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P A S Reed - One of the best experts on this subject based on the ideXlab platform.
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a numerical study of the effects of shot peening on the Short Crack Growth behaviour in notched geometries under bending fatigue tests
International Journal of Fatigue, 2017Co-Authors: Mithila Achintha, Binyan He, P A S ReedAbstract:Abstract The current paper presents a numerical analysis of the effects of shot peening on Short Crack Growth in a low pressure (LP) steam turbine material, FV448. The fatigue behaviour of this material has been experimentally evaluated using a U-notched specimen (representing the fir tree root geometry of the turbine blade) under 3-point bend tests. Two different shot peening intensities were considered in this study: an industrially applied shot peening process and a less intense shot peening process. In the modelling work, a 2-D finite element (FE) model with static Short Cracks has been developed, incorporating both compressive residual stress and strain hardening distribution effects caused by shot peening. Both linear-elastic (LEFM) and elasto-plastic (EPFM) fracture mechanics were used to characterise the Crack driving force in the un-peened and shot-peened conditions, taking into account the effects of stress redistribution caused by residual stress relaxation and Crack opening. The stress intensity factor used in the LEFM approach was calculated using the weight function method, and the equivalent stress intensity factor used in the EPFM approach was calculated from the J-integral, which was evaluated using the Cracked FE model. These results could explain the mechanism of (experimentally observed) retardation of Crack Growth through the shot-peening-affected layer and also quantified this influence on fatigue life. The relative contributions of compressive residual stresses and strain hardening were assessed by investigating them separately. The sub-surface compressive residual stress distribution produced by shot peening could effectively reduce Crack propagation but the strain hardening distribution, in contrast, can accelerate it. However, strain hardening is expected to hinder the Crack initiation process by restricting the plastic deformation during cyclic loading. Predictions of the fatigue life of the shot-peened notched specimens were made based on this numerical analysis. Acceptable results were obtained using both the LEFM and EPFM approaches and the difference between them is discussed.
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effects of shot peening on Short Crack Growth rate and resulting low cycle fatigue behaviour in low pressure turbine blade material
Materials Science and Technology, 2013Co-Authors: Binyan He, K A Soady, B G Mellor, Andy Morris, P A S ReedAbstract:The effect of shot peening on subsequent low cycle fatigue behaviour of a representative low pressure steam turbine blade material has been investigated in bend test samples. An analysis of the Short fatigue Crack Growth behaviour has been conducted. For samples with no stress concentration feature, shot peening was found to have a more evident beneficial effect at lower strain levels than at higher strain levels, whereas for samples with a stress concentration feature, the beneficial effect was retained even at higher strain levels. Preexisting Cracks were observed on the shot peened surface, which started to grow at 10–25% of fatigue life in the low cycle fatigue regime. The Crack propagation rate was slower than that observed in the ground sample, suggesting that the shot peening process delayed Crack propagation. This improvement in fatigue life has been attributed to the significant slowing of small Cracks while growing through surface regions of significant compressive residual stresses and local wo...
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effects of microstructure on room temperature fatigue Crack initiation and Short Crack propagation in udimet 720li ni base superalloy
International Journal of Fatigue, 2008Co-Authors: H T Pang, P A S ReedAbstract:Abstract An assessment of the effects of microstructure on room temperature fatigue Crack initiation and Short Crack propagation in a Ni-base superalloy is presented. The assessment was carried out on microstructural variants of U720Li, including as-received U720Li, U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform on plain bend bars. Tests were conducted in 3-point bend under load control with an R-ratio of 0.1. A maximum load of 95% σy was used in all tests. Room temperature fatigue Crack initiation was noted to occur due to slip band Cracking and from porosity on or just beneath the surface in all materials. Crack propagation was noted to be highly faceted (due to planar slip band Cracking) immediately after Crack initiation followed by a transition to a flatter Stage II type Crack path as Crack length increases. U720Li-LP was noted to show the longest fatigue lifetime, followed by U720Li-LG while U720Li shows the Shortest life. The longer lifetime of U720Li-LP was linked to a higher resistance to both fatigue Crack initiation and Short Crack propagation. U720Li and U720Li-LG show approximately similar Crack initiation resistance although U720Li-LG showed slightly improved Short Crack Growth resistance. The observations have been rationalised in terms of the microstructural characteristics of the materials, and it is believed that larger grain size, larger coherent γ′ precipitate size and higher volume fractions of both coherent and primary γ′ precipitates will improve overall fatigue lifetimes in PM Ni-base alloys which exhibit planar slip characteristics at room temperature.
