Experimental and Numerical Investigation of Mechanical and Thermal Residual Strains in Adhesively Bonded Joints

Abstract:  This paper describes an investigation of residual and mechanical strains in aluminium/aluminium (Al/Al) and aluminium/carbon fibre-reinforced polymer (Al/CFRP) adhesively bonded double-lap joints. Residual strains were measured inside the adherends by means of neutron diffraction (ND) and modelled using finite element analysis (FEA). In the Al/Al joints the measured residual strains were negligible, showing good agreement with FE predicted results. However, considerable strains developed in the Al/CFRP joint because of differential thermal contraction of the two materials during joint manufacture. Although considerable scatter was seen in the ND results, the measured and predicted trends showed similar behaviour and were of comparable magnitude. The paper also reports measurements of internal strains in an Al/CFRP joint under tensile load using ND and of surface strains using moiré interferometry (MI). In general, good agreement was observed between FE predictions, surface strains measured with MI and internal strains measured with ND for the loaded Al/CFRP joint.     

Damage Analyses of Adhesively Bonded Single Lap Joints Due to Delaminated FRP Composite Adherends

Three-dimensional non-linear Finite Element Analyses (FEA) due to an in-plane loading have been performed to evaluate the out-of-plane normal and shear stresses over the overlap region of a Single Lap Joint (SLJ) on different surfaces. These surfaces have been considered as; (i) two interfacial surfaces between the adherends and the adhesive layer, (ii) the mid-surface of the adhesive layer and (iii) two surfaces beneath the surface ply of both the adherends adjacent to the adhesive layer. The critical locations of onset of adhesion, cohesion and delamination failures on the above mentioned surfaces of the SLJ have been determined using suitable damage criteria. A comparative study due to adhesion, cohesion and delamination failures in the SLJ with Fiber Reinforced Polymeric (FRP) composite adherends have been presented. The effects of simultaneous variations of the delamination positions on the out-of-plane peel and shear stress components have been studied by pre-embedding the delamination damages at the critical locations in both the adherends. It has been observed that the possibilities of onset of cohesion failures in the adhesive layer are higher compared to the adhesion and delamination failures. The detailed analyses showed that secondary peaks of out-of-plane stress components (σ _z , τ _yz and τ _xz ) on the mid surface of the adhesive layer appeared at the locations closer to the delamination fronts due to pre-embedded delamination damages. The highest stress magnitudes on the overlap edge of the SLJ have been reduced significantly when the centers of the delamination damages are exactly aligned with the overlap ends of the joint. No significant variations of stress magnitudes have been noticed either when the delaminations are pre-embedded outside the overlap regions or when the delamination damages are completely entrapped within the overlap region.     

Analysis of an Interface Fracture Specimen for Adhesively Bonded Joints

An interface fracture specimen for measuring the fracture toughness of adhesively bonded joints is proposed. It is found to represent a wide range of mode mixity. The specimen is calibrated assuming LEFM conditions and the limit of validity of this calibration as the load increases is also assessed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analysis of Acoustic Emission Propagation in Metal-to-Metal Adhesively Bonded Joints

Acoustic emission (AE) monitoring shows promise as one of the most effective methods for condition monitoring of adhesively-bonded joints. Previous research has demonstrated its ability to detect, locate and classify adhesive joint failure, though in these studies little attention appears to have been paid to the differences in AE wave propagation through the bonded and un-bonded sections of the specimens tested, or to the effects of the wave modes excited or the propagation distances. This paper details an experimental study conducted on large aluminium sheet specimens to identify the effects of the presence of an adhesive layer on AE wave propagation. Three specimens are considered; a single aluminium sheet, two aluminium sheets placed together without adhesive, and an adhesively-bonded specimen. A pencil lead break (PLB) is used as a simulated AE source, and is applied to the three specimens at varying propagation distances and orientations. The acquired signals are processed using wavelet-transforms to explore time-frequency features, and compared with modified group-velocity curves based on the Rayleigh–Lamb equations to allow identification of wave-modes and edge-reflections. The effects of propagation distance and source orientation are investigated while comparison is made between the three specimens. It is concluded that while the wave propagation modes can be approximated as being constant throughout all three specimens, there is a significant change in the received waveforms due to the attenuation of high-frequency components exhibited by the bonded specimen. These findings may be utilised to provide a deeper understanding of acquired AE data, improving the current abilities to identify, locate and characterise damage mechanisms occurring within adhesive joints, ultimately improving safety in the use of adhesive bonding for critical applications.     

Effect of Through-Thickness Stitching on the Fatigue Life of Composite Single-Lap Joints

An exploratory experimental evaluation has been undertaken to investigate the effect of through-thickness stitching on the fatigue life of composite single lap joints. Balanced single lap joints were considered, and the lay-up for the adherends was (0/±45/90)_s Specimens were stitched transversely and through-the-thickness with Kevlar^® thread in a zigzag pattern and were manufactured using the resin transfer moulding (RTM) technique. Preliminary experimental results have indicated that stitches remain intact when the crack starts growing from either side of the overlap. This consequently imparts a significant improvement in the fatigue life of stitched versus unstitched counterparts.     

