Papers by Keyword: LCF

Abstract: In this paper, the static and low-cycle-fatigue (LCF) behavior of wrought samples of 17-4 PH stainless steel (SS) manufactured via Selective Laser Melting (SLM) are presented. On the one hand, several scholars have studied SLM materials and literature reports a huge amount of data as for the high-cycle-fatigue (HCF) behavior. On the other hand, few are the data available on the LCF behavior of those materials. The aim of the present research is to provide reliable data for an as-build 17-4 PH steel manufactured via SLM techniques. Only with quantitative data, indeed, it is possible to exploit all the advantages that this technology can offer. In this regard, both quasi-static (QS) and low-cycle-fatigue tests were performed on Additive Manufacturing (AM) cylindrical samples. Through QS tests, the constitutive low has been defined. Strain-controlled fatigue tests on an electromechanical machine were performed on 12 samples designed according to the ASTM standard. Tests were continued also after the stabilization was reached (needed for the cyclic curve described with the Ramberg-Osgood equation) to obtain also the fatigue (ε-N) curve. Results show that the material has a softening behavior. The Basquin-Coffin-Manson (BCM) parameters were tuned on the basis of the ε-N combinations after rupture.

Abstract: The joining points between martensite packets (laths) and their microstructure in low-carbon martensitic steel were TEM studied. In order to determine the real microstructure of the packet, martensite examinations were conducted before low-cycle fatigue (LCF) tests, considering the structure of the packets and types of their joining. The changes in microstructure occurred in the above places after austenite-martensite transformation were also analyzed. It was shown that after jointing some packets initiate arch-like contours in the laths, exhibiting a presence of local stresses. Several types of joints are considered, including the penetration of laths of one packet into that of neighboring one. It was revealed that the microstructure changes are exhibited in joining points without any external deformation, and result in the localized plastic deformation at LCF. It is assumed that microcrack initiation and commencement of fatigue failure of the material should be expected to happen just in these areas. All the above is explained from point of view of the peculiarities of martensitic transformation.

Abstract: The microstructure changes, development of micro plastic deformation and formation and distribution of slip bands were studied. It is shown that development of micro deformation during LCF depends on loading conditions (amplitude and number of cycles) and microstructureIt is shown that as non-localized as well as localized micro plastic deformation takes place because of structural inhomogeneity. Supposedly, the localized deformation is related to the sites of internal stress concentration accumulated during the LCF.The effect of microstructure of structural steels on the rate of local cyclic deformation, leading to nucleation and growth of slip bands of fatigue cracks, was studied. The interaction of slip bands with precipitates, grain boundaries and low-angle boundaries were also analyzed.The sites of nucleation of primary and secondary slip bands were identified, and the following aspects were considered: 1. the possibility of microcrack nucleation on (or in) slip bands, 2. The kind of slip bands the slip bands may nucleate in, 3. The potential sites (except the slip bands) and reasons of nanocrack formation are specified.

Abstract: High-temperature components, for example turbochargers, are often subject to complex thermal and mechanical loading paths. Non-uniform temperature distribution and constraints by neighboring components result in complex timely varying stress and strain states during operation. In this paper the inelastic behavior of a casting material Ni-resist D-5S in a wide stress, strain rate and temperature ranges is analyzed. The material model including a constitutive equation for the inelastic strain rate tensor, a non-linear kinematic hardening rule and a damage evolution equation is developed. To calibrate the model, experimental databases from creep and low cycle fatigue (LCF) tests are applied. For the verification of the model, simulations of the material behavior under uni-axial thermo-mechanical fatigue (TMF) loading conditions are performed. The results for the stress response and lifetime are compared with experimental data.

Abstract: The behavior of parts subjected to simultaneous thermal and mechanical fatigue loads is an area of research that carries great significance in the power generation, petrochemical, and aerospace industries. Machinery with expensive components undergo varying applications of force while exposed to variable temperature working fluids. An example case is found in steam turbines, which subject stainless steel blades to cyclic loads from rotation as well as the passing of heated gases. Accurate service life prediction is especially challenging due to the thermo-mechanical loading being present on the complex geometric profile of the blades. This research puts forth a method for determining crack initiation lifetimes in variably-notched type 304 austenitic stainless steel specimens subjected to differing fatigue and thermo-mechanical fatigue conditions. A base analytical model and genetic algorithm were used to develop phenomenology-informed predictions that fall within a factor of two of the actual crack initiation times.

Abstract: Metal Matrix Composites (MMC) based on a TRIP (TRansformation Induced Plasticity)- or TWIP (TWinning Induced Plasticity)-steel matrix reinforced with MgO-partially stabilized zirconia (Mg-PSZ) are an interesting research field as both components exhibit a deformation-induced or stress-assisted martensitic phase transformation and twinning, respectively. The present work deals with the fatigue characteristics of a reinforced CrMnNi-steel as a function of the ceramic particle size. Therefore, the particles were classified into three grades (grade 1: <10 μm; grade 2: 10-30 μm; grade 3: 30-50 μm) whereas the volume fraction concerning the composite material was kept constant at 10 vol.%. The composites were produced using the hot pressing technique. The tests were performed under total strain control in a range of 0.2% ≤ Δε_t ≤ 1.2%. The microstructure of fatigued specimens was examined using scanning electron microscopy.

Abstract: The work deals with the transmission electron microscopy (TEM) study of thin films of chromium-nickel Х18Н10 steel. The films were prepared from bulk samples after low cycle fatigue (LCF) tests. Focus was made on the processes accompanying propagation of small microcracks. Particularly, the microstructure changes near the crack tip were analyzed in terms of accommodation processes taking place during crack propagation, such as formation of slip bands, twins etc. The authors conducted crystallographic analysis of the defects formed during crack propagation in correlation with the reasons of their initiation and homogenous length of the slip bands. Thus, the reasons of microcrack deviation from the initial direction were determined. The research has shown that the most convenient microstructure variables in the austenitic crystals of polycrystalline sample, affecting the microcrack deviation, are microstructure, crystallography and the homogenous length of slip bands.

Abstract: Aluminium cast alloys are used for engine components, such as pistons and cylinder heads. The micromechanical properties of an AlSi12 cast alloy under monotonic and cyclic loadings are investigated. Therefore a microstructure-based two dimensional finite element model is generated. The characteristic shape of primary precipitates is analyzed and translated into an artificial microstructure. The quality of the generated microstructure is evaluated based on the stress distribution along the primary particle boundaries. The effect of the temperature dependent material behavior of the aluminium matrix is studied with respect to the resulting stress distribution along the particle boundaries. The results are discussed in terms of a possible change of fracture mechanisms from a brittle type fracture at low temperatures to an increasingly ductile fracture at high temperatures.

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: Alloy 104 is a novel high strength, α+β titanium alloy primarily aimed at aero-engine fan disc applications. Two microstructural variants of Alloy 104 have been assessed. Room temperature tensile strength and elongation have been investigated alongside a more detailed study of low and high cycle fatigue behaviour. The alloy clearly demonstrated an improved fatigue resistance in both microstructural conditions, whilst maintaining forgeability and a comparable density to Ti-6Al-4V. Furthermore, the alloy has been subjected to a load regime with a hold period at peak loads and proven to be insensitive to dwell fatigue.