Fatigue Crack Growth Behavior Research Articles

Microstructure and Fatigue Performance of Ti6Al4V Produced by Laser Powder Bed Fusion after Post-Heat Treatment

With the development of additive manufacturing (AM), the Ti-6Al-4V alloy manufactured by laser powder bed fusion (LPBF) is becoming more widely studied. Fatigue fracture is the main failure mode of such components. During LPBF processing, porosity defects are unavoidable, which hinders the exploration of the relationship between fatigue performance and microstructure. In this study, a laser remelting method was used to reduce porosity defects inside the Ti-6Al-4V alloy. Three annealing treatments (AT) and three solution-aging treatments (SAT) were used to study the effect of the two-phase zone (α + β) microstructure on fatigue life and fatigue crack growth behavior. Fatigue life and crack growth rate (CGR) curves were obtained, and fatigue fracture surface and crack growth fracture surface were analyzed. The results show that microstructure influences fatigue life but has little effect on CGR. Compared with the as-built specimen, the fatigue life of the AT and SAT specimens increased significantly at 850℃ by 101 and 63.7 times, respectively. The thickness of the α lath and the location of crack nucleation together affect the fatigue life. In the stable growth stage, the layered microstructure of α colonies is the most resistant to crack growth.

Fatigue behaviour of multi-spot joints of 2024-T3 aluminium sheets obtained by refill Friction Stir Spot Welding with polysulfide sealant

The aeronautical industry is looking with interest at friction-based welding for many years due to weight-saving capacities, where fusion-based welding problems can be avoided, allowing also the joining of materials considered complicated to weld with other technologies. Refill Friction Stir Spot Welding (refill FSSW) is a solid-state joining process that shows great potential to be a substitute for single-point joining processes like riveting. The main objective of this study is to investigate the mechanical behaviour of multi-spot joints of AA 2024-T3 obtained by refill FSSW with the addition of a sealant commonly used in riveted joints by the aviation industry. A fatigue test campaign was carried out on the joints with and without sealant to evaluate the possible synergy between the refill FSSW and sealant. The fatigue data obtained are statistically analyzed and compared. According to the comprehensive study including macro- and micro-structure, deformation strain field, residual stress distribution as well as the fatigue crack growth behaviour, the underlying history of multi-spot joints fatigue damage is revealed. The overall results prove that producing lap-joints by refill FSSW with sealant improves considerably the fatigue performance.

Dual precipitate simultaneous enhancement of tensile and fatigue strength in (FeCoNi)86Al7Ti7 high-entropy alloy fabricated using selective laser melting

Recent studies have indicated that precipitation-strengthened high-entropy alloys (HEAs) show superior mechanical performance and have been successfully fabricated by additive manufacturing. However, the lack of fatigue and fracture research has limited the engineering applications of additive manufacturing HEAs. This work explored a dual precipitation-strengthened (FeCoNi)86Al7Ti7 HEA with excellent tensile and fatigue strength, prepared through selective laser melting and heat treatment. Compared with the as-built samples, the tensile properties and fatigue endurance limit improved through aging by 48.7% and 30%, respectively. The strengthening mechanism and enhanced fatigue performance were clarified in detail. The improvement in fatigue strength was attributed to the improved resistance of the L12 and L21 precipitates. During deformation, the dislocation shear coherent L12 precipitates reduced slip band energy and inhibited slip band expansion, while the L21 particles acted as obstructions for further slip band propagation, severely limiting the rapid formation and propagation of crack growth. In-situ TEM cyclic tensile-tensile testing also clarified the fatigue crack growth behavior, demonstrating that crack deflection due to L21 precipitate obstruction slowed down the crack growth rate and efficiently promoted the closure of the microcrack tips. This work offers implications for a new strategy to develop additive manufacturing HEAs.