复合材料单搭接头的剪切强度对比实验研究

在常温和湿热高温的环境下,分别进行了三种不同的成碰工艺(RTM、引入缝纫的RTM和胶接)制备的单搭接头的剪切强度实验,研究了成型工艺对单搭接头的剪切强度的影响,并根据实验现象分析了单搭接头的剪切破坏机理.实验结果表明:引入缝纫的RTM成型试样的剪切强度性能最高,而胶接成型试样的剪切强度最低;环境温度对RTM和引入缝纫RTM成型试样的剪切强度影响不大,而对胶接成型试样的剪切强度有较大影响.     

Propagation of Ultrasonic Lamb Waves in Aluminium Adhesively Bonded LAP Joints and in Single Plates

The analysis of the propagation of Lamb waves in multi- and single-layer systems is presented in this work. The study of this propagation problem is developed from matrix formulations, which describe elastic waves in layered media. The behavior of the different modes that propagate in the overlapped region is characterized. It is found that their relative amplitudes can be estimated based on the properties of the incident wave mode. It was verified that the excitation of these modes is ruled by the degree in which the shapes of both modes match each other.

Other result of this method refers to the evaluation of the attenuation of both longitudinal and transversal waves in plates, which is usually difficult, especially in thin plates by using the conventional pulse-echo technique. By using the immersion pitch and catch setup, the total attenuation, the sum of the losses due to the leaking in the fluid and to the material damping can easily be obtained through two measurements at different distances. The leaking losses are calculated by the bulk properties of both the fluid and the plate, and, consequently, the attenuation of the longitudinal and the transversal waves (material damping).     

单向拉伸条件下补片参数对复合材料胶接修复结构的影响

建立含中心半穿透圆孔的损伤金属板修补结构的三维有限元模型,以应力集中系数(Stress Concentration Factor,SCF)和挠度w作为复合材料胶接修复效果的指标,分析单向拉伸条件下,正方形补片的长度、厚度和铺层方式对修复效果的影响。结果表明:补片长度取孔直径的3.5倍、厚度取孔深度的0.6~0.8倍、铺层方式取0°/90°铺层时,复合材料单面修复含损伤裂纹板的效果较好。根据分析结果制备了实验件,进行了单向静拉伸实验,修补实验件的破坏强度比未修补实验件提高了10.1%。     

Experimental and Numerical Investigation of Strain Distributions Within the Adhesive Layer in Bonded Joints

Abstract: We present the results of investigations into the strain distributions in the adhesive layers of epoxy‐bonded aluminium–aluminium and aluminium–carbon fibre reinforced polymer (Al/CFRP) double‐lap shear joints. The area of the adhesive at the end of the overlap region, where high stresses are predicted and failure generally initiates, has been studied at high magnification. 2‐D maps of the longitudinal and engineering shear strains in this region have been obtained experimentally by using high magnification moiré interferometry (HMMI). These results have been compared with the strain maps predicted by 3D elastic and elasto‐plastic finite element (FE) modelling. Excellent agreement is seen between the experimentally measured and FE‐predicted strain distributions, thus validating both methods. In some cases, quantitative comparison of the strains along certain profiles is compromised by noise in the measurements, especially for the Al/CFRP joints, which experienced lower overall strains. The source of such errors is discussed in the paper.     

Damage Mechanisms in Composite/Composite Bonded Joints Under Static Tensile Loading

This paper presents an experimental study of the initiation and development of damage in composite/composite joints. The materials studied are glass fibre reinforced polyester laminates and these are bonded together using an epoxy adhesive. Two types of joint interface are examined, assemblies between woven roving layers and between mat layers. The experimental techniques employed are dye impregnation, extensometry, visual observation and photoelasticimetry. It is shown that the nature of the surface layer does not influence the damage initiation load but does affect subsequent crack propagation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Experimental Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures

This paper reports on recent experimental work to investigate the response of bolted carbon fibre composite joints and structures when subjected to constant dynamic loading rates between 0.1 m/s and 10 m/s. Single fastener joints were tested in both the bearing (shear) and pull-through (normal) loading directions. It was found that the joints exhibited only minor loading rate dependence when loaded in the pull-through direction but there was a significant change in failure mode when the joints were loaded in bearing at or above 1 m/s. Below 1 m/s loading rate the failure mode consisted of initial bolt bearing followed by bolt failure. At a loading rate of 1 m/s and above the bolt failed in a ‘tearing’ mode that absorbed significantly more energy than the low rate tests. A simple composite structure was created to investigate the effect of loading rate on a more complex joint arrangement. The structure was loaded in two different modes and at constant dynamic loading rates between 0.1 m/s and 10 m/s. For the structure investigated and the loading modes considered, only minor loading rate effects were observed, even when the dominant contribution to joint loads came from bearing. It was observed that the load realignment present in the structural tests allowed the joints to fail in a mode that was not bearing dominant, and hence the loading rate sensitivity was not expressed.     

