Fatigue Failure of Rubber-to-Rubber Joints

This paper presents an experimental investigation on fatigue failure of rubber-to-rubber similar and dissimilar joints made from natural rubber and EPDM. The effects of interlinking density, relative proportion of one matrix in the two-component joints, filler loading in the filled part of the joint and strain level on the fatigue life have been studied. Adhesion (peel strength) between similar and dissimilar rubber-to-rubber joints has also been investigated to attempt to determine a correlation between adhesion and fatigue failure.     

Photoelastic Studies on Rubber-to-Rubber Joints

Photoelasticity is a method which yields information on the principal stress difference and orientation in a composite structure. Various problems associated with this technique, especially those concerning the fundamental relationship between the fringe order and the stress, have yet to be investigated. A few studies of this relationship in a universal stress state have been presented, particularly in the field of rubber-to-metal and rubber-to-fabric composites, but no evaluations have so far been made in the field of rubber-to-rubber joints. Applying the photoelastic method, we report our observations on the stress distribution in a natural rubber to natural rubber joint subjected to uniaxial tension. A comparison between the results of experimental photoelastic studies and the corresponding computer modelling has been illustrated. The theoretical displacement pattern of the angular joints of bi-rubber part has also been highlighted.     

Fatigue of rubber-rubber joints and the effect of joint angle

Results of our investigation of the fatigue-to-failure of rubber-rubber joints are presented. Two different types of composites-type I with the angle tip of the stiffer matrix embedded in the softer matrix at an angle, and type II having the reverse configuration-were prepared for the study. The joint angle was varied from 30° to 180°, and it was observed that the more acute the joint angle, the shorter the fatigue life. The purpose of varying the joint configuration was to provide different stress concentrations near the interface. The fatigue strength of various rubber-rubber joints is also related to the strain energy density function. The composite specimen with a higher strain energy density shows a shorter fatigue life. The fatigue life of dissimilar rubber joints and the observed nature of failure of each type of composite are highlighted.     

Failure analysis of AA8011-pultruded GFRP adhesively bonded similar and dissimilar joints

In this work, a comparative failure analysis of aluminum (AA8011/AA8011) and glass fiber reinforced polyester (GFRP/GFRP) based similar and dissimilar joints is presented. The GFRP is prepared using pultrusion technique. Single lap joints are prepared by using Araldite R2011 epoxy as an adhesive. The lap joints are then tested under tension to estimate the average shear strength of the assembly. It is observed that the average bond strength of AA8011/AA8011 is lesser than that of the GFRP/GFRP joint. The failure of similar joints occurred by fracture within the adhesive. The dissimilar joint is failed predominantly by interface debonding. Further, a detailed three dimensional stress analysis of the joints is carried out using finite element method (FEM). The damage analysis of adhesive layer is carried out by coupling FEM with cohesive zone model (CZM). The stress, damage distributions and failure mechanisms are compared for similar joints in detail. A failure mechanism is proposed for AA8011/AA8011 type joint that favours a rapid crack growth in the adhesive after crack initiation, which is responsible for lesser bond strength. The increase in overlap length has positive effect that the peak load increases proportionally with overlap length.     

Single and dual adhesive bond strength analysis of single lap joint between dissimilar adherends

The emerging trends for joining of aircraft structural parts made up of different materials are essential for structural optimization. Adhesively bonded joints are widely used in the aircraft structural constructions for joining of the similar and dissimilar materials. The bond strength mainly depends on the type of adhesive and its properties. Dual adhesive bonded single lap joint concept is preferred where there is large difference in properties of the two dissimilar adherends and demanding environmental conditions. In this work, Araldite-2015 ductile and AV138 brittle adhesives have been used separately between the dissimilar adherends such as, CFRP and aluminium adherends. In the dual adhesive case, the ductile adhesive Araldite-2015 has been used at the ends of the overlap because of high shear and peel strength, whereas in the middle of the bonded region the brittle adhesive AV138 has been used at different dimensions. The bond strength and corresponding failure patterns have been evaluated. The Digital Image Correlation (DIC) method has been used to monitor the relative displacements between the dissimilar adherends. Finite element analysis (FEA) has been carried-out using ABAQUS software. The variation of peel and shear stresses along the single and dual adhesive bond length have been captured. Comparison of experimental and numerical studies have been carried-out and the results of numerical values are closely matching with the experimental values. From the studies it is found that, the use of dual adhesive helps in increasing the bond strength.     

