Showing papers on "Micromechanics published in 1981"

Fracture micromechanics of polymer materials

Abstract: 1 Changes in the mechanical properties of polymer and composite materials during the fatigue process.- 2 The observations of continuum ruptures in polymers under load.- 3 Regularities of submicrocrack origination in loaded polymers.- 4 Structural conditions for submicrocrack generation.- 5 Molecular mechanism of submicrocrack generation.- 6 Localization of the fracture process.- 7 A statistical model of the fracture of polymer materials.- 8 Theory of scattered fracture at the complex stress state.- 9 Fracture of polymer and composite materials during high speed tension.- 10 Analysis of the temperature field during vibrational loading with consideration given to scattered damage.- References.

Micromechanics of Flaw Growth in Static Fatigue: Influence of Residual Contact Stresses

Abstract: Residual contact stresses about indentation flaws are demonstrated to have a strong deleterious effect on specimen lifetime in static fatigue. The underlying basis of conventional fatigue analysis is first examined critically and is argued to be deficient in the way the stress intensity factor for the flaws is related to the characteristic parameters of crack geometry and applied loading. In general, it is necessary to incorporate a residual term into the stress intensity formulation. A modified theory of static fatigue is accordingly developed, in which the residual contact stresses play a far from secondary role in the micromechanics of flaw evolution to failure. Strength tests on Vickers-indented soda-lime glass disks in water environment provide clear experimental confirmation of the major theoretical predictions. Implications of the residual stress effect concerning fracture mechanics predictions of lifetimes for “real” ceramic components under service conditions are discussed.

Micromechanics and kinetics of deformation zones at crack tips in polycarbonate

Abstract: Plane stress deformation zones are grown from cracks produced by electron beam irradiation in thin films of polycarbonate (PC) bonded to ductile copper grids. The kinetics of zone growth in both length and width are followed by optical microscopy while v f, the ratio of zone thickness to film thickness, is followed by optical interference and transmission microscopy measurements. These data allow the zone surface displacement and stress profiles to be computed at various times during growth. There is a stress concentration at the zone tip which relaxes to a uniform stress over the rest of the zone up to near the crack tip. Both the tip stress concentration and uniform zone stress decrease as zone growth proceeds. The zone surface displacement rate follows these changes in stress. It is highest just behind the zone tip where the stress concentration exists and is constant in the uniform stress zone. It decreases markedly with time. While the Dugdale model does not predict the details of the zone micromechanics, in particular the zone tip stress concentration, it does predict qualitatively the correct trends. The crack propagates into the oriented polymer in the deformation zone in the form of a characteristic half diamond shape, which is the analogue of diamond cavities previously observed in the fracture process of oriented bulk polymers.

Thermal Expansion Properties of Composite Materials.

Abstract: Thermal expansion data for several composite materials, including generic epoxy resins, various graphite, boron, and glass fibers, and unidirectional and woven fabric composites in an epoxy matrix, were compiled. A discussion of the design, material, environmental, and fabrication properties affecting thermal expansion behavior is presented. Test methods and their accuracy are discussed. Analytical approaches to predict laminate coefficients of thermal expansion (CTE) based on lamination theory and micromechanics are also included. A discussion is included of methods of tuning a laminate to obtain a near-zero CTE for space applications.

Moisture and Thermal Expansion of Composite Materials

Abstract: : An experimental technique is presented for determining the moisture and thermal expanbsion coefficients of polymers, and polymer-matrix composite materials. Materials tested included Hercules 3501-6 neat epoxy resin, Hercules AS/3501-6 graphite/epoxy composite and Owens-Corning S2 glass fibers in the same Hercules 3501-6 epoxy matrix. Correlations of the experimentally determined moisture and thermal expansion properties with a nonlinear finite element micromechanics analysis are presented. Thermal expansion results for all three materials, both dry and moisture-conditioned, were obtained. Diffusivity constants were also experimentally determined. It is demonstrated that the moisture and thermal expansion of composite materials can be determined experimentally, and predicted numerically with generally good results. (Author)

Analysis of crack propagation as an energy absorption mechanism in metal matrix composites

Abstract: The crack initiation and crack propagation capability was extended to the previously developed generalized plane strain, finite element micromechanics analysis. Also, an axisymmetric analysis was developed, which contains all of the general features of the plane analysis, including elastoplastic material behavior, temperature-dependent material properties, and crack propagation. These analyses were used to generate various example problems demonstrating the inelastic response of, and crack initiation and propagation in, a boron/aluminum composite.

The Construction and Use of Toughness Maps in a Fracture Analysis of the Micromechanics of a Composite Failure.

Abstract: : Analytical expressions are presented which predict the debonding and pull-out lengths observed in brittle fibre composites. These characteristic lengths are combined with models of four toughening mechanisms to calculate the work of fracture of a composite. The results are presented as maps showing not only contours of toughness but also the dominant toughening micromechanism. The toughness is largely determined by six material parameters, each map demonstrating the combined effect of changing two of these simultaneously. Maps are presented for glass-fibres in epoxy and carbon-fibres in epoxy. Their use is demonstrated by showing the effects of hygrothermal aging on the toughness of the composites. (MM)