Stepped-lap Joints Research Articles

Analysis of the effect of impact damage on the repairability of composite panels

The present paper aims to provide further understanding of the behaviour of Carbon Fibre Reinforced Plastic (CFRP) composite panels under high velocity impact and develop design guideline for repair of damaged composite panels in order to increase the aircraft survivability. This work consists of two parts: part one is a combination of experimental investigation and numerical simulation to evaluate the impact of a woven CFRP laminate which were subjected to selected impact velocities (100m/s–500m/s) in order to evaluate the induced impact damage in two different thicknesses of CFRP composite panels (4.125mm and 2.625mm). In part two a finite element model is developed to design a guideline for repairing of a composite panel. In order to achieve this an optimised repair models with variable parameters such as number of steps and length of steps in the stepped lap joints are investigated. The penetration process and also change of kinetic energy absorption characteristics have been used to validate the finite element results. Finite element results were in close agreement with experimental data obtained from different sources.

Fatigue life prediction of composite material's adhesive joints in automotive applications

Composite and adhesive joints are used increasingly in the automotive industry, not only because of the government policy but also their advantages in mechanical properties over traditional materials and joints. An active research area is the fatigue analysis of adhesive joints. In this paper, a methodology to predict the fatigue life of adhesive joint is proposed and implemented into LS-DYNA with the joint modelled using a user-defined cohesive material. Fatigue crack growth rate is used to obtain the fatigue damage accumulation rate in cohesive zone model. Our method is verified by numerical simulations of two commonly used adhesive joints in the automotive industry: single-lap joint and stepped-lap joint. The predicted S-N curve fits well with the experimental data.

The detection, inspection, and failure analysis of a composite wing skin defect on a tactical aircraft

A routine ultrasonic inspection of a lower wing skin revealed a defect between the titanium alloy stepped-lap joint and composite surface. Since the wing skin passed high-tension loads through to the fuselage, the possible presence of such a defect had the potential to compromise other aircraft with the same construction. Naval Air Station (NAS) Jacksonville personnel took the lead in coordinating the approach to identify, diagnose, and manage the defect by synchronizing efforts between non-destructive testing, metallurgy, and composites experts. The defect was identified as a disbond between the splice plate and bond primer that led to a delamination fatigue crack between composite plies. From these results, staff implemented a novel inspection program to search for similar defects in the fleet.

Investigation on Void Effect on Shear Stress Field in Bonded Stepped-Lap Joint

In this paper, an adhesively-bonded stepped-lap joint suffering from a void within its adhesive layer is investigated. The void separates the layer into two sections. The joint is under tensile load and materials are isotropic and assumed to behave as linear elastic. Classical elasticity theory is used to determine shear stress distribution in the separated sections of adhesive layer along the overlap length. A set of differential equations was derived and solved by using appropriate boundary conditions. Finite element solution was used as the second method to verify the obtained results by analytical method. A two-dimensional model was created in ANSYS and meshed by PLANE elements. A good agreement was observed between two methods of solutions. Results revealed that the stepped-lap joint performed better in stress distribution with a void rather than single-lap and double-lap joints.

Design and analysis of concrete pump truck with carbon fibre composite placing boom

Traditional concrete pump trucks are made of high-strength alloy steel, which causes serious welding defects. Carbon fibre composites have high strength, light weight and are good candidates for the design of concrete pump truck placing booms. As the boom operates in a complex alternating stress state, it is designed into two parts in this article. The first part is the beam structure of carbon fibre boom, which is designed and analyzed with finite-element software ANSYS. The results show that it meets the requirements of strength and stiffness when the layer depth is 15 layers, and the stacking sequence is [02/45/08/ −45/03]. The second part is boom end, which is designed as metal encapsulated carbon fibre composites sandwich structure. Finally, the stepped lap joint is chosen to connect the two parts; its parameters are determined, and the shear strength of the bonding layer is verified.

