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.     

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.     

Fatigue behavior of continuous glass fiber composites: Effect of the matrix nature

The effect of the matrix morphology on the fatigue behavior of a continuous glass fiber/polypropylene (GF/PP) composite system was studied by means of stress‐life and mode II cyclic delamination tests. The stress‐life behavior of a GF composite is considerably affected by the nature of the matrix. A two‐stage fatigue damage curve was observed in the composite made with a PP matrix, whereas a three‐stage curve was observed in the composite made with a thermoset polyester matrix. For a fatigue stress higher than 50% of the yield stress, the PP matrix composite showed a considerably longer fatigue life than the thermoset polyester matrix composite. Mode II cyclic delamination tests showed that the morphology itself of the PP matrix also played an important role. Higher fatigue delamination growth rates, at given strain energy release rates, and lower strain energy release rates at failure were obtained for a composite showing a coarse spherulitic morphology and well‐marked interspherulitic regions than for a composite showing a finer spherulitic morphology and less‐marked interspherulitic regions. While the fatigue mode of the composite with a coarse spherulitic morphology was interspherulitic, that of the composite with a finer spherulitic morphology was transpherulitic.     

Cracking energy density calculation of hyperelastic constitutive model and its application in rubber fatigue life estimations

The finite deformation of rubber under multiaxial stress will finally result in its fatigue failure. The ability to predict the effects of complex strain histories on fatigue life is a critical need. The cracking energy density (CED) distribution characteristics in the finite deformation and rubber fatigue life estimated by the CED criterion are investigated. Then the influences of the crack orientation angle θ and the principal stretch ratio λ on the relationship between CED and strain energy density (SED) are obtained. Finally, the results are used for predicting the fatigue life of rubber material and are compared to experimental values. The results indicate that the ratios of the predicted lives based on the CED damage parameter and measured lives are within two times scatter factor and that of the predicted lives based on the SED damage parameter and measured lives are greatly influenced by the crack orientation angle θ. The rubber fatigue life has great relationship with the angle of the crack plane normal vector and the first principal stretch direction. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44195.     

Influence of Wood Grain Direction on Linear Welding

Wood grain orientation differences in the two surfaces to be bonded yield bondlines of different strengths in linear wood welding. End-grain-to-end-grain welds of good strength were obtained for both beech and oak woods. The tendency to defibration in end-grain-to-end-grain welding indicated that for wood densities higher than or equal to the density of beech wood, end-grain-to-end-grain welding is possible and yields sufficient joint strength. A higher density seemed to yield a stronger joint. Wood pieces having other grain directions were also vibration welded. These were: (1) with the grain perpendicular to the wood longitudinal grain direction, (2) with the grain of both wood pieces at 45° to the wood longitudinal grain direction and (3) with the grain of both wood pieces at 45° to the wood longitudinal grain direction but at 90° to each other to form a fishbone-like pattern. The first of these yielded results comparable to end-grain-to-end-grain welding. The other two yielded much lower strength of the joints, indicating that fibre orientation in the interphase composite formed during welding had considerable influence on joint strength. These differences in joint strength have been explained by the very marked effect that anisotropy of the interphase composite has on fibre/matrix composites.     

Investigations on weld joining of sisal CSM‐thermoplastic composites

An experimental study was conducted to investigate joint efficiency of both, butt, and lap joints of sisal CSM reinforced polymer composites. The thermoplastics, HDPE, and polypropylene (PP) were used separately as matrices in composites. Sisal‐HDPE composites exhibited excellent improvement in tensile strength that reached up to 47.5 MPa at 30 phr loading of sisal CSM as compared with 17.7 MPa of HDPE. Significant improvement in HDT was also observed that increased from 60.2 to 75°C on 0 to 30 phr reinforcement of sisal CSM in HDPE. Similar improvement was noticed with PP where in HDT improved from 69 to 87.6°C on incorporation of 0 to 30 phr sisal CSM. Hot tool welding process was employed for joining the composite materials. The joint efficiency of butt joint of HDPE was observed as 30%. It varied from 48 to 59% for lap joints of different sizes. The joint efficiencies of 20 mm lap joints of different compositions were observed as 59, 98, 75, and 58% in 0, 10, 20, and 30 phr Sisal CSM‐HDPE composites, respectively. Welded joint strengthening is attributed to partial reinforcement of interface that occurs during softening of matrix material which allowed spring back of originally pressed fibers followed by their repositioning in the welded part. POLYM. COMPOS., 36:214–220, 2015. © 2014 Society of Plastics Engineers     

