Estimation Of Crack Initiation Life Research Articles

Notch fatigue analysis and crack initiation life estimation of maraging steel fabricated by laser beam powder bed fusion under multiaxial loading

This paper deals with the notch fatigue behaviour and crack initiation life estimation in maraging steel fabricated by laser beam powder bed fusion under multiaxial loading. Tests are conducted in tubular geometries with lateral holes considering different normal stress to shear stress ratios and multiaxial loading levels. The cyclic stress–strain response at the notch-controlled process zone is simulated numerically using two alternative approaches: a generalised isotropic plasticity model with mixed isotropic-kinematic hardening, and a linear-elastic model. Both approaches demonstrated to be suitable for predicting the crack initiation sites, the directions of crack growth, and the fatigue life. Fatigue life was calculated from a SWT-based model combined with the Theory of Critical Distances. Elastic-plastic predictions led to smaller errors but slightly shifted to the non-conservative side.

INVESTIGATING POSSIBILITIES OF CRACK INITIATION LIFE EXTENSION IN JET ENGINES COMPRESSOR DISKS

This paper deals with jet engines compressor disks which have dovetail joints with blades. A compressor disk with reduced fatigue resistance was taken as an example. Two simplified conceptual solutions of the dovetail joint with blades were devised. Based on the low cycle fatigue theory, the crack initiation life of their critical parts with newly-proposed transition rounding at the bottom of dovetail grooves was estimated. Two different flank angles in the dovetail grooves (60° in the critical part that belongs to the first dovetail joint conceptual solution and 55° in the critical part that belongs to the second dovetail joint conceptual solution), two different aviation steels selected for workmanship (13H11N2V2MF and AISI 304 steel) and two load histories (load history LH1 and load history LH2), were taken into account. By load history LH2 an overload of the critical parts was simulated. The results of crack initiation life estimation of the critical parts in the dovetail joint conceptual solutions show that there is a possibility for the crack initiation life extension of the observed compressor disk. In all analyzed variants, it has been shown that the critical part in the second dovetail joint conceptual solution has longer crack initiation life than the critical part in the first dovetail joint conceptual solution. For example, the critical part in the second dovetail joint conceptual solution made of AISI 304 steel, in the case of load history LH1 has 141.55% longer crack initiation life than the critical part in the first dovetail joint conceptual solution made of 13H11N2V2MF steel. In the case of load history LH2 (an overload case) that percent is greater and amounts to 173.15%.

A Methodology for Incorporating the Effect of Grain Size on the Energy Efficiency Coefficient for Fatigue Crack Initiation Estimation in Polycrystalline Metal

Estimating fatigue crack initiation of applied loading is challenging due to the large number of individual entities within a microstructure that could affect the accumulation of dislocations. In order to improve the prediction accuracy of fatigue crack initiation models, it is essential to accurately compute the energy dissipated into the microstructure per fatigue loading cycle. The extent of the energy dissipated within the microstructure as a fraction of the overall energy imparted by loading has previously been defined as the ‘energy efficiency coefficient’. This work studied the energy efficiency coefficient as a factor in the measurement of accumulated plastic strain energy stored at the crack initiation site during cyclic loading. In particular, the crystal plasticity constitutive formulation was known as ’length scale independent’ previously. As a result, a semi-empirical approach was presented whereby the potential effect of grain size can be accounted for without the use of a strain gradient plasticity approach. The randomized representative volume elements were created based on the experimental analysis of grain size distribution. The work was aimed at capturing some of the effects of grain size and utilizing them to complete a semi-empirical estimation of crack initiation in polycrystalline materials. The computational methodology ensured the representative of microstructural properties, including the elastic constant and critical resolved shear stress via appreciable fit achieved with the empirical tensile test results. Crystal plasticity finite element modeling was incorporated into a finite element code to estimate the potential for crack initiation. The energy efficiency coefficient was computed for a class of material with grain size to C11000 electrolytic tough pitch (ETP) copper. This methodology can improve fatigue crack initiation life estimation and advance the fundamental study of energy efficiency coefficient during fatigue crack initiation.

A fatigue crack initiation and growth life estimation method in single-bolted connections

Fatigue life estimation accuracy of mechanical parts and assemblies has always been the source of concern in different industries. The main contribution of this article lies in a study on the accuracy of different multiaxial fatigue criteria, proposing and investigating the accuracy of four optimized fatigue crack initiation life estimation methods—volume, weighted volume, surface and point, thereby improving the multiaxial fatigue life estimation accuracy. In order to achieve the goal, the fatigue lives of bolt clamped specimens, previously tested under defined experimental conditions, were estimated during fatigue crack initiation and fatigue crack growth and then summed together. In the fatigue crack initiation part, a code was written and used in the MATLAB software environment based on critical plane approach and the different multiaxial fatigue criteria. Besides the AFGROW software was utilized to estimate the crack growth share of fatigue life. Experimental and numerical results showed to be in agreement. Furthermore, detailed study and comparison of the results with the available experimental data showed that a combination of Smith–Watson–Topper approach and volume method results in lower error values, while a combination of Fatemi–Socie criterion and surface or point method presents estimated lives with lower error values. In addition, the numerical proposed procedure resulted in a good prediction of the location of fatigue crack initiation.

