Frederick J. McGarry

Matrix and Interface Stresses in a Discontinuous Fiber Composite Model

The method of finite element analysis is applied to an axially symmetrical model of a single filament glass-resin composite under tension. Fiber end geometries are varied by considering ellipsoids of revolution with one axis length equal to the fiber diameter while the second varies from one-tenth to ten times the fiber diameter. Tapered tips with these same axis ratios are considered also. The results show that the best ellipsoidal end shape for minimizing matrix shear stress concentration is one having the longitudinal axis equal to twice the fiber diameter. The shear stress concentration for this geometry, however, is still greater than that of the slowly tapered end, which causes the lowest stress concentration. General stress patterns in the matrix as found by the finite element method are compared to previous analytical and experimental predictions.

The Extension of Crack Tip Damage Zones in Fiber Reinforced Plastic Laminates

The size and character of the damage zone at the tip of sharp notches in fiber reinforced plastic laminates have been investigated. The variables studied were the stress intensity factor, specimen size, laminate thickness, ply thickness, ply orientation, and fiber properties. The damage zone consists of subcracks parallel to the fibers of each ply, in some cases accompanied by delamination between plies. The damage zone is found to increase in extent approximately in proportion to K2 I up to fracture for notch-sensitive laminates. For notch-insensitive laminates, a point is reached where the zone spreads rapidly across the entire specimen prior to fracture. A strong dependence of damage zone size and fracture toughness on ply thickness, fiber orientation, and fiber properties is demonstrated and discussed.

Mechanical behaviour of rigid-rod polymer fibres: 2. Improvement of compressive strength☆

Abstract Under axial compression, both poly(p-phenylene benzobisoxazole) and Kevlar® fibres fail by buckling of fibrils located just beneath the outer surface where lateral constraint is minimal. As the fibrils cascade buckle through the bulk of the fibre, a kink band is formed. A thin, well-adhered high-modulus ceramic coating on the surface increases the axial compressive strength by restraining the fibrils against buckling and kink band initiation. Compressive strength improvement is a function of coating thickness. The coating also reduces the radial thermal expansion coefficient in accord with finite-element predictions.

Mechanical behaviour of rigid rod polymer fibres: 1. Measurement of axial compressive and transverse tensile properties

Abstract Rigid rod polymer fibres have low axial compressive and transverse tensile strengths which can be measured by several methods. The tensile recoil method is tedious but a new fibre cutting device which simplifies and improves the method has been developed. Also a new method to test the transverse strength index of single fibres has been devised. The index is found to be similar among a variety of fibres suggesting that such properties depend more upon interfibrillar morphology than upon interchain properties. The same device can also perform three point bending tests on single fibres. This permits the determination of the fibre compressive modulus which for rigid rod polymer fibres appears to be considerably less than the tensile modulus.

Stress Intensity Factors for Anisotropic Fracture Test Specimens of Several Geometries

Stress intensity factors have been obtained for single-edge-notched, double-edge-notched, and double cantilever beam fracture toughness test specimens using a two-dimensional hybrid stress model finite element analysis. The degree of anisotropy is shown to have a significant effect on the stress intensity factor in some cases, with differing effects for different specimen shapes; however, effects of anisotropy are relatively constant for varying crack lengths of a given shape. An experimental K-calilbration for the double cantilever beam specimen is in good agreement with the analytical prediction, and the effect of geometry on the applied load to cause crack propagation is accurately predicted by the analysis.