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microstructure effects on high temperature fatigue Crack initiation and Short Crack Growth in turbine disc nickel base superalloy udimet 720li
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2007Co-Authors: H T Pang, P A S ReedAbstract:Abstract An assessment of the effects of microstructure on fatigue Crack initiation and Short Crack Growth in a turbine disc nickel-base superalloy at 650 °C in air is presented. U720Li and microstructural variants of U720Li, i.e. U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent γ′ precipitate variant) have been assessed by uninterrupted and replicated Short Crack tests in polished U-notch specimens using a 1-1-1-1 trapezoidal loading cycle at nominal stress levels ranging between 700 and 850 MPa (calculated in the unCracked ligament). Crack initiation was primarily due to porosity on or near the surface but also due to grain boundary oxidation. Initial transgranular Crack Growth across four to six grains in air was noted at Short Crack lengths before oxidation-assisted intergranular Crack Growth modes were established at larger Crack lengths. At a nominal applied stress of 840 MPa, U720Li and U720Li-LP show similar fatigue lifetimes while U720Li-LG demonstrates a significantly improved fatigue lifetime, particularly when lifetimes are compared on a local strain range basis. A larger grain size gave the most significant performance benefits in terms of overall fatigue lifetime under these test conditions.
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elevated temperature Short Crack fatigue behaviour in near eutectic al si alloys
International Journal of Fatigue, 2003Co-Authors: M.r. Joyce, C M Styles, P A S ReedAbstract:This paper considers two candidate automotive piston alloys and highlights the influence of microstructural features on fatigue behaviour. Fatigue initiation and subsequent Short Crack Growth was assessed at 20, 200 and 350 °C. It is shown that both temperature and test frequency have a strong influence on the fatigue performance of the materials tested. The microstructure was quantitatively characterised in terms of the primary Si distribution. Together with post failure analysis, this allowed identification of critical microstructural features affecting both fatigue Crack initiation and early Growth. Large primary Si particles were found to act as preferential initiation sites by Cracking or decohesion (dependent on test temperature) and are also sought out preferentially during Short Crack Growth.
Bernd Oberwinkler - One of the best experts on this subject based on the ideXlab platform.
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modeling the fatigue Crack Growth behavior of ti 6al 4v by considering grain size and stress ratio
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2011Co-Authors: Bernd OberwinklerAbstract:Abstract Ti-6Al-4V is a commonly used titanium base alloy in aerospace applications. The increasing demand for damage-tolerant designs of such components necessitates a detailed knowledge of its Crack Growth behavior. The aim of this research was the characterization and phenomenological modeling of long Crack Growth behavior with respect to microstructure and stress ratio. Therefore, the long Crack propagation was characterized for eight different heat treatment conditions and four stress ratios. For comparison, physically Short Crack Growth tests were also performed. The long Crack Growth threshold was found to be dominated by roughness-induced Crack closure, and the fracture surface roughness is controlled by the primary α-grain size. The reason for this correlation is a near-threshold Crack propagation mode, which is dominated by the transcrystalline fracture of α-grains. This correlation was used to model the Crack Growth threshold with respect to microstructure. A linear relation was determined between the stress ratio and the threshold value, which was also found in this approach. Further presented models cover the Crack Growth behavior in the near-threshold (Stage I) and mid-Growth rate regions (Stage II).
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importance of local microstructure for damage tolerant light weight design of ti 6al 4v forgings
International Journal of Fatigue, 2010Co-Authors: Bernd Oberwinkler, Martin Riedler, Wilfried EichlsederAbstract:Abstract Forged parts made of titanium are generally used in aerospace industry, e.g. for engine mounts, pylon fitting and frame parts, housings, gear box components, engine disks and so on. To achieve damage tolerant together with light weight design of such parts the local microstructure resulting from thermomechanical treatment has to be considered. Therefore a new approach is introduced in this paper containing methodologies for the determination of local microstructural-based S/N-curves. They can be embedded in a lifetime prediction based on local stresses. Long Crack Growth and fatigue tests were performed on Ti–6Al–4V with different types of microstructures and distinct differences were found in respect of fatigue and Crack propagation. It is shown that the microstructural parameters primary α-grain size and (α + β)-content dominate the fatigue behavior based on rotating bending S/N-curves whereas the long Crack Growth is mainly affected by primary α-grain size and connectivity of α-grains. In addition first results regarding characterization of Crack initiation phase and Short Crack Growth of mill-annealed Ti–6Al–4V are presented for clarification of the fatigue process in respect of microstructure.
