Peel Specimens Research Articles

Influence of brazing process and gap size on the fatigue strength of shear and peel specimen

Brazing is a joining technique used in many industries for components that consist of many individual parts. Many of these components are cyclically loaded during service. For this reason, reliable approaches to assess the service life must be developed. For this purpose, it is necessary to gain knowledge about factors influencing the fatigue strength and the damage behavior. In this paper induction, vacuum- and continuous furnace brazed shear and peel specimen with different brazing gap widths are analyzed. Therefore, the specimens were characterized, measuring the geometry of the specimens and surface condition of the brazing radii, and tested under load control using constant amplitudes. It is found that the manufacturing process and the parameters used have a significant influence on the mechanical properties of the brazing material, the surface condition and the profile of the fillet radii. These properties have again an influence on the damage behavior and the fatigue strength. In particular crack-like defects of varying severity, which can extend deep into the brazing material, that are located in highly stressed areas of the fillet radii have a significant influence on the fatigue strength. It is also found that, regardless of the brazing process used, there is a tendency in the area of the brazing gap width for the number and size of defects to increase with increasing gap size, which can negatively affect fatigue strength depending on the damage behavior.

Prediction of spot weld fatigue life using finite element approach

Resistant spot welding is predominantly used in automotive industry for joining sheet metal. Mostly automobile fatigue failure occurs in and around spot welding (80%). So, accurate prediction of the deformation, stresses and fatigue life of spot weld is necessary. In this work finite element model of spot weld and experimental study of coach peel test of spot welded low carbon steel sheets are presented. The property change in various zones of spot weld is found by nano-indentation test followed by inverse finite element modelling. Spot weld failure happens at the interface between nugget and heat affected zone due to the geometric stress concentration and property change that occurs during the welding process. Solid elements are successful in capturing the geometric stress concentration, but they increase the simulation cost and hence a model with combined shell and solid elements is used. For nugget solid elements are used and tore present all other parts of spot weld shell elements are used. Tensile and fatigue tests on coach peel specimen are done as per ASTM standardD903-98. The simulation results are found to be in close agreement with experimental results.

Parametric investigation into fatigue life behaviour of spot welded tensile shear test samples

Resistance spot welding is a widely used joining process in the structural components of various engineering products such as white goods, aerospace equipment and especially automobiles. Hence it is crucial that the mechanical performance of such joints be well understood in order to design and manufacture reliable engineering products. In service, Spot-welded joints undergo fatigue failure under cyclic loadings. In order to examine and improve the fatigue life of spot welded structures, various assessment methods have been used on standardized test samples such as tensile shear (TS), modified tensile shear (MTS), coach peel (CP) and modified coach peel (MCP) specimens. The fatigue life studies have shown that among the most important design variables for spot-welded structures influencing joint strengths, sheet thickness, spot weld nugget diameter, number of spot welds and the joint type can be counted. In this study, a numerical examination was carried out to observe the effects of sheet thickness and nugget diameter on the fatigue life of tensile shear (TS) samples. In the analysis both Morrows mean stress and Coffin Manson approach were utilized. The performance of both methods was verified on the basis of experimental data available in literature. The elastic and plastic strains necessary for the calculations were found out by means of finite element method. The results presented herein provide variation of fatigue life predictions according to selected geometry parameters along with valuable interpretations that can be used as guidelines for designers.

Fatigue life prediction of spot-welded joints with a notch stress approach

Spot-welded joints are widely used in the construction of vehicle structures and frequently become critical locations for fatigue failure. Hence, it is essential to have reliable fatigue life prediction method for the spot-welded joints during vehicle structure design. In this paper, a new notch stress approach is developed for fatigue life prediction of the spot-welded joints. Currently, structural stress methods are widely used in automotive industry for fatigue life prediction of spot-welded joints. However, these methods are not well considering local geometry information. This paper introduces a notch stress based method to overcome the limitation of the structural stress methods. In the notch stress method, stress concentration factors for spot-welded joints are calculated from stress intensity factor equations. Then, the notch stress method is validated with fatigue test results of lap-shear and coach peel specimens.

