Fatigue Crack Growth Characteristics Research Articles

Fatigue crack growth test and characteristics analysis for ultra-thick crack-arrest steel

The fatigue crack growth rate of materials is an essential aspect of the fatigue life prediction. Within the container ship industry, crack-arrest thick steel plates are extensively utilized to mitigate the fracture risks caused by the increasing vessel dimensions. However, it remains deficient in comprehensive investigations into its fatigue crack growth characteristics. This study conducted a sequence of experiments on 85 mm crack-arrest steel (EH47BCACOD) plates. The experiments include tension tests, through-thickness crack growth tests, and surface crack growth tests, while various thickness-based sampling locations, directions, and stress intensity factor ratios are designated. The results illustrate that the crack-arrest steel exhibits better crack growth resistance than conventional marine steels. Regarding non-homogeneity in the thickness direction, variations are observed among ultra-thick plates situated at different thickness locations. The crack growth rate near the surface of plate is lower than that at interior of plate, resembling the behavior exhibited by the tensile test. Furthermore, evident anisotropy is identified in the resistance of fatigue crack growth.

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.

Characterization of fatigue crack growth behavior in welded tubular T-joint

This paper focuses on investigating the fatigue crack growth (FCG) characteristics and residual fatigue life of tubular T-joints which are prone to suffer fatigue damage at the brace and chord intersections under multi-axial stress. Static and fatigue loading tests are performed to investigate the FCG behaviors of tubular T-joints, combining beach marking technique. The evolution of FCG characteristics is studied during the crack growth, convergence and wall penetration through an efficient co-simulation system that established using the multi-scale modeling technique. Fracture morphology analysis is conducted to gain insight into the FCG behaviors combining the scanning electron microscope (SEM) observation. Results indicate that multiple cracks initiate at the crown regions and subsequently evolve circumferentially around the weldments in a doubly-curved shape towards wall-thickness. Interaction effects between adjacent cracks extend the fatigue life along the outer surface, attributed to the premature exposure of converged crack front and the induced variation of stress intensify factor (SIF) distribution along the crack front. The dominant failure mode of tubular T-joints is characterized by an opening mode crack, with the contribution of anti-plane shear mode crack gradually increasing as structural symmetry diminishes. The established co-simulation system shows advantage in capturing the FCG behavior, predicting the fatigue life and characterizing the FCG characteristics with a good balancing of simulation efficiency and calculation accuracy.

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.

Fracture behavior of PH15-5 stainless steel manufactured via directed energy deposition

The tensile properties, fracture toughness, and fatigue crack growth (FCG) characteristics of a directed energy deposited precipitation hardened stainless steel (grade: PH15-5; age hardening heat treatment condition: H900) were examined. In the as-fabricated condition, the alloy contains microfissures that are oriented parallel to the build direction, whose appearance was difficult to be detected using optical microscopy. Due to their relative orientation w.r.t. the loading direction, significant anisotropy in tensile and FCG behavior was noted, with the properties being particularly lower when the loading direction is perpendicular to the crack orientation. Despite the presence of microfissures, the alloy’s fracture initiation toughness is comparable to (or in some cases exceeds) those manufactured using either conventionally techniques or laser powder bed fusion. Activation of the extrinsic toughening mechanisms, such as crack deflection when its mode I direction is perpendicular to the microfissures and a combination of crack bridging and deflection when it is parallel, are the micromechanical reasons for the observed high toughness. The efficacy of such mechanisms is observed to depend on the plastic zone size relative to the microfissure spacing. The understanding developed in this study enables the development of strategies for enhancing the damage tolerance of additively manufactured alloys.

Sensitivity of crack-growth lives to sustained thermal gradients

This paper investigates the effect of path-dependent material properties on predicted crack-growth lives with material properties set at the instantaneous crack-tip location. Specifically, we focus on cracks advancing through thermal gradients with temperature-dependent crack-growth properties. Here, material properties that describe fatigue crack growth under cyclic load and time-dependent crack growth under sustained load have been set to capture the response of the high-temperature alloy ME3. This study investigates the effect of peak temperature, peak stress, temperature gradient, hold time, and time to reach full loading. To perform this investigation, we design a simple example problem that captures key features of fatigue crack growth without introducing unnecessary complexity. Results indicate that increasing lives are associated with lower crack-tip temperatures due to cooling thermal gradients that reduce crack growth per cycle. Furthermore, these results reveal minimal impacts on lives for temperatures with negligible time-dependent crack growth, and larger increases in lives are expected for cracks with more appreciable time-dependent crack growth. Designers may employ these ideas to extend useful lives of applications with strong spatial gradients in the material response, e.g., recent efforts to design internal cooling passages into gas-turbine engines using additive manufacturing.

