Scarf Angle Research Articles

Experimental and Numerical Study of Adhesively Bonded CFRP Scarf-Lap Joints Subjected to Tensile Loads

The tensile performance of adhesively bonded CFRP scarf-lap joints was investigated experimentally and numerically. In this study, scarf angle and adherend thickness were chosen as design parameters. The lap shear strength is not directly proportional to scarf angle and adherend thickness for the brittle adhesive studied in the paper. The major failure mode includes cohesive shear failure and adherend delamination failure. The results present a stepped failure morphology along the bondline in the adhesive layer. A finite element model based on cohesive zone model was established to further investigate the stress distribution of scarf-lap joints with different lap parameters. The numerical results were compared with the experiment results, showing a good agreement, thus verifying the validity of the established numerical model.

Parameters sensitivity and optimization for composite scarf repair

Scarf repair is the preferred method to restore the load-carrying capacity of the damaged structure of fiber-reinforced composite material when stiffness, strength, and flushness are needed. However, shear stress concentration occurs along the scarf bond line coinciding with the location of 0° plies. Previous studies did not consider the sensitivity of parameters systematically and involve the interaction effect. In this paper, sensitivities of scarf angle, adhesive thickness, and stacking sequence are evaluated synthetically through main effect and interaction effect by using the method of design of experiment. The general strategy of decreasing scarf angle needs to remove more materials from parent laminate. Thus, a new strategy of changing the stacking sequence of repair is performed to reduce the adhesive stress concentration by means of the optimization based on genetic algorithm. In spite of this, optimization results may not have good reliability especially when manufacture, load, and other fluctuations happen. Robustness analysis based on Monte Carlo simulation is used to estimate the reliability of optimization results.

The Effect of Joint Form and Parameter Values on Mechanical Properties of Bamboo-Bundle Laminated Veneer Lumber (BLVL)

Bamboo-bundle laminated veneer lumber (BLVL) was produced by joint lengthening technology. The objective of this study was to evaluate the effect of joint form and the values of key parameters on mechanical properties of BLVL. Two joint forms, i.e., finger joint and scarf joint, and two corresponding joint parameters, i.e., finger length and scarf angle, were investigated in laminates. The results indicated that the mechanical properties of jointed BLVL were reduced in comparison to those of BLVL without joints. For finger-joint form, the BLVL member with 14 mm fingers achieved better compressive strength, bending modulus of rupture (MOR), and tensile strength than 19 and 25 mm ones. For scarf-joint form, the 30° scarf-jointed member exhibited the highest bending modulus and tensile modulus of rupture (MOR) in this study, then followed the 45° and 60° ones. The 60° scarf-jointed BLVLs were better than 30° and 45° groups in compression strength. Synthesis of the testing results revealed that finger-jointed BLVLs achieved better bending modulus of rupture than scarf-jointed ones.

On the interfacial strength of bonded scarf joints

The interfacial strength of bi-material bonded scarf joints is evaluated by an integrated experimental and numerical investigation. The effects of the scarf angle, interface dimension and the stress singularity on the interfacial strength were examined using a series of aluminum/PMMA and aluminum/polycarbonate bi-material specimens under static bending moments. The experimental results showed that the maximum principal tensile stress at failure did not vary significantly with interface dimension. The measured maximum principal tensile stress of scarf joints at failure decreases as the scarf angle increases. We correlated fracture initiation with critical values of the stress intensities of the singular fields near the three-dimensional interface corner. The order of the singularity at three-dimensional interface corners and edges was obtained numerically using common finite element methods. Finally, an empirical equation based on interface fracture mechanics was determined by curve fitting of the experimental results to predict the interfacial strength of bonded scarf joints.

