一方向繊維強化プラスチックのクリープ特性と寿命予測―繊維破断部近傍の界面はく離を考慮したクリープ破断モデル―

The effect of water on fiber bridging of polymer matrix composites (PMCs) has been studied for stress-corrosion cracking (SCC). The fiber bridging during crack propagation in the PMC was observed directly using a transparent polymer matrix. Specimens were immersed and weighed just before conducting the crack propagation test. The contribution of mechanical degradation caused by the water to the bridging was measured as a function of weight gain due to water absorption. The critical energy release rate of the matrix decreased with increase of the water absorption. The energy release rate of a bridged crack varied with the water absorption. To investigate the variation in the bridging mechanism under the water environment, we calculated the bridging stress the σt and the bridging contribution to crack propagation resistance ΔG based on a single-fiber bridging mechanism with the interfacial debonding length and the interfacial debonding energy. The debonding length was measured directly, and the debonding energy was obtained in a fragmentation test. The calculation showed that the increase in the interfacial debonding length influenced the fiber bridging mechanism.

Single fiber fragmentation test is one of the methods to evaluate strength of a fiber embedded in matrix resin. This method has two advantages; the Weibull parameter can be obtained from a few tests and an actual strength of the embedded fiber in practice can be obtained. This test evaluates relationship between the fiber strength and the fiber length. However, the fiber length is defined by the length of the gage length divided by the number of fragment points, and the fiber length is overestimated. Therefore, in the conventional evaluation, the fiber strength is not independent to interfacial strength and test speed. To assume the exact fiber length, an interfacial debonding length and a stress recovery length must be considered. In this study, the new corrected evaluation to generalize the fiber strength is suggested. To discuss the validity of the method, several fragmentation tests are performed under the condition of the various interfacial strength and the various test speed.

In this chapter, the time-dependent first matrix cracking stress of fiber-reinforced ceramic-matrix composites (CMCs) is investigated using the energy balance approach. The shear-lag model combined with the interface oxidation model, fiber oxidation model, and fiber failure model is adopted to analyze the microstress distributions in fiber-reinforced CMCs. The relationships between the first matrix cracking stress, interface debonding and slip, fiber fracture, and oxidation time and temperature are established. The effects of the fiber volume, the interface shear stress, the interface debonding energy, the fiber Weibull modulus, the fiber strength on the first matrix cracking stress, the interface debonding length, and the fiber broken fraction are analyzed. The first matrix cracking stresses of C/SiC with strong and weak interface bonding after unstressed oxidation at 700 °C in air atmosphere are predicted for different oxidation time.

7 - Time-dependent nonlinear damage behavior of ceramic–matrix composites

A method of evaluating the interfacial fracture toughness using a single-fibre composite test is proposed. In contrast with the existing techniques, the method takes into account the phenomenon of friction between the fibre and matrix in the debonding zone. A general mathematical solution of the problem and modelling of the friction phenomenon are presented. Finite-element analysis using a “contact” statement is utilized for numerical evaluation of the stress–strain state. The influence of the coefficient of friction and interfacial debonding length is analysed in detail. It is shown that the friction reduces the calculated value of the elastic strain energy release rate for a given debonding length, relative to that obtained when friction is neglected. The magnitude of the difference depends on the coefficient of friction, the elastic properties of the fibre and matrix, and the characteristics of the debonding mechanism. Experimental data on debonding in a series of glass-epoxy single-fibre composites are analysed using the proposed numerical technique to obtain the effects of fibre surface treatments and fibre strain-to-break on the interfacial fracture toughness. © Kluwer Academic Publishers

In this chapter, multiple matrix cracking evolution of fiber-reinforced ceramic-matrix composites (CMCs) is investigated using the micromechanical approach. The shear-lag model is adopted to analyze the fiber and matrix stress distributions in the damaged composite. The fracture mechanics approach is used to determine the fiber/matrix interface debonding length. Combining the critical matrix strain energy criterion (CMSE) and fracture mechanics fiber/matrix interface debonding criterion, the stress-dependent multiple matrix cracking evolution is analyzed for different fiber volume fraction, fiber/matrix interface properties, and matrix cracking characteristic stress. Experimental multiple matrix cracking evolution of unidirectional C/Si3N4, SiC/Si3N4, SiC/CAS, SiC/CAS-II, SiC/SiC, SiC/Borosilicate and mini-SiC/SiC composites are predicted.KeywordsCeramic-matrix composites (CMCs)Multiple matrix crackingInterface debonding

In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process, leading to stress concentrations in the matrix and neighboring fibers and debonding of the interface, which can propagate at high speed over long distances. In our previous work, a fiber break within a two-dimensional fiber array embedded in elastic epoxy matrix (with cohesive interface) was modeled to quantify the effects of these dynamic stresses. The results indicated that the elastic limit of the polymer matrix can be exceeded. In this study, the effects of matrix plasticity on dynamic stress concentrations due to a single fiber break are investigated. For the range of matrix yield stresses considered, the dynamic stress concentrations are significantly higher than corresponding values predicted by a quasi-static model with a pre-broken fiber. Based on the ratio of shear yield strength of the matrix and mode II peak traction of the interface cohesive law, two distinct regimes of damage are shown to exist. Only matrix yielding occurs when this ratio is less than 1.0, while both interfacial debonding and matrix yielding occur when it is greater than 1.0. At higher fiber break strengths, where the elastic matrix model predicts unstable interfacial debonding, reduction in matrix yield strength leads to a transition to stable debonding and arrest. Reducing the matrix yield strength also leads to a lowering of the peak dynamic stress concentrations in adjacent fibers, while spreading the stress concentrations over a larger volume of the composite microstructure.

