Scarf Joints Research Articles

On the design methodology of scarf repairs to composite laminates

Scarf repairs are the preferred method of repairing composite structures, especially when externally bonded patches can no longer meet the stiffness, strength, and flushness requirements. Present designs of scarf repairs are based on two-dimensional analyses of scarf joints, assuming a uniform stress distribution along the scarf. The purpose of this investigation is to first critically assess the adequacy of the current design method, and then to develop an improved design methodology. Strain concentrations in the adhesive and the composite laminates are determined using the elastic–plastic finite element method, taking into account of the influence of stacking sequence, laminate thickness, and adhesive yielding. Three series of experiments have been carried out, involving joints between metallic and composite adherends, with matched and un-matched lay-ups. Based on the experimental findings of joints and the computational analyses of joints and repairs, an improved method has been proposed that accounts for local peak plastic strains due to non-uniform shear strain distribution. Comparison with experimental results confirmed that the new approach provides an improved first-order prediction of repair efficiency of scarf joints.

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.

ＣＦＲＰスカーフ接着継手の衝撃後強度特性

Tension and compression tests of scarf joints were conducted to evaluate the effect of angle of scarf. Impact loadings were applied for some specimens prior to the residual compression and tension test of scarf joints. Such scarf joints were experimentally investigated by using three variations of scarf angles of 3, 4.5, and 6 degrees. The impact loadings at the center of the specimen were with two different energy level for tension specimen, and three for compression. As a result, the statical tensile and compressive strengths of non-damaged scarf joint decreased as the scarf angle increased. The strengths of scarf joint compared with the parent specimen are more significantly decreased in tension than in compression. For the small angle of scarf the impact damage and the residual strength of scarf joints were measured to be similar to those of the parent material. Whereas for the large scarf angle, the delamination of the adhesive layer of scarf joint increased remarkably, then, the residual strength of scarf joint decreased. It is obvious that the delamination of adhesive layer affected the residual strength of scarf joints under tension and compression.

Experimental studies on adhesively-bonded scarf joints: influence of natural ageing on progressive damage

This work characterizes the effects of natural ageing on the micro-mechanical behaviour of two adhesively-bonded scarf joints. The samples studied are made of XC18-type steel with different scarf angles (33 and 6°) and the adhesive is an epoxy resin. Contrary to most experimental studies which determine the strength of bonded joints in terms of their failure load, in this study sensitive strain gauges have been used to measure progressive damage of the adhesive joints. The results show that the damage is closely linked to the mechanical and geometrical properties of the test joints and that ageing increases the load thresholds of the first microcracks initiation and the ultimate failure of both adhesively bonded scarf joints.

Evaluation of the Impact Damages of Scarf-Repaired Composites

A CFRP scarf-repaired specimen is manufactured using the autoclave method on the assumption that it is a part of the repaired composite airframes. Such specimens are composed of a scarf joint and a step-lap joint. Though the in-plane mechanical properties of the scarf-repaired composites have been studied, few researches on out-of-plane behavior have been reported. Thus, the mechanism of the impact damages in the scarf-repaired composites was clarified through two kinds of experiments, the low-velocity impact test and the static indentation test. Through the low-velocity impact test, the damage configuration was observed and the influences of the scarf angle and stacking sequence on impact damage were investigated. As a result, the debonding at the interface between the adhesive layer and the repair laminate was observed in all the specimens. Moreover, damage growth was observed through the static indentation test. The indentation results indicated that the debonding at the interface between the adhesive layer and the repair laminate occurs at the intersection of the adhesive layer and the delamination of the repair laminate.

Improved design methods for scarf repairs to highly strained composite aircraft structure

This paper presents an analytical method to optimise the profile of the scarf joint between dissimilar modulus adherends so that adhesive stresses are approximately uniform along the joint. The optimised scarf repair is expected to enhance joint strength and reduce the amount of material removal. Finite element analyses have been performed to both validate the optimal solution and to evaluate the use of low stiffness patch to repair carbon epoxy composites. In particular, the influence of patch lay-up on adhesive stresses has been investigated.

Parametric study of scarf joints in composite structures

Bonded scarf or stepped repairs are used in composite structures when high strength recovery is needed or when there is a requirement for a flush surface to satisfy aerodynamic or stealth requirements. Scarf repairs are complex to design and require the removal of significant parent structure, particularly for thick skins. A parametric finite element (FE) model has been developed to allow a broad study into the effect of various parameters on the performance of a scarf joint. The stress distribution along the bondline has been investigated, and the sensitivity of peak stresses determined with respect to changes in scarf angle, adhesive thickness, ply thickness, laminate thickness, over-laminate thickness and lay-up sequence. Furthermore, the adhesive stresses resulting from joining matched and mismatched laminates was investigated. The benefit of load by-pass of a repair was also examined using a 3D model of a circular patch. The results of this investigation provide further insight into the stresses that develop in scarf repairs of composite structures under load. This insight may lead to improved design and analysis techniques of scarf joints in composite structures.