English

This paper deals with the stress and failure characteristics of V-shaped epoxy adhesive joints. The effect of scarf angles upon failure morphology was investigated by tensile tests and monitoring using high speed cameras. V-shaped specimens were fabricated with three bond thicknesses, t (i.e. 0.2mm, 0.6mm, and 1.0mm) and various scarf angles (i.e. θ = 30 , 45 , 60 , 75 , and 90 ). From failure surface observation, failure morphology can be divided into five types, consisting of interface failure and/or cohesive failure. Shear stress plays a major role in failure morphology where higher shear stress favors interface failure in the specimens tested. A simple failure criterion is also proposed based on the results obtained. It is found that the proposed criterion can approximately predict the failure stress of V-shaped epoxy adhesive joint specimens. For stress analysis, V-shaped epoxy adhesive joints having bond thicknesses t = 1.0mm, and with various scarf angles (i.e. θ = 30 , 45 , 60 , 75 , and 90 ), were modelled. Stress singularity observed in simulation plays a major role in failure morphology where higher singularity favors cohesive failure in the specimens tested in experiment.

Analysis of the Influence of Geometric Parameters on the Stress Distributions in Adhesively Bonded Scarf Joints Using 2D Models Under Elastic Assumption

The scarf joint is a usual experimental assembly employed to analyze the mechanical behavior of an adhesive. In fact, using a unique type of bonded assembly with a classic tensile testing machine, various tensile-shear loadings of the adhesive can be applied by changing the value of the scarf angle. In this paper, accurate numerical analyses of the stress distributions within the adhesive in scarf joints under elastic assumption using 2D models are developed. Numerical results underline the influence of the adhesive thickness and mainly the influence of the scarf angle on the edge effects, and confirm the presence of an optimal scarf angle associated with very low stress concentrations. Moreover, the use of a suited elastic limit for the adhesive, defined from the two stress invariants, hydrostatic stress and von Mises equivalent stress, allows the more stressed parts of the adhesive with respect to the scarf angle to be defined. These results also underline the possible influences of the edge effects on the experimental results, i.e., possible crack initiations close to the free edges of the adhesive for some scarf angles. Finally, it is shown that a little modification of the free edges of the adhesive (a so-called “cleaning”) can strongly reduce the influences of the edge effects and thus can improve the experimental results for a wide range of scarf angles.

Three-dimensional FEM stress analysis and strength prediction of scarf adhesive joints with similar adherends subjected to static tensile loadings

The interface stress distributions in scarf adhesive joints with similar adherends under static tensile loadings were analyzed using two-dimensional and three-dimensional finite element calculations for the two cases where adhesive length and width of the adherends were held constant. The effects of adhesive Young׳s modulus, the scarf angle and the adhesive thickness on the interface stress distributions were examined. In addition, the joint strength was predicted using the interface stress distributions based on the maximum principal stress theory and von Mises׳ stress criterion. It was found that when the scarf angle was around 60°, the singular stress at the edges of the interfaces was minimal in the 3-dimensional FEM calculations while the singular stresses vanished at 52° in the 2-dimensional FEM calculations. The value of the normalized maximum principal stress at the edges of the interfaces obtained from the 3-D FEM was larger than that from the 2-D FEM. It was found that the maximum principal stress increased at the edge of the interfaces as the scarf angle decreased. In addition, the effect of the adherends where the width was held constant was demonstrated. Experiments to measure strain and joint strengths were carried out. The measured strains were in a fairly good agreement with those obtained from the 3-D FEM calculations. Also, the measured joint strengths were fairly consistent with the predicted results based on the maximum principal stress theory. As a result, the maximum joint strength was observed when the scarf angle was around 60°. The difference in the interface stress distributions was demonstrated between the two cases.

The Influence of Orthotropy and Taper Angle on the Compressive Strength of Composite Laminates with Scarfed Holes

The residual strengths of laminated composites containing notches or holes depend on the characteristic dimension of the notches as well as the laminate properties. This paper presents an experimental and computational investigation of the effect of orthotropy as well as scarf notch angles on the compressive strength of composite laminates made of seven different stacking sequences with the number of plies ranging between 20 and 24. Specimens containing scarfed holes of different scarf angles (6° and 10°) and straight holes of two different diameters (3.175mm and 6.35mm) were tested for comparison. Experimental results of the compressive strengths, normalised by the respective un-notched compressive strength show that the extent of orthotropy, defined as the laminate stiffness ratio along and perpendicular to the main loading direction, has a major effect. Computational modelling was carried out using the finite element method to quantify the residual strength; in which ply fracture is modelled using the continuum damage mechanics approach.

