Papers by Author: Dietmar Eifler

Abstract: The present contribution gives an overview on innovative methods to characterize cyclic deformation and lifetime behavior of metallic materials and hybrid joints based on high precision measurement of electrical resistance, temperature and magnetic properties during fatigue testing. General aim is to minimize the number of fatigue tests for reliable S-N curve calculation. Moreover, instrumented cyclic hardness tests allow short-time assessment of cyclic hardening in case of limited availability of test material. The methods are applied to a wide range of materials, from carbon steels, over cast iron and metastable austenitic steels to ultrasonically welded Al-alloy/polymer matrix composites.

Abstract: High frequency fatigue tests were carried out with a 20 kHz ultrasonic testing facility to investigate the cyclic deformation behavior of Ti6Al4V in the Very High Cycle Fatigue (VHCF) regime in detail. The S,N_f -curve at the stress ratio R = -1 shows a significant decrease of the stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 10⁷ cycles. Microscopic investigations of the distribution of the α-and β-phase of Ti6Al4V indicate that inhomogeneities in the phase distribution are reasons for the internal crack initiation. Scanning electron microscopy as well as light microscopy were used to investigate the internal crack initiation phenomenon in the VHCF-regime. Beside the primary fatigue crack additional defects like micro cracks and crack clusters were observed in the fatigued specimens. SEM-investigations of specimens which were loaded up to 10¹⁰ cycles without failure show irreversible microstructural changes inside the specimens. Two step tests were performed to evaluate the influence of internal fatigue induced defects observed in specimens which did not fail within 10¹⁰ cycles.

Abstract: To realize modern light weight constructions it is more and more necessary to combine the advantages of dissimilar materials. Fusion welding of dissimilar metals is in the most cases difficult or even impossible as a result of different melting points of the materials and the development of undesirable brittle intermetallic phases in the welding zone. This often leads to joint strengths considerable below the tensile strength of the base materials. By using Friction Stir Welding (FSW) as a pressure welding method, it is possible to reduce the development of the intermetallic phases of Al/Mg-joints significantly. But as calculated phase diagrams and high resolution microscopic SEM-investigations have shown it is not feasible to avoid them completely. The intermetallic phases form in the contact area very small continuous layers between the joining partners. On the other side it is known that ultrasonic energy can crack oxide layers. Hence a hybrid welding system at the Institute of Materials Science and Engineering (WKK) at the University of Kaiserslautern was developed called “Ultrasound Supported Friction Stir Welding (US-FSW)” with the aim to shatter the brittle interlayer lines and to scatter fragments in the welding area during the FSW process. Pre-investigations have shown that for US-FSW-joints between Al wrought alloys and Mg cast alloys the strength can be increased up to 30% in comparison to conventional friction stir welds. Currently, further investigations are carried out with joints between AC-48000 and AZ80.

Abstract: Using a low temperature turning process with carbon dioxide cooling in the cutting zone a variation of the morphology at the specimen surfaces of the metastable austenitic steel AISI 347 was realized. In LCF and HCF fatigue tests at ambient temperature and 300 °C the influence of the surface morphology on the cyclic deformation behavior and fatigue life was investigated by the measurement of stress-strain hysteresis. An additional magnetic measurement allows the characterization of the phase transformation from paramagnetic austenite in ferromagnetic α´-martensite during the turning processes and during cyclic loading. The surface morphology was studied in detail by SEM and x-ray investigations.