Effects of scarf joints on bending strength and modulus of elasticity to laminated veneer lumber (LVL)

The role of geometry on the mechanical performance of scarf joints in laminated veneer lumber (LVL) bonded with phenol formaldehyde and melamine formaldehyde (MF) adhesives was investigated. Model joints consists of 3, 4 and 5 mm veneer thicknesses at 30°, 45° and 60° of varying scarf joints for LVL produced from brutia pine ( Pinus brutia Ten) and elm ( Ulmus compestris l.) woods. However, there is little information available concerning the bending strength and modulus of elasticity for LVL, and in particular, scarf joints in these field. In this study, it was aimed to determine the bending strength and modulus of elasticity for LVL. For this purpose, samples were tested according to TS EN 310 standard. It was observed that the highest bending strength (291.5 N/mm 2) and modulus of elasticity (28 101 N/mm 2) were obtained in control (solid wood) samples having three layered LVL, jointed with 30° angle and bonded with MF adhesive. As a result of the effects scarf joints on bending strength and modulus elasticity test, if the scarf angle decreases, the properties of LVL increase.

Fatigue behavior of integrally fabricated joints between titanium matrix composite and titanium alloy

Abstract An integrally fabricated double scarf joint, with a taper ratio of 20:1, between the monolithic titanium alloy and the silicon carbide fiber reinforced titanium matrix composite was studied under the monotonic and fatigue loading (both tension–tension and tension–compression) conditions. Further, the two parent materials forming this functionally graded material system were characterized under the similar loading conditions to compare their performance. In general, the mechanical response of the functionally graded material system was in-between those of the alloy and the composite under all loading conditions. The ultimate tensile strength of the functionally graded material showed a small improvement over that of the alloy; however, its modulus of elasticity in the joint region was about the average of its counterparts of alloy and composite. There was also a small improvement in fatigue life of the functionally graded material relative to that of the monolithic alloy in both tension–tension and tension–compression conditions. The fatigue damage initiated at fiber ends in the joint region and then progressed in the monolithic alloy section. The functionally graded material system containing the integrally fabricated joint between the monolithic titanium alloy and the titanium matrix composite provided the expected performance, i.e., it performed at least similar to its weakest link, i.e., titanium alloy, and it has potential to provide the practical benefits of cost, durability etc.

Tensile failure of adhesively bonded CFRP composite scarf joints

Adhesively bonded scarf joints, comprising unidirectional carbon fibre-reinforced epoxy adherends and AF-163-2 film adhesive of 0.15 mm thick, are tested under uniaxial tensile loading. Test results revealed that failure occurred into two modes, namely fibre fracture and pull-out (Mode A) and cohesive shear failure of the adhesive film (Mode B). Mode A failure was observed for scarf angles less than about 2°, while Mode B failure was observed for scarf angles more than 2°. The knockdown in tensile strength was most prominent for scarf angles less than about 1°. For the largest scarf angle of 5°, the tensile strength dropped by 89% in comparison to the strength values of neat composites with no joints. The knockdown in tensile strength with increasing scarf angle was well described by our FEM predictions, which modeled the unidirectional composite as a homogeneous orthotropic continuum with Hashin–Lee failure criterion. Finally, the effect of adhesive bond line thickness on the joint strength is investigated by finite element simulations.

Comparing dry bond strength of spruce and beech wood glued with different adhesives by means of scarf- and lap joint testing method

Dry bond strength of different adhesives on spruce and beech wood was tested by means of the former DIN standard 53253 (1964)scarf joint and the operative standard lap joint testing method according to EN 302-1 (2004). The failure mode in standard lap joint tests was mainly wood failure, whereas scarf joints failed in the bond line. Due to the high proportion of wood failure, no clear effect of the different adhesives, but a clear effect of wood species on the bond strength was observed for lap joint samples. By contrast, scarf joint testing was far less dependent on wood species and delivered clear differences in the strength of different adhesive systems.

Effect of the PVA gluing on bending strength properties of finger jointed turkey oakwood (Quercus cerris L.)

