Scarf Joints Research Articles

Experimental and theoretical analyses of Iroko wood laminates

Abstract This paper reports the experimental tests, which have been carried out to assess the mechanical properties of Iroko wood laminates used for the construction of a large wooden sailing ship. Three-point bending tests have been carried out on different types of specimens: laminates with 3 layers @ 0° and without scarf joints, laminates with 3 layers @ 0° having the outer layers with scarf joints, and laminates with 4 layers @ 0°/±45°/0° and no scarf joints. The tests have been performed in compliance with the current EN Standards. The analyses of the experimental data allowed the assessment of the mechanical properties of the laminated Iroko wood as well as the influence of scarf joints. The experimental results demonstrated that the presence of scarf joints only affect the strength of the glued laminate, while the stiffness properties in terms of Young modulus in bending and shear modulus, obtained applying the “method of variable support span”, remain essentially the same. The investigated laminates have been also analysed using a 3D computed tomography and an ultrasonic phased array equipment in order to assess the dimensions of possible defects or voids in the adhesive and the dynamic modulus of elasticity. The tomographic measurements of the glue thickness explained the reason of the reduced strength of the scarf joints, due to the inhomogeneity of the glue bond-line. The value of the dynamic modulus of elasticity, obtained by the ultrasonic technique, is slightly higher than the value of the modulus of elasticity obtained by the bending tests. Finally, the experimental findings have been compared with those drawn from both Classical Lamination Theory and Dietz approach, obtaining a good agreement and confirming that the Dietz approach is a fast and easy way to assess the elastic properties of a laminated structure.

Recent progress on damage mechanisms in polymeric adhesively bonded high-performance composite joints under fatigue

A prominent critical problem with the use of adhesively bonded composite scarf joint in aircraft structures is their susceptibility to unpredictable failure. Any efficient and robust design approach for adhesively bonded scarf joints under fatigue loading should account for the damage mechanisms in the joint and this cannot be achieved without understanding damage development with initiation and propagation. Research into damage development using methodical approach were conducted on the stiffness of polymer adhesively bonded composite scarf joint since it is the prime physical parameter that can be measured most accurately. The research produced significant new knowledge of damage development under fatigue as strain hardening and a methodology to characterize damage initiation and propagation in adhesively bonded joints. Under uninterrupted fatigue, stiffness degradation and plasticity effects led to strain hardening of the joint adhesive until failure. But there was in-homogenous stiffness degradation in regions within the joint due to defects and anomalies so that the general stiffness degradation of the scarf joint could be either a stiffness increase or decrease to failure.The changes in the rate of stiffness over the fatigue life of a scarf joint reveal three characteristic regions. Region I & II marked the damage initiation life of scarf joints and this was (66±9)% of the fatigue life for specimens tested, and Region III marked the onset of damage propagation to failure. Mean strain effect was evident in some scarf joint under fatigue cycle. It is shown that the damage initiation & propagation life under variable and constant fatigue loading for scarf joints is independent of the adhesive thickness, the fatigue load application frequency, and the severity of the applied load. Damage propagation in adhesively bonded scarf joint was found to be a combination (1) strain hardening of the adhesive layer leading to brittle cracking of the adhesive layer (2) cracking of the adhesive layer at 45° or 90° to the axial loading, leading to break in stress distribution in the adhesive joint as it matures with strain hardening (3) leading to gradual quasi-static failure of the joint particularly since there was no evidence of direct residual fatigue pattern on the fracture surface either in this research work or from any existing literature for similar loading situation. In all, the paper addressed one of the most critical and important steps that will drive correct quality implementation of design for fatigue in polymer adhesively bonded joints towards addressing the current airworthiness limitations on use of these joints in the aerospace industry and further to expand the scope of application of adhesively bonded joints.

