Fatigue Crack Propagation Tests Research Articles

Experimental Study on the Fatigue Crack Propagation Rate of 925A Steel for a Ship Rudder System.

The low-temperature fatigue crack propagation rate of 925A steel, as a rudder steel for polar special ships, has a crucial impact on the evaluation of the fatigue strength of polar ships. The purpose of this article is to study the fatigue crack propagation rate of 925A steel under different low-temperature conditions from room temperature (RT) to -60 °C. The material was subjected to fatigue crack propagation tests and stress intensity factor tests. The experimental tests were conducted according to the Chinese Standard of GB/T6398-2017. The results show that as the temperature decreases, the lifespan of 925A increases. Within a certain stress intensity factor, as the temperature decreases, the fatigue crack propagation rate decreases. At -60 °C, it exhibits ductile fracture; within normal polar temperatures, it can be determined that 925A meets the requirements for low-temperature fatigue crack propagation rates in polar regions. However, in some extreme polar temperatures below -60 °C, preventing brittle failure becomes a key focus of fatigue design. Finally, the fatigue crack propagation behavior at the microscale of 925A steel at low temperatures was described using fracture morphology. The experimental data can provide reference for the design of polar ships to further resist low-temperature fatigue and cold brittle fracture.

Multi-scale characterization of fatigue crack tip deformation and propagation in QP steel

A novel measurement method for multi-scale characterization of fatigue crack tip deformation and propagation behaviors in high strength and plasticity, multi-phase, fine-grain Quenching and Partitioning (QP) steel was proposed. The fatigue crack propagation (FCP) test system with the macro/micro images acquisition devices was built to measure the FCP rate and crack tip deformation simultaneously. First, at the mesoscale, the evolution characteristics of the crack tip deformation fields, plasticity induced crack closure (PICC) behaviors and plastic zone during the FCP process were analyzed using the sub-image stitching and matching digital image correlation (DIC) method. Then, the FCP models were established with special emphasis on the model considering the PICC effect based on the obtained crack opening load at the mesoscale. Moreover, the corresponding microscale surface crack and fracture morphologies, microstructure phase transformation and deformations were observed and analyzed by electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) techniques.

3D characterization of closure of surface and internal fatigue cracks in nodular cast iron using Digital Volume Correlation of laboratory X-ray tomography images

Digital volume correlation (DVC) of laboratory X-ray tomography images has been used to characterize the closure of artificial internal fatigue cracks in nodular graphite cast iron. DVC allows to produce crack opening displacement (COD) map from 3D tomography images at the sub-voxel scale thanks to the graphite nodules which can be used as internal natural markers. Fatigue samples containing controlled internal defects have been produced in a nodular cast iron by diffusion bonding plates containing laser-machined sharp notches. Specimens with internal cracks and surface cracks were respectively characterized by laboratory tomography at different loads during fatigue crack propagation tests. The local stress intensity factors (SIFs) have been extracted from the measured displacement fields by using Williams series, based on those results the closure levels of internal cracks and surface cracks were comparatively analyzed and discussed. Crack closure levels were found to be linearly related to the location of the surface crack front, and the level of internal crack closure was comparable to that of the surface crack in bulk.

Effect of solution treatment on porosity, tensile properties and fatigue resistance of Al–Cu alloy fabricated by wire arc additive manufacturing

This work studied the effect of solution heat treatment on the microstructure and properties of the Al–Cu alloy fabricated by wire arc additive manufacturing (WAAM), and focused on the influence of porosity and second phase evolution on the tensile properties and fatigue resistance of the alloy. The results show that with solution heat treatment at 530 °C for 1h, Al–Cu alloy fabricated by WAAM with low porosity (2.4 %) and high peak hardness (148.2 HV) can be obtained. Higher solution temperatures significantly increased the porosity, and extending the solution time caused the problems of increased porosity, micropore growth, microcracks as well as cavity. Influence of porosity on fatigue resistance is more significant than that on tensile properties. The results of tensile tests suggest that the effect of porosity on the strength of T6-treated alloy is limited and much smaller than that of the precipitation strengthening induced by precipitates, while the results of the fatigue crack propagation tests imply that porosity plays the dominant role in the fatigue crack propagation behaviors of the T6-treated alloy.

