Fatigue Crack Growth Rate Tests Research Articles

Crack Growth Rate Model Derived from Domain Knowledge-Guided Symbolic Regression

Machine learning (ML) has powerful nonlinear processing and multivariate learning capabilities, so it has been widely utilised in the fatigue field. However, most ML methods are inexplicable black-box models that are difficult to apply in engineering practice. Symbolic regression (SR) is an interpretable machine learning method for determining the optimal fitting equation for datasets. In this study, domain knowledge-guided SR was used to determine a new fatigue crack growth (FCG) rate model. Three terms of the variable subtree of ΔK, R-ratio, and ΔKth were obtained by analysing eight traditional semi-empirical FCG rate models. Based on the FCG rate test data from other literature, the SR model was constructed using Al-7055-T7511. It was subsequently extended to other alloys (Ti-10V-2Fe-3Al, Ti-6Al-4V, Cr-Mo-V, LC9cs, Al-6013-T651, and Al-2324-T3) using multiple linear regression. Compared with the three semi-empirical FCG rate models, the SR model yielded higher prediction accuracy. This result demonstrates the potential of domain knowledge-guided SR for building the FCG rate model.

Analysis of fracture toughness and fatigue crack growth rates of electron beam-welded dissimilar nickel-based superalloys: Mode-I cracks oriented normal to the weld

This paper discusses the experimentally obtained results of fracture toughness and fatigue crack growth rates of dissimilar metal-welded joints of Inconel 718 to Nimonic 80A. The dissimilar metal-welded joints were prepared in butt joint configuration using electron beam welding. Mode-I cracks were oriented normal to the weld. The base metals have different yield strength values (plastic mismatch) and ultimate tensile strength values but have the same elastic modulus and thermal expansion coefficients. The experiments were conducted using single-edge notched bend specimen design. The fracture toughness ( J) and fatigue crack growth rates of dissimilar metal-welded joints were affected by the direction of crack growth, such as cracks moving from stronger metal to weaker metal or vice versa. In comparison to Nimonic 80A (base metal), the J value of dissimilar metal-welded joint with crack direction from Nimonic 80A (stronger) to Inconel 718 (weaker) was lower. Moreover, the J value of dissimilar metal-welded joint was higher than that of Nimonic 80A (base metal) when crack traveled from Inconel 718 (weaker) to Nimonic 80A (stronger). In the fatigue crack growth rate test, the crack grew faster as it approached the weld interface from Nimonic 80A toward Inconel 718 due to crack acceleration. Conversely, the crack grew slowly in the opposite direction due to crack shielding. The onset of these effects began sooner for higher loads than lower loads.

Validation of on-line crack compliance data analysis methods for the residual stress intensity factor

Residual stress can significantly impact the fatigue crack growth rate (FCGR) observed in standard tests. In the present study, implementation of the on-line crack compliance (OLCC) method for determining the residual stress intensity factor, Kres, in FCGR tests is supported by validation data, details of novel data analysis methods, and strategies for data reduction. Slitting measurements on C(T) specimens of quenched (but not stress relieved) and aged aluminum alloy 7050-T74 provide reference profiles of Kres as a function of crack size. The OLCC method quantifies Kres profiles from stress intensity factor control and load control FCGR tests over a range of residual stress states. Novel OLCC data reduction techniques are developed, and data analysis attributes and limitations explored, contributing to an improved methodology for standardization. The results demonstrate that the on-line crack compliance method is a valid means of quantifying Kres during fatigue crack growth rate testing.

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.

Prediction of fatigue life of TC4 titanium alloy based on normalized equivalent initial flaw size model

Ti-6Al-4 V (TC4) is the primary and most widely used titanium alloy in the aerospace industry, mainly for the manufacture of engines and high temperature load-bearing elements in aircraft structural components. In this study, fatigue life and fatigue crack growth (FCG) rate tests of TC4 were carried out at different temperatures and stress ratios. The effect of temperature on the fatigue mechanical properties of TC4 titanium alloy materials was investigated analyzing the test results. The fatigue limit and FCG threshold were obtained from experiments. By the using equivalent initial flaw size method, the growth behavior of small cracks were incorporates in macroscopic FCG rate and a method to predict the fatigue life of TC4 at different temperatures was established. The parameters in the proposed model have a clear physical meaning and can be determined by experiment. Finally, the fatigue test data of TC4 titanium alloy under different test conditions are used to verify the proposed fatigue life prediction model and the prediction results have reasonable accuracy with the test results.