Study on multiple fatigue cracks growth in the web of a hollow compressor impeller

The fatigue crack growth characteristics of a hollow impeller whose thin-walled web has the propensity to nucleate multiple site damage (MSD) were investigated. To study its MSD crack growth, a simulation framework was proposed to accomplish noncoplanar crack coalescence and user-defined growth. It employed a modified linkup criterion and could account for the geometric constraints of cracked bodies, crack closure, and the kinking and twisting angle introduced by KII and KIII. Interaction features of the multisite crack growth were studied by simulating carefully designed cracking scenarios, and the crack cascade’s typical evolution over time and space was deduced. Results showed that larger adjacent cracks would introduce a stronger interaction effect. For crack colonies, the stress intensity factor would increase and remain higher between different merging positions. The crack growth was predominantly in tensile mode (greater than 95%) before coalescence, whereas the in-plane and out-of-plane shear modes increased noticeably to about 30% during and after coalescence. MSD will adversely affect the impeller fatigue evaluation due to faster crack growth, more complex crack profiles, and difficulty in determining the critical crack size due to the jump-like increase of crack colonies that makes applying damage tolerance difficult.

Short fatigue crack growth mechanism in Ni-Co based superalloy at elevated temperatures and in oxidative atmospheres

In this study, to elucidate the oxidation effect on the short fatigue crack growth (SFCG) characteristics of Ni-Co based TMW-4M3 superalloy, fatigue tests were conducted at room/elevated temperature in air/vacuum and three-dimensional microscopic observation of the SFCs using a plasma focused ion beam – scanning electron microscope (PFIB – SEM) system. Fatigue lives tested under vacuum at elevated temperature were comparable to those at room temperature while those tested at elevated temperatures in air showed shorter fatigue life in higher stress regions and longer fatigue life in lower stress regions than the others. In situ observation of SFCs at elevated temperatures in air revealed SFCG deceleration in the small ΔK regions and acceleration in large ΔK regions. SFCs opening/closing behaviours at elevated temperatures measured by digital image correlation (DIC) showed the crack opening stress to be higher at elevated temperature in air, possibly due to oxide-induced crack closure. However, the crack closure effect did not fully explain the difference in FCG rate between room and elevated temperatures in air. Three-dimensional investigations revealed SFCs to form at elevated temperatures in air, showing straight transgranular FCG to be insensitive to microstructure in slow growth regions, and intergranular FCG to precede that in the surrounding material in fast growth regions, in contrast to microstructural SFCs features at room temperature. It appears that slow and straight unique SFCG at elevated temperatures might occur due to intermittent brittle fracture of oxides formed at the crack tip. This suggests a possible role of the oxide layer at the crack tip in broadening the plastic deformation distribution, thus contributing to FCGR deceleration.

Influence of carbon black particle size on fatigue life of rubber compound by varying strain and temperature

AbstractFatigue failure is one of the most critical failure modes in engineering structures. The fracture process proceeds through two major phases: crack nucleation and subsequent crack propagation leading to the ultimate failure. With special reference to rubber products especially tyre, the fatigue crack growth characteristics have been well understood with good approximation by many researchers. But the experimental research on crack nucleation is in the nascent stage. In the present study, we have focused on the role of filler particle size, temperature and strain on crack nucleation and fracture surface morphology of unfilled and carbon black (CB)‐filled natural rubber (NR) compounds. Three different kinds of ASTM grade carbon blacks, namely, N134, N234 and N339 have been used to determine the effect of filler particle size on crack initiation. All compounds have been tested at room temperature (i.e., RT 25°C), 70°C and 100°C with different strain amplitudes (50%, 75%, 100%, and 125%) to understand its effect on damage mechanics. It is observed that at a given strain and temperature, compound with N134 has better fatigue life in comparison to the N234 and N339. It is also found that with increasing temperature, fatigue life is being reduced for all the compounds. Optical microscope and field emission scanning electron microscope (FESEM) along with micro computational tomography (CT) have been utilized to understand the failure mechanism which reveal the presence of initial flaws as filler protrusions that creates micro cracks during fatigue loading and thus leads to crack nucleation.