A hybrid shoulder to achieve a significant improvement in tensile strength and fatigue performance of friction stir welded joints for Al–Mg–Si alloy

In this paper, a hybrid shoulder friction stir welding (HS-FSW) process was developed combining the advantages of rotational shoulder friction stir welding (RS-FSW) and non-rotational shoulder friction stir welding (NRS-FSW). Both static and fatigue performance of the HS-FSW Al–Mg–Si alloy joints were examined and compared with those of the RS-FSW and NRS-FSW joints. The ultimate tensile strengths of the joints welded by HS-FSW are increased by 17.3 % and 6.7 %, compared with those of RS and NRS FSW, respectively. Besides, fatigue strengths of HS-FSW joints grow by respective 25.4 % and 1.8 %. For a better understanding of the significant improvements, the fatigue crack propagation (FCP) test and residual stress measurement were carried out. FCP test indicates that the advancing side (AS) is the most sensitive zone for FCP. The residual stress measurements suggest that the HS-FSW joint has the lowest distribution amplitude of residual stress compared with that of the RS-FSW and NRS-FSW joints. Based on above evolution, an assessment is firstly conducted, which reveals that the axial force and the lower heat input to mitigate the amplitude and the peak value of tensile stress during welding, jointly contribute to the superior imprisonment of the tensile strength and fatigue strength of the HS-FSW joints.

Damage-reliability approach for fatigue crack propagation in MDPE gas pipe

Fatigue crack propagation tests are carried out on arc-shaped specimens prepared from MDPE gas pipes. Damage zone characterization is achieved using the diamond wafer sectioning technique from partially propagated and prematurely arrested cracks. Damage ahead of the crack-tip is used to assess a damage parameter and reliability based on statistical laws and subsequent use the PHIMECA Software. It is shown that a length and a width associated with a corresponding change in thickness at the fracture surface satisfactorily describe the damage zone size. The 3-parameter Weibull model gives the best reliability behavior and a critical lifetime of 82%. When considering separately both the Dugdale model and experimental damage zone measurements, it is possible to establish the evolution of the reliability index as a function of crack length. It is concluded that the reliability index approach based on damage provides a more consistent representation compared to analytical models.

Influence of Microstructure on Tensile Properties and Fatigue Crack Propagation Behavior for Lath Martensitic Steel

This paper addresses the role of multilevel microstructures on the fatigue crack propagation behavior and the tensile properties of lath martensite with different substructure sizes. Microstructure characterization of the alloy was carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron back-scattering diffraction (EBSD), and optical microscopy (OM). Based on the classic Hall–Petch relationship, the results of tensile tests showed that martensitic block is the effective control unit of yield strength. Furthermore, the plasticity of lath martensite is not sensitive to grain size. The tensile deformation mechanisms were also discussed. Fatigue crack propagation tests revealed that the coarse grain has a higher crack propagation threshold and lower crack propagation rate than the fine grain in lath martensitic steel. The change in the plasticity zone ahead of the crack tip leads to the transitional behavior of the fatigue crack propagation rate. When plasticity zone sizes are equal to the block size, the fatigue crack propagation reverts to a stable propagation stage. Finally, an empirical model was established to predict the fatigue crack propagation rate of the stable propagation stage based on the tensile properties of the lath martensitic steel.

Fatigue crack propagation of the gradient surface-modified layer of high-strength steel

A gradient modified layer can be produced on the high-strength steel surface by carburizing heat treatment. In this study, microstructure characterization and fatigue crack propagation test were carried out on the gradient surface-modified layer of the 18CrNiMo7-6 high-strength steel. With increasing depth of the surface-modified layer, the yield strength and the kernel average misorientation value decrease, and the equivalent grain size of the slatted martensite structure and the number of the large-angle boundaries increase. The orientation difference angle of the point to point pre- and post-fatigue at the crack of the transition layer and the matrix layer increases from less than 2° to approximately 10° and about 4°, respectively. With the increase of the depth of the surface-modified layer, the degree of deformation inside the martensitic packet decreases, while the extent of macroscopic deformation of the material increases. Deformation occurs inside the martensitic packet at the main crack, and there is essentially no deformation inside the martensite packets away from the main crack. On both sides of the main crack, a large number of extrusions parallel to the martensitic lath appear. The large-angle boundaries of the original austenite grain boundary and martensite boundary hinder the propagation of the fatigue crack.