Optimisation study of tapered scarf and stepped-lap joints in composite repair patches

Stepped-lap repairs of composite structures can offer an alternative that is easier to perform and less time-consuming to produce than the widely used tapered scarf repair. However, the design of stepped-lap joints must be carefully investigated in order to avoid generating stress concentration regions. This study investigates the influence of joint parameters on peak stresses in the adhesive bondline in tapered scarf and stepped-lap joints. Linear finite element analysis was performed to conduct a parametric study with focus on six joint design parameters: ply thickness, adhesive thickness, taper angle, stacking sequence, overply layup, and overply lap length. Results showed that tapered scarf and stepped-lap joints have a strong sensitivity to ply thickness, taper angle, and stacking sequence. The introduction of overplies provided protection and stiffness at joint tips, and a critical overply lap length was identified. The location of 0° plies in the composite laminates was highlighted as an important factor. The analysis was then extended to three-dimensional FE models for verification. In conclusion, results showed that high stress concentration in stepped-lap joints can be mitigated with the introduction of overplies and appropriate changes in joint design parameters to reduce stress peaks at joint tips and step corners.

曲げモーメントを受ける同種被着体材料による中実円柱および中空円筒段付重ね合わせ接着継手の有限要素法応力解析と強度について

三次元有限要素法（FEM）を用いて曲げモーメントを受ける同種材料中実円柱および中空円筒段付き重ね合わせ接着継手の応力分布を解析し，接着層の縦弾性係数，接着層厚さ，スカーフ角，段数が接着界面での最大主応力分布に及ぼす影響を調べた。同種材料中実円柱および中空円筒段付き重ね合わせ接着継手のどちらの場合も突合わせ部接着界面の外径端部で最大主応力が特異性を示し，荷重は重ね合わせ部より大方突合わせ部で負担されることが分かった。両継手の場合とも最大主応力の値は接着層の縦弾性係数が小さくなるほど，接着層の厚さが小さくなるほど，また段数とスカーフ角が大きくなるほど小さくなることが示され，角状段付き接着継手の特性との比較検討も行った。FEM 計算による継手強度推定値は実験結果とかなりよく一致し，本研究で扱った寸法では同種材料中実円柱継手の方が中空円筒継手よりも，やや強度が大きくなることが示された。

A Comparative Study of Friction Stir Brazing and Furnace Brazing of Dissimilar Metal Al and Cu Plates

Friction stir brazing of 5-mm-thick Al and Cu plates in simple lap and stepped lap joint configurations was carried out in air as a modified process for friction stir lap welding of dissimilar metals, in which a pin-free tool and a 0.1-mm-thick Zn braze foil were used to eliminate both pin wear and keyhole and to achieve interface mixing by a metallurgical reaction route instead of vertical plastic mixing, respectively. The microstructure distribution features of friction stir brazing were characterized and compared with traditional furnace brazing to confirm the metallurgical advantages of the new process. For traditional furnace brazing, poor wettability at the Al side for lower brazing temperature or solidification cracking within too thick solidified microstructure (~150 µm) at higher brazing temperature was found. For friction stir brazing, oxide film removal could be achieved well in air without flux by rapid dissolution of the base metals into the molten braze; both solidified microstructure and solidification cracking could be eliminated by extruding excessive liquid phase out of the interface.

Compressive behaviour of concrete elements confined with GFRP-prefabricated bonded shells

In this paper, a new technique is proposed to confine concrete columns using GFRP-prefabricated bonded shells and a shrinkage-compensating cement mortar. The confinement is performed with three identical shells, each of which contains two stepped lap joints at their ends. The main advantage of this technique is that the shells can be directly assembled on site by structural bonding to cover any column shape. The gap between the shells and the column is filled with shrinkage-compensating cement mortar. To evaluate the efficiency of this technique, several concrete specimens were confined, instrumented and tested under monotonic compression. To simulate the actual strengthening conditions, the load was directly applied on the concrete cross-section. The results show clearly the benefits of an efficient confinement in terms of a noticeable increase in the axial strength and ductility where the stress–strain curves are characterised by a third softening phase and the failure mode which spread over the whole length of the shells.

