Fatigue Crack Growth Properties Research Articles

Study on fatigue crack growth property of abrasive waterjet peened aluminum alloy

Abrasive waterjet peening is a favorable surface treatment method for improving the fatigue resistance of metal materials. An insight into the fatigue crack growth properties of AWJ peened specimens is meaningful for obtaining better strengthening performance. In present work, a numerical model of AWJ peening was established and experimentally validated for investigating the fatigue crack growth characteristics of Aluminum specimens. The effect of peening and loading conditions on the fatigue performance was also analyzed. The results indicated that the stress intensity factor at the peened region was enhanced and the crack propagation was significantly inhibited by the compressive residual stress. The influence of compressive residual stress on the effective stress factor range is greater under lower external load and higher loading ratio. The fatigue life for reaching the crack length of 40 mm is increased by 37%, 60% and 98% after peening by using the intensity of 0.6 mmA, 0.8 mmA and 1 mmA, respectively.

Size effects in fatigue crack growth in confined volumes: A microbending case study on nanocrystalline nickel

Mechanical size effects are a well known phenomenon when the sample volume is reduced or the characteristic length of the microstructure is changed. While size effects in micropillar compression (smaller is stronger) or due to grain refinement (Hall-Petch) are well understood, this is less so in fracture mechanics. Given this lack of knowledge, the main question addressed in this work is: What happens to the fatigue crack growth properties when extrinsic size effects play a role? To answer this question, nanocrystalline nickel cantilevers, ranging in width from 5 to ▪, were subjected to fatigue crack growth. The crack growth rates and stress intensity factors were calculated and the Paris exponent in the stable crack growth regime was determined. It was found that the results scatter more strongly for the smaller cantilevers compared to the larger cantilevers. Results are interpreted in terms of plastic zone size and ligament size which are found to be critical for small cantilevers.

Enhancement of the Microstructure and Fatigue Crack Growth Performance of Additive Manufactured Titanium Alloy Parts by Laser-Assisted Ultrasonic Vibration Processing

AbstractPost-processing techniques can efficiently improve the surface quality, address microstructural defects, and optimize mechanical properties in additively manufactured parts. Surface severe plastic deformation processes such as ultrasonic nanocrystal surface modification (UNSM) integrated with localized laser heating were explored to enhance the surface properties, microstructure as well as the fatigue crack growth properties (FCG) in both directions of built. We successfully induced greater plasticity flow and achieved beneficial refinement of the surface grain structure by precisely controlling the heat and impact energies during surface treatment process. The LA-UNSM process, with the parameters utilized in this study considerably decreased the FCG rates of treated samples, when compared to samples without surface treatment. Improved fatigue crack growth properties along vertical and horizontal orientations after the post-process treatment were attributed to the induced-microstructural changes, improved surface quality, induced compressive residual stresses through gradient structure deformation layer that was prepared on the surface of the material. The fractographic analysis revealed that the cracks mostly originated from the pores in the as-produced state. This observation shows a clear correlation between the surface treatment performed and a substantial improvement in fatigue crack growth resistance.

Effect of water vapour and H2S content on the hydrogen embrittlement sensitivity of low alloy steels for underground storage

A key point for the development of a renewable energy economy at a large scale is the possibility to use the current natural gas network and storage capacity to transport and store hydrogen. This study is dedicated to evaluate the integrity of materials used for underground aquifer storage regarding hydrogen embrittlement in a gaseous environment saturated with water vapour and containing few amounts of H2S. In this environment, the water vapour content in the gas stored may reach saturation at bottom well pressure and temperature. It is known that the presence of impurities in the gas, such as H2O, can promote or inhibit hydrogen embrittlement phenomenon, depending on the nature of the steel. This work investigates mechanical properties of a tempered martensite N80-Q steel, and a ferrite-perlite L360-NB C–Mn steel issued from a completion and a collect tube respectively. The testing environments are NG, NG + H2S, NG + 25%H2 and NG + 25%H2 + H2S saturated in water vapour at 8.5 MPa and room temperature. These environmental conditions aim at replicating the storage service conditions.Fracture toughness and fatigue crack growth properties are assessed. So far, the mechanical behaviour of such steels under hydrogen gas pressure saturated with water vapour and low amount of H2S has poorly been addressed. Regarding toughness properties, the two steels present different behaviour: for the L360 NB crack has not propagated for any testing environments, while cracks propagated in all the tests for the N80 Q. Despite this difference, for the two steels, the toughness does not seem to be affected by hydrogen as the results obtained in NG + 25%H2 + H2O and NG + H2O are similar. Based on the literature C–Mn steels toughness is affected by the presence of dry hydrogen. Hence, the results presented here show that H2O inhibits hydrogen embrittlement as far as toughness is concerned. The L360NB, however, presented a slight decrease in toughness properties with the combination of NG + 25%H2 + H2S + H2O. FCG (fatigue crack growth) results, on the opposite, clearly highlight the influence of hydrogen on the mechanical behaviour of the two steels. The FCG rates are faster from a factor five (resp. 10) in NG + 25%H2 + H2O compared to NG + H2O for the N80 Q (resp. L360). In this article, toughness and FGC results on the two steels are discussed in terms of microstructure and mechanical loading modes, aiming to quantify and better understand the influence of H2O and H2S on the sensitivity of low alloy steels to hydrogen embrittlement.

