Effects of residual stress and interfacial frictional shear stress on interfacial debonding at notch-tip in two-dimensional unidirectional composite

A calculation method based on the shear lag approach was presented to get an approximate estimate of influences of residual stresses and frictional shear stress at the debonded interface on the interfacial debonding behavior at the notch-tip along fiber direction in two-dimensional unidirectional double-edge-notched composites. With this method, the energy release rate for initiation and growth of debonding as a function of composite stress were calculated for some examples. The calculation results showed in outline how much the tensile and compressive residual stresses in the matrix and fiber along fiber direction, respectively, act to hasten the initiation and growth of the debonding when the final cut element in the notch is matrix, while they act to retard them when the final cut element is fiber, and how much the frictional shear stress at the debonded interface reduces the growth rate of the debonding.     

Growth of interfacial debonding in notched two-dimensional unidirectional composite under stress and displacement controls

A simplified calculation method for study of the growth of interfacial debonding between elastic fiber and elastic matrix ahead of the notch-tip in composites under displacement and stress controlled conditions was presented based on the shear lag approach in which the influences of residual stress and frictional shear stress at the debonded interface were incorporated. The calculation method was applied to a model two-dimensional composite. An outline is given of the difference and similarity in the growing behavior of the debonding between the displacement and stress controls, and of the influences of the residual stresses, frictional shear stress, the nature of the final cut component (fiber or matrix) and sample length on the debonding behavior.     

Evaluation on the interfacial fracture toughness of fiber-reinforced titanium matrix composites by push out test

The models for single-fiber push out test are developed to evaluate the fracture toughness G_IIc of the fiber/matrix interface in titanium alloys reinforced by SiC monofilaments. The models are based on fracture mechanics, taking into consideration of the free-end surface and Poisson expansion. Theoretical solutions to G_IIc are obtained, and the effects of several key factors such as the initial crack length, crack length, friction coefficient, and interfacial frictional shear stress are discussed. The predictions by the models are compared with the previous finite element analysis results for the interfacial toughness of the composites including Sigma1240/Ti-6-4, SCS/Ti-6-4, SCS/Timetal 834, and SCS/Timetal 21s. The results show that the models can reliably predict the interfacial toughness of the titanium matrix composites, in which interfacial debonding usually occurs at the bottom of the samples.     

Initial damage induced by thermal residual stress and microscopic failure analysis of carbon-fiber reinforced composite under shear loading

In order to study the mechanical properties and the progressive failure process of composite under shear loading, a representative volume element (RVE) of fiber random distribution was established, with two dominant damage mechanisms – matrix plastic deformation and interfacial debonding – included in the simulation by the extended Drucker–Prager model and cohesive zone model, respectively. Also, a temperature-dependent RVE has been set up to analyze the influence of thermal residual stress. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the in-plane shear fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation. The progressive damage process and final failure mode of in-plane shear model which are based on constitute are very consistent with the observed result under scanning electron microscopy of V-notched rail shear test. Also, a transverse shear model was established as contrast in order to comprehensively understand the mechanical properties of composite materials under shear loading, and the progressive damage process and final failure mode of composite under transverse shear loading were researched. Thermal residual stress changes the damage initiation locations and damage evolution path and causes significant decreases in the strength and fracture strain.     

Interfacial debonding and slipping of carbon fiber-reinforced ceramic-matrix composites under two-stage cyclic loading

In this paper, the interface debonding and frictional slipping of carbon fiber-reinforced ceramic-matrix composites (CMCs) under two-stage cyclic fatigue loading have been investigated using micromechanics approach. Under cyclic fatigue loading, the fiber/matrix interface shear stress degrades with increasing cycle number due to interface wear. The synergistic effect of interface wear and fatigue loading sequence on interface debonding and frictional slipping has been analyzed. Based on the fatigue damage mechanism of fiber slipping relative to matrix, in the interface debonded region, upon unloading and subsequent reloading, the interface debonded length and interface slip lengths, i.e. interface counter-slip length and interface new-slip length, are determined using the fracture mechanics approach. The relationships between interface debonding, interface slipping, interface wear, cycle number, and different loading sequences are determined. There are two types of fatigue loading sequences considered, i.e. (1) cyclic loading under low peak stress for N₁ cycles, and then high peak stress; and (2) cyclic loading under high peak stress for N₁ cycles, and then low peak stress. The effects of peak stress level, interface wear, cycle number, and loading sequence on interface debonding and frictional slipping of fiber-reinforced CMCs have been analyzed. The fatigue hysteresis loops of cross-ply carbon fiber-reinforced silicon carbide composite corresponding to different cycle number under two-stage cyclic fatigue loading have been predicted.     