引張り荷重を受ける異種中実円柱および中空円筒段付重ね合せ接着継手の有限要素法応力解析と強度について

引張り荷重を受ける異種材料中実円柱段付き重ね合せ，および中空円筒段付き重ね合せ接着継手の応力分布を軸対称有限要素法を用いて解析し，被着体の縦弾性係数比，接着層の縦弾性係数，接着層厚さ，スカーフ角，段数などが最大主応力分布に及ぼす影響を調べた。その結果，異種材料中実円柱段付き重ね合せ継手の場合，接着界面の外径端部で最大主応力が特異性を示した。また中空円筒段付き重ね合せ接着継手の場合は，外径端および内径端で最大主応力が特異性を示し，内径端の方が大きくなった。両継手の場合とも最大主応力の値は被着体の縦弾性係数比が小さくなるほど大きく，さらに接着層の厚さが小さくなるほど，また接着層の弾性係数，スカーフ角が大きくなるほど、小さくなる。段数は内径端と外径端で逆の影響を示す。FEM計算による継手強度は実験結果とよく一致し，異種材料中実円柱継手の方が中空円筒継手よりも，やや強度が大きくなることが示された。

Buckling and compressive failure of stepped-lap joints repaired with composite patches

In this study, the buckling and compressive failure of adhesively-bonded stepped-lap joints (with/without composite patches) composed of pultruded glass fiber-reinforced polymer composite laminates was investigated experimentally and numerically. Two-component epoxy adhesive was used for bonding purposes. Composite patches were woven glass-epoxy layer. They were added onto the conventional stepped-lap joints so that additional load transfer paths were created and localized stress concentrations near joint edge were reduced. The axial compression tests were performed and the results revealed that the buckling and failure load of new stepped lap composite joints with composite patches were significantly higher than the conventional stepped lap joints. The influence of the overlap length was evaluated. For both types of joints, a small increase was observed in failure loads by increase of the overlap length. Numerical results showed a very good agreement with experimental results.

Mixed-mode fracture of adhesively-bonded pultruded composite lap joints

The fracture behavior of adhesively-bonded double- and stepped-lap joints composed of pultruded glass fiber-reinforced polymer composite laminates and subjected to axial tension was experimentally investigated and numerically modeled. Two methods were used for the calculation of the strain energy release rate: the experimental compliance method and the virtual crack closure technique. Their results showed good agreement for stepped-lap joints, while significant deviations were observed for double-lap joints due to small stiffness changes. The experimental compliance method results were sensitive to these small changes and the virtual crack closure technique accuracy was affected by the inability of the finite element analysis to accurately model the behavior before visual crack initiation. The dominant fracture mode changed from Mode I to Mode II in the case of stepped-lap joints, while an almost constant mode ratio was retained for double-lap joints in the applied loading range. A non-convex mixed-mode fracture criterion was established for crack initiation and propagation based on the virtual crack closure technique results. Both the experimental compliance method and the virtual crack closure technique proved applicable for the interpretation of the fracture mechanics data of the structural joints examined, provided that stiffness degradation can be accurately described.

Fracture of adhesively-bonded pultruded GFRP joints under constant amplitude fatigue loading

The fracture behavior of adhesively-bonded structural joints under tensile constant amplitude fatigue has been investigated. Double-lap joints (DLJs) and stepped-lap joints (SLJs) composed of pultruded GFRP laminates and an epoxy adhesive were examined. Both joint types exhibited a fiber-tear failure with cracks that initiated at the joint edges in the adhesive–laminate interface and then propagated in the laminates’ mat layers. A linear trend of crack growth and compliance was observed for the DLJs whereas a sigmoid trend was found for SLJs. The experimental compliance method was used for the calculation of the maximum strain energy release rate, G max, and the incremental polynomial method for the calculation of the crack propagation rate, da/dN. Fatigue crack growth (FCG) curves established for both joint types showed that empirical constants are dependent on the joint configuration. Based on this fatigue life modeling, design allowables that correlate the developed damage with fatigue life were derived.

852 曲げモーメントを受ける異種被着体段付き重ね合わせ接着継手の三次元有限要素応力解析と強度推定(接着接合の力学III,オーガナイズドセッション)

852 曲げモーメントを受ける異種被着体段付き重ね合わせ接着継手の三次元有限要素応力解析と強度推定(接着接合の力学III,オーガナイズドセッション)

Progressive failure process of adhesively bonded joints composed of pultruded GFRP

Adhesively bonded joints are being used increasingly in civil engineering structures, especially for joints comprising pultruded glass fiber reinforced polymer (GFRP) laminates. The layered material architecture, however, leads to a complex delamination failure within the pultruded material, thus necessitating understanding of the progressive failure mechanism of such joints under axial tensile loading. In this work, adhesively bonded joints composed of pultruded GFRP laminates, including double and stepped lap joints, were experimentally investigated. The static strengths of joints were obtained and the failure mechanism was understood. Crack propagation and back face strain gages were successfully employed to identify crack initiation and describe crack propagation, even though the failure mechanism was always sudden and brittle. The dominant failure mode for both types of joints was a fiber-tear-off failure that occurred in the mat layers of the GFRP laminates. The critical strain energy release rate was calculated. Different values were obtained for the two joint types due to different combinations of fracture modes.

