Short Fatigue Crack Growth Behaviour Research Articles

In-situ investigation of fatigue crack growth behavior of 34CrNi3Mo high-strength steel

In this study, the short fatigue crack growth behavior of 34CrNi3Mo high-strength steel was investigated through in-situ fatigue testing under scanning electron microscopy. An innovative method was proposed to determine the fatigue crack growth threshold by a changing load strategy, which shows high consistency with the standard test for compact tensile specimens. The difference in misorientation along distinct directions at the crack tip has been identified as a factor controlling crack deflection regardless of slip system types.

Quantitative characterization of short fatigue crack and grain boundary interaction behavior in zirconium

Short fatigue crack growth behavior of zirconium was quantitatively investigated through post-mortem electron backscattering diffraction analyses. Results show that the short fatigue crack growth was mainly transgranular, sensitive to crystallographic orientation which determined the slip systems within grains and often deflected at grain boundaries. The cracks propagated preferentially on prismatic slip planes and pyramidal slip planes due to higher Schmid factor for each grain. A set of crack deflection at grain boundaries was quantitatively analyzed. A combination of the geometrical compatibility factor, m', and twist angle, α, as well as the Schmid factor, m, were proposed to evaluate short fatigue crack growth across grain boundaries. It was found the short fatigue crack could cross grain boundaries on the conditions of m > 0.3, α < 40°, and m' > 0.3.

The Effects of Pits Size and Shape on Fatigue Crack Initiation

Generate and monitor the growth of corrosion pits on the surface of 316L stainless steel using the scanning droplet cell technique and assess the effects of varies factors such as pits size, pits shape, flow rate, and applied stress on pit initiation and growth. Configure of a test system to measure the pit and short fatigue crack growth behaviour, to investigate each stage of the corrosion fatigue damage process occurring at the material surface and compare this with the modified film surface using alternating voltage passivation process (AVPP). Understand the effects of corrosion on cracking behaviour during the different stages of fatigue crack growth.

Short fatigue crack growth behavior in 18Ni marageing steel

In recent decades, it has been found that the fatigue crack growth rate could be slowed down by the zigzag crack growth path in some materials, but the mechanism was not well clarified. In this study, the effect of crack growth path on the fatigue crack growth rate of 18Ni maraging steel under three different heat treatment states was studied, and two factors influencing the fatigue crack growth rate were outlined. It was found that, firstly, the main factor affecting the fatigue crack growth rate was the strength of the material; secondly, the zigzag crack growth path did not slow down the crack growth rate but increased the crack growth rate. Therefore, a two-phase model was established to understand the influence of crack growth path on the growth rate. In different phases, their resistances to the crack growth are different, and the difference in the strength between the two phases determines whether the zigzag cracking path can depress the fatigue crack growth or not. Specifically, when the difference in the strength between the second phase and the matrix is greater than a certain value, the zigzag cracking path leads to the acceleration of fatigue crack growth. The consequences show that not all the zigzag crack propagation paths are beneficial for the higher fatigue cracking resistance, and the specific analysis should be combined with the material microstructure in detail.

Effect of microstructural features on short fatigue crack growth behaviour in SA508 Grade 3 Class I low alloy steel

Aim of the paper is to understand the effect of microstructural features of SA508 Grade 3 Class I low alloy steel (LAS) on short crack propagation rate under cyclic loading. The complex upper bainitic microstructure of this LAS consists of low angle bainitic ferrite lath boundaries and high angle prior austenite grain boundaries (PAGBs). Compared to bainitic ferrite lath boundaries, the PAGBs provided major hindrance to short fatigue crack propagation in the subject LAS. The high angle PAGBs strongly resist the dislocation motion ahead of the crack tip as the crack tip approaches the PAGBs compared to that of low angle bainitic ferrite lath boundaries. This restriction of dislocation motion ahead of the crack tip based on hindrance provided by PAGBs resulted in retardation in short fatigue crack propagation rate along the crack path. The short fatigue crack propagated at stress intensity factor (SIF) range ‘ΔK’ values lower than threshold SIF range ‘ΔKth’ for the long cracks. The growth rate of short fatigue cracks cannot be predicted by Paris law which is applicable for long crack growth. This is due to the fact that crack growth rate undergoes acceleration and retardation in short crack regime because of microstructural effects.

