Double Scarf Research Articles

Effects of repair techniques and scarf angles on mechanical performance of composite materials

Today, composite materials, which have the advantage of strength and lightness ratio, have gained great importance especially in the aeronautical industry and automotive sector. With widespread use of composites, the repair of damages caused by external factors has also become an important research topic. In this study, the effects of different repair methods and scarf angles on the mechanical performance of the material were investigated. Scarf angles of 20°, 30°, and 45° have been selected in order not to create too many scarf areas and to find a quick repair method. Also, a comparison of single scarf and double scarf repair was made to find a more robust solution. The samples were produced from carbon fiber prepreg and with the [45/0/90/0/45]2s fiber orientation. Tensile, compression, and flexural (3-point bending) tests according to ASTM standards were applied to composite samples prepared with single scarf and double scarf repair configurations at different angles. The stress–strain curves obtained as a result of the tests showed that the specimens repaired at 20° had the highest strength. In addition, it has been determined that the samples repaired with double scarf withstand higher forces compared to the samples repaired with single scarf at the same angle.

When it is necessary to perform an additional akin osteotomy with a scarf osteotomy for mild and moderate deformities of hallux valgus?

SummaryIntroduction. Scarf osteotomy is a common method of treating acquired hallux valgus deformity today. At the same time, as a result of scarf osteotomy, the pathological hallux valgus interphalangeal angle (HVIPA) is not elimi-nated. In order to correct the latter, it is possible to per-form an osteotomy of the base of the main phalanx of the 1st finger (Akin osteotomy). There is a limited amount of objective data comparing the radiological and functional results of treatment of patients with mild and moderate HV deformity who underwent solely either scarf osteo-tomy or scarf osteotomy in combination with additional Akin osteotomy. The aim of the study was to compare the clinical and radiological results of isolated scarf osteo-tomy and a double scarf and Akin osteotomy in patients with mild and moderate HV forefoot deformity. Materials and methods. The study included 187 patients (187 feet) with HV anterior deformity of mild and moderate severi-ty, who were divided into 2 groups. The first group, where Akin surgery was performed in addition to scarf osteoto-my, consisted of 86 patients (86 feet). 101 patients (101 feet) were included in the second group, where only scarf osteotomy was performed. According to all indicators, the patients of the groups were comparable and had no significant differences. The exception was the hallux val-gus interphalangeal angle (HVIPA), which was significant-ly larger in the first group A, since the threshold value for performing an additional Akin osteotomy was the HVIPA value of 9° or more. All patients underwent clinical exam-ination, X-ray control and questionnaires of AOFAS, VAS and MOXFQ scales before surgery, as well as on the 12thand 24th months after surgery. An objective assessment of the quality of hallux valgus deformation elimination was carried out according to three angular indicators: hallux valgus interphalangeal angle (interphalageal an-gle — HVIPA), hallux valgus angle (hallux valgus angle — HVA), the first interplatarsal angle (1–2 intermaetatarsal angle — 1–2 IMA). Results. There was no statistically sig-nificant difference in anatomical and functional results of treatment of patients of the studied groups. In addition, there were no significant changes in the studied indica-tors at 12 and 24 months after surgery, both within each group and in comparison of the groups with each other. Conclusion. To achieve good results when correcting the hallux valgus of the first toe, the anatomy of the phalan-ges of the first toe should be taken into account. Liter-ature data, as well as the results of our studies indicate that correction of valgus deformity of the interphalangeal joint with an HVIPA of 9° or more is recommended to pre-vent loss of correction after osteotomy of the first meta-tarsal. Despite the increase in the traumatic nature of the intervention, Akin osteotomy performed in addition to scarf osteotomy according to appropriate indications does not require correction of the postoperative rehabili-tation program, does not adversely affect the anatomical and functional outcome of treatment, does not lead to an increase in the proportion of complications.

Moisture absorption and hygrothermal degradation of composite laminates repaired via double scarf method

The paper focused on the moisture absorption and hygrothermal degradation of composite laminates repaired via double scarf method. Experimental works were carried out on double scarf-repaired composite laminates to elucidate the effect of hygrothermal aging on the failure strength and failure mode. Finite element models were established to study the moisture distribution in the composite laminates and adhesive layer. And a progressive damage model considering the moisture gradient was subsequently proposed to predict the strength and analyze the damage mechanism. At last, the effects of overlap patch and moisture content on the failure strength were discussed.

Mechanical performance of carbon-glass hybrid composite joints in quasi-static tension and tension-tension fatigue

The ever increasing size of wind turbines has given rise to a need for robust glass to carbon joint designs. This study investigates the effect of different ply layups on the static and fatigue behaviour of hybrid glass/carbon fibre composite joints. Uni-directional carbon fibre prepreg was co-cured to 8H glass prepreg using an overlap to thickness ratio of 20:1, where four joint designs were examined: scarf, interleaving and two forms of double scarf. The joints were tested statically in uniaxial tension and dynamically in tension-tension fatigue. Finite element analysis has been performed to provide insight into stress distributions within each joint. The double scarf joint (with glass on the outside) was found to perform best in fatigue and static tension, while the interleaving joint performed second best in fatigue in static tension but poorest in fatigue. For joint designs that will be used under highly stressed cyclic loading conditions, the current study indicates that static tests alone are a poor indicator of the joint performance and fatigue tests are required.