Tetsuo Shoji - One of the best experts on this subject based on the ideXlab platform.
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effects of dissolved hydrogen and surface condition on the intergranular stress corrosion Cracking initiation and Short Crack Growth behavior of non sensitized 316 stainless steel in simulated pwr primary water
Corrosion Science, 2017Co-Authors: Xiangyu Zhong, Shirish Chandrakant Bali, Tetsuo ShojiAbstract:Abstract The effects of dissolved hydrogen (DH) and inner surface finish condition of the hollowed specimen on stress corrosion Crack (SCC) initiation and small Crack Growth behaviors of 316 stainless steel were investigated in high temperature water with various DH levels. The drilled specimen showed higher surface hardness and more enhanced SCC initiation and Short Crack Growth behavior than honed specimen but both showed a similar trend in the water chemistry with different DH levels. Fractographic examinations showed mixed modes of transgranular and intergranular Cracking. The influences of DH and surface condition on SCC behaviors are discussed.
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accelerated test for evaluation of intergranular stress corrosion Cracking initiation characteristics of non sensitized 316 austenitic stainless steel in simulated pressure water reactor environment
Corrosion Science, 2017Co-Authors: Xiangyu Zhong, Shirish Chandrakant Bali, Tetsuo ShojiAbstract:Abstract Accelerated technique has been developed for evaluation of intergranular stress corrosion Cracking (IGSCC) initiation behavior of non-sensitized materials in pressure water reactor environment by means of the implementation of hollowed cylindrical specimens under slow strain rate tensile. Typical IGSCC feature was observed on the fracture surface. The Crack depth distribution showed two peaks feature which relates to the worked hardened layer on the inner surface. The specimens tested at lower strain rate showed higher fraction of IGSCC, larger number of Cracks initiation, Shorter elongation and smaller Crack opening displacement, suggesting the transition behavior of IGSCC initiation and Short Crack Growth.
Michael Vormwald - One of the best experts on this subject based on the ideXlab platform.
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fatigue Crack Growth in cruciform welded joints influence of residual stresses and of the weld toe geometry
International Journal of Fatigue, 2017Co-Authors: Tchoffo D Ngoula, Th H Beier, Michael VormwaldAbstract:Abstract The aim of the present investigation is to calculate the fatigue life of cruciform welded joints by taking into account both the effect of residual stresses and the influence of the weld toe geometry. Two and three dimensional finite element models, with Cracks as initial defects, will be constructed for this purpose. Fatigue Crack Growth analyses are performed by using the node release technique, together with the finite element program ABAQUS. The welding residual stresses, as well as the plasticity induced Crack closure effects, are considered. The effective cyclic J-integral (ΔJeff) is used as Crack tip parameter in a relation similar to the Paris equation for the calculation of the fatigue life. For this purpose, a specific code was written for the determination of ΔJeff at each Crack length configuration. The impact of residual stresses on ΔJeff as well as on the fatigue life during Short Crack Growth is investigated. Results reveal that the influence of residual stresses can be neglected only for large load amplitudes. The calculated fatigue lives are compared with experimental data and a good accordance between both results is achieved. The influences of the weld toe radius and of the weld flank angle are also investigated.
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deformations and damage to metallic materials under multiaxial non proportional loading
Computational Materials Science, 2009Co-Authors: Michael Vormwald, Ralph DoringAbstract:Abstract The responsibility for the safe operation of engineering structures calls for trustworthy data and models to assess fatigue life. Material science develops models on the microscopic scale whereas engineering science has to consider length and time scales which can give accurate quantitative answers. In multiaxial non-proportional fatigue the applied plasticity models should be capable of capturing the non-proportional hardening and ratcheting with adequate accuracy. The capabilities of a previously introduced model are featured with respect to out-of-phase loading of stainless steel. In engineering the meso scale of about 10 μm turns out to be the smallest accessible scale of observation of fatigue damage. At this scale fatigue of metals is seen as Growth of Short surface Cracks. A phenomenological model is proposed for describing the Short Crack Growth taking into account non-proportional straining. Calculated and experimentally determined Crack Growth curves and lives to technical Crack initiation have been found to be in reasonable accordance. Future research activities are indicated.