An investigation of fatigue failure prediction of adhesively bonded metal/metal joints

A fracture mechanics-based model for fatigue failure prediction of adhesive joints has been applied in this work. The model is based on the integration of the kinetic law of evolution of defects originated at stress concentrations within the joint. Final failure can be either brittle (fracture toughness-driven) or ductile (tensile/shear strength-driven) depending on the adhesive. The model has been validated against experiments conducted on single-lap shear joints bonded with a structural adhesive. Three different kinds of adhesives, namely a modified methacrylate, a one-part epoxy and a two-part epoxy supplied by Henkel, have been considered and three different overlap lengths have been tested. Fracture toughness and fatigue crack growth properties of the adhesives have been determined with mode I tests. The number of cycles to failure has been successfully predicted in several cases. It is interesting to notice that in the case of joints loaded at the same average shear stress, the shorter the joint, the longer the duration. This fact is also captured by the model.     

Strength prediction of epoxy adhesively bonded scarf joints of dissimilar adherends

In this study, strength of epoxy adhesively bonded scarf joints of dissimilar adherends, namely SUS304 stainless steel and YH75 aluminum alloy is examined on several scarf angles and various bond thicknesses under uniaxial tensile loading. Scarf angle, θ=45°, 60° and 75° are employed. The bond thickness, t between the dissimilar adherends is controlled to be ranged between 0.1 and 1.2 mm. Finite element (FE) analysis is also executed to investigate the stress distributions in the adhesive layer of scarf joints by ANSYS 11 code. As a result, the apparent Young's modulus of adhesive layer in scarf joints is found to be 1.5-5 times higher than those of bulk epoxy adhesive, which has been obtained from tensile tests. For scarf joint strength prediction, the existing failure criteria (i.e. maximum principal stress and Mises equivalent stress) cannot satisfactorily estimate the present experimental results. Though the measured stress multiaxiality of scarf joints proportionally increases as the scarf angle increases, the experimental results do not agree with the theoretical values. From analytical solutions, stress singularity exists most pronouncedly at the steel/adhesive interface corner of joint having 45-75° scarf angle. The failure surface observations confirm that the failure has always initiated at this apex. This is also in agreement with stress-y distribution obtained within FE analysis. Finally, the strength of scarf joints bonded with brittle adhesive can be best predicted by interface corner toughness, Hc parameter.     

搭接长度对钛合金-芳纶纤维复合材料单搭接接头胶接性能的影响

采用钛合金与芳纶纤维复合材料制备不同搭接长度的单搭接接头。利用数字图像相关技术（DIC）、万能试验机等表征方法,对接头拉伸应变与极限载荷进行表征,研究了搭接长度对异质材料单搭接接头胶接性能与破坏模式的变化规律。结果表明,随着搭接长度的增加,单搭接接头极限载荷提升,胶接强度降低,高搭接长度接头出现渐进损伤;偏心弯矩引起的接头偏移减少,搭接部位纵向应变区域面积占比降低;芳纶纤维复合材料层间破坏模式增多,钛合金?胶层界面破坏模式减少,剥离复合材料层数增加。     

Fatigue lifetime assessment of adhesive joints by ultrasonic and thermal wave imaging