The objective of this study was to examine the influence of finger length, type of glue and finger orientation on bending strength (modulus of rupture and elasticity) of finger jointed turkey oakwood (Quercus cerris L.). Limited information is available on end gluing hardwoods in contrast to softwoods, which have been extensively investigated and industrially utilized (Pena 1999). Non-structural products include furniture, cladding, fencing, internal and external joinery. In structural uses finger jointing is finally the major method to end joint timber for the production of glue-laminated wood (Koch 1972, Jokerst 1981, Nestic and Milner 1993). The other methods to end joint timber (butt and scarf joint) did not find industrial acceptance (River 1994). There are no experimental results for finger-jointed oakwood in Greece. Oak is the most abundant tree species in Greece (746 400 ha) and covers 49% of the broadleaved Greek forests (about 2.5 million ha). Oakwood is produced from coppice forests in small dimensions and utilized almost exclusively as fuelwood and for charcoal production (Stamou 2001). Pena (1999) studied the suitability of producing non-structural finger joints made from European Oak (Quercus petraea L.). He examined the effect of the geometry of finger joint in bending strength, using two different finger lengths (9 and 12 mm). He concluded that modulus of elasticity of the jointed specimens did not differ significantly from the unjointed ones. On the contrary, the jointed specimens presented lower values of modulus of rupture than the unjointed ones (43%).

Yield Behaviour of Rubber-Modified Epoxy Adhesives under Multiaxial Stress Conditions: Effect of Plastic Deformability of the Matrix Phase

In a previous study, the yield stress in the adhesive layer of scarf and torsional butt joints with a rubber-modified adhesive was measured, and the results agreed with those calculated by a finite element method, where Gurson's constitutive equations were applied to the adhesive layer. In this study, to investigate the effect of plastic deformability of the matrix phase on the yielding behaviour of rubber-modified epoxy adhesives, the yield behaviour was investigated by using scarf and torsional butt joints, where a reactive diluent was used to vary the plastic deformability of the matrix phase. The applicability of Gurson's model was examined in terms of the yield stresses calculated by a finite element method with a variety of the model parameters. As a result, it is confirmed that the yield stresses of the rubber-modified adhesives with different matrix phases can be approximately estimated by Gurson's model, and the model parameter's agreement with the experimental data increases with increasing the plastic deformability of the matrix phase.

Theoretical Method to Predict the First Microcracks in a Scarf Joint

ABSTRACT In this work, we have calculated the theoretical threshold (F d) of the first microcracks in the scarf joint. This threshold is particularly important because it marks the end of the elastic behavior of the bonded structure. At this point, the mechanical behavior of the adhesive is nonlinear and becomes dependent on the type of loading applied (dynamic tests, fatigue). Our method takes into account the geometrical variations of the joint as the scarf angle varies. We have used and adapted to our study a theoretical model based on the asymptotic-expansion method. The comparison between the theoretical values and the experimental thresholds determined by strain gauges and acoustic-emission techniques showed that the model accurately forecasts the microcracking of the joint provided that the scarf-angle value is more than 10°. When α is smaller than 10°, the theoretical model can no longer predict the very complex micromechanical behavior at the extremities of the joint, where the sharp ends of the adhesive layer and the metallic adherends coexist and react among themselves.

Fatigue Life Estimation of Adhesively Bonded Scarf Joints Based on a Continuum Damage Mechanics Model

An estimation method of fatigue strength of adhesively bonded joints with various stress triaxialities in the adhesive layer has been proposed based on a damage evolution model for high cycle fatigue. To realize various triaxial stress states, fatigue test was conducted for adhesively bonded butt and scarf joints with various scarf angles bonded by a rubber-modified epoxy adhesive. An equation for estimating the damage evolution in the adhesive layer of the butt and scarf joints was derived from the damage model, where undefined parameters in the equation were determined by comparing the experimentally obtained damage evolution curves of the butt joints with the estimated damage evolution curves. Furthermore, an equation for the estimation of fatigue strength was derived under the assumption that fatigue failure occurs when the damage variable reaches to a critical value. When compared the experimental S-N data of scarf joints with the estimated ones, the estimated fatigue strengths agree well with the experimental data with various scarf angles. This finding suggests that the CDM model is applicable for estimating fatigue strength of adhesively bonded joints with different stress triaxialities.

802 ゴム変成エポキシ接着剤により接着されたスカーフ継手の疲労荷重下における損傷挙動に及ぼす応力多軸性の影響(OS-8A,OS-8 界面と接着・接合の力学)

802 ゴム変成エポキシ接着剤により接着されたスカーフ継手の疲労荷重下における損傷挙動に及ぼす応力多軸性の影響(OS-8A,OS-8 界面と接着・接合の力学)