Fatigue crack growth properties of PMMA material used in underwater transparent structures

ABSTRACT Polymethyl methacrylate (PMMA) is an ideal material used in underwater transparent structures such as submersible windows and the fully-transparent pressure chamber of the sightseeing submersibles. The failure of PMMA structures usually come from small crack-like defects. In this paper, the fatigue crack growth experiments were carried out on PMMA materials with different thicknesses (B = 3, 6, 12 mm) and stress ratios (R = 0.1, 0.3) as well as plane strain fracture toughness test for unstable crack propagation conditions. Thickness effect and dwell time effect are experimentally investigated in addition to load ratio effect. The applicability of an improved crack growth rate model on PMMA material is validated by test results. The application of this model for prediction of crack growth rates can greatly save experimental time under different working conditions and can be applied in fatigue assessment of full-transparent structures under random loading.

Fatigue crack growth characterization of composite-to-steel bonded interface using ENF and 4ENF tests

In this paper, mode II fatigue crack growth properties of the composite-to-steel interface are characterised through different test configurations, namely ENF and 4ENF tests. Different loading types including force control and displacement control methods are compared. An innovative shear strain based method is proposed for monitoring the mode II crack growth at the bi-material interface through Digital Image Correlation (DIC). A 3D finite element model with Virtual Crack Closure Technique (VCCT) is built and used for obtaining the strain energy release rate (SERR) to investigate the effect of geometrical nonlinearity, friction at the interface and steel yielding, as well as to verify the mode mixity. The results show that the standard 3-point bending ENF specimen can be unstable under force control and sweeps narrow SERR range by a single test under displacement control. The 4-point bending 4ENF test shows stable crack propagation and clear SERR developing trend. More pronounced geometrical nonlinearity and friction effect exist for 4ENF test which can be considered when interpreting the Paris curves by a nonlinear finite element model.

The fracture toughness and fatigue crack growth properties of 18Ni 300 maraging steel manufactured by wire + arc additive manufacturing

Wire + arc additive manufacturing (WAAM) was used to manufacture the 18Ni 300 maraging steel component. The microstructure, tensile properties, fracture toughness, and fatigue crack growth resistance of the WAAM-ed 18Ni 300 maraging steel were investigated. The bottom regions showed unique microstructure: martensite, tempered martensite, ferrite, granular perlite, and carbide. The formation of tempered martensite in remelting regions would contribute to the following reasons: the cooling impact of the substrate; the partial remelting and long low tempering time. For the non-remelting regions, much ferrite would contribute to the organic mixture of ferrite and (granular) carbides, providing the environment of forming perlite. Meanwhile, numerous Ni3Mo and Ni3Ti IMCs were observed in the martensite matrix. The tensile properties of different regions and directions were different. The fracture toughness and fatigue crack growth resistance of horizontal sample was greater than the vertical sample due to the unique microstructures in the bottom region and weak interlayer combination. The WAAM-ed 18Ni 300 component had a great matching between fracture toughness and yield strength, which is similar to the SLM-ed 18Ni 300, illustrating the WAAM-ed 18Ni 300 components might have a better application potential in the aerospace equipment field.

