Property Modeling across Transition Temperatures in PMC's: Part I. Tensile Properties

Numerous studies report the effects of temperature on the stiffness and strength of polymer matrix composites (PMC's). Due to the complexity of the relaxation phenomenon in the matrix, these studies are mainly qualitative. In the present paper models were developed that can explicitly relate the mechanical response of the composite to temperature. These models are related to the microstructure of the constituents, and therefore can be applied to any polymer matrix composite. The possibility of using these models was illustrated by the study of the mechanical behavior of carbon fiber AS4/polyphenylene sulfide (PPS) at elevated temperatures. Part I of this paper relates to the modeling of the temperature-dependent composite tensile properties (stiffness and strength). Parts II and III focus on the life prediction of AS4/PPS undergoing static and fatigue end-loaded bending at elevated temperatures.     

Property Modeling across Transition Temperatures in PMC's: Part II. Stress Rupture in End-Loaded Bending

The quasi-static properties (strength and stiffness) of polymer matrix composites (PMC's) exhibit significant temperature dependence in tension. Part I of the present paper introduced a new model enabling the computations of the strength and stiffness of PMC's in tension as an explicit function of temperature. In the second part, this model was used to model the behavior of unidirectional PMC's (AS4/PPS) in end-loaded bending. A new set of stress-rupture experiments was performed on the amorphous composite in bending in order to complete the data available in the literature (based on semi-crystalline composite). A model enabling the computation of the rupture life of the bent specimen as a function of temperature and applied load was established. The theoretical simulations were successfully compared to a set of 7 independent end-loaded bending experiments. This paper demonstrates the validity and the ease of integrating the polymer stiffness-temperature model in mechanics of composite materials, and completes the theoretical and experimental basis for the modeling of the end-loaded behavior of PMC's.     

Hygrothermal ageing effects on the static fatigue of glass/epoxy composites

A study of the effects of water ageing on the static fatigue behaviour of unidirectional glass/epoxy composites is presented. The failure mechanisms associated with fatigue damage were investigated under three-point bending loading. Depending on the ageing temperature, two failure features were identified: either fibre microbuckling on the compressive side of the specimen, or progressive cracking on the tensile side. Microbuckling has been found to be related to the reversible plasticization of the epoxy matrix, as measured by dynamic mechanical thermal analysis. On the other hand, tensile failure was associated with an irreversible weakening of the fibres and interfaces at elevated ageing temperatures. Some similarity is identified between damage processes in static and dynamic bending fatigue.     

Applicability of empirical models for evaluation of stress ratio effect on the durability of fiber-reinforced creep rupture–susceptible composites

Fiber-reinforced polymer–matrix composites are known to exhibit loading rate- and time-dependent mechanical response. Their fatigue strength is determined by a complex interaction of damage processes governed by loading duration and cycle number. Apart from mechanistic approaches, a number of empirical models of various sophistication have been proposed to predict the durability of composites, differing in the amount of experimental data needed for their application. The accuracy of several such models is evaluated by comparing the prediction to the experimentally determined stress ratio effect on fatigue life of glass fiber-reinforced polyester–matrix composite. It is found that the accuracy of prediction generally improves with increasing the amount of test data needed for model calibration. However, the most accurate method of fatigue life estimation, among the selected ones, is by the modified Goodman diagram.     

Dynamic compressive behavior of unidirectional IM7/5250-4 laminate after thermal oxidation

Fiber reinforced high temperature polymer matrix composites are currently gaining wide usage in aircraft structures, especially in airframe and engine inlet casing. The failure of composites in worst-case operational conditions mandates the extensive investigation of the mechanical behavior, and the durability in long-term performance and service life under thermal oxidation. In this work, unidirectional IM7 carbon fiber reinforced high-temperature BMI resin composite (IM7/5250-4) were isothermally aged in air for 2 months at 195 °C and 245 °C, respectively. The dynamic behavior of thermally aged composites was investigated on a split Hopkinson pressure bar (SHPB) in three principal directions. The results indicate that thermal oxidation leads to significant reduction in both stiffness and strength of the composites. Optical micrographs of fracture surface and failure pattern of composite after SHPB impact reveals oxidation induced debonding along the fiber–matrix interface due to oxygen diffusion under long-term exposure to elevated temperatures.     