A Backface Strain Technique for Detecting Fatigue Crack Initiation in Adhesive Joints

A new backface strain technique was developed to detect fatigue crack initiation in adhesive-bonded lap joints. The technique was based on the special strain distribution in single lap joints and detected the fatigue crack initiation by the switch in the direction of the strain variation. Use of this technique not only permits the determination of fatigue crack initiation life in the joint, but also allows the site of crack initiation to be located. With the assistance of this new backface strain technique, a fatigue crack was found to initiate in the adhesive but to propagate towards the interface to continue its growth on the interface and to cause the final separation of the joint along the interface. Measurements of fatigue crack initiation lives at different stress levels indicate that the adhesive-controlled crack initiation took an increasingly greater proportion of the total fatigue life as the stress decreased, so that the lifetime in the long-life regime was dominated by the resistance of the adhesive to fatigue crack initiation.     

Modification of high‐performance polymer composite through high‐energy radiation and low‐pressure plasma for aerospace and space applications

In this investigation, attempts are made to modify a high‐performance polymer such as polybenzimidazole (PBI) (service temperature ranges from ?260°C to +400°C) through high‐energy radiation and low‐pressure plasma to prepare composite with the same polymer. The PBI composites are prepared using an ultrahigh temperature resistant epoxy adhesive to join the two polymer sheets. The service temperature of this adhesive ranges from ?260°C to +370°C, and in addition, this adhesive has excellent resistance to most acids, alkalis, solvents, corrosive agents, radiation, and fire, making it extremely useful for aerospace and space applications. Prior to preparing the composite, the surface of the PBI is ultrasonically cleaned by acetone followed by its modification through high‐energy radiation for 6 h in the pool of a SLOWPOKE‐2 (safe low power critical experiment) nuclear reactor, which produces a mixed field of thermal and epithermal neutrons, energetic electrons, and protons, and γ‐rays, with a dose rate of 37 kGy/h and low‐pressure plasma through 13.56 MHz RF glow discharge for 120 s at 100 W of power using nitrogen as process gas, to essentially increase the surface energy of the polymer, leading to substantial improvement of its adhesion characteristics. Prior to joining, the polymer surfaces are characterized by estimating surface energy and electron spectroscopy for chemical analysis (ESCA). To determine the joint strength, tensile lap shear tests are performed according to ASTM D 5868–95 standard. Another set of experiments is carried out by exposing the low‐pressure plasma‐modified polymer joint under the SLOWPOKE‐2 nuclear for 6 h. Considerable increase in the joint strength is observed, when the polymer surface is modified by either high‐energy radiation or low‐pressure plasma. There is further significant increase in joint strength, when the polymer surface is first modified by low‐pressure plasma followed by exposing the joint under high‐energy radiation. To simulate with spatial conditions, the joints are exposed to cryogenic (?196°C) and high temperatures (+300°C) for 100 h. Then, tensile lap shear tests are carried out to determine the effects of these environments on the joint strength. It is observed that when these polymeric joints are exposed to these climatic conditions, the joints could retain their strength of about 95% of that of joints tested under ambient conditions. Finally, to understand the behavior of ultrahigh temperature resistant epoxy adhesive bonding of PBI, the fractured surfaces of the joints are examined by scanning electron microscope. It is observed that there is considerable interfacial failure in the case of unmodified polymer‐to‐polymer joint whereas surface‐modified polymer essentially fails cohesively within the adhesive. Therefore, this high‐performance polymer composite could be highly useful for structural applications in space and aerospace. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1959–1967, 2006     

Residual static strength and the fracture initiation path in adhesively bonded joints weakened with interfacial edge pre-crack

This paper deals with the application of fracture mechanics approaches for predicting the residual static strength and the crack kinking angle of adhesively bonded joints containing interfacial edge pre-cracks. The interfacial cracks are created due to different factors such as inappropriate surface preparation which cause a significant reduction of the joint strength. To investigate the residual strength of interfacial cracked adhesive joints and predict the crack kinking angle, three different approaches including the maximum tangential stress (MTS), the minimum strain energy density (SED) and the maximum tangential strain energy density (MTSED) were assessed. To this end, single lap joints (SLJs) containing a brittle adhesive material and with different pre-crack sizes and various substrate thicknesses were manufactured and tested. The results were also verified by applying fracture mechanics approaches on previously published experimental data. According to the results, it was concluded that in mode II dominant cases, the predictions of kinking angle using the MTS method was in good agreement with the experimental observations, while in mode I dominant cases the mentioned approach provided poor predictions. It was also found that the SED criterion could be a precise model for predicting the crack extension angle in mode I dominant conditions. The results also showed that the MTS criterion predicts the residual static strength of interfacial cracked adhesive joints very well.     

