Effect of Adhesive Type and Thickness on the Lap Shear Strength

The effect of the adhesive thickness on the bond strength of single-lap adhesive joints is still not perfectly understood. The classical elastic analyses predict that the strength increases with the adhesive thickness, whereas experimental results show the opposite. Various theories have been proposed to explain this discrepancy, but more experimental tests are necessary to understand all the variables.

The objective of the present study was to assess the effect of the adhesive thickness on the strength of single-lap joints for different kinds of adhesives. Three different adhesives were selected and tested in bulk. The strain to failure in tension ranged from 1.3% for the most brittle adhesive to 44% for the most ductile adhesive. The adherend selected was a high-strength steel to keep the adherends in the elastic range and simplify the analysis. Three thicknesses were studied for each adhesive: 0.2, 0.5, and 1 mm.

A statistical analysis of the experimental results shows that the lap shear strength increases as the bondline gets thinner and the adhesive gets tougher.     

Effect of Bondline Thickness on Mixed-Mode Debonding of Adhesive Joints to Electroprimed Steel Surfaces

Structural applications of adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for techniques to assess adhesive joint strength, particularly along bondline interfaces where compliant adhesives contact more rigid metallic surfaces. The present study investigates the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel adherend surfaces. Three bondline thicknesses, representative of structural automotive joints, were evaluated for unprimed and primed bondlines. Experimental results for static load versus debond extension were input to finite element analyses for computing debond parameters (strain energy release rates). The debonds always initiated at a through-the-thickness location that had the greatest peel component of strain energy release rate. The total strain energy release rate values correlated well with trends in joint strength as a function of bondline thickness.     

Delamination propagation analyses of spar wingskin joints made with curved laminated FRP composite panels

Initiation and propagation of inter-laminar delamination in adhesive bonded spar wingskin joint (SWJ) made with laminated fibre-reinforced plastic (FRP) composite curved panels have been studied employing three-dimensional finite element analyses. In-plane and out-of-plane normal and shear stress distributions are seen to be highly three-dimensional in nature. Tsai-Wu coupled stress failure criteria have been employed to identify critical locations of onset of delamination-induced damage. This occurs underneath the toe-end of the spar overlap and at the inter-laminar surface between the first and second plies of the curved FRP wingskin panel. Significant edge effects on the joint strength have been observed due to the curvature geometry of the composite wingskin panels. Non-linear finite element analyses have been carried out for study of delamination propagation using contact and multi point constraint (MPC) elements. The use of contact elements prevents inter-penetration of delaminated surfaces. Whereas, sequential release of MPC elements facilitates computation of opening, sliding and cross-sliding modes of delamination-induced strain energy release rates (SERR) by using virtual crack closure technique. Variation in delamination lengths significantly effects the variation of peel and inter-laminar shear stresses and different modes of SERRs. Variations on the two delamination fronts are seen to be quite different indicating dis-similar propagation rates. The Mode I SERR (G_I) predominantly governs the delamination propagation in the SWJ.     

3D FE delamination induced damage analyses of adhesive bonded lap shear joints made with curved laminated FRP composite panels

This paper deals with the evaluation of inter-laminar stresses in the adhesive layer existing between the lap and the strap adherends of lap shear joints (LSJ) made with curved laminated fibre reinforced plastic (FRP) composite panels for varied embedded delaminations between the first and second plies of the strap adherend. Non-linear finite element analyses have been carried out using contact and multi point constraint (MPC) elements. The use of contact elements ensures avoidance of inter-penetration of delaminated surfaces. Sequential release of MPC elements facilitates computation of individual modes of Strain Energy Release Rates (SERR). The effects of varied delamination lengths on variations of peel and inter-laminar shear stresses and different modes of SERR are seen to be very significant. Their variations on both the delamination fronts, for each size of the delamination, are found to be much different from each other indicating different propagation rates at the two delamination fronts. The structural integrity of the LSJ in the presence of delaminations, thus, can be predicted with adaptive finite element (FE) simulations. It is further seen that the peak stress magnitudes and SERRs are higher in the LSJs made with curved FRP composite panels as compared to the flat laminates. This may be due to the stiffening effects induced by the curvature geometry of the curved composite panels.     

