Influence of mode mixity and loading conditions on the fatigue crack growth behaviour of an epoxy adhesive

Adhesive joints usually experience mixed mode and mostly cyclic stresses conditions during their service life. The aim of the current research is to investigate the fatigue behaviour of a structural epoxy adhesive. Pure modes I and II and mixed mode tests were carried out to study the fracture and fatigue crack growth (FCG) behaviour of the adhesive. Compliance‐based beam method was considered for experimental fracture energy measurement. The effects of load level and load ratio on the mode I FCG behaviour and Paris law parameters were also investigated. Result showed that the effect of load level on fatigue crack propagation is more pronounced for lower R ratios. It was found that when the crack faces are closer during the unloading process, the difference between the R² and G_min/G_max is higher. Some possibilities are the crack closure phenomenon, difficulty in measuring the G_min , and the employed data reduction approach.     

金属与多孔性材料复合用胶粘剂的研制

本文介绍金属 -多孔性复合板材用改性EVA粘合剂的合成方法 ,讨论了原料配比、反应温度、A组分与B组分的配比对粘合剂性能和剥离强度的影响。     

The influence of mode mixity and adhesive system on the fatigue life of adhesive joints

In real applications, adhesive joints are commonly subjected to fatigue and mixed mode loading conditions. The aim of the current work is to experimentally analyse the influence of mode mixity on the fatigue strength of joints with an epoxy‐based adhesive. Different adhesive systems (acrylic and epoxies) were considered for pure mode I fatigue loading conditions. To achieve this, Arcan joints with an epoxy adhesive were manufactured and tested at different mode mixities. Based on stiffness degradation, monitored during the tests, damage evolution was calculated for different loading conditions and for all the tested adhesives. Finally, fatigue envelopes were constructed for different fatigue life regimes. Results show that a shear loading component reduces both the static strength and fatigue life of the joints. A small reduction rate of the stiffness was found throughout the most part of the life until a sudden drop was observed, indicating a smooth damage evolution.     

Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite

Axial fatigue tests have been performed at three different stress ratios, R, of ?1, 0 and 0.4 using smooth specimens of an aluminium alloy composite reinforced with SiC particulates of 20 μm particle size. The effect of stress ratio on fatigue strength was studied on the basis of crack initiation, small crack growth and fracture surface analysis. The stress ratio dependence of fatigue strength that has been commonly observed in other materials was obtained, in which fatigue strength decreased with increasing stress ratio when characterized in terms of stress amplitude. At R=?1, the fatigue strength of the SiC_p/Al composite was the same as that of the unreinforced alloy, but at R= 0 and 0.4 decreased significantly, indicating a detrimental effect of tensile mean stress in the SiC_p/Al composite. The modified Goodman relation gave a fairly good estimation of the fatigue strength at 10⁷ cycles in the unreinforced alloy, but significantly unconservative estimation in the SiC_p/Al composite. At R= 0 and 0.4, cracks initiated at the interfaces between SiC particles and the matrix or due to particle cracking and then grew predominantly along the interfaces, because debonding between SiC particles and the matrix occurred easily under tensile mean stress. Such behaviour was different from that at R=?1. Therefore, it was concluded that the decrease in fatigue strength at high stress ratios and the observed stress ratio dependence in the SiC_p/Al composite were attributed to the different fracture mechanisms operated at high stress ratios.     

基于振动疲劳试验的复合材料螺栓连接预紧力松弛特性

在强迫弯曲振动试验的基础上,建立了基于模态参数(共振频率和阻尼比)表征螺栓连接结构动态性能的分析方法和试验测试手段;通过施加不同初始预紧力和激振频率,探究碳纤维/环氧复合材料螺栓连接预紧力松弛的时变行为及其影响因素。结果表明:在10h振动疲劳过程中,螺栓初始预紧力越小,激振频率越大,连接件预紧力松弛程度越大;振动疲劳损伤会导致连接结构刚度衰退、阻尼增加;复合材料螺栓连接松弛受到材料黏弹性以及界面摩擦的共同影响,其中约50%的松弛是由复合材料黏弹性效应引起的。      

