Maximum Interfacial Shear Stress Research Articles

Interfacial shear stress distribution in model composites: the effect of fibre modulus

The micromechanics of reinforcement have been investigated for a continuous intermediate-modulus (IM) carbon fibre embedded in an epoxy resin (MY-750). The embedded single-fibre (fragmentation) geometry was employed as the loading configuration. A laser Raman spectroscopic method was used to obtain the fibre strain distribution along the embedded fibre fragments, at various levels of applied strain. The interfacial shear stress distribution along the fibre was derived through a balance of forces analysis. A number of parameters, such as the maximum interfacial shear stress at each level of applied strain and the fibre debonded length, were evaluated. The maximum interfacial shear stress of the IM fibre system was found to increase by 80%, compared with the high-modulus fibre system examined previously, while the distance from the fibre end where the interfacial shear stress maximizes was significantly shorter. The debonded length was found to increase only marginally up to an applied strain of 1.8%, followed by a dramatic rate of increase between 1.8% and 2.5% of applied strain.

Embedded-end debonding during fiber pull-out

Abstract Interfacial debonding at the embedded end rather than at the loaded end of the fiber during fiber pull-out is considered in the present study. When the embedded-end face of the fiber is not bonded to the matrix, the local maximum interfacial shear stress occurs not only at the loaded end but also at the embedded end. Specifically when both the Young's modulus ratio and the radius ratio of the fiber to the matrix are large, the interfacial shear stress at the embedded end can exceed that at the loaded end, and this results in embedded-end debonding. The calculated interfacial shear stresses at the embedded end based on different models are compared and the criterion for embedded-end debonding is described.

Comparison of interphase models for a crack in fiber reinforced composite

The influence of a nonhomogeneous interphase on fracture mechanics of a fiber reinforced composite is studied. The stress intensity factor at the crack tips, maximum interfacial shear and normal stresses, maximum cleavage stress in the matrix and load diffusion along the length of the fiber are studied as a function of the fiber width, the interphase thickness, and the relative stiffness properties of the fiber, the matrix and the interphase. The normal stresses at the interface, which represents the possibility of debonding of the interface, is lowest for interphase thicknesses of the order of one-tenth of the fiber-diameter, when the crack is in the stiffer material. These normal stresses are highest at such interphase thicknesses if the crack is in the less stiffer material. The results obtained by using the nonhomogeneous interphase model are also compared with five other interphase models used in the literature for the interphase, namely the perfect, the homogeneous, the distributed uncoupled shear and normal springs, and the distributed shear springs. It is found that the trends of the above parameters as a function of interphase thicknesses are different for the spring and continuum models, if the crack is in a stiffer material.

Interfacial Shear Stress Distribution in Model Composites Part 2: Fragmentation Studies on Carbon Fibre/Epoxy Systems

The micromechanics of reinforcement of a model composite system con sisting of a continuous high-modulus (HM) carbon fibre embedded in an epoxy resin have been investigated. The composite was subjected to incremental tensile loading up to full fibre fragmentation, while the strain in the fibre was monitored at each level of load using a laser Raman spectroscopic (LRS) technique. The average strain in the fibre increased linearly with applied matrix strain up to a value of 0.8 %, when the first fibre fracture oc curred. After fracture, the strain in the fibre was found to build from the tips of the fibre breaks, reaching a maximum value in the middle of each fragment. The shape of the load transfer profiles at the locality of the fibre tips indicated that the stress transfer efficiency had been affected by the fracture process. The length of interfacial debonding at the point of fibre fracture was found to be driven by the strain energy of the fractured fragments. The interfacial shear stress (ISS) distributions at various levels of applied load along in dividual fragments, have been derived from the load transfer profiles using a balance of forces analysis. The shape of the ISS profiles confirmed that interfacial debonding initiated from the tips of the fibre breaks, whereas good fibre/matrix adhesion was retained around the mid-length of each fragment. By increasing the applied strain to 1.8%, the maximum ISS values also increased in spite of the presence of debonding at the fibre tips. An upper ISS limit of 42 MPa was calculated at this point. Further increases of the applied strain to 5% resulted in significant reductions in the values of the maximum ISS, as well as an in crease, of the frictional slip towards the middle of each fragment. Finally, by employing the assumptions of the conventional fragmentation test, the calculated value of the nominal interfacial shear strength at the point of full fragmentation was lower by a factor of 2 than the value measured by the LRS method.

