A technique for the determination of interfacial properties from debond length measurement

Fiber-matrix interfacial debonding is observed and the debond length is directly measured during flexure tests performed on transparent SiC fiber-reinforced borosilicate glass composites. The relationship among the debond length, applied stress, and interfacial properties is investigated both experimentally and theoretically. A new technique based on debond length measurement is introduced for measuring fiber-matrix interfacial properties such as interfacial shear strength, frictional shear stress, and interfacial debond energy. Analytical models are employed for the new technique to interpret the experimental data. Fiber pushout technique is also employed to measure the interfacial properties independently. It is shown that these two different techniques of debond length measurement and fiber pushout test for measuring the interfacial properties can provide comparable results.     

Interface debond crack growth in tension–tension cyclic loading of single fiber polymer composites

Fiber/matrix interface debond crack growth from a fiber break is defined as one of the key mechanisms of fatigue damage in unidirectional composites. Considering debond as an interface crack its growth in cyclic loading is analyzed utilizing a power law, where the debond growth rate is a power function of the change of the strain energy release rate in the cycle. To obtain values of two parameters in the power law cyclic loading of fragmented single fiber specimen is suggested. Measurements of the debond length increase with the number of load cycles in tension–tension fatigue are performed for glass fiber/epoxy single fiber composites. Analytical method in the steady-state growth region and FEM for short debonds are combined for calculating the strain energy release rate of the growing debond crack. Interface failure parameters in fatigue are determined by fitting the modeling and experimental results. The determined parameters for interface fatigue are validated at different stress levels.     

In situ detection of fiber break and analysis of its effect on stress transfer during tensile tests of a metal matrix composite

In situ observation by synchrotron X-ray topography was carried out of the notched and unnotched SCS-6 fiber-reinforced aluminum alloys during tensile tests. Fiber breaks occurring at different loading levels were successfully detected. Effects of debond length and plastic deformation after one fiber break on the stress transfer mechanism were then analyzed by three-dimensional finite element models. Implications of these experimental and numerical results were suggested.     

Advanced Evaluation of Push‐In Data for the Assessment of Fiber Reinforced Ceramic Matrix Composites

Fiber reinforced ceramics are ready to be used in wide spread applications due to their unique combination of typical ceramic properties and enhanced fracture toughness. This outstanding combination is only achievable if appropriate fiber–matrix interfaces are ascertained. The interfacial state can favourably be measured by the fiber push‐in test. This method is reviewed in this paper and further elaborated in order to derive quantitative prediction of the composite performance from fiber–matrix interface bonding and friction data. The problem of overestimating the frictional forces due to the Poisson effect is overcome by means of analysing the hysteresis area from two load–unload‐cycles. Significant dependence of interface friction on fiber diameter is detected as it is predicted taking into account the effect of fiber roughness on the interface clamping stress. A statistical procedure is applied to determine the critical debond stress which is the upper limit to prevent brittle fiber failure.     

Microstructural evaluation and fiber debonding in aligned ceramic-matrix composites

The slice compression test (SCT) represents a new approach for the estimation of the interfacial shear stress and debond length in fiber-reinforced ceramics. The technique is based upon multiple measurements and simultaneous loading of a large number of fibres in the test specimen. The specimen slice is compressed between a hard ceramic anvil and a ductile metal to produce interfacial debonding and slippage as a result of the fiber/matrix elastic mismatch. The extent of fiber protrusion under maximum load and the residual protrusion of the fibres after load relaxation are the experimentally determined parameters and have been measured for a laminate reinforced with Nicalon SiC fiber (LASIII). The minimum applied stress for the initiation of debonding was estimated. In addition, the microstructure of the ceramic composite was quantitatively characterized to determine numerical values of the microstructural parameters which could affect the interfacial shear stress. The fiber diameters and spatial fiber distribution were determined to obtain the degree of order of the fibers within the composite.     