ANSYS软件对板式换热器结构强度的分析

用 ANSYS 有限元软件对在液压试验条件下板式换热器的框架结构强度进行分析,得出整体结构的变形情况和等效应力分布情况,得到液压试验时板式换热器受力的普遍规律,为板式换热器的结构开发和优化设计提供了参考依据。     

Effect of the aspect ratio of the pre-existing rectangular adhesion failure on the structural integrity of the adhesively bonded single lap joint

Non-linear three dimensional (3-D) finite element analyses (FEA) of the single lap joints (SLJs) having pre-existing rectangular adhesion failure in the interface of the strap adherend and the adhesive have been carried out. The effect of the size, the shape and the aspect ratio of the pre-existing rectangular adhesion failure on (i) the strength, (ii) the interfacial stresses and (iii) the strain energy release rates (SERRs) in the vicinity of the adhesion failure front have been presented in this research work. The SLJ is subjected to uniformly applied tensile load. The adherends are made with very high strength steels and the adhesive is a commercially available AV119. The analyses of the adhesion failure propagation have been carried out by sequentially releasing the constraints of the nodes ahead of the pre-existing adhesion failure front in finite element model. The SERR values in the vicinity of the adhesion failure fronts are computed using the virtual crack closure technique (VCCT) for assessment of the structural integrity of the SLJ. The strength of the SLJ, the interfacial stresses, and the three modes of strain energy release rates (SERRs) have been found to be significantly affected by the shape and size of adhesion failures. The SERRs and interfacial stresses along the rectangular adhesion failure front are compared with the corresponding values around the circular adhesion failure front of same area, pre-existing in the SLJ. It is observed that the circular and rectangular adhesion failures of the same area will have dissimilar growth rate and the mode II is the dominant failure mode. The total strain energy release rate and the failure strength, computed from the 3-D FEA of the SLJ is in good agreement with the experimental fracture toughness of the AV119 adhesive and the experimentally obtained failure loads, respectively.     

异形压力管道的应力分析及评价

以有限元法和分析设计准则为基础,分析了某异形压力管道复杂的应力分布情况。针对有限元计算的准确性问题,通过应变测试与有限元计算相比较的方法研究了该压力管道在0.43和0.55 MPa压力下一些特殊部位的应力强度,发现有限元结果与实际应变测试一致。研究表明该异形压力管道在0.8 MPa内压下时,虽然在三角区肋板尖端处的薄膜应力和弯曲应力较大,但均在JB4732-1995《钢制压力容器-分析设计标准》允许范围内,对管道运行基本没有影响,及能保证其安全稳定的运行。     

Finite Element Analysis of an Adhesive Joint Using the Cohesive Zone Model and Surface Pattern Design of Bonding Surfaces

Adhesive joints have been widely used in the automotive and aerospace fields in order to reduce the weights of products. The strength of adhesive joints, accordingly, needs to be increased and their behaviour should be predicted in order to achieve accurate designs. Studies to improve the strength of adhesive joints via surface treatment methods or by using two adhesives with different mechanical properties have been conducted. Various modeling methods also have been studied to predict the behaviour of adhesive joints. Unfortunately, the relationship between the bonding surface roughness and adhesive joint strength needs to be further clarified in order to be applied in practical design. As analyzing the relationship through a conventional finite element method assuming perfect bonding is challenging, the behaviour of the adhesive joints may be analyzed using a cohesive zone model or interface modeling methods from an integrating released energy point of view.

The strength of adhesive joints can be improved via micro-patterning due to the mechanical interlocking effect. Therefore, in this study, a micro-pattern was fabricated to improve the strength of adhesive joints. Various pattern-sized single leg bending joints and end notched flexure joints were manufactured and experimented upon. In this study, characteristics of each pattern surface were independently classified and modeled with a cohesive zone model. Finite element analyses were then performed and simulation results were compared with experimental results. The numerical results satisfactorily describe the experimental results, and failure loads were predicted with a maximum relative error of 8%. From these results, it may be concluded that the present findings can be applied to practical design and that the failure load can be predicted via a finite element analysis.     

