Near-threshold Fatigue Crack Growth Rate Research Articles

Assessing fatigue crack growth thresholds for a Ti-6Al-4V (STOA) alloy using two experimental methods

Fatigue crack growth rate behavior in the threshold and near-threshold regimes for a Ti-6Al-4V solution-treated and over-aged (STOA) alloy was investigated using two test procedures: i) accelerated load reduction (ALR) utilizing faster load-shed rates than recommended in the ASTM standard E647 load-reduction (LR) test procedure, and ii) compression pre-cracking constant-amplitude (CPCA) or load-increasing (CPLI) and load-reduction (CPLR). Fatigue crack growth tests were conducted on compact, C(T), specimens (B = 6.35 mm) with two different widths (W = 51 and 76 mm) at two stress ratios (R = 0.1 and 0.7). The crack-growth-rate test data were compared to previous test data produced from the same batch of material using the ASTM LR and CPCA test procedures that were tested on C(T) specimens (W = 25, 51 and 76 mm) over a wide range in stress ratios (R = 0.1, 0.4, and 0.7).Although none of the load-reduction test procedures provided a good representation of the threshold value (ΔKth) at low R, the CPCA test procedure was the most promising. The LR, ALR, and CPLR test procedures were influenced by prior loading history, leading to higher ΔKth values and slower crack growth rates in the threshold regime due to a rise in the crack-opening loads. The LR and ALR tests at R = 0.1 and shed rates of −0.08, −0.32, and −0.95 mm−1 yielded ΔKth values that were 15 to 38 % higher than the estimated threshold stress-intensity factor range (ΔK*th)R=0.1 based on the crack-closure model (FASTRAN). The compression pre–cracking test procedures are more accurate alternatives for developing threshold and near-threshold fatigue crack growth rates. Specifically, the CPLR test procedure produced a ΔKth value that was 8–18 % higher than (ΔK*th)R=0.1. Additionally, LR and ALR tests produced different ΔKth values as a function of the specimen width for a given load ratio, which violates classical fracture mechanics principles. In addition, the spread in the ΔK-rate data in the low-rate regime was larger than the spread in the mid-rate regime as a function of R, a term referred to as “fanning”. In contrast, the CPCA test procedure produced consistent crack growth rates over the same range of ΔK values examined, independent of the specimen width and these data did not exhibit “fanning” with R. Further research is necessary to develop standard test procedures capable of providing a more definitive representation of the ΔKth value in the threshold regime. However, FASTRAN can serve as a crucial tool in aligning the actual ΔKth values with those obtained through CP testing, thereby bridging the gap and enhancing the reliability of the results.

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.

Effect of compression precracking on the near threshold fatigue crack propagation in an AISI 316L stainless steel

This work addresses an alternative test procedure based on fatigue precracking under compression to mitigate the potential effects of crack closure on near-threshold fatigue data and, thus, to obtain more consistent and reliable ΔKth-values. The method relies on performing a compression precracking, in which both maximum and minimum loads are compressive, thereby resulting in a tensile residual stress field at the crack tip. Fatigue crack growth (FCG) tests conducted on an AISI 316L stainless steel provide the experimental data upon which the near-threshold fatigue crack growth rate data for different test techniques can be determined. The experimental program includes FCG tests conducted on conventional C(T) specimens with crack size to specimen width ratio a/W=0.2, single edge notched SE(B) specimens under three-point bending with a/W=0.2 and two different span sizes, S, characterized by S/W=4 and 8. Overall, the present investigation shows that combining the compression precracking technique with the utilization of non-standard bend specimens having increased span improves evaluation of the near-threshold fatigue crack growth rate data for the tested AISI 316L steel, thereby providing more accurate ΔKth-values.

