Effect of interfacial debonding on the failure behavior in a fiber-reinforced composite subjected to transverse tension

By using computational micromechanics, macroscopic stress–strain curves for a glass/epoxy lamina subjected to transverse tension were determined in this paper. To compute stress for given strain, a finite element model of a three-phased unit cell with the hexagonal symmetry was employed. Mixed mode debonding conditions between reinforcement and matrix were modeled by a bilinear cohesive law. A stress transfer between matrix and fiber was simulated by a inhomogeneous interphase. A detailed analysis of a debonding growth was also presented. Parametrical studies showed that a choice of values for cohesive parameters as well as debonding locations do have an influence on the macroscopic response of material. An ability of the proposed model to simulate the softening behavior of the material under transverse tension and to predict final failure was demonstrated.     

On the determination of interfacial stresses in a composite material

The conventional finite element method has been modified to allow the elastic stresses along the fibre matrix interfaces of a composite to be determined with improved accuracy. The results obtained by this 'modified' method are compared with both a photoelastic and some traditional finite element solutions.     

Correlation of fiber pull-out strength and interfacial vibration damping techniques by micromechanical analysis

Adhesion between fiber and matrix in fiber-reinforced polymer composites plays an important role both in controlling mechanical properties and in the overall performance of composites. This suggests that analytical and experimental methods to characterize the interface can be used to predict the mechanical performance of the material. To this end, vibration damping techniques have been used as a non-destructive method to evaluate interfacial effects on composites. According to the theory of energy dissipation, the quality of the interfacial adhesion can be evaluated upon separating the predicted internal energy dissipation associated with perfect adhesion from the measured internal energy dissipation of a composite system; this enables the quantification of interfacial adhesion. A micromechanics-based model for evaluating the adhesion between fiber and matrix from the damping characteristic of a cantilever beam was developed that shows an inverse relationship between the damping contributed by the interface and its adhesion strength. A simple optical system was used to measure the damping factor of unidirectional fiber-reinforced-polymer composites. Cantilever beam specimens containing either a single glass fiber or three types of single metallic wires embedded in an epoxy resin matrix were tested. A correlation was found between the measured interfacial adhesion strength directly from microbond pull-out tests and the micromechanics-based calculations from vibration damping experiments.     

Analysis of first ply failure in composite laminates

The mechanics of transverse cracking in an elastic fibrous composite ply is explored for the case of low crack density. Cracks are assumed to initiate from a nucleus created by localized fiber debonding and matrix cracking. It is found that cracks may propagate in two directions on planes which are parallel to the fiber axis and perpendicular to the midplane of the ply. In general, crack propagation in the direction of the fiber axis controls the strength of thin plies, while cracking in the direction perpendicular to the fiber axis determines the strength of thick plies. The theory relates ply thickness, crack geometry and ply toughness to ply strength. It predicts a significant increase in strength with decreasing ply thickness in constrained thin plies. The strength of thick plies is found to be constant, but it may be reduced by preexisting damage. Results are illustrated by comparison with experimental data.     

Time-dependent interfacial failure in metallic alloys

Time-dependent intergranular brittle fracture has now been studied experimentally in a number of alloy systems, and the generic features are becoming clear. Mobile surface-adsorbed elements are caused to diffuse inward along grain boundaries under the influence of a tensile stress, and this can lead to sub-critical crack growth by decohesion. Oxygen is found to play this role in nickel-base superalloys and intermetallics, as well as in a precipitation-strengthened Cu–Be alloy. Crack-growth rates lie in the range 10⁻⁷–10⁻⁴ m sec⁻¹. The same kind of cracking is found in steels treated so that free sulfur is able to segregate to the surface, as well as in Cu-Sn alloys, in which the embrittling element is surface-segregated Sn. The latter has been studied in bicrystals, and the importance of the variation in diffusivity with grain boundary structure has been documented. Hydrogen-induced cracking is a special case of an extremely mobile embrittling element and is responsible for much of the brittleness found in intermetallics. The effect of boron in retarding brittle behavior in Ni₃Al has been shown to result partly from its interaction with hydrogen. This is a prime example of how segregated solutes can be used to ameliorate the tendency for diffusion-controlled brittle fracture.     