Strain visualization of growing short fatigue cracks in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint: Intergranular cracking and crack closure

Physically short fatigue crack growth behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint was investigated in-situ in SEM to reveal effects of crack closure and tortuous crack growth path on crack-tip straining behavior with the help of high resolution digital image correlation technique. The crack growth path and associated fatigue damage at microstructural scale was characterized by electron backscatter diffraction and electron channeling contrast imaging techniques. Results showed that the short crack growth was influenced by both local microstructure and global strength gradient. The short crack could deflect intergranularly when meeting with one large grain due to misfit deformation and strain localization. The crack closure, which was accompanied during loading and unloading in terms of crack surface contact, shielded the real crack-tip and manifested itself well in both compressive strain at crack wake and strain development at crack-tip. It is indicated that strain localization at lathy boundaries and formation of sub-grains was responsible for the fatigue of short cracks.

Tracking hydrogen embrittlement using short fatigue crack behavior of metals

Understanding hydrogen embrittlement phenomenon that leads to deterioration of mechanical properties of metallic components is vital for applications involving hydrogen environment. Among these, understanding the influence of hydrogen on the fatigue behaviour of metals is of great interest. Total fatigue life of a material can be divided into fatigue crack initiation and fatigue crack growth phase. While fatigue crack initiation can be linked with the propagation of short fatigue cracks, the size of which is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack growth behaviour using conventional CT specimens. Extensive literature is available on hydrogen effect on long fatigue crack growth behaviour of metals that leads to the change in crack growth rate and the threshold stress intensity factor range (ΔKth). However, it is the short fatigue crack growth behaviour that provides the fundamental understanding and correlation of the metallic microstructure with hydrogen embrittlement phenomenon. Short fatigue crack growth behaviour is characteristically different from long crack growth behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as a strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. To this end, a novel experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged materials involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs a curves. Various metallic materials ranging from austenitic stainless steel (AISI 316L) to reactor pressure vessel steel (SA508 Grade 3 Class I low alloy steel) and line pipe steels (API 5L X65 & X80) are studied in this work.

Effects of compressive residual stress on short fatigue crack growth in a nickel-based superalloy

The effects of compressive residual stress on short fatigue crack growth in Inconel 718 have been investigated. Using two different indentation procedures, controlled plane-strain compressive residual stress fields were applied to short through-thickness cracks, which had been generated by machining away wakes of long cracks that had been grown down to threshold levels at a stress ratio of 0.1. Short fatigue crack growth tests were conducted at stress ratios of 0.1 and 0.7. At a stress ratio of 0.1, the residual stress-free testpieces demonstrated typical short fatigue crack growth behaviour, indicated by a reduction of threshold value and increases in fatigue crack growth rates compared to those of long fatigue cracks. A significant decrease in short crack growth rates was observed within the compressive residual stress region of indented testpieces, together with an increase of threshold values. At a high stress ratio of 0.7, any similar decrease in rate is barely observable. This indicates that the effects of compressive residual stress on short fatigue crack growth are monotonic in sign and could perhaps be simulated by a crack closure approach similar to that applied for long fatigue cracks. However the superposition principle which incorporates the compressive residual stress as a negative stress intensity factor, despite many successful applications to long fatigue cracks, is found to be inappropriate here. Hence one should be cautious when extending such a methodology to short fatigue cracks as it may result in optimistic life estimations.

