Michael Stoschka

Effect of High-Strength Filler Metals on Fatigue

Fatigue life of welded joints is in general independent from the material strength. High-strength materials are only beneficial in the low-cycle-fatigue region due to their increased yield limit. This property leads to their application, for instance, in welded mobile crane structures. The high-cycle fatigue limit, however, depends mostly on the geometry and the metallurgy of the notch. Therefore, an optimized weld process is required to achieve a certain fatigue strength. This paper contributes to the obtainable fatigue limits for thin-walled, high-strength joints regarding an optimization of the gas metal arc weld process for fillet welds without additional post-treatment. A methodology is designed to manufacture welded specimens with minimized production scatter. The specimens were carefully analysed by metallographic studies, hardness, distortion and geometric weld toe measurements. The detailed analysis enables a profound link between experimental fatigue life and weld process settings. For the assessment of the fatigue life of thin-walled specimens, the nominal stress approach and the notch stress method are used. The thin wall thickness is considered in the nominal approach by a thickness correction factor. The experimental results showed that the highest fatigue strength for the specific specimen design in ‘as-welded’ condition can be obtained when using a high-strength metal-cored wire filler in combination with a three-component mixed gas.

Effect of high-strength filler metals on the fatigue behaviour of butt joints

Welded structures made of high-strength steel offer benefits in fatigue strength for finite life applications. The high-cycle fatigue limit, however, depends mostly on the geometry, and the metallurgy of the notch is of little account. Therefore, an optimised weld process is required to achieve an improvement in the fatigue strength. This paper contributes to the field of fatigue behaviour of thin-walled, high-strength steel butt joints, with regard to an optimisation of the gas metal arc weld process. An existing methodology was extended to manufacture welded specimens with minimised production scatter. The majority of the butt joint samples were dynamically tested, with the root surface ground flush to plate, to study the effect of the weld process on fatigue. The investigated specimen were carefully analysed by metallographic studies along with hardness, distortion, and weld toe topography measurements. This facilitated in finding a relation between experimental fatigue life and the weld parameters. The nominal stress approach, including a benign, nonconservative thinness correction, and the recommended notch stress concept, were applied to assess the fatigue behaviour of the thin-walled, high-strength steel butt joints. The experimental results showed that in case of high-quality welds with negligable geometric notch factor, a small, but distinct influence of the filler metal on fatigue is observable. The highest fatigue strength for the investigated butt joint design was obtained with a high-strength metal-cored wire filler in combination with a three-component shielding gas.

Numerical fatigue assessment of welded and HFMI-treated joints by notch stress/strain and fracture mechanical approaches

Abstract Post-treatment methods enhance the fatigue strength of welded high-strength steel joints significantly. In industrial applications, one commonly applied technique is the high frequency mechanical impact (HFMI) treatment. Attained local benefits increasing fatigue strength are the compressive residual stress state, the reduced notch effect at the weld toe, and additionally the local work hardening of the material. This paper presents the set-up of a closed simulation loop including structural weld simulation, numerical computation of the HFMI-process, and a numerical evaluation of the local fatigue life. A thermo-mechanical coupled weld process simulation using Sysweld is built-up to assess the course of residual stress and strain due to welding. The resulting local material behavior is transferred as mechanical cards to the solver Abaqus for the subsequent numerical simulation of the HFMI-process. Hence, major influences such as transient change of material parameters during welding and subsequent cool-down, effect of process dependent clamping conditions and changing contact characteristics are considered. Finally, a numerical evaluation of the local fatigue behavior by the local stress/strain approach and by crack propagation is performed by the aid of the software packages Femfat, nCode and Franc2D. A comparison of the simulated fatigue life with experimental test results proofs their basic applicability; but also numerical limitations of the presented simulation tools are determined. The major benefit of the established simulation chain is the opportunity to study different weld process and HFMI-treatment parameters in regard to fatigue strength without the need of comparably expensive fatigue tests.

Effect of post-weld heat treatment on the fatigue strength of HFMI-treated mild steel joints

Welding as a thermo-mechanical joining process generally induces residual stresses and distortion in welded components or structures. Mechanical post-treatment methods as the high-frequency mechanical impact treatment (HFMI) reduces the geometrical notch and introduces compressive stresses in the locally treated weld toe area, whereas post-weld heat treatment (PWHT) globally affects the whole structure. In this paper, the fatigue strength of HFMI-treated transverse non-load-carrying attachments and cruciform joints made of structural mild steel S355 before and after PWHT is investigated. Comprehensive tumescent fatigue tests and evaluation of notch topography, residual stress and distortion show the influence of the investigated post-treatment methods. To analyse the effect of distortion on the resulting stress condition during the fatigue tests, simulations and strain gauge measurements are carried out for different load cases. Finally, a local fatigue assessment based on the effective notch stress approach shows that an additional PWHT is not beneficial for fatigue strength. As an increase in distortion of the samples, and an influence on the base material properties, caused by the heat-treatment is not observable, the decrease in fatigue is mainly caused by the entire relieve of manufacturing induced (as-welded/HFMI-treated) prior compressive residual stresses to an almost zero stress value.

A Novel Two-Disc Machine for High Precision Friction Assessment

The concept of two-disc model testing has proven to deliver valuable information for the applicability of new technologies, such as surface structuring, coatings, alternative fluids, or advanced materials, in actual machine elements. In this article an advanced two-disc machine with sophisticated control technology for dry and lubricated setup is presented. (i) All involved components are controlled via a powerful PLC unit leading to the possibility of realising extremely accurate SRRs down to . (ii) High-speed data acquisition allows local insight into tribological phenomena by providing 72 data points along one shaft rotation. (iii) Several lubrication scenarios such as fluid, mixed, and starved lubrication, as well as dry contacts, can be considered. (iv) Raw-data of all sensors, including normal force, friction force, vibration speed, stiffness (wear), infrared temperature, contact potential, and motor speed, is presented. Some example results of dry polymer-steel tribosystems and lubricated experiments are shown to elucidate the capabilities of the novel test rig.

Evaluation of fillet weld properties and fatigue behaviour in dependence of welding parameters

Numerous different design codes can be used to describe the durability of welded structures. One wide spread approach is the local notch stress approach, which calculates the fatigue lifetime in dependency of the notch stress factor using different effective radii. To calculate the fatigue behaviour by using the local notch stress approach, the RIMS-concept is commonly used [1] . The evaluation of the influence due to the welding process parameters, especially for high-strength steels, the effect of both the geometrical and metallurgical notch is studied in a parametric way for selected weld joints. Experimental fatigue tests have been performed to investigate the link between fatigue life and manufacturing process dependent weld toe notch design. To be able to capture the influence of welding parameters, as energy input per unit length, welding velocity, angle of blowpipe, size and shape of the heat input zone in a numerical way, a local coupled thermo-mechanical simulation is build-up. The complexity of this modelling increases very strong by the temperature dependency of the multitudinous phase material properties. The material and manufacturing properties were adjusted by comparison of the temperature profiles. This experimental based procedure defines the simulation base for more complex welding seams.