Various nondestructive evaluation (NDE) methods are frequently employed to inspect the adhesive bonds of aircraft structures in service. The literature on the capability of various NDE techniques reveals a deficiency in linking NDE test parameter characteristics of the frequency or size of defects to critical failure properties such as the lifetime and the strength of adhesive bonds. In this study an attempt has been made to develop such correlations. A specimen geometry was employed so as to permit cleavage-type debonding under fatigue loading. This geometry and loading configuration provide for a simple fatigue testing program and simple analytical methods. Damage by flexural fatigue aging of these adhesively bonded specimens was induced at different intervals of their fatigue lifetime. The specimens were composed of materials that were commonly used in actual aircraft production during the 1970s. Pulse-echo ultrasonic C-scanning and thermal wave imaging were performed to inspect the adhesive joints at various percentages of the fatigue lifetime. A novel low-frequency ultrasonic method was used for making the C-scans; this technique was immune to signal amplitude changes due to interference phenomena caused by bond thickness variation. A direct correlation of the ultrasonic parameter (size of the debonded area) with the percentage lifetime of the adhesive joints was tentatively established. It was also found that this correlation was consistent when the scanning was conducted from either the top surface or the bottom surface of the adhesive joints. A similar correlation between the size of the debonded area and the percentage of fatigue lifetime of the adhesive joint was found using thennal wave imaging. Thus, it appears that the measurements obtained from both techniques are consistent.     

Fatigue strength and failure analysis of weld-bonded joint of stainless steel

Quasi-static tests of spot welded and weld-bonded joints with 1.5 mm-thick SUS304 stainless steel sheets were conducted. Joint weld diameters were measured using scanning acoustic microscopy. Fatigue tests were performed to obtain the fatigue lives of two joint types subjected to different stress levels. The equations of load-life curves were obtained by nonlinear regression using a three parameter power function. Scanning electron microscopy was used to explore fatigue failure mechanisms of the joints. The results illustrate that nugget diameters of weld-bonded joints were smaller than those of spot welded joints. Their shear strength was lower, but weld-bonded joints showed a better fatigue performance than that of spot welded joints. Two fatigue failure modes were observed via testing: eyebrow failure mode and substrate fracture.     

Fatigue durability assessment of automotive adhesive joints by an in situ corrosion fatigue test

Fatigue and corrosion damage are the major concerns of automotive adhesive joints, yet literature reports few works about the in situ fatigue durability of adhesive joints in corrosive environment. This study presents an investigation on the fatigue durability of automotive adhesive single lap joints by an in situ corrosion fatigue test. The joints were manufactured with commercial coated AA5754-O aluminum sheets bonded by a toughened epoxy structural adhesive. An in situ corrosion chamber was designed and employed to simulate a humid and corrosive environment by spraying 5% saline solution or distilled water mist around the joints’ overlap area during fatigue testing. The test results show that in the 5% saline solution mist environment, the joints’ fatigue lives encountered a great loss for about an order in magnitude compared to the joints tested in laboratory environment. The difference of fatigue lives between 5% saline solution mist test and distilled water mist test is insignificant. The fracture surface analysis by scanning electron microscope and EDX techniques indicates that the adhesive joints failed interfacially in the corrosive environment, which differs from the cohesive failure mode in the laboratory environment.     

The Fatigue and Durability Behaviour of Automotive Adhesives. Part III: Predicting the Service Life

In Part I [1] a fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The results were plotted in the form of the rate of crack growth per cycle, da/dN, versus the maximum strain-energy release-rate, G_max, applied in the fatigue cycle, using logarithmic axes. In Part II [2] the mechanisms of failure were considered, particularly the mechanisms of environmental attack. The present paper, Part III, discusses the use of the relationship between da/dN and G_max, which can be obtained in a relatively short timescale, to predict the fatigue lifetime of (uncracked) single-overlap joints cyclically loaded in tension. An analytical and a finite-element model have been derived to predict the number of cycles of failure, N_f, for lap joints and, particularly when the latter model was used to deduce the value of the strain-energy release-rate, G, in the lap joints, the agreement between the theoretical predictions and the experimental results is found to be very good.     