Examining T-peel specimen bond length effects: Experimental and numerical explorations of transitions to steady-state debonding

The broad versatility of the T-peel test, widely used for characterizing adhesion across a plethora of adhesives, adherends, and geometries, results in a range of responses that may complicate meaningful interpretation. This research effort, involving several specific specimen types, was undertaken to investigate concerns that commonly used configurations may not always result in plateaus in the force-displacement response. An experimental and numerical study is conducted on debonding of T-peel specimens having 75 mm bond length and 0.81 mm thick adherends made of either 6061 aluminum (Al) or one of the three steels (G70 70U hot dip galvanized, E60 elctrogalvanized (EGZ), 1010 cold-rolled steel) bonded with either LORD® 406 or Maxlok™ acrylic adhesive. For the EGZ and the Al adherends, specimens with a bond length of 250 mm and adherend thickness of 1.60 mm are also examined. Effects of adherend materials and thicknesses, bond lengths, and adhesives on test results are examined using three metrics to interpret the T-peel bond performance. A limited correlation is found between the commonly used “T-peel strength” and the energy dissipated per unit debond area. For these two metrics, the relative performances of the CRS and the Al specimens are quite different. Quasi-static plane strain deformations of the test specimens are analyzed by the finite element method (FEM) and a cohesive zone model using the commercial software, ABAQUS, to help interpret the test data. Numerical results provided energies required to elastically and plastically deform the adherends, and help determine the transition from non-steady-state to steady-state debonding. The FE simulations also facilitate determination of the fraction of the crosshead displacement at which steady-state debonding occurs. Results reported herein could help engineers select appropriate peel specimen dimensions for extracting meaningful data for the adhesive performance and comparisons.

The robustness of Al-steel resistance spot welding process

Several types of materials are commonly used in automotive body structures, driving the need for joining of dissimilar material joints of aluminum alloys to steels by various techniques including resistance spot welding. Al-steel spot welds can exhibit variable structures and properties, even when produced with the same welding schedule, welding equipment, and with the same combination of materials. Laboratory test conditions can correct for this variability; however, this is not always a realistic representation of production conditions. This study evaluated the influence of production conditions such as gaps between sheet metal, angles between the sheet metal and welding electrode, changes in current, and cap length on the microstructural and mechanical properties of Al-steel spot welds. A designed experiment was used to weld tensile (lap) shear, coach peel, and metallographic specimens under various combinations of production conditions. Comparative t-test and quadratic regression statistical analyses were performed to quantify the production conditions that significantly affect the robustness of the Al-steel spot welding process. Compared to analogous Al-Al specimens, the Al-steel tensile shear specimens exhibited greater strengths and were robust to being produced under various production conditions. However, the Al-steel coach peel specimens were sensitive to some of the production conditions, especially those that combined off normal sheet angles and gaps between the metal sheets. These differences in behavior are discussed, including the underlying fundamental metallurgical and physical mechanisms.

A notch stress method for fatigue life prediction of spot-welded joints

Spot-welded joints are widely used in the construction of vehicle structures and frequently become critical locations for fatigue failure. Hence, it is essential to have reliable fatigue life prediction method for the spot-welded joints during vehicle structure design. In this paper, a new notch stress approach is developed for fatigue life prediction of the spot-welded joints. Currently, structural stress methods are widely used in automotive industry for fatigue life prediction of spot-welded joints. However, these methods are not well considering local geometry information. This paper introduces a notch stress based method to overcome the limitation of the structural stress methods. In the notch stress method, stress concentration factors for spot-welded joints are calculated from stress intensity factor equations. Then, the notch stress method is validated with fatigue test results of lap-shear and coach peel specimens.