A physically short fatigue crack growth approach based on low cycle fatigue properties

This work proposes an improved and simplified fatigue life equation called LAPS, formulated from low cycle fatigue data of smooth specimens and the Rice-Kujawski-Ellyin asymptotic field, with proper crack opening functions for closure effects. The model captures both long and physically short fatigue crack growth behavior, but the emphasis of this contribution is on the modifications for physically short cracks based on empirical models found in the literature. The predictions for physically short cracks from this model coincide well with experimental data for the railway axle used steel 25CrMo4. The predictions for long cracks match well with data from a variety of different metals. This makes the model a suitable alternative, e.g. to NASGRO, for engineering applications.

Effect of intermetallic particles and grain boundaries on short fatigue crack growth behaviour in a cast Al–4Cu–3Ni–0.7Si piston alloy

AbstractThe short fatigue crack growth behaviour in a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post‐mortem analysis of crack profiles. Crack–microstructure interactions have been clearly delineated using a combination of optical microscopy, scanning electron microscopy and electron backscatter diffraction. Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy.

Fatigue life prediction of 316 stainless steel in simulated PWR primary water by time domain analysis based on short fatigue crack growth behavior

Fatigue life prediction of 316 stainless steel in simulated PWR primary water by time domain analysis based on short fatigue crack growth behavior

Observation and modeling of physically short fatigue crack closure in terms of in-situ SEM fatigue test

Abstract Crack closure and opening behavior of physically short fatigue crack tip was investigated in microstructures with different grain sizes by using the in-situ SEM fatigue test. A gradually decreased crack closure zone near the crack tip was observed which depended on the microstructures, crack size and stress levels. A new parameter involving the load level, grain size, specimen dimensions and crack length was proposed to interpret the physically short crack closure development. The contribution of the gradient crack closure zone on the physically short fatigue crack growth behavior was evaluated by using the modified partial crack closure model. A generalized fatigue crack growth driving force was finally determined and correlated well with the physically short crack growth rate.

Micromechanisms of short fatigue crack growth in an Al–Si piston alloy

The short fatigue crack growth behaviour of a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post-mortem analysis of the crack profiles. Crack-microstructure interactions have been clearly delineated using a combination of optical microscopy and scanning electron microscopy. Results show that intermetallic particles and eutectic Al–Si regions play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and Al–Si eutectic regions. Crack deflection at intermetallics and eutectic Si is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy.

Effects of shot peening on short crack growth rate and resulting low cycle fatigue behaviour in low pressure turbine blade material

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 workhardening developed by the shot peening process. Once cracks in the notch root have penetrated this region in the depth direction, faster crack growth rates, similar to those observed in the ground case, were seen.

Three-dimensional in situ observations of short fatigue crack growth in magnesium

Three-dimensional (3D) short fatigue crack growth behaviour in a cast magnesium alloy, Elektron 21, was studied using a combination of X-ray diffraction contrast tomography (DCT) and microtomography at the European Synchrotron Radiation Facility. A 3D map of grain shapes and crystallographic orientations was produced in a miniature fatigue specimen using DCT. A focused ion beam instrument was used to introduce small notches in selected grains. Synchrotron microtomography was used to study the evolution of fatigue cracks from these notches during interrupted in situ fatigue cycling. Stage I crack growth occurred preferentially on the basal plane of the magnesium hexagonal close packed crystal, with local crack growth rates between 4 and 40 nm cycle −1. Retardations in the local crack growth rate were observed in certain grains and at certain grain boundaries. The observed interactions between the crack and the polycrystalline microstructure can be explained using Schmid factors and a development of the tilt–twist model of Zhai and Wilkinson (Zhai T, Wilkinson AJ, Martin JW. Acta Mater 2000;48;4917).

