Fatigue Crack Growth Rate Measurements Research Articles

The Influence of Residual Stress on Fatigue Crack Growth Rates in Stainless Steel Processed by Different Additive Manufacturing Methods

The properties and microstructure of Type 304L stainless steel produced by two additive manufacturing (AM) methods—directed energy deposition (DED) and powder bed fusion (PBF)—are evaluated and compared. Localized heating and steep temperature gradients of AM processes lead to significant residual stress and distinctive microstructures, which may be process-specific and influence mechanical behavior. Test data show that materials produced by DED and PDF have small differences in tensile strengths but clear differences in residual stress and microstructural features. Measured fatigue crack growth rates (FCGRs) for cracks propagating parallel to and perpendicular to the build directions differ between the two AM materials. To separate the influences of residual stress and microstructure, K-control test procedures with decreasing and constant stress intensity factor ranges are used to measure FCGRs in the near-threshold regime (crack growth rates ≤ 1 × 10−8 m/cycle). Residual stress is quantified by the residual stress intensity factor, Kres, measured by the online crack compliance method. Correcting the FCGR data for differences in Kres brings results for specimens of the two AM materials into agreement with each other and with results for wrought specimens, when the latter are corrected for crack closure. Differences in microstructure and tensile strength have an insignificant influence on FCGRs in these tests.

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.

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.

On the fatigue crack growth behavior of Ti–10V–2Fe–3Al in gaseous hydrogen

The fatigue crack growth behavior of Ti–10V–2Fe–3Al in gaseous hydrogen (H2) was assessed through comparative experiments conducted in laboratory air and 8.3 MPa H2. The measured fatigue crack growth rate (da/dN) versus applied stress intensity factor range (ΔK) relationships and observed fracture morphologies for laboratory air and H2 were comparable up to ΔK ≈ 6.9 MPa√m, when tested at a load ratio of 0.1 and frequency of 10 Hz. At higher ΔK values, significant crack deflection and subsequent catastrophic failure occurred in the specimen tested in H2. This degradation was not observed in a specimen pre-exposed to 8.3 MPa H2 for 96 h and then immediately tested in laboratory air. X-ray diffraction of the failed H2-tested specimen revealed that the material remnants were predominantly composed of TiH2, suggesting that hydride formation was the catalyst for catastrophic failure in H2. The mechanistic implications of these results and their impact on current material compatibility assessments for Ti alloys in hydrogen service are then discussed.

A means for industry to determine the economic life of bonded joints under representative operation flight loads

The present paper shows that the variability in the cyclic fatigue crack growth (FCG) rate in structural adhesives that is frequently observed in laboratory test results can be captured by using a Hartman-Schijve methodology. To this end the Hartman-Schijve equation has been used to access an ‘upper-bound’ FCG rate curve that (a) encompasses all the experimental data, (b) provides a conservative, ‘worst-case’ FCG rate curve, and (c) accounts for the experimental scatter that is frequently seen under fatigue loading. The importance of allowing for the variability in the measured FCG rate is illustrated by considering FCG in a bonded double-overlap joint subjected to an industry-standard, combat-aircraft flight-load fatigue-cycle spectrum (i.e. FALSTAFF).

A finite element study of fatigue crack propagation in single lap bonded joint with process-induced disbond

This paper presents a method for predicting fatigue crack propagation in adhesive bonded composite joints with an initial full-width disbond using finite element analysis and numerical integration of the material's fatigue crack growth rate law. Fatigue tests were conducted on single lap joints. Crack lengths were monitored from four runout corners. In-situ crack growth measurements were performed by ink injection to identify the crack front profile during fatigue loading. The crack growth was modelled using a fracture mechanics criterion considering two different crack propagation patterns. The material's fatigue crack growth rate law was determined experimentally using the standard double cantilever beam and end notch flexure specimens. Using the total strain energy release rate and the two crack scenarios, the numerical model predicted the lower and upper bounds of the measured fatigue crack growth rates of the lap joint.

