Fatigue Strength Of Lap Joints Research Articles

Implicit gradient approach for numerical analysis of laser welded joints

This paper analyzes the fatigue strength of laser welded steel joints by applying the implied gradient method. This method is adopted using the same procedure proposed for studying arc welded joints. The fatigue scatter band of laser joints, obtained from numerical analysis of experimental data taken from the literature, is different from the relative curve of arc welded joints. However, at high cycles fatigue the two bands maintain exactly the same average values and the same scatter. The fatigue strength of lap joints with different weld patterns confirms the use of the proposed generalized fatigue scatter band for laser joints.

A model for fatigue strength of welded lap joints

ABSTRACTThe most advanced structural frames of cars and motorcycles are made from welded box beams of aluminium alloys. A recent technological solution for the beams consists of press‐formed metal sheets joined by welded single‐lap joints. Because the frames undergo cyclic loading, the evaluation of the fatigue endurance of this kind of connections plays a crucial role in the development of the frames. The present research deals with the evaluation of the fatigue behaviour of welded lap joints, subjected to tensile and bending loads. Fractographic evidence and numerical analyses show that the fatigue strength is controlled by the presence of defects and inhomogeneities at the tip of the weld root. In particular, these defects, which appear to lie on the maximum Kθθ path, increase the stress intensity factor (SIF) at the weld root. Such an analysis, together with a statistical description of the population defects, allows us to develop a methodology to predict the fatigue strength of lap joints. A simple method for SIF computation based on finite‐element (FE) analyses and structural stresses at the lap joints is then presented.

Finite element analysis of the effect of weld geometry and load condition on fatigue strength of lap joint

Many welded lap joints are subject to fluctuating loads, and fatigue failure plays a dominant role in the failure of such structures. Based on the concepts of linear elastic fracture mechanics, the effects of weld geometry, load conditions and the boundary constraints on fatigue strength of a ferrite–pearlite steel lap joint were investigated in this paper using the finite element method. Paris's power law was used to predict the fatigue life of the joints. Various weld geometry including the leg length, flank angle and the size of lack-of-penetration were considered during the calculation of fatigue strengths. The loads include tension, bending and their combinations. It was found that the existence of a root crack has no influence on the fatigue strength of the joint, under the relevant load conditions. The existence of a toe crack is also of no influence on the fatigue strength of the joint if the applied loads, e.g. DOB>0 in this paper, produce a compressive stress field at the top region of the main plate. For a lap joint with a free transverse ( Y direction in this paper) boundary constraint at the main plate, a joint with a smaller size of lack-of-penetration, a reasonably large weld leg and smaller flank angle is recommended to be used in engineering practice, in order to obtain a higher fatigue strength. For a lap joint, with transverse fixed boundary constraint at the main plate, the fatigue strength increases with a decrease of weld size but the influence of flank angle depends on the type of load carried. It was also found that the size reduction in FE model is a significant influence on the calculated fatigue strength; the use of reduced size FE model gives a much higher overestimate of fatigue strength of the joint.

レーザ溶接された重ね継手の疲労強度について

In recent years YAG laser is to be applied to welding of car body as light weight construction used thin plates. Lap joint is used generally on welding two thin plates as compared with butt joint. Because groove precision is necessary for butt joint. So, fillet weld has been adopted to lap joint by MAG welding that is conventional welding way. Narrow bead width and deep penetration shape are characteristics of laser welding. But, these characteristics are weak points in case of fillet weld by laser welding, because minute positioning becomes necessary. When it applies laser welding to lap joint, deep penetration shape that is a characteristic of laser welding is able to weld lapped two plates and it does not need positioning precision. In this studies, it picks up fatigue strength of lap joint of thin plates and it examined a rational joining measurement of laser welded joint by an experiment and numerical analysis. As a result, it got some knowledge about joint width of lap joint and it shows a major conclusion to the following. The radius of curvature on weld toe of YAG laser fillet welded joint is larger than that of MAG fillet welded joint and stress concentration on weld toe of YAG laser fillet welded joint is relaxed more than MAG fillet welded joint. So, fatigue strength of lap joint by YAG laser fillet weld is larger than that by MAG fillet weld in case of almost same leg length. Using a mode I stress intensity factor of nonpropergating crack, it was able to predict fatigue strength of lap joint, fillet welded joint by YAG laser and fillet welded joint by MAG in an error of approximately 23%. If it makes joint width of lap joint larger than plate thickness, a stress intensity factor becomes almost constant value. Accordingly, it needs to make joint width larger than plate thickness. Slit length of lap joint hardly cause influence to fatigue strength.

217 レーザ溶接された重ね継手の接合寸法と疲労強度(O.S.5 : 溶接・接合材の強度および力学的特性の評価)

217 レーザ溶接された重ね継手の接合寸法と疲労強度(O.S.5 : 溶接・接合材の強度および力学的特性の評価)

フィレットを持つ重ね合せ接着継手の応力分布と疲労強度

In adhesive bonded lap joints, stress in the adhesive layer is concentrated at the lap end because of difference in elastic moduli of constituents and abrupt thickness change. Hence, in order to improve the joint stength by reducing the stress concentration in the adhesive layer, it is practiced in industry to form it with a fillet of adhesive spew or to round the corners of the adherend. However, there are few studies on the fatigue strength charactristics of these improved adhesive joints.In order to investigate the relation between the fatigue stength of these joints and the local stress distribution near the lap end, a series of fatigue tests were conducted on the adhesive bonded lap joints with an adhesive fillet plus a rounded adherend, and the local stress near the adherend corner was analysed by FEM. Furthermore, the fatigue crack propagation behavior was observed with an optical microscope.As a result, It is confirmed that the fatigue strength of lap joints with different corner radii can be standardized by the maximum value of maximum shear stress near the lap end.

Fatigue Life Estimation of Adhesively Bonded Lap Joints

This paper describes a method of estimating the fatigue life of adhesively bonded lap joints on the basis of the stress analysis in adhesive layer with finite element method. First, cyclic tensile fatigue tests were conducted for adhesively bonded lap joints with different lap length and adhesive layer thickness. The results were evaluated from the viewpoint of the maximum values of both tensile and shear stress obtained numerically, instead of the apparent stress. Then these standardized fatigue strength were compared with those of adhesively bonded butt joints of a thin wall tube under cyclic tensile and fully reversed torsional load conditions. The results indicate that fatigue strength of lap joints evaluated from the maximum tensile stress of the adhesive layer agrees well with the fatigue strength of adhesively bonded butt joints of thin wall tube under cyclic tensile load condition. It is confirmed that fatigue strength of lap joints can be estimated adequately based on the fatigue strength of the butt joint of thin wall tube and the numerical results for the stress state of adhesive layer.