Notch fatigue fracture and crack growth behaviors on a steel sheet under out-of-plane bending

For industrial products made of sheets, it is crucial to prevent the formation of fatigue cracks and the rapid destruction by stress risers. To carry out tests under out-of-plane bending load, this study used not only smooth sheet specimens made of a common structural steel, but also specimens with a through-wall circular hole in the center and specimens with a V-shape notch on one edge of the plate. The fracture mechanics parameters were investigated while fatigue crack initiation, growth, and the process leading up to a break were observed. In the high cycle region, the notch factor of the Hole specimen was 1.07, and that of the V-notch specimen was 2.22. With the same initial set stress amplitude, the V-notch specimen had fewer cycles at the time of crack initiation and faster crack growth rate than the Hole specimen. The state and the extent of crack initiation and growth were affected by the strain concentration and cyclic loading levels with different notch geometries. The threshold and unstable crack growth stress intensity amplitudes were identified, while the fracture mechanism and affecting factors were examined. By extending the Paris-Erdogan equation, a crack growth rate prediction model was proposed and validated with good approximation accuracy. It becomes possible to quantitatively predict the occurrence of fatigue, crack propagation, fracture, and fatigue life in industrial machinery and structures made of similar materials in out-of-plane bending.

Fatigue crack growth behavior of marine steel under variable amplitude loading-combining DIC technique and SEM observation

This paper experimentally studies the fatigue crack growth (FCG) behavior of AH36 grade marine steel under variable amplitude loading. FCG testing is conducted using compact tension specimens. Digital image correlation technique (DIC) is employed to study the evolution of strain distribution and cyclic fatigue characteristics. Fractographic analysis is performed to explore the overload retardation mechanism. Results indicate that the FCG rate varies significantly under overload application, leading to the extension of fatigue life by 1.03, 1.18 and 2.42 times under the overload ratios (OLRs) of 1.5, 2.0 and 2.5. Reasonable evaluation of the fatigue performance depends on the loading history. Extension of the fatigue life for periodic overload (POL) cases is not a simple superposition of that under two single ones, the interaction effect between the adjacent tensile overloads cannot be neglected. Greater plastic dissipated energy corresponds to more severe plastic deformation at larger OLRs and crack lengths. The larger strain accumulation at the crack tip should account for the greater FCG resistance and more evident overload retardation effect. The fresh fracture surface is destroyed at a certain region after overload due to the premature contact of the crack flanks, confirming that overload induced crack closure exists.

Study of fatigue crack growth behaviour of Al-Mg and Al-Mg-Sc-Zr alloys

ABSTRACT Fatigue crack growth behaviour of Al-Mg and Al-Mg-Sc-Zr alloys is studied under different orientations and stress ratios. The microstructures and the fracture morphologies of each alloy are observed. For both alloys, they have Brass, S and Goss texture; while only Al-Mg alloy has Cube texture which indicates recrystallisation; as fatigue crack propagated, intergranular fracture transformed into transgranular fracture accompanied with gradually coarser crack surfaces. Al-Mg alloy is featured by large recrystallised grain, favourable plasticity, rougher crack surfaces, and with relatively strong crack growth resistance before rapid growth. While Al-Mg-Sc-Zr alloy with small grain size and relatively smoother crack surfaces, shown cyclic stability at higher ΔK level.

Fatigue crack growth behavior of 42CrMo high-strength steel with tempered sorbite/bainite microstructures: Roles of grain and constituent in microstructures