Seam Welding of Aluminum Sheet Using Ultrasonic Additive Manufacturing System

Ultrasonic welding was investigated as a method of joining 0.076 in. (1.93 mm) thick aluminum 6061 flat sheet material. Joints were produced with ultrasonic additive manufacturing (UAM) equipment in a modified application of the ultrasonic welding process. Through joint design development, successful welds were achieved with a scarf joint configuration. Using a design of experiments (DOE) approach, weld parameters including weld amplitude, scarf angle, and weld speed were optimized for mechanical strength. Lower angles and higher amplitudes were found to provide the highest strengths within the levels tested. Finite-element studies indicate that 5 deg and 10 deg angles produce an increased relative motion of the workpieces as compared to 15 deg, 20 deg, and 25 deg angles, likely leading to increased strength. Successful joints showed no indication of voids under optical microscopy. As-welded joints produce tensile strengths of 221 MPa, while heat treated joints produce tensile strengths of 310 MPa, comparable to heat treated bulk material. High-temperature tensile testing was conducted at 210 °C, with samples exhibiting strengths of 184.1 MPa, similar to bulk material. Room temperature fatigue testing resulted in cyclic failures at approximately 190,000 cycles on average, approaching that of bulk material.

Bearing Capacity of a Scarf Joint

The bearing capacity of a scarf joint depends on the length of the joint and the type of connection. It is possible to use circular bolts or squared dowels. Both connection tools used in the repairs of historic timber structures are made of wood according to the requirements of the heritage authority.

Experiment and design methods of composite scarf repair for primary-load bearing structures

A relative thick plate of composite being repaired by scarf bonding was substituted as the scarf joints and studied under tensile load in this paper. The experimental investigation told us the ultimate capability of load and failure mechanism of composite scarf joints. In order to explain failure mechanism for scarf bonding repair of composite structures distinctly, FEM based on continuum damage mechanics (CDM) was carried out. CDM equations were derived by adding the delamination mode and 3D Hashin criteria based on strain into the composite constitutive relation. Furthermore, a modified semi-analytical method (MAM) was developed to obtain shear stress distribution in theory. MAM can improve the accuracy of Harman and Wang method, solve the stress asymmetry, obtain secondary peak of shear stress, and increase the highest peak of stress. In the end, the discussions for the methodologies of MAM, CDM and linear FEM were executed to compare the errors of predicting ultimate load. The discussion revealed that MAM has sufficient accuracy as an analytical methodology which could be utilized to design composite scarf repairs in a short cycle simply.

The numerical assessment of a full-scale historical truss structure reconstructed with use of traditional all-wooden joints

Abstract This paper focuses on description of the mechanical behavior of the historical gothic truss of St. James's Church in Brno. The numerical approach using finite element analysis (FEA) provided virtual assessment of the truss with a prediction of its behavior after simulated restoration using joints at locations of possible failure. The historical truss was subsequently analyzed by both beam truss structure and detailed 3D solid lap scarf joint modeled by reduction technique using substructuring. Static analyses were carried out using the finite element method (FEM) in order to establish a reliable numerical model and assess the static risks. The finite element models in ANSYS software assume fully orthotropic material properties of wood (Norway spruce and European beech) with elastic behavior. Results portrayed very good design of the assessed truss in the global mechanical behavior despite the rigidity of joints varied in longitudinal and transverse directions of the frames. Changes in global truss behavior were observed, but the changes in objective vertical displacement were not high. The differences based on rigidity level were not more than 7% of maximum vertical displacement of beams. The minor differences were recognized in the global truss behavior owing to new positions of implemented joints in the truss. Analyses showed each member in the truss contributes to global truss rigidity and stability to different degree. Further, analyses showed areas in the truss where it was necessary to correct joints orientation.

