Measured Crack Growth Rates Research Articles

Characterization of fatigue crack growth in directed energy deposited Ti-6Al-4V by marker load method

Fatigue crack growth properties of materials are crucial for evaluating damage tolerance in additive manufacturing (AM) metallic structures. However, the unique microstructures and defects of AM materials result in highly complex fatigue crack growth behaviors. Currently, there is a lack of systematic fatigue crack growth rate measurement methods specifically targeting this characteristic. Therefore, taking directed energy deposited Ti-6Al-4V titanium alloy as the object of the study, fatigue crack growth tests were conducted in three orthogonal build orientations of the material using marker load method. Additionally, the visual measurement and compliance methods were also employed to measure fatigue crack growth rates, and the anisotropy of fatigue crack growth property was analyzed. Subsequently, anisotropic fatigue crack growth behaviors were characterized by optical microscope, scanning electron microscope, confocal microscope, and electron backscatter diffraction, suggesting that the microstructure is the primary factor affecting overall fatigue crack growth. Furthermore, nanoindentation tests were conducted to obtain the micromechanical properties within and among columnar grains in different build orientations, clarifying the homogeneity and anisotropy of mechanical properties. Finally, a fatigue crack growth rate measurement method based on marker load method was established, and the advantages of this method in AM materials and structures were summarized by comparing the results with those obtained using these two mature methods.

The impact of loading rate on chloride induced stress corrosion cracking of 304L stainless steel

Abstract The influence of applied loading rate (dK/dt) on stress corrosion cracking (SCC) behavior in annealed 304L stainless steel immersed in 4.7 M MgCl2 solution was assessed at varying temperatures from 23 °C to 70 °C. Measured crack growth rates obtained under rising K loading (dK/dt > 0) are compared to those obtained during static K testing. A rising K-based loading protocol was found to yield more conservative crack growth rates across all temperatures, with the variation in crack growth rates (and therefore the dependence in loading rate) decreasing with increasing environmental severity.

Evaluation of residual stress reproducibility and orientation dependent fatigue crack growth in powder bed fusion stainless steel

The complex thermal gradients of additive manufacturing (AM) result in residual stress and distinctive grain morphologies that influence mechanical performance and contribute to concern regarding the fatigue properties of AM parts. In this study, residual stress, microstructure, and fatigue crack growth rate (FCGR) results were compared in AM Type 304L stainless steel produced by laser powder bed fusion (PBF) on different systems using similar process parameters. Residual stress measured in the build direction was remarkably consistent in all PBF builds. Backscatter electron large area images revealed similar grain morphologies in the different builds, all of which exhibited elongated grains in the build direction and inhomogeneous grain size and shape. Fatigue crack growth investigated both parallel and perpendicular to the build direction revealed higher measured crack growth rates in the near-threshold regime in both orientations of the PBF material compared to wrought material. The difference in near-threshold fatigue crack growth rates is attributed primarily to the influence of processing-induced residual stress quantified by the residual stress intensity factor. These values revealed consistent FCGR effects in each orientation across specimens extracted from different builds and were then used to reveal a convergence of the corrected FCGR data of all PBF and wrought specimens.

Environmentally Assisted Cracking of High-Strength Nickel-Based Alloys Under Cathodic Polarization

Environmentally assisted cracking of various high-strength nickel-based alloys used for subsea components was studied from the perspective of fatigue crack growth rate (FCGR) and static crack growth rate (SCGR) as a function of several critical variables in 3.5 wt% NaCl (pH = 8.2) under cathodic polarization. This paper compiles data from earlier sources as well as more recent work that has been performed in this area. The crack growth rate (CGR) measured under various loading modes, i.e., cyclic, static, and varying stress intensity (K) profiles revealed evidence of susceptibility to hydrogen embrittlement. The measured crack growth rates under cyclic loading exhibited a strong dependence on loading frequency, and no evidence of a plateau in FCGR at low frequencies. The SCGR for various alloys was a function of not only the applied value of K but also of the applied strain rate . In general, Alloy 725 exhibited the highest susceptibility to hydrogen embrittlement, while Alloy 718 exhibited the highest resistance as reflected in both the values of threshold stress intensity factor (Kth) and SCGR values. SCGR was also measured as a function of the applied potential, and first principles modeling suggested that it was related to water adsorption under the crack tip conditions. A crack tip strain rate-based model was applied to rationalize the effect of various loading modes on the measured crack growth rates.

