Stress Concentrations In Composites Research Articles

Smart Materials Based on a High and Low Temperatures SME-Alloys

The analysis of the nature of low plasticity of composites with strengthening of hard phases are shown that plastic shear constraint in thin interlayer of the binding phase does not provide the small scale of the structural level of deformation. As a result, a material undergoes a brittle fracture. Therefore, one should find another mechanism for relaxation of internal stress concentrations in composites during loading.The aim of a present paper is to illustrate the concept of how use a shape memory alloys with phase transformation for metal matrix composites. It has been studied shape memory based alloys and deformation and fracture of MMC with phase transformations in matrix.It have been shown that if plastic deformation is considered as local structural transformation and instability in the material is provided from the very beginning, this will result in a general increase of plasticity due to a decrease in the scale of the structural level of plastic deformation.Deformation tests of composites near the phase transition temperature show that there are different transformations induced by a highly non-uniform stress state of the binding phase. Under loading the “quasi-amorphous state” is formed in the binding phase after formation of a microcrystalline, highly misoriented structure with the characteristic size of crystallites less than 10 nm. This structure has high plasticity and strong hardening. It governs an efficient transfer of external load to solid particles, inducing dislocation glide even in typically brittle titanium carbide particles during non-uniform loading.

A micromechanical study of stress concentrations in composites

Random and periodic representations of composite microstructures are inherently different both in terms of the resultant range of stresses that each phase carries as well as the total load over the entire volume comprising both matrix and fiber phases. In this study, an algorithm was developed to generate random representative volume elements (RVE) with varying volume fractions and minimum distances between fibers. The random microstructures were analyzed using finite element models (FEM) and the results compared to those for periodic microstructured RVEs in terms of the range of stress values, maximum stress, and homogenized stiffness values. Using a large number of random RVE analyses, a meaningful estimation for range and average maximum stress in the matrix phase was achieved. Results show that random microstructures exhibit a much larger range of stress values than periodic microstructures, resulting in an uneven distribution of load and distinct areas of high and low stress concentration in the matrix. It is shown that the maximum stress in the matrix phase, often responsible for failure initiation, is largely dependent on the random morphology, minimum distances between fibers, and volume fraction. Moreover, it is shown that the predicted overall load-carrying capacity of the matrix changes depending on the use of random or periodic microstructures.

Stress Concentrations in Composites with Microvascular Channels

Microvascular channels in fiber-reinforced composites offer various functionalities ranging from self-healing and damage monitoring, to active thermal management. However, the tradeoff between extended functionalities and mechanical performance at vascularized composites is still an issue. In this study, a three dimensional finite element model is developed to investigate the stress concentrations generated around macro-vascular channels for various channel configurations and lamination sequences. Results indicate that the stress distribution around vascular channel is same for symmetric stacking configurations spite of having different layer just above the channel and different resin pocket dimensions. The effect of changing the vascule diameter is mostly observed in UD 0 configuration.

Micro-scale finite element analysis of stress concentrations in steel fiber composites under transverse loading

Steel fibers, with their high stiffness and high ductility, have a potential to provide a new range of properties in polymer composites, in comparison with carbon and glass fiber composites. However, the high stiffness contrast between the steel fiber and the polymer matrix plus the fiber’s non-circular cross-section are likely to generate high stress concentrations in a composite under transverse loading. In the present study, these stress concentrations are analyzed using finite element modeling and compared with the case of carbon and glass fiber composites. The study is performed for an isolated fiber and multiple fibers in hexagonal and random packings with 40% and 60% of fiber volume fractions. According to the results, in spite of a high contrast between the stiffness values of steel and glass fibers, no significant difference between the transverse stress concentrations was observed for steel and glass fibers in the hexagonal packing due to the difference in material properties. Differences in stress concentrations were noted for the case of randomly packed fibers. The polygonal cross-section of steel fibers was found to introduce extreme stress concentrations.

