Scarf Joints Research Articles

The influence of the bevel angle on the micro-mechanical behaviour of bonded scarf joints

In this work, both experimental and theoretical analyses are presented of the influence of the scarf angle (α) on the micro-mechanical behaviour of a scarf-joint bonded structure loaded in uniaxial tension. The specimens studied were made of mild steel for the adherends and of an epoxy resin for the adhesive. This study not only considers the ultimate strength of the specimens, but also characterizes the various stages of the gradual adhesive damage and takes into account the threshold for the initiation of the first microcracks and the threshold for the start of flaw propagation as the fundamental parameters. These thresholds were determined by two experimental methods used simultaneously: extensometry with electrical strain gauges, and acoustic emission. Some of the experimental results are also compared with some theories that the authors have developed previously in this field, in order to determine their limits of application.

Dynamic deformation of lap joints and scarf joints under impact loads

The propagation of stress waves and the concentration of dynamic stress under impact loading in single lap joints, tapered lap joints and scarf joints, which were bonded adhesively, were investigated analytically and experimentally. Viscoelastic properties of cured adhesive resin were obtained from the variation of strain waves which propagated in a rod of cured resin. The compliance of the cured adhesive resin depended on the frequency of the strain waves. The viscoelastic properties obtained in the test were formulated by Voigt viscoelastic models. The stress distribution and the time variation of stress and strain in the joints under tensile impact loading were calculated using the finite element method, considering the viscoelastic properties of the adhesive. Stress concentration occurred in both the edges of the adhesive layers of the single lap joints and the tapered lap joints. The stress concentration in the scarf joints was relatively small. The stress occurred in the taper lap joints was smaller than that in the single lap joints when the taper lap joints had enough taper length. Impact test on the joints, which had adherends of aluminum alloy, was conducted to verify the pertinence of the analyses. The deformation of the tapered lap joints and the scarf joints showed good agreement on the analytical results. The deformation of the single lap joints was more complicated than the analytical results.

Evaluation of the fatigue strength of adhesively bonded CFRP/metal single and single-step double-lap joints

Generally, the adhesive layer of an adhesively bonded joint is under a state of concentrated multiaxial stress. Hence, stress multiaxiality and concentrated stress distributions must be taken into account in estimating the strength of adhesive joints. In our previous study [1], a fatigue-failure criterion was investigated which took account of the stress multiaxiality in adhesively bonded CFRP/aluminum butt, scarf and thick-adherend lap-shear joints, which have uniform stress distributions in the adhesive layer. Another parameter governing the strength characteristics of the adhesive joints is the concentrated stress distribution due to the stress singularity at the corners of the adhesive/adherend interface. Recently, stress-singularity parameters have been used to evaluate the strength of adhesive joints. However, there are few studies evaluating the fatigue strength from this point of view. In this study, a method of estimating the fatigue strengths of adhesively bonded single-lap, cracked single-lap and single-step double-lap joints composed of CFRP and an aluminum alloy is proposed which is based on two stress-singularity parameters.

An investigation of the stress singularity near the free edge of scarf joints

The singular stress field which develops at the interface corner of a scarf joint between two bonded elastic solids is investigated. Depending upon the scarf angle and the material elastic properties, the singular stresses at a radial distance r from an interface corner may be expressed in the form Hr λ−1 , where λ−1 is the order of the stress singularity and H is the intensity of the singularity. The intensity H of the singularity at the interface corner of scarf joints subjected to a remote uniform tension is evaluated for various material combinations and a range of scarf angles using a combination of the finite element method and a path independent contour integral. Two types of scarf joints are considered: (i) a scarf joint between two long bi-material strips and (ii) a scarf joint consisting of a thin elastic layer sandwiched between two substrates. The role of the intensity H, which determines the amplitude of the stress field within the singularity zone, is examined and the implications of the results for the initiation of joint failure are discussed.

Theoretical and Experimental Analysis of the Scarf Joint Bonded Structure: Influence of the Adhesive Thickness on the Micro-mechanical Behavior

In this work, we have studied the influence of the adhesive thickness on the micro-mechanical behavior of a scarf joint bonded structure loaded in uniaxial tension. Adherends are made of mild steel containing 0.18% Carbon (French Standard XC18), the adhesive is a two-component epoxy resin with a 5800 MPa elastic modulus. The experimental method is based on strain gauge measurements and acoustic emission. It makes it possible to determine the following zones: —the areas of the joint where the start of microcracks occurs (threshold Fd); —the areas where crack propagation occurs (threshold Fg) up to the failure (threshold Fr). The experimental results confirm the good correlation between the different thresholds. They also show that there is an optimal adhesive thickness close to 0.1 mm, which confers to the scarf joint the greatest resistance to microcrack initiation and crack extension. We have compared our experimental measurements with the main theories in this domain to determine their limits and their fields of application, particularly in the angular singularities regions near the ends of the lap.