A deep learning-based fatigue crack growth rate measurement method using mobile phones

This paper proposes a fatigue crack growth rate (FCGR) measure method based on deep learning. Crack data sets of different scales are first collected by a camera, and are then used to train the faster region-based convolutional neural network (Faster R-CNN), respectively. A novel global and local dual-scale Faster R-CNN is further constructed and integrated into the mobile phone to predict the crack length during the entire loading cycle. The fatigue crack growth rate constants are finally obtained by integrating fracture mechanics. The proposed method can effectively measure the propagating small cracks during cyclic loading and extend to non-standard specimens.

Crack Growth Behavior in High pH Carbonate/Bicarbonate Environments: Application of Crack Tip Strain Rate Modeling

Aging pipeline infrastructure is subject to various degradation modes, in particular, there is a growing concern about stress corrosion cracking (SCC) from external environments. The current program focused on developing data in high pH carbonate/bicarbonate environments as a function of temperature, applied potential, and loading modes in three different pipeline steels. Crack growth rate (CGR) measurements indicate that the susceptibility to SCC increases with increasing temperature and is higher under low-cycle fatigue compared to constant K conditions. The CGR behavior at each temperature was a strong function of the applied potential, coinciding with the potential range of the active-passive transition in the polarization scans. The potential at which a peak in crack growth rate was observed, coincided with the peak value of the ratio of the current density from the fast to slow scans. The CGR data were fitted to a crack tip strain rate-based model to account for both the cyclic as well as static CGR. The model developed was able to successfully predict prior data in the literature generated under various loading conditions.

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

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

Influence of Deposition Strategies on Residual Stress in Wire + Arc Additive Manufactured Titanium Ti-6Al-4V

Wire + arc additive manufacturing (WAAM) is a modern manufacturing process that has opened new possibilities for rapid builds and reductions in material wastage. This paper explores residual stress in WAAM Ti-6Al-4V walls built using three different deposition strategies: single bead, parallel path, and oscillation path. The effect of interlayer hammer peening and interlayer temperature was investigated for the single bead walls. We also examined the residual stress in compact-tension (C(T)) coupons extracted from large builds (walls) with crack orientation either parallel with or perpendicular to the build direction. This type of sample is often used for the measurement of the fatigue crack growth rate. The contour method was used for experimental determinations of residual stress. In addtion, residual stress in the C(T) coupons was estimated by finite element (FE) analysis. A good agreement was achieved between the contour method and FE analysis. The oscillation-path wall had the lowest residual stress values. For the single bead walls built with various process conditions, residual stress was significantly reduced after removing the substrate. A interlayer temperature of 110 °C resulted in much higher residual stress values in the wall (both tensile and compressive) compared to the continuous build, with much higher interlayer temperature.

Stress corrosion crack propagation affected by microstructures for nuclear steam turbine rotor steels in the simulated environment

The propagation behavior of stress corrosion cracking (SCC) for two CrNiMoV steels in the simulated environment of low-pressure nuclear steam turbine rotor was systematically investigated by microstructural characterizations, electrochemical measurements, slow strain rate tensile tests, and crack growth rate (CGR) measurements. The results showed that both the corrosion susceptibility and stress corrosion susceptibility increased with the increase of localized residual strain, yield strength, and the decrease of grain size. With the higher stress corrosion susceptibility, the SCC CGR of Steel A was higher than Steel B in the corrosive solution at 80 °C, however, at 180 °C the SCC CGR of Steel A was lower than Steel B under the lower applied K. The change of SCC CGR was strongly affected by crack branches, crack deflections and plastic deformation during the SCC propagation, which may have some correlations with the microstructural distribution.

Combined approach for fatigue crack characterisation in metals

In this work a comprehensive characterisation of the fatigue crack growth of an Al 2024 alloy is presented. A powerful Christopher-James-Patterson (CJP) model is used for multi-parameter characterisation of the crack state at a few different locations through the fatigue crack growth. The CJP model of crack tip displacements and stress fields was proposed in order to better capture the influences on the applied elastic stress field of the plastic enclave that is generated around a growing fatigue crack. The model does this through a set of elastic stresses applied at a notional elastic-plastic boundary, and it has been shown to accurately model plastic zone shape and size, whilst its ability to predict the effective range of stress intensity factor during a fatigue cycle has been verified. Thus, the model can predict the driving force component of the crack and additional components that account for different shielding mechanisms. CJP model is combined with full-field non-contact Digital Image Correlation (DIC) so that the model can be fed at any moment with real experimental information. The results are then correlated with Paris law data for Al 2024 alloy through Scanning Electron Microscopy (SEM) observations of the fracture surface. In this work the effect of crack length and load level are thoroughly studied and validated with fatigue crack growth rate measurements.