Different effects of multiscale microstructure on fatigue crack growth path and rate in selective laser melted Ti6Al4V

AbstractDuring the selective laser melting (SLM) process of Ti6Al4V, a special structure can be formed with columnar prior β grains along the building direction and fully martensitic α′ within the β grain. To investigate the influence of such special structure on the fatigue crack growth (FCG) rate, Ti6Al4V specimens fabricated by SLM were heat‐treated at two different temperatures in this study. The columnar grains were retained, and the martensite was decomposed when heat‐treated below the β transus. It is found that all the SLM features were removed when heat‐treated above the β transus. FCG rate tests were subsequently performed at room temperature, and it was found that the prior β grains affected the macroscopic fracture morphology, but there was no discernible influence on the FCG rate. The morphology of the α phase affected the crack growth path and the FCG rate. Changes in the strength‐toughness relationship induced by heat treatment can help understand the decrease in the FCG rate.

Investigation of the fatigue crack growth behavior of S355 steel weldments of motor hangers of high-speed trains

The behavior of fatigue cracks in welded details of motor hangers is investigated through crack growth numerical simulation under constant amplitude loading. The fatigue crack growth rate (FCGR) parameters of S355J2W steel are measured. FCGR tests are conducted on compact tension (CT) specimens. FCGR increases as the stress ratio increasing. Test data are compared with the reference curve of standards IIW and BS7910. Crack growth calculating results under mode I and mixed-modes conditions are compared. The crack growth path by simulation is in good agreement with the real crack path. The results also revealed that, even though fatigue cracks in the motor hanger are usually caused by mixed-modes conditions, mode I still governs the propagation.

Effect of Internal Hydrogen on the Fatigue Crack Growth Rate in the Coarse-Grain Heat-Affected Zone of a CrMo Steel

The effect of internal hydrogen in the fatigue crack growth rate of the coarse grain region of a 2.25Cr1Mo steel welded joint was analyzed in this work. The microstructure of the coarse grain region was simulated by means of a heat treatment able to provide the same microstructure with a similar prior austenite grain size and hardness to the one in a real welded joint. The fatigue crack growth rate was measured under standard laboratory conditions using compact tensile (CT) specimens that were (i) uncharged and hydrogen pre-charged in a hydrogen pressure reactor (under 19.5 MPa and 450 °C for 21 h). The influence of fatigue frequency was assessed using frequencies of 10 Hz, 0.1 Hz, and 0.05 Hz. Additionally, two load ratios (R = 0.1 and R = 0.5) were applied to analyze their influence in the da/dN vs. ∆K curves and therefore in the fatigue crack growth rate. The embrittlement produced by the presence of internal hydrogen was clearly noticed at the beginning of the fatigue crack growth rate test (ΔK = 30 MPm), obtaining significant higher values than without hydrogen. This effect became more notorious as the test frequency decreased and the load ratio increased. At the same time, the failure mechanism changed from ductile (striations) to brittle (hydrogen decohesion) with intergranular fracture (IG) becoming the predominant failure mechanism under the highest loads (R = 0.5).

Evaluating a natural gas pipeline steel for blended hydrogen service

An X70 natural gas pipeline steel that is being considered for blended natural gas/hydrogen gas service was evaluated in a high-pressure hydrogen gas environment. Fracture toughness testing and fatigue crack growth rate testing were conducted according to ASTM standards and the results are presented in the context of ASME B31.12 qualification. While a reduction in fracture toughness was observed, the fracture toughness and fatigue behavior were within acceptable bounds for hydrogen service.