Short fatigue crack growth behavior under mixed-mode loading

Mixed-mode loading represents the true loading condition in many practical situations. In addition, most of the fatigue life of many components is often spent in the short crack growth stage. The study of short crack growth behavior under mixed-mode loading has, therefore, much practical significance. This work investigated short crack growth behavior under mixed-mode loading using a common medium carbon steel. The effects of load mixity, crack closure, and load ratio on short crack growth behavior were evaluated by conducting experiments using four-point bending specimens with several initial KII/KI mixed-mode ratios and two load ratios. Cracks were observed to grow along the paths with very small KII/KI ratios (i.e. mode I). The maximum tangential stress criterion was used to predict the crack growth paths and the predictions were found to be close to the experimental observations. Several parameters including equivalent stress intensity factor range and effective stress intensity factor range were used to correlate short crack growth rates under mixed-mode loading. Threshold values for short cracks were found to be lower than those for long cracks for all the mixed-mode loading conditions. Crack closure was observed for the entire crack length regime with all load mixity conditions at R ≈ 0.05 and for short crack regime under high load mixity condition at R = 0.5. Several models were used to describe mean stress effects and to correlate crack growth rate data.

Short fatigue crack growth behaviour of a ferritic steel weld metal

The present study deals with the short fatigue crack growth behaviour of a ferritic steel (nuclear grade SA 333 Grade 6 steel) weld metal at two load ratios R of 0.1 and –1. Single edge notched weld joint specimens were tested at different stress levels. The acceleration and deceleration in short crack growth with increasing stress intensity factor range was explained in terms of microstructure of weld metal. Acicular ferrite and grain boundary ferrite acted as barriers to crack growth. Non-propagating cracks were observed due to multiple cracks and blocking by ferrite grain boundaries. Transition crack length, the limiting crack length above which the crack exhibited a typical long crack behaviour, was determined to be 1 mm at R=0.1 and 1.5 mm at R=2 1.

101 Ti-6Al-4V合金平滑材の微小疲労き裂発生・進展挙動に関する研究(OS11-1 疲労破壊制御・機構解明への展開,オーガナイズドセッション11 地方ですべき、地方から世界に向けて発信できる材料力学的研究)

101 Ti-6Al-4V合金平滑材の微小疲労き裂発生・進展挙動に関する研究(OS11-1 疲労破壊制御・機構解明への展開,オーガナイズドセッション11 地方ですべき、地方から世界に向けて発信できる材料力学的研究)

Studies on short fatigue crack growth behavior of a plain carbon steel using a new specimen configuration

A simple experimental technique has been developed to examine short crack growth behavior vis-a-vis microstructural features of a material. The procedure involves the use of a new specimen configuration, fatigue testing with the help of a rotating bending machine and optical microscopic examination of crack profiles and their associated microstructures using an image analyzer. The growth characteristics of several small fatigue cracks in a 0.25% carbon steel have been studied using this technique. The critical crack lengths above which linear elastic fracture mechanics may be applicable, the fatigue threshold values at the transition of short to long cracks and the influence of microstructure on the crack path have been examined. The estimated critical crack lengths are considered to be dependent on the nature of the short cracks; but the fatigue threshold values obtained from short crack growth experiments are found to lie in a narrow range. The cracks were found to have affinity to grow through interfaces, but the length of a crack passing through a specific phase was found to be approximately proportional to the amount of the latter.

Application of the EBSD Technique to Study the Initiation and Propagation of Short Cracks / Einsatz der EBSD-Technik zur Untersuchung der Entstehung und Ausbreitung von kurzen Rissen

By means of electron back scattered diffraction (EBSD) in the scanning electron microscope (SEM) microstructurally short fatigue crack initiation and propagation has been characterised in the metastable beta-titanium alloy TIME-TAL®LCB (Ti-6.8Mo-4.5Fe-1.5AI). It was shown that local crystallographic orientation of individual grains influences the crack initiation and crack growth mechanisms. Grain boundaries are classified into high- and low-angle grain boundaries dependent on the orientation relationship of two neighbouring grains. During cyclic loading intergranular and transgranular cracks preferentially initiate at grain boundaries with highmisorientation angles because of stresses due to elastic anisotropy. Furthermore low- and high-angle grain boundaries influence the short fatigue crack growth behaviour, increase and decrease of crack propagation rate respectively, in different way.