Location Of Crack Initiation Site Research Articles

On the fatigue behavior of low-temperature gaseous carburized 316L austenitic stainless steel: Experimental analysis and predictive approach

Low-temperature gaseous carburization is a surface modification method for austenitic stainless steels. In order to investigate the effects of low-temperature gaseous carburization on the fatigue behavior of AISI 316L, fully reversed axial fatigue tests were performed at room temperature on specimens with various remaining case depths. The fatigue performance of AISI 316L could be significantly improved; a 15% higher endurance limit is achieved after low-temperature gaseous carburization. After removal of the outer, brittle part of carburized case by electropolishing, the improvement of the fatigue performance is reduced. Fractography showed that for untreated specimens, fatigue cracks always initiated on the surface. For the carburized specimens, however, the locations of crack initiation sites depend on the applied stress levels. Compressive residual stresses in the case move the crack initiation site to the sub-surface; the lower the applied stress, the deeper the initiation site. A quantitative analysis of the effect on fatigue behavior forms the basis for a life prediction model, which can accurately predict the fatigue life of AISI316L steel after low temperature carburization.

Effect of friction in a single-tooth fatigue test

The primary objective of the single tooth fatigue test is to generate gear-tooth bending fatigue data at a comparatively low price. Assuming a negligible influence on fatigue data, the effect of friction is not evaluated. The aim of this study is to evaluate the effect of tooth friction on tooth bending fatigue life and on the location of the crack initiation site in a single-tooth fatigue test of case carburized steel spur gear. By using Neuber’s rule, tooth root stresses predicted by means of two-dimensional elastic finite element analysis are translated into estimates of elastic-plastic stress and strain behaviour. The fatigue life and location of crack initiation site are predicted on the basis of the strain-life approach with consideration of mean stress, surface finish and residual stress effects. Two load models are considered. In the first load model, which represents the operating principle commonly used in practice, the test tooth is loaded through the actuating arm. In the second one, which is proposed in this study, the test tooth supports the reaction through the fixed anvil. In both load models, the test tooth is contacted at the highest point of single tooth contact, while the other one is contacted at a lower position along the tooth profile. The computational model is validated against experimental crack initiation lives from literature. The study revealed that friction has a more significant impact on tooth bending fatigue life and location of the crack initiation for the first load model as opposed to the second one.

Effect of Relative Humidity on Crack Growth Mechanism of Hot Dip Galvanized Steel in Atmospheric Environment

A hot dip zinc galvanizing is currently used as an effective corrosion protection technique for steels. There are a lot of reports concerning to the corrosion of coated metals. However, due to its difficulties for evaluation, the corrosion fatigue strength of galvanized steel has not been investigated. For this reason, the effect of galvanizing on the fatigue strength has not been understood yet. In the previous work, we studied the fatigue of zinc galvanized S45C steel at room humidity. In order to investigate the atmospheric corrosion fatigue strength of galvanized steel, we conducted the fatigue test focusing on the fracture mechanism of hot-dip galvanized steel in the corrosive atmospheric environment. The substrate of the test material was S45C steel rod. Prior to the galvanizing process, S45C steel rod was pickled to remove the flux. The specimens were immersed in the galvanizing bath at 450℃ for 3 min. The corrosive environment was prepared in a glass sealed container controlled by the humidity control system. Load controlled fatigue tests were carried out at a load ratio of 0.01 and a frequency of 10 Hz (sine wave). SEM observation was performed to study the morphology of fracture surface and to determine the crack initiation sites. In the cross sectional observation of galvanized layer, the Zn-Fe alloy layer was constructed in delta-one (δ 1 ) phase and zeta (ζ) phase, and the top of surface was eta (η) phase which was comprised of the pure zinc layer. When the maximum stress was 90% of yield stress, the fatigue strength at RH=80% was mostly equal to that at room humidity (approximately RH=30%). However, the great difference in the fatigue strength was observed between the specimen at room humidity and that at RH=80% when the maximum stress was less than 80% of yield stress. The number of cycles to failure at RH=80% were increased approximately two times compared with that at room humidity. Stage II was defined as the main crack stable propagation region and the shape of fracture surface at stage II at RH=80% was different from that at room humidity. Multiple crack initiation sites at room humidity were observed on the η phase or a Zn-Fe alloy layer/η phase interface. Those crack initiation sites can be distinguished individually. They were existed some fracture surfaces of stage II, and these shapes were all crescent. On the other hand, there were multiple crack initiation sites at RH=80%, and every crack initiation site was located on the top of the surface. The location of crack initiation sites existed close to each other. Although some crack initiation sites were observed, those were existed in a fracture surface of stage II. The shape was elliptical and was the same with that of S45C steel. This suggests that those cracks coalesced with each other in the process of multiple cracks growing, and finally a fracture surface of stage II was formed. The oxygen contents of oxide formed on the fracture surface when the maximum stress was less than 80% of yield stress showed a larger value at RH=80% than that at room humidity. We consider that the galvanized layer in high-humidity corrosive atmosphere had been hardened by the oxide layer and resulted in the increase of fatigue strength.