Marker load-aided bidirectional fatigue crack growth rate measurement via a semi-elliptical surface crack

It is always impractical to measure the fatigue crack growth rate in the thickness direction of a plate using a standard specimen because of the limited plate thickness. To address this issue, we proposed a method for crack growth rate measurements utilizing a semi-elliptical surface crack, the size of which is determined via marker bands. This method allows the crack growth rates both in the surface direction and the thickness direction to be measured in a single test. The core of the method involves the load spectrum development and the crack growth data analyses. In the load spectrum development, the number of marker load cycles in a load block is estimated through a crack growth equation including the crack closure effect, while the interval of marker load blocks is determined using the crack length measured in a pilot test. The estimated parameters can be amended using the crack growth data acquired by fractographic analyses, should unclear marker bands or unreasonable band spacing occur. In the crack growth data analyses, a closed stress intensity factor solution for a surface-cracked plate with clamped ends was developed, facilitating stress intensity factor calculations. Finally, we demonstrated an application of the proposed method, i.e. fatigue crack growth rate measurements for Ti-6Al-4 V plates. Fine and legible marker bands were observed by fractography, and the resulting da/dN-ΔK curve was consistent with the curves obtained by using standard specimens, justifying the proposed method.

Fatigue crack growth of a railway wheel

Abstract Typically, fatigue crack propagation in railway wheels is initiated at some subsurface defect and occurs under mixed mode (I–II) conditions. For a Spanish AVE train wheel, fatigue crack growth characterization of the steel in mode I, mixed mode I–II, and evaluation of crack path starting from an assumed flaw are presented and discussed. Mode I fatigue crack growth rate measurement were performed in compact tension C(T) specimens according to the ASTM E647 standard. Three different load ratios were used, and fatigue crack growth thresholds were determined according to two different procedures. Load shedding and constant maximum stress intensity factor with increasing load ratio R were used for evaluation of fatigue crack growth threshold. To model a crack growth scenario in a railway wheel, mixed mode I–II fatigue crack growth tests were performed using CTS specimens. Fatigue crack growth rates and propagation direction of a crack subjected to mixed mode loading were measured. A finite element analysis was performed in order to obtain the K I and K II values for the tested loading angles. The crack propagation direction for the tested mixed mode loading conditions was experimentally measured and numerically calculated, and the obtained results were then compared in order to validate the used numerical techniques. The modelled crack growth, up to final fracture in the wheel, is consistent with the expectation for the type of initial damage considered.

A modified loading method for separating the effect of residual stress on fatigue crack growth rate of welded joints

AbstractA novel loading method is proposed to remove the effect of residual stress on measured fatigue crack growth rate in welded joint, using which the influence of empirical crack growth rate correlation equations on the results can be avoided. The total stress intensity factor range and the effective stress ratios corresponding to the combined effect of applied and residual stress are kept constant during the fatigue crack growth test, by changing the applied maximum and minimum loads with the increase of crack length and changes in residual stress based on the superposition rule. The feasibility of the proposed method was validated by tests of compact tension specimens containing residual stresses introduced by four‐point bending. The loading method was then applied to a test of welded compact tension specimen with coupled effect of residual stress and microstructure change on fatigue crack growth rate. The fluctuation in the separated crack growth rate reflects the effect of microstructure change on fatigue crack growth rate.

Tensile and fatigue qualification testing of ITER-CS conduit alloy JK2LB

The ITER Central Solenoid (CS) coils utilize cable-in-conduit conductor (CICC) and the conduit alloy is JK2LB. The production grade conduit alloy (and it's welds) must meet strict requirements for strength, toughness, fatigue crack resistance, and fabricability. The conduit alloy must retain good mechanical properties after additional fabrication steps such as welding, coil winding strain and exposure to the Nb3Sn superconductor's reaction heat treatment. Here we present data from cryogenic tensile, fracture toughness, fatigue crack growth rate, and axial fatigue tests of JK2LB alloy and conduit butt welds, before and after the exposure to the reaction heat treatment. The tests of specimens removed directly from the conduit provide confirmation of the materials properties and the effect of the cold work and aging. The 4 K fatigue performance is extremely important to the reliability of the CS and is covered both by axial cyclic fatigue tests and the fatigue crack growth rate measurements.