Numerical Investigation of Fatigue Crack Propagation Behaviour of 550E High-Performance Steel

The fatigue crack propagation behaviour of Q550E high-performance steel (HPS) is studied in this paper. Static tensile testing and fatigue crack propagation testing were carried out, and the results were compared with those of Q235. Finite element models were developed and verified against the experimental results. The impacts of the initial crack angle, crack depth ratio, stress ratio, thickness, and corrosion pitting on the fatigue crack propagation behaviour of the HPS were analysed. The results show that the fatigue life of Q550 was reduced by 18% due to the corrosion pitting, but it did not change the crack propagation path. When the stress intensity factor is higher than a certain value, the fatigue performance of Q235 is better than that of Q550E. The initial crack angle of 52.5° is the critical angle of the crack stress intensity factor. The steel tends to fracture as the crack depth ratio increases, and more attention should be paid to the effective crack length in engineering practice. An increasing stress ratio leads to a smaller stress intensity factor, and the thickness affects the stress intensity factor in the later stage. The crack stress intensity factor around the corrosion pits gradually decreases along the thickness direction, and the crack tips around the corrosion pits tend to reach the yield state initially, accelerating the fatigue fracture of the specimen and ultimately leading to a decrease in fatigue life.

The effect of surface treatment and orientation on fatigue crack growth rate and residual stress distribution of wire arc additively manufactured low carbon steel components

The directed energy deposition (DED) processes, such as laser metal deposition or Wire Arc Additive Manufacturing (WAAM), are gradually becoming the preferred method for fabrication of large-scale components using metal additive manufacturing (AM) technology. In this work, the possibility of fatigue life enhancement in WAAM built low carbon steel components, by means of rolling and laser shock peening surface treatment techniques, was investigated. A series of fatigue crack propagation tests were performed on surface treated ER70S-6 and ER100S-1 WAAM built specimens, and the results were analysed and compared with the untreated materials tested under the same loading conditions. The obtained results were interpreted in terms of the sensitivity of the cracking behaviour to the specimen orientation and extraction location. Furthermore, the residual stress profiles were measured, before and after applying the surface treatment techniques, and the effects of locked-in residual stresses on the fatigue performance of WAAM built components were discussed. Finally, a detailed texture analysis was performed on the surface treated and untreated regions of both WAAM built materials considered in this work. The obtained results from this study provide an insight into the advantages and disadvantages of various surface treatment techniques for fatigue life enhancement of WAAM built components with the view to extend the application of this advanced manufacturing technology to a wider range of industrial applications.

Fatigue crack propagation behavior of the grain size transition zone in a dual microstructure turbine disc

Constant load amplitude andΔK fatigue crack propagation (FCP) tests have been carried out to investigateFCP behavior of grain size transition zone in a dual microstructure turbine disc. The results show grain size exerts more significant influence in the time-dependent FCP regime, but the effect of grain size is relatively limited in the cycle-dependent FCP regime.Larger local plastic deformation zone is found in fine-grained region,which is believed to be the reason that causes more significant grain boundary oxidation along with contribution from more grain boundaries acting as short-circuit diffusion paths of oxide-forming elements, resulting in accelerated FCP.

Temperature dependent transition from Mode I to crystallographic cracking in a Ni-base single crystal superalloy

To study the temperature dependent transition from Mode I to crystallographic cracking, fatigue crack propagation tests at room temperature, 300 °C and 450 °C, and crystal plasticity finite element analysis are conducted for a Ni-base single crystal superalloy by adopting compact specimens with four different crystallographic orientations. Through experimental examination, the stress intensity factor range (ΔKI) at the critical point of transition scatters for different crystallographic orientations and cannot explain the transition behavior. The slip plane for transition at different temperatures keeps the same. By crystal plasticity finite element assessment, the slip systems on the slip plane for transition are quite activated comparing with other slip systems. The distributions of the plastic shear strains along 12 slip systems at the critical point of transition exhibit the same pattern at different temperatures, which explains the slip plane for transition at different temperatures keeping the same. A damage parameter considering slip plane activity and normal stress on the slip plane can explain the transition regardless of crystallographic orientations. The damage parameter at the critical point of transition increases with the temperature, which indicates that the resistance for transition to crystallographic cracking increases with the temperature.