Initiation sites for cracks developed from pits in a shot-peened 12Cr martensitic stainless steel

A serial sectioning procedure was carried out on a shot-peened 12Cr martensitic stainless steel to determine the location of fatigue crack initiation sites from pits of varying depth. Multiple cracks, including fully-developed cracks (surface-breaking cracks extending beyond the pit base) and non-fully-developed cracks, were observed for pits of depth greater than 70μm. If the longest crack at each side of the pit was assumed to be the dominant crack, most dominant cracks did not initiate from the pit base despite a propensity for macro-stress and strain localisation to the pit base. These results indicate that common assumptions about crack initiation from pits in shot-peened steel can be misleading. Knowledge of the location of crack initiation sites and how the crack develops is required to characterise the mechanical driving force for the crack prior to attainment of a fully-developed crack.

The Influence of Corrosion Pits and Cold Expanded Fastener Holes on the Fatigue Life Aluminium 7075-T651

The Royal New Zealand Air Force (RNZAF) utilised the split sleeve cold expansion process to increase the fatigue life of fastener holes in the wings of the C130 transport fleet. As part of the validation of the fatigue improvements offered by the process the Defence Technology Agency conducted a series of fatigue tests on cold expanded fastener holes in aluminium 7075-T651, including specimens with corrosion induced after the cold expansion process had been performed. This research conducted an analysis of fatigue crack origins and modelled the stress concentration factors generated as a result of the corrosion pits. These results were used to explain the differing fatigue life and s-n curves produced by corroded and non-corroded fatigue specimens and the location of crack initiation sites around corroded cold expanded fastener holes.

An investigation into the location of crack initiation sites in alumina, polycarbonate and mild steel

The effects of notch root radius on fracture toughness and crack initiation sites have been investigated in this paper using three different classes of materials. Data on alumina which represent ceramics, mild steel from the metals ffeily and polycarbonate representing plastics were obtained and analysed. The locations of crack initiation sites have been pinpointed by scanning electron microscopy. These identified sites more or less are located within the critical process zone or the theoretical plastic zone. The critical process zone size (Dc ) or the theoretical plastic zone size (RYF ) are independent of the notch root radius unlike the plain‐strain fracture toughness of notched specimens [KIC (ρ)]. The authors emphasize why the parameters Dc and RYF are useful for a quantitative evaluation of the reliability of structural materials.

Interaction of local elastoplasticity with hydrogen: embrittlement effects

Abstract The finite element method was used to solve the coupled elastic–plastic boundary value problem and transient hydrogen diffusion initial boundary value problem. Solutions were obtained at room temperature and under plane strain deformation in the neighborhood of a blunting crack tip under small scale yielding conditions and in the neighborhood of a rounded notch in a 4-point bend specimen. A discussion of the finite element results in conjunction with different mechanisms of hydrogen embrittlement is presented. If a critical amount of hydrogen is required for hydrogen induced crack initiation, the present results predict locations of crack initiation sites at steel bend specimens which are in agreement with experimental observations on the occurrence of the first microcracking event.

Enhanced hydrogen concentrations ahead of rounded notches and cracks—competition between plastic strain and hydrostatic stress

The finite element method was used to solve the coupled elastic–plastic boundary value problem and transient hydrogen diffusion initial boundary value problem. Solutions were obtained at room temperature and under plane strain deformation in the neighborhood of a blunting crack tip under small scale yielding conditions and in the neighborhood of a rounded notch in a four-point bend specimen. The hydrogen population profiles in both normal interstitial lattice sites (NILS) and trapping sites were calculated and conditions for the predominance of the total amount of hydrogen by either of the populations were studied. A discussion of the finite element results in conjunction with different mechanisms of hydrogen embrittlement is presented. If a critical amount of hydrogen is required for hydrogen induced crack initiation, the present results predict locations of crack initiation sites at steel bend specimens which are in agreement with experimental observations on the occurrence of the first microcracking event.

FATIGUE LIFE PREDICTION OF NOTCHED COMPOSITE COMPONENTS

Abstract— The local stress/strain approach has been used to predict the fatigue lives of notched composite components. The method was based on a microstress analysis and the application of a multiaxial fatigue parameter incorporating the alternating strain components on the critical plane. This parameter was able to correlate the fatigue lives obtained under a variety of multiaxial loading and geometrical configurations, enabling a generalized fatigue life curve to be determined on the basis of limited experimental data.The ability of the multiaxial fatigue parameter to relate the fatigue behaviour of composites was illustrated by predicting the locations of crack initiation sites in a unidirectional silicon carbide fibre reinforced titanium plate containing a circular hole tested under constant amplitude cyclic loading. The same approach was also successfully employed to predict the fatigue lives of graphite reinforced epoxy composite tubes with circular holes tested under several combinations of cyclic tension and torsion.