ELASTIC BRIDGING FOR MODELING FATIGUE CRACK PROPAGATION IN A FIBER-REINFORCED TITANIUM MATRIX COMPOSITE

Abstract— A model based upon linear elastic bridging and fiber crack tip shielding is proposed for predicting fatigue crack growth in a SCS-6/Ti-6–4 composite. The model is characterized by the fiber/matrix debond length rather than the fiber/matrix interfacial frictional shear strength used in most current fatigue models. Finite elements combined with fracture mechanics are applied for computing the local stress intensity. The local stress intensity in the matrix is then utilized to predict crack growth in the composite via comparison to monolithic fatigue crack propagation data for a similar Ti-6–4 matrix material.     

Fracture behavior and acoustic emission in bending tests on single-fiber composites

The bending fracture mechanisms and interfacial behavior of single-fiber composites (s.f.c.) with different fiber surface treatments and embedded fiber positions were investigated in three-point bending with simultaneous acoustic emission monitoring. Microfractures occurring at fiber breakages were examined by AE parameters and observations by a polarized microscope. As a result, it was found that AE signals in a bulk resin specimen were almost not detected, while many AE events were monitored in the s.f.c. bending specimens. The number of AE events was in good agreement with the number of fiber breakages, except for specimens with an embedded fiber near the compressive surface. Using AE parameters, especially the peak frequency and its power energy obtained by a power spectrum analysis, failure modes can be identified. A transition of failure mode from fiber break accompanied by a matrix crack and debonding to buckling is observed when the stress in the embedded fiber changes from tension to compression. The debond length is very long near the loading point for the specimens with a fiber near the tensile surface, but it decreases with increasing distance from the loading point. The debond length is small for the specimen with an embedded fiber near the neutral plane since the strain in the fiber decreases. Furthermore, a model for debonding failure is proposed and the maximum interfacial shear stress is derived. It is confirmed that fiber fragment lengths for the specimens with a fiber near the tensile side can be also expressed by the Weibull distribution as done in s.f.c. tensile tests.     

Determination of fiber/matrix adhesion using the Outwater–Murphy single fiber specimen

Fiber/matrix (F/M) interface toughness values of carbon/vinylester 411 and glass/vinylester 411 has been determined using the Outwater–Murphy (OM) single-fiber compression test specimen. The OM test specimen consists of a rectangular block of matrix with a centrally embedded fiber with a hole drilled through the specimen and the fiber. Upon loading of the specimen in compression, the fiber may debond at the hole edge. The interface toughness can be determined from the measured values of the critical load for debonding. The fiber/matrix debond toughness values for dry carbon/vinylester and glass/vinylester were 51.2 and 38 J/m², respectively, in reasonable agreement with previously published data determined using other test methods. Testing of water immersed carbon/vinylester 411 and glass/vinylester 411 OM specimens revealed significant reductions of the fiber/matrix adhesion.     

碳纤维布加固混凝土梁的剥离破坏

碳纤维布加固混凝土结构中剥离破坏是一种重要的破坏形式,通过九根碳纤维布加固混凝土梁的模型试验,考察了剥离承载力及剥离模式。根据试验结果和力学理论知识,对碳纤维布加固混凝土梁的剥离破坏进行了极限应力分析,建立了极限状态的判据和相应的碳纤维布剥离承载力计算方法,在计算方法中考虑了粘结正应力与剪应力的综合作用以及U形箍的横向剪切变形,与试验结果的比较表明,方法具有较高的精度,完善了碳纤维布加固混凝土梁的设计理论。     

Determination of interfacial mechanical properties of ceramic composites by the compression of micro-pillar test specimens

A novel method to determine the fiber-matrix interfacial properties of ceramic matrix composites is proposed and evaluated; where micro-pillar samples containing inclined fiber/matrix interfaces were prepared from a SiC fiber-reinforced SiC matrix composites and then compression-tested using the nano-indentation technique. This new test method employs a simple geometry and mitigates the uncertainties associated with complex stress state in the conventional single-filament push-out method or tensile unloading–reloading hysteresis loop analysis method for the determination of interfacial properties. Based on the test results using samples with different interface orientations, the interfacial debond shear strength and the internal friction coefficient are explicitly determined and compared with values obtained by other test methods. SEM observation showed that micro compression caused an adhesive type of debonding between the fiber and the pyrolytic carbon interface. The results suggest that the debonding/failure behavior of the micro-pillars followed the Coulomb fracture criterion. The determined interfacial debond shear strength is ~100 MPa, which appears to be smaller than that determined from fiber push-out test for similar composite systems. The difference can be explained by the effect of normal stress (clamping stress) on the apparent interfacial debond shear strength.     