Investigation on fatigue crack propagation behaviour of U71Mn and U75V rails using peridynamics

Fatigue crack propagation in rails is an important factor that increases the cost of railway maintenance and threatens operating safety. Research into fatigue crack propagation rules for different rail materials helps to provide scientific guidance for rail selection and maintenance plans. Based on the results of fatigue crack propagation tests on U71Mn and U75V rails, the bond fatigue fracture criterion was established for the two materials, and their accuracy was verified. Using ordinary state-based peridynamic theory, a model was established to analyse the process of crack propagation in the two types of rails. The behaviour of U71Mn and U75V rails was then simulated with different wheel loads and wheel-rail friction coefficients, and the characteristics and rules for fatigue crack propagation in the two types of rails were analysed in a comparative manner. The results showed that the material type exerted little impact on the crack propagation path within the rails and that the U71Mn rail, which showed a lower crack propagation rate than the U75V rail, is more suitable for high-speed railways. The rate of crack propagation increased with both wheel load and wheel-rail friction coefficient. Certain measures, such as track regularity improvement, could be employed to reduce dynamic wheel-rail impact, while others, such as lubrication, could be adopted to decrease wheel-rail friction coefficients to prolong rail service life.

Effect of rafted microstructure and its temperature dependency on fatigue crack propagation in a single‐crystal Ni‐base superalloy

AbstractEffects of rafted microstructure and its temperature dependency on fatigue crack propagation (FCP) in a single‐crystal Ni‐base superalloy are experimentally investigated. FCP tests are conducted at room temperature, 450°C, and 700°C for two types of pre‐rafted specimens, and their FCP behaviors are compared with that of a specimen with cuboidal γ′ precipitates. It is found in the experiments that there is no significant influence of rafted microstructures on FCP rate at room temperature, while a clear influence is pronounced at 450 and 700°C depending on the coarsening direction of rafted microstructures and the level of stress intensity factor range. The temperature dependency in the effect of rafted microstructure is discussed with special considerations on the microscopic FCP behavior affected by the morphology of γ′ precipitates, the coherency of the γ/γ′ interface, and temperature‐dependent slip characteristics of γ and γ′ phases.

Investigation on the Fatigue Crack Propagation of Medium-Entropy Alloys with Heterogeneous Microstructures

The behavior and the mechanism of fatigue crack propagation in CrCoNi medium-entropy alloys (MEAs) with heterogeneous microstructures were investigated in this paper. After cold-rolling and recrystallization annealing at different temperatures and times, five sets of heterostructured specimens were acquired with different recrystallization levels. Then, the structure characterizations of these five sets of specimens were carried out by nanoindentation testing and electron back-scatter diffraction (EBSD) mapping. Finally, the fatigue crack propagation tests were conducted on single edge crack specimens of these different heterogeneous microstructures. The experimental results indicate that the crack propagation rates of specimens with partial recrystallization microstructures are higher than those with complete recrystallization microstructures, and the effect on fatigue crack thresholds of these specimens is the opposite. The fatigue cracks grow along the slip planes or twin boundaries in recrystallization grains (RGs), which induced crack deflections and the roughness-induced crack closure effect. For this reason, the area percentage of recrystallization and the grain size of RGs have a great effect on the value of the fatigue crack growth threshold.

Temperature and ΔK dependence of grain boundary effect on fatigue crack propagation in a two-dimensional polycrystalline Ni-base superalloy

Effects of grain boundaries (GBs) on fatigue crack propagation (FCP) in a cast Ni-base superalloy were experimentally investigated using “two-dimensional” polycrystalline specimens extracted from a directionally solidified material. FCP tests were conducted at room temperature (RT), 700 °C and 900 °C under the stress intensity factor range, ΔK, increasing conditions, and distinctive FCP behavior both in grain interiors and around the GBs was observed in-situ. At RT, the crack propagated in the shearing mode along slip planes. At 700 °C, the cracking mode transitioned from the opening mode to the shearing mode with the increasing ΔK. At 900 °C, only the opening mode cracking occurred. The shearing cracks at RT and in the high ΔK region at 700 °C were retarded near the GBs, whereas the opening mode crack in the low ΔK region at 700 °C was not affected by the GBs. The retardation was attributed to a transition from the shearing mode to the opening mode induced by the slip plane incompatibility at the GBs, and the degree of the retardation showed a good correlation with the size of the shearing crack plane. At 900 °C, the crack propagated along high angle GBs with higher FCP rates than those in grain interiors. The acceleration was greater at the GBs with larger misorientation, which was attributed to higher GB energy. In high ΔK regions, secondary cracks were nucleated along high angle GBs ahead of main crack tips possibly because of external stress normal to the GBs and/or the strain incompatibility at the GBs.