Fracture behavior of laser powder bed fusion fabricated Ti41Nb via in-situ alloying

The microstructures and the mechanical properties, including fatigue crack growth (FCG) and mode I fracture toughness (KIC), of in-situ alloyed Ti41Nb (wt.%) fabricated using the laser powder bed fusion (LB-PBF) were investigated. A small hatch spacing - fast scanning strategy was utilized for obtaining parts with minimum porosity and unmelted Nb particle combination. The mesostructure of the fabricated blocks comprises columnar grains with alternating Nb-poor regions (NPRs), which occur due to the short residence time of materials in the melt that limits the melting and diffusion of Nb, and matrix layers. Extensive ωiso precipitates were noted in the microstructure, which were attributed to the large-build related heat accumulation during the LB-PBF process that stems from the combination of large sample size and high energy density input. Consequently, ductility of the as-built parts was low. The FCG rate at high ∆K was strongly influenced by the layers of NPR and hence depend on the orientation whereas the near-threshold FCG rate was strongly influenced by the local slip systems ahead of the crack tip. Significant anisotropy in KIC, a result of the crack deflection by the NPR/matrix interfaces, was observed. The mesostructure induced toughening mechanism improves the fracture toughness of the resultant alloy despite its brittle failure response. The potential of in-situ alloying to induce toughening through the mesostructure control was discussed.

Near-threshold fatigue crack growth rates of laser powder bed fusion produced Ti-6Al-4V

When Ti-6Al-4V is processed by laser powder bed fusion (LPBF), a material with a martensitic microstructure is obtained. Moreover, the presence of internal stresses, an outer surface with relatively high surface roughness and the presence of remnant porosity all influence the fatigue life of high cyclically loaded components. The majority of investigations on LPBF produced Ti-6Al-4V have focussed on a fatigue life method providing valuable, albeit limited, insight into fatigue failure mechanisms. Near-threshold fatigue crack growth rates are vital for describing fatigue crack initiation mechanisms and how these are influenced by residual stress, a martensitic microstructure and build orientation. This study investigates near-threshold fatigue crack growth rates of LPBF produced Ti-6Al-4V. The study makes use of full-size compact tension specimens for fatigue crack growth rate investigations, the contour method for residual stress measurement, and scanning electron microscopy with electron backscatter diffraction to consider both morphological and crystallographic texture. Results show anisotropic near-threshold fatigue crack growth rates that are dependant on residual stress levels and load-ratios. Fracture is predominantly governed by transgranular quasi-cleavage mechanisms, and the fracture path is directed by the columnar prior beta-grain structure resulting in orientation-dependant crack closure effects. Residual stresses result in crack opening that causes a shift of near-threshold fatigue crack growth rates. An intrinsic ΔKth of ~1.6 ± 0.2 MPa√m and critical Kmax of ~ 3 MPa√m is measured that is independent of the stress state but dependant on orientation. It is shown that this anisotropy is linked to morphological texture and to a lesser extent the crystallographic texture of LBPF produced Ti-6Al-4V.

Near-Threshold Fatigue Crack Growth Rates of Laser Powder Bed Fusion Produced Ti-6Al-4V

When Ti-6Al-4V is processed by laser powder bed fusion (LPBF), a material with a martensitic microstructure is obtained. Moreover, the presence of internal stresses, an outer surface with relatively high surface roughness and the presence of remnant porosity all influence the fatigue life of high cyclically loaded components. The majority of investigations on LPBF produced Ti-6Al-4V have focussed on a fatigue life method providing valuable, albeit limited, insight into fatigue failure mechanisms. Near-threshold fatigue crack growth rates are vital for describing fatigue crack initiation mechanisms and how these are influenced by residual stress, a martensitic microstructure and build orientation. This study investigates near-threshold fatigue crack growth rates of LPBF produced Ti-6Al-4V. The study makes use of full-size compact tension specimens for fatigue crack growth rate investigations, the contour method for residual stress measurement, and scanning electron microscopy with electron backscatter diffraction to consider both morphological and crystallographic texture. Results show anisotropic near-threshold fatigue crack growth rates that are dependent on residual stress levels and load-ratios. Fracture is predominantly governed by transgranular quasi-cleavage mechanisms, and the fracture path is directed by the columnar prior beta-grain structure resulting in orientation-dependent crack closure effects. Residual stresses result in crack opening that causes a shift of near-threshold fatigue crack growth rates. An intrinsic ΔKth of ~1.6 ± 0.2 MPa√m and critical Kmax of ~ 3 MPa√m is measured that is independent of the stress state but dependent on orientation. It is shown that this anisotropy is linked to morphological texture and to a lesser extent the crystallographic texture of LBPF produced Ti-6Al-4V.