High-tensile joints of continuously fusion bonded hybrid structures

Tailored hybrid structures manufactured within a continuous production process, for instance in a continuous roll forming line, offer a high potential for the automotive industry, due to its weight reduction compared to pure metal parts. Therefore, the hybrid roll formed parts consist of steel sheets reinforced by carbon fibre reinforced thermoplastic tapes (CFR-TP).Both materials are joined by fusion bonding whereby the surface of the thermoplastic matrix is melted on the steel surface. The pre-treatment of the steel with a primer, customized for joining of steel and CFR-TP, is a promising approach, due to its easy implementation into continuous coil-coating processes. Major challenges consist in achieving sufficient joint strength as well as developing of a testing method for the evaluation of the adhesion between both materials.Relating to joint strength, various process parameters (i.e. temperature of steel and CFR-TP) are examined. The fracture pattern are analysed optically by microscopy to detect possible primer damage. Furthermore, the evaluation is carried out by mechanical testing of peel specimens within climbing drum peel test to determine the production-related influence in a continuous process. The examinations show the usability of climbing drum peel test and the increase of joint strength with higher energy input.

地層剥ぎ取り技法を用いた地球「現象」の実物標本化と博物館資料の新たな可能性

地層剥ぎ取り技法を用いた地球「現象」の実物標本化と博物館資料の新たな可能性

地層剥ぎ取り技法を用いた箱根火山起源噴出物の実物標本化 : 神奈川県立生命の星・地球博物館における露頭情報の収集・保存・活用(＜特集＞火山噴火史解明のための露頭データベース構築の検討(3))

地層剥ぎ取り技法を用いた箱根火山起源噴出物の実物標本化 : 神奈川県立生命の星・地球博物館における露頭情報の収集・保存・活用(＜特集＞火山噴火史解明のための露頭データベース構築の検討(3))

Strength analysis of aluminium foil parts made by composite metal foil manufacturing

A number of different parts for mechanical testing were produced using composite metal foil manufacturing (CMFM). This process is a combination of laminated object manufacturing and brazing technology. Aluminium is one of the toughest metals to join and CMFM can achieve this task with ease. By using aluminium 1050 foils of 0.1 mm thickness, various parts were made according to British and International Standards, including lap joints, peel specimens, dog-bone specimens and tested for their mechanical properties. A special, 80 % zinc and 20 % aluminium by weight, brazing paste was utilized for joining the foils together. The test of the single lap joints show that none of the specimens failed at the bonded area and the failure was always due to fracture of the parent metal. Cohesive failure was also observed for the single lap joints by using 10 mm thick aluminium metal plates. It helped in calculating the lap shear strength which is a useful design parameter. The peel test showed good bond consistency in all the specimens with an average peel strength of 20 MPa. Comparative tensile test was conducted with a dog-bone specimen machined from a solid block of aluminium 1050 and specimens made with CMFM. The results showed that the specimens made by CMFM fracture at force values that are higher than that of the parent metal. This demonstrates that CMFM has the capability to produce high quality and stronger parts as compared to conventional machining methods employed for the production of metal parts. The effect of using different number of layers for the same cross-sectional area has also been investigated.

Laser-powered tape placement process – simulation and optimization

The laser-assisted thermoplastic tape placement process for unidirectional carbon fiber reinforced polyetheretherketon (CF PEEK) tapes was investigated. In this process, tapes are melted, laid-up on a tool, and consolidated under a roller. As an out of autoclave process, thermoplastic tape placement has high potential for aerospace manufacturing industry. To optimize laser settings for high efficiency and reproducibility, a thermal simulation was created. Process windows are calculated based on critical process temperatures. A parameter study was conducted to investigate the influence of lay-up speed and laser settings, especially laser beam height and angle of impact, on the process. For different lay-up speeds, optimum values and limits of laser settings are found to gain autoclave quality with high material output and high reproducibility. Peel specimens have been produced to verify simulated optimum parameters, which showed good agreement. Economical potential of thermoplastic tape placement was shown in a direct comparison to thermoset tape placement.