单向碳/碳复合材料拉-拉疲劳寿命及剩余强度预测模型

基于预测单向复合材料纵向拉伸强度的随机核模型,引入纤维单丝剩余强度二参数Weibull模型及纤维单丝与基体界面剩余强度模型,研究建立了单向复合材料纵向拉-拉疲劳寿命及剩余强度的预测模型。对经过一定次数拉-拉疲劳载荷循环后的纤维束抽取其纤维单丝进行剩余强度拉伸试验,建立了纤维单丝剩余强度的二参数Weibull模型,测试单向碳/碳（C/C）复合材料的纤维与基体界面强度。通过单向C/C复合材料算例分析表明,92.5%、90.6%和87.5%应力水平下对数预测寿命与对数试验寿命比值分别为0.79、1.00和1.11,表明所建立的寿命预测模型用于预测单向C/C复合材料疲劳寿命是可行的;纵向拉伸剩余强度预测值与试验值误差在10%以内,吻合较好,表明所提出的剩余强度预测模型具有较高的精度。     

Fatigue Life Prediction of Carbon Fiber-Reinforced Ceramic-Matrix Composites at Room and Elevated Temperatures. Part I: Experimental Analysis

This paper presents an experimental analysis on the fatigue behavior in C/SiC ceramic-matrix composites (CMCs) with different fiber preforms, i.e., unidirectional, cross-ply and 2.5D woven, at room and elevated temperatures in air atmosphere. The experimental fatigue life S???N curves of C/SiC composites corresponding to different stress levels and test conditions have been obtained. The damage evolution processes under fatigue loading have been analyzed using fatigue hysteresis modulus and fatigue hysteresis loss energy. By comparing the experimental fatigue hysteresis loss energy with theoretical computational values, the interface shear stress corresponding to different peak stress, fiber preforms and test conditions have been estimated. It was found that the degradation of interface shear stress and fibres strength caused by oxidation markedly decreases the fatigue life of C/SiC composites at elevated temperature.     

The effect of temperature on fatigue damage of FRP composites

The present study intends to investigate the effect of temperature on cumulative fatigue damage (D) of laminated fibre-reinforced polymer (FRP) composites. The effect of temperature on fatigue damage is formulated based on Ramkrishnan–Jayaraman and Varvani-Farahani–Shirazi residual stiffness fatigue damage models. The models are further developed to assess the fatigue damage of FRP composites at various temperatures (T). This task is fulfilled by formulating the temperature dependency of Young’s modulus (E) and ultimate tensile strength (σ_ult) as the inputs of the models. Temperature-dependant parameters of Young’s modulus and ultimate tensile strength are found to be in good agreement with the experimentally obtained data when used for unidirectional, cross-ply and quasi-isotropic FRP laminates. The proposed fatigue damage model is evaluated using six sets of fatigue damage data. The proposed temperature-dependent model was also found promising to predict the fatigue damage of unidirectional (UD) and orthogonal woven FRP composites at different temperatures.     

Life extension technology for steam pipe systems—II. Development of life prediction methodology

Part II of this paper addresses the development of a fracture mechanics based life prediction methodology of steam pipes which operate at elevated temperatures but in the sub-creep temperature range. Elastic-plastic fracture mechanics concepts were employed to establish the remaining life prediction methodology and inspection criteria of steam pipes. Leak-before-break analyses were utilized to determine the flaw inspection criteria. Both tension and bending type loading conditions were considered in the life prediction analysis. The life assessment technology is concerned with the fatigue crack growth life of circumferential cracks in a pipe. The material properties of the A106B steam pipe steel reported in Part I of this paper were used to predict the fatigue life of steam pipes. The effects of operating parameters (e.g. stress and temperature), pipe size, and material properties on the remaining life and inspection intervals of steam pipes can be quantitatively evaluated.     

Constituent damage mechanisms in metal matrix composites under fatigue loading, and their effects on fatigue life

Load controlled fatigue experiments were performed on 8-ply unidirectional ([0]₈) SCS-6-Ti-15-3 metal matrix composites (MMCs) at different temperatures, and the results were interpreted in terms of the overall three-regime framework of fatigue. The emphasis was on understanding the mechanisms and mechanics of constituent damage evolution, and their effects on fatigue life. Most tests were performed at an R-ratio of 0.1, but limited fully-reversed (R = −1) tests were conducted. In regime 1, damage was fiber failure dominated, but the exact mechanisms were different at room and elevated temperatures. In regime 2, observation of matrix cracks and persistent slip bands provided convincing evidence of matrix dominated damage. Weak fiber-matrix interfaces contributed to crack bridging. However, fiber fracture also played an important role in regime 2; tension-tension and tension-compression tests showed similar lives on a maximum fiber stress basis, although the strain range, which primarily controls matrix crack growth, was almost double for R = −1 compared with R = 0 or 0.1. Good agreement was obtained from the different R-ratio tests, between the MMC and matrix data, and data at room and elevated temperatures, when compared based on the strain range in the tension part of a cycle. Analyses and observations of fiber pull-out lengths and fiber fractures in the matrix crack wake provided evidence of fiber damage; the analyses also helped to explain increased fiber bridging with fiber volume fraction. Issues of fatigue life prediction are briefly discussed.     

