Theoretical, experimental and computational mechanics of fracture in constrained interlayers

The constraint of a thin silver interlayer is used to create high triaxial stresses to evaluate the applicability of theoretical models for ductile fracture. Rice and Tracey's model for cavity expansion under high triaxial states of stress and Huanget al.'s model for cavity instability were considered. The experimentally determined _m / _y values suggest that further investigation of the Huanget al. theory is warranted. Microstructural analysis revealed that multiple cavities were initially present in the silver interlayers, and the number and size of the cavities increased as failure was approached. Finite element analysis and experimental results showed excellent agreement in a computational determination of cavity instability. Thus, it appears that ductile fracture in constrained thin interlayers can be explained with unstable cavity growth.     

Elastic-plastic fracture of an orthotropic plate with a crack under biaxial loading

Institute of Mechanics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Prikladnaya Mekhanika, Vol. 26, No. 8, pp. 93–99, August, 1990.     

Elastoplastic fracture of an isotropic plate with a crack under biaxial loading

Institute of Mechanics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Prikladnaya Mekhanika, Vol. 25, No. 7, pp. 86–91, July, 1989.     

Crack growth in unidirectional graphite-epoxy under biaxial loading

Center-notched coupons of 16-ply unidirectional AS4/3501-6 graphite-epoxy were loaded in tension to failure in order to determine the direction of initial crack growth, the load required to cause initial crack growth, and the load required to cause failure. Fifteen-degree off-axis tensile coupons provided a far-field biaxial state of stress away from the notch. Specimens of three different length-to-width ratios were tested in order to vary the biaxial stress states. Two notch configurations were used, one perpendicular to the loading direction, the other perpendicular to the material fiber direction. In all of the cases studied experimentally, crack growth was parallel to the fiber direction. An anisotropicelasticity crack-tip stress analysis was subsequently implemented in order to compare experimental crack-growth directions to those predicted by three crack-extension-direction criteria. Of the three criteria studied, (Strain-energy density, the tensor polynomial, and the normal stress ratio), only one, the normal-stress-ratio criterion, provided crack-growth-direction predictions that agreed with the experimental results. Paper was presented at the 1985 SEM Spring Conference on Experimental Mechanics, held in Las Vegas, NV on June 9–14.     

Dynamic crack curving and branching under biaxial loading

A 16-spark-gap camera was used to record the dynamic photoelastic patterns of ten centrally cracked, Homalite-100 specimens which fractured under ten initial biaxial-stress ratios ranging from 3.7 to 0, some of which do not exist in normal fracture specimens. The dynamic photoelastic patterns of curved cracks were used to verify the previously developed dynamic-crack-curving criterion. Cracks which immediately curved upon propagation in three specimens under abnormally high inital biaxial loading were used to verify the static counterpart of the dynamic-crack-curving criterion under these extreme loading conditions. A previously developed dynamic-crack-branching criterion was also verified by four dynamic photoelastic results involving cracks which eventually branched under the lower initial biaxial loading.     

A piezoelectric biaxial loading device for interfacial fracture experiments

In this paper we describe the development of a new biaxial loading device for investigating mixed-mode fracture at bimaterial interfaces. The new device makes use of piezoelectric actuators and specially arranged flexures to provide independent displacements normal and tangential to the interface. Capacitive probes and special washer load cells were used for measuring the displacements and reactive loads, respectively. A closed-loop circuit was formed with a personal computer to control the applied displacements to within 10 nm. Preliminary experiments with quartz/epoxy/aluminum sandwich specimens with cracks growing between the quartz and the epoxy found that the intrinsic toughness of this interface was 30% lower than the value for a glass/epoxy interface. Crack opening interferometry measurements having a resolution of 30 nm revealed the presence of a cohesive zone whose size was about 0.5 μm.     

Stability of layered coatings under biaxial thermomechanical loading

The three-dimensional problem of stability of layered coatings at normal and high temperatures and small precritical strains is formulated and solved and the characteristic equations are derived. The temperature dependence of the physical and mechanical parameters of specific layered coatings is established experimentally. Three-layer coatings on a homogeneous substrate at different temperatures are considered as an example. Recommendations for establishing the optimal service conditions for layered structural members with coatings are formulated.     

