Inertia and damping influence the values of the stress intensity factors (SIFs) at high-frequency loading and they must be included in computations. In the present study, different dynamic simulation procedures were carried out for two types of specimen geometries and the achieved SIF values were compared. Fast computation procedures such as harmonic modal analysis and direct steady-state analysis were compared to the computationally expensive transient dynamic analysis. Two different methods for calculating the SIF, the J-integral and the crack tip opening displacement (CTOD) methods, were applied and compared and the results showed a near perfect agreement in calculation of the mode I SIF. The Rayleigh damping model was introduced into the dynamic computation to investigate its effect and the results revealed a clear effect on the SIF at 20 kHz frequency. The fast direct steady-state analysis showed good agreement to both harmonic modal and transient analysis with the different damping values used and is, after this study, the recommended procedure.

This paper investigated the microstructure and fatigue behavior of PM-HIPed Inconel 625. The microstructure was composed of γ phase and (Mo, Nb) carbonitrides located mostly on prior particle boundaries. Despite the presence of these carbonitrides, the samples showed good tensile properties with high elongation. Two different surface conditions, pickled and machined, were used for high cycle fatigue testing under a four-point bending test. The results indicated that pickled samples had 6% lower fatigue strength (at 106 cycles) with three times higher standard deviation compared to the machined ones. Fatigue failure mechanisms were found to be dependent on surface conditions and showed different failure modes due to non-metallic oxide inclusions and surface defects in samples with machined and pickled surfaces, respectively. The effect of type, size, and location of defects, multiplicity of crack initiations, as well as surface roughness were analyzed and discussed.

Top hammer drilling is a common method to drill holes in rock formations in mining and civil engineering applications. Failure of drilling machine components has a significant impact on the cost and period of the operation. Internal components of percussive hammers experience extreme loading conditions during their service life. The focus of the present case study is to characterize failure mechanisms of two cylindrical impact pistons subjected to impact loading. The investi-gated components were manufactured from two different steel grades, a surface hardened low alloyed high strength steel and a through hardened cold work tool steel.Failure of both pistons started with degradation of the impact surfaces in term of cavitation erosion and localized surface fatigue phenomena. Subsequently, chipping and removal of material from impact surfaces resulted in formation of semi-spherical holes and craters on both surfaces.Radial and hoop cracks started to develop from cavities on the impact surface. The radial cracks then propagated parallel to the impacting surface in the longitudinal direction of the piston. Once the cracks formed at the impact surface, the damage was controlled by impact fa-tigue. Fatigue beach marks were identified on the fracture surface of failed component.

High-nitrogen-chromium alloyed powder metallurgy (PM) tool steels offer many attractive features including high strength and corrosion resistance. The PM route offers various advantages such as advanced alloy composition, high homogeneity, and well-defined size distribution of hard phase particles. This study presents microstructure and mechanical properties of a PM Cr-Mo-V-N alloy. The conventional manufacturing route for this alloy is hot isostatic pressing (HIP) followed by hot working. To investigate the possibility of near-net-shape manufacturing, a comprehensive comparison of the performance was made between steels produced by as-HIPed and HIPed followed by hot working. Both steel types were heat treated in the same way to obtain martensitic matrix with limited retained austenite. In the present investigation, microstructure and phase analyses were performed by X-ray diffraction and scanning electron microscopy. Mechanical tests were carried out by hardness measurements and tensile fatigue tests in the very high cycle fatigue regime using ultrasonic fatigue testing. 

Until 1970´s, fatigue properties of materials were usually studied and evaluated up to the HCF regime (N<10⁶ cycles) beyond which failure was not expected. However, in the late decades of the 20^th century proof of fatigue failure in the VHCF regime (N>10⁶ cycles) was presented by different material scientists. This led to an aroused interest in developing a new testing technique where a very high number of loading cycles is achieved within reasonably short time. The ultrasonic fatigue testing system was developed for this purpose, testing materials in the VHCF regime, where specimens are loaded at 20 kHz reaching 10¹⁰ cycles in less than a week. In the ultrasonic fatigue testing system, an electric sinusoidal signal is generated and then converted to a mechanical sinusoidal vibration. The mechanical vibration is led through a magnifying horn to the specimen. The system vibrates at resonance frequency, hence all individual parts of the system are designed and dimensioned according to the system resonance frequency.

Fatigue testing commonly implies testing for fatigue strength and testing fatigue crack growth. The staircase test method is used to determine a materials fatigue strength at a certain fatigue life. To measure the crack growth rate the crack is continuously monitored and measured and the stress intensity at the crack tip is computed for the growing crack. FEM has been used when the specimen geometry is complicated or when dynamic simulations are required.