How defects in an adhesive layer influence the fatigue strength of bonded steel-sheet specimens

Fatigue specimens were manufactured with a number of artificial bond defects. The defects were different configurations of non-bonded areas within the nominally perfectly-bonded region. These defects included variants with a reduced degree of bond filling, bonds with a channel without adhesive, zig-zag-shaped bond edges, and finally bond areas with smooth corners. Fatigue testing was performed with the bond loaded in the peel mode. The stiffness was measured for the bonds and was found to vary significantly with defect type and size. The fatigue strength was also found to differ substantially for different bond defects. A finite element model combined with fracture mechanics was used to predict the stiffness drop during fatigue testing of the bonded specimens and to predict fatigue life. The effect of defects in the bond on the stiffness and life was predicted in a satisfactory way.     

Damage behaviour of laminated composites during fatigue loading

This study deals with the modelling of damage evolution in the carbon/epoxy laminated composites under static and fatigue loading. A cumulative damage model is developed on the basis of damage evolution due to static and fatigue during cyclic loading. A continuum damage mechanics (CDM)‐based damage model coupling with the micromechanics has been utilized to predict the fatigue behaviour of laminate composites. A multicriterion approach has been introduced to predict the damage behaviour in the longitudinal, transverse, and shear direction at the ply scale. Extensive experimental results on T300/EPL1012 carbon/epoxy laminates are prepared to characterize under static and fatigue loading and to evaluate the proposed model in different conditions. The obtained results show that at the beginning of the cyclic loading, the damage grows suddenly and increases until final failure, which justifies the proposed method is able to predict the evolution of the damage due to static and fatigue loading separately during cyclic loading. The obtained results show that considering damage due to static loading leads to more accurate results, particularly in low‐cycle fatigue.     

Flexural behaviour of UHTCC‐layered concrete composite beam subjected to static and fatigue loads

As an engineered material, ultra‐high toughness cementitious composite (UHTCC) exhibits the characteristics of pseudo strain hardening and multiple cracking under uniaxial tension. It can be applied as the reinforcing and protective material of concrete structures. In this paper, static and fatigue flexural tests were carried out on UHTCC‐layered concrete composite beams, for which UHTCC layer was used on the tension side. Under both static and fatigue loads, plane section assumption was suitable for such composite beams, and a good bond strength was achieved between the two layers. For static specimens, the UHTCC layer enhanced the ductility of the concrete layer. While under cyclic loads, because of the reinforcing effect of UHTCC, more than one crack were formed in the concrete layer, which led to a ductile deformation. Furthermore, the fatigue damage process of the composite beam was analysed.     

填充物、胶膜和Z-pin对复合材料T型接头强度影响对比

为提高复合材料T型接头结构的拉伸强度,对接头中胶膜属性、圆弧区填充物属性和Z-pin增强三种结构参数对T型接头强度的影响进行了研究。设计了两种不同胶膜属性、两种不同填充材料和有无Z-pin的同尺寸试验件,完成拉伸试验,测得极限位移和极限拉伸强度,并进行了对比分析,同时研究了不同T型接头的损伤演化过程。结果表明：J299胶膜复合材料T型接头的极限位移和极限载荷相比于J116B胶膜分别提高了57.8%和64.7%;ZXC195增强芯复合材料T型接头的极限位移和极限载荷相比于单向带材料分别提高了51.7%和30.3%;Z-pin钉对复合材料T型接头的极限位移和极限载荷分别提高了190.8%和31.9%。三种结构参数均只影响接头的极限载荷和极限位移的大小,接头的整体刚度没有改变。胶膜属性对接头极限载荷的提高影响最大,而Z-pin对接头的极限位移提高影响最大。     

Analysis of adhesive bonded composite lap joints with transverse stitching

The effect of transverse stitching on the stresses in the adhesive is investigated using an adhesive sandwich model with nonlinear adhesive properties and a transverse stitching model for adhesive bonded composite single-lap and double-lap joints. Numerical results indicate that, among all stitching parameters, thread pretension and stitch density have significant effect on the peel stresses in the adhesive; increase in the thread pretension and the stitch density leads to a decrease in peel stress in the adhesive, while an increase in other parameters generally results in a negligible reduction in peel stress. The effect of stitching was found to be negligible on the shear stresses in the adhesive. Thus it is concluded that stitching is effective for the joints where peel stresses are critical and ineffective for those where shear stresses are critical.     