Improved description of low-cycle fatigue behaviour of 316L steel under axial, torsional and combined loading using plastic J-integral

Low-cycle fatigue behaviour and fatigue crack kinetics of the 316L austenitic stainless steel were studied under cyclic axial, torsional and in-phase combined loading using hollow cylindrical (tubular) specimens with a small hole for crack initiation. The concept of plastic.J-integral was used, which was shown in previous studies to unify the crack growth rate data for several different materials. Dependencies of Jp on crack length were determined by extensive finite element modelling considering non-linear material behaviour according to the cyclic stress–strain curve. Locally deflected cracks were modelled in accordance with amplitudes of the axial and torsional components of combined loading. The measured crack growth rate diagrams for all types of loading and for various loading amplitudes were unified using amplitude of Jp. Fatigue lives under torsional loading were much longer than under axial loading for the same equivalent plastic strain amplitude, which was explained by higher crack driving forces in terms of Jp under axial loading than under torsional loading. Fatigue lives estimated by crack propagation based on a master curve in terms of Jp,a were in a good agreement with those obtained experimentally under all types of loading. The used concept can reduce the experimental program to obtaining of material data only for axial loading, which can then be used for prediction of behaviour under in-phase multiaxial loading. The von Mises formula for multiaxial low-cycle fatigue loading εeq,p2 = εp2 + γp2 / 3 was modified so that the fatigue lives under axial, torsional and combined loading were characterized in a matching way. Using the formula εp,Nf2 = εp2 + γp2 / 25, the fatigue life data fell on a single Coffin-Manson curve.

Structural fatigue and fracture of shape memory alloy actuators: Current status and perspectives

Shape Memory Alloy (SMA)-actuators are efficient, simple, and robust alternatives to conventional actuators when a small volume and/or large force and stroke are required. The analysis of their failure response is critical for their design in order to achieve optimum functionality and performance. Here, (i) the existing knowledge base on the fatigue and overload fracture response of SMAs under actuation loading is reviewed regarding the failure micromechanisms, empirical relations for actuation fatigue life prediction, experimental measurements of fracture toughness and fatigue crack growth rates, and numerical investigations of toughness properties and (ii) future developments required to expand the acquired knowledge, enhance the current understanding, and ultimately enable commercial applications of SMA-actuators are discussed.

Mechanistic fatigue in Ni-based superalloy single crystals: A study of crack paths and growth rates

The mechanistic basis of microstructurally short crack paths and growth rates is investigated in Ni-based superalloy single crystals using Crystal Plasticity (CP) and eXtended Finite Element (XFEM) analyses of experimental edge-cracked samples over a range of crystal orientations. Crack paths are determined to be those along crystallographic slip systems within which the slip is highest. Crack growth rates are determined by the crack tip critical stored energy density. Experimental observations of tortuous crack paths and their dependence on crystal orientation are reasonably well captured by the mechanistic model. Key features of alternating and straight crack paths are reproduced. The experimentally measured crack growth rates as a function of crystal orientation are also captured by the mechanistic model and controlled by the crack tip critical stored energy density which was found to be 385 Jm−2 in the Ni-based superalloy single crystals analysed. A new methodology for determination of critical stored energy density and mechanistic quantification of short crack growth rates is presented.

The Effect of Loading Rate on the Environment-Assisted Cracking Behavior of AA7075-T651 in Aqueous NaCl Solution

The influence of loading rate on the environment-assisted cracking (EAC) behavior of AA7075-T651 immersed in 0.6 and 1.0 M NaCl solution was assessed at applied potentials ranging from −800 to −1200 mVSCE via a slow-rising stress intensity (K) testing methodology. Measured crack growth rates under rising K loading are compared to those obtained using a fixed K protocol, which revealed that rising K-based testing consistently yields increased crack growth rates relative to static K approaches across all tested conditions. However, relative to other alloy systems, EAC in AA7075-T651 is only modestly loading rate-dependent, as demonstrated by testing conducted at fixed dK/dt ranging from 0.25 to 2.0 MPa√m/h. The implications of the observed results are considered in the context of current EAC testing specifications, with specific focus on the conservatism and efficiency of rising K-based approaches.