A synthesis of data from steel spot welded joints of reduced thickness by means of local SED

In some previous works two fatigue scatterbands were given in terms of the local strain energy density (SED) for welded joints made of structural steel and aluminium alloy. The material-dependent radius R0 of the control volume was identified with reference to conventional arc welding processes. It resulted equal to 0.28mm for welded joints made of structural steel tested under prevailing mode I, with fatigue failures both at the weld root and the weld toe. The scatterband did not include data from thin welded joints, the main plate thickness being greater than 6mm. The SED approach is used here to summarise more than 400 fatigue data from steel spot welds in lap–shear joints, coach–peel specimens and cross–tension specimens of small thickness. Due to the typical crack initiation and propagation phases, an appropriate choice to summarise the data in a narrow scatterband seems to be the use of a toroidal control volume. The volume circumferentially encapsulates the slit tip along the weld spot nugget. The SED value at 2 million cycles is found to match that of the previous scatterband for welded joints of higher thicknesses.

GS28 スポット溶接継手の疲労寿命に及ぼすナゲット径と打点位置変動の影響

In this paper, the influence of variation of nugget diameter and spot welding location on fatigue life of the spot-welded joints was investigated. The relationship between nominal structural stress, which is used as fatigue life estimation parameter, and influence factors of fatigue life on single spot-welded joints under eccentric tensile shear and triple spot-welded joints under peel load was quantitatively clarified, and fatigue tests of these specimens were conducted. Following the increase of the distance of eccentricity of spot welding location from the center line of tensile shear specimen, the influence factors of fatigue life became highly sensitive. The sensitivity of peel specimen was larger than one of tensile shear specimen.

Evaluation of adhesive properties of three resilient denture liners by the modified peel test method

The characteristics of adhesive properties between a denture base and resilient denture liner were investigated by a modified peel test with an L-shaped metal attachment. Three commercially resilient denture lining materials, namely GC Reline Soft (S), GC Reline Extra Soft (ES), and GC Reline Ultra Soft (US), were evaluated. Acrylic resin (GC Acron) was used as denture base material. Peel specimens consisting of the denture base acrylic resin and resilient denture liner were tested after storage for 1 and 30 days in distilled water at 37°C. The modified peel test method gave load–displacement curves and work of adhesion (WA) values of the denture base material and resilient denture liner. The WA of specimens after 1 day of storage ranged from 1.71 to 2.55Nmm−1 and increased in the order from US to S to ES. On the other hand, the WA of specimens after 30 day of storage ranged from 1.44 to 2.47Nmm−1 and increased in the order from US to ES to S. US had significantly lower WA after 1 and 30 days of storage than did S and ES (P<0.05). Comparison of the WA between 1 and 30 days, reveals large differences for ES and US, but not for S. This could be explained by the difference in failure modes. Within the limitations of this investigation, it was concluded the modified peel test is effective for evaluating the adhesion between denture base material and a resilient denture liner.

Fatigue performance of spot welded and weld bonded advanced high strength steel sheets

Fatigue properties and failure characterisation of spot welded and spot weld bonded (WB) high strength steels, such as advanced high strength steels (AHSS) DP600 GI, TRIP600, and high strength low alloy (HSLA) steel HSLA340Y GI have been studied. Tensile shear and coach peel specimens were used in the present study. Both constant amplitude sinusoidal loading and random loading fatigue tests were conducted. The results show that the fatigue strength, under constant amplitude sinusoidal fatigue loading, of spot welded DP600 GI and TRIP600 specimens was approximately the same as that of HSLA340Y GI specimens in the case of tensile shear and coach peel type specimens for both low and high cycle fatigue. However, spot WB specimens show significantly higher fatigue strength than that of spot welded specimens, especially at low load condition (high cycles). Fatigue tests were also conducted on wide width DP600 GI and HSLA340Y GI spot welded specimens and no significant difference in fatigue strength between regular size specimens (39 mm in width) and wide specimens (125 mm in width) was observed. Random loading fatigue tests show that DP600 GI has a lower fatigue strength compared to HSLA340Y GI. Crack initiation and propagation during the fatigue loading history has been experimentally determined and discussed.