Fatigue crack growth under overload/underload in different strength structural steels

Aiming at the different strength-level structural steels, fatigue crack growth rate (FCGR) tests were conducted under a series of loading conditions, i.e., constant amplitude load (CAL), single overload (OL), single underload (UL), or periodic underloads (PULs). Meanwhile, the effects of various baseline loading stress ratios on the FCGR were considered. To analyze the effects of crack-tip behavior on the crack propagation following an OL/UL, the crack-tip strain fields and crack opening displacement were obtained by digital image correlation (DIC). It was found that when the OL/UL-amplitude was constant, the fatigue crack propagations under the OL/UL were affected by both steel strengths and baseline loading stress ratios. In general, an increase in the steel strength grade indicated a lower crack retardation/acceleration during the OL/UL process, which was more evident under the lower baseline stress-ratio condition. The higher the strength, the lower the crack-opening stress level, the smaller the plastic zone size, and the higher the crack-tip residual strain. Correspondingly, there were different variations in the instantaneous FCGR and OL/UL-affected cycles. When the OL/ULs are considered in the design stage, our work can guide the steel strength grade selection and provide a better understanding of the crack growth retardation/ acceleration mechanisms.

Thermo-mechanical coupling analysis of edge-cracked rubber specimen focusing on the crack tip: Experimental observation and numerical simulation

The fatigue performance of rubber composite cannot be ignored in the field of engineering. Fatigue crack growth rate (FCGR) test by using a planar tensile specimen with a crack or cracks is often used for rubber fatigue characterization. Thermo-mechanical coupling characteristics of rubber composite under dynamic loading are significant and need to be considered. In our work, an interesting experimental phenomenon was observed during rubber FCGR test. The temperature around crack tip region is the highest at the beginning, and then the high temperature region shifts from the crack tip to the central region of the planar tensile specimen, and finally a steady-state temperature distribution is achieved. To explain such observation, a thermo-mechanical coupling method based on finite element analysis (FEA) for the edge-cracked rubber specimen was established. The simulation result was consistent with the experimental observation, and can explain the heat build-up phenomenon of the edge-cracked rubber well.

Fatigue life assessment of high-strength steel wires: Beach marks test and numerical investigation

High-strength steel wires are widely used in large-span bridges. Fatigue damage of steel wires is becoming more and more prominent under adverse environment and repeated load. This paper performed the fatigue crack growth rate tests of high strength steel wires under different stress ratios. The striated fracture morphology was obtained by stepwise fatigue loading method. The fatigue crack growth rate of steel wire was determined by measuring fracture stripe width by optical microscope. The experimental curves of fatigue crack growth rate versus stress intensity factor range under various stress ratios were obtained. The fracture morphology of steel wires after fatigue was observed by scanning electron microscopy. The characterization parameters of fatigue crack growth and critical fracture required for numerical simulation were obtained based on experimental results and statistical method. Following that, a numerical fatigue crack propagation model was established combined with ABAQUS and FRANC3D. The numerical model was validated by experimental observations in the present study and open literature. A parametric analysis method considering initial pit depth and stress amplitude was proposed to evaluate the residual life of steel wires. The results showed that the linear elastic fracture mechanics and numerical simulation-based method can be used to assess the fatigue life of virgin and corroded steel wires.

Effect of crack-tip deformation on fatigue crack growth: A comparative study under overload/underload conditions

With the aim to provide the structural design stage with data on the crack retardation/acceleration effects caused by overload/underload and to explore the corresponding mechanisms, we conducted a series of comparative fatigue crack growth rate (FCGR) tests on S355J2W+N steel. The various amplitudes of overload/underload were investigated comprehensively under different stress-ratio baseline loads for single and periodic overloading/underloading processes. Moreover, the crack-tip deformation behaviors (i.e., the strain/stress fields and crack closure responses following the overload/underload) were observed using a digital image correlation (DIC) technique and calculated through the finite element analysis. The relevant fracture morphologies and crack-tip dislocation structures were characterized and analyzed. The results showed that the crack propagation cycles could be influenced to different degrees by a single overload (OL), a single underload (UL), and periodic underloads (PULs). Compared with the transient crack acceleration induced by a UL, a lasting crack growth retardation occurred following an OL. The OL&UL sequences had various effects on the changes in FCGR. Furthermore, a higher stress-ratio baseline loading and larger-amplitude OL increased the retardation effect; a lower stress-ratio baseline loading and larger-amplitude UL enhanced the acceleration effect. The degrees of crack retardation or acceleration following the OL/UL were determined by the changes in the crack closure and residual stress state near crack tip, which were closely related to the baseline loading stress ratio and local residual strain characteristics. Correspondingly, the OL and PULs also changed the dislocation density and structures in the microscopic view.