Interfacial studies on carbon/thermoplastic model composites using laser Raman spectroscopy

Interfacial studies were carried out on a model composite system consisting of a short carbon fibre embedded in a polycarbonate matrix. While the composite was being strained, the local strain along the fibre was monitored using a Raman spectroscopic technique. The residual compressive strain in the fibre due to fabrication was found to be −0.45%. Subsequent loading of the composite up to 0.55% in tension resulted in a complex stress field consisting of tension at the fibre ends and compression in the middle of the fibre. The fibre strain at different levels of applied load was converted to interfacial shear stress (ISS) distribution along the fibre by employing a simple equilibrium analysis. The shape of the ISS profiles indicated a predominantly frictional type of load transfer from the matrix to the fibre. Finally, the maximum ISS value of 15 MPa was found to be unaffected by the amount of strain experienced by the composite.

Interfacial Mechanical Properties of Fiber-Reinforced Ceramic Composites

The interfacial properties such as interfacial shear strength, residual clamping stress and coefficient of friction of SiC fiber-reinforced borosilicate glass composites were evaluated using a single-fiber pull-out test. Maximum interfacial shear stress was adopted as an interfacial debond criterion and Poisson's effect of the fiber due to the applied stress was considered in the present evaluation. During the single-fiber pull-out test, AE signal was measured to understand the relationship between load-time curve and interfacial debonding behavior. Interfacial debonding behavior was classified into two types that are with and without stress transfer at the debonded interface. When the shrinkage of the fiber induced by the applied tensile stress overcomes the clamp due to interfacial residual radial stress, abrupt interfacial fracture occurs. Critical condition for these behavior was derived and influence of interfacial debonding behavior on the fiber bridging mechanism was discussed.

Interfacial Shear and Peel Stresses in Multilayered Thin Stacks Subjected to Uniform Thermal Loading

An analytical approach has been developed to evaluate the interfacial shear and peel stresses in multilayered thin stacks subjected to uniform temperature variation. The approach, which is based on an extension of Suhir’s bimetal thermostat model, provides a system of coupled linear second order differential equations used in solving for the interfacial stresses. Once these stresses have been determined, the normal stress in each layer and the deflection of the stack can be readily obtained. Two numerical examples are used to demonstrate the capability of the approach. The first example deals with a five-layered symmetric double-shear solder joint. The results are compared to a nonsymmetric three-layered solder joint to study the effect of bending and the interactions of material properties. One of the interesting features observed is that the maximum interfacial shear stress does not necessarily occur at the edge. The other numerical example is a four-layered transistor stack mounted on a substrate. The structural behavior is analyzed, and the effect of geometric dimension is examined. The present approach is shown to provide more accurate results than those of others based on the same assumptions. In addition, applications of the approach to more general stack configurations are also discussed.

Characterizing the fibre/matrix interface of carbon fibre-reinforced composites using a single fibre pull-out test

A single fibre pull-out technique has been used to determine the interfacial bond strength of carbon fibres embedded in epoxy resin. A method is presented whereby the maximum interfacial bond shear stress may be evaluated from the simple measurement of the pull-out force and fibre dimensions. Additional information may also be obtained concerning the interface morphology. The results obtained demonstrate the feasibility of the method and good agreement between theory and experiment.

Intrinsic Material Limitations in Using Interphase Modification to Alter Fiber-Matrix Adhesion in Composite Materials.

AbstractA very large percentage of studies seeking to improve fiber-matrix adhesion to alter composite properties are directed at the formation of primary chemical bonds between the reinforcement surface and the matrix. The dynamic events that occur when the fiber-matrix interface is formed lead to the creation of an interphase which can have properties quite different from the matrix in addition to any chemical bond formation.This study has been directed at elucidating the role of these interphase properties themselves on fiber-matrix adhesion. A reinforcement (AS-4 carbon fiber) with a quantifiable surface chemistry and an epoxy matrix have been kept constant through a series of experiments in which the distance between crosslinks of the matrix has been changed. The wettability of the fiber and the degree of chemical bonding to the fiber have not changed but the interfacial shear strength measured for each of these systems has decreased with decreasing matrix modulus. It will be shown that the properties of the matrix and the residual stresses created during processing limit the maximum interfacial shear stress that can be supported by the fiber-matrix interphase.

A phenomenological theory of interface shear stresses generated by hydrostatic pressure in martensitic materials

The application of hydrostatic pressure to martensitic materials generates a shear stress at the martensite-austenite interface due to the different elastic properties of the martensite and austenite phases. A closed-form solution for the maximum interface shear stress is developed for a single martensite plate in an infinite austenite matrix. The magnitude of this interface shear stress is a function of the applied hydrostatic pressure, the geometry of the martensite plate inclusion, and the elastic properties of the phases. Interface shear stresses are calculated and compared to hydrostatic pressure data from 0 to 2.3 kbar. It is found that the interface shear stress is of the correct order of magnitude, when combined with the pressure-volume-change term, to account for the large depression of the austenite start temperature with pressure (−30°C/kbar). The shear modulus and Poisson's ratio are reported for the temperature range of 22°–350°C for both the austenite and a 95 per cent martensite specimens.