Fatigue Crack Growth Threshold Research Articles

A qualitative analytical investigation of geometrically nonlinear effects in wind turbine blade cross sections

This paper analytically investigates the Brazier effect on asymmetric thin-walled sections subject to biaxial bending. In the latter case a torsional moment – in this paper referred to as Brazier torsion – is induced, which proved to be a vital part of the solution. By means of a generic cross section, that was inspired by a wind turbine blade, it is demonstrated that geometric nonlinear effects can induce an in-plane opening deformation in re-entrant corners that may decrease the fatigue life. The opening effect induces Mode-I stress intensity factors which exceed the threshold for fatigue crack growth at loads well below the load-carrying capacity of the beam.The findings in this paper are twofold: Firstly, the investigated analysis procedure can be integrated into the design process of wind turbine blade cross sections. Secondly, the proposed approach serves as a basis for computationally efficient numerical analysis approaches of structures that comprise complex geometry and anisotropic material behaviour – such as wind turbine rotor blades.

Experimental investigation and numerical simulation on fatigue crack behavior of bridge steel WNQ570 base metal and butt weld

The experimental study on the fatigue crack growth rate and the threshold of high strength bridge steel WNQ570 base metal and butt weld has been performed. Two data processing methods are conducted to derive fatigue crack growth parameters of 95% survival probability. It turns out that the fatigue crack growth rate based on each specimen data is larger than that based on group method of data processing in terms of normal stress intensity range (10–70MPam0.5). The fatigue crack growth rates increase and the fatigue crack growth thresholds decrease with increasing stress ratios. For WNQ570 of the batch utilized in this research, the fatigue crack growth rate of base metal is lower than butt weld. The fatigue crack performance of WNQ570 base metal is also better than other common bridge steels including HPS485W, 14MnNbq, Q345qD and the universal steel performance provided by BS7910 and IIW-1823-07. Based on the measured FCGR parameters, numerical simulation by Franc3D and ABAQUS can precisely predict the fatigue life of compact tension specimens, which demonstrates that simulation method is appropriate to be applied to fatigue assessment practice of whole steel bridges based on linear elastic fracture mechanics (LEFM).

302 Effect of hydrogen on fatigue crack growth threshold of carbon steels

302 Effect of hydrogen on fatigue crack growth threshold of carbon steels

Experimental investigation on fatigue crack behavior of bridge steel Q345qD base metal and butt weld

Abstract The experimental study on the fatigue crack growth rate (FCGR) and the fatigue threshold of a widely-used bridge steel in China, Q345qD, has been performed. Both Q345qD base metal and butt welds are tested. Two data processing methods are performed to derive the characteristic fatigue crack growth parameters of 95% survival probability. These parameters can provide essential input data for the fatigue assessment of steel bridges using Q345qD. It turns out that the fatigue crack growth rate based on each specimen data and small sample technique is larger than that based on group method of data processing in terms of normal stress intensity range (10–70 MPa m 0.5 ). The fatigue crack growth rate is found to increase and the fatigue crack growth threshold is found to decrease as the stress ratio increases. For Q345qD of the batch utilized in this research, the fatigue crack growth rate of base metal is slower than butt weld. The fatigue crack propagation performance of Q345qD base metal is also better than other bridge steels including HPS485W, 14MnNbq, Q500qE and the universal steel performance provided by BS7910 and IIW-1823-07. In addition, the small sample technique proposed in this paper can provide some references for dealing with the uncertainties of the material tests within the limitation of the number of the specimens.

Mechanics and mechanisms of fatigue in a WC–Ni hardmetal and a comparative study with respect to WC–Co hardmetals

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC–Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC–Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path.

Near threshold fatigue crack growth in ultrafinegrained copper

The near threshold fatigue crack growth in ultrafine-grained (UFG) copper at room temperature was studied in comparison to conventional coarse-grained (CG) copper. The fatigue crack growth rates da/dN in UFG copper were enhanced at ΔK ≤ 7 MPa√m compared to the CG material. The crack closure shielding, as evaluated using the compliance variation technique, was shown to explain these differences. The effective stress intensity factor amplitude AKeff appears to be the same driving force in both materials. Tests performed in high vacuum on UFG copper demonstrate the existence of a huge effect of environment with growth rates higher of about two orders of magnitude in air compared to high vacuum. This environmental effect on the crack path and the related microstructure is discussed on the basis of fractography observations performed using scanning electron microscope and completed with field emission scanning electron microscope combined with the focused ion beam technique.