On the use of energy methods for interpretation of results of single-fiber fragmentation experiments

_We consider fragmentation experiments as a set of experimental results for fiber break density as a function of applied strain. This paper explores the potential for using fracture mechanics or energy methods in interpreting fragmentation experiments. We found that energy does not control fiber fracture; instead, fiber fracture releases much more energy than required to fracture the fiber. The excess released energy can lead to other damage mechanisms such as interfacial debonding. By assuming that all the excess released energy causes interfacial debonding and balancing energy using the energy release rate for debonding, we were able to determine interfacial toughness from fragmentation experiments. A reliable determination of interfacial toughness requires prior knowledge of interphase stress-transfer properties, fiber failure properties, actual damage mechanisms, and the coefficient of friction at the interface.     

Influence of heat treatment on microstructure and mechanical properties of the interface in an EN AW-6082/1.4310 composite extrusion

The presented paper deals with a unidirectional steel wire reinforced aluminum matrix composite manufactured by composite extrusion. The main objective of this work was to determine the effect of heat treatment, and the influence of long solution annealing times on the composites interface regarding microstructural changes and the resulting interface strength. For evaluation of the microstructure high resolution transmission electron microscope (TEM) investigations accompanied with electron dispersive X-ray spectroscopy (EDX) were performed. It could be shown that diffusion from the steel wire into the aluminum matrix occurs and that the diffusion paths as well as particle formation is influenced by the preceded heat treatment. Diffusion paths in the range of 40–150 nm could be observed for Al, Fe, Cr and Ni. After annealing times over 5 h an extensive growth of an intermetallic reaction layer was found. The mechanical properties of the interface were determined by push-out-tests and tensile tests radial to the interface, which provided the debonding shear strength σ_deb and for the latter experiment the interfacial radial strength σ_IR. It has become apparent that debonding shear strength is highly influenced by matrix properties. In radial tensile tests the failure is predominantly controlled by the chemical bond of the interface. It was shown that interface strength of specimen with small reaction zones of about 3 μm were beneficial for the mechanical behavior in both loading conditions. Longer annealing times showed a drastic decrease of interface shear strength. It was concluded from EDX measurements and in comparison with literature that the reaction zone is dominated by the growth of Al₅Fe₂ (η-phase).     

Numerical simulation of micro-scratch tests for coating/substrate composites

In this paper the micro-scratch test is simulated by ANSYS finite element code for thin hard coating on substrate composite material system. Coulomb friction between indenter and material surface is considered. The material elastic-plastic properties are taken into account. Contact elements are used to simulate the frictional contact between indenter and material surfaces, as well as the frictional contact after the detachment of coating/substrate interfaces has taken place. In the case of coating/substrate interfaces being perfectly bonded, the distributions of interfacial normal stress and shear stress are obtained for the material system subjected to normal and tangential loading. In the case of considering the detachment of interfaces, the length of interfacial detachment and the redistribution of stresses because of interfacial detachments are obtained. The influences of different frictional coefficients and different indenter moving distances on the distributions of stresses and displacements are studied. In the simulation, the interfacial adhesion shear strength is considered as a main adhesion parameter of coating/substrate interfaces. The critical normal loading from scratch tests are directly related to interfacial adhesion shear strengths. Using the critical normal loading known from experiments, the interfacial adhesion shear strength is obtained from the calculation. When the interfacial adhesion shear strength is known, the critical normal loading is obtained for different coating thicknesses. The numerical results are compared with the experimental values for composite materials of thin TiN coating on stainless steel substrate.     

