Pure Shear Stress State Research Articles

A shear test method to measure shear strength of metallic materials and solder joints using small specimens

The asymmetrical four-point bend (AFPB) shear test, which is the subject of this paper, was originally developed by Slepetz et al. to evaluate the shear properties of large metallic specimens and polymer composites. Similar to the Iosipescu test, the AFPB test also provides a nearly pure-shear stress state with an unnotched specimen. However, since the AFPB test involves point loading as opposed to distributed surface loading in the case of the Iosipescu test, the shear force generated in this method depends on the fixture dimensions. Therefore, its use has been very limited. Recently, it was shown that this method could be used to evaluate high temperature shear properties of small ceramic composite joints. Thus, as an extension to the earlier aforementioned work, the goal of this study was to investigate the application of AFPB test method to small metallic test specimens. In this work, the Iosipescu shear test was also used to check the accuracy of results from the AFPB tests. The stress analysis of this test method by finite element modeling (FEM) will be published elsewhere.

３軸応力下における接着層の降伏および破壊挙動

Stress states in adhesive layer of bonded joints are usually three-axial. However, yield and failure behavior under triaxial stress conditions have yet been clarified sufficiently. Two types of adhesive joints are available for investigating yield and fracture characteristics. One is scarf joint which has considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied by changing scarf angle. Another is butt joint with thin wall tube in which considerably uniform pure shear stress state in the adhesive layer can be realized under torsional load condition. Furthermore, stress triaxiality which appeared in the adhesive layer of most of adhesive bonded joint can be reproduced by tensile tests of the scarf joint and torsional tests of the butt joint with thin wall tube. In this study, the effects of stress triaxiality on yield and failure stress were investigated by above mentioned tests using three kinds of adhesive with various ductility. The results showed that the yielding and failure criterion was changed by stress triaxiality and that the fracture mechanism of adhesive layer with homogeneous adhesive was different from the one with heterogeneous adhesive. Based on these experimental results, the method to predict the yielding and failure criteria were proposed by using the triaxiality parameter.

Micromechanics of shear deformations in cracked bonded joints

Experiments and analytical analysis were carried out to elucidate the process of crack propagation in adhesively bonded joints loaded in mode II. The adhesive used was a toughened epoxy resin, with the bond thickness varying from a few micrometers to 0.6mm. The development of a plastic deformation zone at the crack tip was monitored in real-time using a high-magnification video camera. Within the plastic zone the adhesive shear strain, determined from scratch marks applied to the specimen edge, was uniform across the bond except for several bond thicknesses long region just ahead of the crack tip where, depending on bond thickness, noticeable strain gradients may develop. The experimental results suggest that the critical shear strain at the crack tip is a viable fracture criterion. A simplified analysis for the cracked bond which is based on the technical theory of beams/plates and which considers nonlinear adhesive behavior was developed. The model prediction for the increase in the plastic deformation zone with load and the distribution of shear strain within the zone agreed well with the experimental results. An expression for the energy dissipated by the advancing crack was derived which accounted for the nonlinearity in the load vs. deflection curve observed in the fracture experiments and allowed Gnc to be calculated from easily measurable test parameters. Considerable work has been done to determine the evolution of damage at the crack tip in engineering materials, and the parameters controlling crack propagation. The majority of these works employed monolithic specimens loaded in mode I. Unfortunately, such test configuration gives rise to a number of complex phenomena, including surface or free-edge effects (i.e. the observed surface deformations may greatly depart from the deformations in the specimen's interior), a three-dimensional stress field within the damage zone, and large strain gradients at the crack tip which may well exceed the resolution capability of the detecting apparatus. To date, no universally accepted criterion seems to have emerged for predicting the material resistance to crack propagation. In this work, a fracture approach based on testing the material under severe spatial constraint and a pure shear stress state was developed in order to remove some of the aforementioned difficulties. The End-Notch Flexure (ENF) adhesive bond specimen shown in Fig. 1 was used here to monitor the deformation process at the crack tip. This test, originally developed for studying