Characterization of Surface Fatigue Crack Nucleation and Microstructurally Small Crack Growth in High Strength Aluminum Alloys

It is well established that fatigue crack nucleation and small crack growth in high strength aluminum alloys are highly influenced by the surrounding microstructure including grain boundaries, texture, inclusion barriers, among other factors. As such, specific and targeted experimental and computational methods are necessary to accurately capture and predict the discrete behavior of microstructurally small fatigue cracks. In this study, surface fatigue crack nucleation and microstructurally small crack growth in high strength aluminum alloys, commonly used in aerospace applications, are evaluated through a holistic approach encompassing fatigue testing, crack measurement, and computational prediction of crack growth rates. During fatigue testing, crack shapes and growth are quantified using a novel surface replication technique that is applied to investigate crack nucleation, as well as to collect validation data that includes an accurate description of crack shape during crack propagation, a challenging and essential component in predicting crack growth. Computational simulation of fatigue crack growth in non-straight, complex surface crack arrays typically requires high fidelity analysis using computationally expensive methods to account for the mathematical and geometrical complexities inherent in the solution. A dislocation distribution based technique has been previously demonstrated to rapidly and accurately predict the stress intensity factors for through cracks of complex shape. This method was expanded and investigated as an approach for rapidly predicting the crack growth rate of kinked and tortuous surface crack arrays, using the crack configuration and bulk material properties as inputs. To investigate the accuracy and effectiveness of this characterization approach, surface crack growth in AA7075-T7351 was experimentally analyzed and modeled under high cycle and low cycle fatigue conditions. This comprehensive approach was determined to be an expedient and applicable method for characterizing and evaluating the nucleation and crack growth rate of non-planar microstructurally small and short crack configurations.

Microstructure, mechanical properties and IASCC susceptibility of stainless steel baffle bolts after 30 years of operation in a PWR

Neutron irradiation in the core of a nuclear power plant can change the material properties of stainless steel significantly. In this paper cold worked AISI 316 baffle bolts retrieved from Doel 1 PWR were investigated with the aim to assess the structural degradation due to neutron irradiation by analyzing microstructure, mechanical behavior and stress corrosion susceptibility. The baffle bolts had received neutron doses ranging from 6 to 59 dpa covering a major part of a nuclear power plant's lifetime. First, microstructural investigations were carried out to reveal the defect and dislocation density and the amount of void swelling. Then mechanical properties were determined using tensile tests and fracture toughness tests. The irradiation assisted stress corrosion cracking (IASCC) susceptibility was investigated by slow strain rate testing and crack growth rate testing. Results were compared with literature values and found to be in good agreement. Main conclusions were that void swelling was small even for the highest dpa values and that the fracture toughness values saturated above 15 dpa. The material was susceptible to IASCC as shown by SSRT and crack growth rate measurements exhibiting intergranular fracture morphology.

Accelerating fatigue simulations of a phase-field damage model for rubber

Phase-field damage models are able to describe crack nucleation as well as crack propagation and coalescence without additional technicalities, because cracks are treated in a continuous, spatially finite manner. Previously, we have developed a phase-field model to capture the rate-dependent failure of rubber, and we have further enhanced it to describe failure due to cyclic loading. Although the model accurately describes fatigue failure, the associated cyclic simulations are slow. Therefore, this contribution presents an acceleration scheme for cyclic simulations of our previously introduced phase-field damage model so that the simulation speed is increased to facilitate large-scale simulations of industrially relevant problems. We formulate an explicit and an implicit cycle jump method, which, depending on the selected jump size, reduce the calculation time up to 99.5%. To circumvent the manual tuning of the jump size, we also present an adaptive jump size selection procedure. Thanks to the implicit adaptive scheme, all material parameters are identified from experiments, which include fatigue crack nucleation and crack growth. Finally, the model and its parameters are validated with additional measurements of the fatigue crack growth rate.

Investigation of the Enhancement Interactions between Double Parallel Cracks on Fatigue Growth Behaviors.

