Short Crack Growth Behaviour Research Articles

Modelling short crack growth under creep-fatigue interaction for different dwell times

The creep-fatigue interaction (CFI) short crack growth behavior of 316H steel under different dwell times at 550℃ was investigated in this work. The dwell time effect of the CFI short crack growth behavior is analyzed systematically, the results revealed that compared to the long cracks, the short cracks is more sensitive to the creep damage effect during the dwell period. Meanwhile, a CTOD-based CFI short crack growth rate predication model is proposed by introducing the creep effect during the dwell time. Compared with conventional fracture mechanics approach, the CTOD-based approach demonstrated a superior capability in providing a unified prediction result for the creep-fatigue interaction in short crack growth rates under different dwell times.

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.

In-situ observation of environmentally assisted crack initiation and short crack growth behaviour of new-generation 7xxx series alloys in humid air

In-situ observation of environmentally assisted crack initiation and short crack growth behaviour of new-generation 7xxx series alloys in humid air

Plastic stress concentration effects in fatigue strength

Elastoplastic (EP) stress gradient factors (SGF) ahead of notch tips are used to evaluate the actual notch effects in fatigue strength due to their tolerance to non-propagating short cracks, which depends on the notch stress concentration factor, on the stress gradient ahead it, and on the material. Considering the significant role of local plasticity in the growth behavior of short cracks within the notch plastic zone, a sound mechanical methodology is proposed to account for the effect of EP stress and strain fields in the actual fatigue stress concentration factor value, which includes its tolerance to short cracks. To validate the proposed methodology, life predictions based on numerical EP SGF are compared to experimental SN data of notched specimens made of different materials tested under uniaxial loads collected from the literature.

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.

Experimental characterization of the short crack growth behavior of a ductile cast iron (DCI GJS-500) affected by intergranular embrittlement at temperatures nearby 400 [formula omitted

In this investigation, the fatigue and crack growth behavior of the ductile cast iron material (DCI) GJS-500 was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests, out-of-phase thermomechanical fatigue (OPTMF) tests as well as low cycle fatigue tests combined with the replica testing method (LCF-R) within the temperature range RT–500 °C. The studied material exhibits an embrittlement at temperatures nearby 400 °C, leading to a significant higher fatigue crack growth rate and a reduced lifetime, when compared with RT and 500 °C. A possible explanation for the observed higher fatigue crack growth rate coupled with a significant lifetime reduction, is intergranular embrittlement.

Experimental investigation of the damage characteristics of two cast aluminium alloys: Part II – LCF/HCF and TMF/HCF loading with special focus on the short crack growth behavior

In this work, the influence of superimposed high cycle fatigue on the LCF/HCF and TMF/HCF lifetime is investigated for two cast aluminium alloys of the types AlSi7 and AlSi12. The replica technique is used to examine the short crack growth behavior under pure LCF and LCF/HCF loading. The observed short crack growth evolution explains the observed lifetime reduction with increasing HCF amplitudes.

Effect of microstructure on short fatigue crack growth of wire arc additive manufactured Ti-6Al-4V

In the present paper, the short fatigue crack behavior in wire arc additive manufactured (WAAMed) Ti-6Al-4V was investigated. It was found that the crack in the colony microstructure showed the highest fluctuation range (213 μm) because the crack deflected at the colony boundary. While the crack in the duplex microstructure showed the lowest fluctuation range (48.3 μm) because the crack cut through the αp (primary α) and αs (secondary α) directly. In addition, the short crack growth behavior was determined by the highest Schmid factor (M) and the burgers orientation relationship (BOR), which indicated that the short crack tended to pass through the α/β interface along with the basal slip systems within the shortest path distance. It was suggested that the short crack behavior with colony microstructure was determined by the α/β interface while the crack growth behavior with duplex microstructure was determined by the critical resolved stress (τCRSS) and the αs/β interface.

Short-Crack Growth Behavior in Additively Manufactured AlSi10Mg Alloy

Short-Crack Growth Behavior in Additively Manufactured AlSi10Mg Alloy

Growth behavior of short fatigue cracks in a unidirectional SiC fiber-reinforced titanium matrix composite under spectrum loading

In metal matrix composites, the propagation of matrix crack normally leads to load redistribution within the fiber and matrix. The variation of matrix load, on the other hand, affects the growth of the crack in the metal matrix. In this paper, an approach to predict the matrix crack growth rate of a unidirectional SiC fiber-reinforced titanium matrix composite (TMCs) during spectrum loading is developed. The matrix crack was regarded as a mode I central crack. The effects of crack closure and high load hysteresis caused by the variability of were considered. Based on the Forman model, the matrix crack growth rate was also obtained. The results show that the growth rate of matrix crack decreases with the increase of the number of cycles, which can be attributed to the decrease of the stress intensity factor of the matrix crack resulting from the decrease of the load exerted on the matrix. The decreased load of the matrix was caused by the increase of the fiber bridging stress and the redistribution of the stress within the composite when the matrix crack length increases.

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.

