Crack Growth Behavior in Cemented Carbide by Repeated Thermal Shock

Effect of stress ratio on fatigue lifetime and crack growth behavior of WC–Co cemented carbide

In the present work, fatigue crack growth (FCG) behavior of modified 9Cr-1Mo steel has been investigated near fatigue threshold and Paris regime at room temperature (RT) and 650 °C. The modified 9Cr-1Mo steel was subjected to normalizing at 1050 °C followed by tempering (650-750 °C) and warm rolling at 550 °C followed by tempering in order to improve its precipitation state and refine microstructure. The microstructural parameters such as grain size, block size, lath width and precipitate size were examined and correlated with FCG and tensile properties of investigated samples. The results showed that decreasing tempering temperature significantly reduces the fatigue threshold at RT while increases the yield strength at elevated temperature (i.e., 650 °C). Increase in fatigue threshold is found to increase with the increase in block size, whereas high density of fine precipitates contributed the high temperature strength. The FCG rate in Paris regime was less affected by heat treatment and rolling process at RT. However, rolled sample tested at 650 °C shows a decrease in FCG rate as compared to heat-treated samples. In terms of higher FCG resistance and yield strength, warm rolling followed by tempering at low temperature (i.e., 700 °C) can be considered optimum. For the numerical simulation of FCG behavior, extended finite element method (XFEM) along with Paris law has been implemented in ABAQUS using python script. The simulated FCG behavior is found in good agreement with the experimental results.

In this study, the fatigue crack growth (FCG) characteristics of the short surface crack was studied at various stress ratios, R, using the fine grained cemented carbide. The FCG law for the material was studied using two parameters, the maximum stress intensity factor Kmax and the stress intensity factor range ΔK. It was found that the relationship between the rate of FCG, da/dN and ΔK depends on the R value. In the da/dN vs. Kmax relation, at the region of high crack growth rate, the FCG rate was nearly constant by the value of Kmax without depending on R. However, in the low FCG rate region, the FCG rate tended to accelerate as R became lower. Therefore, it was difficult to specify da/dN as a single value at different R ratios using either ΔK or Kmax from the low speed region to the high-speed region. For that reason, the FCG behaviour of the material was studied based on the crack growth law, da/dN = AKmaxmΔKn. The values of m and n were obtained as 6.6 and 1.4, respectively. Therefore, it was found that the FCG rate of the cemented carbide was strongly dependent on Kmax rather than ΔK, like the FCG characteristics of ceramics.

The influence of frequency on fatigue crack growth (FCG) behavior was investigated for three differently heat-treated 17-4 PH stainless steels, namely unaged A, peak-aged H900, and overaged at 673 and 773 K, and at frequencies ranging from 0.002 to 20 Hz. At 773 K, all heat-treated conditions exhibited similar FCG behavior in which no frequency effect was observed at frequencies higher than 2 Hz and the fatigue crack growth rates (FCGRs) increased with decreasing frequency below 2 Hz. The increase in FCGR at a lower frequency at 773 K was thought to be caused by a time-dependent, oxidation-assisted cracking mechanism. At 673 K, for a given heat-treated condition, the FCGRs increased with decreasing frequency at higher frequencies and leveled off at lower frequencies. Such an anomalous FCG behavior was attributable to a dynamic strain aging (DSA) effect. At a given frequency, when the temperature was increased from 673 to 773 K, the FCGR increased in Condition H1150, but decreased in Conditions A and H900 due to an in-situ overaging and precipitate-coarsening effect during the FCG test for the latter.

