Intensity Of Singular Stress Field Research Articles

Strength analysis due to thermal loading and tensile loading when metals are bonded by heat-curing adhesives

Heat-curing adhesives are widely used after being cured by heating to a temperature higher than room temperature. To evaluate the adhesive strength, therefore, it is necessary to consider both the thermal stress generated during heat curing and external loads such as tensile stress. Butt joint specimens are essential for evaluating tensile adhesive strength but also thermal strength. The interfacial strength can be discussed from the stress intensity factor (SIF) of a fictitious edge interfacial crack assumed at the interface end. This is because the SIF is controlled by the intensity of singular stress field (ISSF) at the crack-free interface end and a constant term associated with the thermal load. In this paper, a useful thermal SIF solution is proposed by superposing the SIF under tensile stress and the SIF under uniform interface stress associated with thermal loading. This general SIF expression provided under arbitrary material combination can be applied for predicting the tensile strength σc and critical temperature change ΔT without performing new FEM calculations. The usefulness of the expression is confirmed through the adhesive strength of Aluminum/Epoxy butt joint experimentally obtained. Once the critical SIF K1C can be obtained from the tensile strength σc and the temperature change ΔT, the adhesive strength can be expressed as K1C = constant of an assumed fictitious interface, and this can be used to predict critical σc for various temperature change ΔT and for various adhesive bondline thickness h.

Adhesive strength improvement by providing steps in joints and differentiating initial and final debonding stresses

Step adhesive joints have a special characteristic quite different from other joints. When initial delamination occurs in other joints such as butt, lap and scarf joints, final failure always occurs. In these cases, the external stress causing initial delamination σcInitial is equal to the final failure stress as σcFinal = σcInitial. However, in step joints, the final failure stress σcFinal can be greater than the initial delamination stress σcInitial < σcFinal. To clarify the adhesive improvement mechanism, first, this paper discusses the ISSF (Intensity of Singular Stress Fields) for fully bonded step joint by varying the number of steps NS. Second, the singular stress field causing 2nd debonding is discussed by analyzing partially delaminated step joint. The results show that 2nd debonding requires larger external load than the initial debonding as σcInitial < σcFinal. This is because under the same external load the singular stress causing the 2nd deboning is smaller than the one causing the initial debonding. When NS≥6 and suitable overlap length, the final bond strength σcFinal can be more than 3.6 ∼ 4.4 times larger than the initial delamination stress σcInitial≪σcFinal resulting in much larger bond strength.

Cohesive failure verification and fracture origin identification when the adhesive strength is expressed as a constant ISSF

The aim of this paper is to explain that the ISSF evaluation method can apply when the fracture is cohesive as well as an interface fracture. The cohesive fracture usually occurs very close to the adhesive joint's interface controlled by the intensity of singular stress field (ISSF). This is the reason why the adhesive strength can be evaluated as a constant value of ISSF. In this study, the fracture origin is confirmed at the interface end to verify the ISSF evaluation method. Next, the cohesive fracture near the interface is confirmed since the slight amount of adhesive remains near the polished streaks of the adherend steel. Finally, it is found that the cohesive fracture very close to the interface guarantees the appropriate adhesive strength which can be expressed as a constant value of the ISSF.

Intensity of singular stress field for bonded body with dissimilar adherends

In this study, the intensities of the singular stress field (ISSFs) at the interface corner and interface edge in 3D BJ models with similar/dissimilar adherends are calculated by changing the adhesive layer thicknesses by the proportional method. The differences between the ISSFs for the 3D BJ models with similar/dissimilar adherends are discussed. The ISSF for the BJ with dissimilar adherends is larger than that for the BJ with similar adherends. The differences between the ISSFs for the 3D BJ models with similar/dissimilar adherends increase as the adhesive layer thickness decreases. When the adhesive layer thickness is 0.05 mm, the ISSF at the interface corner in the BJ with dissimilar adherends is about 1.7 as large as that for the BJ with similar adherends. Then, the remarkable differences between the singular stresses at the interface corner and along the interface edge in the BJ with dissimilar adherends are found.