Results from fatigue tests in the VHCF regime have showed that interior non-metallic inclusions are the most common initiation sites. This led to further investigations on the inclusion type, shape and size and the effect they have on the fatigue strength. An inclusion initiated fatigue crack normally creates a fish-eye with a Fine Granular Area (FGA) around the inclusion. It has been confirmed that 90-99% of the total fatigue life is due to the formation of the FGA area.

This study contains an assemblage of theories and models for, and results of, different VHCF tests.

Very-high-cycle-fatigue (VHCF) strength properties are of interest to several technical applications assessed globally at different laboratories with long-life fatigue testing capabilities. Also, VHCF failure mechanisms are a scientific topic with remaining open research questions. Herein, three automotive bar grade steels are studied with respect to VHCF strength and initiation mechanisms. A microalloyed ferritic-pearlitic steel (38MnSiV5, 870 MPa tensile strength), a quenched and tempered martensitic steel (50CrV4, 1410 MPa tensile strength), and a carburizing steel (16MnCr5, 1180 MPa core structure tensile strength) are studied to reveal characteristics regarding initiation and VHCF failure mechanisms. A 20 kHz ultrasonic fatigue testing instrument is used to obtain fatigue lives up to and above 10(9) load cycles in uniaxial loading. Hour-glass specimens, smooth or notched, are tested at R = -1 and R = 0.1. Fatigue strength and stress life (SN)-diagram data are achieved, and crack initiation and growth mechanisms are studied using primarily field-emission gun-scanning electron microscopy (FEG-SEM). Fatigue strengths are explained by a modified life-dependent Murakami-expression, the Haigh diagram, and notch sensitivity. Interior and surface crack initiations by surface defects, triple points, and inclusions are found. The fine granular area (FGA) to fish-eye crack growth transition conditions are explored and schematic descriptions are given.

Mechanical fatigue failure occurs in components subjected to cyclic loading. A crack initiates at critical regions in the component and propagates during repeated loading. The expected fatigue life depends on the level, type and frequency of the loading. Generally, as implied by the Wohler’s SN curve, higher applied cyclic load leads to lower fatigue lifes and vice versa. When designing mechanical components carrying cyclic loading, engineers take into account the fatigue limit, i.e. the material specific maximum load allowed for the desired fatigue life. In automotive machinery, components are often required to withstand a very high amount of load cycles before failing. Hence, fatigue data for the very high cycle fatigue (VHCF) regime becomes significant.

In this thesis, an ultrasonic fatigue testing system, with 20 kHz loading frequency, was used to determine the fatigue properties of three high strength low alloyed automotive steels (a ferritic-pearlitic, a martensitic and a carburizing martensitic steel) in the VHCF regime. Theoretical modelling taking into account the influence of the high load frequency was developed and utilized to control the experimental testing. Fatigue strength, crack initiation mechanisms and crack propagation behaviour in the VHCF regime were studied. More specifically, fatigue strength (σ_N) at 10⁸ cycles, in both uniaxial and bending loading, crack growth rate (da/dN) and threshold behaviour (ΔK_th) in the low stress intensity factor regime were determined using specially designed specimens and test rigs.

The fatigue failure of the automotive steels in the VHCF regime, revealed fracture surfaces with fine granular area (FGA) formation close to the initiation point, with a transition to flat transgranular crack growth inside the fish-eye area. The stress intensity thresholds of FGA and fish-eye transitions are described and related to crack tip plastic zone sizes and steel strength.

The effect of damping at 20 kHz, on automotive steels, was thoroughly studied by experimental measurements and theoretical investigations. The effect of introducing damping in dynamic analysis of stress and stress intensity computations for fatigue strength and crack growth testing at 20 kHz was clarified. Best practice for theoretical modelling and computation of stress intensities in crack growth testing at 20 kHz, including the effect of damping, along with guidelines for best practice testing procedure are provided.

In automotive machinery, components are often required to withstand a very high amount of loading cycles before failing. Hence, fatigue data for the very high cycle fatigue (VHCF) regime are of importance for the engineers designing such components. In this thesis, the VHCF properties of three different high strength microalloyed automotive steel grades are investigated. Through theoretical computation and modelling, as well as experimental testing, knowledge of fatigue strength as well as crack initiation and propagation behaviour is gained.