An FE analysis was used to determine the elastic and elastic-plastic stress distributions at 3 different keyhole notch geometries in compact tension specimens. This enabled an estimate to be made of the local cyclic stress ratio, R^ocai (<7min.ioca/<W.iocai)» variation within the notch plastic zone with increasing distance from the notch root. This was carried out for both plane stress and strain conditions. Use of the local stress ratio was made to truncate, where appropriate, the value of the stress intensity range, AK, for nominal stress ratios of 0.1 and 0.7. Fatigue crack growth life predictions for small corner cracks at notches, based on this approach are shown to be more accurate than when based on the nominal stress ratio (R) value. INTRODUCTION An important element in the structural integrity assessment of many components or structures involves the prediction of fatigue crack growth (FCG) behaviour whilst under the influence of stress concentration features. This is particularly so in gas turbine applications where the material may be subjected locally to stresses greater than the elastic limit at notches, fastener holes or other necessary design features. Fatigue life assessment must include an understanding of the influence of the notch geometry and the resultant stress field on both the crack formation (if not already present) and subsequent crack propagation phases of life. For the purposes of analysis the crack propagation phase may be sub-divided into Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 80 Localized Damage growth through a notch plastic zone followed by growth through the elevated elastic stress field [1]. Earlier Studies (2,3) have been concerned with establishing reliable methods of fatigue life prediction of notched components with physically short fatigue cracks under essentially elastic conditions at the notch root. Experiments were carried out with keyhole notched compact tension (CT) specimens in nickel base superalloys in which measured FCG rates were used to infer values of stress intensity range (AK). When these inferred values of AK were compared with those estimated using an analysis developed by Chell [4], it was shown that in the absence of microstructural short crack effects and any crack interaction mechanisms, the theoretically based AK predictions were highly conservative This paper focuses on the modification of the cyclic stress ratio (R = ^min/^max) locally at the notch root due to confined notch plasticity and its influence on fatigue crack growth rate and consequent fatigue life. FINITE ELEMENT ANALYSIS OF KEYHOLE NOTCH COMPACT TENSION SPECIMENS. A finite element analysis was performed using the Pafec Interactive Graphics, System, PIGS, and finite element software, PAFEC-FE. Three two dimensional meshes of 8 noded isoparametric quad elements were constructed (Figure 1 is typical) to represent the following keyhole notch geometries. Table 1 Keyhole Notch Dimensions Notch Depth (mm) 10.4 12.5 13.25 Notch Radius 2.0 2.75 3.25 (mm) Since the specimens possess a plane of symmetry half the specimen was modelled and the nodes in that plane restrained in the direction normal to the plane. In order to prevent rigid body motion a single node in the symmetry plane was restrained in the appropriate direction. Three tensile loads were determined which would generate a notch plastic zone of approximately 1.0 mm in extent, as measured from the notch root for each of the notch geometries. These loads were applied to their associated mesh as a series of point loads over two elements. Typical elastic stress distributions are shown in Figures Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 Localized Damage 81 2(a) and (b). The same loads were used in deriving the elastic plastic stress distributions shown in Figure 3(a) and (b) for plane stress and in Figures 4(a) and (b) for plane strain, where the non-linear material behaviour was simulated as a series of linear responses. STRESS RATIO MODIFICATION The FE distributions of elastic and elastic-plastic stresses provide a means of establishing a local stress ratio distribution by using the principle of superposition. Under conditions where plasticity is generated at a notch the stress ratio within the notch plastic zone is different to that of the applied remote load cycle since within the enclave the maximum stress is limited to some value dependant on the stress state. As the specimen is unloaded, it recovers elastically and may cause compressive stresses at the notch root' if the stress range of the cycle is of sufficient magnitude. From a knowledge of the elastic and elastic-plastic stress distributions it is possible to estimate the variation of the local stress ratio around the notch. The technique used in this estimation is illustrated in Figure 5. A proportion of the elastic stress distribution, corresponding to the nominal stress ratio, is taken at a number of positions along the projected crack path. These ranges are then superimposed onto the elastic-plastic stress distribution at the relevant positions along the crack path thus enabling a distribution of minimum local stress to be determined. Hence, given that the maximum is that shown in the elastic plastic stress distribution, the distribution of local stress ratio can be determined along the crack path. The local stress ratio distributions for the two notch geometries and nominal stress ratios are shown in Figures 6(a) and 6(b). For a nominally applied stress ratio of 0.1 both notch geometries clearly show a region close to the notch root, where the local stress ratio is negative, as the distance from the notch root increases the local stress ratio recovers to a value above that applied remotely. Beyond this the local stress ratio decays towards the nominally applied stress ratio. For a nominal stress ratio of 0.7 the same suppression, peak and decay of local stress ratio is observed, however, the magnitude of the elastic stress range is not sufficient to generate local compressive stress at the notch root. RESULTS AND DISCUSSION Load control fatigue crack growth tests were conducted at the two nominal stress ratios of 0.1 and 0.7 using keyhole CT specimens in Waspaloy with small corner cracks at the notch bore extremities. Full details of which are given in (5) and (6). A stress intensity factor solution due to Chell (4) was used to correlate the growth rates and life predictions made, the results of which are shown in Figure 7(a). The degree of conservatism associated with Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 82 Localized Damage the R = 0.1 data is somewhat greater than that associated with the R = 0.7 data. It is suggested that this is due to the effect of the interaction between the notch plastic enclave and the surrounding elastic bulk discussed above. By considering the stress ratio modification (Figure 6) and assuming that closure occurs at zero stress the values of AK for R = 0.1 are truncated. Since no local compressive stresses are generated at a nominal stress ratio of 0.7 only the R = 0.1 data is affected. The resulting life prediction plot (Figure 7(b)) clearly shows that the inclusion of the effects of local R decreases significantly the degree of conservatism of the R = 0.1 life predictions to a level equivalent to the R = 0.7 data. CONCLUSIONS Elastic and elastic-plastic finite element analyses have been used in the determination of local stress ratio modification due to confined notch plasticity. Fatigue crack growth life predictions for small corner cracks at notches, based on this local approach are shown to be more accurate than when based on the nominal stress ratio.

