Wolfgang Nierlich

Hydrogen Accelerated Classical Rolling Contact Fatigue and Evaluation of the Residual Stress Response

The accelerating effect of absorbed hydrogen on subsurface rolling contact fatigue is studied by bearing rig testing under HF electric current passage. The additional chemical loading enhances dislocation mobility and microplasticity. Structural changes occur in a premature stage, as shown by comparative XRD residual stress analysis. Dark etching regions appear to be a precursor for phase transformation in regular white etching bands. Fatigue crack initiation is demonstrated.

Service Loading Analysis of Wind Turbine Gearbox Rolling Bearings Based on X-Ray Diffraction Residual Stress Measurements

Rolling bearings in wind turbine gearboxes occasionally fail prematurely by so-called white etching cracks. The appearance of the damage indicates brittle spontaneous tensile stress induced surface cracking followed by corrosion fatigue driven crack growth. An X-ray diffraction based residual stress analysis reveals vibrations in service as the root cause. The occurrence of high local friction coefficients in the rolling contact is described by a tribological model. Depth profiles of the equivalent shear and normal stresses are compared with residual stress patterns and a relevant fracture strength, respectively. White etching crack failures are reproduced on a rolling contact fatigue test rig under increased mixed friction. Causative vibration loading is evident from residual stress measurements. Cold working compressive residual stresses are an effective countermeasure.

EVIDENCE AND ANALYSIS OF THERMAL STATIC STRAIN AGING IN THE DEFORMED SURFACE ZONE OF FINISH-MACHINED HARDENED STEEL

After heat treating, finish machining of the hardened steel represents the last manufacturing step of machine elements. The practically most important operation of grinding is applied to achieve edge zone compressive residual stresses, best surface quality and dimensional accuracy. Metal removal involves high plastic deformation work. Glide and intersection processes raise the density and produce lower energy substructures of dislocations. The temperature and time behavior of post-machining thermal treatment is analyzed on ground and honed martensitic SAE 52100 rolling bearing steel. Microstructure stabilization is reflected in a large XRD peak width decrease on the surface. The kinetics are modeled by rate-controlling carbide dissolution as the carbon source for Cottrell-type segregation at dislocations. This thermal static strain aging is verified by the formation of a slight white etching surface layer. The model is extended to also consider superimposed thermal dislocation recovery. Both effects are separable. In rolling contact fatigue tests under mixed friction conditions, air reheating below the tempering temperature, which avoids hardness loss, leads to a significant lifetime increase. The effect also occurs after cold working.

X-Ray Diffraction Residual Stress Analysis: One of the Few Advanced Physical Measuring Techniques That Have Established Themselves for Routine Application in Industry

The conventional procedure of X-ray diffraction (XRD) residual stress measurement is improved by means of a modification of the beam path of the diffractometer and an iterative technique that includes a pre-analysis of the nearpeak line-profile. The achieved short measuring times of 5 and around 10 min per residual stress value and retained austenite content, respectively, serve as precondition for routine industrial applications over the last three decades within SKF. The line width represents a measure of material ageing within the lifetime cycle of a rolling bearing: calibration curves for the (near-) surface and the sub-surface failure mode are presented. Material response analysis permits differentiation of these failure modes and between low- and high-cycle fatigue.

Finite Element Analysis of the Rolling-Sliding Contact of Vibrationally Loaded Bearings Based on a Micro Friction Model

Mixed friction acting in a rolling contact increases the v. Mises equivalent stress and shifts the maximum towards the surface. Tangential stresses are superimposed to the stress distribution. The resulting position of the maximum v. Mises stress depends on the magnitude of the friction coefficient and is located directly on the surface from values of about 0.25 upwards. The impact of three-dimensional machine vibrations on rolling bearings in operation can cause severe mixed friction running conditions. Residual stress distributions measured on indentation-free raceways indicate high friction coefficients of up to greater than 0.25. The surfaces reveal smoothing of the finishing structure but no adhesive wear. The simulation of the vibrationally loaded rolling-sliding contact is based on the tribological model of localized friction coefficient. This approach avoids seizing by allowing for increased friction only in intermittently changing subareas of the contact at low sliding speed. The macroscopic friction coefficient, meeting a mixing rule, does not exceed 0.1. The finite element method (FEM) is used for the stress analysis. In the first step, a simplified FEM model involves a circumferentially oriented band of high friction coefficient from 0.2 to 0.5 within a cylindrical roller contact. The resulting depth distributions of the v. Mises equivalent stress during overrolling and the corresponding residual stresses are evaluated below the inner ring raceway of the bearing. The features of the FEM model are discussed in detail. The increased sliding friction in the band shifts the maximum of the v. Mises equivalent stress to the surface. Compressive residual stresses are induced in the edge zone. Depending on the applied Hertzian pressure, an additional subsurface peak occurs. First results of the finite element analysis are presented.

Service Loading of Rolling Bearings in Wind Turbine Gearboxes: X-ray Diffraction Material Response Analysis of White Etching Crack Premature Failures

ABSTRACT Rolling bearings in wind turbine gearboxes occasionally fail prematurely due to white etching cracks. The appearance of preparatively opened fracture faces indicates damage initiation from the surface in most cases due to brittle spontaneous tensile stress–induced cracking. Under the influence of decomposition products of the penetrating lubricant, branching crack growth is subsequently driven by corrosion fatigue. The material response analysis of rolling bearings from all gearbox locations, based on X-ray diffraction residual stress measurements, reveals vibrations in service as the root cause of surface crack initiation. The occurrence of high local friction coefficients in the rolling contact is described by a tribological model. Depth profiles of the equivalent shear and normal stresses are respectively compared with the measured residual stress patterns and a relevant fracture strength. White etching crack failures are experimentally reproduced on a rolling contact fatigue test rig under increased mixed friction. Causative vibration loading is evident from material response analysis. The generation of compressive residual stresses by cold-working the surface is proven to be an effective countermeasure.