A Bilinear Constitutive Model for Isotropic Bimodulus Materials

Proper formulation of stress-strain relations, particularly in tension-compression situations for isotropic biomodulus materials, is an unresolved problem. Ambartsumyan’s model [8] and Jones’ weighted compliance matrix model [9] do not satisfy the principle of coordinate invariance. Shapiro’s first stress invariant model [10] is too simple a model to describe the behavior of real materials. In fact, Rigbi [13] has raised a question about the compatibility of bimodularity with isotropy in a solid. Medri [2] has opined that linear principal strain-principal stress relations are fictitious, and warned that the bilinear approximation of uniaxial stress-strain behavior leads to ill-working bimodulus material model under combined loading. In the present work, a general bilinear constitutive model has been presented and described in biaxial principal stress plane with zonewise linear principal strain-principal stress relations. Elastic coefficients in the model are characterized based on the signs of (i) principal stresses, (ii) principal strains, and (iii) on the value of strain energy component ratio ER greater than or less than unity. The last criterion is used in tension-compression and compression-tension situations to account for different shear moduli in pure shear stress and pure shear strain states as well as unequal cross compliances.

A Model for the Diffusion of Moisture in Adhesive Joints. Part II: Experimental

Abstract The concentration dependence and temperature dependence of the diffusion coefficient, as discussed in Part I, are validated with literature data on poly(styrene) and on poly(vinylacetate). The effect on diffusivity, of a uniaxial tensile stress state and of a biaxial tensile stress state, is measured with permeation tests on stretched poly(ethyleneterephthalate) (PET) films. The influence of semi-crystallinity is briefly discussed. Further, diffusivity measurements under a tensile stress state, under a compressive stress state, and under a pure shear stress state are performed on Ultem® polyimide films, using a modified sorption technique. Good agreement between theoretical predictions and experiment is found. Finally, predictions by the solubility model discussed in Part I are compared with data on low density polyethylene and on Ultem polyimide.

先端複合材料パネルの面内せん断試験法の研究

The mechanical in-plane shear testing method for advanced composite panels has not yet been authorized internationally because of its difficulty to present the uniform shear stress field and the reliable shear properties. In the present paper, the several revisions of the traditional picture-frame loading fixture are proposed to use for the in-plane shear testing method. The experiments on unidirectional and cloth-reinforced CFRP specimens and the numerical analyses by the use of FEM were performed in order to verify the advantages of these various revisions. As a result, the uniform pure shear stress state and hence the reliable higher shear strengths were obtained, because there is no large stress concentrations along edges and at corners, compared with other testing methods which used the traditional simple picture-frame fixture, two-rail or three-rail shear fixtures and JIS method. That is, in these shear tests which are used most widely at present, the lower shear strengths were obtained because of the premature failure due to stress concentration and nonuniform stress distribution. Further, it seemed that JIS method for GFRP did not give a shear fracture under combined stress conditions in unidirectional composites.

An Analysis of the Entropy of Thermally Activated Dislocation Motion Based on the Theory of Thermoelasticity

AbstractAn analysis is made of the entropy of thermally activated dislocation motion using the continuum theory of thermoelasticity. A relation for the activation entropy, which is valid for non‐linear, anisotropic elastic solids is derived. The activation entropy is expressed in terms of the general stress state in the solid and is found to be linear in the first temperature derivative of the elastic compliances, a relation often applied to the special case of linear, isotropic elasticity, but not previously derived for the more general case treated here. It is shown that, under the special conditions where pure shear or pure hydrostatic stress states are associated with both the initial and activated states, the present result can be reduced to the one often found in the past for the case of linear isotropic elasticity. However, these special conditions are never satisfied either in the thermally activated motion of dislocations or in diffusion. This suggests that the previous result is not applicable to these situations.

Influence of a tension stress field introduced in the elastohydrodynamic contact zone on rolling contact fatigue

The rolling contact fatigue life of modified NU 209 roller bearing inner races subjected to tensile stress mechanically induced in the contact zone, has been investigated using the four-contact fatigue machine. A high tensile stress of 80 MN/m 2 by a clamp pressure, acting on the inner surface of the specimen, has a catastrophic effect on fatigue life; all specimens burst earlier than pitting failure would have occurred at a near zero tensile stress level. A theoretical study of the influence of the axi-symmetrical stress field, induced by the steady pressure applied inside the hollow rotating cylinder on the stress distribution in the e.h.d. contact zone of cylinders showed a significant increase of the resultant shear stresses caused by tension. The pure shear stress state may exist in the contact zone when the tensile stress level is high.