The fatigue crack growth (FCG) behaviour of SiC and B4C/B reinforced Ti-6 Al-4 V metal matrix composites loaded in the transverse direction as a function of modifications of the interface between the fibre and matrix was studied. The interface chemistry, modified by sulphur diffusion during thermal cycling treatment, changed the FCG in air, dry nitrogen and hydrogen environments when compared with the as-received specimens. The FCG rates tend to be higher in a humid environment. The SEM fractrography indicates that the FCG in humid air was by an increased amount of fibre splitting. The FCG in dry nitrogen environment was more often by interface debonding with some fibre splitting and fiber fracture. The FCG rates in dry hydrogen for both as-received and heat-treated specimens were intermediate between the observed rates for dry nitrogen and humid air. During FCG in laboratory air, the sulphur-enriched interface of the specimens thermal cycled in a sulphur environment reacts with the humidity in air to degrade the interface cohesion, resulting in complete separation of the interface from the matrix and the fibre at low strains. This inability of the interface to sustain any strain further increases the FCG rates in the matrix. The results show that the interface does transfer load during fatigue cycling either in an inert environment or if the interface has a minimal amount of impurities.

Overload-induced anisotropy of fatigue crack growth in laser deposited Ti–6Al–4V alloy

The unusual near-threshold FCG behavior of a single crystal superalloy and the resolved shear stress as the crack driving force

Effect of laser parameters on fatigue crack growth behavior of AZ31B magnesium alloy TIG welded bionic joint

A model for predicting fatigue crack growth behaviour of a low alloy steel at low temperatures

Probabilistic evaluation on fatigue crack growth behavior in nickel based GH4169 superalloy through experimental data

Predicting fatigue crack growth rates and thresholds at low temperatures

Effects of pearlite morphology and specimen thickness on fatigue crack growth resistance in ferritic–pearlitic steels

Fatigue Crack Growth (FCG) behaviour in a Single-Edge-Notched Tension (SENT) specimen of 2024-T3 aluminium alloy under a standard mini-FALSTAFF spectrum load sequence was experimentally determined. Further, the FCG behaviour was predicted using cycle-by-cycle method and compared with experimental results. Prediction procedure involved are rain-flow counting of fatigue load cycles, estimation of crack driving force for each of the counted cycle and prediction of crack extension per cycle from constant amplitude crack growth rate equation. In the present work, a new crack driving force (CDF) K*eff involving Kujawski’s crack driving force K* in conjunction with Elber’s crack closure concept was used to account for load interaction effects. FCG prediction was also made using conventional CDF ΔKeff (Elber’s) approach. A good correlation was observed between experimental and predicted FCG behaviour under spectrum loads by the proposed K*eff approach. Also, this prediction was observed to be better than that predicted by conventional ΔKeff approach.

When the flaws are detected in Japanese nuclear power components by in-service inspection, structural integrity assessment are performed in the technical judgment on continuous service. If cyclic loading is assumed, fatigue crack growth analysis should be conducted based on the Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers Code (JSME FFS Code). However, fatigue crack growth analysis for BWR components consisting of Ni-base alloy is currently impossible, since the reference curve of fatigue crack growth rate for Ni-base alloy in BWR water environment is not yet prescribed in the JSME FFS Code. In this study, fatigue crack growth behavior of Ni-base alloy used for Japanese BWR plants in BWR water environment was investigated. Based on the experimental data, the fatigue crack growth rate curve was evaluated. Four test parameters of material, corrosion potential, stress ratio and load rising time were considered. As a result of fatigue crack growth tests, the effects of all test parameters on the fatigue crack growth behavior were found. A Mean curve of fatigue crack growth rate in Paris law format, which was a function of stress ratio and rising time, was formulated based on crack growth data in normal water chemistry (corrosion potential was over 150 mVSHE) for weld metal and heat affected zone (HAZ), respectively. A reference curve of fatigue crack growth rate was also formulated by the statistical treatment considering the scatter of crack growth rate. Further, in order to determine the threshold stress intensity factor range ΔKth of reference curve of fatigue crack growth, ΔK decreasing tests were conducted under the test condition of 1 second of rising time. As a result, the threshold value of ΔK was evaluated based on the ASTM E 647, and the ΔKth of the reference curve was conservatively determined considering the margin.