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an experimental evaluation of three critical plane multiaxial fatigue criteria
International Journal of Fatigue, 2007Co-Authors: Yanyao Jiang, Olaf Hertel, Michael VormwaldAbstract:Abstract The Fatemi–Socie criterion, the Jiang criterion, and a Short Crack Growth based criterion were evaluated using the combined axial-torsion fatigue testing results obtained from extensive experiments on thin-walled tubular specimens made from S460N. The Fatemi–Socie criterion combines the maximum shear strain amplitude with a consideration of the normal stress on the critical plane. The Jiang criterion makes use of the plastic strain energy on a material plane as the major contributor to the fatigue damage. By assuming an initial Crack length, the Short Crack model attributes the fatigue life to the action of a Crack driving force, namely the effective cyclic J-integral. The results show that all the three criteria correlated well with the experimental observations in terms of fatigue life predictions. A great discrepancy was found between the predicted Cracking directions and the observed Cracking orientations.
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deformation behaviour Short Crack Growth and fatigue livesunder multiaxial nonproportional loading
International Journal of Fatigue, 2006Co-Authors: J Hoffmeyer, Ralph Doring, Timm Seeger, Michael VormwaldAbstract:Abstract Experimental results of a research project on Short Crack Growth under multiaxial nonproportional loading are presented. Fatigue lives, Crack Growth curves and the deformation behaviour of hollow tube specimens and notched specimens were investigated under combined tension and torsion loading. The results served as basis for the development of a cyclic plasticity model [Doring R, Hoffmeyer J, Vormwald M, Seeger T. A plasticity model for calculating stress–strain sequences under multiaxial nonproportional cyclic loading. In: Comput Mater Sci. 28(3–4);2003:587–96; Doring R, Hoffmeyer J, Seeger T, Vormwald M. Constitutive modelling of nonproportional hardening, cyclic hardening and ratchetting. In: Proceedings of the seventh international conference on biaxial/multiaxial fatigue and fracture, DVM, Berlin; 2004. p. 291–6; Hoffmeyer J. Anrisslebensdauervorhersage bei mehrachsiger Beanspruchung auf Basis des Kurzrisskonzepts. PhD-Thesis, TU Darmstadt; 2004.] and a Short Crack model [Hoffmeyer J. Anrisslebensdauervorhersage bei mehrachsiger Beanspruchung auf Basis des Kurzrisskonzepts. PhD-Thesis, TU Darmstadt; 2004; Doring R, Hoffmeyer J, Seeger T, Vormwald M. Fatigue lifetime prediction based on a Short Crack Growth model for multiaxial nonproportional loading. In: Proceedings of the seventh international conference on biaxial and multiaxial fatigue and fracture, DVM, Berlin; 2004. p. 253–8]. Stress–strain paths including nonproportional hardening and experimental fatigue lives of the unnotched specimens under different loading cases are discussed and compared with calculations. Load-time-sequences were in-phase, 45° and 90° out-of-phase loading with constant and variable amplitudes, torsion without and with superimposed static normal stress, and strain paths like box, butterfly, diamond and cross path. For the notched specimens fatigue lives under 0° and 90° out-of-phase loading are compared with calculations based on finite element results and the Short Crack model. During some tests the initiation, Growth and orientation of Short Cracks was studied using the plastic replica technique.
Daniel Kujawski - One of the best experts on this subject based on the ideXlab platform.
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a new δk kmax 0 5 driving force parameter for Crack Growth in aluminum alloys
International Journal of Fatigue, 2001Co-Authors: Daniel KujawskiAbstract:Abstract In this paper, a new mechanical driving force parameter for long- and Short-Crack Growth rate correlation is proposed. This new parameter, (ΔK+Kmax)0.5, does not utilize disputable Crack closure data, instead it is calculated as a geometric mean of the positive part of the applied stress intensity factor (SIF) range, ΔK+, and the corresponding maximum value of the SIF, Kmax. The proposed parameter correlates fairly well the R-ratio effects on the threshold condition and fatigue Crack Growth rate at the low and intermediate stress intensities for six aluminum alloys investigated.
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a fatigue Crack driving force parameter with load ratio effects
International Journal of Fatigue, 2001Co-Authors: Daniel KujawskiAbstract:Abstract In this paper, a fatigue Crack driving force parameter, ( K max ) α (Δ K + ) 1− α with load ratio effects on Crack Growth modeling is proposed. This new Crack driving force does not invoke disputable Crack closure data. Instead, it is calculated solely using the positive part of the range of the applied stress intensity factor (SIF), Δ K + , and the corresponding maximum value of the SIF, K max . It unifies the overall Crack rate prediction methodology regarding load ratio effects for both the long- and Short-Crack Growth behaviors. The predictions are compared with the experimental data from the literature and the agreement is found to be fairly good.