Crack Growth Threshold Research Articles

Effect of hydrogen-gas pressure on threshold of hydrogen-induced crack growth for carbon steel JIS-SM490B, low-alloy steel JIS-SCM435 and stainless steel JIS-SUS304

Effect of hydrogen-gas pressure on threshold of hydrogen-induced crack growth for carbon steel JIS-SM490B, low-alloy steel JIS-SCM435 and stainless steel JIS-SUS304

Fatigue crack growth behavior of thermo-mechanically processed AA 5754: Experiment and extended finite element method simulation

Fatigue and fracture parameters such as sigmoidal curve, fatigue crack growth threshold, stress intensity factor (SIF), and Paris constants are evaluated for thermo-mechanically processed Al-Mg (AA 5754) alloy at different stress-ratio. Experimental and numerical techniques are utilized for fracture and fatigue studies. Cryorolling and metal inert gas (MIG) welding processes are adopted to alter its mechanical and fatigue properties. The effect of cyclic stress ratio, thermo-mechanical processing, and MIG welding has been investigated on crack growth rate of the alloy. Extended finite element method (XFEM)-based program is developed to simulate both two-dimensional (2D) and three-dimensional (3D) crack geometries for quasi-static crack growth.

Notch fatigue and crack growth resistance of Ti-6Al-4V ELI additively manufactured via selective laser melting: A critical distance approach to defect sensitivity

This work is aimed at exploring the applicability of the theory of critical distances to the notch fatigue prognosis of Ti-6Al-4V ELI additively manufactured via selective laser melting. Particular attention is paid on investigating how the defectiveness influences the material critical length and the path followed by the fatigue cracks after nucleation. Fully-reversed axial fatigue tests are carried out on notched specimens of different severity and macrocrack growth tests are performed to determine the crack growth threshold ΔKth. The critical distance lengths are estimated according to a critical distance inverse search procedure and compared with the value deduced from ΔKth.

Effect of graphite morphology on the fatigue and fracture resistance of ferritic ductile cast iron

In this paper, four material variants of an EN-GJS-400-type ferritic ductile cast iron (DCI) are produced by varying solidification time and chemical composition. In this way, experimental batches with different size of graphite nodules and spheroidal graphite fraction are obtained. The material variants are characterized through static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests. It is shown that by increasing the mean diameter of the spheroidal graphite particles fatigue strength, tensile strength and total elongation decrease, while fatigue crack growth threshold and fracture toughness increase. On the contrary, reducing the spheroidal graphite fraction owing to degeneration into chunky graphite results in lower tensile and fatigue properties as well as fracture toughness, while the near-threshold fatigue crack growth resistance is improved. This effect is discussed by the light of crack closure, fracture surface roughness and fractographic analyses.

Microstructural effects on the sub-critical fatigue crack growth in nano-bainite

Bainitic transformation at low temperatures in high carbon steels leads to a dual phase nanostructure of bainitic ferrite and retained austenite. Strengths above 1 GPa can be easily achieved in these steels while also preserving significant amount of ductility. However, in order to be used in most structural applications, microstructure tailoring for high damage tolerance under fatigue loading is of utmost importance. In the current study, we have developed three distinct bainitic microstructures by austempering at 250, 300 and 350 °C respectively after similar austenitization treatment. Sub-critical fatigue crack growth investigation for all the specimens has been carried out under ambient environmental conditions. It has been found that the specimen with the coarsest morphology of bainitic ferrite and retained austenite and the highest retained austenite content shows the largest crack growth threshold. Threshold has been found to be reached for the specimens when the crack tip opening displacement attains the respective bainitic lath thickness for different isothermal conditions. In Stage II, the specimens with the largest lath thickness and highest austenite content showed marginally slower crack growth rates than the other specimens. This can be attributed to transformation induced plasticity that is expected to be more severe in the specimens with the highest austenite content. The specimens transformed at the highest isothermal temperature also showed the highest plane strain fracture toughness deduced from the stress intensity value at which the crack propagates catastrophically.

