Material Fatigue

© Photo Fraunhofer IWM

Fatigue Fracture

Shot Peening Simulations

Welding Simulations

Load-bearing components and structures in all industrial sectors such as automotive, vehicle, railway, aerospace, steel and bridge constructions are exposed to complex stresses. Many technical cases of damage can be attributed to material fatigue due to cyclic mechanical loading. In order to ensure the reliable use of critical components, the assessment of material fatigue is of decisive importance.

In order to fulfil the requirement for lightweight, resource-efficient and fatigue-resistant components, it is necessary to consider the properties of the component surface layer during the design phase of a construction. On the basis of our material and damage models for components and structures subjected to fatigue loading, we perform reliable service lifetime calculations so our customers are able to fully benefit from the potential of the strength reserves of their construction materials under extreme loads.

 

 

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Controlling material fatigue

Services

Welding Simulation, Residual Stresses, Component Distortion and Microstructure

Mechanical Surface Layer Strengthening

Increasing the Lifetime of Welded Joints by High Frequency Mechanical Impact

Publications

Controlling material fatigue

We:

determine the static and fatigue strength as well as the safety of components

support you in material selection and design optimisation for cyclic loaded components

investigate the lightweight potential of load-bearing components and structures thus avoiding over-dimensioning and thereby saving costs for material, production and transport

support you in the material substitution for existing and new constructions

help you to improve the endurance limit of metallic components by mechanical surface treatment and to avoid fatigue fractures

assist you to handle production-related material defects in a safe and reliable way

advise you with respect to continuation of component operation and the lifetime extension of metallic structures

support you with numerical welding simulation to determine the residual stress state and the distortion of your components in order to evaluate their lifetime

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Services

Numerical and experimental stress and strain analysis of components under static and cyclic loading

Numerical component assessment under static and cyclic loading experimentally supported by component tests

Numerical and experimental strength and safety analysis

Welding simulation and strength analysis of welded joints under consideration of residual stress and distortion

Numerical simulation of shot-peening, deep rolling and high frequency mechanical impact and resulting residual compressive stresses in the surface-near layer of metallic structural materials

Calculation of service lifetime of mechanical surface-treated (such as shot-peened, deep rolled and high frequency mechanical impact) components including the quantification of the lifetime improvement and the estimation of the cost-benefit ratio

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FEM calculation of residual stress in a welded pipe in comparison with the residual stress measurement by neutron diffraction method (ND)

Welding Simulation, Residual Stresses, Component Distortion and Microstructure

For the assessment of the fatigue strength of welded joints a realistic estimation of the damaging effect of welding residual stresses are of fundamental importance. A necessary element is the evaluation of the welding residual stress field and its possible degradation under mechanical loading. By means of validated numerical evidence the influence of residual stresses on the structural integrity may be quantified and the design of fatigue-resistant light-weight components facilitated.

For this purpose we develop suitable material models describing the material behaviour during the welding process and the resulting residual stress and distortion field precisely. If the residual stress field causes damage depends strongly on its behaviour during operation. A stable welding residual stress field can affect the material-related fatigue strength and the service lifetime of components and structures.

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Schematic representation of mechanical surface treatment methods

Generation of strengthening and compressive residual stress by shot-peening

Mechanical Surface Layer Strengthening

The surface layer of metallic components and structures is of decisive importance for their lifetime. In all industrial sectors, where components are exposed to high stress such as automotive, vehicle and railway construction, aerospace, steel and bridge construction, drive technology and gear manufacturing, combustion engines, steam and gas turbines, compressor and pump manufacturing, tool and mold making as well as medical engineering the material condition of the surface layer plays an elementary role. The reason for this is that damage processes such as fatigue and corrosion often initiate at the surface. Hence they can be retarded or accelerated depending on the stress state of the surface layer, which has a significant impact on the component lifetime.

A lot of mechanical surface treatment methods (Figure 1) have been developed during the recent decades. These methods use the effect of the elastic-plastic deformation of the surface layers of metallic components by use of suitable impact tools such as balls, hammers, rollers, waterjet and laser. This deformation results in strengthening and compressive residual stress in a thin surface layer. Such beneficial layers may serve the purpose to significantly increase the fatigue resistance of the entire component.

Therefore we develop material models and simulation tools which help to optimise the material behaviour and the component lifetime by use of a customized surface treatment. We derive correlations between material behavior, component geometry and process-related influencing factors of mechanical surface treatment methods on the residual stress field and the surface layer strengthening by combining experimental results with material and process simulation. Finally we determine the increase of lifetime of cyclic loaded components as a function of the process parameters.

Shot-peening

Deep rolling

High frequency mechanical impact

Increasing the Lifetime of Welded Joints by High Frequency Mechanical Impact

In general welds have significantly lower fatigue strength in comparison to the adjacent base material. This is mainly caused by the notch effect due of the weld seam geometry. Furthermore changing material and microstructure conditions in the weld zone (metallurgical notch effect) and tensile residual stress in regions which are susceptible to cracking reduce the fatigue strength of welded joints.

In recent years High Frequency Mechanical Impact or HFMI method has achieved great significance in practice. In this method a hardened cylindrical pin with a round tip strikes on a component with high speed respectively frequency >90 Hz, whereby the seam notch geometrically will be flattened, the surface will be strengthened and high compressive residual stresses will be induced. The effectiveness of the HFMI method to increase the service lifetime respectively the fatigue strength of welded joints as well as their applicability has been confirmed by numerous studies. By an effective post-treatment of welds not only constructive problems of new designs can be solved but also the use of high-strength steels will be facilitated.

Plastic deformation of the weld seam by means of High Frequency Mechanical Impact

FEM simulation of the generation of compressive residual stress in a weld toe by use of high frequency mechanical impact method

Numerical lifetime estimation of weld joints which were treated with high frequency mechanical impact method (red line) and der experimental verification (blue)

Publications

Hemmesi, K.; Farajian, M.; Siegele, D.; Numerical investigation of welding residual stress field and its behaviour under multiaxial loading in tubular joints; Advanced Materials Research 996 (2014) 788-793 Link

Foehrenbach, J.; Hardenacke, V.; Farajian, M.; High frequency mechanical impact treatment (HFMI) for the fatigue improvement: numerical and experimental investigations to describe the condition in the surface layer; Welding in the World 60/4 (2016) 749-755 Link

Farajian, M.; Nitschke-Pagel, T.; Siegele, D.; Welding Residual Stress behavior in Tubular Steel Joints under Multiaxial Loading; HTM Journal of Heat Treatment and Materials 69/1 (2014) 6-13 Link

Farajian, M.; Nitschke-Pagel, T.; Dilger, K., Relaxation of welding residual stresses – Part I: under quasi–static loading, International Journal of Microstructure and Materials Properties 7/1 (2012) 3-15

Farajian, M. ; Nitschke-Pagel, T.; Wimpory, R.C.; Hofmann, M.; Klaus, M.; Residual stress field measurements in welds by means of X-ray, synchrotron and neutron diffraction, Materials Science and Engineering Technology, 42/11(2011) 996-1001

Farajian, M.;  Wimpory, R.C.; Nitschke-Pagel, T.; Relaxation and stability of welding residual stresses in high strength steel under mechanical Loading, Steel Research International, 81/12 (2010) 1137-1143

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