The influence of high load frequency on both theoretical computation and experimental testing of fatigue properties was analyzed and conclusions were drawn. An ultrasonic fatigue testing system at 20 kHz load frequency was used to perform uniaxial and bending fatigue, and crack growth testing.

During the past decades, Very High Cycle Fatigue (VHCF) research has developed into an active and prioritized research area. An increased interest in testing up to 107-1010 load cycles, realized within a reasonably short amount of time, has been enabled by the development of 20-30 kHz ultrasonic fatigue testing equipment. Here, a study is presented on fatigue crack propagation at 20 kHz of three different automotive steels tested at R=-1 and R=-0.24 load ratios. However, the high load rates provokes new challenges, as theoretically finding the best practice method to compute the stress intensity factor considering 20 kHz dynamic effects (inertia forces and damping). Calibrating the fatigue load system, monitoring, controlling and performing precise measurements of the growing crack during the tests are some examples of experimental challenges encountered. Here, a best practice method for computing the stress intensity factor is presented together with a complete 20 kHz fatigue crack growth testing procedure.Three different bar steel grades have been tested; a ferritic-pearlitic, a quenched and tempered martensitic and a carburizing steel grade. Crack propagation test results differentiated between the three steel grades and were depending on load ratio R. The obtained test results at 20 kHz were found to agree with results of the same steel grades tested at conventionally used load frequencies. The SEM fractography analysis revealed ductile transgranular crack propagation mechanisms, also this in agreement with the same steel grades tested at lower frequencies.

The 20 kHz load frequency enables fatigue tests for very high cycle fatigue life, 109-1013 cycles, within conveniently short time. In automotive applications, many components are subjected to flexural loading and hence bending fatigue is an important test mode. Ultrasound fatigue test instruments have been used successfully in several assessments of fatigue strength and more commonly in uniaxial loading. Here, a 3-point bending fatigue test rig operating in resonance at 20 kHz load frequency has been designed to test plane specimens at R=0.1 loading. The test rig design and stress calculations are presented. Testing for fatigue strength was conducted using the staircase method with 15 specimens of each steel grade, specimens reaching 108 cycles were considered run-outs giving fatigue strength at 108 cycles. Additional 15 specimens of each grade were tested for S-N curves with the upper limit above 109 cycles. Two different common automotive steels, 38MnSiV5, a micro-alloyed ferritic-pearlitic steel, and 16MnCr5, a carburizing martensitic steel, were tested. The fatigue strengths achieved from the staircase testing are 340 and 419 MPa stress amplitudes for the 38MnSiV5 and 16MnCr5 steels, respectively. The S-N curves of the steels appear to be quite flat in the tested life range 107 - 109.

Fatigue properties are evaluated in a large span of fatigue lives ranging from a few load cycles to more than 1013 load cycles. If the interest is focused on fatigue lives above 10(7) load cycles, we speak of the very high cycle fatigue (VHCF) range. For evaluation of properties in the VHCF range one often needs to use higher load frequencies to be able to perform testing within a reasonable time. Therefore, the influence of load frequency on fatigue strength and fatigue crack growth is an important issue, both from testing and design perspectives. Within an EU-RFCS research project on the frequency influence on high strength steel fatigue properties the present study has been conducted on fatigue crack growth testing to determine threshold values and crack growth material parameters. The testing was analyzed by FE-computation to determine geometry factors for AK-determination. The testing was performed in a 20 kHz ultrasound resonance instrument. In such a system the whole load train needs to be designed to run at a resonance frequency of 20 kHz, and it implies that the specimen needs to be designed and computations performed by dynamic computational methods. As the crack grows the dynamic response of the specimen will change, and hence calculation to obtain the geometry factor is made with a progressing crack length. A uniaxial tensile load at 20 kHz frequency is applied to a single edged notched side-grooved flat specimen. The specimen dimensions are calculated in order to have a resonance frequency of 20 kHz, which is the frequency used for the experiments. Dynamic FEM computation, with a 3D-model and a quarter symmetry was used with one of the symmetry planes parallel to and in the crack growth line. To avoid crack surface interpenetration during the simulations a rigid thin sheet was introduced and used as a counter-face to the crack surface. The solution obtained was then combined with the breathing crack model proposed by Chati et. al. (1997) in order to solve for the irregularities observed when crack surface interpenetration occurs. Finally, the whole load train was considered. Thus, also the computed frequencies were very close to frequencies observed in experiments. The computation of stress intensities was made for varying crack lengths in a series of simulations. The geometry factor relation was determined and used in 20 kHz crack growth testing to control the actual stress intensity at the advancing crack tip. Comparison of computations and experimental results were made.