双点腐蚀缺陷对管道剩余强度的影响

以含缺陷管道模型为例,应用有限元软件对其进行应力分析,探讨腐蚀缺陷几何参数对管道剩余强度的影响,定量给出了缺陷深度和纵横比(长、短轴之比)变化对管道局部应力的影响作用。     

某型号换热器回程水室应力分析及强度校核

研究了某型号管壳式换热器的回程水室,运用有限元软件Ansys分析了该回程水室在稳态下的温度场及其在温度载荷与压力载荷共同作用下的应力场。根据ASME规范和相关判据对回程水室关键部位进行了应力分析及安全评定。计算结果表明,该回程水室的设计在安全范围之内,符合规范要求,这为回程水室的工程设计提供了理论基础。     

某型号换热器前法兰应力分析及评定

主要针对某型号换热器前法兰的应力进行了分析,利用有限元软件分析了该型号换热器前法兰的温度场以及在热机耦合载荷下的应力场。根据ASME规范和相关判据对前法兰关键部位进行了应力分析及安全评定。计算结果表明,该前法兰最大的应力强度值为209.76 MPa小于安全强度值,故该设计在安全范围之内,符合规范要求,同时为前法兰的工程设计提供了理论基础。     

Effect of Bond Thickness on Fracture Behaviour in Adhesive Joints

To study the effects of bond thickness on the fracture behaviour of adhesive joints, experimental investigation and finite element analysis have been carried out for compact tension (CT) and double-cantilever-beam (DCB) specimens with different bond thickness. Fractography and fracture toughness exhibited apparent variations with bond thickness. Numerical results indicate that the crack tip stress fields are affected by bond thickness due to the restriction of plastic deformation by the adherends. At the same J level, a higher opening stress was observed in the joint with a smaller bond thickness (h). Beyond the crack tip region, a self-similar stress field can be described by the normalized loading parameter, J/hσ₀. The relationship between J and crack tip opening displacement, δ, is dependent on the bond thickness. The strong dependence of toughness upon bond thickness is a result of the competition between two different fracture mechanisms. For small bond thickness, toughness is linearly proportional to bond thickness due to the high constraint. After reaching a critical bond thickness, the toughness decreases with further increase of bond thickness due to the rapid opening (blunting) of the crack tip with loading. A simple model has been proposed to predict the variation of toughness with bond thickness.     

Effect of Bond Thickness on Fracture Behaviour in Adhesive Joints

To study the effects of bond thickness on the fracture behaviour of adhesive joints, experimental investigation and finite element analysis have been carried out for compact tension (CT) and double-cantilever-beam (DCB) specimens with different bond thickness. Fractography and fracture toughness exhibited apparent variations with bond thickness. Numerical results indicate that the crack tip stress fields are affected by bond thickness due to the restriction of plastic deformation by the adherends. At the same J level, a higher opening stress was observed in the joint with a smaller bond thickness (h). Beyond the crack tip region, a self-similar stress field can be described by the normalized loading parameter, J/hσ₀. The relationship between J and crack tip opening displacement, δ, is dependent on the bond thickness. The strong dependence of toughness upon bond thickness is a result of the competition between two different fracture mechanisms. For small bond thickness, toughness is linearly proportional to bond thickness due to the high constraint. After reaching a critical bond thickness, the toughness decreases with further increase of bond thickness due to the rapid opening (blunting) of the crack tip with loading. A simple model has been proposed to predict the variation of toughness with bond thickness.     

A Fracture Mechanics Approach to Characterizing Cyclic Debonding of Varied Thickness Adhesive Joints to Electroprimed Steel Surfaces

Applications of adhesive bonding for automotive structures have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for design techniques to assess the influence of bondline thickness on adhesive joint strength. One design approach currently being used is based on limiting shear stresses in the adhesive while designing to eliminate peel stresses. Another design approach is based on fracture mechanics and accounts for shear and peel stresses and both static and fatigue modes of failure. The present study applies fracture mechanics to investigate the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel surfaces. Three bondline thicknesses equal to 0.254, 0.813, and 1.27 mm were evaluated for unprimed and primed bondlines. For the experimental portion of the study, debond growth rates (da/dN) were measured using a remote imaging system over a range of applied cyclic loads. Corresponding changes in the strain release rates (ΔG) were calculated, through finite element analyses, as a function of debond length and applied load level. The computations for ΔG applied a finite element formulation to determine both the peel component, ΔG_i, and the shear component, ΔG_ii. When computed ΔG values were plotted against the measured debond growth rates, da/dN, the results showed a power law relationship which characterizes the debond behavior of a given material system and bondline thickness.     