Scarf repairs to highly strained graphite/epoxy structure

Scarf joints representative of repairs to graphite/epoxy (gr/ep) honeycomb structure, typical of that used in the F/A-18 horizontal stabilator, have been investigated using mechanical tests and finite-element (FE) modelling. The load capacity of such scarf repairs is marginal when tested under hot/wet conditions compared to the required design ultimate strain of 5200 μ ε. An analysis of the scarf joint using FE methods predicts that high stresses will occur at ply drop-offs and at the top of the scarf and good correlation is observed between the FE models and experimental results. Detailed predictions of the shear stress distribution within the joint have also been obtained from the FE models. These show that the shear stress in the adhesive is not uniform over the length of the scarf, as would be the case for isotropic adherends, but rather varies with position by up to 250%, due to the varying longitudinal compliances of the plies within the gr/ep adherends. Experimental evidence indicating the occurrence of stress relaxation or creep within the scarf joints is also reported. An F/A-18 stabilator containing a scarf repair has been loaded satisfactorily to design limit load without failure. External patches have also been shown to be effective as temporary repairs.

Optimum regimes for the laser strengthening of threaded joints working under cyclic loads

Threaded joints that are subject to fatigue loading in service are used in a wide range of equipment. One application is in the scarf joints of drill strings. Traditional methods of increasing the fatigue strength of scarf joints (quenching after heating by high-frequency current, surface plastic deformation of the bottom of the thread) sometimes result in an unacceptable change in the geometric parameters of its profile. Thus, it is useful to examine the feasibility of employing modern local methods of strengthening. One of these methods is laser heat treatment (LTT).

Substrate volume and stress gradient concepts in mechanical adhesion: analysis of single straight sections

The primary objective of this study was to optimize the geometry and volume of the substrate in lap, butt, and scarf adhesive joints under various loading and boundary conditions by finite element analysis. The finite element models are validated by evaluating for convergence and by calculating theoretical values using the strength of materials and applied elasticity. The results of this study are significant in optimizing mechanical adhesion because any real joint surface may contain a combination of the above model joints on a smaller scale. Studying the behavior of these basic models will help in optimizing the topography of surfaces to be bonded. As a novel approach for design purposes, new parameters involving stress times joint volume and stress divided by joint volume as well as stress gradients are considered. Finite element meshes of the basic lap, butt, and four scarf joints (30°, 45°, 60°, 75°) having 25.4 mm adhesive length are completed. Upon validation of these models, they arc subjected to various loading and boundary conditions and results obtained. A similar study is done by reducing the adhesive length by 50% in all the scarf joints to compare the trends with the original 25.4 mm adhesive length models. Also, several models are created to evaluate the effect of the volume of substrate in the lap and butt joints. Considering the fact that large strain gradients result in failure, this study focuses not only on the magnitudes, but also on the gradients of stresses and strains in optimizing the joint strength. This study has concluded that there exist significant differences in the interfacial stresses with respect to the scarf angle, considering lap and butt to be special cases of scarf with 0° and 90° angles, as well as with respect to the adhesive material properties, adhesive thickness, and loading conditions.

３軸応力下における接着層の降伏および破壊挙動

Stress states in adhesive layer of bonded joints are usually three-axial. However, yield and failure behavior under triaxial stress conditions have yet been clarified sufficiently. Two types of adhesive joints are available for investigating yield and fracture characteristics. One is scarf joint which has considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied by changing scarf angle. Another is butt joint with thin wall tube in which considerably uniform pure shear stress state in the adhesive layer can be realized under torsional load condition. Furthermore, stress triaxiality which appeared in the adhesive layer of most of adhesive bonded joint can be reproduced by tensile tests of the scarf joint and torsional tests of the butt joint with thin wall tube. In this study, the effects of stress triaxiality on yield and failure stress were investigated by above mentioned tests using three kinds of adhesive with various ductility. The results showed that the yielding and failure criterion was changed by stress triaxiality and that the fracture mechanism of adhesive layer with homogeneous adhesive was different from the one with heterogeneous adhesive. Based on these experimental results, the method to predict the yielding and failure criteria were proposed by using the triaxiality parameter.