In this paper, interactions of double parallel cracks were studied by performing experiments and numerical simulations. Fatigue crack propagation tests were carried out to measure crack growth rates in the specimens with double parallel cracks or a single crack. Finite element method was adopted to calculate stress intensity factors at the crack tips. Results show that the double parallel cracks at different positions present a shielding effect or enhancement effect on crack growth rates and stress intensity factors. When the double parallel cracks are offset, crack interactions mostly behave as enhancement effects. Empirical formulas were obtained to calculate the stress intensity factor at the “dangerous” crack tip of the double parallel cracks. By modifying the material parameters in Paris equation of the single crack, the double parallel cracks are simplified into a single crack with the same crack growth rates.

Use of DC voltage fluctuation method to investigate real-time Mode I and Mode II subcritical crack growth behavior in gypsum rock

Subcritical crack growth strongly influences the mechanical behavior of rock and the precursory phase of geohazards and catastrophic failure. In earlier studies, indirect measuring methods were used to investigate the subcritical crack growth behavior of rock materials. However, these methods cannot fully reflect the whole process of subcritical crack growth of geomaterials. Therefore, in this study, a new method (DC voltage fluctuation method) was developed which can monitor the whole process of crack growth and measure the crack growth rate in real time. This method is based on the monitoring principle of change in voltage caused by the change in resistance. Then, the whole processes of Mode I and Mode II subcritical crack growth of gypsum specimen were monitored in real time by the self-designed fracture strain gauge, which consists of 10 breakable resistance wires, and a DC voltage signal monitor. Furthermore, using the measured crack growth rates and the stress intensity factors of varying crack path lengths, the parameters of the modified Charles model were fitted. The results show that: (1) the subcritical crack growth process mainly includes three stages: cracking latent period (primary stage), steady-state crack growth stage (secondary stage), and accelerated crack growth stage (tertiary stage). The cracking latent period lasts for the largest proportion of the entire fracture process duration. The duration of each stage decreases gradually with increasing applied load. (2) In the steady-state crack growth stage, both Mode I and Mode II crack growth rates increase with increasing applied load. However, the crack growth rates in the accelerated stage are basically the same with increasing applied load. When a specimen is subjected to a given constant applied load, the subcritical crack growth rate of Mode II is higher than that of Mode I in the secondary and tertiary stages. (3) The parameters A and n of the subcritical crack growth model (modified Charles model) (secondary stage) increase with increasing external applied load. The parameters are closely associated with the size of fracture process zone at the crack tip. The results of this study can provide a deeper insight into the macroscopic failure of rock, and also provide the parameters for the time-dependent damage evolutional process of underground constructions.

Hydrogen Embrittlement of Low Alloy Steels Under Cathodic Polarization

Hydrogen embrittlement of low alloys steels at three different strength levels (745 Mega Pascals [MPa], 904 MPa, and 1,166 MPa) were evaluated under cathodic polarization. Crack growth rate measurements were performed under constant stress intensity (K) conditions, as a function of applied K values as well as applied potential to characterize the behavior of the three different steels. At −1,050 mVSCE saturated calomel electrode (SCE), the threshold stress intensity (Kth) value increased from 44 MPa√m to 60 MPa√m as the yield strength decreased from 1,166 MPa to 745 MPa. The crack growth rate at 66 MPa√m and −1,050 mVSCE decreased from 3 × 10−5 mm/s to 4 × 10−8 mm/s as the yield strength decreased from 1,166 MPa to 745 MPa. For the 1,166 MPa steel at low values of K, the crack growth rate decreased by two orders of magnitude as the potential decreased from −1,000 mVSCE to −950 mVSCE. At higher values of K, the effect of potential on the crack growth rate was not as significant. The 745 MPa steel in general exhibited slow crack growth rate values (2 to 4 × 10−8 mm/s) over the range of K values and applied potentials in which it was evaluated. Water adsorption on fresh metal surfaces in the estimated crack tip chemistry was modeled using density functional theory. The variation in crack growth rate with applied potential at low and intermediate values of K correlated with the fractional coverage of water adsorption on the fresh metal surface. It is proposed that the water reduction reaction and the subsequent generation of hydrogen are the rate limiting steps in the slow subcritical crack growth rate processes for low alloy steels under the conditions evaluated. For the higher values of K, where the crack growth rate showed a weak dependence on applied potential, water reduction, and generation of hydrogen are likely not the rate limiting steps.