Fatigue performances of selective laser melted Ti-6Al-4V alloy: Influence of surface finishing, hot isostatic pressing and heat treatments

In this study, a detailed evaluation on the effects of different surface finishing processes, hot isostatic pressing (HIP) and heat treatments on the fatigue performance of SLMed Ti-6Al-4V is conducted. It is found that surface finishing processes of turning, grinding, grinding followed by sandblasting and polishing can effectively reduce the surface roughness of as-SLMed surfaces (Rz = 68.66 μm) and significantly improve the fatigue performances. The HIP, HT-920 and HT-850-550 treatments can significantly improve the ductilities of SLMed Ti-6Al-4V due to the coarsening of α laths and formation of β grains. The ultimate tensile strengths (UTSs) and yield strengths (YSs) of HIPed and HT-920 specimens reduce more than 140 MPa, while those of the HT-850-550 specimens show no obvious reduction. The SLMed Ti-6Al-4V specimen has a fatigue limit lower than 300 MPa, while the HT-920 and HT-850-550 treatments enhance the threshold of crack growth and increase the fatigue limit to 350–400 MPa. The reduction or elimination of pores and the modification of microstructure in HIPed specimens both significantly extend the period of fatigue crack initiation, leading to an increased fatigue limit of 450–500 MPa.

The response of accumulated stress and microstructural refinement on cyclic deformation and associated mechanism in high strength bainite-martenite dual-phase steel

We elucidate here the response of accumulated stress and microstructural refinement on the ultra-high cyclic deformation and associated crack initiation mechanism in high strength bainite-martenite dual-phase steel, where non-inclusion induced crack initiation is the dominated failure mode. The non-inclusion induced crack initiation involved nucleation of microvoids in the coarse bainitic lath when the applied stress accumulated. The density of the microvoids increased with the accumulation of applied stress, and the microvoids that were near the grain boundary connected to form a microcrack. Failure occurred when the microcracks combined to form a macrocrack. The premise of crack propagation is that the total stress intensity factor at the crack tip is larger than granular bright facet (GBF) crack growth threshold value. A mathematical criterion is proposed for non-inclusion induced cracking. The study suggested that the probability of crack initiation is reduced when the microstructural constituent, bainitic lath, is refined.

Fatigue limit of carbon and Cr[sbnd]Mo steels as a small fatigue crack threshold in high-pressure hydrogen gas

The fatigue limit properties of a carbon steel and a low-alloy CrMo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the CrMo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtained SN data inferred that the fatigue limit was not negatively affected by hydrogen in either of the steels. Observation of fatigue cracks in the unbroken specimens revealed that non-propagating cracks can exist even in 115-MPa hydrogen gas, and that the crack growth threshold is not degraded by hydrogen. The experimental results provide justification for the fatigue limit design of components that are to be exposed to high-pressure hydrogen gas.

Mean stress and plasticity effect prediction on notch fatigue and crack growth threshold, combining the theory of critical distances and multiaxial fatigue criteria

AbstractIn the present work, we propose a robust calibration of some bi‐parametric multiaxial fatigue criteria applied in conjunction with the theory of critical distances (TCD). This is based on least‐square fitting fatigue data generated using plain and sharp‐notched specimens tested at two different load ratios and allows for the estimation of the critical distance according to the point and line method formulation of TCD. It is shown that this combination permits to incorporate the mean stress effect into the fatigue strength calculation, which is not accounted for in the classical formulation of TCD based on the range of the maximum principal stress. It is also shown that for those materials exhibiting a low fatigue‐strength‐to‐yield‐stress ratio σfl,R = −1/σYS, such as 7075‐T6 (σfl,R = −1/σYS = 0.30), satisfactorily accurate predictions are obtained assuming a linear‐elastic stress distribution, even at the tip of sharp notches and cracks. Conversely, for any materials characterized by higher values of this ratio, as quenched and tempered 42CrMo4 (σfl,R = −1/σYS = 0.54), it is recommended to consider the stabilized elastic‐plastic stress/strain distribution, also for plain and blunt‐notched samples and even in the high cycle fatigue regime still with the application of the TCD.

Oxide induced crack closure in the near threshold regime: The effect of oxide debris release

Within this work the influence of oxide particle transport during fatigue loading on the fatigue propagation rate was investigated in detail by means of load-shedding tests in the near threshold region (crack propagation rate between 0.1 and 10 nm per cycle). To this purpose, standard fatigue specimens (SENB - single edge notch bending) were machined. At selected specimens, a stretchable nitrile rubber patch was glued onto the sample surfaces in the ligament region. This patch was expected to prevent the emergence of iron oxide particles near the sample surface, and indeed subsequent fracture surface analysis showed obvious differences in the shape of the arising oxide debris layer. It is shown that the build-up of oxide debris on the fracture surface is higher in the specimen with the nitrile rubber patch applied. This leads to increased oxide induced crack closure and, in turn, to a higher fatigue crack growth threshold. In this context, also the influence of the specimen thickness on the fatigue crack growth threshold is examined. Finally, also the influence of the load ratio on the oxide debris layer is briefly discussed.

