by Chai, Herzl

The evolution of surface damage in bilayers due to cyclic spherical indentation in the presence of incompressible lubricant is studied using an all-transparent glass/polycarbonate system as a model for more practical applications such as dental crowns and rolling contact fatigue. In situ observations and post-mortem material sectioning reveal that inner cone cracks evolve sequentially from the contact edge inward by slow growth in a process controlled by stress shielding from preceding cracks. The embryonic cracks are then accelerated by the action of fluid pressure into the flexural tensile stress at the lower part of the coating, where crossover fracture leading to delamination between the coating and substrate may ensue.A consistent FEM brittle fracture analysis incorporating multiple cracks, rate-dependent toughness and liquid pressure is used to follow the damage evolution in the coating. Crack trajectories are determined incrementally under the dual constraint K
_I = K
_II = 0, which maximize the tension at the crack tip upon the application of fluid pressure. The latter, evaluated at each increment with the aid of a fluid entrapment model, helps drive the leading crack past the compression zone beneath the contact via a hydraulic pump like action. In the early stages of fracture, the liquid pressure is reasonably well approximated by the Hertzian radial surface stress at the crack mouth. Fluid trapped in secondary cracks accentuate the compression beneath the contact. This helps squeeze more liquid into the tip of the leading crack in a zipping like action, which further enhance the crack driving force in the far field. The analytic predictions generally collaborate well with the tests.

DOI: 10.1007/s10704-006-9047-0
Online Date: 1/30/2007
Print publication date: 1/1/2007
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by Ruckert, C. O. F. T.; Tarpani, J. R.; Filho, W. W. Bose; Spinelli, Dirceu

This paper discusses the relationship between striation spacing, i.e., the microscopic crack propagation rate, as measured in postmortem fractographic inspection of fatigue fractured surfaces, and the macroscopic crack propagation rate, i.e., da/dN, as monitored during fatigue crack growth tests. Compact tensile specimens C(T) in prevalent plane-strain conditions were extracted in LT orientation from the center of a 2-in. thick rolled plate of a SAE-AMS 7475-T7351 Al alloy. Testpieces were fatigue tested according to ASTM-E647 standard, at room temperature in a servo-hydraulic closed-loop MTS testing machine operating with the unloading elastic compliance technique. da/dN-ΔK data points were collected in the Paris’ law validity region, with crack growth rates typically ranging from 0.18 to 2.02 µm/cycle. Topographical survey was conducted on the test specimen fracture surfaces in a scanning electronic microscope in order to determine striation spacing created during the fatigue test. Macro- and micro-crack growth rates were compared and good correlation have been obtained for the data within the range of ΔK assessed in the study. Results of crack growth rates have been quantitatively evaluated in terms of fatigue life estimation.

DOI: 10.1007/s10704-006-9029-2
Online Date: 1/23/2007
Print publication date: 12/1/2006
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by Saito, Y.; Yoda, M.; Imamura, S.

Creep crack growth rates were measured using centrally cracked tension specimens of thin polypropylene film with different crack lengths at various stresses and temperatures. The creep crack growth rates were correlated with the stress intensity factor. There was the region of the minimum constant crack growth rate which occupied more than 70% of the total creep failure life. This constant creep crack growth rate characteristics were analyzed on the basis of the stress-dependent Arrhenius type thermally activated process.

DOI: 10.1007/s10704-006-9046-1
Online Date: 1/19/2007
Print publication date: 1/1/2007
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by Butcher, Cliff; Chen, Zengtao; Worswick, Michael

Numerical simulation of stretch flange forming of Al–Mg sheet AA5182 was performed using the upper and lower bound constitutive models of Gurson–Tvergaard–Needleman (GTN) and Sun and Wang, respectively. Stress and strain-controlled nucleation rules are adopted for both models. The lower bound model of Sun and Wang has been extended to include the void coalescence criterion of Tvergaard and Needleman to form the so-called Sun–Tvergaard–Needleman (STN) model. Upper and lower bound formability predictions are combined to create a predictive formability band as actual formability lies between these limits. The resulting formability predictions are compared with experimental results and an appropriate void nucleation stress and strain suggested.

DOI: 10.1007/s10704-006-9044-3
Online Date: 1/18/2007
Print publication date: 12/1/2006
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by Sankar, Ramaswamy; Lesser, Alan J.

This paper deals with modeling of the interaction in overlapping cracks that the authors have earlier identified to be generic to a wide range of polymeric systems (Ramasamy and Lesser, J Polym Sci B Phys, 2003). A complex stress function method is used for evaluating stress intensity factors for interacting cracks. The interaction between two parallel overlapping cracks is considered first. It is shown for this case that the stress intensity factor can fall below the threshold value when there is sufficient overlap, leading to arrest of crack growth at the overlapping tip. Then the interaction in a doubly periodic infinite array of cracks is considered. The interaction in the array is found to be non-linear. However, at a given stress level, the highest density of stable cracks is related to the threshold value for crack propagation K_th though a simple set of equations. It is also shown that in an infinite array of cracks, the energy release rate criterion for crack growth is different from the stress intensity factor criterion due to a reduced stiffness of the material.

