Fatigue Life Of Composites Research Articles

Low Pressure Casting Process of FeCrSi/A366.0 Alloy Composites and Their Characterization

Low pressure casting process was considered for fabrication of FeCrSi metal fiber reinforced A366.0 aluminum composites. FeCrSi/A366.0 alloy composite was fabricated by applied pressure 0.8MPa. The microstructure features, tensile strength and fatigue life of composites were investigated from room temperature to high temperature. It was confirmed that the FeCrSi metal fiber did indeed have a strengthening effect on the composite, lending it good mechanical properties and a good fatigue life at high temperatures.

Cover Picture: Macromol. Mater. Eng. 8/2006

Cover: The picture on the cover shows the flex fatigue life of carbon black filled SBR with 0–5 phr nanodispersed clay and ESEM images of flexing fracture surfaces of carbon black filled SBR without clay or with 5 phr clay. Addition of clay greatly improves the flex fatigue life of composites due to the increased hysteresis and tear energy leading to the advantage of clay with high aspect ratio in blunting crack over carbon black. Further details can be found in the article by Y.‐P. Wu, W. Zhao, and L.‐Q. Zhang* on page 944.

A Simple Fatigue Life Model for Three-dimensional Fiber-Polymer Composites

A simple model is proposed for predicting the fatigue life of three-dimensional (3D) fiber-polymer composite materials for certain cyclic load conditions. The fatigue life can be easily calculated using the model with three empirically determined constants: (i) the ultimate strength of the 3D composite, (ii) the ultimate strength of the two-dimensional (2D) laminate, which is identical in every respect to the 3D composite (e.g., same fiber layup pattern, fiber volume content) except for the through-thickness reinforcement, and (iii) the slope parameter (m) of the fatigue-life (S-log N) curve for the 2D laminate. It is shown that the S-log N curves for stitched composites under cyclic tension-tension and compression-compression loading and for z-pinned composites under cyclic bending loading can be accurately predicted using this model. This model can also approximate the tensile fatigue life of composite lap joints reinforced by stitches or z-pins. However, the model is not reliable in predicting the fatigue life of 3D composites in all cases. It is only accurate when the fatigue damage mechanisms and failure mode of the 3D composite are not altered significantly by the through-thickness reinforcement. Consequently, a limitation of the model is that a priori knowledge of the fatigue damage process of the 3D composite is needed before it can be used with confidence to predict fatigue life. This problem is exasperated by the paucity of information available on the fatigue mechanisms of many types of 3D composites. Until this problem is resolved, the model should only be applied to those types of 3D composites in which their fatigue processes have been determined.

Fatigue reliability analysis of composites based on residual strength

The fatigue reliability of Gr/PEEK composites has been investigated in the present study. In the experimental investigation, constant amplitude loading, multiple-level loading and random loading have been considered step by step during the fatigue tests. In particular, a residual strength degradation model was presented for predicting the fatigue life of composites subjected to constant amplitude fatigue loading. The accumulation of fatigue damage was determined, using the residual strength degradation model. The residual strength distribution after an arbitrary fatigue cycle can be represented by a two-parameter Weibull distribution function. A fatigue reliability analysis method for arbitrary load history was thus developed through the Weibull distribution function. Good correlation between theory and experiment was obtained for constant amplitude loading, two-level loading, three-level loading and random loading. The result of the present study can be used as a basis for the reliability design of mechanical components made of Gr/PEEK composites.

Fatigue Behavior of Impacted Plain-Weave Glass/Epoxy Composites under Tensile Fatigue Loading

The goals of this paper are to identify the impact damage behavior of plain-weave E-glass/epoxy composites and predict the fatigue life of the composites with impact-induced damage under constant amplitude loading. To identify these behaviors, the low velocity impact and fatigue after impact tests are performed for glass/epoxy composites having two types of fiber orientations. The impact damage behavior is dependent on the fiber orientation of the composites. The fatigue life of the impacted composites can be identified through the prediction model, which was proposed on the carbon/epoxy laminates by authors regardless of fiber orientations.

Failure in Composite Materials due to Volumetric Expansion of Freezing Moisture

A model is developed for predicting volumetric expansion induced cracking in orthotropic composite materials due to freezing of trapped moisture in a slender rectangular flaw region. Conformal transformation and the complex function method are used to obtain the stress distribution in the matrix at the interior boundary. The stress field in the rectangular inclusion is derived by solving for two important variables characterizing the expanded equilibrium boundary determined by the principle of minimum strain energy. The compressive stress acting on the long side of the rectangular inclusion acts as a crack driving force., The model is used to predict the occurrence of cracking due to volumetric expansion of ice in a specific composite, EXTREN, that has been observed in experiments. The model can be adapted to predict fatigue life of composites under freeze-thaw conditions.

An empirical model for fatigue behavior prediction of glass fibre-reinforced plastic composites for various stress ratios and test frequencies

Current models used to predict the fatigue life of glass fibre-reinforced plastic composites do not accurately consider the effects of load stress ratios and load frequencies. These models usually require significant amount of experimental data to establish a set of characteristic fatigue curves for a given composite. This paper proposes a fatigue model for glass fibre-reinforced plastic composites that includes the non-linear effect of stress ratio and load frequency on the fatigue life. The model can be used to predict the fatigue behavior of a composite material using a well-defined minimum number of tests. Fatigue data from the literature and selected research laboratories were used to test the model. Predictions were found to be in good agreement with all experimental data adequately accounting for the influence of test frequency and stress ratio on the fatigue life of composites.

