Fatigue Strength Of Composite Materials Research Articles

Evaluation of fatigue strength of carbon fiber reinforced plastics (CFRP) with various types of hybrid matrices

Polymer composite materials (PCM) have found wide application in various industries due to the ability to create low-weight products with specified operational properties. During operation, composite products are exposed to static and cyclic loads, climatic and many other factors. Evaluation of the fatigue strength of composite materials and the influence of various additives and modifiers on it is an urgent scientific and practical task. The article describes the technology of obtaining PCM with various types of hybrid matrices formed by the main binder material and the material representing an independent "liquid" phase in the composite structure. Based on the analysis of the kinetics of curing, anaerobic polymer material (Loctite 638), silicone elastomer (Unisil-9628) and synthetic wax were selected as the materials of the components of the "liquid" phase. Fatigue strength assessment tests were carried out by applying cyclically varying tension-compression loads to the samples. The load during cyclic tests was 70% of the static tensile strength of the samples. The residual strength was evaluated by testing the tensile strength of the samples until complete destruction after cyclic loading. The results of fatigue strength tests of carbon fiber plastics with various types of hybrid matrices (formed by various components of the "liquid" phase) are presented. The analysis of the results showed that the use of an anaerobic polymer material as a component of the "liquid" phase of the hybrid matrix makes it possible to increase both the initial static strength of the material (by ~ 1%) and the residual strength after cyclic loading (by ~ 11%) compared with these indicators obtained during the testing of control samples. After performing cyclic loading, carbon fiber plastics with anaerobic polymer material and silicone elastomer have an increase in residual strength compared to previously performed static tensile tests by ~ 8% and ~13%, respectively. The use of an anaerobic polymer material and silicone elastomer as a component of the "liquid" phase makes it possible to increase the modulus of elasticity of carbon fiber plastics after cyclic loading by ~ 13% and 5%, respectively, compared with the results of preliminary static tests.

Fatigue fracture criterion of composite materials

The first investigations on the fatigue strength of composite materials were carried out on cyclic tension of plastics reinforced by fiberglass. Since that time, intensive studies of fatigue strength of composite materials were begin. The experimental data obtained for different types of composite materials shown, that the fatigue fracture processes are governed by the accumulation of defects of different nature. The main mechanisms that determine the durability of composites is the kinetics of the damaged state development until the final fracture. At the same time, according to the experimental results, the damage accumulation curve of the total damage is defined by increased function of the time (number of loading cycles) until to the moment of macro fracture. In this paper, based on these studies and the conception of scattered damage and fracture, fatigue strength criterion is formulated. Fatigue criterion coefficients are concretized and damage accumulation curves are plotted, which depends on the number cycles and stress level. The comparison with experimental results on fatigue of glass-reinforced plastics and carbon fiber reinforced plastics is given.

A Fatigue-Fracture Criterion for Composite Materials

The fatigue strength of composite materials was first investigated under cyclic tension of plastics reinforced by fiberglass. Since that time, intense investigations of the fatigue strength of composite materials have begun. The experimental data obtained for different types of composite materials show that the fatigue-fracture processes are induced by the accumulation of defects of various nature. The main mechanisms for determining the durability of composites are the kinetics of development of the damaged state until final fracture. At the same time, according to the experimental results, the total-damage-accumulation curve is an increased function of time (the number of loading cycles) until the moment of macrofracture. In this study, on the basis of these investigations and the concept of scattered damage and fracture, the fatigue strength criterion is formulated. The fatigue criterion coefficients are specified, and the damage-accumulation curves are plotted in dependence on the number of cycles and the stress level. A comparison with the experimental results on the fatigue of glass-reinforced plastics and carbon-filled plastics is given.

Fuzzy approach in modeling static and fatigue strength of composite materials and structures

For composite materials and structures static strength, fatigue damage and durability demonstrate a scatter factor of results larger than for isotropic materials. To characterize it the fuzzy set approach is proposed. Two different mechanical descriptions of fatigue life are used in order to describe the uncertainty and randomness of parameters characterizing the fatigue damage and finally the fatigue durability. The theoretical predictions representing the lower and upper bounds of a fatigue life are compared with experimental data. In general, the present analysis shows that the fuzzy set description allows us to take into account much more parameters than classical deterministic or statistical methods. The scatter of the results is significantly dependent on the form of the used approximations of fatigue lives.