An investigation into the crack initiation and propagation behaviour of bonded single-lap joints using backface strain

In this paper, the backface strain (BFS) measurement technique is used to characterise fatigue damage in single-lap adhesive joints subjected to constant amplitude fatigue loading. Different regions in the BFS plots are correlated with damage in the joints through microscopic characterisation of damage and cracking in partially fatigued joints and comparison with 3D finite element analysis (FEA) of various crack growth scenarios. Crack initiation domination was found at lower fatigue loads whereas crack propagation dominated at higher fatigue loads. Using the BFS and fatigue life measurement results, a simple predictive model is proposed which divides the fatigue lifetime into different regions depending upon the fatigue load. The model can be used with experimental BFS measurements to determine the residual life of the joint in different regions of damage progression during the fatigue life.     

Damage Parameters of Adhesive Joints with General Triaxiality Part I: Finite Element Analysis

In part 1 of this paper, the triaxiality function of several adhesive joints was investigated using the finite element method. The main aim of this investigation was to decide on the most suitable joint type for fatigue experimental tests, which can be used to control triaxiality in adhesive layers. These fatigue experimental tests are presented in this paper (part 2). In part 1 of this paper, three types of adhesive joints were considered. These were: butt joint, cleavage joint and scarf joint. The geometry of the butt joint cannot be manipulated in order to control triaxiality, and therefore triaxiality is controlled through the tension/torsion loading ratios. It was found that the type of load had an effect on the map of triaxiality in the adhesive layer. For the cleavage joint, it was found that triaxiality can be manipulated by changing the adhesive bondline angle. However, the relationship between triaxiality and bondline angle was found to be complex. For the scarf joint, the triaxiality can also be manipulated in a similar way to that used for the cleavage joint by changing the bondline angle. The scarf joints has many advantages over cleavage joints including: (a) simpler relationship between triaxiality and bondline angle and (b) easier manufacturing process related to the geometry of the joint. It was shown that the scarf joint is the best practical choice to control triaxiality in fatigue experimental tests.     

Torque Transmission Capabilities of Bonded Polygonal Lap Joints for Carbon Fiber Epoxy Composites

Although carbon fiber epoxy composite materials have excellent properties for structures, the joint in composite materials often reduces the efficiency of the composite structure because the joint is often the weakest area in the composite structure.

In this paper, the effects of the adhesive thickness and the adherend surface roughness on the static and fatigue strengths of adhesively-bonded tubular polygonal lap joints have been investigated by experimental methods. The dependencies of the static and fatigue strengths on the stacking sequences of the composite adherends were observed.

From the experimental investigations, it was found that the fatigue strength of the circular adhesively-bounded joints was quite dependent on the surface roughness of the adherends and that polygonal adhesively-bonded joints had better fatigue strength characteristics than circular adhesively-bonded joints.     

Torque Transmission Capabilities of Bonded Polygonal Lap Joints for Carbon Fiber Epoxy Composites

Although carbon fiber epoxy composite materials have excellent properties for structures, the joint in composite materials often reduces the efficiency of the composite structure because the joint is often the weakest area in the composite structure.

In this paper, the effects of the adhesive thickness and the adherend surface roughness on the static and fatigue strengths of adhesively-bonded tubular polygonal lap joints have been investigated by experimental methods. The dependencies of the static and fatigue strengths on the stacking sequences of the composite adherends were observed.

From the experimental investigations, it was found that the fatigue strength of the circular adhesively-bounded joints was quite dependent on the surface roughness of the adherends and that polygonal adhesively-bonded joints had better fatigue strength characteristics than circular adhesively-bonded joints.     