Interphase behavior of titanium matrix composites at elevated temperature

This paper examines the effect of temperature and thermal exposure on the interphase behavior of continuous fiber reinforced titanium metal matrix composites. The system considered is SCS-6/Timetal-21S. Elevated temperature fiber push-out tests were conducted to determine the effect of test temperature on interphase shear properties. Corresponding variations of debonding shear strength and frictional shear stress with test temperature are presented and discussed. Thermal exposure, both in a vacuum and an air environment, has been conducted on specimens, with temperatures up to 650°C and exposure times of up to 100 h. The resulting size and composition of the interphase have been examined. Fiber push-out tests were carried out at room and elevated temperature on the aged specimens. Results are discussed in terms of the influence of relaxation and oxidation on the debond shear strength. Using the experimentally determined interphase shear properties, the interphase toughness has been calculated and discussed in relation to interface decohesion models.     

An efficient BEM to calculate weight functions and its application to bridging analysis in an orthotropic medium

An efficient boundary element method to calculate crack weight functions is developed. The weight function method is applied to bridging effect analysis in a single-edge notched composite specimen by using a bridging law which includes both interfacial debonding and sliding properties between fiber and matrix in ceramic matrix composites. A numerical method to solve the distributed spring model treating bridging fibers as stress distribution to close the crack surface is provided to determine the bridging stress, debond length, crack opening displacement and stress intensity factor.     

Relationship between fiber flaw spectra and the fragmentation process: A computer simulation investigation

The single-filament-composite (SFC) fragmentation test can be utilized to provide quantitative information on the fiber strength distribution and the fiber/matrix interface shear stress, which are important properties that control the performance of fiber composites. An accurate interpretation of the fragmentation data, however, is difficult owing to the stochastic nature of the fragmentation process, as well as the complex interplay between the fiber flaw strength variation and the stress transfer zones about every broken flaw. In this work we have developed a computer simulation approach that models the fragmentation process by explicitly incorporating considerations regarding the strength and spatial distributions of the flaws. The effect of stress variation along the fiber length is accounted for by incorporating specific fiber-loading models. From the simulation it is demonstrated that the fragmentation data may be used to produce a rough sketch of the underlying flaw strength spectrum. An examination of the fragment size statistics suggests that appropriate analysis of the SFC data may be utilized to detect the occurrence of matrix/fiber interface yielding or debonding. An alternative methodology for mapping out the flaw strength distribution by means of a multiple-long-fiber failure test is also presented for comparison purposes.     

A model for fiber length attrition in injection-molded long-fiber composites

Long-fiber thermoplastic (LFT) composites consist of an engineering thermoplastic matrix with glass or carbon reinforcing fibers that are initially 10–13 mm long. When an LFT is injection molded, flow during mold filling degrades the fiber length. Here we present a detailed quantitative model for fiber length attrition in a flowing fiber suspension. The model tracks a discrete fiber length distribution at each spatial node. A conservation equation for total fiber length is combined with a breakage rate that is based on buckling of fibers due to hydrodynamic forces. The model is combined with a mold filling simulation to predict spatial and temporal variations in fiber length distribution in a mold cavity during filling. The predictions compare well to experiments on a glass–fiber/PP LFT molding. Fiber length distributions predicted by the model are easily incorporated into micromechanics models to predict the stress–strain behavior of molded LFT materials.     

An Investigation of Interfacial Fatigue in Fiber Reinforced Composites

Based on the shear-lag model and the modified degradation formula for coefficient of friction, the interfacial fatigue and debonding for fiber reinforced composites under cyclic loading are studied. The loading condition is chosen as the kind that is the most frequently used in fiber-pull-out experiments. The stress components in the debonded and bonded regions are obtained according to the maximum and minimum applied loading. By the aid of theory of fracture mechanics and Paris formula, the governing equation is solved numerically and the interfacial debonding is simulated. The relationships between the parameters (such as the debond rate, debond length, debond force) and the number of cycles are obtained.     