Plastic buckling of cylindrical shells under biaxial loading

The predictions for plastic buckling of shells are significantly affected by the plasticity model employed, in particular in the case of nonproportional loading. A series of experiments on plastic buckling of cylindrical aluminum alloy shells under biaxial loading (external pressure and axial tension), with well-defined loading and boundary conditions, was therefore carried out to provide experimental data for evaluation of the suitability of different, plasticity models. In the experiments, initial imperfections and their growth under load were measured and special attention was paid to buckling detection and load path control. The Southwell plot was applied with success to smooth the results. The results show that axial tension decreases resistance to buckling under external pressure in the plastic region due to softening of the material behavior. Comparison with numerical calculations usingJ ₂ deformation and incremental theories indicate that both theories do not predict correctly plastic buckling under nonproportional loading.Babcock (SEM Member), deceased, was Professor of Aeronautics and Applied Mechanics, California Institute of Technology, Pasadena, CA 91125.     

Instability analysis of non-homogeneous materials under biaxial loading

In this paper, we formulate the instability problem of non-homogeneous materials under biaxial loading, in the sense that the mechanical parameters, and more specifically the strain hardening or softening and the strain-rate sensitivity, are spatially dependent functions. We first study the time behavior of the process and present the evolution of strain non-uniformities before the critical time, to show that it depends on the interplay between the spatial distribution of mechanical parameters and the initial non-uniformities. We next study the instability modes of a material exhibiting inhomogeneities along one direction by an effective instability analysis [Dudzinski, D., Molinari, A., 1991. Perturbation analysis of thermoviscoplastic instabilities in biaxial loading. International Journal of Solids and Structures 27 (5), 601–628] adapted to the non-homogeneous case. This method is based on a suitable non-uniform reference solution and allows to select the localization modes activated at different deformation levels for different “deformation paths” of the material zones.     

Plane-stress fracture testing of finite sheets under biaxial loads

A procedure which combines the Williams series-type stress- and displacement-field expressions at the crack-tip neighborhood with a suitable numerical scheme away from the crack-tip was employed in the determination of the plane-stress fracture properties of four finite 7076-T6 aluminum sheets containing cracks emanating from a circular hole under four biaxial loads. The compatibility of the analytical and numerical displacements at the nodal points along the boundary of the crack-tip neighborhood was utilized in formulating displacement-continuity expressions containing some undetermined constants which solution depends on the nature of the boundary loading conditions. By linear superposition of the displacement due to remote uniaxial load and the displacements due to remotely applied transverse load in the neighborhood of the crack-tip, biaxial-displacement-continuity expressions containing these important fracture properties—namely, the opening Mode I stress-intensity factorK, the nonsingular stress term associated with the stresses in the direction parallel to the plane of cracksA and the integration termB associated with the displacement in this direction—were evaluated. Because no known biaxial testing of this geometry had been reported prior to this research, the analytical procedure was used to select the optimum geometry required in a biaxial fracture test of a finite-sheet specimen containing cracks emanating from a circular hole. This geometric optimization of the specimen guaranteed uniformity of stress all over the volume of specimen and also made the alteration of the existing MTS test fixtures unnecessary. Four square sheets of 7075-T6 aluminum alloy containing a central hole with two collinear cracks emanating radially at the edge of the hole were then fabricated in accordance with the analytically determined geometric requirements. The biaxial fracture test was then conducted under four biaxial load factors (λ) of 0.0, 0.5, 1.0 and 1.5. The fracture toughness obtained in this research was compared with those reported for uniaxial loading of large panels. It was found that there is a good correlation between the reported fracture toughness and this work.     