Effect of consolidation on adhesive and abrasive wear of ultra high molecular weight polyethylene

Total hip replacement (THR) is widely performed to recover hip joint functions lost by trauma or disease and to relieve pain. The major cause of failure in THR is the wear of the ultra high molecular weight polyethylene (UHMWPE) component. The dominant wear mechanism in THR occurs through adhesion and abrasion. While poor consolidation of UHMWPE is known to increase the incidence of a different damage mode, delamination, which is the dominant wear mechanism in tibial inserts but uncommon in THR, the effect of consolidation on adhesive and abrasive wear of UHMWPE is not clear. In this study UHMWPE resin was subjected to hot isostatic pressing under a pressure of 138MPa at different temperatures (210 degrees C, 250 degrees C, and 300 degrees C) to achieve varying degrees of consolidation. The extent of consolidation was determined by optical microscopy using thin sections, and by scanning electron microscopy using cryofractured and solvent etched specimens. Wear behavior of the samples with varying degree of consolidation was determined using a bi-directional pin-on-disc machine simulating conditions in a hip joint. Increasing the processing temperature decreased the incidence of fusion defects and particle boundaries reflecting the powder flakes of the virgin resin, improving the consolidation. However, the bi-directional pin-on-disc wear rate did not change with the processing temperature, indicating that adhesive and abrasive wear is independent of the extent of consolidation in the range of parameters studied here.

The origin of the β transition and its influence on physical ageing

Abstract Volume dilatometry was used to explore the origin of the β transition ( T β ) and its influence on physical ageing. The results show that volume recovery does occur even at temperatures below T β , a phenomenon indicating the existence of molecular rearrangements and cooperative motions below this point. Lowering the ageing temperatures through T β progressively retards short-term molecular rearrangements and cooperative motions. Nonetheless, the T β quench does not incur any excess free-volume trapping, and its associated change in specific volume follows a linear relationship with respect to temperature. The existence of T β is time-dependent, which leads to the conclusion that the formation of T β has a kinetic origin.

Crack Propagation Modes in Injection Molded Fiber Reinforced Thermoplastics

Abstract : The modes of crack propagation are reported for injection molded short glass and carbon fiber reinforced thermoplastics. The matrices ranged from ductile to brittle, including Nylon 66, polycarbonate, polysulfone, poly(amide-imide), and polyphenylene sulfide; fiber contents were 30 or 40 % by weight. The main crack is found to grow in a fiber avoidance mode, bypassing regions of agglomeration of locally aligned fibers. The local mode of crack tip advance varied with matrix ductility and bond strength. The fracture toughness and fatigue resistance of each material are related to the mode of crack growth.

Three-Dimensional Solution for a Through-Thickness Crack in a Cross-Plied Laminate

A three-dimensional solution is given to the problem of a through-thickness edge crack in a thin 90/0/0/90 laminate under uniform tension normal to the crack direction. The solution is obtained from a three-dimensional finite-element analysis based on the hybrid stress model, and formulated through the Hellinger-Reissner variational, principle. The results indicate that the classical 1/√r stress singularity is maintained for the in-plane stresses which vary through the thickness and are discontinuous at the ply interface. The interlaminar shear and normal stresses also increase rapidly as the crack tip is approached; the interlaminar shear stresses are always maximum at the ply interface, while the interlaminar normal stress may be maximum at the interface or at the laminate center. The in-plane stresses follow a similar distribution and give an average through-thickness stress-intensity factor similar to two-dimensional predictions. The interlaminar stresses show a strong interaction of the crack tip singular and free-edge effects, as well as a strong influence of the degree of biaxiality of the in-plane stresses. The results suggest several aspects of subsequent subcritical crack extension.

Fracture Testing of Injection-Molded Glass and Carbon Fiber-Reinforced Thermoplastics

A study is reported of the application of linear elastic fracture toughness testing techniques to short glass and carbon fiber-reinforced injection-molded thermoplastics. The materials were typical of high-temperature engineering thermoplastics and included polycarbonate, polysulfone, nylon 6/6, polyphenylene sulfide, and poly(amide-imide). Tests were run on specimens of various sizes and shapes with machined notches and fatigue cracks. Results are reported for modes of crack growth, applicability of linear elastic fracture mechanics, and test peculiarities. Attempts to correlate the matrix toughness and fiber length with composite toughness are discussed.