DOI: 10.1007/s10704-006-9043-4
Online Date: 1/18/2007
Print publication date: 12/1/2006
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by Ayhan, A. O.; Kaya, A. C.; Nied, H. F.

Many important interface crack problems are inherently three-dimensional in nature, e.g., debonding of laminated structures at corners and holes. In an effort to accurately analyze three-dimensional interface fracture problems, an efficient computational technique was developed that utilizes enriched crack tip elements containing the correct interface crack tip asymptotic behavior. In the enriched element formulation, the stress intensity factors K
_I, K
_II, and K
_III are treated as additional degrees of freedom and are obtained directly during the finite element solution phase. In this study, the results that should be of greatest interest are obtained for semi-circular surface and quarter-circular corner cracks. Solutions are generated for uniform remote tension and uniform thermal loading, over a wide range of bimaterial combinations. Of particular interest are the free surface effects, and the influence of Dundurs’ material parameters on the strain energy release rate magnitudes and corresponding phase angles.

DOI: 10.1007/s10704-006-9040-7
Online Date: 1/18/2007
Print publication date: 12/1/2006
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by Zhang, Yang; Chen, Zengtao

Failure of ductile materials is often related to coalescence of microscopic voids. The stress triaxiality is one of the primary factors that influence the coalescence. In the present work, a 3D unit cell model is employed to investigate this effect. The cell model contains two aligned voids. A coalescence criterion is proposed in which the critical void volume fraction is expressed in terms of stress triaxiality.

DOI: 10.1007/s10704-006-9045-2
Online Date: 1/17/2007
Print publication date: 1/1/2007
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by Nishioka, Toshihisa; Zhou, Zhi-Dong; Yu, Jia-Huan

The report of Lambros and Rosakis [(1995) J Mech Phys Solids 43(2): 169–188] has focused attention on steady-state transonic interfacial crack growth accounting for the phenomenon of crack face contact in elastic/rigid bimaterial but could not handle issues relating to energy transmission across the interface. The present paper attempts to provide a complete explicit expression of the asymptotic fields induced by transonically propagating interfacial crack in elastic/elastic bimaterial for in-plane case. The energy distribution on the contact area, crack tip and two singular characteristic lines is analysed thoroughly and compared with the dynamic separated J-integrals. The length of the contact zone is also discussed briefly by establishing energy fracture criterion that satisfies contact condition. The two-dimensional in-plane asymptotic deformation field surrounding the contact area of a crack propagating transonically along an elastic/elastic bimaterial interface is observed and discussed thoroughly.

DOI: 10.1007/s10704-006-9035-4
Online Date: 1/11/2007
Print publication date: 12/1/2006
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by Jin, Xiaoqing; Keer, Leon M.; Chez, Eugene L.

Cracking of a fluid-filled subsurface crack is studied by means of the distributed dislocation technique within the framework of two-dimensional linear elastic fracture mechanics. The Griffith crack was initially opened by the application of hydrostatic pressure of an incompressible fluid within the crack. A moving Hertz line contact load distribution is applied at the surface of the half-plane in the presence of friction. The stress intensity factors at the tips of the fluid-filled crack are analyzed with the restriction that due to the fluid incompressibility there is no change of the crack-opening volume. When the crack starts to propagate/kink, numerical results show that the internal fluid pressure will be relieved, and as the ratio of the branched crack length to main crack length increases, the elastic strain energy release rate decreases. The crack growth is assumed to be arrested when the energy release rate is below a certain value. Based on the energy criterion, predictions are attempted for determining the load position where the crack propagation/kink commences as well as the growth increment of the branch crack before it is arrested. A step-by-step crack path is constructed to simulate the growth pattern of the fluid-filled crack under moving Hertzian loading.

DOI: 10.1007/s10704-006-9026-5
Online Date: 1/11/2007
Print publication date: 12/1/2006
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by Sevostianov, Igor; Sabina, Federico J.; Bravo-Castillero, Julian; Guinovart-Diaz, Raul; Rodriguez-Ramos, Reinaldo

The paper addresses the problem of the connection between effective elastic and conductive properties of a fiber reinforced material with both phases (the matrix and the fibers) being transversely-isotropic. The exact solution for a square array of fibers and approximate solutions for randomly located parallel fibers are constructed and compared. The results allow one to predict the entire set of macroscopic elastic stiffnesses through one or two measurement of thermal or electrical conductivity. As a side result, it is shown that the mutual positions of inhomogeneities produce only a minor effect and that applicability of the non-interaction approximation is much wider than expected.

DOI: 10.1007/s10704-006-9039-0
Online Date: 1/10/2007
Print publication date: 12/1/2006
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