Microstructural differences through the thickness have always been an attention focus for large-scale steel structures, since they could significantly influence fatigue crack propagation (FCP) characteristics. Here, the effects of microstructure in FCP behaviors of 42CrMo steel were analyzed with emphasis on two different tempered sorbite/bainite (TS/B) microstructures: (i) fine TS/B structures (M42); (ii) coarse TS/B structures (M52). The results show that the FCP rate is relatively slower for M52 than for M42 in the regime through the later stage of near-threshold (I-L) to the early stage of Paris-regime (Ⅱ-E) with low ΔK, but is fairly similar for the two cases in the other ΔK regimes. Under relatively low ΔK, the FCP path of M52 is more tortuous with numerous micro-cracks in B and larger-sized crack deflections at the carbides and boundaries compared to M42, especially in the I-L where fatigue micro-cracks can deflect at the single coarse carbide, thereby resulting in roughness-induced crack closure and crack retardation behavior. Theoretical analysis shows that FCP behaviors are closely related to the cyclic plastic region size (Δrp) at the crack-tip. In the early stage of near-threshold region, Δrp is restricted within a grain-size scale for both steels, thus leading to a similar FCP rate. However, from the I-L to the Ⅱ-E, coarse B/ferrite-grains in M52 are always larger than the Δrp of fatigue crack-tip, and thereby lead to numerous micro-cracks formation and crack deflections at the carbides and B boundaries, inducing crack closure. In the later stage of Paris region, B/ferrite-grains in the two cases are much smaller than Δrp, so that FCP behaviors depend on the microstructure group effects, or overall strength and fracture toughness of materials, which cause the close FCP rate.

Influence of crack configurations on fatigue crack growth behavior of Al 2024 alloy

The effect of inter crack distance on fatigue crack growth (FCG) on the multi-crack computational domain in Al 2024 was simulated using the extended finite element (XFEM) method. An in-house developed MATLAB code was used to simulate the experimentally reported FCG behavior of side edge crack and center cracked specimen. Numerical simulation was performed by using Nasgro constitutive model and this model was chosen over Paris equation due to the capability of predicting all regions (i.e., Region I, II, and III) of FCG regime. The empirical model constants of the Nasgro equation were calculated by using a genetic algorithm (GA). The genetic algorithm was implemented to optimize the model constants by minimizing the average error between simulated and experimental FCG regimes. This method was then extended to the multi-crack computational domain to investigate the effect of crack spacing on FCG rate. Al 2024 alloy is widely used in structural applications in the aerospace and automobile sector. Therefore, the prediction of fatigue crack growth behavior, especially in the high cycle fatigue region, becomes very important for predicting the damage tolerance of the material. In high cycle conditions, the stresses involved are lower than the yield stress, and hence the Linear elastic fracture mechanics (LEFM) approach was used. This LEFM approach was coupled with the XFEM method for comparing the fatigue crack growth behavior in the single crack and multi crack domains in the present work.

Toward developing Ti alloys with high fatigue crack growth resistance by additive manufacturing

Fatigue crack growth behaviors were investigated by three-point bending tests for TA19 alloy fabricated by laser metal deposition and four kinds of heat-treated samples. The crack growth resistance of the TA19 samples in the near-threshold regime and Paris regime was evaluated through the experimental characterization and theoretical analysis of the interaction between fatigue crack and α/β phase interface, columnar prior-β grain boundary and colony boundary. The results show that in the near-threshold regime, the fatigue crack propagation threshold and resistance increase with the increase of widths of lamellar αp phases and colonies, and the decrease of the number of α laths with an angle (φ) relative to the applied stress direction ranging from 75° to 90°. In the Paris regime, the fatigue cracking path can be deflected at colony boundaries or columnar prior-β grain boundaries. The larger the deflection angle, the more tortuous the cracking path and the lower the fatigue crack growth rate. The angle (γ) of the columnar prior-β grain growth direction relative to the build direction affects not only φ of different α variants, but also the fatigue cracking path deflection angle (θij) at columnar prior-β grain boundaries. An optimal combination of γ = 0°-15°-0°-15° for several adjacent columnar prior-β grains is derived from the theoretical analysis, and that can effectively avoid φ being in the range from 75° to 90° and make θij as large as possible. Such findings provide a guide for the selection of scanning strategies and process parameters to additively manufacture Ti alloys with high fatigue damage tolerance.