Abstract: The austenitic steel X6CrNiNb1810 (AISI 347) was investigated in isothermal total strain-controlled tests at ambient temperature and T = 300 °C in the LCF-and HCF-range. The phase transformation from paramagnetic austenite (fcc) into ferromagnetic α´-martensite ́(bcc) leads to cyclic hardening and to an increase in fatigue life. At 300 °C no α´-martensite formation was observed in the LCF-range and the cyclic deformation behavior depends basically on cyclic hardening processes due to an increase of the dislocation density, followed by cyclic saturation and softening due to changes in the dislocation structure. In the HCF-range an increase in fatigue life was observed due to ε- and α´-martensite formation. Measurements of the mechanical stress-strain-hysteresis as well as temperature and magnetic properties enable a characterization of the cyclic deformation behavior and phase transformation in detail. The changes in the physical data were interpreted via microstructural changes observed by scanning-and transmission-electron-microscopy as well as by x-ray investigations. Additionally electromagnetic acoustic transducers (EMATs) developed from the Fraunhofer Institute of Non-destructive Testing (IZFP) Saarbrücken were used for an in-situ characterization of the fatigue processes.

Abstract: In this research work specimens of the metastable austenitic steels AISI 304 and AISI 347 with one phase (fully austenitic) and two phase (austenitic-α ́-martensitic) microstructure were monotonically loaded at ambient temperature. Using stress-strain and temperature measurements the deformation behavior was characterized in detail. To study the influence of morphology of deformation induced α ́-martensite on the stress-strain response a phase field model for α ́-martensite transformations was developed. With this approach it was possible to model the two phase austenite-α ́-martensite microstructure and investigate the deformation behavior on the micro level. With optical microscopy, magnetic and x-ray measurements the microstructure characterization of fully austenitic and austenitic-α ́-martensitic steels was realized.

Abstract: In this study, a series of experimental studies was conducted to investigate the fatigue behavior of Ti-6Al-4V alloy at room temperature. Specifically, by inspecting the cylindrical specimens with a circumferential notch of different depths (20-200µm) and notch root radii (20-100µm), the notch effect was systematically investigated with tension-compression fatigue tests (R = –1). To quantify the effects of small notch, the -parameter model was adopted and its applicability for Ti-6Al-4V alloy was examined. Finally, the fatigue characteristics are discussed in conjunction with the behavior of small fatigue cracks at notches.

Abstract: The deformation induced α´-martensite formation in the metastable austenitic steels AISI 304, AISI 321 and AISI 348 was investigated in tensile and low cycle fatigue tests at ambient and low temperature. By means of stress strain and magnetic measurements, the mechanical behavior and phase transformation were characterized. The susceptibility of deformation induced α´-martensite formation depends on the chemical composition, the temperature and the degree of cumulated plastic strain. On the basis of comprehensive experimental data a mathematical model was developed to describe and predict the α´-martensite formation under cyclic loading in the temperature range -60 °C to 25 °C. The influence of test temperature and austenite stability of the model parameters was studied.

Abstract: In this investigation specimens of the austenitic steels AISI 304, AISI 321 and AISI 348 were investigated in fatigue tests in the temperature range -60°C ≤ T ≤ 550°C. A detailed microstructure-based characterization of the cyclic deformation behavior of austenitic steels was performed by means of stress-strain hysteresis, electrical resistance and magnetic measurements. Up to ambient temperature the occurring deformation induced martensite formation was measured in-situ with a ferritescope during cyclic loading. The temperature range for dynamic strain aging was reliably identified by means of a temperature increase fatigue test with one single specimen.

Abstract: Friction stir welding (FSW) is a new solid-state welding process that can produce low-cost and high-quality joints of especially aluminum and mgnesium alloys. The welding zone consists of different regions with characteristic microstructuralal details such as a weld nugget, a thermo-mechanically-affected zone (TMAZ) and a heat-affected zone (HAZ). Tension-compression fatigue tests were performed using FSW aluminum alloy AA5454 sheet specimens at a stress ratio of –1. To investigate the propagation behavior of small fatigue cracks in those regions, an artificial defect was introduced into different defined locations in the FSW specimens as well as into the parent material specimens. The crack propagation rates depended on the defined locations and were a function of the hardness; that is, the lower the hardness was, the higher the propagation rate was. The crack paths were mostly perpendicular to the applied stress axis, but some crack paths exhibited deviations by the influence of the local anisotropy of the microstructure.