Mechanical behavior of interlocking multi-stepped double scarf adhesive joints including void and disbond effects

Surface topographical effects on the mechanical behavior of interlocking multi-stepped double scarf adhesive joints under tensile load were studied. For this purpose, finite element analysis (FEA) of the joint geometry at 10 different step angles was carried out. In the second stage, the effects of substrate voids and adhesive delaminations on the interfacial strength were studied for the scarf angle of 32.2° by FEA simulation as well as experimentally. For the cases of the missing steps (voids) and delamination (absence of bonding induced by release agent) the ratios of maximum stresses (principal, von Mises, normal, shear and transverse) between the completely bonded and altered (void or delaminated) joints were compared with the failure load ratios for the same joints to interpret the mechanism of failure. The results revealed that except for the normal stress, the maximum stress ratios reach a maximum value and then decrease with increasing scarf angle. FEA analysis with the voids showed that the strength of the joint not only depends on their size, but also on their location in the joint. When the experimental results were compared with the FEA using the stress ratio between the unmodified (completely bonded) and modified (void or disbond) cases, the results indicated that the normal stress dominates the failure behavior of the 32.2° scarf angle joint. Comparison of the experimental results for the void, and disbond cases revealed that the disbond cases can possess higher joint strength in comparison to the void cases. This finding could not be predicted by FEA, and was attributed to the presence of friction at the interface subsequent to delamination.

Numerical analysis on load-bearing capacity and damage of double scarf adhesive joints subjected to combined loadings of tension and bending

The load-bearing capacity and the damage level of the double scarf joint (DSJ) under combined loadings of tension and bending were investigated numerically, which takes into account the effects of scarf angle and adhesive type. A finite element method (FEM), which includes a mixed-mode cohesive zone model (CZM) with a bilinear shape, was employed to govern the interface separation behaviors. At the point corresponding to the ultimate loading, it was observed that the interface damage level of DSJ with the ductile adhesive is higher and more uniform than that of the joint with the brittle one. More than that, the numerical results illustrated that the failure of DSJ is controlled not only by the ultimate loading, but also by the applied displacement until complete failure. Therefore, the failure energy, which is defined as the integral of the loading with respect to the displacement, was adopted to estimate the joint performance. Subsequently, the numerical results showed that the failure energy of the joint with the ductile adhesive is higher than that of the joint with the brittle one. Furthermore, all the discussed characteristic parameters of a DSJ with a given adhesive, including ultimate loading, the von-Mises equivalent stress and interface damage level corresponding to the ultimate loading, and the failure energy, were inversely proportional to the scarf angle. Finally, through comparing with the existing experimental measurements, the adoptive method was validated.

Experimental and Analytical Modeling of Scarf Repaired Composite Panels

Experiments and finite element analysis (FEA) were performed on scarf-patch repaired composite panels. Tensile static tests were performed on pristine and repaired panels to evaluate their tensile strength. The obtained results showed that the repaired panels restored tensile strength by 95% of the pristine value. Subsequently, a verification finite element (FE) model was established. Two additional models, one with homogenized mechanical properties of the laminate and one that considered that the ply-by-ply properties were built to simulate the experimental repairs. The predictions of the two models agreed reasonably well with the experimental results and the optimum scarf angle was found at 2.5°.

