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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.5, #3)
On the Role of Lengthscale in the Prediction of Failure of Composite Structures: Assessment and Needs by S. Mark Spearing; Paul A. Lagace; Hugh L. N. McManus (pp. 139-149).
The role of modeling in the design of structural composite components against failure is discussed. Composite materials fail due to damage processes operating at several lengthscales. The interactions between these processes offer the principal challenges to applying mechanism-based models at structural scales beyond the ply level. A methodology is proposed to increase the efficiency of the design process, analogous to the 'building block' approach, which provides a framework for integrating mechanism-based models with the current experimentally-based design process. Available models are reviewed and their key elements identified. General concepts are illustrated via a discussion of the particular issues pertaining to notched components. Key steps needed to allow the evolution of the design process to the envisioned process are identified.
Keywords: design; composite structures; failure; lengthscale; damage; models
Stress Rupture of PMC's in End-Loaded Bending by Blair E. Russell; Céline A. Mahieux; Kenneth L. Reifsnider (pp. 151-159).
This paper introduces a method to study the time dependent behavior of polymer matrix composites. An end-loaded bending method and fixture are developed to emphasize the contribution of changes in matrix properties to the behavior of fiber dominated composites loaded in the fiber direction. This method has distinct advantages over other methods such as tensile stress rupture and three- and four-point bending rupture methods. This paper discusses the design and fabrication of an end-loaded bending fixture. A brief analysis is presented which relates strain level to end-to-end distance, eliminating the need for strain gauges. Time-dependent rupture in bending of polymer matrix composites is reported.
Keywords: end-loaded bending; stress rupture; matrix characterization; polymer matrix composites; carbon fiber/polyphenylene sulfide (PPS)
Analysis of the Edge Effect on Flow Patterns in Liquid Composites Molding by A. Hammami; R. Gauvin; F. Trochu; O. Touret; P. Ferland (pp. 161-173).
In liquid composites molding, the cutting of the fiber preform is often not sufficiently precise leaving a small clearance between the reinforcement and the mold edges. This clearance creates a preferential flow path for the resin which may disrupt the filling of the mold cavity. Experience has shown that even a small clearance of 1 or 2 mm could have a significant effect, especially if the preform has a high fiber content. A model is thus needed to predict this channeling effect in order to take it into account in computer simulations of the mold filling process.This paper presents a model to describe this phenomenon. The idea is to characterize simultaneously the flow in the channel and through the reinforcement. The model is derived using Navier–Stokes equations in the open channel and Darcy's law in the porous preform. From this model, an equivalent porous medium was defined for which an equivalent permeability tensor can be computed as a function of the channel geometry. Numerical simulations performed with the computer software RTMFLOT developed in our laboratory have shown that in some limit cases, i.e., a large channel (5–6 mm) or for very low porosity reinforcements, the transverse flow from the channel to the preform can be neglected in the model while still obtaining quite a good prediction of the flow edge effect. In other cases, however, namely for a clearance of intermediate size (2–3 mm) which is the most common case in RTM, or for a higher porosity of the reinforcement, the transverse flow from the channel to the rest of the preform must be taken into account. Experimental data to validate the proposed model are also presented.
Keywords: resin transfer molding; edge effect; preform; numerical simulation
A Comparative Study of the Mechanical Performance and Cost of Metal, FRP, and Hybrid Beams by Patrick Kim (pp. 175-187).
Aluminum, FRP, and aluminum/FRP hybrid box beams designed for minimum weight are compared. The FRP beams consist of ±45° angle-ply windings with an intercalated unidirectional layer in the flanges. The hybrid beams consist of aluminum for the predominantly shear-loaded webs and part of the flange, and FRP in the parts of the flange with the maximum tensile or compressive stresses. All-CFRP beams have the lowest weight for a desired stiffness or strength, providing up to nearly 70% weight saving compared to aluminum beams. Hybrid beams are slightly heavier than all-composite beams, with up to 56% weight saving compared to aluminum, but they are easier and less expensive to produce. A hybrid beam can be produced at the same cost as an equivalent aluminum beam at a higher, more realistic CFRP price than an all-CFRP beam. Other advantages of aluminum/CFRP hybrids are increased ductility, a high fatigue resistance, and joining using conventional techniques for metals.
Keywords: hybrid structure; structural optimization; weight savings; comparative performance; cost analysis
Comparison of Interlaminar Fracture Toughness in Unidirectional and Woven Roving Marine Composites by P. Compston; P.-Y. B. Jar (pp. 189-206).
This paper investigates the effect of fibre lay-up and matrix toughness on mode I and mode II interlaminar fracture toughness (GIc and GIIc) of marine composites. Unidirectional and woven roving fibres were used as reinforcements. Two vinyl ester resins with different toughness were used as matrices. Results from both modes showed toughness variation that is consistent with matrix toughness. Values of GIc were not significantly influenced by fibre lay-up except at peak load points in the woven roving/brittle-matrix composite. Each peak load point, caused by interlocked bridging fibres, signified the onset of unstable crack growth. For unidirectional specimens, crack growth was stable and GIc statistically more reliable than woven roving specimens, which gave fewer GIc values due to frequent unstable crack growth. Mode II tests revealed that, except for crack initiation, GIIc was higher in woven roving composites. This was due to fibre bridging, perpendicular to the crack growth direction, which encouraged stable crack growth and increased energy absorption. Mode II R-curves were obtained for the woven roving specimens. These R-curves provide additional information useful for characterising delamination resistance. The paper concludes that composites with woven roving fibres show similar mode I delamination characteristics to the unidirectional composites; but their mode II delamination characteristics, after crack initiation, are quite different.
Keywords: interlaminar fracture toughness; mode I; mode II; woven glass fibre; matrix toughness