Experimental investigation on anisotropic fatigue crack growth characteristics of Q420C steel

The hot-rolled steel sheets often exhibit anisotropy in their mechanical characteristics due to the grain’s elongation along the rolling orientation. The anisotropy characteristic also appears in the fatigue crack growth behaviour. However, it has not been thoroughly studied for some types of steel materials although they have been widely used for wind turbine towers. In this study, the longitudinal fatigue crack growth behaviour of the hot-rolled steel sheet (Q420C) is investigated by an experimental programme. A total of six specimens were tested under constant amplitude loading cases with different load ratios. It was found that longitudinal cracks are approximately 15% faster than transverse cracks at the load ratio of 0.1; at the load ratio of 0.3, they are approximately 22% faster. The anisotropy of crack growth rates is attributed to differences in the microstructure of the material. A comparison of the crack growth rate is carried out for Q420C steel and some widely used American-made steel materials. Furthermore, the applicability of design curves in the British Standard BS 7910 is explored, the crack closure phenomenon is analysed and the fatigue crack growth mechanism is discussed.

Superior Comprehensive Mechanical Properties of a Low-Carbon Medium Manganese Steel for Replacing AISI 4330 Steel in the Oil and Gas Industry.

A low-carbon medium manganese steel (0.12C-3.13Mn) containing Cr, Ni, Mo, V, and Cu elements was designed to replace the AISI 4330 steel applied in the oil and gas industry. The mechanical properties, microstructures, and fatigue crack growth rate were comparatively analyzed using uniaxial tension tests, microstructure characterization, and compact tension with fatigue crack growth characterization. The results showed that the ductility and -40 °C impact energy of 0.12C-3.13Mn steel were better than AISI 4330 steel (from 115 J to 179 J), while the yield strength of 957 MPa of the former was lower than the latter of 1060 MPa after being subjected to the same tempering process. The microstructure of 0.12C-3.13Mn steel was composed of a mixture of tempered martensite, reversed austenite, and nanosized precipitation particles, while the microstructure of S4330 steel contained ferrite and large-size Fe3C with lath and near-spherical morphologies. Compared to Cr-rich Fe3C, (V, Mo)C and Cu-rich particles have smaller sizes and, thus, provide more strengthening increment, leading to a higher yield ratio. The impressive fatigue-resistance property was obtained in 0.12C-3.13Mn steel because the threshold value was 5.23 MPa*m1/2 compared to the value of 4.88 MPa*m1/2 for S4330 steel. Even if the fatigue crack grew, the stress intensity factor range of 0.12C-3.13Mn steel was obviously wider than that of AISI 4330 steel due to the presence of reversed austenite and secondary cracks. Overall, the AISI 4330 steel could be replaced with the designed 0.12C-3.13Mn steel due to the similar strength and better ductility, low-temperature toughness, and fatigue-resistance property.

Fatigue crack propagation analysis in an aero-engine turbine disc using computational methods and spin test

This article presents a study on applying the fracture mechanics-based method to predict the fatigue crack growth (FCG) characteristics of the turbine disc in a turboshaft engine. According to the stress analysis results, the electric discharge machining (EDM) method introduced notches for initiating cracks of a specific size into the critical locations of the turbine disc. The FCG of these cracks under cyclic load was predicted utilising a three-dimensional (3D) numerical analysis method and a simplified rectangular plate model with relevant fracture mechanics properties. Meanwhile, the full-scale spin test was performed using a pre-flawed turbine disc under intentional loads at an isothermal temperature. After the test, the crack surface fractography and the quantitative fracture striation analysis revealed the practical crack propagation behaviour. There was a relatively small difference between the experimental results and the 3D finite element method, which indicated the good predictability of FEM for turbine disc crack growth. Furthermore, the rectangular plate models could predict the front stage of steady-state crack growth but were overall conservative due to their inability to precisely calculate the effects of crack growth and structure geometry on the stress intensity factor.

