by De, Deepankar; Narasimhan, R.

An interface fracture specimen for measuring the fracture toughness of adhesively bonded joints is proposed. It is found to represent a wide range of mode mixity. The specimen is calibrated assuming LEFM conditions and the limit of validity of this calibration as the load increases is also assessed.

DOI: 10.1023/A:1007699613421
Print publication date: 8/1/1998
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by Momber, Andreas W.

The paper alternatively discusses the concrete cylinder compressive test. Basically, the fracture is treated as a single comminution process. After compressive testing, the main concrete debris as well as the samples of fine-grained particles, including grain-size distribution and grain shape, are analyzed. The fine-grained samples are distributed according to a R(osin)-R(ammler)-S(perling)-distribution. The distribution parameters depend on the mechanical properties of the concrete. The shpae factor increases as the compressive strength increases. Generally, with an increase in the strength properties, the fracture becomes non-regular due to a ‘homogenization’ of the fracture process.

DOI: 10.1023/A:1007637912513
Print publication date: 8/1/1998
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by Shenjie, Zhou; Shuxun, Sun; zhiyuan, Cao

This paper concerns the dual boundary contour method for solving two-dimensional crack problems. The formulation of the dual boundary contour method is presented. The crack surface is modeled by using continuous quadratic boundary elements. The traction boundary contour equation is applied for traction nodes on one of the crack surfaces and the displacement boundary contour equation is applied for displacement nodes on the opposite crack surface and noncrack boundaries. The direct calculation of the singular integrals arising in displacement BIEs is addressed. These singular integrals are accurately evaluated with potential functions. The singularity subtraction technique for determining the stress intensity factor K_I, K_II and the T-term are developed for mixed mode conditions. Some two-dimensional examples are presented and numerical results obtained by this approach are in very good agreement with the results of the previous papers.

DOI: 10.1023/A:1007561523565
Print publication date: 8/1/1998
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by Hofinger, Ines; Oechsner, Matthias; Bahr, Hans-Achim; Swain, Michael V.

A well-known four-point bending test has been modified such that the critical energy release rate
$$\mathcal{G}_c $$
for delaminating cracks propagating at the interface of a thin, brittle layer bonded to a substrate can be measured. The energy release rate
$$\mathcal{G} $$
required for crack delaminating at those interfaces is obtained by attaching a stiffening layer to the layer system. Another advantage of this modification is that segmentation of the layer and plastic deformation of the substrate during bending are avoided. The interface fracture energy
$$\mathcal{G}_c $$
of a plasma sprayed ZrO₂-ceramic layer on a flame sprayed high alloyed steel substrate has been measured.

DOI: 10.1023/A:1007530932726
Print publication date: 8/1/1998
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by Du, J.; Thouless, M.D.; Yee, A.F.

The effects of a process zone on toughness and on R-curve behavior were investigated for a model, rubber-modified epoxy polymer. The system studied was one in which the bridging mechanism of toughening does not operate. The characteristic features of R-curve behavior, a rise in toughness with crack extension until an approximate steady-state is reached, were observed using double-cantilever-beam tests. The evolution of the process zone was studied using transmission-optical microscopy. As the crack grew, the process zone appeared to fan out until it reached a steady-state thickness; it then remained a uniform size upon further crack advance. The features of the experimental R-curves were shown to be directly correlated to the evolution of the process zone. Furthermore, the effect of the portion of the process zone in the crack wake was examined by a series of experiments in which the wake was partially removed, and the R-curve re-established by subsequent loading. These experiments demonstrated that removal of the crack wake caused the crack-growth resistance to drop. The toughness then built back up to the steady-state value as the crack wake re-developed. This unambiguously demonstrated a contribution to toughening from the crack wake despite the absence of any observable bridging mechanism. These results support the accepted notion that an extrinsic toughening mechanism is responsible for the increased toughness observed upon adding rubber particles to an epoxy matrix