Effects of various adherend surface treatments on fatigue behavior of joints bonded with a silver-filled electronically conductive adhesive

Conductive adhesives have been used in a variety of electronic packaging applications. This paper presents an investigation into the effects of various adherend surface treatments on the fatigue and failure behaviors of adhesively-bonded joints. For this purpose, single-lap joints were fabricated using specimens with adherend surfaces modified employing various chemical and mechanical modification techniques, and tested under a spectrum of fatigue and environmental conditions. The results of our work indicate a profound influence of the adherend surface on both the fatigue behavior and also the moisture ingress mechanism into the joint. Finally, experiments were conducted to assess the effect of adherend surface condition on the moisture ingress mechanism.     

Prediction of crack length and crack growth rate of adhesive joints by a piezoelectric method

As adhesive joints have been widely used for fastening thin adherends, the damage tolerance design of adhesive joints has become important, and the estimation of initiation and propagation of a fatigue crack in the adhesive has become necessary. However, the measurement of crack length of tubular joints has been difficult because the observation of crack initiation and growth in the adhesive layer by conventional methods is not easy. In this work, a prediction method for the fatigue crack length in the adhesive layer of tubular single-lap adhesive joints was developed by the piezoelectric method. In order to obtain the relationship between the fatigue crack length and the piezoelectric signal, finite element analysis was conducted and verified by experiments. The damage of the adhesive joints was monitored by the piezoelectric method during torsional fatigue tests on tubular single-lap adhesive joints. Using the damage monitoring signals and the relationship between the fatigue crack length and the piezoelectric signal, a method for predicting fatigue crack growth in the adhesive layer of tubular single-lap adhesive joints was developed.     

Elastoplastic Finite Element Analysis and Strength Evaluation of Adhesive Butt Joints of Similar and Dissimilar Hollow Shafts Subjected to External Bending Moments

Stress distributions and deformation of adhesive butt joints are analyzed by an elastoplastic finite element method when the joints of similar and dissimilar shafts are subjected to external bending moments. The effects of the ratio of Young's modulus for the adherends to that for an adhesive and the effects of the adhesive thickness on the interface stress distribution are investigated. Joint strength is predicted by using the elastoplastic interface stress distributions. It is found that the singular stress at the edge of the interfaces increases with an increase of the ratio of Young's modulus. Measurement of strains in joints and experiments on joint strength were conducted. The numerical results are in fairly good agreement with the experimental results. It is observed that the joint strength for dissimilar shafts are smaller than those for similar shafts. A fracture of dissimilar adhesive up-bonded shafts occurred from the interface of the adherend with smaller Young's modulus. It is seen that joint strength increases as the adhesive thickness increases.     

Static and fatigue behavior of adhesive joints in SMC-SMC composites

This paper discusses the static and fatigue behavior of adhesively bonded single lap joints in SMC-SMC composites. Effects of lap length and adhesive thickness on the static and fatigue strength of SMC-SMC adhesive joints are studied. Effects of SMC surface preparation and test speed on the joint performance are evaluated. Finally, the effect of water exposure on the joint durability is also investigated. Results show that the static behavior of adhesive joints in SMC-SMC composites is significantly influenced by the lap length and adhesive thickness. With an increase in lap length from 12.7 mm to 38.1 mm, the joint failure load increases by 37%. The joint failure load also increases with the adhesive thickness, but it reaches a maximum at an adhesive thickness of 0.33 mm and then decreases. However, lap length and adhesive thickness have negligible effect on the ratio of fatigue strength to static strength. The fatigue strength at 10⁶ cycles is approximately 50% to 54% of the static strength for various adhesive thicknesses and lap lengths investigated in this study. Adhesive failure, fiber tear or combination of these two failure modes are observed during both static and fatigue tests.     