Fatigue Characteristics of 7050-T7451 Aluminum Alloy Friction Stir Welding Joints and the Stress Ratio Effect.

The fatigue crack initiation and growth characteristics in 7050-T7451 aluminum alloy butt joints subjected to different stress ratios and owing to friction stir welding (FSW) were investigated using fatigue tests for stress ratios of 0.1, 0.3, and 0.5. The difference between the fatigue crack initiation in the base material (BM) and FSW joints, related to coarse secondary phases, was explored using scanning electron microscopy (SEM). Accordingly, Al23CuFe4, Al7Cu2Fe, and Al2Mg3Zn3 were the preferred joint crack initiation locations, whereas Mg2Si was the major fracture initiation point of the parent material, and cracks tended to propagate along dense, coarse secondary phases, becoming more pronounced for larger cracks. In addition, as the stress ratio increased, non-Mg2Si phase fracture initiation points appeared in the BM. Meanwhile, the quantity of non-Mg2Si phases in the joints continued to increase, and the crack initiation sites became increasingly concentrated in the TMAZ-HAZ region.

High-temperature fatigue crack growth characteristics of IN718 Ni-based alloy treated by laser peening

The characteristics of fatigue crack growth (FCG) of laser peened IN718 Ni-based alloy at high temperature with different laser power densities were studied. The residual stress relaxation and micro-structure evolution of the samples at high temperature after laser peening (LP) were analyzed by X-ray diffraction technique and scanning electron microscopy (SEM), respectively. Meanwhile, SEM was used to characterize the fracture morphologies of the samples. The strengthening mechanism of LP on the high-temperature FCG resistance of IN718 Ni-based alloy was discussed. The results showed that large compressive residual stress, fine grains and complex grain boundaries induced by LP could significantly enhance the resistance of FCG at 700 °C. As a result, the FCG rate was decreased and fatigue life of the sample was increased. In addition, the laser power density was found to be inversely proportional to the average spacing of fatigue striations and directly proportional to the size and depth of dimples, which indicated that the increase in laser power density can improve the high-temperature FCG resistance of IN718 Ni-based alloy.

Fatigue crack growth characterization of Inconel 718 after additive manufacturing by laser powder bed fusion and heat treatment

Numerous studies were conducted on the tension–compression fatigue behavior of Inconel 718 (IN718) fabricated with laser powder bed fusion (LPBF). Contrary, only a few fatigue crack growth rate (FCGR) studies are available in the literature. Considering the above, LPBF manufactured IN718 is heat treated in variants including (H) and excluding (A) hot isostatic pressing. FCGR tests are performed at stress ratios R = 0.1 and R = 0.5. Both variants show similar FCGR and fracture modes at both stress ratios, despite a significant difference in ductility. Furthermore, A and H show similar FCGR at R = 0.1 and better performance at R = 0.5 compared to wrought IN718.

Fatigue crack growth in additively manufactured Hastelloy X - Influences of crack orientation and post-fabrication treatments

Numerous studies have been conducted in the recent past to assess different nickel-based superalloys fabricated using laser-based powder bed fusion (PBF-LB). Most of these works focused on the processability, microstructural evolution, post-fabrication treatments and the mechanical behaviour of components manufactured by PBF-LB under static loading. However, limited efforts have been made to investigate the fatigue crack growth (FCG) characteristics of PBF-LB built alloys, thereby imposing a barrier to their use as critical components. As such, the aim of the present study was to evaluate and understand the effect of build orientations (0° and 90°) and post-fabrication treatments (solution heat treatment and hot isostatic pressing) on the FCG behaviour of Hastelloy X (HX) fabricated by PBF-LB process. The results from the FCG tests showed that the build orientations had no discernible influence on the fatigue threshold and FCG rate within near-threshold and Paris regions. Nevertheless, the post-fabrication treatments led to an increase in the fatigue threshold and crack growth resistance within the near-threshold region because of the alleviation of detrimental tensile residual stress alongside the crack growth direction. In conjunction with FCG tests, scanning electron microscopy and electron backscattering diffraction were used to investigate the crack path and fracture surfaces within near-threshold and Paris regions.