Investigation of Mechanical Properties and Impact Strength Depending on the Number of Fiber Layers in Glass Fiber- reinforced Polyester Matrix Composite Materials

Abstract In this study, glass fiber-reinforced composite materials were produced by hand lay-up with different number of layers of glass fiber, and tensile, flexural and impact fatigue strength of composite materials were investigated. The samples were also metallographically characterized. Depending on the number of fiber layers of samples mechanical properties and impact fatigue strength were examined. Due to the increase in the number of fiber layers, the mechanical properties of the samples were found to be improved.

Dissipated Energy of Composite Materials—Part I: Cyclic Dynamic Stress

Abstract This paper presents a method for the estimation of the vibratory fatigue strength of composite materials that is based on the accumulated dissipated energy. The accumulated dissipated energy was measured with a special measuring technique, which is able to catch quasi-continuously the mechanical properties of the tested samples. The curves of the accumulated dissipated energy were approximated with an equation that includes several parameters. These parameters were identified for experiments with different types of load and for samples with different contents of micropores. For samples that reach more than 106 load cycles, the accumulated dissipated energy in one-load-stage tests is a linear function. With this, the fatigue limit of the samples can be predicted.

The accumulated dissipated energy of composites under cyclic-dynamic stress

This paper presents a method for the estimation of the vibratory fatigue strength of composite materials, which is based on the accumulated dissipated energy. The accumulated dissipated energy was measured with a special measuring technique that is able to catch quasi-continuously the mechanical properties of the tested samples. The samples were made of glass-fiber epoxy foams reinforced with pretreated woven fabrics. The curves of the accumulated dissipated energy were approximated by an equation that includes several parameters. These parameters were identified for experiments with different types of load and for samples with different contents of micropores. In the equation for samples that reach more than 106 load cycles, the accumulated dissipated energy in one load stage tests turns out to be a linear function. With this function the fatigue limit of the samples can be predicted.

Fatigue Strength of Particulate-Reinforced Die-Cast Aluminum Alloy Composites at Room and Elevated Temperatures.

Particulate-reinforced aluminum alloy composites, using NiAl intermetallic compound and SiC ceramic particles as the reinforcement phase, were fabricated by a die-casting method. Through high cycle fatigue tests, the fatigue strength of composite materials in the as cast condition was investigated at room and elevated temperatures of 200°C and 300°C. Fatigue strengths of each material gradually decreased with increase of temperature. Composite materials showed a higher fatigue limit, as compared with that of the matrix Al alloy within the tested temperature range. The NiAl particle reinforced material showed a good fatigue limit up to 200°C. At 300°C, the fatigue strength was dominated by the matrix strength. It was found that these fatigue behaviors were associated with the fracture surface observations.

An approximate model of predicting the anisotropy of high-cycle fatigue strength of composite materials

1. We proposed a calculation-empirical model for the approximate evaluation of the anisotropy of the high-cycle fatigue strength of the CMs which takes into account simultaneously the variation of loading frequency and type of deformation. In the experimental part, the complete model requires determination of fatigue strength in respect of nonzero pulsed tensile loading and compression in the orthogonal reinforcement directions and under the angle Φ=45°, and in the simpler variant only in the preferential reinforcement direction (Φ=0°). For calculations it is necessary to obtain experimental data on the anisotropy of the limit of the short-term static strength and also long-term static strength in tension and compression in the reinforcement directions. 2. The results show that the calculated and experimental data for the woven and oriented glass fiber-reinforced plastics in pulsed tension, compression, and symmetric tension-compression in the directions Φ=15, 30, 45, and 90° are in satisfactory agreement.

Determination of the fatigue strength of composite materials under plane stress state conditions

Determination of the fatigue strength of composite materials under plane stress state conditions