The failure mechanism and the pull-out strength of a bond-type anchor near a free edge

A bond-type anchor bolt has been used as a post-installed type anchor. Recent developments in construction technology have diversified the use of this type of anchor even in a new construction site. Since this type of anchor exhibits a complex failure mode involving cone and bond failure, its design method is still to be investigated in depth. Among the various conditions to be considered, we focus on the effect of a free edge on the pull-out strength both experimentally and analytically. We propose a new method to estimate the cone failure strength using the theory of linear fracture mechanics. The experimental results coincide well with the analytical estimation. The reduction of the strength is much larger than expected by the analysis based on the current design method.     

A simplified FE model for pull-out failure of spot welds

A simplified model is developed to simulate the pull-out failure behavior of spot welds in full vehicle crash simulations. The failure is only considered in the base steel which is characterized with the Gurson model. The parameters of the Gurson model are optimized from modeling several different test types of base steel. The weld region is represented by a single solid element, and its properties are optimized from modeling the coach-peel and lap-shear joints. The simplified model is validated by modeling the spot-welded coupon tests. It is indicated that an acceptable time step size for full vehicle crash simulations can be achieved with the model.     

Two stages of interfacial reaction in B-Al composite

Using an original technique, new data for the interfacial reaction in B-Al composites have been obtained. Two stages of the boron fibre/aluminium interface reaction were revealed. In Stage 1, due to the aluminothermal reduction of the oxide film on the boron fibre, finecrystalline rhombohedral boron, alumina and dispersed aluminium borides are formed. The latter are formed by the reaction with the-fine-crystalline boron and do not affect the fibre strength but can contribute to a more intensive interaction. In Stage 2, borides are formed by direct reaction with the fibres, thus seriously reducing their strength.     

Apparent interfacial failure in mixed-mode adhesive fracture

Aluminium-epoxy adhesive specimens constructed with the bond at 45? to the direction of loading appear to fail very close to the interface. The actual locus of failure was investigated by¹⁴C labelling of the epoxy polymer and also by Auger spectroscopy profile analysis. Both techniques indicated a residual film of polymer a few hundred angstroms thick on the aluminium surface. The fracture energy of these specimens was determined and found to be affected by the surface roughness of the aluminium. The mixed-mode fracture energy (G _{I,II) C} ^45° of these specimens in the absence of any surface roughness effect (polished surfaces) was 140 J m^?2 compared to 136 J m^?2 for the same polymer in simple opening-modeG _{I C} adhesive fracture. The “interfacial” failure and the effect of surface finish on fracture are discussed in terms of the applied stress directing the failure toward the interface but the approach of the crack to the boundary being limited by the size of the crack tip deformation zone.     

Study of shear-induced interfacial crystallization in polymer-based composite through in situ monitoring interfacial shear stress

In order to further investigate shear-induced interfacial crystallization of polymer-based composites, an improved fiber-pulling device was designed and built. Its peculiar characteristic is that a force transducer is assembled to in situ monitor the variation of interfacial shear stress between the polymer matrix and pulling fiber. Thus, the relationship between interfacial shear stress and the subsequent crystalline morphology can be quantitatively established. In the preliminary study via this device, isotactic polypropylene (iPP)/glass fiber composite was adopted as a model system. The results indicate that interfacial crystallization kinetics is promoted by the presence of interfacial shear stress. Furthermore, there are two thresholds of interfacial shear stress for interfacial crystalline morphology. To be specific, one (0.017 MPa) is for the induction of iPP nucleation, above which α-form iPP crystals are obviously encouraged during the subsequent isothermal crystallization; the other is for the generation of β-form iPP crystals (0.042 MPa), above which β-form crystals are favored to be triggered in the transcrystalline region.     