An experimental investigation of failure initiation in bonded joints

The effects of cure temperature on the tensile strength of scarf joints between aluminium substrate and epoxy resins are examined. The measured tensile strengths showed strong dependence on the adhesive layer thickness, cure temperature and the scarf angle. A failure initiation criterion is proposed for bonded joints subjected to a combination of thermal and mechanical loading, based upon the critical value of a free-edge stress intensity factor. Free-edge failure occurs when the free-edge intensity factor attains a critical value irrespective of the cure temperature and layer thickness.

ANALYSIS OF FREE‐EDGE STRESS AND DISPLACEMENT FIELDS IN SCARF JOINTS SUBJECTED TO A UNIFORM CHANGE IN TEMPERATURE

The role of free‐edge stresses in controlling the initiation of failure from the interface corner of a scarf joint subjected to a uniform change in temperature is examined. In general, the stress field can be expressed by σij = Hrλ−1 + σij0 , where r is the radial distance from the interface corner, λ − 1 is the order of the stress singularity, H is the intensity of the singularity, and σij0 is a non‐singular constant stress. A combination of the finite element method and a path‐independent integral is used to evaluate the magnitude of H for two joint configurations: (i) a scarf joint between two long bi‐material strips; and (ii) a scarf joint consisting of a thin elastic layer sandwiched between two substrates. The magnitude of H is linearly dependent on a non‐dimensional constant function a; the magnitude of a decreases with increasing level of mismatch in the elastic properties of the bonded materials. A comparison between the values of H evaluated by the path‐independent integral method and the commonly used extrapolation method indicate that the extrapolation method could be in error by as much as 25%.

Fatigue properties of jointed wood composites: Part I. Statistical analysis, fatigue master curves and constant life diagrams

The primary aim of this work was to assess the fatigue performance of scarf-jointed laminated wood composites used to manufacture wind turbine blades and establish simple fatigue design procedures. Laminates made from poplar (Populus canadensis/serotina), Khaya (Khaya ivorensis) and beech (Fagus sylvatica), incorporating typical scarf joints, were assessed under reversed loading (R =−1). Scarf joints were found to be great equalizers of fatigue performance for wood species with different static strengths. Poplar was investigated at several other R ratios (+3, −3, −0.84 and 0.33). The application of 95% survival probability limits derived from pooled data increases the statistical reliability of σ–N curves and gives an improved estimate of a material's minimum performance. The σ–N curves derived for all three wood species at R =−1 were normalized with respect to ultimate compressive strength values and found to be practically coincidental. This allowed the derivation of a master curve for a generic scarf-jointed wood laminate under reversed load conditions. This relationship was verified using data from the literature and found to be a good predictor of fatigue performance. The construction of simple triangulated constant life diagrams based on static tensile and compressive tests and fatigue testing at R =−1 brings about a rapid assessment of the overall fatigue performance of any wood composite. These can then be used in the fatigue design or life prediction of wood composites under cyclic loading.

Fatigue failure criterion of adhesively bonded CFRP/metal joints under multiaxial stress conditions

Abstract In order to investigate the fatigue failure criterion of adhesively bonded CFRP/metal joints under multiaxial stress conditions, a series of fatigue tests was carried out on three kinds of adhesively bonded joint specimens; butt joint, scarf joint and thick-adherend lap-shear joint, where these joints have considerably uniform stress distributions in the adhesive layer. In addition, stress distributions of these joints were analysed by the finite element method. The results show that the fatigue limits of respective joints are governed by the maximum principal stress except when negative hydrostatic pressure acts on the adhesive.

THE ROLE OF SCARF ANGLE IN THE PERFORMANCE OF ALUMINUM MATRIX COMPOSITE JOINTS

The role of geometry on the mechanical performance of scarf joints in Al-matrix composites reinforced with continuous polycrystalline alumina fibers was investigated. Model joints consisting of thin metal interlayers at varying scarf angles between composite sub-elements were designed, manufactured and tested to study the relevant deformation and failure phenomena. Specimens were produced by pressurized infiltration of molten Al-4.5%Mg into fiber preforms containing prescribed discontinuities. In this way, matrix continuity through the interlayer was ensured and defects normally introduced in actual joining operations were minimized. Reference composite specimens were produced in the same manner and exhibited tensile strengths of the order of 700 MPa, compared with a matrix yield strength, σ0, of approximately 100 MPa. The strength of the scarf joints with aspect ratios of 17–48 and angles below 45° was relatively constant, ∼310 ± 10 MPa, but increased at higher angles, reaching over 485 MPa at 75°. Failure of these joints was typically by debonding at the composite-interlayer interface. A simple analytical model suggests that failure occurs when the stress normal to the interlayer reaches a critical value, although the details of the debonding mechanism are still unclear. Conditions leading to a transition from debond failure to plastic collapse in joints of low aspect ratios are also discussed. © 1997 Acta Metallurgica Inc.