Fatigue crack growth and mechanical properties of laminated TC4/TB8 titanium alloys fabricated by diffusion bonding

Fatigue crack growth and mechanical properties of laminated TC4/TB8 titanium alloys fabricated by diffusion bonding

Antagonistic fatigue crack acceleration/deceleration phenomena in Ni-based superalloy 718 under hydrogen-supply

Mechanical properties of structural alloys, including Ni-based superalloy 718 (Alloy718), are degraded when hydrogen (H) is supplied: hydrogen embrittlement (HE). The presence of H notably deteriorates fatigue crack growth (FCG) property, which renders the growth rate much higher and shortens the lifetime of the components operating in the hydrogenating environment. Hence, the mechanisms behind such acceleration phenomenon in FCG should be understood comprehensively toward developing promising alloys resistant to hydrogen occlusion. In particular, Alloy718 has a meager resistance to HE, even regularly displaying superior mechanical and physical performances. Notwithstanding, the present study unveiled that the FCG acceleration by dissolved H in Alloy718 can be negligible. An abnormal deceleration of FCG can instead be pronounced by optimizing the metallurgical state, a hopeful prospect in Ni-based alloys applied to the hydrogenating environment.

High cycle fatigue properties and fatigue crack propagation behavior of a bainite railway axle steel

Carbide-free bainite (CFB) steel with metastable austenite and refined microstructures has been demonstrated to be a promising alternative to conventional pearlitic rail and wheel steels, due to its outstanding static mechanical properties and performance in service. However, the exploration on its potential application in railway axles is scarce, and the effect of microstructures on the fatigue crack growth (FCG) behavior of CFB steels is not well understood. In this work, a CFB axle steel containing retained austenite (RA) and refined bainite is designed by austempering and low-temperature tempering. The static mechanical properties, high cycle fatigue (HCF) properties and FCG properties of the conventional high-speed railway axle steel (grade DZ2) and the CFB steel are evaluated. The tensile strength, uniform elongation and HCF strength of CFB steel are about 87%, 24% and 60% higher than that of the DZ2 steel, respectively. The FCG behavior of the two axle steels is almost equivalent in the Paris law region. In the DZ2 steel, crack deflection and secondary cracks inhibit the main crack growth, the grain bridging caused by the intergranular crack growth delays the crack opening at the crack tip. The crack propagates transgranularly in the CFB steel. When the crack propagation direction is parallel to the long axis of the bainite lath, the crack propagates preferentially along this direction. However, when the directions are nearly perpendicular, a zig-zag crack path formed near the fine bainitic lath can effectively prevent crack growth. Furthermore, the deformation induced martensitic transformation ahead of the crack tip enhances the crack-tip shielding.

Microstructure evolution and fatigue properties subjected to optimized coverage areas of laser shock peened TB10 titanium alloy

Laser shock peening (LSP) is an advanced surface modification technology, which can induce the high-strain-rate plastic deformation on the metal surface to improve mechanical properties. In this work, the effects of LSP on the microstructure, microhardness, residual stress and fatigue crack growth (FCG) properties of TB10 titanium alloy were investigated. The results showed that LSP could induce high density dislocations and realize the nanocrystallization on the titanium alloy surface. The magnitudes of compressive residual stress and microhardness increased with increasing LSP energy and impact times. The fatigue test showed that the LSP coverage area significantly affected the FCG performance, especially the fatigue life increased by 40.2 % when the LSP area was perpendicular to the cracks. This was attributed to the fact that the FCG rate of early cracks was significantly reduced by a wider region of the compressive residual stress field perpendicular to the crack direction.

Study on the Fatigue Crack Initiation and Growth Behavior in Bismuth- and Lead-Based Free-Cutting Brasses

Several studies have been conducted on the fatigue behavior of copper and 7-3, and 6-4 brasses. However, there have been fewer studies on the fatigue behavior and fatigue crack growth (FCG) properties of free-cutting brass, primarily because emphasis has been placed on the development of lead-free free-cutting brass. In this study, fatigue experiments were performed in the atmosphere at room temperature using three types of free-cutting, two types of bismuth (Bi)-based (with different grain sizes), and lead (Pb)-based brasses. It was found that lead-free Bi-based free-cutting brass had approximately the same fatigue performance as that of Pb-based free-cutting brass. It was also clarified that the addition of Bi or Pb initiated fatigue cracks, and that the crack growth period occupied most of the fatigue life. Differences in the FCG behavior of the three free-cutting brasses were observed in the low ΔK range. The modified linear fracture mechanics parameter M was used to quantitatively analyze the fatigue life and FCG behavior (short surface cracks). A comparison between the calculated and experimental results showed that M was useful.