Fatigue crack growth in locking compression plates

Fatigue crack growth in orthopaedic locking compression plates (LCP), under four-point bending, has been simulated by the extended finite element method (xFEM) to assess LCP optimal geometry in respect to remaining life. Loads corresponded to those occurring in human tibia during gait cycle for different body weights (60, 90 and 120 kg). Experimental investigation consisted of tensile testing and fatigue crack growth rate testing of Ti-6Al-4 V alloy, as used for LCP, by standard testing using tensile testing machine and Fractomat, respectively. Results showed that the remaining life of LCPs strongly depend on plate geometry and patient body weight (load).

Multi-physics FE-analysis and measurements for thermo-mechanical fatigue crack growth rate testing applications

The presented methodology in this study is addressed to in-phase (IP) and out-of-phase (OOP) loading cycles in stationary and transient thermo-mechanical fields. The subject of the numerical and experimental study is a single edge notch tension (SENT) specimen produced from a high-temperature nickel-based alloy ХН73М. In order to determination a local thermo-mechanical stress-strain rate and displacement fields a new algorithm for the multi-physics numerical calculations developed and implemented incorporates Maxwell 3D, Fluent and Transient Structural modules of ANSYS 2021R1. The employment of proposed algorithm to represent the cyclic history associated with the TMF conditions in the experiments, multi-physics finite element (FE) modelling of the stress, strain and displacement fields in the SENT specimen was performed. Additionally, time dependent non-uniform temperature fields were determined with the same cyclic variations and magnitudes as in the experimental OOP and IP cycling. As a complement to the FEM computations, the infra-red thermography temperature distribution measurements was implemented for the TMF state in the experiments in the SENT specimen. The comparison multi-physics FE-analysis and direct measurements shown in the present study is intended to contribute to a better understanding of the different mechanisms driving TMF crack growth and the address the outstanding questions associated with basic methodology.

Calibration and validation of extended back-face strain compliance for a wide range of crack lengths in SENB-4P specimens

Compliance equations based on back-face strain or crack mouth opening displacement, and potential drop techniques are widely used to calculate fatigue crack growth rate. Standard ASTM E647 for fatigue crack growth rate testing does not include compliance based equations for single edge notched four-point bending (SENB-4P) specimens. Equations developed based on finite element (FE) analysis have been reported in literature; however, they are limited to the long crack propagation regime (i.e., relative crack length ratios a/W > 0.15). No compliance relations for crack growth in the physically short crack regime were found in literature. This work reports on a complementary numerical-experimental study towards the development of compliance equations with an extended application range in terms of relative crack length. FE simulations have been used to calibrate a back-face strain compliance equation for calculation of relative crack length in the range 0.05 ≤ a/W ≤ 0.5 for SENB-4P specimens. Four point bending fatigue tests were performed on SENB specimens extracted from 50 mm thick welded steel plates. Direct current potential drop (DCPD) for crack monitoring was used as benchmark and validation of the crack lengths determined from a back-face strain gage.