Application of three-dimensional spline variational analysis for composite repair

Abstract The purpose of this research was to apply a non-commercialized spline-based (B-spline analysis method or BSAM) computer program to model and predict strain fields in two composite repairs, the scarf joint and the stepped-lap joint, subjected to static tensile loading. Test specimens with scarf and stepped-lap joints were fabricated using quasi-isotropic 16 ply panels made from IM6/3501-6 pregreg with a [+45 2 /0 2 /−45 2 /90 2 ] S lay-up. Moire interferometry, a high-resolution full-field optical technique used to measure displacements on a surface, was used to determine experimental strain data. Full-field strains along the free-edge were predicted and compared to experimental strain data. Line plots are generated from predicted and experimental data extracted at several points across the width of the repair to provide a quantitative comparison. Overall, both the full-field strain images and line plots show good agreement between the BSAM analysis and the experimental data.

804 静的曲げモーメントを受ける異種材料段付き重ね合わせ接着継手の有限要素応力解析(接着接合の力学)

804 静的曲げモーメントを受ける異種材料段付き重ね合わせ接着継手の有限要素応力解析(接着接合の力学)

Stress Analysis of Asymmetric Co-Cured Stepped-Lap Joints

Abstract Manufacturing of large adhesively-bonded structural joints in the marine composites industry requires careful design and manufacturing to ensure durability of the joint. When the adherends to be joined are both fiber-reinforced laminates, an alternative to adhesive bonding is to co-cure the joint. This process eliminates the adhesive layer and the two interfaces between the adhesive and the adherends. The continuous matrix surrounding the fibers on either side of the joint influences the load transfer and the corresponding stresses developed in the joint. In this study, finite element analysis was used to investigate the stresses developed in a co-cured stepped-lap joint made from adherends with unequal stiffness. Two different approaches were used to model asymmetric joints with uniform step lengths: a homogeneous approach that modeled each adherend as transversely isotropic, and a constituent-level approach that modeled the resin and idealized fibers individually. It was found that the co-cured stepped lap joint behaves very much like a series of adhesively bonded lap joints. The shear stress distribution for step length to ply thickness ratios greater than about 10 exhibits a parabolic shape with peaks at the ends and lower values in the center, which is very similar to the stress distribution in adhesively bonded stepped lap joints. It was observed that the peak and minimum stresses approach a steady value with increasing step length, implying that there may be a limiting strength for an asymmetric co-cured stepped lap joint.

804 衝撃及び静的引張荷重を受ける異種段付き重ね合わせ接着継手の三次元有限要素応力解析(OS-8A,OS-8 界面と接着・接合の力学)

804 衝撃及び静的引張荷重を受ける異種段付き重ね合わせ接着継手の三次元有限要素応力解析(OS-8A,OS-8 界面と接着・接合の力学)

Evaluation of fatigue characteristics for adhesively-bonded composite stepped lap joint

The stepped lap joint for composite structure were manufactured and tested under static and fatigue tensile load. With test results, it was found that crack was initiated at the end of overlap and propagated through the delamination of composite adherend. The average tensile load of the joint increased with the increasing of the number of step and edge angle of adherend, while the peak shear stress at the end of overlap decreased. But the average tensile load did not increase and was converged when the ratio value of joint length against the number of step was less than 6. The fatigue endurance limit of stepped joint was found 30% of static tensile load regardless with the geometry of specimen but the fatigue life of each load level increased significantly with the increasing of the number of step to exceed 6, the ratio value of joint length against the number of step, while the fatigue life decreased significantly when the ratio was less than 6. The ratio 6 was the critical value to be able to increase the static tensile load effectively with the increasing of step number for each joint length. The critical crack propagation distance was observed 5–7 mm from the end of overlap.