Influence of plasticity on the fatigue lifetime prediction of adhesively bonded joints using the stress-life approach

The use of adhesive bonding for designing lightweight load-bearing components has increased in recent decades. In this paper the influence of plasticity on the lifetime prediction of bonded joints using the stress-life approach was investigated. The adhesive was a toughened epoxy for structural applications. Stress calculations were performed using finite element analysis. Three material models were employed, a linear-elastic model and two elastoplastic models: Von Mises (pressure independent) and Drucker–Prager (pressure dependent). Effective stress was calculated using the theory of critical distances. Lifetime predictions were based on SN curves from literature for scarf and single-lap joints at four different temperatures (−35, −10 °C, RT, +50 °C). The material properties were acquired from uniaxial tensile quasi-static experiments on bulk adhesive specimens. These experiments showed a reduction in the values of Young’s modulus and yield stress with increasing temperature. A model was proposed based on an Arrhenius-type equation in order to fit the yield stress as a function of temperature. The model showed good agreement to the experimental findings. Regarding lifetime predictions (a) the influence of critical distance was higher for single-lap joints than scarf joints and (b) the prediction errors were lower for elastoplastic modelling than linear-elastic modelling, especially for single-lap joints.

Load carrying capacity of halved and tabled tenoned timber scarf joint

The stress distribution of timber tenoned scarf joint subjected to tension force was analysed by means of finite element method. This analysis was compared and validated with results from an experimental work on 42 specimens of Scots pine wood (Pinus sylvestris L.) with 48 × 148 mm in cross-section and 1932 mm, in length, manufactured with a tenoned scarf joint in the middle of its length. Specimens were divided in eight groups corresponding to different joint geometries. A plane finite element analysis using ANSYS software was made considering the elasticity moduli, Poisson ratios and friction coefficients obtained in previous research works for the same material. Three different failure modes were analysed: compression parallel to the grain in the notch area, shear parallel to the grain in the heel surface, and the combination of tension and bending moment in the effective cross-section of the piece, which is limited by the crack initiation due to tension perpendicular to the grain. Equations for the verification of this joint are proposed as a practical conclusion.

Tensile behavior of scarf joints after impact in different locations

Tensile behavior of scarf joints after impact in different locations

Fracture Behaviour of Adhesives Modified with various Rubber Content under Combined Stress Conditions

Rubber-modified adhesives have attracted special interest recently due to their property of dramatically increase in fracture toughness of adhesively bonded joints, where the fracture toughness increases with the rubber content. However, an increase in rubber content decreases the joint strength without initial flaws. Besides, most adhesively-bonded joints have combined normal and shear stresses concentrated in the adhesive layer. Hence, the effect of rubber content on adhesively-bonded joints should be clarified under combined stress conditions. In this paper the aim is to characterize the rubber content dependent behaviour of the rubber-modified adhesive under combined stress conditions. Tensile tests were conducted for adhesively-bonded butt and scarf joints with various rubber contents, where these joints can cover the combined stress conditions in the adhesive layers of most joints in industrial applications. The influence of the rubber content on the joint strength is represented in the normal stress-shear stress diagram.

Damage tolerance investigation of high-performance scarf joints with bondline flaws under various environmental, geometrical and support conditions

The criticality of disbonds on the load-carrying capacity of scarf joints under different environmental conditions was experimentally investigated. Tests were conducted on scarf joints containing bondline flaws of varying lengths and locations under room-temperature (RT), cold-dry (CD) and hot-wet (HW) conditions. The results showed that the residual strength of scarf joints under RT and CD condition decreases exponentially with the size of the bondline flaw up to a threshold length and remains almost unchanged for larger bondline flaws. In contrast, those specimens under HW condition were less sensitive to the presence of disbonds. The effects of scarf angle, adherend composite layup and support condition on the load-carrying capacity of scarf joints with a bondline flaw were also investigated. For specimens where the eccentricity was counteracted by a sandwich support, the residual strength of the joint improved considerably. Fractographic analysis using Micro Computed Tomography and SEM were carried out to examine the effects of the environmental condition on the failure mechanisms of scarf joints.

Tailoring of bonded composite scarf joint interface for impact damage mitigation and stiffness compatibility

A novel composite scarf joint is successfully fabricated with thermoplastic core shell microparticles incorporated into the interlaminar interface regions with a dual objective of mitigating the impact damage and improving the stiffness compatibility of the joint. The impact analysis of the joints revealed that the incorporation of microparticles led to a significant improvement of the impact load bearing capacity of joint and reduced the extent of the damage area. It is also observed from flexural tests that the microparticles reduces the stiffness of the laminate proportional to the weight fraction of the particles and thereby help design a joint adherend with controlled matching stiffness. This engineered scarf joint adherend configuration has the potential to minimise stiffness mismatch between a fatigue worn-out damaged composite part and the new scarf repair patch laminate designed for it. This shall help ensuring an easy restoration of uniform load distribution in the newly bonded composite scarf joints and repairs.