Fracture and fatigue behavior of WC–Co and WC–CoNi cemented carbides

The fracture and fatigue characteristics of several cemented carbide grades are investigated as a function of their microstructure. In doing so, the influence of binder chemical nature and content (Co and 76wt.% Co–24wt.% Ni), as well as carbide grain size on hardness, flexural strength, fracture toughness and fatigue crack growth (FCG) behavior is evaluated. Mechanical testing is combined with a detailed inspection of crack–microstructure interaction, by means of scanning electron microscopy, in order to evaluate crack-path tortuosity and to discern fracture and fatigue micromechanisms within the metallic binder. Results show that CoNi-base hardmetals exhibit slightly lower hardness but higher toughness values than Co-base grades. Meanwhile, flexural strength is found to be rather independent of the binder chemical nature. Regarding FCG behavior, experimental results indicate that: (1) FCG threshold (Kth) values for coarse-grained grades are higher than those measured for the medium-grained ones; and (2) fatigue sensitivity levels exhibited by CoNi- and Co-base cemented carbides, for a given binder mean free path, are similar. These findings are rationalized on the basis of the increasing relevance of crack deflection mechanisms as microstructure gets coarser and the evidence of similar fatigue degradation phenomena within the binder independent of its chemical nature, respectively.

Fatigue crack growth behavior of a coarse- and a fine-grained high manganese austenitic twin-induced plasticity steel

The fatigue crack growth behavior of a coarse-grained (CG) and a fine-grained (FG) high manganese austenitic twin-induced plasticity (TWIP) steel has been investigated at room temperature. Crack growth tests were performed at stress ratios of 0.1 and 0.6 under the control of stress intensity factor range using three-point bending specimens. The results indicate that at the two stress ratios, the CG steel exhibits a higher fatigue crack growth resistance than the FG steel in both the near threshold and Paris regimes. Furthermore, a decreased stress ratio and an increased grain size both lead to an increased fatigue crack growth threshold. Microstructural observations reveal that cracks propagate more tortuously in the CG steel than in the FG steel, accompanied by rougher fracture surfaces, which tends to generate more roughness-induced crack closure and thus a higher fatigue threshold value. Additionally, the CG steel shows much larger plastic zone sizes ahead of the crack tip than the FG steel, suggesting that plasticity-induced crack closure may also play an important role in decreasing the fatigue crack growth rate in the CG steel. By excluding the crack closure effects, the CG steel still demonstrates a higher effective crack growth threshold than the FG steel; this is considered to be due to the increased planarity of slip in the CG steel, as compared with that in the FG steel.

Fatigue small crack growth threshold determination of a high-Nb TiAl alloy at different temperatures by in-situ observation

The purpose of this paper is to estimate the fatigue crack growth threshold of a high-Nb TiAl alloy at the different temperatures based on scanning electron microscopy (SEM) in-situ observation. The results indicated that the fatigue crack growth threshold ΔKth of a nearly lamellar high-Nb TiAl alloy with 8% Nb content at room temperature and 750°C was determined as 12.89 MPa·m1/2 and 8.69 MPa·m1/2, respectively. The effect of the elevated temperature on the fatigue crack growth threshold cannot be ignored. At the same time, the early stage of fatigue crack propagation exhibited multicrack initiation and bridge-link behavior.

An atomistic investigation into the nature of near threshold fatigue crack growth in aluminum alloys

Despite decades of study, the atomic-scale mechanisms of fatigue crack growth remain elusive. Here we use the coupled atomistic–discrete dislocation method, a multiscale simulation method, to examine the influence of dislocation glide resistance on near-threshold fatigue crack growth in an aluminum alloy. The simulations indicate that the threshold increases with an increase in dislocation glide resistance, and that a transition in the crack growth direction can occur when dislocation nucleation is inhibited by other nucleated dislocations. Three main mechanisms of fatigue crack propagation are observed: cleavage along the primary slip plane, crack extension by dislocation emission, and crack extension by opening along lattice defects.

