Advanced Materials Research Vols. 891-892

Abstract: The present study documents that at loading amplitudes close to the fatigue limit, cyclic irreversible plastic deformation in form of slip band generation in the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318 LN) mainly takes place in few austenite grains without any microcrack initiation in these grains. This was shown by means of focused ion beam (FIB) cutting in combination with high resolution scanning electron microscopy (SEM) at pronounced extrusion-intrusion-pairs in several austenite grains. Investigations by means of confocal laser scanning microscopy (CLSM) revealed that the slip band density in these grains increases with the number of loading cycles and remains constant in the very high cycle fatigue (VHCF) regime. Under such loading conditions, fatigue cracks frequently initiate in the ferrite phase due to anisotropy stresses which are strongly superimposed by stress intensifications at the tip of austenite slip bands. TEM investigations revealed that austenite slip bands, which are piling up against phase boundaries, cause localized dislocation generation and motion in neighboring ferrite grains. The cyclic irreversible motion of these dislocations on several parallel slip planes is correlated with the stage of fatigue crack initiation. A crystal plasticity model based on a finite element program, which considers anisotropic elasticity, allows for the determination of crack initiation sites in real microstructures according to the above mentioned mechanisms. Crystallographic orientations, measured by means of the electron back scatter diffraction (EBSD) technique, serve as input parameters for the calculations regarding microcrack initiation as well as for the analysis of the subsequent short fatigue crack propagation, which is strongly affected by microstructural barriers such as grain and phase boundaries.

Abstract: Today in many cases ultrasonic testing machines with a frequency of f ≈ 20 kHz are used for investigations of the fatigue behaviour up to the very high cycle regime (VHCF-regime). A question that arises is if the results of these high frequency fatigue tests are comparable to conventional fatigue tests. This paper compares the fatigue behaviour of a quenched and tempered steel 50CrMo4 in two different tempered conditions investigated at low frequencies (f ≤ 400 Hz) on a servohydraulic testing machine and at a high frequency (f ≈ 20 kHz) on an ultrasonic fatigue testing machine. Effects which can occur because of the different testing techniques and testing frequencies are investigated. A concept is derived to describe the frequency effect caused by the strain rate. The estimations are compared with results of the fatigue tests.

Abstract: In Ti–6Al–4V alloy, fatigue properties have been widely investigated, and the origin of fatigue fracture is usually at the surface in the high stress and lower fatigue life region, whereas in low stress and longer fatigue lifetimes origins are generally sub-surface in nature. Very high cycle fatigue tests were conducted, and observation of fracture surfaces revealed that a unique fine concave and convex agglutinate (hereinafter called Granular Region) formed on the fracture surface of sub-surface fractures. The granular region was not observed on the fracture surface of surface fractures. To clarify the formation mechanism and process of forming the granular region, which is a unique phenomenon in the very high cycle fatigue, fatigue tests using specimens with an artificial surface defect were conducted in air and vacuum. The fatigue tests were based on the idea that the environment around a sub-surface fatigue crack is a vacuum-like environment. During the tests, fracture surfaces were intentionally contacted in air and vacuum under different loading conditions. Fracture surface observations revealed that repeated contact of the fracture surfaces and a vacuum environment are necessary for the formation of the granular region. A mechanism for the formation of the granular region will be proposed.

Abstract: The use of advanced high strength steels (AHSS) in the automotive body-in-white is increasing. Those steels are predominantly joined by resistance spot welding. For the performance of the whole body-in-white, the fatigue behaviour is of high interest, especially as during production, weld imperfections such as cracks and manufacturing-related gaps cannot be avoided. In this study the TRIP steel HCT690 was used as it is a typical advanced high strength steel in automotive production. The investigation into the influence of cracks was split depending on the crack location in the weld area. Surface cracks in the electrode indentation area as well as in the heat affected zone were produced during welding and analyzed. The results showed that surface cracks independent of their position have no effect on the fatigue life. The produced internal imperfections have shown only a marginal impact on the fatigue life. It was ascertained that gaps of 3 mm lead to a significant drop in fatigue life compared to gap free shear tension samples under a load ratio R of 0.1. This fact was attributed to decreased stiffness, higher transverse vibration and higher rotation between the sheets. Furthermore, FE-simulations have shown an increase in local stresses in gapped samples.

Abstract: Difficult-to cut-materials are associated with premature tool failure, most likely in the case of complex geometries and this shapes. However, Nickel-based alloys are commonly used in high-temperature and aerospace applications, where thin deep holes are often required. Then, the only viable manufacturing solution relies on non-contact processes, like electrodischarge (ED) drilling. Morphology of ED machined surfaces is significantly different than obtained by metal-cutting operation and is known to jeopardize fatigue strength, but the extent needs to be gauged and related to the process parameters. Aim of the paper is to study the effect of holes (0.8 mm diameter, aspect ratio 10) produced by ED drilling on the fatigue life of Inconel 718. Rotating bending fatigue tests are carried out on specimens drilled under two ED setups, as well as with a traditional cutting tool. Specimens free from holes are fatigued under the same conditions for comparison. Based on previous studies, extremal ED parameters are selected, giving best surface finish versus highest productivity. S-N curves show that the ED process causes a decrease of the fatigue resistance with respect to traditional drilling, whereas the effect of different ED setups is negligible. Maximum productivity can thus be pursued with no threat to fatigue performance. The fatigue limit variation is quantified by using the superposition effect principle: ED drilling causes an increase of the stress concentration factor around 25% if compared to traditional drilling. The macroscopic fatigue behavior is integrated with a study of the effects of the different drilling processes in the micro-scale, by means of a microstructural and fractographic analysis.