Interfacial debonding behavior of a rigid particle-filled polymer composite

Glass beads, non-modified and modified with coupling agent, were filled separately into high density polyethylene to obtain composite materials with different interfacial adhesion strengths. In situtensile tests reveal the damage mechanisms, which are mainly induced by the interfacial debonding. The interfacial debonding process is observed and studied. The debonding stress is found to be linearly related to the opening angle formed at two poles of the particles. Initial and final opening angles, in addition to the corresponding debonding stresses, are measured. The interfacial fracture energy obtained by using the Griffith fracture theory is found to be 0.028 J m^-2 and 0.058 J m^-2 for mechanical anchorage and physical entanglement across the interface, respectively. The stronger the interfacial adhesion, the smaller is the maximum opening angle and greater the debonding stress.     

Continuum Micro-mechanics of Estimating Interfacial Shear Strength in Short Fiber Composites

A new continuum approach to micro-mechanics of short fiber composites yielded two separate methods of estimating the apparent interfacial shear strength and fiber orientation efficiency. The methods exploit the compilation of the effects of fiber length distribution and interfacial shear strength on strengthening efficiency into a function of strain. The In-Built Method derives a unique combination of apparent interfacial shear strength and fiber orientation efficiency being able to reproduce the experimental stress–strain curve of a short fiber reinforced composite with a very low residual standard deviation. The Boundary Method accomplishes rapid interfacial shear strength screening in materials selection by constructing and utilizing the proposed selection chart.     

Relation between interfacial shear strength and compressive strength of carbon fiber/epoxy resin composite strands

The mechanism with which the fiber-matrix interfacial strength exerts its influence on the compressive strength of fiber reinforced composites has been studied by measuring the axial compressive strength of carbon fiber/epoxy resin unidirectional composite strands having different levels of interfacial shear strength. The composite strands are used for experiments in order to investigate the compressive strength which is not affected by the delamination taking place at a weak interlayer of the laminated composites. The interfacial strength is varied by applying various degrees of liquid-phase surface treatment to the fibers. The efficiency of the compressive strength of the fibers utilized in the strength of the composite strands is estimated by measuring the compressive strength of the single carbon filaments with a micro-compression test. The compressive strength of the composite strands does not increase monotonically with increasing interfacial shear strength but showes lower values at higher interfacial shear strengths. With increasing interfacial shear strength, the suppression of the interfacial failure in the misaligned fiber region increases the compressive strength, while at higher interfacial shear strengths, the enhancement of the crack sensitivity decreases the compressive strength.     

Interfacial adhesion between semi-crystalline polymers (polypropylene and nylon-6): in situ compatibilized interface and fracture mechanism

The interfacial fracture toughness between semi-crystalline polymers (polyamide/polypropylene) were studied to understand the failure mechanisms at the interface, especially when the interface was reinforced by an in situ compatibilizer. Based on the observation of the interface using scanning electron microscopy and wide angle X-ray spectroscopy, it was revealed that crystalline structure of polypropylene was not affected by the in situ compatibilizer at the interface. The reinforcing mechanism could be qualitatively identified by investigating the evolution of fracture toughness as a function of annealing time and temperature. The adhesion strength increased with the annealing time. Depending on the annealing temperature, the fracture toughness passed a peak value and then reached a plateau after some bonding time. As long as the chain length of the compatibilizer is long enough to form entanglements with the molecules at both bulk sides, the fracture at the interface is decided by the balance between adhesion strength at the interface and cohesive strength in the weak modulus side; the failure locus follows the lower one. Thus, adhesive failure occurred first when the reaction at the interface did not occur long enough to provide high adhesive strength at the interface, but the cohesive failure occurred in the crack propagation side after the adhesive strength value became higher than the cohesive strength value.     

Assessment of interface deformation and fracture in metal matrix composites under transverse loading conditions

The transverse properties of unidirectional metal matrix composites (MMCs) are dominated by the fiber/matrix interfacial properties, residual stresses and matrix mechanical response. In order to monitor and study, in situ, the failure of interfaces in titanium-based composites subjected to transverse loading conditions, an ultrasonic imaging technique has been developed. The interface was imaged ultrasonically and the change in ultrasonic amplitude with the transverse loading was monitored, indicating the sensitivity of the technique to fracture and deformation of interfaces. This change in amplitude has been explained in terms of the multiple reflection theory of ultrasonic waves. The multiple reflection theory enabled estimation of the interfacial deformation and debonding as a function of loading. The ultrasonic technique was also used in conjunction with finite element modeling in order to quantify the fiber/matrix interfacial transverse strength in situ in MMCs.     