The influence of corrosion pits on the fatigue life of 17-4PH steam turbine blade steel

Fatigue life tests on smooth and pre-pitted specimens and near threshold fatigue crack growth rate measurements are performed on 17-4PH steam turbine blade steel. The influence of environment and stress ratio is investigated. A decrease in the fatigue life and a slowdown of the crack growth rate in the near threshold regime is found in salt solution compared to air. The latter can be explained by oxide-induced crack closure. The critical stresses for fatigue failure in the presence of corrosion pits can be calculated using a methodology that was already successfully applied to 12% Cr steam turbine blade steel.

Microscopic Surface Studies of Modified Nickel-Based Superalloy 718 after Corrosion Fatigue in Simulated Sour Environment

The Unites States predicted 60% growth in energy demand by 2030 makes oil and natural gas primary target fuels for energy generation. The fact that the peak of oil production from shallow wells (< 5000 m) is about to be reached, thereby pushing the oil and natural gas industry into deeper wells. However, drilling to depths greater than 5000 m requires materials with high strength-to weight ratio, excellent corrosion resistance in H2S-contating sour environments, and high toughness. Ni-based superalloy, also called Alloy 718, has been considered for use as a bottom-hole component during offshore drilling due to its good general corrosion resistance and lack of magnetism. However, its susceptibility to environmentally assisted cracking (EAC) has not been studied for microstructures with increased high strength. To fill this technology gap, the National Energy Technology Laboratory in collaboration with DNV is currently studying mechanisms of fatigue crack growth rate (FCGR) for Alloy718, oil grade version, in simulated sour well environments. The FCGR behavior of oil grade Alloy 718, was investigated by monitoring crack growth rate in deaerated 0.2% and 20 % NaCl solutions, respectively, in contact with H2S and CO2 as a function of frequency. The partial pressures for H2S (pH2S) was 692 kPa and for CO2 (pCO2) was 3458 kPa in each solution. Each fatigue experiment was performed at 177°C at ΔK=1392 Nmm-3/2 and R=0.2 in the frequency sequence 1Hz/ 0.3Hz/ 0.1Hz/ 0.03Hz/ 0.01Hz/ 0.003Hz followed by ΔK=522 Nmm-3/2 and R=0.7 in the sequence 1Hz/ 0.3Hz/ 0.1Hz/ 0.03Hz/ 0.01Hz in an autoclave. Surface investigations were augmented by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). In this study, research focused on surface analyses supported by the fatigue crack growth rate measurements. SEM backscatter electron (BSE) micrograph of the fracture surface after the fatigue experiment carried in 0.2% NaCl containing CO2 and H2S revealed no accumulation of the corrosion products The EDS chemical analysis near the crack tip found mostly nickel. In general, only transgranular crack growth was observed over the investigated ΔK and frequency scans. SEM investigations of the fracture surface of the sample after the fatigue test in the 20% NaCl containing CO2 and H2S. sour environment at 177°C revealed the presence of secondary cracks, bright inclusions, and beach marks. The EDS chemical analysis of the bright inclusion, showed a (Nb, C)-rich phase. Also, corrosion products were not detected near the crack tip. It appears that the passive layer is disturbed by the cyclic stress in each solution. The presence of secondary cracks at the crack tip of Alloy 718 in 20% NaCl/pH2S = 692 kPa/ pCO2=3458kPa indicates that this environment is more aggressive than 0.2% NaCl/ pH2S = 692 kPa/ pCO2=3458kPa.