Stress Rupture of PMC's in End-Loaded Bending

This paper introduces a method to study the time dependent behavior of polymer matrix composites. An end-loaded bending method and fixture are developed to emphasize the contribution of changes in matrix properties to the behavior of fiber dominated composites loaded in the fiber direction. This method has distinct advantages over other methods such as tensile stress rupture and three- and four-point bending rupture methods. This paper discusses the design and fabrication of an end-loaded bending fixture. A brief analysis is presented which relates strain level to end-to-end distance, eliminating the need for strain gauges. Time-dependent rupture in bending of polymer matrix composites is reported.     

Behaviour of composite sandwich foam-laminated glass/epoxy under solicitation static and fatigue

The purpose of this work is the study of the mechanical behaviour in 3-point bending, in static and fatigue mode, of two different composite sandwiches laminate foam with unidirectional reinforcements and crossed SMS [0₄] and SMS [0/90₂/0]. These two sandwich composites were developed in laboratory. The specimens are made of fiberglass and epoxy resin for skins and PVC foam for the core. The tests in the static mode permitted the determination of the mechanical characteristics necessary to the fatigue tests and the mechanical behaviour of the materials. The fatigue tests made in controlled force gave curves which inform about the lifetimes of the materials. The observations of the fracture surfaces revealed the different modes of damages causing the material fracture. The lifetimes are characterized by the Wöhler curves. This study evidenced the influence of the reinforcement orientation on endurance of the studied material.     

An approach to fatigue life modeling in titanium-matrix composites

A review of the procedures developed by the author and his colleagues over the last several years for predicting elevated-temperature fatigue life of metal-matrix composites is presented. Modeling approaches involve concepts of both linear and non-linear summation of damage from cycle-dependent as well as time-dependent mechanisms. The analyses, further, treat the micromechanical stresses in the constituents as parameters in the life prediction models. The material characterized is SCS-6/Timetal®21S, a metastable beta titanium alloy reinforced with continuous SiC fibers. Modeling is applied to isothermal fatigue at different frequencies and temperatures, and thermomechanical fatigue (TMF) under both in-phase and out-of-phase loading conditions at different temperature ranges and maximum temperatures. Experimental data are used as the basis for determining the parameters embedded in the models. The numerical results, in turn, provide insight into the dominant mechanisms controlling fatigue life under a given condition. The capability to correlate experimental data from a wide variety of test conditions for several versions of a damage summation model is demonstrated.     

复合材料层合板冲击后压-压疲劳寿命预测方法

针对冲击后复合材料层合板, 发展了含冲击初始损伤层合板的压-压疲劳寿命预测方法。该方法基于无损单向板的力学性能和疲劳特性, 对不同铺层参数、 不同几何尺寸以及不同冲击条件下层合板的疲劳寿命进行预测。为消除人为假设冲击损伤造成的误差, 对层合板在冲击载荷及冲击后疲劳载荷作用下的破坏进行全程分析, 即把冲击后层合板的实际损伤状态直接作为疲劳分析的初始状态。同时基于逐渐损伤思想, 推导了含冲击初始损伤层合板的应力分析过程, 建立了相应的三维逐渐累积损伤模型, 开发了参数化的复合材料层合结构冲击及冲击后疲劳破坏模拟程序, 为复合材料层合结构的抗冲击设计及其疲劳损伤扩展行为研究提供了较好的技术平台。      

Estimate Interface Shear Stress of Unidirectional C/SiC Ceramic Matrix Composites from Hysteresis Loops

The tensile-tensile fatigue behavior of unidirectional C/SiC ceramic matrix composites at room and elevated temperature has been investigated. An approach to estimate the interface shear stress of ceramic matrix composites under fatigue loading has been developed. Based on the damage mechanisms of fiber sliding relative to matrix in the interface debonded region upon unloading and subsequent reloading, the unloading interface reverse slip length and reloading interface new slip length are determined by the fracture mechanics approach. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulatd in terms of interface shear stress. By comparing the experimental hysteresis loss energy with the computational values, the interface shear stress of unidirectional C/SiC ceramic composites corresponding to different cycles at room and elevated temperatures has been predicted.     

Fatigue analysis of unidirectional GFRP composites under combined bending and torsional loads

Fatigue behavior of unidirectional glass fiber reinforced polyester (GFRP) composites at room temperature under in-phase combined torsion/bending loading was investigated. All fatigue tests were carried out on constant-deflection fatigue machine with frequency of 25 Hz. A 30% reduction from the initial applied moments was taken as a failure criterion in the combined torsion/bending fatigue tests of the composite materials. A series of pure torsional fatigue tests were conducted to construct the failure contour of GFRP composites using different failure theories. The obtained S–N curves from combined torsion/bending tests were compared with both, pure torsion fatigue test results and published results of pure bending fatigue tests of GFRP rods. Pictures by scanning electron microscope were used to closely examine the failure mode of the tested specimens under combined torsion/bending loading.