Effect of temperature on the joint strength of a silyl-modified polymer-based adhesive

ABSTRACTTemperature is a very important factor that must be fully considered in the study on the adhesive joint strength. In this paper, a silyl-modified polymer-based adhesive ISR 70-08 which is widely used in engineering was studied. Dog-bone specimens were fabricated and tested at −40°C, room temperature (RT), and 90°C. Results show a decrease in the main mechanical properties with increasing temperature. Butt joints (BJs), single-lap joints (SLJs), and Scarf joints (SJs) were fabricated and tested at different temperatures. A quadratic polynomial expression was an ideal choice to express the joint strength as a function of temperature which was obtained using the least-squares method. Temperature combinations of −40°C, 0°C, and 90°C were obtained to study the effect of temperature on the joint strength more easily for this adhesive. A three-dimensional surface, consisting of temperature, adhesive angle, and joint strength was presented to facilitate the application of bonding structures in engineering

Approximate numerical model for adhesive scarf joints in wooden beams

The purpose of this paper is to formulate an approximate numerical model for adhesive scarf joints in wooden beams under the assumption that the joint is deformable. This approximate model is founded on observations resulting from the general solution of the plane linear theory of elasticity. It is assumed that wood is orthotropic. The joint can be subjected to a complex load combination including an axial force, a shear force and a bending moment. Within this framework, analytical relations for stresses in the adhesive, as well as stresses and displacements of the adherends, were derived in a form useful for engineering practice. A numerical example proves the high accuracy of the approximate model when compared to the general model based on the plane linear theory of elasticity.

Influence of Temperature on the Fatigue Behaviour of a Toughened Epoxy Adhesive

Toughened epoxy adhesives are widely used for structural applications, especially in the design of lightweight products. During their service life, bonded joints are exposed to cyclic loading under a broad range of temperatures. In this study the influence of temperature on the fatigue behaviour of a toughened epoxy adhesive was investigated by experimental determination of SN curves under tension–tension cyclic loading. Bulk adhesive specimens and single lap, thick adherend and scarf joints were tested at a stress ratio of R = 0.1 and a frequency of f = 7 Hz at five different temperatures (−35°C, −10°C, RT, +50°C, +80°C). Dynamic mechanical analysis (DMA) was conducted to evaluate the temperatures of the phase transitions in the adhesive. Based on the experimental findings an interpolation method was proposed for estimating SN parameters at temperatures unaffected by phase transitions. The fatigue tests showed there was a reduction in fatigue lifetime with increasing temperature. DMA results indicated the adhesive underwent two phase transitions. The results from interpolations for different types of samples showed excellent agreement with the experimental findings, confirming the validity of the method, providing no phase transitions are within or close to the interpolation range.

Estimation of the Lifetime of Bonded Joints under Cyclic Loads at Different Temperatures

The fatigue lifetime of joints bonded with a toughened epoxy adhesive has been estimated using the stress-life method, based on experimental SN curves. Five temperatures (35°C, −10°C, RT, +50°C, +80°C) were considered. The estimations were performed using a direct approach, for joints having near-homogeneous stress distributions (scarf joints), and; an optimised approach, for joints having inhomogeneous stress distributions (single lap joints). The stress distributions were calculated considering linear-elastic material behaviour using analytical methods and finite element analysis. For inhomogeneous stress distributions the effective stress was defined in terms of the theory of critical distances. The calculations were performed considering: (i) uniaxial stress, evaluating the maximum principal stress, and considering (ii) multiaxial stress, for the influence of the relationship between hydrostatic and deviatoric stresses. The estimations were validated with fatigue experiments under tension–tension loading at a stress ratio of 0.1 and a frequency of 7 Hz. The experimental results showed a reduction of fatigue lifetime with increasing temperature. The estimated values indicate a better quality of prediction for the multiaxial stress and that it is more preferable to use a base SN curve having a stress distribution as similar as possible to the stress distributions of the components to be designed.