Study of fatigue crack growth characteristics of TIG welded AZ31B magnesium alloy joint based on laser bionic treatment

AbstractIt is crucial to obtain better fatigue property for the safe application of materials, while nature provides new structural design ideas for effectively improving fatigue property. In this study, a “soft‐hard” alternating bionic structure was fabricated on the surface of TIG welded AZ31B Mg alloy joints by laser bionic treatment (LBT). Microstructure of welded joints was tailored by LBT, resulting in fine equiaxed grains with random orientations and increased microhardness. The fatigue crack growth threshold of the welded joints with bionic structure is increased by more than 40%, and crack growth rate was lower. LBT could significantly affect crack growth behavior and presented obvious crack deflection, resulting in a rougher fracture surface and more second cracks and branches. LBT significantly improved the fatigue crack growth resistance, inducing a series of strengthening mechanisms. The present work opens a new alternative to improve the fatigue properties of welded joints.

Study on Antifatigue Crack Growth Characteristics of Ball-End Milling Bionic Surface

The surface morphology of parts has an important influence on service performance, such as wear resistance, fatigue resistance, and corrosion resistance. For example, the fatigue fracture resistance of the surface of the automobile cover mold directly determines the quality of the workpiece and the service life of the mold. In nature, the pit-shaped non-smooth form of the surface of certain organisms has good fatigue resistance. Using laser processing and other methods to prepare a pit-shaped bionic non-smooth surface on the surface of the part is an effective method to improve the fatigue resistance of the mold surface. Ball-end milling can form a regularly distributed pit-like surface morphology, and has the advantages of high processing efficiency, wide operating range, and low production cost compared with those of laser processing and preparation technology. It provides a new processing method for efficient milling preparation of bionic surfaces. In this paper, the morphology of the quadrilateral pits on the surface of the dung beetle is used as the prototype of the bionic design, and the mechanism of the antifatigue crack propagation characteristics of the quadrilateral pits of the bionic dung beetle is revealed and analyzed. The fatigue crack growth has a good retarding effect. At the same time, through numerical simulation and experimental research on the fatigue crack growth characteristics of the surface of the bionic pit, the results show that the source of fatigue cracks often starts inside the surface of the processed bionic pit, and the surface of the quadrilateral pit on the surface of the bionic dung beetle has good antifatigue crack growth characteristics.

Fatigue fracture characterization by cyclic material forces in viscoelastic solids at small strain

The study at hand introduces a new approach to characterize fatigue crack growth in small strain linear viscoelastic solids by configurational mechanics. In this study, Prony series with n-Maxwell elements are used to describe the viscoelastic behavior. As a starting point in this work, the local balance of energy momentum is derived using the free energy density. Moreover, at cyclic loading, the cyclic free energy substitutes the free energy. Using the cyclic free energy, the balance of cyclic energy momentum is obtained. The newly derived balance law at cyclic loading is appropriate for each cycle. In the finite element framework, nodal material forces and cyclic nodal material forces are obtained using the weak and discretized forms of the balance of energy momentum and cyclic energy momentum, respectively. The crack driving force and the cyclic crack driving force are determined by the nodal material forces and the cyclic nodal material forces, respectively. Finally, numerical examples are shown to illustrate path-independence of the domain integrals using material forces and cyclic material forces. The existence of the balance of energy momentum and cyclic energy momentum are also illustrated by numerical examples.

On the influence of test speed and environment in the fatigue life of small diameter nitinol and stainless steel wire

To better understand the complex interplay of speed and environment on metals commonly used in implants, rotary bend fatigue tests were conducted on stainless steel and nitinol wires. A range of alternating strains was tested to create ε-N curves at two speeds (physiologic and accelerated) and in three environments (deionized water at body temperature, phosphate buffered saline at body temperature, and laboratory air at ambient room temperature). Results indicate that speed and environment can affect the observed fatigue life in nuanced ways. An electropotential monitoring technique was demonstrated to characterize fatigue crack growth which may be useful in future investigations.