Predicting Interfacial Strengthening Behaviour of Particulate-Reinforced MMC — A Micro-mechanistic Approach

The fracture properties of particulate-reinforced metal matrix composites (MMCs) are influenced by several factors, such as particle size, inter-particle spacing and volume fraction of the reinforcement. In addition, complex microstructural mechanisms, such as precipitation hardening induced by heat treatment processing, affect the fracture toughness of MMCs. Precipitates that are formed at the particle/matrix interface region, lead to improvement of the interfacial strength, and hence enhancement of the macroscopic strength properties of the composite material. In this paper, a micro-mechanics model, based on thermodynamics principles, is proposed to determine the fracture strength of the interface at a segregated state in MMCs. This model uses energy considerations to express the fracture toughness of the interface in terms of interfacial critical strain energy release rate and elastic modulus. The interfacial fracture toughness is further expressed as a function of the macroscopic fracture toughness and mechanical properties of the composite, using a toughening mechanism model based on crack deflection and interface cracking. Mechanical testing is also performed to obtain macroscopic data, such as the fracture strength, elastic modulus and fracture toughness of the composite, which are used as input to the model. Based on the experimental data and the analysis, the interfacial strength is determined for SiC particle-reinforced aluminium matrix composites subjected to different heat treatment processing conditions.     

Roles of interfaces between carbon fibers and epoxy matrix on interlaminar fracture toughness of composites

The effect of atmospheric-pressure plasma treatment on high strength PAN-based carbon fibers had been studied in terms of fiber surface energetics and mode I and II interlaminar fracture toughness of unidirectional carbon fibers/epoxy matrix composites. The surface characterization of plasma treated carbon fibers was investigated by X-ray photoelectron spectroscopy (XPS) and contact angles. As a result, the plasma treatment changed the surface properties of the carbon fibers, mainly through formation of oxygen functional groups like hydroxyl, carbonyl, and carboxyl groups. According to contact angle measurements, it was observed that plasma treatment led to an increase in surface free energy of the fibers, mainly due to the increase of its specific component. Fracture toughness test results employing double-cantilever beam (DCB) and end notched flexure (ENF) specimens also showed that the increase in specific components or hydrogen bonding between the –OH groups on carbon fibers and the =O ring in epoxy matrix resins played an important role in improving the degree of adhesion at interfaces, resulting in an increase in the interfacial fracture toughness of the composites studied.     

单层单晶石墨烯与柔性基底界面性能的实验研究

单晶石墨烯具有更优异的力学及电学性能,有望成为新一代柔性电子器件的核心材料.因此,有必要从实验的角度精细分析化学气相沉积法制得的大尺度单晶石墨烯与柔性基底复合结构的界面力学行为.本文通过显微拉曼光谱实验方法测量了不同长度的单层单晶石墨烯/PET(聚对苯二甲酸乙二醇酯)基底的界面力学性能参数及其在长度方向上界面边缘的尺度效应.实验给出了石墨烯在PET基底加载过程中与基底间黏附、滑移、脱黏三个界面状态的演化过程与应力分布规律.实验发现,单晶石墨烯与柔性基底间由范德瓦耳斯力控制的界面应变传递过程存在明显的边缘效应,并且与石墨烯的长度有关.界面的切应力具有尺度效应,其值随石墨烯长度的增加而减小,而石墨烯界面传递最大应变以及界面脱黏极限则不受试件尺度的影响.     