Influences on the thermomechanical fatigue crack growth of the nickel alloy 617*

Abstract The following contribution deals with the experimental investigation and theoretical evaluation of fatigue crack growth under isothermal and non-isothermal conditions at the nickel alloy 617. The microstructure and mechanical properties of alloy 617 are influenced significantly by the thermal heat treatment and the following thermal exposure in service. Hence, a solution annealed and a long-time service exposed material condition is studied. The crack growth measurement is carried out by using an alternate current potential drop system, which is integrated into a thermomechanical fatigue (TMF) test facility. The measured fatigue crack growth rates results in a function of material condition, temperature and load waveform. Furthermore, the results of the non-isothermal tests depend on the phase between thermal and mechanical load (in-phase, out-of-phase). A fracture mechanic based, time dependent model is upgraded by an approach to consider environmental effects, where almost all model parameters represent directly measureable values. A consistent description of all results and a good correlation with the experimental data can be achieved.

Stochastic nonlinear fatigue crack growth predictions for simple specimens subject to representative ship structural loading sequences

Recent work by the authors investigated an extension of the finite element analysis of plasticity-induced crack closure to non-stationary, ship structural loading sequences by taking advantage of their inherent time-dependent nature in which the larger loading cycles tend to be clustered together. In doing so, first-order load interactions are presumed to arise from the random occurrence and severity of physical storms encountered by ships and offshore structures throughout their service lives. This material hysteresis is captured through a time-dependent crack “opening” level (Kop) which is based on the evolution of a rate-independent, incremental plasticity model simulating combined nonlinear kinematic and isotropic hardening. The result is a mechanistic rather than phenomenological numerical model requiring only experimentally measured fatigue crack growth rates under constant amplitude, cyclic loading (e.g., ASTM E647-13) and a full material constitutive model defined through experimental push–pull tests for the same material. This approach permits a consideration of material behaviors which are physically relevant to structural steels, yet necessarily omitted in the similar application of a strip-yield model.The present paper generalizes the model originally proposed by the authors to now consider arbitrary storm model loading sequences taken from high-fidelity, time-domain seakeeping codes. To predict the fatigue fracture induced by variable amplitude stress records with upwards of 5×106 time-dependent cycles, a consistent modeling reduction is applied based on the Ordered Overall Range (OOR) or racetrack counting method. The resultant crack growth behavior is demonstrated to converge remarkably well for sufficiently small refined mesh sizes. Using this model, and by considering different arrangements of the same stress record, the importance of nonlinearities (i.e., those associated with ship response as well as material hysteresis) are emphasized.

Fatigue life estimation of pitted 12% Cr steam turbine blade steel in different environments and at different stress ratios

The influence of corrosion pits on the endurable fatigue loading in different environments and at various stress ratios has been investigated for 12% Cr steam turbine blade steel. Very high cycle fatigue measurements were performed using ultrasonic fatigue testing technique with superimposed static load at stress ratios ranging from R=0.05 to R=0.9. Fatigue crack growth rate (FCGR) measurements in the near threshold regime and S–N tests were conducted at a temperature of 90°C in air, de-aerated 300ppbCl− solution and aerated 6ppmCl− solution. The influence of corrosion pits on the fatigue limit was determined with artificially generated corrosion pits. It was found that the FCGRs in solution are lower and the threshold stress intensity factor ranges ΔKth are higher than in air. Fracture surface investigation with scanning electron microscope and roughness measurement were made which suggest oxide and roughness induced crack closure as the most appropriate explanation. Data evaluation of fatigue life tests with pre-pitted specimens supports the applicability of treating pits as effective cracks. An estimation for the stress intensity factor range of pits which allows determining the endurable fatigue loading was empirically found.

The environmentally small/short crack growth effect: current understanding

The key factors that impact on the evolution of stress corrosion and corrosion fatigue cracks in the small crack regime are highlighted and a detailed description is given of the mechanistic basis of the environmentally small/short crack effect. Stress corrosion and corrosion fatigue crack growth rate measurements in the small and short crack regime, principally on steels, are then reviewed and the results discussed in the context of current understanding of the controlling processes.