An artificial neural network modeling approach for short and long fatigue crack propagation

Fatigue crack growth-based damage modeling approaches have received great interest due to its critical importance in the industry. However, a substantial deficiency of an explicit fatigue damage model to quantify the accurate fatigue crack growth behavior remains due to the complex fatigue crack growth behavior in different length scales. This complexity arises from the fact that fatigue crack growth (FCG) in different length scales depends on many damage controlling parameters. Machine learning-based fatigue damage modeling approaches have received noticeable attention for fatigue crack growth analysis due to their abilities to account for numerous damage parameters simultaneously. In the presented paper, a radial basis function artificial neural network (RBF-ANN) model has been developed to predict the FCG behavior, including the short and long crack regimes. The presented RBF-ANN model has been trained and verified by experimental data sets of Ti-6Al-4V titanium alloy, 2024-T3 and 7075-T6 aluminium alloys. The predictions showed that the RBF-ANN model has a good interpolation capability to predict the nonlinearity of both short and long crack growth behavior. However, the model shows poor extrapolation capability for accurate short crack growth predictions for cases that there are limited data sets in hand. The model effectiveness greatly depends on sufficient available input data.

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.

A Surrogate Approach to Reveal Microstructural Mechanisms Controlling the 3D Short Crack Growth in a Ti-6Al-4V Alloy

Microstructural features and short crack growth behavior were characterized and linked in a Ti-6Al-4V by employing X-ray micro-tomography combined with EBSD serial sectioning. Statistical analysis was used to rank the contributing features to the crack behavior. Afterwards, by creating surrogate models, the microstructural mechanism controlling the short crack behavior were revealed. Short crack preferably grows inside the predominant α phase above the average microstructural fraction. A high number of grains in contact with cracked α grains elongated in the loading direction may impose a constraint on the crack opening resulting in low crack growth rates. As the crack front becomes larger, the increase in the shear stress field away from the cracked grain leads to crack bifurcations, resulting in a decrease in crack driving forces with low crack growth rates. This leads to a preferable growth in α+β phase and along the interface above the average microstructural fractions.

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.

Effect of shot peening on short crack propagation in 300M steel

The short crack growth behaviour of high strength martensitic 300M steel was studied for as-machined, polished and 2 peening conditions subjected to axial fatigue at 2 stress amplitudes. It was observed that prior austenite grain boundaries acted as barriers to short crack propagation in peened specimens within the peening induced compressive residual stress layer, causing crack decelerations upto 6 grains. Using the da/dN-ΔK trend of the short cracks, fatigue lives of the peened specimens were predicted at both the stress amplitudes. These analytical predictions (which reflect the effect of microstructure and compressive residual stresses) were within 15% of the experimental fatigue lives.

On the behaviour of non-propagating cracks in steel and aluminium castings

Casting defects as well as sharp notches can serve as initial points for fatigue cracks under cyclic loading. On the contrary, the cyclic material crack resistance counteracts to crack propagation. Hence, an incipient crack can arrest completely, even though a constant cyclic load is applied. Considering a short propagating crack, the local stress intensity factor, which represents the crack growth driving force, increases due to the crack extension, yet crack arrest may occur. Such an arresting of growing cracks can be traced back to an increase of the materials resistance in terms of crack closure effects. This paper deals with the evaluation of the cyclic short crack behaviour and the assessment of non-propagating cracks for cast aluminium and cast steel in terms of the Frost diagram. At first, single edge notched bending specimens are machined out of both cast materials and investigated in regard to their fracture mechanical short and long crack growth behaviour under varying stress ratios. Moreover, the data is analysed in terms of the short crack resistance (R-curve) and the subsequent long crack growth behaviour. Secondly, detectable surface porosity defects, regarding the aluminium material, and sharp surface notches, as in case of cast steel, are investigated by a linear elastic finite-element simulation to evaluate the local stress concentration factors numerically. Finally, Frost diagrams are set up for both materials and well approved by the detected non-propagating cracks. A final comparison of the investigated fatigue test data points and the Frost diagrams reveals a sound agreement validating the applicability of the presented approach.

Effects of 3D microstructural distribution on short crack growth behavior in two bimodal Ti–6Al–4V alloys

In situ X-ray micro-tomography was performed to analyze the short crack growth in the 3D microstructure of two bimodal Ti–6Al–4V alloys with predominant α+β and α phase volume fractions. Short crack strongly interacts with the local microstructure and it grows into the predominant phase above the average microstructural fraction in each alloy. The higher volume fraction and size of the α grains in the microstructure induce crack growth with lower variations and larger deflection lengths inside the α grains. As the short crack length increases, the larger deflections left behind the crack front induces higher crack closure that decreases the crack driving forces. As a result, the short crack remains sensitive to the local microstructural features at higher lengths.

Welding parameter selection and short fatigue crack growth of dissimilar aluminum alloy friction stir welded joint

The welding process parameters of dissimilar 2024-T4 and 7075-T6 aluminum alloy friction stir welded (FSWy) joints are selected by the metallographic microstructure, the mechanical properties, and the hardness distribution. The joints with rotational speed of 900 rpm and advancing speed of 100 mm/min that have the highest tensile strength are used to carry out the experimental fatigue research. The short crack growth behavior of dissimilar FSW joints is analyzed through the replica technique. The stage of the crack initiation and stable growth accounts for about 60~80% of total fatigue life. The short crack grows rapidly once it reaches the crack size of approximately 350~450 μm. The cracks are prone to initiate at the defects of subsurface in the weld nugget zone and propagate in a quasi-cleavage fracture mode.