粘合剂AIR-1对海上漂浮疏浚胶管外胶性能和粘合强度的影响

研究了粘合增进剂AIR-1用量对海上漂浮疏浚胶管NR/SBR/EPDM外胶的影响。结果表明,加入粘合增进剂AIR-1后,胶料的门尼粘度增大,硫化时间和焦烧时间缩短。当AIR-1用量为4.0份时,硫化胶的力学性能最好,交联密度最大,AIR-1的加入没有影响硫化胶的耐臭氧老化性能,胶管成品的粘合强度明显提高。     

Effect of Temperature on the Quasi-static Strength and Fatigue Resistance of Bonded Composite Double Lap Joints

Fibre reinforced polymer composites (FRP's) are often used to reduce the weight of a structure. Traditionally the composite parts are bolted together; however, increased weight savings can often be achieved by adhesive bonding or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods and concerns about long-term performance. The authors have attempted to address these issues by studying the effects of fatigue loading, test environment and pre-conditioning on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However, it was recognised that in some applications short-overlap joints will be used and these joints might behave quite differently. In this work, double-lap joints were tested both quasi-statically and in fatigue across the temperature range experienced by a jet aircraft. Two variants on the double-lap joint sample were used for the testing, one with multidirectional (MD) CFRP adherends and the other with unidirectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperatures. It was proposed that this was because at low temperature the strength was determined by the peak stresses in the joints, whereas, at high temperatures, strength is controlled by creep of the joints which is determined by the minimum stresses in the joint. This argument was supported by the stress analysis.     

Effect of Temperature on the Quasi-static Strength and Fatigue Resistance of Bonded Composite Double Lap Joints

Fibre reinforced polymer composites (FRP's) are often used to reduce the weight of a structure. Traditionally the composite parts are bolted together; however, increased weight savings can often be achieved by adhesive bonding or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods and concerns about long-term performance. The authors have attempted to address these issues by studying the effects of fatigue loading, test environment and pre-conditioning on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However, it was recognised that in some applications short-overlap joints will be used and these joints might behave quite differently. In this work, double-lap joints were tested both quasi-statically and in fatigue across the temperature range experienced by a jet aircraft. Two variants on the double-lap joint sample were used for the testing, one with multidirectional (MD) CFRP adherends and the other with unidirectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperatures. It was proposed that this was because at low temperature the strength was determined by the peak stresses in the joints, whereas, at high temperatures, strength is controlled by creep of the joints which is determined by the minimum stresses in the joint. This argument was supported by the stress analysis.     

Effect of Adherend Recessing on the Tensile Strength of Single Lap Joints

Bonded joints are gaining importance in many fields of manufacturing owing to a significant number of advantages to the traditional methods. The single lap joint (SLJ) is the most commonly used method. The use of material or geometric changes in SLJ reduces peel and shear peak stresses at the damage initiation sites. In this work, the effect of adherend recessing at the overlap edges on the tensile strength of SLJ, bonded with a brittle adhesive, was experimentally and numerically studied. The recess dimensions (length and depth) were optimized for different values of overlap length (L _O), thus allowing the maximization of the joint's strength by the reduction of peak stresses at the overlap edges. The effect of recessing was also investigated by a finite element (FE) analysis and cohesive zone modelling (CZM), which allowed characterizing the entire fracture process and provided joint strength predictions. For this purpose, a static FE analysis was performed in ABAQUS® considering geometric nonlinearities. In the end, the experimental and FE results revealed the accuracy of the FE analysis in predicting the strength and also provided some design principles for the strength improvement of SLJ using a relatively simple and straightforward technique.     

金属材料表面能的测算及其对粘合强度的影响

本文提出了基于双液法接触角测定的固体表面能色散成份γ_s~d计算的改进方法,并测算了经不同表面处理的钢、铁、铝等合金及玻璃。氧化铝陶瓷,石英等高表面能材料的γ_s~d值,研究其对粘合性能的影响。