Evaluation of Fatigue Crack Growth Rates and Threshold Conditions in an Al 2124-T851 Alloy at Different Load Asymmetry Considering Microstructure Orientation

Results of a fairly comprehensive experimental programme aimed at evaluation of fatigue crack growth (FCG) rates in an aircraft Al 2124 alloy in T851 state, after hot rolling, solution annealing, small plastic strain and artificial ageing are presented and discussed. Measurement was performed not only in the region of stable crack growth, but also in the threshold region enabling to estimate threshold values of stress intensity factor range using method of regression analyses. FCG rates were evaluated in both L-T and T-S directions to evaluate sensitivity of FCG resistance on different microstructure orientation. Different load asymmetry conditions R = Fmin / Fmax were used, namely R = 0.1 and 0.6. Repeated measurements in more than one specimen at each condition enabled to evaluate reproducibility, scatter of measurement and to perform eventually probabilistic assessment. The reproducibility of measurement was particularly good for L-T orientation. In this case, threshold values were somewhat higher and FCG rates lower in comparison with the T-S orientation. In the contrary, cyclic fracture toughness in T-S orientation was slightly higher. The results are discussed form the viewpoint of residual fatigue life in aircraft components considering probabilistic aspects.

Measurement of Local Initiation, Early Growth and Retardation of Physically Short Fatigue Cracks Using Amended DCPD Method

Different experimental methods are being used in laboratories for automatic measurement of fatigue crack growth rates and threshold values. Such data belong to mechanical properties essential for an assessment of residual life of components and structures containing cracks of length more than several millimetres. However, if the material contains small crack-like defects like inclusions or pores, knowledge about local resistance against fatigue growth of physically short cracks becomes very important. Damage mechanisms are even more complicated in case of a presence of subsurface residual stress field, e.g. due to technological effects. Experimental difficulties connected with investigation of short fatigue crack growth (FCG) can be reduced in case of use of automatic indirect methods. The aim of the work described in this paper was to explore possibilities and limits of the use of DCPD (direct current potential drop) method for physically short crack measurement and to use the optimised method for an evaluation of local damage process of initiation and early growth of short fatigue crack in residual stress field induced by shot peening.

A comparative study of the fatigue behavior of two heat-treated nodular cast irons

The mechanical strength of nodular cast iron (NCI) can be improved by heat treatment. Isothermal Ductile Iron (IDI) competes with Austempered Ductile Iron (ADI) for applications subject to dynamic loading. The preliminary metallographic analysis of ADI and IDI showed similar graphite nodule characteristics but different metallic matrix structure. An extensive mechanical characterization of the two materials determined and compared (i) the high cycle fatigue strength, (ii) the fatigue crack growth rates and threshold ΔKth, (iii) the typical crack propagation mechanisms, and (iv) the material heterogeneity in terms of local tensile tests.

Influence of dissolved hydrogen on the fatigue crack growth behaviour of AISI 4140 steel

In this investigation, the influence of dissolved hydrogen on the fatigue crack growth behaviour of a low alloy steel (AISI 4140) was examined. The investigation also examined the influence of microstructure on the fatigue threshold and the fatigue crack growth behaviour of this material. Compact tension (CT) specimens prepared from AISI 4140 steel as per ASTM standard E-647; were tested in two different heat treated conditions, i.e., cold rolled and annealed as well as austempered condition. The specimens were precharged with hydrogen using ex situ cathodic charging technique at a constant current density for three different time periods ranging from 150 to 250h. The samples were then subjected to fatigue crack growth test in the threshold region as well as in the linear region. The near threshold fatigue crack growth rate, fatigue threshold and the crack growth rate in the linear region of these specimens were determined. The fatigue crack growth behaviour of the hydrogen charged samples were compared with specimens without any dissolved hydrogen. The current test results show that the near threshold crack growth rate increases with the increase in dissolved hydrogen content. It was also observed that the fatigue threshold decreases as the dissolved hydrogen content increases. The crack growth rate was found to be higher in the austempered samples compared to the annealed samples. An interesting phenomenon was observed in the case of annealed specimens charged with hydrogen for 250h. These specimens had unusually high fatigue threshold (ΔKth).

GSM Modem and Industrial Camera Aided Unattended Automatic Fatigue Testing Technology

In order to reduce the time and labor cost, an unattended testing system which can be monitored remotely for fatigue crack growth threshold test was developed. Compliance technique is used to measure fatigue crack length automatically. GSM modem and industrial camera are utilized to realize remote monitoring. Fatigue crack growth threshold test on J2 steel, which is used on the wheel of high-power locomotive, has been carried out with the developed system. The testing process and results demonstrated the safety and efficiency of the system.