Failure Mode and Failure Load of Adhesively Bonded Composite Joints Made by Glass Fiber-Reinforced Polymer

Failure mode and failure load of composite joint design has become a very vital research area for being a weak part of composite structures in aircraft segments. So the objective of this study is to investigate the effect of different parameters on the shear strength and failure mode of adhesively bonded single-lap and scarf joints. The parameters included the overlap length, adherend thickness, and adhesive thickness of single-lap joints, and also the effect of single and double scarf angles of scarf joints. The results showed that in single-lap joints, the increasing overlap length led to increase shear strength until the length-to-width ratio becomes same value. The final failure mode of most tested thin adherends bonded joints with similar specimens was the delamination, while the interfacial failure mode occurred in thick adherends bonded joints. The shear strength increased when the adhesive thickness was reduced. On the other hand, in scarf joints, the ultimate shear strength was obtained when the scarf angle was around θ = 18°, while when scarf angle reached θ = 75°, the shear strength was decreased. The failure mode of scarf joints occurred at the interfaces between the scarf angles.

Numerical analysis of effects of adhesive type and geometry on mixed-mode failure of adhesive joint

In the present study, the effects of the adhesive type and geometry (adhesive thickness and scarf angle) on mixed-mode failure of double scarf adhesive joint (DSJ) under uniaxial tensile loading were numerically examined using the finite element subroutine which coupled with a mixed-mode cohesive zone model (CZM). Especially, the effects of the adhesive type, which actually represent the influences of the cohesive parameters in mode I and mode II, on the mechanical properties of DSJ were discussed systematically. The numerical results reveal that the ultimate tensile loading and the necessary energy for failure of DSJ are controlled by the intrinsic components in mode I and mode II with different rates. Accordingly, the mathematical expressions for the ultimate tensile loading and the failure energy of DSJ with respect to the thickness-dependency cohesive parameters in two modes (I and II) and the scarf angle were deduced to identify each contribution in each mode component for a given type of adhesive. In addition, the numerical results also demonstrate that relationship between the interface damage level (corresponding to the ultimate tensile loading) and the adhesive thickness is not monotonous. However, as an increase of the adhesive thickness, the uniformity of damage level distribution is enhanced. Furthermore, the variation of the interface damage level with respect to the scarf angle is also not monotonous for each adhesive thickness. It can be concluded that the effects of the scarf angle and the adhesive thickness on the mixed-mode failure of DSJ are coupled rather than independently.

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.

Analysis and Design of Viscoelastic Adhesively Bonded Tubular Joint

From the survey of worksabout the study of viscoelastic adhesively bonded tubular joint, it was found that a little amount of data is available about the effects of viscoelastic properties of adhesive material on the overall performance of adhesively bonded joints. In the current investigation, stress analysis was carried out for six different geometries of adhesively bonded tubular joints under quasi-static internal pressure, taking into consideration the viscoelastic properties of the adhesive material. Lifetime investigation was carried out for the adopted joints under open and closed end conditions, based on the experimental data of the mechanical properties for the viscoelastic adhesive. Finite element method was adopted through the current investigation considering non-linear viscoelastic behavior of the adhesive. The effects of joint geometry and loading conditions on the lifetime and the equivalent stresses were investigated. From the obtained results it was found that double scarf with external sleeve and stepped adhesively bonded joints are the most appropriate joints under open end condition. In addition for closed end condition butt jointwith external sleeve and external recessed sleeve joint are the most appropriate joints.

Extensometric and Thermo-Elasticimetric Measurements to Characterize the Damage Evolution in Adhesively Bonded Joints: Application for the Adhesively Bonded Double Scarf Joint

In this article, two nondestructive experimental methods are compared in order to evaluate the influence of a singularity, created by geometry, on the damage evolution in the adhesively bonded joints. The first experimental method uses the “back face technique” to measure the disturbances of the strain fields induced by the cracks. The second experimental method uses an infrared focal plane array camera to measure the thermo-elastic field in the geometrical singularity. First of all, both methods are carried out independently: the analysis of the stress/strain and the thermo-graphic analysis. Then, these two methods enabled us to generate a database, the use of which improves the diagnosis of the damage state of the adhesively bonded joints.

Comparison between experimental and numerical study of the adhesively bonded scarf joint and double scarf joint: Influence of internal singularity created by geometry of the double scarf joint on the damage evolution

The objective is to investigate the influence of singularity created by the internal geometry of the double scarf joint, on the damage evolution to the adhesively bonded joint. Several finely instrumented tests are carried out to characterize the damage process unlike the ultimate failure of the adhesively bonded joint. The approach is to compare the micromechanical behavior of adhesively bonded scarf joint and double scarf joint, with the same scarf angle value. Experimental results are obtained by strain gauge measurements to distinguish three damage thresholds, which characterize the damage evolution of the adhesively bonded joint. Numerical results use a damage criterion in order to predict progressive damage evolution. The numerical results show the viability of the failure criterion and the complementarity between experimental and numerical works allows refining our final conclusions.

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.