Investigating Fatigue Damage Evolution In Adhesively Bonded Structures Using Backface Strain Measurement

Predicting the service life of adhesive joints under fatigue loading remains a major challenge. A significant part of this task is to develop laws that govern the crack initiation phase. This paper contributes to this area through the development and application of the backface strain technique. A numerical study was carried out to investigate the effect of key parameters on the technique and to determine optimum gauge specification and location. Calibration curves were then produced relating the change in strain to the extent of damage. These numerical studies were then validated by undertaking a series of fatigue tests on both aluminium and GRP (glass-reinforced polymer)-bonded joints. Following various degrees of predicted damage the joints were carefully sectioned, polished, and studied using optical microscopy. The predicted and observed damage showed close correlation. The fatigue tests have also indicated that, for unmodified joints (intact fillets), even at high loads (50% static failure load) there was an initiation phase that accounted for about half the fatigue life of the joint. Removal of the adhesive fillet has been found to eliminate the initiation phase and consequently reduce fatigue life.     

Investigating Fatigue Damage Evolution In Adhesively Bonded Structures Using Backface Strain Measurement

Predicting the service life of adhesive joints under fatigue loading remains a major challenge. A significant part of this task is to develop laws that govern the crack initiation phase. This paper contributes to this area through the development and application of the backface strain technique. A numerical study was carried out to investigate the effect of key parameters on the technique and to determine optimum gauge specification and location. Calibration curves were then produced relating the change in strain to the extent of damage. These numerical studies were then validated by undertaking a series of fatigue tests on both aluminium and GRP (glass-reinforced polymer)-bonded joints. Following various degrees of predicted damage the joints were carefully sectioned, polished, and studied using optical microscopy. The predicted and observed damage showed close correlation. The fatigue tests have also indicated that, for unmodified joints (intact fillets), even at high loads (50% static failure load) there was an initiation phase that accounted for about half the fatigue life of the joint. Removal of the adhesive fillet has been found to eliminate the initiation phase and consequently reduce fatigue life.     

Cyclic creep response of adhesively bonded steel lap joints

The viscoelastic nature of polymeric adhesives means that the effect of fatigue frequency has to be treated cautiously. However, this subject has received limited attention and very few studies can be found. Therefore, this work aims at investigating the cyclic creep response of adhesively bonded steel lap joints. Load-controlled fatigue tests were performed with shear stresses of 9.1, 7.4, and 6.3 MPa, which are typically low cycle fatigue stresses. Only during the last 20% of fatigue life can we observe an increase in the cycle hysteresis area due to the decrease of the shear stiffness caused by the failure mechanisms. Under fatigue load, the maximum/minimum strain curves exhibit a shape being similar to that of the steady creep curves, in which occurs a second stage with nearly constant strain rate, independently of the number of cycles and increasing with the load range. A linear relationship between the log cyclic creep rate and the log of the number of cycles to failure was observed, indicating that fatigue behaviour is strictly related to cyclic creep.     

The Fatigue and Durability Behaviour of Automotive Adhesives. Part II: Failure Mechanisms

In part I [1] a fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The cyclic fatigue tests were conducted in a relatively dry environment, of 23°C and 55% RH, and in a “wet” environment, namely immersion in distilled water at 28°C. The “wet” fatigue tests clearly revealed the significant effect an aggressive, hostile environment may have upon the mechanical performance of adhesive joints, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The present paper, Part II, discusses the modes and mechanisms of failure for the two adhesive systems in both the “dry” and “wet” environments. The failure surfaces of the joints tested in Part I have been examined using a variety of analytical techniques and the surface chemistry and morphology compared with that of the “as prepared” (i.e. non-bonded) metal surfaces and cured adhesive. In the present investigation use has been made of an elemental mapping form of X-ray photoelectron spectroscopy (EM-XPS) along with conventional XPS. The surface topography has been examined using scanning electron microscopy and atomic force microscopy. Also, cross-sections of the joints have been studied using the transmission electron microscope. The results reveal that for both the aluminium alloy and EG steel joints that the failure path is complex, and is associated with electrochemical activity (i.e. corrosion) in the case of the latter joints when tested in the “wet” environment. In part III [2], the results presented in the earlier papers will be used to predict the lifetime of single-overlap joints subjected to cyclic fatigue loading.