Fracture of interface cracks under combined loading

This paper presents a study of the fracture behaviour of interfacial cracks under combined mode I and mode II loading condition. Experiments were conducted using scarf joints, in which a wide range of mode ratio is obtained by varying the slant angle. Brittle epoxy adhesive was used in the bonding so that plastic deformation is not a major factor. The observed crack paths are inter-layer with the crack running parallel to the interface. The experimental results suggest that the existing criteria established for homogeneous materials are not applicable to the bi-material system. A fracture criterion is proposed based on the principal strain and the hydrostatic tension, reflecting the sensitivity of brittle adhesives to pressure. It is shown that the theory can also provide good correlation to some of the experimental results obtained using different configurations.

Fracture criterion of mixed mode in adhesively bonded joints of Al/Steel dissimilar materials

A method of strength evaluation applying fracture mechanics in the adhesively bonded joints of Al/Steel dissimilar materials was investigated in this paper. Various shapes of adhesively bonded Al/Steel scarf joints focussing on fracture criterion of mixed mode crack were prepared for the static tests. Also, stress intensity factors of the interface cracks in adhesively bonded joints of Al/Steel dissimilar materials were analyzed with 2-dimensional elastic program of boundary element method(BEM), and the experiment of fracture toughness were carried out under various mixed mode conditions. From the results, the fracture criterion and method of strength evaluation by the fracture toughness in adhesively bonded joints of Al/Steel dissimilar materials were proposed.

Fatigue Failure Criterion of Adhesively-Bonded Joints Under Combined Stress Conditions

Abstract A study was conducted to investigate fatigue failure criteria for adhesively-bonded joints under combined stress conditions. Two types of adhesively-bonded joint specimens were used: the scarf joint and the butterfly-type butt joint. Both types of joints have considerably uniform combined stress distributions in the adhesive layer. Furthermore, the stress distributions of these joints were analyzed by a finite element method. The results showed that the maximum principal, the von Mises equivalent and the maximum shear stresses in the uniform stress region of the adhesive layer at the endurance limit are correlated with the principal stress ratio.

Effect of Joint Configuration on Impact Deformation of Taper Lap and Scarf Joints.

This paper discusses the dynamic strain and stress distribution of adhesively bonded joints having different configurations. The selected configurations are tapered lap and scarf joints. The dynamic deformation of the joints under impact loading is calculated using the finite element method. The calculated strains are compared with the experimentally measured dynamic strains. The bending effect for the tapered lap joints are more remarkable than those in the scarf joints when the joints are subjected to impact loading. Stress waves propagate in a complicated manner due to the bending effect and the reflection of the waves for the tapered lap joints; however the waves propagate smoothly for the scarf joints. The dynamic stress in the adhesive layer increases for the tapered lap joints with increasing in time, but the change in dynamic stress with respect to time is negligible for the scarf joints.

Effect of adhesive layer thickness on fatigue strength of adhesively bonded butt, scarf and butterfly type butt joints

Fatigue tests under multiaxial stress conditions were conducted for the three types of butt, scarf and butterfly type butt joints in a variety of scarf angle and adhesive layer thickness

Strength Evaluation of Bonded Dissimilar Materials by Using Stress Singularity Factor

Recentrly advantages in composite and light weight material techniques have led to the increased use of bonded dissimilar materials such as ceramics/metal bonded joints, IC package, brazing, coating and soldering in the various industries. It is required to analyze the evaluation method of fracture strength and design methodology of bonded joints in dissimilar materials. Stress singularity according to changes of scarf angles for bonded scarf joints in dissimilar materials was investigated by the boundary element method and static experiments. In this paper, effect of the stress singularity factors at the interface edges of scarf joints on various dissmilar materials combinations were investigated by analysis of its stress and stress singularity index using 2-dimensional elastic program of boundary element method. And the variations of stress singularity index by changes for Young's modulus ratios of materials and scarf angles were investigated. Also, it is found that stress singularities at bonded interface edges are disappeared for certain combination of scarf angle in a pair of bonded dissimilar materials. As the results, it is proposed that the strength evaluation by using stress singularity factors, , considering stress singularity at the interface edges of bonded dissimilar materials, is very useful.