressive strengths along the fiber (1) and transverse to the fiber (2 and 3) directions (F1t, F1c, F2t, F2c, F3t, F3c), and shear strengths (F12 or F6, F23 or F4, F31 or F5). There is a plethora of macromechanics failure theories with varying degrees of validity when compared with various experimental results. Extensive reviews of such theories have appeared recently [1], [2], [3]. Reference [2] contains the results of a 12-year “World Wide Failure Exercise” where 19 theories were evaluated. The various theories were ranked with respect to their predictive capabilities in specific cases, but no definitive conclusion has been reached as to the best general approach to failure prediction. One reason for the continuing uncertainty is the lack of adequate and reliable experimental data, especially in three dimensions, for full evaluation of the various theories.
In the present investigation, in-plane and through-thickness tensile, compressive, and shear tests were conducted on unidirectional and textile carbon fiber composite materials at various orientations with the principal material axes to determine their constitutive and failure behavior. The failure modes and strengths were analyzed and the applicability of various failure theories was investigated.
Section snippets
Material
Two forms of a carbon/epoxy material were investigated, a unidirectional lamina (AS4/3501-6) and a carbon-fabric/epoxy composite (AGP 370-5H/3501-6). Both were obtained in prepreg form. The fabric reinforcement was a five-harness satin weave of AS4 carbon fibers with the same fiber count in both the warp and fill directions. Thick laminates of these materials were prepared by stacking 200 prepreg plies of the unidirectional and 80 plies of the fabric prepreg, bagging the assembly, and curing in
Multi-axial characterization
Three-dimensional material characterization was conducted to study the behavior under multi-axial states of stress. This involves through-thickness testing which is more problematic than in-plane testing because it is difficult to fabricate material of uniform quality in sufficiently thick sections. It is also difficult to introduce the loading without the deleterious influence of end effects and stress concentrations. An overview of through-thickness test methods was given by Lodeiro et al. [4]
Failure theories
The results obtained were evaluated based on three types of failure criteria, non interactive or limit criteria (maximum stress), fully interactive criteria (Tsai-Hill, Tsai-Wu), and failure mode based and partially interactive theories (Hashin–Rotem and NU) [3], [5]. The latter is a recently developed interlaminar failure theory at Northwestern University (NU) based on interlaminar matrix strain criteria [3], [8], [9].
According to the maximum stress theory, failure occurs when any one of the
Failure plane orientation
The failure plane orientation under tranverse loading provides important information on the failure mode of the material. In some theories such as the Puck theory, it serves as an intrinsic material property which must be determined experimentally [11]. Christensen and DeTeresa provided an analytical approach for calculation of the failure plane orientation related to the relevant failure criterion [12]. It is maintained that the angle between the loading axis and the failure plane ranges
Summary and conclusions
A unidirectional and a fabric composite were characterized mechanically under in-plane and through-thickness tensile and compressive loading. Stress–strain behavior, strengths, and failure modes were recorded for the various loading conditions, including biaxial states of stress. Strength results were evaluated with classical failure theories and with a recently developed theory for interfiber/interlaminar failure (NU theory). The new theory was in better agreement with experimental results
Acknowledgement
The work described here was sponsored by the Office of Naval Research (ONR). We are grateful to Dr. Y.D.S. Rajapakse of ONR for his encouragement and cooperation.
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