Cycle sequence sensitivity of near threshold fatigue under programmed loading – A fractographic study

Statistically representative programmed block renditions of a transport aircraft wing load spectrum were used to study cycle sequence effects on near-threshold fatigue crack growth rate. Less frequent higher load cycles served as markers that permitted unambiguous estimation of crack growth rate under individual load levels in the load spectrum. Small cycle crack growth rates down to threshold levels are extremely sensitive to the peak-valley sequence of preceding large load cycle, suggesting the need to model the effect of near-tip residual stress on threshold stress intensity in handling load spectra with a large statistical content of smaller load cycles.

Effect of Overload on the Near-Threshold Fatigue Crack Growth Rate in a 2024-T3 Aluminum Alloy: I. Effect of the Character, the Magnitude, and the Sequence of Overload on the Fatigue Crack Growth Rate

The results of overload and underload tests on aluminum specimens in the near-threshold crack growth rate region are discussed. These tests are performed to choose a model for predicting the fatigue life. The influence of the character and the magnitude of overload and underload on the fatigue crack growth time is analyzed. The obtained crack growth curves and the fatigue lives cannot be explained in terms of classical models, which makes it necessary to develop a new model.

Crack Size and Shape Considerations Related to Near-Threshold Fatigue Crack Growth Rate Behavior

Abstract It is well recognized that crack size or load history or both have an important influence on near-threshold fatigue crack rate behavior at a low stress ratio. This investigation will compare the behavior of a radial crack, such as a surface flaw or a corner crack, to that of a linear crack to see if crack shape plays a role in physically small crack to long crack behavior. Recent research has unveiled a methodology to experimentally partition plasticity from roughness- and oxide-induced crack closure, showing that nonplasticity-induced closure mechanisms have a greater influence at low Kmax by orders of magnitude. Knowledge of this is key to understanding differences between long crack and physically small crack behavior in the near-threshold regime. In this study, the well behaved 2024-T351 aluminum alloy was used to generate threshold data using numerous specimen geometries, flaw geometries, crack sizes, and test procedures to highlight not only the significance of crack size but also the role of crack shape. It will be shown both experimentally and analytically that a radial crack (surface flaw, corner crack) has significantly less remote closure and, therefore, a lower threshold than a through crack (compact tension, middle crack tension), even if the crack size is the same.

Fatigue Crack Growth from Compression Pre-Cracks

Abstract The compression pre-crack method can be used to develop near-threshold fatigue crack growth rates much quicker than the ASTM standard load shed method. The compression pre-crack method is faster than the standard method and eliminates load history effects that are of concern when the standard method is used. However, crack growth developed using the compression pre-cracking approach is susceptible to different load history effects. A suite of tests using the compression pre-crack method that were performed on AISI 9310 steel are presented in this paper. An approach for identifying the critical crack growth increments required to develop valid data is presented, and the effects of three potential mechanisms are evaluated. For these tests, the residual stress field resulting from the initial compression field appears to be the more important effect on crack growth behavior. Crack growth increments measured from the notch tip that are 2.5 times larger than the size of the plastic zone resulting from the compression pre-cracking are required in order to develop valid data on the fatigue crack growth rate.

Fatigue crack growth behavior of X2095 Al–Li alloy

Microstructures and micro-textures of X2095 Al–Li alloy in as-received/superplastic state were characterized by means of SEM/BDS, X-ray diffraction and orientation imaging microscopy (OIM). It was observed that the microstructure of the alloy was typical of a particulate-reinforced composite material, consisting of aluminum matrix and homogeneously distributed T B(Al 7Cu 4Li) particles with a volume fraction of about 10%. Brass-type texture was the dominant texture component. Both constant amplitude and near-threshold fatigue crack growth rates of the alloy in the L–T and T–L orientations were determined at different stress ratios. Particular attention was paid to the role of the T B phase in the fatigue crack growth. When a fatigue crack approached a T B particle, the crack basically meandered to avoid the particle. The T B particles thus provided a strong resistance to the propagation of fatigue crack by promoting crack deflection and the related crack closure effects. The fatigue crack propagation behavior has been explained by the microstructural features, micro-textures, cracking characteristics and crack closure effects.