An improved 2D model for bonded composite joints

A growing number of structural applications and repairs require strong adhesively bonded joints between composite parts. The viability of the finite elements method (FEM) for the design and analysis of such joints has been shown by several researchers (Composites 13(1982) 29, mechanics and mechanisms of damage in composites and multi-materials, ESIS11, Mechanical Engineering Publications, London, 1991, Structural adhesive joints in engineering, Elsevier Applied science Publishers, Amsterdam, 1984). Most of the proposed plane stress models for adhesively bonded joints have traditionally relied on effective lamina properties for the composite adherends because one of the primary goals was the study of adhesive stresses and failure criteria (J. Adhes. 42 (1993)). Such work has also been usually carried out on mainframe computers. However, with the rapid developments in computing, commercial FE codes now exists for PC applications. These present new possibilities, namely the construction of models which are closer in their approximation to the structures they represent. Within this context, a new 2D plane stress approach has been developed which allows ply-by-ply modelling of composite adherends in a joint. It is a development of the traditional 2D plane stress model used by several authors such as Siener (Compos. Mater.: Testing Des. 10 (1992) 444) in investigating of composite repair joints. This paper reports the application of this new approach to the modelling of scarf joints between composite adherends under tensile loading. Various scarf angles were investigated and ultimate joint failure predicted. The results of the analysis were compared to the experimental results published by Adkins and Pipes (Proceedings of the Fourth Japan–US Conference on Composite Materials, 1988, pp. 845–854). The main aim of work was to develop a model which could provided more information than conventional models. The new approach offers the prospect of accurately modelling more realistic joints because, through the retention of the laminated nature of the composite adherends, scope now exists for the application of more physically based failure criteria. This work has shown that it is possible to model adhesively bonded composite joints using an improved 2D plane stress model which retains the laminated nature of the composite adherends. There was good agreement between the predicted failure loads and the experimental results for 3.0°, 6.2° and 9.2° scarf joints. These predictions are dependent on the type of failure criteria used. For the composite adherends, the maximum stress criterion faired better in predicting the 3.0° scarf joint strength.

An improved 2D model for bonded composite joints

A growing number of structural applications and repairs require strong adhesively bonded joints between composite parts. The viability of the finite elements method (FEM) for the design and analysis of such joints has been shown by several researchers (Composites 13(1982) 29, mechanics and mechanisms of damage in composites and multi-materials, ESIS11, Mechanical Engineering Publications, London, 1991, Structural adhesive joints in engineering, Elsevier Applied science Publishers, Amsterdam, 1984). Most of the proposed plane stress models for adhesively bonded joints have traditionally relied on effective lamina properties for the composite adherends because one of the primary goals was the study of adhesive stresses and failure criteria (J. Adhes. 42 (1993)). Such work has also been usually carried out on mainframe computers. However, with the rapid developments in computing, commercial FE codes now exists for PC applications. These present new possibilities, namely the construction of models which are closer in their approximation to the structures they represent. Within this context, a new 2D plane stress approach has been developed which allows ply-by-ply modelling of composite adherends in a joint. It is a development of the traditional 2D plane stress model used by several authors such as Siener (Compos. Mater.: Testing Des. 10 (1992) 444) in investigating of composite repair joints. This paper reports the application of this new approach to the modelling of scarf joints between composite adherends under tensile loading. Various scarf angles were investigated and ultimate joint failure predicted. The results of the analysis were compared to the experimental results published by Adkins and Pipes (Proceedings of the Fourth Japan–US Conference on Composite Materials, 1988, pp. 845–854). The main aim of work was to develop a model which could provided more information than conventional models. The new approach offers the prospect of accurately modelling more realistic joints because, through the retention of the laminated nature of the composite adherends, scope now exists for the application of more physically based failure criteria. This work has shown that it is possible to model adhesively bonded composite joints using an improved 2D plane stress model which retains the laminated nature of the composite adherends. There was good agreement between the predicted failure loads and the experimental results for 3.0°, 6.2° and 9.2° scarf joints. These predictions are dependent on the type of failure criteria used. For the composite adherends, the maximum stress criterion faired better in predicting the 3.0° scarf joint strength.

514 ゴム変成エポキシ系接着剤により接合されたスカーフ継手の疲労強度推定(OS 接着・界面(2))

514 ゴム変成エポキシ系接着剤により接合されたスカーフ継手の疲労強度推定(OS 接着・界面(2))

Modeling of cohesive failure processes in structural adhesive bonded joints

This paper presents an approach to predicting the strength of joints bonded by structural adhesives using a finite element method. The material properties of a commercial structural adhesive and the strength of single-lap joints and scarf joints of aluminum bonded by this adhesive were experimentally measured to provide input for and comparison with the finite element model. Criteria based on maximum strain and stress were used to characterize the cohesive failure within the adhesive and adherend failure observed in this study. In addition to its simplicity, the approach described in this paper is capable of analyzing the entire deformation and failure process of adhesive joints in which different fracture modes may dominate and both adhesive and adherends may undergo inelastic deformation. It was shown that the finite element predictions of the joint strength generally agreed well with the experimental measurements.