Damage mechanisms and life prediction in high temperature fatigue of a unidirectional SiC–Ti composite

Isothermal fatigue tests are performed in the longitudinal direction at 450°C on a unidirectional SiC/Ti composite. Three major damage mechanisms are identified: the matrix cyclic softening which overloads fibers leading to their progressive rupture; the interfacial degradation of these broken fibers and their oxidation by the environment. Damage kinetics are estimated using microscopic observations and acoustic emission. Finally, a micromechanical model is used in order to understand the respective influence of these damage mechanisms. It is shown that, at a sufficient load level, fatigue fracture of the composite strongly depends on the interfacial degradation kinetics and that fiber oxidation by the environment generates more progressive damage and considerably reduces the composite fatigue life compared to loading under vacuum.

Effects of truncation and elimination on composite fatigue life

An experimental investigation has been carried out regarding load-sequence effects on the fatigue life of composite structures. Different elimination levels were considered for various load spectra and for constant amplitude block loading. The spectra considered were an early design spectrum for the Gripen fighter aircraft, spectra associated with the aft fitting of the fin and to the upper fitting of the wing, and the compression dominated spectrum “Short Inverted Falstaff”. Both experimental and calculated results show, that the elimination level can be set to approximately 50% of the maximum range occurring in the load sequence. This means that considerable time and cost reductions in structural verification testing can be achieved. This will also have impact on a life prediction methodology since only a characteristic number of loading states need to be considered. The resulting life for block testing, especially for a load ratio of R=−1, is highly influenced by elimination of high load ranges in the sequence. Spectrum fatigue test results accounting for ranges up to 90% of maximum range, the remaining load states eliminated, can be mapped on constant amplitude data. At chosen elimination levels, approximately 80–90% of loading states were eliminated. Calculation results with the same technique, i.e. accounting for a characteristic number of cycles, show promising results.

Fatigue life prediction of fiber-reinforced titanium matrix composites

A micromechanical model was developed to predict the fatigue life of SiC fiber-reinforced titanium matrix composites. The propagation of matrix cracks was modeled by calculating the stress concentration factor at the matrix crack tip in a fiber bridged center matrix crack. The residual stiffness was then predicted by using the partial-crack shear-lag model, and the post-fatigued tensile strength was predicted by using the load carrying capacity of fibers proposed previously for brittle matrix composites. Finally, the catastrophic failure and fatigue life of the composite was determined by Monte Carlo simulation. An integrated computer simulation code was developed to simulate the evolution of fatigue damage, degradation of mechanical properties, and to predict fatigue life. The predicted matrix crack propagation rates, residual stiffness, residual tensile strength, and fatigue life were also correlated with experimental results.

Isothermal fatigue behaviour of a [90°] 8 SiC/Ti-15-3 composite at 426°C

The transverse fatigue behaviour of a unidirectional, SiC/Ti-15-3 composite (35 v/o SiC, [90°] 8 was evaluated at 426°C. The fatigue behaviour of the composite along the fibre direction ([0°] 8) and of unreinforced Ti-15-3 alloy were also studied for purpose of comparison. The [90°] 8 composite fatigue life was much shorter than the [0°] 8 life. Furthermore, the [90°] 8 fatigue life was also found to be far lower than that of the unreinforced Ti-15-3 alloy. A simple, one-dimensional model for [90°] 8 fatigue behaviour indicated that the short life of the composite in this orientation resulted, in large part, from weak fibre-matrix bond strength. This conclusion was supported by fractographic evidence showing numerous fatigue initiation sites along the fibre-matrix interfaces.

Ultrasonic and Mechanical Characterizations of Fatigue States of Graphite Epoxy Composite Laminates

Abstract : Fiber reinforced composites are inherently nonhomogeneous materials in which fabrication procedures can affect their service reliability without effect on their visual appearance. Williams and Doll observed in a unidirectional graphite fiber epoxy composite that a small change (14 deg c) in the precure temperature resulted in significant changes in trans fiber compressive fracture strength and the transfiber compression-compression fatigue life. Because (for some modes of) fatigue the failure of graphite fiber composites tends to be sudden and occurs without any visible evidence of damage any means of nondestructively monitoring fatigue damage or predicting fatigue behavior of graphite fiber composites is likely to enhance their effective use. In the investigation conducted by Williams and Doll they observed that the ultrasonic attenuation of the composite in the as fabricated state can be an indicator of the composite fatigue life. The purpose of the present study is to further explore the relationship between ultrasonic attenuation and fatigue survivability of graphite fiber epoxy composites fabricated under various processing temperatures and pressures.

An analysis of the fatigue behavior of Al-Al3Ni composites

Commercial purity Al-Al3Ni eutectic composites have been prepared by directional solidification at growth rates ranging from 9.63 x 10-3 to 1.0 mm/s. The composites were tested under rotating bending fatigue conditions and the results were analyzed using a model taking into consideration the difference in Poisson’s ratio of the phases, interfiber spacing and premature fracture of fibers. The calculated fatigue lives were shown to be in good agreement with the experimental results at all stress levels. Both the experimental results and calculations show that the composite fatigue life is not sensitive to large changes in the growth rate. The fatigue limit was also found to be insensitive to the mode of fatigue loading and notches. At high stress levels, however, smooth specimens tested under reversed bending exhibited longer lives than notched specimens tested under cyclic tension-tension conditions.