Martin Leitner

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.

Crack propagation analysis and rehabilitation by HFMI of pre-fatigued welded structures

This paper deals with a crack propagation analysis of welded structures and rehabilitation after pre-fatigue loading by the high-frequency mechanical impact (HFMI) post-treatment technique. The investigated specimen type is a thin-walled longitudinal stiffener made of mild steel S355. Fracture mechanical calculations are primarily performed on the basis of the weight-function approach. Thereby, the local residual stress condition at the weld toe is considered by the aid of a structural weld simulation, whereas the numerically evaluated residual stress distribution in depth agrees well to X-ray measurement results. The fracture mechanical analysis illustrates that by incorporating the residual stress state, the calculated lifetime is in good accordance to the conducted fatigue test results. By application of the HFMI-treatment as rehabilitation method, it is found that the beneficial post-treatment effect increases especially by a reduction of the applied load-level. Hence, particularly for minor nominal stress ranges near the high-cycle fatigue region, the mechanical post-treatment as repair method is utmost effective leading to almost equal fatigue strength as for the HFMI-treated specimens without pre-cycling. Finally, proposals for the crack growth assessment of welded structures and a conservative application of HFMI as rehabilitation method for mild steel joints are provided.

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.

Statistical analysis of micropore size distributions in Al–Si castings evaluated by X-ray computed tomography

Abstract In general, micropore size acts as one of the most significant influencing factors on the fatigue strength of aluminium castings. Hence, an in-depth knowledge of the occurrence of micropore sizes and their local distributions in different locations in complexly-shaped lightweight components is of great interest to the casting industry. In this work, the local properties of AlSi8Cu3 and AlSi7Cu0.5Mg cylinder heads and AlSi8Cu3 crankcases were analyzed. Extensive X-ray computed tomography (CT) scans of three specimen positions revealed significant differences in micropore size and distribution. Two CT scan resolutions were selected, with respect to different micropore size populations in the cast components, to enable accurate detection of the microporosity, in addition to an adequate scanning volume, in order to achieve a statistically approved parameter study. Thereby, specimen positions exhibiting smaller mean micropore sizes were scanned at 3 μm/voxel scanning resolution and ones with larger micropore sizes at 8 μm/voxel. A statistical assessment of all of the alloy specifications and specimen positions indicates that the general extreme value and lognormal distribution appropriately describe the micropore size distributions. Finally, an extensive sensitivity study is presented, aimed at examining micropore size characteristics, such as the porosity, sphericity, maximum and mean values and standard deviation, and to investigate their relationships in the investigated cast specimens.

Ermüdungsfestigkeit hochfester Stahlschweißverbindungen

ZusammenfassungDurch die ständig wachsenden Forderungen nach einer Reduzierung des Gewichts und Steigerung der Lebensdauer bei geschweißten Konstruktionen ist eine Verbesserung der Ermüdungsfestigkeit von Schweißverbindungen unerlässlich. Experimentelle Untersuchungen und begleitende numerische Simulationen schaffen die Basis, um die von Prüfkörpern abgeleiteten Ergebnisse auf komplexe Bauteile übertragen zu können. Hierbei werden Einflüsse wie Schweißprozessparameter, Stützwirkung, prozessbedingte Fehlstellen und Nachbehandlungsverfahren auf die lokalen Charakteristika in geschweißten hochfesten Verbindungen untersucht, wodurch eine Optimierung unter Leichtbauaspekten möglich ist.AbstractDue to the permanently increasing requirements to reduce weight and enhance the lifetime of welded structures, an improvement of the fatigue strength of welded joints is indispensable. Experimental investigations and accompanying numerical simulations provide the basis to enable the transfer of small scale specimen results to complex components. Thereby, influences like weld process parameters, fatigue support effect, process depended defects, and post-treatment techniques are analyzed with regard to local properties for high-strength steel joints, whereby an optimization based on light-weight design is facilitated.

Evaluation of wood material models for the numerical assessment of cutting forces in chipping processes

Generally, wood chipping represents an important procedure in the wood processing and forestry industry. To improve structural components like chipping tools, knowledge of the properties of local timber including resistance against chipping as well as the dynamically acting process forces is of utmost significance. The aim of this work is to experimentally evaluate service-induced stresses on machinery parts to create a numerical material model, which is capable of revealing similar resistance against cutting as natural wood. To this end, a small-scale cutting machine has been designed, incorporating a bladeholder with strain gauges applied, measuring the resulting mechanical stresses during the chipping process by focussing on different wood species. Spruce is utilized as a variety with a lower density and European beech for higher density timber applications. The test results demonstrate a distinct difference by cutting both materials, whereby European beech indicates more than twice the resistance against chipping compared to spruce. Setting two different, relatively acute rake angles on the cutting tool does not reveal a fundamental difference for chipping. To evaluate the numerical wood material model, an isotropic ductile damage model, usually applied to ductile metals, was implemented in this study. Based on a sensitivity study of the material properties in the course of the numerical simulation, a possible approach is presented that explains how to change the cutting resistance, depending on the blade movement direction and the angle of the main grain of the timber. In a comparison of different types of mechanical stress from the numerical analysis and experimental tests, the results exhibit strong correlation. Element damage and deletion correspond at similar load levels, exhibiting a deviation of no more than 24%.

Evaluation of wood cutting forces in dry and wet conditions by small-scale chipping tests applying different analysis methods

ABSTRACT The aim of this paper is to study the effect of high moisture content in comparison to dry timber on the resulting cutting forces based on experimental small-scale chipping tests. Therefore, a wood chipper for single cuts is designed and different species of Austrian locally growing trees are utilized. The test specimens are investigated in almost dry and soaked wet conditions. The resistance of wood is measured utilizing a force sensor and the signal during the cutting process is subsequently analysed by two different methods. The results reveal that the mean value of the acting force during cutting is 38–81% minor compared to the maximum force. Even though the peak of the dynamically acting load is measured for just a comparably small time range, it reveals an impact on the fatigue behaviour of the tool as well as the tool supporting material. Hence, an approach of evaluated load spectra is applied to include the load distribution of the chipping process. The effect of dry and wet wood on the cutting resistance is examined, whereby wood exhibiting a high moisture content of 30–40% changes the acting load up to 98%, depending on the method of analysis.