Crack Growth Life Prediction Based on Monte-Carlo Method for High Cycle Fatigue Dynamic Component

The sensibility analysis of the factors to crack growth life has been done. The results show that the input parameters have the following precedence ordering: fatigue crack growth threshold, fracture spectrum, initial crack, fracture toughness, the sensibility values are 11.25, 8.5417, 0.8333, 0.1125, respectively. The model parameters have the following precedence ordering: n, p, C, q. the sensibility values are 6.0417, -3.9583, 1.25, 0.1812, respectively. The reliability analysis was conducted by Monte-Carlo method, the results show that the crack growth life accord with lognormal distribution. The lives with different reliability were obtained. The reliability analysis results of the crack growth life has provided the data for a hybrid approach based on a mixture of the traditional safe-life and damage tolerance techniques which were used as an optimal strategy for ensuring the helicopter structural integrity.

Fatigue Crack Growth Behavior of 316LN Stainless Steel with Different Nitrogen Contents

Fatigue crack growth (FCG) tests were carried out at ambient temperature in air for 316 L (N) stainless steel varying with nitrogen (0.08, 0.14 and 0.22 N (wt %)) designated as 8N, 14N and 22N. The crack growth was monitored with direct current potential drop technique (DCPD) and crack closure estimated by compliance technique. The threshold stress intensity factor range (Δ Kth ) for 8N, 14N and 22N steels were found to be 5.1, 10.5 and 5.78 MPam 1/2 respectively. The effective threshold stress intensity factor range (Δ Keff,th ), (i.e., after correcting for crack closure) were found to be 4.5, 7.7, and 4.4 MPam 1/2 respectively. Both the extrinsic and intrinsic thresholds at the 14N steel were found to be higher than that at other nitrogen contents viz, 8N and 22N. A model has been proposed for quantifying slip irreversibility using the fatigue fracture surface roughness parameters. The trend in variation of FCG thresholds with different nitrogen levels follows that of slip reversibility estimated using this model. The results imply that the lower slip irreversibility indicated higher fatigue thresholds at 14N.

Mixed mode delamination growth of multidirectional composite laminates under fatigue loading

Fatigue delamination growth of multidirectional laminates with 45°/−45° fracture interface was experimentally studied. Delamination tests under three mixed I/II mode ratios were carried out via mixed mode bending test apparatus. Significant R-curve effects on the fatigue crack growth rates and thresholds due to fiber bridging and crack branching were observed, which make the traditional Paris Law unsuitable. In this paper, a formerly developed modified Paris Law with the normalized strain energy release rate as the fracture governing parameter is investigated again. The results from the multidirectional delamination tests fit the modified Paris Law very well, and hence have validated it by experiments. Moreover, the thresholds normalized to delamination resistance were utilized to evaluate the critical fatigue crack growth conditions. By the combination of test data for both 0°/5° and 90°/90° laminates, an interface-independent normalized threshold was proposed, which was found linearly decreasing with mode mixture ratios.

Fatigue crack growth in interstitially hardened AISI 316L stainless steel

Fatigue crack growth in interstitially hardened 316L stainless steel has been investigated in air and 0.6M NaCl. Large compressive residual stresses introduced by “colossal” carbon supersaturation resulting from low temperature carburization increase the fatigue crack growth threshold from 8MPa√m to 10MPa√m, and retard the crack growth rate by a factor of two in the low stress intensity range. Fractography revealed no observable difference between the fracture surfaces of the bulk material and the hardened layer, either in air or the corrosive environment.

Effect of welding residual stresses on fatigue crack growth thresholds

Fatigue crack growth thresholds ΔKth were determined for friction stir welded butt joints made from aluminium alloys AA2024 and AA6013. Plotting the thresholds as a function of load ratio R showed distinctly higher amounts for welded joints as compared to those for parent material at small load ratios, but differences became smaller with increasing load ratio, until thresholds became finally identical for the highest R values. Applying Döker’s concept of two controlling parameters, namely ΔK and Kmax[1], and plotting ΔKth versus Kmax, however, revealed that the effective threshold ΔKth,eff determined at very high R ratios was nearly independent on the alloy and, simultaneously, was identical for parent material and respective welded joints. Thus, differences in threshold behaviour were only caused by the second threshold Kmax,th, which was significantly higher for welded joints as compared to parent material. Differences in Kmax,th coincided with compressive residual stresses determined by cut-compliance measurements in terms of stress intensity factors Krs acting at the crack tip. Based on the analytical approach described by Döker [1], only one characteristic Krs value was needed to calculate all thresholds of welded joints for 0⩽R⩽1 provided a base material master curve is available.