A convenient evaluation of intensity of singular stress field at the interface corner in single lap joint with fillet

A convenient evaluation of intensity of singular stress field at the interface corner in single lap joint with fillet

Meshless analysis of the stress singularity in composite adhesive joints

Adhesives are an exceptionally well-suited method for joining composites. Unlike other methods, such as bolting or riveting, adhesives do not introduce holes in their joining material. This is a significant advantage in the case of composites because the holes required by bolting or riveting induce stress concentrations and can also lead to tears, burrs or delamination. A point of concern in adhesive joints is the adhesive/adherend interface corner where a stress singularity occurs, and failure usually initiates. Thus, it is crucial to study this stress singularity to better understand adhesive joints’ mechanical behaviour.The goal of this work is to validate the application of the Intensity of Singular Stress Fields (ISSF) criterion to meshless methods, in this case, the Radial Point Interpolation Method (RPIM). With this purpose, eight overlap lengths (LO) in single-lap joints (SLJ) composed of Carbon Fibre Reinforced Polymer (CFRP) and bonded with a brittle adhesive were experimentally and numerically tested. Furthermore, an extrapolation based method is implemented to determine the critical stress singularity components (Hc) necessary for the strength predictions. In the end, the experimental and numerical results are compared to assess the suitability of the method. It was found that the ISSF criterion can be accurately applied to meshless methods and composite materials successfully, given the simplicity of the method applied.

Analysis of Intensity of Singular Stress Field at the Stepped-Lap Joint

Analysis of Intensity of Singular Stress Field at the Stepped-Lap Joint

Fracture mechanics approach to stress singularities in composite adhesive joints

Structural design has significantly changed over the years driven by a weight reduction goal. In that sense, composite materials established themselves as the material of excellence in most engineering areas, replacing wood, steel and aluminium. Connection processes also experienced a transformation, with adhesive bonding standing out. Those new materials and techniques require deep research until they could be applied to structures. These studies led to the appearance of different methods for evaluating material and bond performance. Fracture mechanics is an approach based on material discontinuities or defects. Recently, a new fracture mechanics based technique arose called Intensity of Singular Stress Fields (ISSF). It hinges on the Stress Intensity Factors (SIF) approach but does not require an initial crack. This investigation aims to evaluate the applicability of this technique to composite materials. For that, Single-Lap Joints (SLJ) made from Carbon Fibre Reinforced Polymer (CFRP) bonded with a brittle adhesive and eight different overlap lengths (LO) are analysed. The numerical simulations and strength predictions are performed through the Finite Element Method (FEM) and MATLAB software. Finally, the numerical predictions are compared to the experimental data. It can be concluded that the ISSF is applicable to orthotropic materials.

Proportional method for evaluating intensity of singular stress field for butt joint with adhesive spew

In this paper, a proportional method for evaluating an intensity of singular stress field (ISSF) at an interface corner in a butt joint (BJ) with an adhesive spew is proposed. Square-shaped inclusion models under bi-axial tensile stress and under shear stress are used as reference models. The ISSFs for the BJ are calculated by superposing those for the reference models. The reference and BJ models are analyzed by FEM under the same mesh division and the same material combination, and the stresses applied to the reference models are determined by the FEM stresses at the interface corner. The validity of the present method is examined by applying to the BJ composed of the steel and the epoxy resin. The asymptotic solution obtained by the method is in agreement with the FEM stress distributions. Then, the ISSFs for the BJ are calculated by changing adhesive spew similarly. It is shown that the ISSF normalized by the adhesive thickness is constant independent of the thickness.

Reference solution and proportional method to calculate intensity of singular stress field (ISSF) at the interface corner where reinforced fiber enters resin matrix

To evaluate fiber/matrix bonding properties, the average shear stress is generally used as the interface strength without considering the intensity of singular stress field (ISSF) at the interface ends. In this paper, therefore, how to calculate the ISSF is described in detail at the fiber entry point of the single-fiber under pull-out test. Recent studies showed the ISSF method is quite useful for controlling the debonding strength. As a reference solution, the ISSFs at the fiber entry point are tabulated and illustrated under arbitrary material combination for the specific geometry. By applying the proportional method with this reference solution, the ISSF can be calculated for other geometries conveniently. As an example, the ISSFs of Glass fiber and Carbon fibers are shown to be obtained from the reference solution within 1 percent error. Utilizing those results, the difference between the pull-out test and the micro-bond test is discussed by varying the reinforced fiber properties.