Effect of adhesive thickness, adhesive type and scarf angle on the mechanical properties of scarf adhesive joints

The effects of adhesive thickness, adhesive type and scarf angle, which are determined as the main control parameters by the dimensional analysis, on the mechanical properties of a scarf adhesive joint (SJ) subjected to uniaxial tensile loading are examined using a mixed-mode cohesive zone model (CZM) with a bilinear shape to govern the interface separation. Particularly, the adhesive-dependence of the vital cohesive parameters of CZM, which mainly include initial stiffness, total fracture energy and separation strength, is introduced emphatically. The numerical results demonstrate that the ultimate tensile loading increases as the adhesive thickness decreases. Cross the ultimate tension, the joint loses the load-bearing capacity when adopting the brittle adhesive but sustains partial load-bearing capacity while selecting the ductile adhesive. In addition, for the joint with the ductile adhesive, the maximum applied displacement until the complete failure of it is directly proportional to the adhesive thickness, which is different from the case using the brittle adhesive. Taking the combination of the ultimate loading and applied displacement into account, failure energy is employed to evaluate the joint performances. The results show that the failure energy of the joint with the brittle adhesive increases as the adhesive thickness decreases. Conversely, the situation of the joint using the ductile adhesive is vice versa. Moreover, the effect of the adhesive thickness becomes more noticeable with decreasing the scarf angle owing to the variation of the proportion of each component of the mixed-mode. Furthermore, all the characteristic parameters (the ultimate tensile loading, the maximum applied displacement and the failure energy) that adopted to describe the performances of SJ increase as the scarf angle decreases. Finally, the numerical method employed in this study is validated by comparing with existing experimental results.

Stress Analysis and Failure Characterizations of V-Shaped Epoxy Adhesively Bonded Joint

This paper deals with stress and failure characteristics of V-shaped epoxy adhesive joint. Effect of scarf angle upon failure morphology was investigated by tensile test and monitoring using high speed camera. V-shaped specimens were fabricated having bond thicknesses, t = 1.0 mm and various scarf angles (i.e. θ = 30 ̊, 45 ̊, 60 ̊, 75 ̊, and 90 ̊). From failure surface observation, failure morphology can be divided into 5 types consisting of interface failure and/or cohesive failure. Stress singularity plays a major role in failure morphology where higher singularity favors cohesive failure in the specimens tested.

Strength prediction and reliability of brittle epoxy adhesively bonded dissimilar joint

This paper deals with strength and failure prediction as well as reliability issues of adhesive joints of brittle epoxy bonding of two dissimilar adherends. Effects of bond thickness and scarf angle upon the strength of such joints are also addressed. Three kinds of adhesive joints, i.e., butt, scarf and shear joints, are considered. It is found that the strength prediction of various adhesive joints under consideration can be done by establishing interface corner toughness, Hc, parameter. For adhesive joints with an interfacial crack, fracture toughness, Jc, or interfacial toughness, Kc, can be used as a fracture criterion depending on the fracture type observed. The predicted strengths based on these fracture criteria (i.e., Hc, Jc and Kc) are in good agreement with experimental data obtained. Weibull modulus is a suitable parameter to define the strength reliability of adhesive joints. From experimental data, scarf joint of 45° is identified to be preferable since it satisfies both outstanding load-bearing performance and tolerable reliability. In addition, the Weibull statistical method has made possible the strength reliability determination of non-cracked adhesive joints.

FEM stress analysis and strength prediction of scarf adhesive joints under static bending moments

The stress distributions at the interfaces in the scarf adhesive joints under static bending moments were analyzed using two-dimensional and three-dimensional finite element (FEM) calculations. The effects of the scarf angle, adhesive Young's modulus and the adhesive thickness on the interface stress distribution were examined. It was found that the singular stress at the edges of the interfaces decreased as the adhesive Young's modulus increased and the adhesive thickness decreased. The singular stress at the edges of the interfaces obtained from the 3-D was larger than that from the 2-D FEM. The joint strength was also predicted using the elasto-plastic 3-D FEM calculations. For verification of the FEM calculation results, the strains in the adherends and the joint strengths were measured. The measured results of the strains and the joint strengths were fairly consistent with the results obtained from the 3-D FEM calculations and indicated that the rupture bending moment (joint strength) was the maximum when the scarf angle was around 60°.