The results showed that, the unidirectional glass fiber reinforced polyester composites have poor torsional fatigue strength compared with the published results of pure bending fatigue strength. Endurance limit value (calculated from S–N equation at N = 10⁷ cycles) of GFRP specimens tested under combined torsion/bending loading equals 8.5 times the endurance limit of pure torsion fatigue. On the other hand the endurance limit of combined torsion/bending fatigue strength approximately half the fatigue limit of pure bending fatigue strength. The predicted values of combined torsion/bending fatigue strength at different number of cycles, using the published failure theory are in good agreement with the experimental data. For the investigated range of fiber volume fractions (V_f) it was found that higher stress levels are needed to produce fatigue failure after the same number of cycles as V_f increases.     

Damage Monitoring of Unidirectional C/SiC Ceramic-Matrix Composite under Cyclic Fatigue Loading using A Hysteresis Loss Energy-Based Damage Parameter at Room and Elevated Temperatures

The damage evolution of unidirectional C/SiC ceramic-matrix composite (CMC) under cyclic fatigue loading has been investigated using a hysteresis loss energy-based damage parameter at room and elevated temperatures. The experimental fatigue hysteresis modulus and fatigue hysteresis loss energy versus cycle number have been analyzed. By comparing the experimental fatigue hysteresis loss energy with theoretical computational values, the interface shear stress corresponding to different cycle number and peak stress has been estimated. The experimental evolution of fatigue hysteresis loss energy and fatigue hysteresis loss energy-based damage parameter versus cycle number has been predicted for unidirectional C/SiC composite at room and elevated temperatures. The predicted results of interface shear stress degradation, stress–strain hysteresis loops corresponding to different number of applied cycles, fatigue hysteresis loss energy and fatigue hysteresis loss energy-based damage parameter as a functions of cycle number agreed with experimental data. It was found that the fatigue hysteresis energy-based parameter can be used to monitor the fatigue damage evolution and predict the fatigue life of fiber-reinforced CMCs.     

单向聚酯帘线增强橡胶材料疲劳特性研究

利用自行建立的试验系统, 首次对单向聚酯帘线增强橡胶材料进行了疲劳测试, 研究了应变、频率和温度对疲劳损伤累积的影响, 并给出了疲劳寿命预报方程, 为评价轮胎的疲劳特性、预报轮胎的疲劳寿命提供了有效的手段。      

Effects of Temperature,Oxidation and Fiber Preforms on Fatigue Life of Carbon Fiber-Reinforced Ceramic-Matrix Composites

In this paper, the effects of temperature, oxidation and fiber preforms on the fatigue life of carbon fiber-reinforced silicon carbide ceramic-matrix composites (C/SiC CMCs) have been investigated. An effective coefficient of the fiber volume fraction along the loading direction (ECFL) was introduced to describe the fiber architecture of preforms. Under cyclic fatigue loading, the fibers broken fraction was determined by combining the interface wear model and fibers statistical failure model at room temperature, and interface/fibers oxidation model, interface wear model and fibers statistical failure model at elevated temperatures in the oxidative environments. When the broken fibers fraction approaches to the critical value, the composites fatigue fracture. The fatigue life S–N curves and fatigue limits of unidirectional, cross-ply, 2D, 2.5D and 3D C/SiC composites at room temperature, 800 °C in air, 1100, 1300 and 1500 °C in vacuum conditions have been predicted.     

A MECHANISTIC-BASED THERMOMECHANICAL FATIGUE LIFE PREDICTION MODEL FOR METAL MATRIX COMPOSITES

The framework for developing a mechanistic-based life prediction model for metal matrix composites is described. For a composite consisting of unidirectional silicon carbide fibers in a titanium aluminide matrix, SCS-6/Ti-24A1-1INb (at%) [0]₈, three dominant damage mechanisms were identified: (1) matrix fatigue damage, (2) surface-initiated environmental damage, and (3) fiber-dominated damage. Damage expressions were developed for each mechanism along with a method for determining the constants. The damage is summed to obtain the total life. The model is capable of making predictions for a wide range of histories, including isothermal fatigue at different frequencies and stress-ratios, thermomechanical fatigue (TMF) under in-phase and out-of-phase cycling conditions, thermal cycling at constant stress, and stress holds at either maximum or minimum stress. Considering the wide range of cyclic conditions, the predictions compare favorably with experiments. In addition, the controlling damage mechanism for each history is predicted.