Effect of fibre volume fraction on fatigue behaviour of glass fibre reinforced composite

The aim of this paper is to study the fatigue behavior of GFRP composites manufactured by vacuum bagging process by varying the volume fraction. Constant‐amplitude flexural fatigue tests were performed at zero mean stress, i.e. a cyclic stress ratio R=?1 by varying the frequency of the testing machine. The relationship between stiffness degradation rate and fibre volume fraction, was observed, and the influence of volume fraction on the tensile strength was also investigated. The results show that, as the volume fraction increases the stiffness degradation rate initially decreases and then increases after reaching a certain limit for the volume fraction. Graph between volume fraction and Young's modulus shows that as the volume fraction increases Young's modulus also increases and reaches a limit and then it decreases with further increase in volume fraction, due to the increase in fibre content which changes the material properties of the composite material. The obtained results are in agreement with the available results.     

The effect of prior fatigue on the creep behaviour of type 321 stainless steel

An investigation was made of the influence of prior elevated-temperature fatigue (at 400°C, 650°C and 750°C) on the creep behaviour of type 321 stainless steel stressed at 200 MPa at 650°C. Prior fatigue was found to accelerate the creep rate, increase the creep ductility, and decrease the time to failure. The acceleration in creep rate was ^∝e^1.8t^0.68, where e is the strain amplitude and t is the duration of the fatigue test. Transmission electron microscopy showed that the dislocation structures produced by prior fatigue tended to persist throughout the subsequent creep test. A stress-aided recovery mechanism is suggested to account for these effects.     

Simulation of fatigue performance of cross-ply composite laminates

The fatigue life of cross-ply composite laminates was evaluated using a statistical model. A modified shear-lag analysis was applied to describe the cycle-number-dependent stiffness reduction and consequent stress redistribution processes in the laminates resulted from both progressive transverse matrix cracking in transverse plies and local delamination at tips of transverse cracks. From the strength degradation behaviour and the static strength distribution of 0° plies as well as the fatigue behaviour of 90° plies, the fatigue life of cross-ply laminates with various types of lay-up can be simulated from the model. Predictions of fatigue performance are compared with experimental data for [0/90₂] _s , [02/90₂] _s and [0₂/90₄] _s graphite/epoxy cross-ply laminates: good agreements are obtained.     

Interlaminar crack onset in co‐cured and co‐bonded composite joints under mode I cyclic loading

Composite joints exhibit different behavior in regard to delamination resistance when dealing with fatigue phenomenon. This research work focuses on an investigation to understand the failure mechanisms on the interfacial strength domain for delamination onset in cocured and cobonded joints. The analysis was based on strain energy release rate versus number of cycles plots that were obtained from fatigue tests in mode I with a stress ratio R = 0.1. The analysis encompassed from the microscopic to mesoscopic level obtained from scanning electron microscopic, and the images processed to extract the most relevant fracture patterns. The main difference between the two technologies was the stress concentration at the crack tip in which the cobonded joint presents a fabric carrier that blunts the adhesive layer, then delaying the delamination. This paper provides important information and guidelines to aid designers in the selection of the best composite joint for high‐performance structural applications.     

On the linear three-dimensional behaviour of composite beams

This paper describes a method of evaluating the linear stiffness matrix of a three-dimensional beam which accounts for all the possible couplings involved in the use of composite laminations appropriate to satisfy design requirements. The starting point of the procedure is represented by a complete evaluation of the beam cross section properties by use of a two-dimensional finite element analysis. A mass matrix of the beam is also evaluated as well as the external forces, to account for overall thermal deflections.