Application of small geometry specimens to determine the fatigue crack growth anisotropy of Ti–6Al–4V additively manufactured for repair

Titanium alloys are key materials for aerospace and turbine industries. Direct energy deposition (DED) is one additive manufacturing method of interest for titanium alloy repair technology. One notable feature of directed energy deposition (DED) technique is the anisotropy of the microstructure due to the build directions and local heating during layer-by-layer formations. To explore the anisotropy in fatigue crack growth (FCG) properties, single-notched, compact specimens were cut and machined from thin “repaired coupon”, i.e., half AM and half wrought Ti–6Al–4V coupon, perpendicular and parallel to the build layers; the geometry was smaller than that required by standard crack growth testing procedures. Using these small coupons, fracture properties were attained for crack growth in either parallel or perpendicular directions with respect to the build layers. Direct current potential drop (DCPD) was utilized to monitor and measured crack length during fatigue. Optical and scanning electron microscopy was performed to investigate the failure mechanisms and microstructural morphology of polished cross-section and fractured surfaces, respectively. The results show that the crack growth direction parallel to the build layer (perpendicular to the build direction) had lower FCG resistance compared to crack growth direction perpendicular to the build layer (parallel to the build direction). The difference in the FCG rate is attributed to the microstructural features and the materials growth directions. Therefore, these clearly findings show that microstructure characteristics have strong influence on fatigue crack growth behavior in both directions of growth as observed in this study.

Resistance of pearlite against hydrogen-assisted fatigue crack growth

Recent progress in realizing hydrogen society requires higher-strength steels having decent hydrogen embrittlement resistance via optimized microstructural controls. The present communication first reports the fatigue crack growth (FCG) property of pearlitic steel in a high-pressure, i.e., 90 MPa, hydrogen gas environment to probe this standard microstructure's potency for becoming a constructive reinforcing agent in future hydrogen-compatible structural materials. As-transformed eutectoid steel with 1080 MPa-tensile strength exhibited hydrogen-induced FCG acceleration to a lesser extent relative to martensite with an equivalent strength level and occasionally to pure ferritic iron. Furthermore, the superiority over martensite was augmented under slow loading-rate conditions, suppressing time-dependent cracking, a critical issue in implementing defect tolerant design on high-strength steels for hydrogen gas applications.

Effect of aging on the fatigue crack growth properties of carbon-polyamide 6 thermoplastic composites using the multi ΔG-control method

This paper focuses on the fatigue crack growth behavior of unidirectional C/PA6 composites subjected to hydrolytic aging at temperatures ranging from 100 to 140 °C. The fatigue crack growth properties were determined using a multi-ΔG control test method. The latter is fully automatic and allows measurement of crack growth rates at various ΔG levels. It was shown that the fatigue crack growth properties were highly affected by aging. Whatever the aging temperature, the crack growth rate was increased by about a decade after aging, and the fatigue crack growth properties are highly dependent on the molar mass. Finally, it was shown that the traditional Paris law does not allow a full description of the behaviour so the Hartman-Schijve relationship was applied to characterize the fatigue response.

Assessment of mechanical and fatigue crack growth properties of wire + arc additively manufactured mild steel components

AbstractA study has been conducted to evaluate the mechanical and fatigue crack propagation properties of wire + arc additively manufactured ER70S‐6 components. A parallel‐built deposition strategy was employed to fabricate the additively manufactured wall. The hardness values were slightly higher at the bottom and top of the wall due to the presence of Widmanstätten ferrite and carbides. The characterization of mechanical properties in both orientations; parallel and perpendicular to the deposition direction showed a marginal difference in yield strength and ultimate tensile strength. The crack growth rates were correlated with linear elastic fracture mechanics parameter ΔK and compared with an oscillation‐built deposition strategy from the literature. The crack growth rates of both deposition strategies were found to be very similar to each other. Furthermore, it has been demonstrated that the variability in the crack growth histories can be reasonably well captured by using the NASGRO crack growth equation.