Fractographic identification of fracture origin mainly controlled by the intensity of singular stress field (ISSF) in prismatic butt joint with corner fillet

In this study, the fracture origin is identified for the prismatic butt joints whose debonding condition can be expressed as a constant value of the ISSF. The ISSF variation is newly analyzed along the interface side by considering the real specimen geometry with chamfer at the corner. The detail fractographic observation shows that most of the fracture starts from the maximum ISSF region at the interface side instead of the interface corner. When the bondline thickness h is larger, the fracture origin can be seen at sub-surface because the stress triaxiality decreases inside of the specimen. The fracture origin under thermal loading can be estimated similarly because of the coincidence the ISSF variations.

スカーフ継手における特異応力の強さ(ISSF)を用いた接着強度の評価法

スカーフ継手における特異応力の強さ(ISSF)を用いた接着強度の評価法

Adhesive strength evaluation of stepped-lap joint based on the intensity of singular stress field (ISSF)

Adhesive strength evaluation of stepped-lap joint based on the intensity of singular stress field (ISSF)

Adhesive strength evaluation of scarf joint based on the intensity of singular stress field (ISSF)

Adhesive strength evaluation of scarf joint based on the intensity of singular stress field (ISSF)

Analysis of Intensity of Singular Stress Field for Single Lap Joint with fillet

Analysis of Intensity of Singular Stress Field for Single Lap Joint with fillet

Fracture Origin of Prismatic Butt Joints in Relation to the Intensity of Singular Stress Field (ISSF)

接着接合は重量・価格・生産性の面で優れており，その接着強度は，界面端部に生じるISSF＝一定によって表現される。しかし，実際の接着構造は3次元形状を有するため，はく離が生じる起点を正確に把握し，その強度を適切に予測することが製品の信頼性を確保する上で重要である。本研究では，角柱状突き合わせ継手試験片において接着界面縁のどの部分から破壊が生じるのか実験的に明らかにし，その破壊起点が滑らかな界面縁のISSFの分布のどこで生じるのか，正確な解析をもとに考察した。接着界面コーナー部の周辺ではISSFの極大値が局所的に生じるため，破壊起点となりにくく，ほとんどの破壊起点はコーナー部以外の界面縁で生じる。

Variation of intensity of singular stress field (ISSF) along the interface outer edge of prismatic butt joint and debonding condition expressed by the ISSF

In our previous study, the adhesive strength of butt joints having three-dimensional (3D) geometries was investigated by using the intensity of singular stress field (ISSF) in two-dimensional modelling. In this paper, by considering the 3D geometry, the ISSF variation along the butt-joint interface side is discussed to explain the experimental results. The results show that the critical ISSF distributions when debonding occur are almost the same and independent of the adhesive bondline thickness. The validity of the 2D modelling is investigated experimentally for two kinds of brittle and ductile adhesives considering the location of the maximum ISSF. It is found that the adhesive strength can be expressed as a constant value of the ISSF at the center side and also at the corner of the adhesive interface.

Intensity of singular stress fields (ISSFs) in micro-bond test in comparison with ISSFs in pull-out test

Micro-bond test is often used to investigate fiber/matrix bonding behavior. In this experiment, the average shear stress is generally used as the interface strength without considering the singular stress. Therefore, in this paper, the intensity of singular stress field (ISSF) is newly analyzed at the fiber entry/exit points. The obtained ISSFs at the fiber entry point in micro-bond test are compared to the single fiber pull-out under the same fiber geometry. The results show that care should be taken for the previous micro-bond test geometry since the ISSF varies sensitively depending on the testing geometry. To control the initial fiber/matrix debonding and evaluate the bonding behavior correctly, suitable testing geometries are proposed in micro-bond testing.

Adhesive strength evaluation for three-dimensional butt joint in terms of the intensity of singular stress field on the interface outer edge

Our previous studies analyzed the adhesive strength of butt joint systematically in terms of the intensity of singular stress field (ISSF) by using two-dimensional modeling. In consideration of the actual specimen geometry, in this paper, the adhesive strength of butt joint is analyzed by using three-dimensional modeling. A mesh-independent numerical method combined with the finite element method (FEM) is used to calculate the ISSF. The ISSF distribution on the interface outer edge is analyzed, the effect of adhesive thickness is investigated. The ISSF at the interface vertex is analyzed by using a small fillet instead. The difference between 2D and 3D models for adhesive strength evaluation is discussed. The usability of 2D modeling is investigated.

Reduction of intensity of singular stress field at interface end in lap joint

Reduction of intensity of singular stress field at interface end in lap joint