Methodology to evaluate stress corrosion cracking in ethanol environments, applied to circumferential welds on API 5 L steel pipelines

This work presents an experimental methodology developed to perform fatigue crack growth (FCG) and slow strain rate (SSR) tests in ethanol environments aiming to evaluate stress corrosion cracking (SCC) susceptibility of circumferential welds on steel pipelines. FCG and SSR specimens were machined from a welded pipe and the notches were properly designed to promote crack propagation in the different regions of the weld. Tests were carried out keeping the crack-tip region fully immersed in an ethanol solution, which was fueled by a circulation system to ensure replenishment and aeration throughout the test. When applied to a welded API X70 steel pipe, this experimental methodology proved to be an efficient and simple method to achieve relevant and important informations on environmentally assisted crack growth and SCC susceptibility. The method developed here is inserted in the aspects as follows:•Perform tests in slow strain rate and cyclic bend loading in circulating ethanol, to promote the fracture in the different regions of a circumferential weld joint of a steel pipe.•Investigate sensitivity to stress corrosion cracking (SCC) of these different weld regions in ethanolic environment.•This method presents constructive details of a suitable apparatus which the experiments can be easily replicated.

Fatigue Crack Growth Behavior and Fracture Toughness of EH36 TMCP Steel.

The fatigue crack growth behavior and fracture toughness of EH36 thermo-mechanical control process (TMCP) steel were investigated by fatigue crack growth rate testing and fracture toughness testing at room temperature. Scanning electron microscopy was used to observe the fracture characteristics of fatigue crack propagation and fracture toughness. The results indicated that the microstructure of EH36 steel is composed of ferrite and pearlite with a small amount of texture. The Paris formula was obtained based on the experimental data, and the value of fracture toughness for EH36 steel was also calculated using the J-integral method. The observations conducted on fatigue fracture surfaces showed that there were a lot of striations, secondary cracks and tearing ridges in the fatigue crack propagation region. Additionally, there existed many dimples on the fracture surfaces of the fracture toughness specimens, which indicated that the crack was propagated through the mechanism of micro-void growth/coalescence. Based on the micromechanical model, the relationship between the micro-fracture surface morphology and the fracture toughness of EH36 steel was established.

Hydrogen embrittlement resistance of TWIP (twinning-induced plasticity) steel in high pressure hydrogen environment

The behavior of as-received and cold-rolled Fe–25Mn–10Ni–0.65C twinning-induced plasticity steel in hydrogen environment was investigated using fatigue crack growth rate tests. The results show that TWIP steel shows weak hydrogen embrittlement susceptibility, which is due to the formation of deformation twins at the crack tip without strain-induced martensite. Hydrogen can promote the formation of deformation twins and may accumulate in the twin beam, which causes fatigue cracks in TWIP steel to propagate along the twin and slip bands in hydrogen environment. Cold rolling can substantially improve the hydrogen embrittlement resistance and fatigue resistance of TWIP steel.

Microstructure evolution and mechanical behavior of a lean duplex stainless steel aged at 475oC

In this research, the effects of isothermal treatments at 475 °C on the microstructure and mechanical properties of a UNS S32304 lean duplex stainless steel were investigated. Samples of the alloy under as received and aged conditions were analyzed by atomic force microscopy and magnetic force microscopy in order to evaluate the ability of these techniques to identify important aspects that characterize microstructural changes caused by aging. The mechanical behavior was evaluated by tensile tests, hardness tests, Charpy impact tests, crack extension resistance tests, force controlled axial fatigue tests and fatigue crack growth rate tests. The fracture surfaces of all tested specimens were analyzed by scanning electron microscopy. The results indicate that the α-phase spinodal decomposition occurred, with a tendency to saturation for a not too long aging time. The analysis by atomic/magnetic force microscopy allowed to identify characteristics regarding the preferential phase dissolution of the α phase that distinguish the alloy in the as received condition from the aged alloy. As a consequence of the formation of the α’ phase, the absorbed impact energy and the fracture toughness of the alloy decreased substantially. On the other hand, the tensile strength and the fatigue resistance increased significantly. • Possibility to effectively follow spinodal decomposition in a LDSS using the Scanning Probe Microscopy technique. • Short aging time after which the spinodal decomposition and the mechanical behavior of the steel tend to saturate. • Higher values for LDSS impact strength and fracture toughness compared to other duplex stainless steel grades. • An adequate heat treatment can provide interesting values of tensile, toughness and fatigue resistance for the LDSS.