Influence of work-hardening on fatigue crack growth, effective threshold and crack opening behavior in the nickel-based superalloy Inconel 718

In this study, the influence of rotary swaging induced cold work on long fatigue crack growth, threshold and opening behavior in the nickel-based superalloy Inconel 718 was investigated. Ambient temperature experiments were carried out using arc-shaped tension specimens according to ASTM standard E399. The direct current potential drop (DCPD) method was used for crack length measurements and crack opening force evaluation. Fatigue crack growth rates (FCGR) measured at load ratios of R=0.2,R=0.5 and R=0.7 were found to increase due to work-hardening in continuous (increasing ΔK) and load shedding (decreasing ΔK) tests. Effective fatigue crack growth thresholds (ΔKth,eff) were determined according to the Kmax=const. procedure and found to decrease monotonically with increasing degree of work-hardening. Compared to the non-deformed material, crack opening stress intensities Kop were reduced in work-hardened states at three different crack lengths. To quantify the detrimental effects of cold work, the effective stress intensity factor range (ΔKeff) model was applied. SEM and TEM micrographs were taken to complement the study.

Competition mechanisms of fatigue crack growth behavior in lath martensitic steel

AbstractThe fatigue crack growth behavior in 20CrMnTi lath martensitic steel with a hierarchical structure was investigated at room temperature. The microstructure was quantitatively characterized by optical microscopy, scanning electron microscopy, electron backscattering diffraction, and transmission electron microscopy. Crack growth tests were performed at a stress ratio of 0.1 under the control of a stress intensity factor range using compact tension specimens. The results show that the lath martensite with medium grain size quenched in ice salt water possessed the highest fatigue crack propagation resistance, which was attributed to the relatively high intrinsic propagation resistance and extrinsic propagation resistance. Meanwhile, the results indicate that the intrinsic propagation resistance increased with the grain size, while the extrinsic propagation resistance showed an inverse tendency. In addition, the effect of substructure on the fatigue propagation properties is discussed. The deflection angle of block interfaces in the crack growth path indicated the block unit had a significant effect on the fatigue crack propagation behavior of lath martensitic steel. Based on the Taylor model, the block size (db) was considered to be the effective grain size for controlling the crack growth threshold value (∆Kth).

Development of Fatigue Crack Growth Thresholds for Austenitic Stainless Steels Exposed to Air Environment for ASME Code Section XI

Fatigue crack growth thresholds ΔKth define stress intensity factor range below which cracks will not grow. The thresholds ΔKth are useful in industries to determine durability lifetime. Although massive fatigue crack growth experiments for stainless steels in air environment had been reported, the thresholds ΔKth are not codified at the American Society of Mechanical Engineers (ASME) Code Section XI, as well as other fitness-for-service (FFS) codes and standards. Based on the investigation of a few FFS codes and review of literature survey of experimental data, the thresholds ΔKth exposed to air environment have been developed for the ASME Code Section XI. A guidance of the thresholds ΔKth for austenitic stainless steels in air at room and high temperatures can be developed as a function of stress ratio R.

Using acoustic emission to understand fatigue crack growth within a single load cycle

Current methods for prediction of fatigue crack growth are based on empirical correlations which do not take the crack growth behaviour within a single cycle into account. To improve these prediction methods, more understanding of the physical mechanisms of crack growth is required. In this research the acoustic emission technique was used to investigate the crack growth behaviour during a single fatigue cycle. It was found that crack growth can potentially occur both during loading and unloading, but only while the strain energy release rate is above a crack growth (CG) threshold value. The results suggest this CG threshold value is the same in both quasi-static and fatigue loading. Further work is necessary to fully understand the link between the received acoustic emission signals and the actual crack growth processes. Nevertheless, the paper shows the potential of acoustic emission to provide more insight into the physics of crack growth.

Mixed mode II and III fatigue crack growth in a rail steel

Rolling contact fatigue cracks in rails undergo non-proportional mixed-mode I + II + III, in variable proportions along their front. In order to determine the crack growth thresholds and kinetics in mixed-mode II/III, asymmetric four point bending tests are run on a rail steel with different angles between the crack front and the shearing load, so as to vary the mode mixity ratio. For sufficiently high loading ranges, these tests give rise to coplanar shear-mode crack growth. The effective stress intensity factors (SIFs) are derived by an inverse method from the measured crack face relative displacements. It appears to be 10–70% lower than the nominal SIFs and to allow a reasonable correlation of the measured crack growth rates. The local application, ahead of the crack front, of shear-driven or tension-driven fatigue damage models – after 3D elastic-plastic computations of local stress and strain ranges – allows a successful prediction of crack fronts paths and growth rates.