SEM环境下纤维推出技术结合电子束云纹技术表征复合材料界面细观力学性能

纤维推出技术是研究复合材料界面细观力学性能的常用方法。本文将该方法在SEM环境下与电子束云纹技术相结合开发一套基于SEM环境下的纤维推出实验系统。利用该系统测试了SiC/Ti-15-3复合材料的界面剪切强度、摩擦应力、摩擦系数及残余应力分布等细观力学性能。结果表明:对于厚度为500 μm的SiC/Ti-15-3复合材料界面剪切强度为35 MPa,摩擦应力为32.8 MPa,纤维与界面间的摩擦系数为0.082,径向残余应力为?400 MPa。该系统在SEM环境可以实现直径为几微米的纤维推出,扩展了纤维推出技术的应用范围,提高了纤维推出过程的对准精度,减小了测量误差。并且与电子束云纹技术相结合,实时测量纤维推出后界面残余应力分布情况,为复合材料界面的设计、评估及优化提供必要的实验方法。      

Crack bridging and fibre pull-out in polyethylene fibre reinforced epoxy resins

An investigation has been undertaken of the stress distributions in high-performance polyethylene fibres bridging cracks in model epoxy composites. The axial fibre stress has been determined from stress-induced Raman band shifts and the effect of fibre surface treatment has been followed using untreated and plasma-treated polyethylene fibres. It is found that when the specimen is cracked, the fibres do not break and stress is transmitted from the matrix to the fibre across the fibre/matrix interface. A debond propagates along the fibre/matrix interface accompanied by friction along the debonded interface. The axial stress distributions in the fibres can be analysed using a partial-debonding model based upon shear-lag theory and it is found that the maximum interfacial shear stress at the bond/debond transition is a function of the debond length. The debonding process has been modelled successfully in terms of the interfacial fracture energy-based criterion developed by Hsueh for the propagation of a debond along a fibre/matrix interface accompanied by constant friction along the interface.     

界面性能对陶瓷基复合材料拉伸强度的影响

基于陶瓷基复合材料拉伸试验现象引入了主裂纹损伤带的概念, 并将其宽度定义为界面脱粘长度. 由于界面性能对纤维应力集中有较大影响, 并且控制着材料的断裂模式, 分别给出了脆性断裂和韧性断裂的强度计算公式, 并引入了应力集中系数和界面脱粘能量释放率. 分析结果表明, 拉伸强度随着应力集中系数和界面脱粘能量释放率的增大而减小. 文中公式给出的预测值与试验值吻合较好, 表明断裂时纤维所承担的应力用脱粘段纤维平均应力来衡量是合适的.     

界面剪切强度的AE监测及统计模型评估

本文成功地运用声发射(AE)技术测得玻璃丝单纤维复合材料(SFC)试验的临界断裂长度,并运用统计分析方法建立了韦伯分布纤维强度的修正界面剪切强度评估模型.本模型的计算结果与D.Jaques和J.P.Favre等模型作了比较,表明该模型描述界面剪切强度是比较切合实际的.     

Shear-lag analysis of notched laminates with interlaminar debonding

This study considers a method of analysis for predicting the fracture behavior of a notched, unidirectional lamina in the presence of surface constraint layers with debonding between the unidirectional ply and the constraint layers. Two particular cases are presented, the first being a debonded zone of finite width with no longitudinal damage in the unidirectional ply. This solution is then extended to include longitudinal matrix yielding and splitting in the unidirectional ply at the crack tip. The analysis is based on a materials modeling approach using the classical shear-lag assumption to describe the shear transfer between fibers. The fracture behavior of the laminate is studied as a function of initial crack length, the relative physical and geometric properties of the constraint plies and the unidirectional lamina, and width of the debonded zone. The results indicate that debonding can reduce the maximum fiber stress at the crack tip on the order of ten percent. This effect is maximum for a debond width of two or three fiber spacings and is independent of the initial crack length. As the debond width grows beyond this point, the maximum stress increases. For widths of about ten fiber spacings or more, the maximum fiber stress is larger than for the fully bonded case. In the presence of longitudinal matrix damage the same general behavior is found; however, the location of the maximum fiber stress is quite complex. In some cases with large matrix damage and a high constraint ratio, the maximum fiber stress can occur at the end of the debonded zone away from the crack tip.