Some examples are presented and the results are compared either with their theoretical counterparts or with numerical results obtained by means of a full three-dimensional finite element analysis.     

Micrographic studies on adhesively bonded scarf repairs to thick composite aircraft structure

The hard-patch approach to scarf repairs involves adhesively bonding a pre-formed patch into the scarf cavity. This approach has several potential advantages compared with the conventional soft-patch approach, which involves forming the patch from pre-preg and co-bonding it with the adhesive during cure of the patch directly in the repair cavity.Two methods for producing the hard-patch were investigated. The first was the moulded approach where the patch was laid up in a mould and cured prior to bonding in the repair cavity. The development and implementation of the moulded hard-patch repair technique on an F/A-18 horizontal stabiliser is described. The second approach involves machining the patch from a composite panel using digitised data obtained from the use of surface profiling equipment to capture the scarf cavity surface. Micrographic techniques were used to assess critical features of the bond-line produced from the different techniques. The results are compared with microscopic studies from a second F/A-18 horizontal stabiliser that was repaired much earlier using the soft-patch approach. Each repair is assessed in terms of the consolidation of plies along the bond-line and the conformity of the patch to the repair cavity as well as adhesive uniformity and porosity.     

Analytical and experimental study of load attractions and fatigue crack growths in two-sided bonded repairs

With an increasing use of bonded composite patches for repairing fatigue and other damage on metallic aircraft structures, a research program under the sponsorship of the US Air Force Research Laboratory has been conducted for the advancement of this bonded composite technology. Through this research program, analytical methods for design and analysis of bonded patches have been developed and integrated into a PC-based computer code called CRAS (Composite Repair of Aircraft Structure) software. These analytical methods are reviewed briefly in this paper and they are validated with test results.     

Finite-element analysis and experimental study on tensile fatigue behaviour of bitumen–stone adhesion

Tensile fatigue behaviours of bitumen–stone adhesion were investigated using a dynamic mechanics analyser under stress‐controlled mode at two temperatures of 5 and 25°C and various controlled‐stress levels. Failure characteristics including interfacial failure and cohesive failure were examined using image analysis of fracture surfaces. Finite‐element analysis on stress distributions was conducted under different temperatures, film thickness and interfacial bonding conditions. A Coulomb–Mohr like criterion in combination with shear and normal stresses is proposed to deal with the extreme thin adhesive layer, which can be further simplified into an adhesive zone without significant loss of accuracy for stress analysis.     

各向异性导电胶用新型导电复合粒子的制备

采用无钯活化的化学镀的方法成功地制备出外镀银/铜/环氧树脂新型导电复合粒子,并对导电复合粒子进行了光学显微镜、SEM和EDS分析.结果表明:制备的导电微粒密度小,具有良好的导电性能,可以满足各向异性导电胶应用需求.     

Thermo-mechanical fatigue behaviour of welded tubular parts made of ferritic stainless steel

Car exhaust manifolds are critical components subjected to cyclic thermo-mechanical fatigue (TMF) during function. To reduce design costs, robust numerical design tools are required to assess their behaviour and lifetime. Manifolds are constructed by welding several ferritic stainless steel tubular parts together. TMF behaviour of a 1.4509 steel in welded and unwelded conditions is assessed under various loading conditions. Unified elasto-viscoplastic constitutive laws are developed. The specific thermo-mechanical behaviour of the heat-affected zone (HAZ) is also taken into account for welded steel. The reliability of the proposed models in predicting the mechanical response, in particular in the welded zone, is investigated. The local strains of the welded area are measured using a digital image correlation technique. Hence, several numerical models are implemented in ABAQUS and different areas are analysed to reproduce the mechanical behaviour of the heat-affected zone. Results are discussed and compared with experiments to validate the proposed model of the mechanical response of a welded component.