Study of tensile failure mechanisms in scarf repaired CFRP laminates

Bonded scarf repairs are used in composite structures when high strength recovery is desired or when there is a requirement for a smooth surface to satisfy aerodynamic requirements. Experimental and finite element study is carried out to understand the damage propagation and ultimate strength of scarf patch repaired CFRP laminates under uni-axial tensile load here. The ultimate strength and damage propagation in scarf repaired laminates has been investigated with respect to change in scarf angle and patch diameter. It is revealed from the results that damage is initiated in adhesive film at the bonded interface of 0° plies of parent laminate and patch in and then propagates in circumferential direction around scarf patch. It is found that some damage occurs in the parent laminate before the damage initiation in the adhesive film. After the adhesive film failure, the damage in parent laminate quickly propagates transversely to the free edge sides of the laminate and failure occurs. The results of this investigation provide further insight into the damage mechanisms in scarf repairs of composite structures under tensile load. This study may be helpful in improving the design and analysis techniques for scarf patch repair of composite structures.

The investigation of the effect of angle on the failure load and strength of scarf lap joints

In this study, the effect of scarf angle on tensile strength of an epoxy adhesive was investigated both experimentally and numerically. Tensile tests were carried out using Scarf Lap Joints (SLJs). Commercially available epoxy (Atlac 580) was used as adhesive and fiber reinforced epoxy/glass (Hgw 2372 Grade G10 EP GC 203) was used as adherends. The stress analyses were performed using a three-dimensional Finite Element Method (FEM). The FEM analyses were executed with Ansys (V.14.0.1). Experimental results were compared with the FEM results and found quite reasonable. The results have indicated that when overlap angle increases, the failure load values also decrease in all joints.

Prediction of Strength and Failure for Brittle Epoxy Adhesive Joints of Dissimilar Adherends

This paper deals with the strength and failure prediction as well as the reliability issues of adhesive joints of brittle epoxy bonding two dissimilar adherends. The effects of bond thickness and scarf angle upon the strength of such joints are also addressed. Three kinds of adhesive joints, i.e., butt, scarf and shear joints are considered. It is found that the strength prediction of various adhesive joints can be done by establishing the interface corner toughness, Hc parameter. Weibull modulus is suitable to define the reliability of adhesive joints. The scarf joint of 45° is identified to be preferable since it satisfies both outstanding load-bearing performance and tolerable reliability. Hence, both applications of Hc parameter and Weibull statistical method allow the strength and failure predictions of adhesive joints with fairly improved reliability.

Reliability analysis of adhesive bonded scarf joints

A probabilistic model for the reliability analysis of adhesive bonded scarfed lap joints subjected to static loading is developed. It is representative for the main laminate in a wind turbine blade subjected to flapwise bending. The structural analysis is based on a three dimensional (3D) finite element analysis (FEA). For the reliability analysis a design equation is considered which is related to a deterministic code-based design equation where reliability is secured by partial safety factors together with characteristic values for the material properties and loads. The failure criteria are formulated using a von Mises, a modified von Mises and a maximum stress failure criterion. The reliability level is estimated for the scarfed lap joint and this is compared with the target reliability level implicitly used in the wind turbine standard IEC 61400-1. A convergence study is performed to validate the FEA model, and a sensitivity analysis on the influence of various geometrical parameters and material properties on the maximum stress is conducted. Because the yield behavior of many polymeric structural adhesives is dependent on both deviatoric and hydrostatic stress components, different ratios of the compressive to tensile adhesive yield stresses in the failure criterion are considered. It is shown that the chosen failure criterion, the scarf angle and the load are significant for the assessment of the probability of failure.

J045032 接合部に波型形状およびスカーフ角度を有する突き合わせ接着継手の引張強度特性

In this study, we have suggested the new joint configuration, which has scarf angle and a wave-shaped surface at a bonding part. At first, mechanical property in the presented joint subjected to static tensile load is analyzed, using finite element method (FEM). FEM analyses to clarify the stress field at the bonding part are carried out. To confirm the reliability of the analytical results, experiments for strain distribution measurement by using strain gauges are performed. In addition, effects of curvature radius of waved surface at bonding part on stress distribution were examined. As a result, stress concentrations at edges of the bonding part of the presented joint were expected to decrease lower than the conventional waved butt joint. Furthermore, it was thought that the stress concentration decreased at center of bonding part as the curvature radius increased.