Experimental investigation of the fatigue crack growth behaviour in SLM additively manufactured 17-4 PH stainless steel specimens

Experimental investigation of the fatigue crack growth behaviour in SLM additively manufactured 17-4 PH stainless steel specimens

Effect of Stress Ratio on Fatigue Crack Propagation Behavior of Single Crystal Superalloy

In order to study the impact of different stress ratio on the fatigue crack growth behavior of DD6 single crystal superalloy, the fatigue crack growth rate test of different stress ratio (R = 0.05, 0.2, 0.5, 0.6) at 530 °C was conducted, the influence of stress ratio on crack growth rate was studied, and the growth of crack extension rate was corrected by normalization method. The results show that the section includes the fatigue fracture section perpendicular to the load and an angle to the load. The flat section is the main section representing the crack extension. The oblique section is formed with the change of the crack extension load state, and its occurrence is related to the dispersion of (da/dN)−Δk data, reflecting the uniqueness of single crystal. The Paris formula better characterized the crack expansion behavior of DD6 alloy, and the stress is more sensitive than the crack growth rate in the early stable growth phase. Crack growth rate normalized characterization equation constructed under different stress ratios are predicted to be within the 1.3-fold dispersion band.

Fracture toughness and fatigue crack growth behaviour of laser powder bed fusion (LPBF) built Ti-6Al-4V alloy through XFEM

Fracture toughness and fatigue crack growth behaviour of laser powder bed fusion (LPBF) built Ti-6Al-4V alloy through XFEM

Study on fatigue crack growth behaviour of DEDed Ti-6Al-4V alloy through XFEM

Study on fatigue crack growth behaviour of DEDed Ti-6Al-4V alloy through XFEM

Effect of Pre-Strain on Crack Growth Behavior of Zr702/TA2/Q345R Composite Plate

The influence of pre-strain on the fatigue crack growth behavior of Zr702/TA2/Q345R composite plate is studied by experiments and the finite element method. The crack perpendicular to the interface and the through-wall crack are considered at the same time. For the crack perpendicular to the interface, the monotonic plastic zone and cyclic plastic zone at the crack tip are used to study the influence of pre-strain on the plastic zone. Furthermore, the influence of pre-strain on the evolution of the plastic damage at the crack tip is analyzed in detail by studying the variation in the initial plastic energy and equivalent plastic strain. For the through-wall crack, the effect of pre-strain on the propagation behavior of cracks on both sides and the whole crack is studied systematically. The results show that the strengthening of the cracks on the Zr702 side is significantly higher than that on the Q345R side, so the inhibitory effect of pre-strain on the whole crack of the through-wall specimen is mainly due to the increase in the resistance to crack growth on the Zr702 side.

Material damage integration approach for efficient modelling of high cycle fatigue

The recognition of the risk of fatigue failure and its study, particularly fatigue crack growth (FCG) behaviour of engineering materials, is not neoteric, and the majority of approaches for investigating the problem are empirical and dated. With recent computational advances, the cohesive zone modelling (CZM) approach for FCG analysis has become popular especially amongst researchers owing to its flexibility of use particularly within the finite element framework. However, the use of the CZM for explicit cycle-by-cycle high cycle FCG analysis of real structural components is still largely computationally prohibitive. Thus, this study presents a novel material integration (MI) approach to accelerate fatigue crack propagation within the cyclic cohesive zone modelling (CCZM) framework. Using a bilinear cohesive law, the proposed technique is compared with the Linear Extrapolation (LE) technique and assessed for three models of different bulk-interface element discretisation and deformations. The results show that the MI technique offers a more consistent approximation to the accelerated fatigue damage computation and, more importantly, better convergence characteristics for the different models under tension, mixed mode and bending deformations. These outcomes underline the computational benefits of the proposed MI technique in assessing the FCG behaviour of real structural components within the CCZM framework.

