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The high-fidelity generalized method of cells composite micromechanics model is extended to include constituentscale progressive damage via a proposed damage model. The damage model assumes that all material nonlinearity is due to damage in the form of reduced stiffness, and it uses six scalar damage variables (three for tension and three for compression) to track the damage. Damage strains are introduced that account for interaction among the strain components and that also allow the development of the damage evolution equations based on the constituent material uniaxial stress--strain response. Local final-failure criteria are also proposed based on mode-specific strain energy release rates and total dissipated strain energy. The coupled micromechanics-damage model described herein is applied to a unidirectional E-glass/epoxy composite and a proprietary polymer matrix composite. Results illustrate the capability of the coupled model to capture the vastly different character of the monolithic (neat) resin matrix and the composite in response to far-field tension, compression, and shear loading. DOI: 10.2514/1.45671

The reusable launch vehicle thrust cell liner, or thrust chamber, is a critical component of the space shuttle mainengine. It is designed to operate in some of the most severe conditions seen in engineering practice. These conditions give rise to characteristic deformations of the cooling channel wall exposed to high thermal gradients and a coolant-induced pressure differential, characterized by the wall's bulging and thinning, which ultimately lead to experimentally observed 'dog-house' failure modes. In this paper, these deformations are modeled using the cylindrical version of the higher-order theory for functionally graded materials in conjunction with two inelastic constitutive models for the liner's constituents, namely Robinson's unified viscoplasticity theory and the power-law creep model. Comparison of the results based on these two constitutive models under cyclic thermomechanical loading demonstrates that, for the employed constitutive model parameters, the power-law creep model predicts more precisely the experimentally observed deformation leading to the 'dog-house' failure mode for multiple short cycles, while also providing much improved computational efficiency. The differences in the two models' predictions are rooted in the differences in the short-term creep and relaxation responses. DOI: 10.1061/(ASCE)0893-1321(2005)18:1(28) CE Database subject headings: Constitutive models; Simulation; Spacecraft; Liners; Deformation.

The generalized method of cells is a micromechanics model that is generally quite accurate at the macrolevel but not always accurate at the microlevel. This is due to the absence of so-called shear coupling, which provides the required bridge between macroscopically applied normal (shear) stresses and the microscopic shear (normal) stresses necessary for an accurate estimate of microlevel quantities. To overcome this deficiency, a new micromechanics model has been developed for the response of multiphase materials with arbitrary periodic microstructures, named high-fidelity generalized method of cells in part because it employs the same microstructural discretization as the original generalized method of cells. The model's framework is based on the homogenization theory, but the method of solution for the local fields borrows concepts previously employed in constructing the higher-order theory for functionally graded materials, in contrast with the typical finite element-based solution strategies. The model generates the average stress-strain response of heterogeneous materials such as ceramic, metal, and polymeric matrix composites, as well as the internal or microlevel stress and strain fields, with excellent accuracy. The model is employed to investigate the response of a metal matrix composite with locally irregular, but periodic, fiber distributions, and it is shown that irregular architectures affect shear and normal stress-strain response in a different manner. The new model's ability to capture these differences is attributed to the shear-coupling effect absent in the original model.

In the optimal design of a modern gun barrel, there are two main objectives to be achieved: increasing its strength-weight ratio and extending its fatigue life. This can be carried out by generating a residual stress field in the barrel wall, a process known as autofrettage. It is often necessary to machine the autofrettaged cylinder to its final configuration, an operation that will remove some of the desired residual stresses. In order to achieve a residual stress distribution which is as close as possible to the practical one, the following assumptions have been made in the present research on barrel analysis: A von Mises yield criterion, isotropic strain hardening in the plastic region in conjunction with the Prandtl-Reuss theory, pressure release taking into consideration the Bauschinger effect and plane stress conditions. The stresses are calculated incrementally by using the finite difference method, whereby the cylinder wall is divided into N-rings at a distance [delta]r apart. Machining is simulated by removing rings from both sides of the cylindrical surfaces bringing the cylinder to its final shape. After a theoretical development of the procedure and writing a suitable computer program, calculations were performed and a good correlation with the experimental results was found. The numerical results were also compared with other analytical and experimental solutions and a very good correlation in shape and magnitude has been obtained. [DOI: 10.1115/1.1593078]