Timo Björk

The Need for a Weld Quality System for Fatigue Loaded Structures

Historically, production technology researchers and structural design researchers have had only limited dialogue and each group has focused on their own narrow field of interest. However, the drive to increase welding speed and the desire to increase the use of higher strength steel for fatigue-loaded structures have led to the development of an integrated research approach that includes joint efforts in several key technologies: high-speed welding processes, high strength materials, cost-effective NDE, post-weld treatments and FE-based design assessment tools. The current paper focuses on the need to develop a suitable classification system for high quality welds. Current weld quality systems for welds were developed primarily with static loading in mind and are not directly suitable for fatigue loaded components. The need for a multi-technical approach and implementation of a design for purpose philosophy are emphasised.

Capacity of Fillet Welded Joints Made of Ultra High-Strength Steel

The ultimate load-bearing capacity of typical fillet welded joints made of ultra high-strength steel (UHSS) S960 has been investigated. The aim of the work has been to assess the validity of current design rules for UHSS and possibly define new design criteria. Experimental testing and nonlinear finite element analysis (FEA) were applied to define the capacity of fillet welded joints. Joint geometries and material properties were measured for both filler and base materials. In comparison with current design rules, the experimental results showed that the fillet welded joints had adequate load-carrying capacity presuming that adequate weld quality is achieved in the joint fabrication. Load-carrying capacities and rupture modes in welds defined by FEA were in agreement with experimental results. The experimental deformation capacities of some of the joints were found to be critical, but the capacities can be improved by the use of undermatched filler metals. Heat input control is essential in the fabrication of welded connections made of UHSS and thus an additional failure criteria should be considered in design codes due to the softening effect in HAZ.

Effects of weaving technique on the fatigue strength of transverse loaded fillet welds made of ultra-high-strength steel

The effect of weaved weld toe geometry on the fatigue strength of S960 steel grade fillet welds has been studied by means of experimental testing and finite element analysis. The comparisons have been made between normal straight weld toe lines and weaved weld toe lines by using the weld toe radius and structural stress fatigue class (FAT) value as reference values. The effective notch stress approach was utilized to conduct finite element (FE) analyses on weaved weld toe geometry, and the results were compared with the experimental results. The FE analysis showed quite good agreement with the experimental test results, although residual stresses or heat-affected zone (HAZ) softening in welded joint were not included in the modeling. The experimental tests also indicated that it is possible to achieve high fatigue properties for ultra-high-strength steel (UHSS) fillet welded joints without any postweld treatments. By taking account all the essential factors—joint geometry, welding metallurgy, and residual stresses—the reached fatigue strength of UHSS weldments can be much higher than normal fatigue resistance values presented in standards and recommendations.

Development of Data Sheets for Statistical Evaluation of Fatigue Data

Instructions were developed for statistical analysis of the laboratory fatigue data step-by-step. Practical cases for solving several questions raised in the treatment of test data, which involved estimation of necessary sample size, verification of the statistical equivalence of the collated sets of data, and determination of characteristic curves in different cases, were given by using the methods and formulae in the document IIW-XIII-2138-06 (best practice guide on the statistical analysis of fatigue data) as a demonstration of various statistical methods of developing a sound procedure for creating reliable calculation rules for the fatigue analysis.

Rotation capacity of fillet weld joints made of high-strength steel

The capacity of the welded joint can be defined in terms of load-carrying and deformation capacities. For ultra-high-strength structural steels, such as S960 QC, the deformation capacity is a crucial structural property. In this paper, the rotation capacity of fillet weld joints was investigated experimentally and analytically. The rotation capacity of the fillet weld joints under pure bending load is calculated analytically by a model using the elastic peak stress and plastic limit state. The experimental test proved that the plastic load-carrying capacities required by the standards were reached. Additionally, the tests showed that sufficient deformation capacities can be obtained, although fully plastic rotation hinges could not always be reached. The experimental results were in good agreement with the analytically calculated capacities. However, there is a need for some design limitations before more results are available.