Numerical Analysis of Brittle Crack Initiation at Stress Concentrations in Composites

As potential locations for failure initiation, stress concentrations require sophisticated analysis models. In composites, not only geometrical discontinuities may cause high local stresses, but also material discontinuities or even a combination of both. A universally applicable hybrid approach utilizing finite fracture mechanics is employed in this study to model crack initiation at stress concentrations. It overcomes deficiencies of classical criteria. The model is implemented in a versatile finite element procedure. Examples demonstrate the capability of the model to cover relevant effects for simple configurations as well as for glass-ceramic sealing joints in solid oxide fuel cells subject to thermomechanical loading.

Effects of particle arrangement on stress concentrations in composites

The effects of particle arrangement on stress concentrations within particle-reinforced composites are investigated. Both periodic and random particle arrangement models are employed to calculate stress concentration factors for a composite with a filler volume fraction of 0.3. A self-consistent scheme is used to minimize boundary effects on stress concentrations. It is found that a random particle arrangement predicts significantly greater stress concentrations than periodic particle arrangements because of particle clustering. The stress concentration factor does not have a direct relationship with filler volume fraction, and the stress concentration surrounding a particle is dominated by the nearest-neighbor particle.

Stress concentrations in composites with interface sliding, matrix stiffness and uneven fiber spacing using shear lag theory

The stress concentrations near a single fiber break in a unidirectionally reinforced fiber composite are investigated using a shear lag theory within the framework of finite elements. A model for uniformly spaced, well bonded fibers embedded in a matrix that cannot carry axial loads that was formulated previously is first introduced. The solution of this problem involves Fourier transforms and requires only a two-dimensional numerical integration. The work described in the current paper characterizes the stress concentrations around a single fiber break in the presence of fiber/matrix interface sliding, axial matrix stiffness and uneven fiber spacing. Due to the introduction of these complicating factors, the model no longer lends itself to the simple Fourier transformation solution method. For the case of interface sliding a new method is developed to handle sliding in any shear lag system. For the cases of axial matrix stiffness and uneven fiber spacing a finite element code specifically written for this problem is used to determine the fiber stresses. The results are discussed in the context of global versus local load sharing, and the effects on composite failure.

A dependent bundles model for estimating stress concentrations in fibre-matrix composites

This paper pursues the interest in characterizing stress concentrations in composites for the purpose of providing more reliable input into statistical theories about composite strength. A previous paper estimated stress concentration parameters in a fibre-matrix composite by numerical maximum likelihood, using the chain of bundles model of Harlow and Phoenix. An assumption of the model is that bundles are independent. Here, a model is proposed which allows for a dependence across bundles, usually due to the extent to which a fibre becomes unstressed in the region of a break. The model is demonstrated using experimental data on a carbon-epoxy hybrid composite, and arrays of tungsten-cored silicon carbide fibres embedded in resin. It is shown, via the method of maximum likelihood, how inferences may be made about stress concentrations and also parameters describing single fibre strength.

Introduction of the characteristic dimension for describing the scale effect under conditions of stress concentration in composites

1. Different approaches to describing the scale effect under conditions of stress concentration are based on the introduction of the characteristic dimension of the material. 2. In a number of modern carbon-fiber plastics, holes have practically the same effect on static and fatigue strength, which indicates that the characteristic dimension of the material is independent of the loading regime. 3. The assumption of “blunting” of the hole and the introduction of the characteristic dimension as the length of the zone of prefailure make it possible to calculate the dependence of the strength on the dimension and the shape of the hole.

Origin of moisture effects on crack propagation in composites

AbstractA study has been made of the origin of unexpected moisture effects on crack extension in fiberglass laminates. Water immersion has been found to greatly reduce the rate of crack growth under constant loading while increasing the rate under cyclic loading, the latter effect being the expected one. Observations were made of the extension of the stable damage zone at the tip of precut notches in wet and dry environments. The damage zone size is postulated as a critical element in the relaxation of high stress concentrations in composites, such as those at notch or crack tips. Under constant load, moisture is shown to greatly expand the interply delamination region in the damage zone, thus reducing the local fiber stresses and increasing crack resistance. Under cyclic loading moisture has little effect on the delamination region, which is large even for dry environments, and the only effect is weakening of the material and acceleration of cracks. Severe hygrothermal conditions can so weaken the material that the crack resistance is reduced under constant loading as well.