Application of the dissipated energy criterion to predict fatigue crack growth of Type 304 stainless steel following a tensile overload

This paper presents the results of numerical simulations of fatigue crack growth performed using three-dimensional elastic–plastic finite element analysis. A simple node release scheme is used to simulate crack advancement. The crack front is assumed to be straight. Crack growth following a tensile overload is simulated. The total energy dissipated per cycle is calculated directly from the finite element analysis and used to predict fatigue crack growth. For comparison, fatigue crack growth rate experiments were performed on Type 304 stainless steel C( T) specimens to determine the effect of a single tensile overload. The dissipated energy per cycle is found to correlate well with the measured fatigue crack growth rate following an overload.

An inverse method for evaluating weld residual stresses via fatigue crack growth test data

This paper presents an inverse method for calculating the thermal residual stresses in welded specimens via measured fatigue crack growth rates. Firstly, fracture-mechanics superposition law has been used to extract the stress intensity factor due to residual stress contribution from measured crack growth rate. Secondly, a so-called B matrix has been established by performing finite element analysis. Residual stress distribution is then determined by solving linear algebraic equations relating the B matrix and residual stress intensity factors obtained from crack growth test data. The inverse method has been validated by a well-established residual stress distribution and corresponding stress intensity factor, and then applied to an M(T) sample in 2024-T3 alloy with a longitudinal weld. Agreement with the measured residual stresses is reasonably good and reasons for certain differences between the calculated and measured are discussed.

Study on the surface crack growth behavior in 14MnNbq bridge steel

Three-dimensional crack closure correction methods are investigated in this paper. The fatigue crack growth tests of surface cracks in 14MnNbq steel for bridge plate subjected to tensile and bending loadings are systematically conducted. The experimentally measured fatigue crack growth rates of surface cracks are compared with those of through-thickness cracks in detail. It is found that the crack growth rates of surface cracks are lower than those of through-thickness cracks. In order to correct their differences in fatigue crack growth rates, a dimensionless crack closure correction model is proposed. Although this correction model is determined only by the experimental data of surface cracks under tensile loading with a constant ratio R = 0.05, it can correlate the surface crack growth rates with reasonable accuracy under tensile and bending loadings with various stress ratios ranging from 0 to 0.5. Furthermore, predictions of fatigue life and crack aspect ratio for surface cracks are discussed, and the predicted results are also compared with those obtained from other prediction approaches. Comparison results show that the proposed crack closure correction model gives better prediction of fatigue life than other models.

Mode I fatigue fracture behavior of Divinucell H-30 structural foam – A non-linear approach

Abstract This article describes an experimental and computational study of the non-linear fatigue fracture behavior of Divinucell H-30 polymeric closed-cell structural foam under mode I loading conditions. Cyclic fatigue crack growth tests were performed using foam Compact Tension (CT) specimens at room temperature. The tests were conducted under a load control at a frequency of 2 Hz. The load ratio was R = 0.1. The measured fatigue crack growth rates were related to the J -integral range through the power law relationship. The basic idea of the study was that the calculation of the J -integral using the finite element method should be coupled with damage analysis. For this purpose, the Tsai–Wu failure criterion was implemented into a two-dimensional finite element model of the CT specimen. The verification of the model was carried-out through comparisons between predicted and measured results. The model was used to simulate the mechanical response and study the damage distribution in the CT specimen. The J -integral was calculated over a path not passing through the damage zone determined by the failure criterion. It was shown that taking into account non-linear deformation due to damage improves the fatigue fracture performance of the H-30 foam. This finding was explained with increased energy dissipation as a result of non-linear deformation in the crack tip damage zone. The present work contributes toward developing non-linear methods for analyzing the fatigue fracture performance of structural foams.