Experimental fatigue characterization and elasto-plastic finite element analysis of notched specimens made of direct-quenched ultra-high-strength steel:

The low-cycle fatigue behavior of a direct-quenched ultra-high-strength steel was experimentally characterized and numerically modeled. Fatigue and cyclic parameters were obtained by conducting strain-controlled low-cycle fatigue tests on uniform-gage specimens. Surface residual stresses were minimized and axial deflection eliminated by optimization of machining parameters and post-machining electro-polishing. The steel material showed cyclic softening and decrease in yield strength. Cyclic softening, stabilized response, and the cyclic stress–strain curve were numerically simulated using finite element analysis with a model capable of describing nonlinear kinematic-isotropic hardening. The results showed good agreement with experimental values and validated the model’s ability to simulate the softening and cyclic stabilization of the material under investigation. The same numerical method was then used in elasto-plastic stress–strain analysis of notched specimens made of the same material to make fatigue life predictions. Estimated lives were compared with predictions made by analytical approximations such as the linear rule, Neuber’s rule, and the strain energy density method and verified by experimental data. Finite element analysis using stabilized cyclic response yields the most accurate predictions and, thus, provides an effective tool for the fatigue analysis of this material.

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

Abstract 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. Pariss 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.

A novel method for fatigue assessment of steel plates with thermally cut edges

The fatigue strength of thermally cut edges shares many similar features with the fatigue strength of a welded joint. Current design standards are valid for geometrically simple, straight, flame-cut edges made of mild steel. However, the applicability of current recommendations is questionable if the geometry is more complicated, if a different (thermal) cutting process is employed, or if any high-strength steel is used. In this study, conventional fatigue analysis methods are applied in order to explain the experimental results obtained by conducting fatigue tests on a cut component made of different steel grades. Since those methods did not explain the experimental results properly, a novel fatigue analysis method for plates with a thermally cut edge was created. This method is a parallel approach to that recently developed at Lappeenranta University of Technology for analyzing fatigue of a welded joint. In addition to the general stress range, this method also takes into account the structural and local geometry of the cut detail, surface roughness, residual stresses due to the cutting process, the external stress ratio, and the effect of the steel grade. The method agreed well with the test results and can be recommended as a new general approach for analyzing the fatigue of cut edges.

On the critical plane of axially loaded plate structures made of ultra-high strength steel

A number of different theoretical approaches are commonly used to define the critical plane of axially loaded plate structures. The critical plane is the cross-section where yielding and failure occur in quasi-static axial loading. Conventional approaches such as maximum shear stress or octahedral shear stress yield criterion seem to work well in terms of load carrying capacity but they do not correctly predict the failure path in practice. A new approach based on removal of the stress parallel to the critical plane is presented in this work. This method was applied to define the effect of the incline of the butt weld on the axial loading capacity of the plate structure. The base material in this study is ultra-high strength steel, where softening in the weld has specific effects on the behaviour and capacity of the joint. The presented theoretical approach showed good agreement with experimental results and FE-calculations. The inclined angle of the weld should be 60 degrees in order to obtain any benefits as regards the capacity of the structure. In addition to butt welds, the results can be generalized to other types of joints, which increases the importance of knowledge of the critical plane considerably.

The Influence of Residual Stresses on the Fatigue Strength of Cold-Formed Structural Tubes

Rectangular hollow sections (RHS) are widely used in load-carrying structures due to their good load transfer behavior and aesthetic form. During fabrication of a cold-formed rectangular hollow section (CFRHS), the concave inside corner surfaces experience significant compressive plastic strains. The resulting tensile residual stresses in the corner region significantly reduce the fatigue strength in some design applications. Sections cut from CFRHSs with two different wall thicknesses were studied using distortion fatigue loading and using X-ray diffraction residual stress measurements. Residual stresses and fatigue strengths were virtually identical for the two thicknesses even though the corner radius to thickness ratio was different. Residual stresses were found to significantly influence those corners subjected to compression-compression loading. Fatigue strength of these corners was improved by a stress relief heat treatment, but the treatment had no significant effect on corners subjected to tension-tension loading.