In this paper, the stress-dependent matrix multiple fractures of fiber-reinforced ceramic-matrix composites (CMCs) with fiber oxidation and fracture is investigated. The shear-lag model is combined with the fiber oxidation and fracture models, and the fiber/matrix interface debonding criterion is adopted to determine the micro-stress field of the damaged fiber-reinforced CMCs. The effects of the fiber volume, interface shear stress, interface debonding energy, oxidation temperature and time on the stress-dependent matrix multiple fractures, interface debonding and oxidation, and fiber fracture are discussed. When the fiber volume fraction and the interface shear stress in the debonding region increased, the matrix first cracking stress and saturation cracking stress increased, the saturation matrix cracking space decreased, and the matrix multiple fracture evolution rate increased. When the interface debonding energy and the interface shear stress in the oxidation region increased, the matrix cracking density increased. When the oxidation temperature and oxidation duration increased, the matrix cracking density decreased. The experimental matrix multiple fractures of unidirectional C/SiC, SiC/SiC, mini-SiC/SiC, SiC/CAS, SiC/CAS-II, and SiC/borosilicate composites are predicted.

The fatigue hysteresis behavior of unidirectional, 2D cross-ply and woven, and 2.5D woven fiber-reinforced ceramic-matrix composites (CMCs) are analyzed. Based on the fiber/matrix interface debonding and sliding behavior, the fiber/matrix interface debonding and sliding lengths are determined using the fracture mechanics approach. The fiber/matrix interface debonding ratio and interface sliding ratio are determined for different interface slip cases. The effects of fiber volume fraction, peak stress, matrix crack spacing, interface shear stress, interface debonded energy, fibers failure, fiber Poisson contraction, fiber strength, fiber Weibull modulus, matrix cracking mode, applied cycle number and fiber/matrix interface wear on the fatigue stress–strain hysteresis loops and the fiber/matrix interface debonding and sliding are discussed. The experimental cyclic fatigue stress–strain hysteresis loops of unidirectional SiC/CAS, SiC/1723 and C/SiC, 2D cross-ply SiC/CAS and woven SiC/SiC, and 2.5D woven C/SiC composites under cyclic loading/unloading tensile and tension–tension fatigue loading are predicted.

This paper proposes a strength prediction method for unidirectional glass fiber reinforced plastics (GFRPs) after hydrothermal aging: immersion in deionized water at 80°C. First, the strength degradation of the constituents (i.e., the glass fiber and the fiber/matrix interface) of unidirectional GFRP after hydrothermal aging was evaluated from the fiber strength and the interfacial shear stress by using a single-fiber composite (SFC). Both the fiber strength and the interfacial shear stress had a tendency to decrease in the early stage of hydrothermal aging and to saturate toward certain values with long-term aging. In addition, the tensile strength of the unidirectional GFRP was measured after hydrothermal aging. The residual strength of the unidirectional GFRP also had a tendency to decrease sharply in the early stage of hydrothermal aging and to saturate toward a certain strength with long-term aging. Finally, the residual strength of the unidirectional GFRP after hydrothermal aging was predicted using the global loading sharing (GLS) model, by considering the degradation of both the glass fiber and the fiber/matrix interface. The predicted results indicated good agreement with the experimental data while considering not only the degradation of the fiber reinforcement but also the fiber/matrix interface adhesion. It was concluded that the GLS model applied considering the degradation of the GFRP constituents would be a suitable and a simple model to predict the residual strength of the unidirectional GFRP after hydrothermal aging.

Influence of kink bands on the tensile strength of flax fibers

6 - Stochastic loading-dependent nonlinear damage behavior of ceramic–matrix composites

Evaluation of interfacial and mechanical properties of glass fiber and p-DCPD composites with surface treatment of glass fiber

Injection molding of long fiber-reinforced thermoplastics is a well-established method in automotive industry to produce high quality structural parts in mass production without the need of further finishing, where the performance is strongly dependent on the fiber length. It is known that the processing parameters have to be carefully chosen as they directly influence the final fiber length and can therefore have a negative effect on the resulting mechanical properties of the part. Particularly in terms of impact behavior, the fiber length is seen as a key factor. The aim of the present work is to quantify the effects of the injection molding parameters on the impact behavior of long fiber reinforced compounds. In a first step the resulting fiber length of injection molded glass fiber reinforced polypropylene is analyzed as a function of the injection velocity, holding pressure, revolution speed and back pressure. The single steps of fiber length analysis are carefully investigated in terms of feasibility and accuracy, in order to assure that a reproducible and reliable test method is employed. Subsequently, the impact behavior of the PP-GF compounds is quantified by falling dart experiments, and the results are then correlated to the fiber length and the processing parameters to establish a fundamental processing-structure-property relationship. The investigation of the fiber length analysis shows that a high pyrolysis temperature leads to embrittlement of the glass fibers and a decrease of tensile strength of the single fibers. This may consequently lead to incorrect characterization of the fiber length due to fiber breakage. The results of the statistical investigation of the processing parameters indicate a significant influence of the back pressure on both impact energy and fiber length.

Measurement of interfacial shear stress has been difficult on a composite with short-fiber, since the fiber length is shorter than the critical length. In this study, a mechanical model is proposed for the composites and the interfacial shear stress is evaluated with viscoelastic propeties of the model. Injection molded composite specimens are used in short-term creep tests. The coefficients in a viscoelastic model are evaluated by the optimization analyses of experimental results. The strain shared by fiber for a given macroscopic strain is analysed from the change of the coefficients for elements in the mechanical model. The stress in fiber is obtained by substituting the mean stress into Cox's relation. The interfacial shear stress, from the shear-lag analysis, is compared with the prediction after Cox. As a result, the maximum shear stress was found to be bigger than that of Cox's prediction and also least affected by fiber volume fraction.