A Fatigue Analysis of a Hydraulic Francis Turbine Runner

In this work, the estimation of crack initiation life of a hydraulic Francis turbine runner is presented. The life prediction is based on the local strain approach to predict the initiation life. First, the analysis is carried out in air and in water condition and the runner’s natural frequencies were calculated using the finite element (FE) method. The analysis in air is compared with experimental analysis in order to have a representative model of real runner and subsequently the numerical analysis was perform in water. In the case of the runner immersed in water, the added mass effect due to the fluid structure interaction (FSI) is considered. Second, the static and dynamic stresses were calculated according to life estimation. For the calculation of static stresses, the pressure distribution of water and the centrifugal forces were applied to the runner. The dynamic stresses were estimated for interactions between the guide vane and the runner. Lastly, the estimation of the crack initiation life of the runner was obtained.

Estimation of Crack Initiation Life in Low Cycle Fatigue by Stress Redistribution Locus Method for Circumferential Notched Bar of Low Carbon Steel (S20C)

The stress redistribution locus (SRL) method has been proposed to make a reasonable estimate of inelastic behavior under elevated temperatures employing both the elastic Finite Element (FE) analysis and a unique hyperbolic curve, where dimensionless equivalent strain and stress are normalized by elastic ones in the elastic FE analysis. In order to investigate applicability of the SRL method to components under the elastic-plastic state, low cycle fatigue tests were conducted employing a smooth specimen and four types of circumferentially notched bars. The number of cycles to crack initiation estimated by the SRL method was compared with the experiment. This showed that when an adequate elastic region remained inside the notch root, the crack initiation life could be well predicted by the SRL method. However when a ratio of the plastic area to the entire section of the notch root occupied over approximately 0.3%, an equivalent strain range on a surface of the notch root estimated by the SRL method became smaller than the one calculated by using the detailed inelastic FE analyses. On this occasion the crack initiation life estimated by the SRL method was overestimated.

Fatigue crack initiation life of fine grain brass H62

Fine grain alloys possess excellent properties entailing high strength and toughness. Fine brass H62 is made by re-crystallization with grain size ranging from 5 to 10 μm. Fatigue initiation life is investigated from specimens tested on Instron 1341 machine a frequency of 30 Hz. Furthermore fatigue crack initiation life of this fine brass H62 is predicted by the energetic approach. It is found that: (1) the finer the grain size, the longer the fatigue life, which is due to its higher toughness; (2) intergranular cracking is the main mechanism of fatigue failure. Concave pits were found in the zone of fatigue crack propagation; and (3) the energetic approach gave acceptable fatigue crack initiation life estimation.

応力再配分軌跡(SRL)法による環状切欠き材(S20C)の低サイクル疲労・き裂発生寿命の推定

応力再配分軌跡(SRL)法による環状切欠き材(S20C)の低サイクル疲労・き裂発生寿命の推定

Fatigue crack initiation life estimation in a steel welded joint by the use of a two‐scale damage model

ABSTRACTThis work deals with the fatigue behaviour of S355NL steel welded joints classically used in naval structures. The approach suggested here, in order to estimate the fatigue crack initiation life, can be split into two stages. First, stabilized stress–strain cycles are obtained in all points of the welded joint by a finite element analysis, taking constant or variable amplitude loadings into account. This calculation takes account of: base metal elastic–plastic behaviour, variable yield stress based on hardness measurements in various zones of the weld, local geometry at the weld toe and residual stresses if any. Second, if a fast elastic shakedown occurs, a two‐scale damage model based on Lemaitre et al.'s work is used as a post‐processor in order to estimate the fatigue crack initiation life. Material parameters for this model were identified from two Wöhler curves established for base metal. As a validation, four‐point bending fatigue tests were carried out on welded specimens supplied by ‘DCNS company’. Two load ratios were considered: 0.1 and 0.3. Residual stress measurements by X‐ray diffraction completed this analysis. Comparisons between experimental and calculated fatigue lives are promising for the considered loadings. An exploitation of this method is planned for another welding process.

06/01397 Comparison of crack initiation life estimation procedures under creep-fatigue conditions: O'Donnell, M. P. Nuclear Engineering and Design, 2005, 235, (17–19), 1989–2001

06/01397 Comparison of crack initiation life estimation procedures under creep-fatigue conditions: O'Donnell, M. P. Nuclear Engineering and Design, 2005, 235, (17–19), 1989–2001

Local strain computation of crack initiation life of an unstiffened steel tubular T-joint

This paper presents the results of the fatigue crack initiation life estimation for offshore tubular welded joints. The results are obtained by using the fatigue strength exponent, b, and fatigue ductility exponent, c, computed by a backcalculation procedure from an extensive experimental crack initiation estimation of both small scale and medium scale welded steel tubular joints. A general purpose finite element package, ABAQUS, was used for the stress analysis. This allows for the accurate determination of the critical location where the crack/cracks is/are likely to initiate. From the stress analysis, the stress and strain concentration factors (SCF and SNCF) along the intersection of the tube were obtained. The peak SCF and SNCF were used in modified Neuber's rule and the result introduced into a Manson-Coffin model for crack initiation life computation. The results obtained were compared with the experimental results previously obtained at the Canadian Cooperative Fatigue Studies Program.