Experimental study on fatigue crack propagation in balcony opening corners of a cruise ship

The balcony opening corner in a cruise ship is subjected to highly stress concentration and alternating loading during operation, which is prone to cause fatigue crack initiation and further crack propagation. These initiated cracks are difficult to find in time due to these opening corners being hidden in the decoration materials. It is necessary to assess the residual fatigue lives accurately and provide reasonable repairing plans for these cracked opening corners to ensure safety of the cruise. In this paper, in order to study the fatigue crack propagation (FCP) law of the balcony opening corner, make up the lack of the FCP experiment of the balcony opening corners, correlate the FCP behavior of the full scale model with the reduced scale model, and provide an accurate FCP live prediction method for balcony opening corners, reduced scale models based on the similarity theory and the corresponding fixtures are designed. FCP tests of these reduced scale models with small initial cracks are carried out. a-N curves of the cracks at different notch locations in balcony opening corners under different stress ratios are obtained. Formulas of stress intensity factors (SIFs) of cracks at the midpoint of the three types of corners are given by fitting the data obtained by a serial of FE analysis of the cracked corners. Further, the fatigue lives of the tested specimens are calculated and compared with the test data. It is shown that the prediction results fit well with the model test results. The procedure of model test and experimental data can provide a reference for detailed design and precise residual FCP life assessment of opening corners in ship hulls.

Fatigue crack propagation in thin balcony opening corners in a cruise ship accounting for warping deformation

Accurately predicting the fatigue crack propagation (FCP) life of thin balcony opening corner structures with warping deformation using the FCP theory is a major challenging task. Herein, a procedure for analyzing the effect of warping deformation on the FCP in thin balcony opening corner structures is presented. According to the proposed method based on the modesuperposition to simulate the warping deformation (MSSWD), the stress intensity factors (SIFs) of cracks at the midpoint of balcony opening corners with warping deformation are appropriately modified. The FCP tests with reduced-scale models are also performed to provide experimental verification data for the given procedure. The fatigue lives of the specimens under various loads are calculated based on the modified SIFs and unique curve model and then are compared with the test data. The obtained results reveal that this approach can precisely predict the FCP lives of thin balcony opening corners with warping deformation.

Thermomechanical and isothermal fatigue crack propagation testing method for 316LN austenitic stainless steel

AbstractInvestigating fatigue crack growth behavior of materials is crucial to the life design and safety evaluation of engineering structures. During the practical service condition, the temperature keeps changing, which cannot be studied by the traditional fatigue crack growth test method at constant temperature. In this study, we focus on constructing a new thermomechanical fatigue (TMF) crack propagation testing method, developing a thermal deformation compensation method, and proposing an equivalent compliance method suitable for variable temperature conditions based on ASTM E647 standard to measure the crack length. Furthermore, the load‐ and strain‐controlled TMF tests under different phase angles are carried out to prove the validity of the new method.

Influence of Sample Wetting Method on ESC-Behavior of PMMA under Dynamic Fatigue Crack Propagation

Environmental stress cracking (ESC) is one of the most prominent failure mechanisms for polymer components. The high sensitivity of plastics in regard to environmental influences has always meant that plastics as materials have been viewed very critically in outdoor applications. Recently, the massive occurrence of microplastics in the environment means that questions about the long-term stability of plastic parts and the studies of plastic fragmentation are of great scientific interest. ESC behavior also plays an important role in connection with the formation of microplastics. In this work, the influence of two different sample wetting methods on ESC behavior was investigated. In case A, the sample was in situ wetted with the medium during the measurement by using a sponge. In case B, the sample was wetted by storage in the medium prior to measurement. Different stress cracking agents (SCA) were examined for polymethylmethacrylate (PMMA). Fracture-mechanical fatigue crack propagation (FCP) tests were carried out to quantitatively determine the sensitivity to ESC. Correlations between the absorption behavior and the ESC behavior of the SCA and the resulting morphological phenomena were established. Depending on the wetting method, significant differences in FCP were observed. The in situ wetting of the samples (case A) during the FCP measurement with ethylene glycol (EG) and with deionized water (DI) led to a significant shift in the crack propagation curves to higher ∆K—compared to the PMMA reference. In the case of n-heptane (NH), a more brittle crack propagation behavior was observed due to the chemical interaction with PMMA. The previously immersed samples (case B) give different results. Storage in NH and EG showed no influence on the crack propagation behavior. Samples immersed in DI showed a completely different course of crack growth. At a certain load, a sudden deceleration of the crack propagation and thus a horizontal curve could be seen. Above a certain ∆K value, crack growth began again. Depending on the immersion time (14, 30, or 60 days), this so-called stepped behavior shifted to lower da/dN values.