A simple testing technique for fracture under biaxial stresses

Evidence is accumulating to suggest that the fracture toughness of and cyclic crack-propagation rates in a material may be affected by stresses acting parallel to the crack plane. This effect contradicts the justifiable assumption, implicit in fracture-mechanics theory, that only loads causing a stress singularity at the tip of a crack can affect its behavior. More extensive investigation of this important problem involves the development of special testing equipment and specimens. This article offers a simple design for such a system, which has proved in practice to be highly reliable and of adequate accuracy. Preliminary tests on polymethylmethacrylate (PMMA) under biaxial fatigue and ramp loading are described, to demonstrate the technique itself and the phenomena under investigation. The results suggest that, for this material at least, the effects of transverse stresses are indeed slight.     

Contact mechanics of shell structures under local loading

The results of theoretical and experimental studies of contact interaction in frame-and-shell structures under local loading are analyzed. Contact problems for elements of frame-and-shell structures and structures with various foundations such as supports are solved. Critical states (local instability and ultimate plasticity) of frame-and-shell structures under local loading are examined. Numerous experimental results are presented     

Behavior of graphite/epoxy plates with holes under biaxial loading

An experimental investigation was conducted to study the behavior under biaxial tensile loading of quasiisotropic graphite/epoxy plates with circular holes and to determine the influence of hole diameter on failure. The specimens were 40 cm×40 cm (16 in.×16 in.) laminates of [0/±45/90] _s layup. Four hole diameters, 2.54 cm (1.00 in.), 1.91 cm (0.75 in.), 1.27 cm (0.50 in.) and 0.64 cm (0.25 in.), were investigated. Deformations and strains were measured using strain gages and birefringent coatings. Equal biaxial loading was introduced by means fo four whiffle-tree grip linkages and controlled with a servohyraulic system. Initially, the circumferential strain is uniform around the boundary of the hole. Subsequently, with increasing load, regions of high strain concentration with nonlinear response develop at eight characteristic locations 22.5 deg off the fiber axes. Failure in the form of cracking and delamination initiates at these points. Maximum strains at failure on the hole boundary reach values up to twice the ultimate strain of the unnotched laminate. The effect of hole diameter on strength was described satisfactorily using an average biaxial-stress criterion. Good correlation was also obtained with theoretical predictions based on a tensor-polynomial failure criterion for the lamina and a progressive degradation model.     

Finite strip buckling analysis of curved plate assemblies under biaxial loading

A finite strip method is presented for calculating the linear buckling stresses of structural assemblies of long, thin plate components which, in general, are curved and which are rigidly joined together at their longitudinal edges. It is assumed that on buckling under the action of a biaxial direct stress field the perturbation forces and displacements all vary sinusoidally in the longitudinal direction. A stiffness matrix relating the amplitudes of the perturbation forces and displacements is developed for the curved strip on the further assumption of relatively high-order polynomial variations of the displacement components around the plate width. Numerical results are presented of the application of the curved strip in calculating the buckling stresses of plates, cylinders, panels and formed sections.     

Fracture of brittle geomaterial with a circular hole under biaxial loading

The influence of boundary conditions on the fracture of brittle geomaterial in the stress concentration zone under biaxial loading with account for the size effect is theoretically and experimentally studied. The calculation results are compared with the experimental data.     

An experimental investigation of plastic flow under biaxial stress

Due to the extreme complexity of the problem, it has been necessary to formulate theories of plasticity which grossly simplify the material behavior. While, in many cases, these theories give satisfactory results, there are other cases in which they fail. Therefore an investigation has been initiated with the purpose of evaluating presently used theories and assessing the possibilities for improvement for the cases in which the classical theories are not satisfactory. The present paper describes a system for testing, which includes a data-acquisition system and a computer program for comparisons between test and theory. A few test results are included which verify the adequacy of the system and illustrate some facets of the problem.     

On dynamical fracture mechanics in the case of polyharmonic loading by SH-waves

We solve a fracture-mechanics problem for a cracked material under polyharmonic loading (waves of various lengths propagating through an elastic material). The frictional contact interaction of the finite crack edges in a plane is analyzed for the case of normal incidence of two harmonic shear waves with different frequencies. The forces of contact interaction and displacement discontinuity are determined. The effect of the wave frequency on the stress intensity factor for different normalized wave numbers is examined