Interfacial adhesion and micro-failure phenomena in multi-fiber micro-composites using fragmentation test

Statistical fragments and micro-failure modes in the multi-fiber-reinforced micro-composites were investigated by fragmentation test. The specimen consisted of three fibers using carbon fibers (CFs) and glass fibers (GFs), embedded in the epoxy resin with three-dimensional arrangement. Fracture morphology and micro-failure behavior from the progressive fragmentation of fibers and fiber/matrix interfacial adhesion were observed via polarized-light microscope. The interfacial shear strength of CF/epoxy micro-composites is higher than that of the GF/epoxy micro-composites measured by the single fiber fragmentation test. The results show that fragment number on monofilament demonstrates obvious differences between multi-fiber and single fiber systems, and the location of the breakpoint is determined by the CFs that fracture firstly, indicating clustering fracture modes. This is because stress concentration around the breakpoints influences the stress redistribution on the adjacent fibers. Distinct micro-failure modes were observed in three-fiber and the hybrid systems, where matrix cracks around the CFs and interfacial debonding occurs around the GFs. The mixture of CFs and GFs demonstrates distinctive hybrid effect by the changes of the fragment number and initial fracture strain of fibers in hybrid systems.     

Is there any contradiction between the stress and energy failure criteria in micromechanical tests? Part II. Crack propagation: Effect of friction on force-displacement curves

In micromechanical tests for estimating fiber-matrix interfacial properties, such as the pull-out and microbond tests, fiber debonding from a matrix is often accompanied by friction in debonded areas. In the present study, force-displacement curves, which are usually recorded in these tests, were modeled with taking interfacial friction into consideration. The friction stress was assumed, as a first approximation, to be constant across the interface. Two different approaches to interfacial failure were used: the shear-lag approach with a stress-based debonding criterion (the ultimate interfacial shear strength) and the linear elastic fracture mechanics approach using the critical energy release rate as a condition for crack propagation. The force-displacement curves derived from both models are in good agreement with each other and with experimental micromechanical data. It was shown that any pull-out and microbond experiment comprises four stages: (1) linear loading up to the point where debonding starts; (2) stable crack propagation with friction-controlled debonding; (3) catastrophic debonding; and (4) post-debonding friction. Stable crack propagation was shown to be controlled by both friction and release of residual thermal stresses. An algorithm for estimating both fiber-matrix adhesion and interfacial friction from the microbond and pull-out tests data has been proposed.     

Influence of γ-ray radiation grafting on interfacial properties of aramid fibers and epoxy resin composites

This research used Co⁶⁰ γ-ray radiation to modify Armos fibers in 1,2-epoxy-3-chloropropane. After the treatment, the interlaminar shear strength (ILSS) values of aramid/epoxy composites were improved by about 20%. Surface elements of Armos fibers were determined by XPS analysis, which indicated that the oxygen/carbon ratio was increased. The surface of the fibers treated was rougher than that of the untreated fibers when examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Fourier transform infrared (FT-IR) spectra confirmed that the epoxy group was grafted onto the fibers. The wettability of the fibers' surface was also enhanced by the treatment. Nanoindentation technique analysis showed that the nanohardnesses of the various phases (the fiber, the interface and the matrix) in the composite, whose fibers were treated, were correspondingly higher than those in the composite, whose fibers were untreated. The results indicate that γ-ray irradiation grafting technique, which is a suitable batch process for industrialization, can modify the physicochemical properties of Armos fibers and improve the interfacial adhesion of its composite.     

Interface end theory and re-evaluation in interfacial strength test methods

The experimental results of fragmentation, micro-indentation, pull-out and microdebond tests often exhibit large discrepancies. Since all specimens of the four test methods all have interface ends, the singularity theory of the interface end should be used to evaluate the exactness of the test methods. The eigenvalues of the specimens for the micro-indentation test, pull-out test and microdebond test are calculated and investigated. The results show that the stress singularity of the interface end depends on the Dundurs' parameters and the wedge angles. The interfacial shear strength (IFSS) obtained from the tests loses its rationality if the stress is singular at the interface end. In further analysis, for a carbon fiber-epoxy resin composite, it is found that the microdebond test gives the most reliable IFSS results, if the wedge angle of the resin droplet is less than 40°; the results from the pull-out test are dubious, due to the stress singularity at the interface end. In the micro-indentation test, there is a critical matrix stiffness value for a given fiber, above which the stress at the interface end will be non-singular. The fragmentation test assumes the interfacial shear stress on the fiber fragment of critical length is the IFSS. If debonding does not occur at the interface end, then apparently, the interfacial shear stress on the fiber fragment of critical length is less than the true value of IFSS.