Fatigue life uncertainty of adhesively bonded composite scarf joints – an airworthiness perspective

ABSTRACTAdhesively bonded repairs provide a highly structurally efficient and cost-effective means of restoring residual strength to aircraft components. However, gaining airworthiness approval for bonded repairs to primary structures is a significant problem. This is largely because of the failure of current non-destructive inspection techniques to detect weak or non-durable adhesively bonded joints. Due to the presence of undetectable defects and anomalies, recent airworthiness policy ignores the contribution of adhesively bonded joints to the fatigue durability of repaired load-carrying aircraft structures. The key requirement for airworthiness is to demonstrate an acceptable low probability of repair patch disbonding during the remaining life of the structure. In order to satisfy this requirement, it is necessary to identify and control all manufacturing defects and anomalies that influence the durability of the bonded joint. In this study, a methodology has been developed to control manufacturing defects including porosity, unbonded area, and adhesive thickness and flatness variation of bond area. To evaluate the effectiveness of the developed methodology, fatigue tests were conducted, and corresponding uncertainty was analysed. It was found that these defects and anomalies have a significant influence on the fatigue life and fatigue life uncertainty of bonded joints, with minimal effect on their static strength.

Mechanical Performance and Contact Zone of Timber Joint With Oblique Faces

The goal of the work was to evaluate mechanical performance of full-scale timber beams containing scarf joint with a dowel. Work focused on standard testing using modular system to obtain effective stiffness and strength of the beams with and without the joint. The work further researched a contact zone between two timber parts of the joint – at the scarf face. This was carried out using non-destructive optical technique – digital image correlation (DIC) and newly developed algorithm. The joint was made of Norway spruce, dims. 6×0.2×0.24 m and was loaded by two modes: a) 3-point bending and b) 4-point bending. During the loading, a sequence of images was acquired for further investigation of contact zone using the proposed algorithm. The joint with scarf and dowel provided enough effective stiffness, ie. 73–93% for 3-point bending test and 71% for 4-point bending with respect to MOE measured on reference solid beams. Effective strength of the joint was also relatively high and in a range of 55% and 60% with respect to reference solid beams in both 3-point and 4-point bending tests. Contact length differed for loading modes. Mean contact length in symmetrical 4-point bending was about 40%, for asymmetrical 3-point bending test, it was approx. 20% on face closer to support and 44% on a face closer to loads.

Failure modes in halved and tabled tenoned timber scarf joint by tension test

The structural behaviour of halved and tabled tenoned scarf joint subjected to tensile stresses was experimentally analysed to find the different failure modes. A total of 42 specimens of Scots pine wood (Pinus sylvestris L.) were made. Specimens were divided into 8 groups corresponding to different joint geometries. Three different failure modes were observed: compression parallel to the grain in the notch area, shear parallel to the grain in the heel surface, and by cracking starting in the reduced cross-section. The load that caused crack initiation was obtained, and the influence of the length of the overlapping surface was analysed.

3D models of specimens with a scarf joint to test the adhesive and cohesive multi-axial behavior of adhesives

In this paper, accurate numerical analyses of the stress distributions within the adhesive in scarf joints under elastic assumption using 3D models are developed. An elastic limit model that takes into account the hydrostatic stress and von Mises equivalent stress, permit to define the more stressed parts of the adhesive with respect to the scarf angle. It is shown that high stress concentrations are generated near the edges and the corners of the specimen at the interface between the substrate and the adhesive. The use of ronded substrates does not lead to decrease significantly these stress concentrations. It is shown that cleaning the free edge of the adhesive changes the location of edge effects without completely erasing them. A modification of the geometry, has been proposed. This modification leads to a quasi-homogeneous distribution of the stresses in the thickness of the adhesive with stresses that tend to 0 near the edges.