by Motola, Yael; Banks-Sills, Leslie; Fourman, Victor

Fracture tests carried out on unpoled and poled PZT-5H four-point bend specimens are presented in this paper. The crack faces were parallel to the poling direction. Both mechanical loads and electric fields were applied to the poled specimens. The experimental results were analyzed by means of the finite element method and a conservative M-integral including the crack face boundary conditions. Fracture tests on four-point bend PIC-151 specimens with the crack faces perpendicular to the poling directions were also analyzed here; the experimental results were taken from the literature. A mixed mode fracture criterion is proposed for piezoelectric ceramics. This criterion is based upon the energy release rate and two phase angles. This criterion was implemented with experimental results from the literature and from this investigation. Excellent agrement was found between the fracture curve and the experimental results of the specimens with the crack faces perpendicular to the poling direction. With some scatter, reasonable agreement was observed between the fracture curve and the experimental results of the specimens with crack faces parallel to the poling direction.

DOI: 10.1007/s10704-009-9392-x
Online Date: 8/30/2009
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by Xu, Yangjian; Yuan, Huang

The extended finite element method (XFEM) combined with a cyclic cohesive zone model (CCZM) is discussed and implemented for analysis of fatigue crack propagation under mixed-mode loading conditions. Fatigue damage in elastic-plastic materials is described by a damage evolution equation in the cohesive zone model. Both the computational implementation and the CCZM are investigated based on the modified boundary layer formulation under mixed-mode loading conditions. Computational results confirm that the maximum principal stress criterion gives accurate predictions of crack direction in comparison with known experiments. Further popular multi-axial fatigue criteria are compared and discussed. Computations show that the Findley criterion agrees with tensile stress dominant failure and deviates from experiments for shear failure. Furthermore, the crack propagation rate under mixed mode loading has been investigated systematically. It is confirmed that the CCZM can agree with experiments.

DOI: 10.1007/s10704-009-9391-y
Online Date: 8/30/2009
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by Drozdov, A. D.; Christiansen, J. deC.

Observations are reported on isotactic polypropylene in uniaxial tensile tests with various strain rates, relaxation tests with various strains, and creep tests with various stresses at ambient temperature. Constitutive equations are derived for the viscoelastic–viscoplastic responses and damage of a semicrystalline polymer at three-dimensional deformations. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. The model is applied to predict creep-failure diagrams in the entire interval of stresses. A phenomenological approach is proposed to determine a knee stress, at which transition occurs from ductile to brittle rupture. Accuracy of this method is evaluated by numerical simulation.

DOI: 10.1007/s10704-009-9384-x
Online Date: 8/26/2009
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by Zhu, Ming-Liang; Xuan, Fu-Zhen

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode.

DOI: 10.1007/s10704-009-9386-8
Online Date: 8/22/2009
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by LLorca, Javier; Cox, Brian N.

DOI: 10.1007/s10704-009-9383-y
Online Date: 8/13/2009
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by Bansal, Sandeep; Nath, S. K.; Ghosh, P. K.; Ray, S.

The methods of measurement of stretched zone width (SZW) have been examined and its relative insensitivity to thickness and length of pre-fatigue crack in CT specimens, encourages its use to mark the end of blunting in the J–R curve. On the basis of SZW, a mathematical equation has been determined for the slope of blunting line, also as a function of strain hardening coefficient, in the limited context of SA333 Gr. 6 steel and HSLA steel. The internal consistency of the 0.2 mm offset blunting line equation has been demonstrated by comparing the estimated and the observed initiation fracture toughness. The equation has been validated also with the results from literature. The initiation fracture toughness obtained for SA333 Gr. 6 steel following 0.2 mm offset line equation determined here compares well with the results reported in earlier investigations.

DOI: 10.1007/s10704-009-9381-0
Online Date: 8/1/2009
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by Debecker, B.; Vervoort, A.

The layered structure of slate rock induces strength anisotropy. The strength in the direction of the layers (schistosity) is considerably smaller than in any other direction. A series of loading tests on circular samples and another series of loading tests on rectangular samples are performed to examine fracture patterns in slate. The tests are monitored by visual recordings and by recording acoustic emission. The processing of this data results in localization. This allows identification and analysis of the occurrence and propagation of the individual fractures. It is shown that the strength anisotropy on μ-scale is the key factor behind the strength anisotropy on sample scale, as well as behind the deformation behaviour of the sample. In addition, it is observed that a small variability in the layer direction can affect the fracture pattern considerably.

DOI: 10.1007/s10704-009-9382-z
Online Date: 8/1/2009
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