Studying the Effect of Different Environmental Conditions on the Tensile Strength of RGO Reinforced Adhesively Bonded Butt Joints

Journal of Failure Analysis and Prevention - Nanoscale reinforcements are increasingly being used to improve mechanical, electrical and thermal behavior of polymers. In this study, the effects of...     

Fracture Analysis of Double-Side Adhesively Bonded Composite Repairs to Cracked Aluminium Plate Using Line Spring Model

A line spring model is developed for analyzing the fracture problem of cracked metallic plate repaired with the double-sided adhesively bonded composite patch. The restraining action of the bonded patch is modeled as continuous distributed linear springs bridging the crack faces provided that the cracked plate is subjected to extensional load. The effective spring constant is determined from 1-D bonded joint theory. The hyper-singular integral equation (HSIE), which can be solved using the second kind Chebyshev polynomial expansion method, is applied to determine the crack opening displacements (COD) and the crack tip stress intensity factors (SIF) of the repaired cracked plate. The numerical result of SIF for the crack-tip correlates very well with the finite element (FE) computations based on the virtual crack closure technique (VCCT). The present analysis approaches and mathematical techniques are critical to the successful design, analysis and implementation of crack patching.     

混凝土后锚钢筋的静动力粘结特性与工程应用 第十三届全国结构工程学术会议特邀报告

混凝土后锚钢筋又称植筋。在工程实际中,后锚钢筋能有效地解决新老混凝土连接、钢筋漏埋、错埋等钢筋生根问题,被广泛应用于结构加固改造中。结合单锚钢筋、群锚钢筋的粘结锚固性能试验和极限承载能力分析,研究了后锚钢筋与混凝土的粘结机理、粘结破坏形态与静力承载能力计算方法,并讨论了胶层厚度,植筋深度与直径等对静力承载能力的影响;结合植筋钢筋混凝土受弯构件和压弯构件在低周反复周期荷载作用下的滞回特性,分析研究了植筋混凝土构件的延性和抗震性能,并与常规整体浇注构件的特性进行了对比分析,验证了在植筋锚固长度大于20D的情况下,植筋构件的极限承载能力、极限变形能力和延性等主要力学指标都与整浇构件相当,植筋构件的抗震性能是可靠的。     

Enhanced Failure Load Bearing in Adhesively Bonded Strap Repairs: Numerical Analysis and Experimental Results

Journal of Failure Analysis and Prevention - Adhesively bonded repairs are frequently used to repair aluminum structures due to their attractive options compared to traditional methods such as...     

复合材料胶接修补件力学性能的实验研究与数值模拟

进行复合材料修补的铝合金板的静强度实验,测定载荷-位移曲线,分析破坏机理,并讨论了胶层材料性能、复合材料补片性能与厚度等因素对修补件静强度的影响;建立了修补件的三维有限元模型,模拟修补件的载荷-位移曲线和应力分布,验证了模型的有效性;根据应力分布计算结果和失效准则,预测初始损伤及裂纹产生的位置,并估算破坏强度,预测结果...     

Stress Concentration in Adhesive Layer of Adhesively Bonded Piezoelectric Pipe-Joint System

In this paper, a stress-transfer model of an adhesively bonded piezoelectric pipe-joint system subjected to an axial tensile loading is presented. Based on the model, the computational formula of the shearing stress in the adhesive layer of the system is derived. The formula is employed to investigate the stress distribution in the adhesive layer. It shows that there is a large stress concentration at the two ends of the adhesive layer. The smaller the thickness and the larger the shearing modulus of the adhesive layer is, the larger the stress concentration will be.     

钢结硬质合金/中铬钢复合材料的磨料磨损性能

为了解决严苛工况下的磨损问题,利用WC颗粒和中珞钢粉末,采用热压烧结法制备了局域化增强的钢结硬质合金/中珞钢复合材料,观察分析了复合材料界面,研究了试验载荷对复合材料相对耐磨性的影响规律及其磨损机理。结果表明:采用热压烧结法制备的局域化增强复合材料界面结合良好,增强区域组织致密,WC颗粒分布均匀;复合材料的耐磨性远高于中珞钢的,且其相对耐磨性随着试验载荷的增加而增大;在较高试验载荷下,复合材料的耐磨性优势更为显著,加载30 N时复合材料的相对耐磨性达到中鎔钢的3.7倍;在磨损过程中,凸出磨损面的增强区域对基体区域的有效保护作用和基体区域对增强区域的有效支撑作用,两者相互配合是复合材料具有优异耐磨性的主要原因;复合材料的磨损失效机理主要是显微切削和多次塑变疲劳断裂。     

铝合金表面特性对其胶接性能影响的研究进展

不同的表面处理方法会导致铝合金表面的表面理化特性发生改变,从而对铝合金板材与胶黏剂的界面结合强度以及胶接接头的耐腐蚀性能有很大的影响。本文从铝合金表面粗糙度、微观织构、表面氧化层和涂层化学特性等方面入手,对铝合金胶接接头的界面强度和耐腐蚀性能影响的研究现状进行了综述。探讨了铝合金胶接研究发展趋势,并认为铝合金表面理化特性的参数化表征以及表面特性与胶接性能的关系模型建立等方面是今后研究的重要方向。