DOI: 10.1023/A:1007530801531
Print publication date: 8/1/1998
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by Keat, William D.; Larson, Michael C.; Verges, Melody Arthur

A procedure is presented which is well suited for three-dimensional subsurface crack identification in a half-space through the inversion of measured surface displacements. The investigation began with the linear, forward problem of generating contour maps of surface deformation produced by a fracture of known geometry and loading which is embedded in a finite medium. The fundamental solutions for tensile and shear multipoles in a half-space provided an efficient mathematical representation of the three-dimensional fracture. The inverse problem of crack identification centers on the development of a hybrid of the Marquardt–Levenberg algorithm. Initial guesses for the constrained set of search variables were determined heuristically from the correspondences between crack geometry and loading and the resulting uplift at the free surface. Physical measurements of surface deformation were taken for a cube of transparent acrylic polyester in which a fracture was hydraulically pressurized. Displacements induced at the surface of the specimen, which were measured by laser interferometry, had a strong correlation with predictions of the computational model (coupled with a finite element discretization). Numerical tests demonstrate the robustness of the inverse methodology even in the presence of the random and systematic errors corresponding to the experimental interferometric measurements.

DOI: 10.1023/A:1007518209688
Print publication date: 8/1/1998
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by Yan, Cheng; Mai, Yiu-Wing

The growth of a single cylindrical hole ahead of a blunt crack tip was studied using large deformation finite element analysis in three-point bend specimens with different precrack depth. The effect of small second phase particles was taken into account by incorporating Gurson’s constitutive equation. The effects of strain hardening and the initial distance from the hole to the crack tip were also investigated. The results show that the variation of crack tip opening displacement with load is not sensitive to constraint level. The effects of constraint on the growth of hole and ductile initiation toughness are diminished with decreasing initial distance from the hole to the blunt crack tip.

DOI: 10.1023/A:1007404319651
Print publication date: 8/1/1998
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by Bobet, Antonio; Einstein, Herbert H.

The crack pattern, as well as crack initiation, -propagation and -coalescence observed in experiments on gypsum specimens with pre-existing fractures in uniaxial, biaxial, and tensile loading are satisfactorily predicted with the numerical model presented in this paper. This was achieved with a new stress-based crack initiation criterion which is incorporated in FROCK, a Hybridized Indirect Boundary Element method first developed by Chan et al. (1990). The basic formulation of FROCK is described, and the code verified for both open and closed pre-existing fractures either with only friction or with friction and cohesion. The new initiation criterion requires only three material properties: σ_crit, the critical strength of the material in tension; τ_crit, the critical strength of the material in shear; r₀, the size of the plastic zone. The three parameters can be determined with the results from only one test. Predictions using this model are compared with experiments on gypsum specimens with pre-existing fractures loaded in uniaxial and biaxial compression performed by the authors. Specifically, wing crack and shear crack initiation, crack propagation, coalescence stress and -type as well as the crack pattern up to coalescence can be modeled. The model can also duplicate experimental results in compression and tension obtained by other researchers. These results show that stress-based criteria can be effectively used in modeling crack initiation and crack coalescence.

DOI: 10.1023/A:1007460316400
Print publication date: 8/1/1998
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by Wan, Kai-Tak; Lim, Soon-Chong

The mechanical behavior of a thin blistering film under a uniform pressure changes from a bending to a stretching membrane as the thickness and flexural rigidity decrease. An analytical solution is found for the elastic response and the strain energy release rate based on an average membrane stress approximation.

DOI: 10.1023/A:1007612016147
Print publication date: 8/1/1998
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by Ryvkin, Michael; Nuller, Boris

It is shown that the previously developed model for the direct cutting of cellular plane allows to investigate also the more general case of 3D oblique cutting after replacing the coefficient of friction at the cutter faces by its effective value. The numerical results illustrate the advantages and disadvantages of the oblique cutting regime.

DOI: 10.1023/A:1007603815239
Print publication date: 8/1/1998
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