Abstract: To meet the future demands of the aerospace industry with respect to safety, productivity, weight, and cost, new materials and joining concepts have being developed. Recent developments in the metallurgical field now make it possible to use laser-weldable Al-alloys of the 2xxx series such as AA2198 with a high structural efficiency index due to their high strength and low density. AA2198 holds the promise of providing a breakthrough response to the challenges of lightweight design in aircraft applications. Laser beam welding as an efficient joining technology for fuselage structures is already established in the aircraft industry for lower fuselage panels because the welded panels provide a higher buckling strength and lower weight compared with the classical riveted designs. The key factor for the application of laser-welded AA2198 structures is the availability of reliable data for the assessment of their damage tolerance behavior. In the research presented, the mechanical properties with regard to fatigue and fatigue crack propagation of laser beamwelded AA2198 joints and four-stringer panels were investigated. It was found that the fatigue endurance limit of laser beamwelded AA2198T3 is approximately 25 % below the endurance limit of the base material. With regard to the fatigue crack propagation behavior, the laser beam welded four-stringer panels with T-joints show a fatigue life increased by a factor of 1.7 compared with the base material. This work shows that high-quality laser beam welds of AA2198 can be produced on a large scale using the laser beam welding facilities of the Helmholtz-Zentrum Geesthacht.

Abstract: The friction stir welding (FSW) is a dynamically developing version of the pressure welding processes. Nowadays, the knowing of the properties and the behaviour of the welded joints is an important direction of the investigations, especially under cyclic loading. The research work aimed (i) to demonstrate the behaviour of the friction stir welded joints under cyclic loading conditions; (ii) to determine fatigue limit or design curves for aluminium alloys and their welded joints made by FSW process. Experiments were performed on 5754-H22 and 6082-T6 aluminium alloys and their welded joints. Both high cycle fatigue (HCF) and fatigue crack propagation (FCG) tests were executed on both base materials and their welded joints. Statistical behaviour of the base materials and welded joints was represented by the cutting of the specimens and the using of different crack paths. HCF limit curves were determined based on staircase method. FCG limit curves can be determined by own developed six step method. The investigations and their results were compared with each other and with the results can be found in the literature.

Abstract: The design of welded aluminium structures subjected to fatigue loading is usually carried out on the basis of reference fatigue curves published in design codes. The reference curves are usually relevant to dynamic loading in ambient air and make provision for the presence of a corrosive environment by downgrading the detail category for a particular joint type. This investigation studied the corrosion-fatigue behaviour of Al 5083-H111 welded using ER5356 wire. Comparison between experimentally determined fatigue curves and reference fatigue design curves published in Eurocode 9 confirms that 5083-H111 welds display significantly lower reference fatigue strengths than unwelded base material. Immersion in a NaCl solution during testing reduced the reference fatigue strengths even further. The reduction in detail category number recommended in Eurocode 9 for aluminium butt welds on immersion in sea water appears suitable (or even marginally conservative) for the 5XXX series Al-Mg-Mn welds examined in this investigation.

Abstract: The influence of geometrical notch and weld defects (pores and incomplete fusions) on the fatigue behaviour at very high numbers of loading cycles is shown for welded samples made of the base material EN AW-5083 and EN AW-6082 and the filler material S Al 5183. High frequency fatigue tests with specimens highest stressed cross-sections representing different welding zones show no endurance limit up to 2·10⁹ load cycles. The weakest link of the weld seams are the geometrical notch and (if present) pores and incomplete fusions in the seam interior and at the surface. Considering weld defects in the filler material as fatigue crack initiating notches, a threshold value for the cyclic stress intensity factor (ΔK_min) of 0.9 MPam was found. Using ΔK_min the fatigue life of the samples is discussed on the basis of stress amplitude, the projected area of the defect and its position. X-ray examinations revealed a good correlation between the failure-relevant defect areas and the overall fatigue life of welded samples.

Abstract: To improve the fatigue properties of structural steel, a novel surface modification process which combines high-frequency induction heating (IH) with fine particle peening (FPP) was developed. IH-FPP treatment was performed on the surface of structural steel specimens (0.45%C) at temperatures from 600 to 750 °C, with peening times of 60 and 120 s. To determine the characteristics of the treated surfaces, the microstructure was observed using an optical microscope and a scanning electron microscope. Vickers hardness and residual stress distributions were also measured. The characteristics of fine-grained microstructures were examined by electron backscatter diffraction. Furthermore, in order to investigate the effect of the grain refinement achieved by IH-FPP treatment, rotational bending fatigue tests were performed on treated specimens. Results showed that IH-FPP treatment created fine-grained microstructures beneath the surfaces of steel samples. The average ferrite grain size was 4.06 μm for a treatment temperature of 700 °C, and finally 0.76 μm for 600 °C . This was due to dynamic recrystallization in the processed region. IH-FPP treated specimens exhibited a higher fatigue strength than untreated specimens. As almost no compressive residual stress was measured in the treated or untreated specimens, the increase in fatigue strength resulting from IH-FPP treatment was due solely to grain refinement.