A multiscale model to predict fatigue crack growth behavior of carbon nanofiber/epoxy nanocomposites

In this study, we developed a multiscale model for predicting the fatigue crack growth behavior of carbon nanofiber (CNF)/epoxy nanocomposites. Here, we focus on considering the effect of experimentally observed microscopic toughness mechanisms (CNF debonding, subsequent plastic nanovoid growth, and pull-out of CNFs) to predict the fatigue crack growth behavior of CNF/epoxy nanocomposites. The predictions of the multiscale model exhibited a high degree of conformity with experimental data. The results of this study are expected to elucidate the complex phenomenon of fatigue crack growth in nanocomposites with randomly distributed CNFs and provide highly accurate predictions.

Fatigue Crack Growth Behavior of Nitrogen-Alloyed Low-Carbon Austenitic Stainless Steel at Room Temperature

Fatigue Crack Growth Behavior of Nitrogen-Alloyed Low-Carbon Austenitic Stainless Steel at Room Temperature

Effect of post-weld heat treatment on mechanical properties and fatigue crack growth behaviour of friction stir welded 7075-T651 Al alloy

The effect of post weld heat-treatments (PWHT) on the mechanical and fatigue crack growth of friction stir welded 7075-T651 aluminum alloy was investigated. The PWHT of natural aging (NA), artificial aging at 190 °C for 24 h (HT1), and 423 °C for 12 h (HT2) resulted in significant improvement in the mechanical properties of the welded joint as compared to the base metal. The highest improvement in mechanical strength is seen in HT1 post heat treatment followed by NA and HT2 post heat treatments. The ductility of welded joint for PWHT at 190 °C with a soaking period of 24 h is about 70 % of the base metal. Crack growth tests at constant amplitude loading (CAL) and single tensile overload (OL) with an overload ratio of 1.8 were conducted at a stress ratio of 0.1. A difference in the crack growth rates after application of overload is seen in all PWHT conditions. Different heat treatments did not change the growth behaviour in base metal in both the loading cases. However, a slight difference in crack growth rates is observed due to the application of overload. The crack growth acceleration followed by delayed retardation is observed due to the application of overload. The post weld heat treatment enhances the retardation effect due to tensile overload and the degree of enhancement depends on the PWHT.

Thermal fatigue crack growth behavior of Ni60 coating and bonding area on 20CrNiMo alloy strengthened by laser shock processing

After the laser cladding repair of the brake disc surface in high-speed trains, thermal fatigue cracks in the cladding layer and bonding area are easily observed during the service process. With the aim of solving this problem and decreasing hidden risks, effects of laser shock processing (LSP) on the thermal fatigue life and crack growth behavior of the coating and bonding area of the brake disc were investigated. The results indicate that LSP can refine the columnar grain structures of the cladding layer and bonding area while simultaneously converting residual tensile stress into compressive stress; as a result, the thermal fatigue performance of the brake disc repaired by laser cladding is enhanced. After strengthening by LSP, on the one hand, grain refinement made the structure more uniform and generated more grain boundaries; on the other hand, a pre-set compressive stress field in the cladding layer and bonding area existed. Both aspects were beneficial to dissipate the alternating stress generated by the thermal fatigue process, dissipating the driving force of crack extension and promoting the transformation of thermal fatigue cracks from lateral growth along the interface between the cladding layer and bonding area to the growth into the substrate.

Two‐parameter analysis of fatigue crack growth behavior in structural acrylic adhesive joints

AbstractConducting displacement‐controlled fatigue double cantilever beam tests of a structural acrylic adhesive, two different slopes were observed in the fatigue crack growth (FCG) relationship of the FCG rate versus the strain energy release rate (SERR) parameters depending on the loading conditions. Because fatigue is a two‐parameter problem where stress waveforms contain cyclic and monotonic terms, the contribution of each term toward FCG needs to be clarified. Therefore, each slope was explained using SERR with load range and mean load. In addition, cases were observed where the slope changed with an increasing number of cycles. Accordingly, the FCG rate changes. Understanding the phenomenon is important to estimate the crack position from an engineering point of view. Therefore, FCG was categorized into three types using amplitude and mean displacement. After a sufficient number of fatigue cycles, amplitude played a more dominant role than R‐ratio or mean displacement to determine crack location.