An investigation of the fatigue properties of the aluminium-copper alloy AA2014-T6

ABSTRACT An approach to determine the fatigue behaviour in the heat-affected zone (HAZ) of AA 2014 has not been investigated exhaustively since the HAZ has been treated as one region. In this study, fatigue crack growth (FCG), fatigue strength and low cycle fatigue (LCF) properties of the base metal (BM) and two regions of the HAZ were studied. The two regions of the HAZ were thermally simulated at 590°C and 650°C representing regions of the weldment 4 mm and 5 mm from the weld centreline respectively. The hardness tests and microstructure characterisation of the base metal and the two regions of the HAZ were also carried out. The HAZ region thermally simulated at 590°C was found to offer the least resistance to FCG, had shorter LCF life and lowest value of fatigue strength compared to both the BM and the HAZ region thermally simulated at 650°C. The cause of weakness in this region was associated with the dissolution of strengthening phases. The degradation of the fatigue properties of AA2014 occurred severely at 590°C due to the dissolution of CuAl2 precipitates in the aluminium matrix. The HAZ region thermally simulated at 590°C was confirmed to be the weakest link in the HAZ.

Study on fatigue crack growth behavior of selective laser‐melted Ti6Al4V under different build directions, stress ratios, and temperatures

AbstractThe experimental study of fatigue crack growth (FCG) behavior in Ti6Al4V alloy manufactured by selective laser melting (SLM) was carried out on an in situ fatigue testing machine. Two specimen orientations relative to the build direction (horizontal and vertical), two temperatures (room temperature, RT, and 400°C), and two stress ratios (0.1 and 0.5) were considered, and the FCG curves with the threshold values were determined. The results showed that the FCG properties were affected by the stress ratio due to different degree of crack closure and temperature with grain softening at 400°C in threshold and Paris region. The threshold value is highly dependent on the build direction of the alloy, which was caused by the anisotropic microstructure, while in the Paris region, the effect of material direction can be negligible and FCG rate tends to be consistent. The fracture mechanism at different stages was discussed and revealed by the fracture morphology observation using scanning electron microscopy.

Fatigue crack growth behavior and morphological analysis of natural rubber compounds with varying particle size and structure of carbon black

AbstractCarbon black (CB) is an essential ingredient of any rubber compound to achieve the desired strength, stiffness, wear, and fatigue resistance. Depending on the function of a tire component (tread, sidewall, apex, etc.), different types of CBs, varying in particle sizes and structures, are used. On prolonged exposure to cyclic loading, rubber compounds lose their strength due to mechanical fatigue. In this study, the fatigue crack growth (FCG) behavior of unfilled and CB‐filled natural rubber compounds is investigated with varying particle size and structure (N115, N134, N220, N234, N330, and N339). FCG properties have been measured using a Tear and Fatigue Analyzer under various strain levels and temperatures. Microscopic analysis revealed that compounds with lower particle size and high structure CB showed better distribution and dispersion of CB throughout the whole matrix. Lower particle size with higher surface area displayed superior FCG resistance compared to the higher particle size CB. FCG of above compounds has also been studied at three different temperatures such as room temperature (25°C), 70°C, and 100°C. Significant increase in FCG rate was observed with increase in temperature due to the thermo‐oxidative degradation and reduction of strain‐induced crystallization.

Effects of shot peening with different coverage on surface integrity and fatigue crack growth properties of 7B50-T7751 aluminum alloy

Engineering design methods based on the assumption of crack growth and the application of fracture mechanics principles are commonly used in aerospace engineering for the design of aluminum structures. In aerospace manufacturing, surface shot peening is one of the most widely used techniques for improving the fatigue performance of structural components. This work reviews the effect of different shot peening coverage on surface integrity and fatigue crack growth (FCG) properties of 7B50-T7751 aluminum alloy. The shot peening was applied to middle-crack tension (MT) specimens with four kinds of peening coverage: 100%, 300%, 600% and 1000%, respectively. The results showed that shot peening treatment reduced specimen surface roughness, moreover, surface shot peening promoted a maximum microhardness increased by 28.1% and induced compressive residual stresses in order of −412 MPa compared to untreated specimens. FCG experiment results showed that the effect of shot peening coverage on da/dN-ΔK curves was quite limited at constant amplitude loading. With the increase of shot peening coverage from 100% to 1000%, the FCG rate decreases first and then increase. At 300% of surface shot peening coverage, the FCG rate is the lowest, which results in the longest fatigue crack growth life.