Peel Behaviour of Aircraft Fuel Tank Sealants: the Effect of Peel Angle, Sealant Layer Thickness and Peel Rate

The peel performance of two aircraft fuel tank sealants was comprehensively investigated by means of a modified peel specimen previously developed. Experiments were carried out at five different peel angles in the range from 90 to 180° using seven sealant layer thicknesses in the range from 0.1 to 5 mm. The effect of the peel rate was also investigated at a fixed peel angle and sealant layer thickness. The results were analysed in terms of the peel energy. Both peel angle and sealant layer thickness were found to affect significantly the measured peel energy of the sealants in a coupled way. In particular, it was found that the peel energy increased linearly with the sealant layer thickness, for the range considered in this study, and the rate of this increase was also found to increase as the peel angle was varied from 90 to 180°. For very small sealant layer thicknesses (0.1 mm) there was no effect of the peel angle on the measured peel energy. The results were explained in terms of the amount of energy these materials dissipated upon deformation up to fracture and a relationship for the prediction of the peel energy was proposed. The peel rate was found to affect only slightly the measured peel energy, in the range studied.

Crack initiation and kinking behaviours of spot welded coach peel specimens under cyclic loading

The present study is carried out to investigate fatigue crack initiation and kinking behaviours of spot welded coach peel (CP) specimens of low carbon steel sheets subjected to cyclic loading by experimental and finite element analysis methods. Evaluations of fatigue crack growth stages were performed by crack tip plastic strains and J integral analyses and also by microhardness measurements on process zone. According to the experimental and analytical results, fatigue crack initiation and growing stages in the spot welded CP specimens can be divided to three stages. Stage I corresponds to 'gap sharpening stage' observed at the beginning steps of cyclic loading with crack growing on the interface plane between the overlapped sheets. Stage II corresponds to kinked crack initiation and propagation through the sheet thickness observed after applying a certain number of loading cycles. Stage III corresponds to crack propagation through the width of the specimens observed at the final step of the fatigue crack propagation. The FE results of the crack paths and crack locations are in good agreement with those of experimental observations.

The use of a modified peel specimen to assess the peel resistance of aircraft fuel tank sealants

The aim of this study was to comprehensively review the traditional peel specimen for assessing adhesion of aircraft fuel tank sealants. A modified peel specimen was proposed that overcomes the inherent problems of the traditional test, which are associated with inaccurate geometry definition and clamping, and avoids any subjective interaction during testing from the knife cuts used to re-initiate the crack at the metallic interface. Artificial defects, which are easy to apply consistently, were introduced during manufacturing of the peel specimens and it was shown that these can act as precise knife cuts during testing. Two aircraft fuel tank sealants were tested using two different peel angles (135° and 180°), several sealant layer thicknesses (1.5, 2, 3 and 4 mm), various substrates (aluminium and mild steel) and chromate epoxy primers (low and conventional VOC). For those specimens that failed cohesively, the peel resistance increased significantly with the peel angle and the sealant layer thickness. On the other hand, the use of chromate epoxy primers did not affect either the peel resistance of the sealants or the failure mode.

Fatigue properties and failure characterisation of self-piercing riveted 6111 aluminium sheet joints

Aluminium has been long considered as an alternative material for steel in the automobile industry. However, welding of aluminium is more difficult and prone to defects compared with steel. Therefore other methods of joining aluminium sheets are being investigated. In the present investigation, the fatigue properties of self-piercing riveted joints in 6111 aluminium alloy sheets has been studied using tensile shear and coach peel type specimens. Different thickness combinations of aluminium sheet joints and different load ratio R were also investigated in the fatigue tests. Fatigue crack initiation and growth were observed and analysed. It has been found that the maximum fatigue load increases with increasing sheet thickness in both tensile shear and coach peel specimens, but this increase is smaller for high fatigue cycles. With the same maximum fatigue load, specimens under load ratio of R=0·3 have a longer fatigue life than that of specimens under R = 0·1. In most of the cases (>90%), specimens failed in the upper sheet (with the rivet head) during fatigue loading in the case of equal thickness combination and thick upper sheet combination, for both tensile shear and coach peel specimens. As for different thickness combination rivet joints, the thinner sheets are usually the places where failure occurs whichever it is the upper sheet or lower sheet in the combination.