Cyclical fatigue of annealed and of thermally tempered soda-lime-silica glass

We present experimental and theoretical investigations on the cyclic fatigue of annealed and of thermally tempered soda-lime-silica glass. Static fatigue due to subcritical crack growth at micro cracks significantly decreases the macroscopic strength of soda-lime-silica glass and causes a time-dependent strength reduction. A subsequent thermal tempering process is typically used to induce residual surface compression stresses, which inhibit the crack growth of surface cracks, and corresponding bulk tension stresses. From the experimental results we show that the existing models for static fatigue used in linear elastic fracture mechanics can be used for the lifetime prediction of cyclically loaded annealed glass and thermally tempered glass, although the (static) crack growth exponent slightly decreases in cyclic loading. The equivalent duration of tensile stress at the crack tip of a micro crack governs the crack growths and not the number of cycles. The threshold for subcritical crack growth determined from the cyclic experiments was found to be in good agreement with data from literature. But unlike in strength tests with singular and quasi-static re-loading, it could be found that periodic loading with load free intervals does not lead to a strength increase by crack healing effects. Based on the results, an engineering design concept for cyclically loaded glass is presented.

Large plastic zones and extensive influence of notch under near-threshold mode II and mode III loading of fatigue cracks

Plastic zone shapes and sizes were determined for a cylindrical specimen for mode II and mode III crack testing made of ARMCO iron. Three methods of calculation were applied: Irwins approximate solution, HRR field and finite element method. The plastic zones were found to be very large under mode II and mode III loading even at the loading level corresponding to fatigue crack growth threshold. The difference between sizes of plastic zones under tensile-mode and shear-mode loading was explained by different stress gradients and corresponding stress concentration factors of notches under shear and tensile type loading. This is also the reason for a much further influence of notch on stress intensity factors for mode II and mode III cracks emanating from the notch than the influence under mode I loading. Preliminary results for reversed plastic zone size indicated that it is smaller than the theoretical size of 1/4 of the monotonic zone which explained the presence of large monotonic zone at threshold loading.

115 MPa水素ガス中におけるSCM435の水素誘起き裂進展下限界値<i>K</i><sub>I,H</sub>を求める試験方法

115 MPa水素ガス中におけるSCM435の水素誘起き裂進展下限界値<i>K</i><sub>I,H</sub>を求める試験方法

Relationship between unusual high-temperature fatigue crack growth threshold behavior in superalloys and sudden failure mode transitions

An investigation of high temperature cyclic fatigue crack growth (FCG) threshold behavior of two advanced nickel disk alloys was conducted. The focus of the study was the unusual crossover effect in the near-threshold region of these type of alloys where conditions which produce higher crack growth rates in the Paris regime, produce higher resistance to crack growth in the near threshold regime. It was shown that this crossover effect is associated with a sudden change in the fatigue failure mode from a predominant transgranular mode in the Paris regime to fully intergranular mode in the threshold fatigue crack growth region. This type of a sudden change in the fracture mechanisms has not been previously reported and is surprising considering that intergranular failure is typically associated with faster crack growth rates and not the slow FCG rates of the near-threshold regime. By characterizing this behavior as a function of test temperature, environment and cyclic frequency, it was determined that both the crossover effect and the onset of intergranular failure are caused by environmentally driven mechanisms which have not as yet been fully identified. A plausible explanation for the observed behavior is proposed.

High-R low growth rate fatigue crack propagation at elevated temperatures

While fatigue cracking is widely considered as the primary cracking mode under cyclic loading conditions, this is not the case for high-R (Kmin/Kmax) low growth rate fatigue crack propagation at elevated temperatures. Creep cracks can sometimes be observed in such situations when the stress intensity factor range ΔK is around the fatigue crack growth threshold stress intensity factor ΔKth. A Time Dependent Failure Assessment Diagram (TDFAD) has been adopted in this study for predicting creep cracking in high-R cyclic loading conditions at elevated temperatures. In addition, attention is also paid to oxide-induced crack closure which is greatly influential at high temperatures. Three power plant steels with different chromium contents, namely 1%Cr, 9%Cr and 18%Cr steels, have been investigated at temperatures between 550°C and 650 °C. The experiments were performed by reducing ΔK continuously to ΔKth. Good accordance has been found between fracture surface observations and the TDFAD approach predictions of creep crack initiation. Apart from creep cracking, high-R ΔKth values have also been found to be influenced by oxide-induced crack closure at elevated temperatures. The enhancement is studied through the relationship between the crack tip opening displacement and stress intensity factor.