The interfacial reaction and microstructure in a ZK60-based hybrid composite

The oxidation behavior of B₄C particles was investigated by measuring the weight changes during isothermal heating at 650 to 800°C. The interfacial microstructure and fractography of the B₄Cp and SiCw reinforced ZK60 magnesium hybrid composite was studied using a scanning electron microscope (SEM), a transmission electron microscope (TEM) and an electron energy loss spectrometer (EELS). It was found that the oxidation of B₄C particles in air at the temperatures investigated produced an amorphous B₂O₃ surface layer. The oxidation kinetics was limited by the diffusion rate of oxygen through the liquid B₂O₃ layer. The oxidation layer was found to dominate the interfacial reaction, the microstructure and the mechanical properties of the hybrid composite. The rod-like MgB₂ and the granular MgO were produced due to the reaction between the B₂O₃ and the magnesium matrix. The intimate bonding between the MgB₂ and both the reinforcements and the matrix is believed to be the main reason for the improvement of the mechanical properties of the composite.     

Scattering of antiplane shear waves in a fiber-reinforced composite medium with interfacial layers

Summary This paper deals with the scattering of antiplane shear waves in a metal matrix composite reinforced by fibers with interfacial layers. We assume same-size cylindrical inclusions and same-thickness interface layers with nonhomogeneous elastic properties. The effective complex wave numbers follow from the coherent wave equation which depends only upon the scattering amplitude of the single scattering problem. Effective elastic constants can be obtained from phase velocities of coherent waves. Numerical calculations for an SiC-fiber-reinforced Al composite are carried out, and the effect of interface properties on scattering cross section, phase velocity, attenuation of coherent plane wave, and effective elastic constant is shown graphically.     

Thermodynamic analysis of fatigue failure in a composite laminate

We put forward a thermodynamic approach for analyzing fatigue failure in a composite laminate. We show that fatigue is an irreversible progression of increasing entropy that accumulates until it reaches a critical value called the fracture fatigue entropy (FFE) at the onset of failure. Extensive series of fatigue tests are carried out that involve load-controlled tension-tension, and displacement controlled fully-reversed bending fatigue with three different stress ratios as well as constant- and variable-loading. The role of hysteresis energy in the entropy generation is investigated. FFE values are calculated based the experimental data obtained for temperature and hysteresis energy of a woven Glass/Epoxy (G10/FR4) laminate. The concept of tallying entropy accumulation and the use of FFE are useful for determining the fatigue life of composite laminates undergoing cyclic loading.     

Probabilistic analysis of a pull-out test

This paper presents a sensitivity analysis of the pull-out strength of reinforcement embedded in concrete. Considering both European and French design codes, this failure strength depends on the variability of uncertain parameters such as Young’s modulus of concrete and yield stresses of materials (concrete and steel); moreover, two failure modes can be observed in the studied experimental test. A methodology allowing the characterization of the sensitivity of the pull-out strength to these uncertain parameters is derived. These parameters are modeled by Lognormal random variables. Results show the evolution of the pull-out strength for different anchorage lengths. Probability density functions of the random variable modeling the failure strength are computed using probabilistic methods. A finite element model is also built to quantify uncertainties concerning failure modes, computing 95% confidence intervals.     

毛白杨木粉/聚丙烯复合材料界面相容性的介电弛豫解析

为解析木塑复合材料的界面相容性机制,通过介电弛豫过程分析研究不同硅烷偶联剂添加量的毛白杨木粉/聚丙烯复合材料的温度谱及频率谱,并计算介电弛豫过程中的表观活化能和热力学量。结果表明:在添加硅烷偶联剂的毛白杨木粉/聚丙烯复合材料中能观察到基于木材细胞壁无定形区中伯醇羟基的回转取向运动的弛豫过程;弛豫强度随硅烷偶联剂添加量增大先减少而后缓慢增大;随偶联剂添加量的增大,弛豫时间分布峰呈先变宽、变低,然后再变尖、变高趋势;表观活化能、活化焓、活化自由能和活化熵随硅烷偶联剂添加量增加先增大后减小。表观活化能在硅烷偶联剂添加量(质量比)为2.0%时达到最大值(28.12kJ/mol),与未添加偶联剂的毛白杨木粉/聚丙烯复合材料的(13.86kJ/mol)相比增加2倍以上,活化焓在硅烷偶联剂添加量从0%时的12.09kJ/mol增大到2.0%时的26.35kJ/mol,增大了117.9%,说明弛豫过程中伯醇羟基回转取向运动需要克服的能垒增加,毛白杨木粉与聚丙烯塑料的相容性更好,结合更紧密,界面强度更强,性能更加稳定。