Oil palm wood flour filled natural rubber composites: fatigue and hysteresis behaviour

The fatigue and hysteresis behaviour of oil palm wood flour (OPWF) filled natural rubber composites was studied. The stress at any strain decreased with increasing OPWF loading in the composites. As the filler loading increased, the poor wetting of the OPWF by the rubber matrix gave rise to poor interfacial adhesion between the filler and rubber matrix. Results also indicate that the composite with the highest loading of OPWF was the most sensitive towards changes in strain energy, and hence exhibited the highest hysteresis. Thermal ageing not only reduced the fatigue life, but also increased the hysteresis of the composites. © 2000 Society of Chemical Industry     

全钢子午线轮胎疲劳破坏现象仿真与优化设计研究

基于有限元的轮胎疲劳寿命评价方法,建立295/80R22.5全钢子午线轮胎的有限元模型。使用ABAQUS隐式分析方法获得轮胎的应变、应力以及应变能密度等基本参数后,根据轮胎实际使用中的破坏情况选择影响疲劳寿命的评价指标。结果表明:高速耐久工况下,应选择应变能密度和帘线张力作为影响疲劳寿命的评价指标,而且适当增加带束胶厚度能有效提高轮胎的疲劳寿命;低速重载工况下,应当选择帘线和橡胶间的剪应力(层间剪应力)作为影响疲劳寿命的评价指标,且适当降低胎体帘布层和补强层的高度能有效提高轮胎的疲劳寿命。     

Mechanical behaviors of four‐step 1 × 1 braided carbon/epoxy three‐dimensional composite tubes under axial compression loading

This article focuses on the quasistatic axial compression behavior and the consequent energy absorption of three different types of carbon/epoxy braided composite tubes. The focus is to evaluate the effect of sample length and braiding angle on the energy absorption and failure mechanism of the braided composite tubes. All tubes were manufactured with carbon fiber through four‐step 1 × 1 braiding process and epoxy resin. Quasistatic axial compression tests were carried out to comprehend the failure mechanism and the corresponding compressive load–displacement characteristics of each braided composite tube. The quasistatic compression test parameters such as the compression peak load and the energy absorption of all these composite tubes were compared. It was found that as the length of the sample increased, the peak load reduced and the energy absorption of the braided tubes at 45° braiding angle was considerably higher than that of other braiding angles of 25° and 35°. The failure modes included matrix crack along the braiding angle, fiber breakage, bulging and debonding between yarns. POLYM. COMPOS., 37:3210–3218, 2016. © 2015 Society of Plastics Engineers     

Fatigue and failure behavior of silver-filled electronically-conductive adhesive joints subjected to elevated temperatures

The use of adhesives to replace mechanical connectors and other joining methods has enjoyed rapid growth in recent years. There are a number of issues of concern in the design of joints bonded using electronically-conductive adhesives (ECAs). One of these is the cyclic fatigue behavior of conductive adhesive interconnects under different environmental conditions, in which fatigue failure might occur due either to mechanical or thermal stresses varying in a cyclic manner. This paper addresses the effect of elevated temperatures on the fatigue and failure behavior of ECAs. For this purpose, joints were prepared using stainless steel adherend specimens bonded with a commercial ECA, and tested using monotonic and cyclic loadings, at two elevated temperatures, namely 50^°C and 90^°C. When the temperature was increased to 90^°C, close to the glass transition temperature of the adhesive, we observed consistently parallel fatigue curves at different load ratios (R = P _min /P _max) for joints, as in the case of 50^°C test condition, along with significant reduction in fatigue lives. Joint failure mechanisms were also analyzed using optical techniques, and joint conductivity measurements.     

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.     

Effect of Temperature on the Joints Strength of an Automotive Polyurethane Adhesive

In this paper, the performance of an automotive polyurethane adhesive was studied through adhesive joints tests. Butt joints and single lap joints were fabricated and tested at seven temperature measuring points (TMPs). It is shown that both the tensile strength and lap shear strength decrease with the increasing of temperature. Quadratic polynomial expression obtained by the least square method can represent the tensile and lap shear strength as a function of temperature very well. ?40°C, 0°C, and 90°C were selected as the most ideal TMPs for this adhesive through the comparison of the residual sums of squares of 35 fitting curves with different combination of TMPs. Scarf joints with adhesive angles of 60° and 30° were fabricated and tested at ?40°C, 0°C, and 90°C. It also showed a decrease in joint strength with the increasing temperature. Joint strength as a function of adhesive angle is presented. It was found to closely follow a linear behaviour. A three-dimensional surface, consisting of temperature, adhesive angle, and joint strength, is presented finally to facilitate the design of automotive bonding structures.