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.

Fracture Toughness and Fatigue Crack Growth Characteristics of Aluminum Alloy Die Castings.

Effects of a surface layer with chill structure, specimen thickness, shape of mold and powder lubrication in a sleeve on fracture mechanic characteristics were investigated for aluminum alloy die castings ADC12 with two different types of molds (flat and staircase shapes). The results are as follows. In the case of a flat mold, a decrease of thickness improves mechanical properties. A surface layer with chill structure has a substantial influence on mechanical properties, but little influence on fracture toughness. In the case of the staircase mold, the mechanical properties are the same as for the flat mold. On the other hand, the powder lubrication in the sleeve improves the fracture toughness and reduces the scatter in fracture toughness. There is little influence of both the surface layer with chill structure and the specimen orientation on the fatigue crack growth resistance, but with decrease of specimen thickness, the fatigue crack growth resistance decreases, especially in the region of near-threshold fatigue crack growth rate. In addition, the lubricating method has no effect on the fatigue crack growth resistance for the staircase-shapes die castings.

Fatigue crack growth behavior of a new single crystal nickel-based superalloy (CMSX-4) at 650 C

CMSX-4 is a recently developed rhenium containing single crystal nickel-based superalloy. This alloy has potential applications in many critical high-temperature applications such as turbine blades, rotors, nuclear reactors, etc. The fatigue crack growth rate and the fatigue threshold data of this material is extremely important for accurate life prediction, as well as failure safe design, at elevated temperatures. In this paper, the fatigue crack growth behavior of CMSX-4 has been studied at 650 C. The investigation also examined the influence of [gamma][prime] precipitates (size and distribution) on the near-threshold fatigue crack growth rate and the fatigue threshold. The influence of load ratio on the fatigue crack growth rate and the fatigue threshold was also examined. Detailed fractographic studies were carried out to determine the crack growth mechanism in fatigue in the threshold region. Compact tension specimens were prepared from the single crystal nickel-based superalloy CMSX-4 with [001] orientation as the tensile loading axis direction. These specimens were given three different heat treatments to produce three different [gamma][prime] precipitate sizes and distributions. Fatigue crack growth behavior of these specimens was studied at 650 C in air. The results of the present investigation indicate that the near-threshold fatigue crack growth rate decreasesmore » and that the fatigue threshold increases with an increase in the [gamma][prime] precipitate size at 650 C. The fatigue threshold decreased linearly with an increase in load ratio. Fractographs at 650 C show a stage 2 type of crack growth along [l brace]100[r brace] type of crystal planes in the threshold region, and along [l brace]111[r brace] type of crystal planes in the high [Delta]K region.« less

Approach to Predict Near-Threshold Fatigue Crack Growth Rate in Heat-Treatment Residual Stress Field.

Fatigue crack growth rates in a residual stress field can be predicted by using a superposition technique. This approach needs an analysis of residual and contact stress intensity factors as the authors proposed in previous papers. Objectives of this study are to discuss an approach for predicting the fatigue crack growth rates near the threshold where the oxide induced crack closure is dominant and to evaluate experimentally an effect of the residual plastic stretch on the crack surface contact. It is concluded that this approach gives a good prediction by the using upper bound of base metal crack growth rate data, and the residual plastic stretch has an effect on the crack surface contact stress.

A comparison of test methods for the determination of fatigue crack growth rate threshold in titanium at elevated temperature

The near-threshold fatigue crack growth rate behavior of Ti-8A1-1Mo-1V at elevated temperature was investigated using three different test methods and two different specimen geometries. The first two methods were performed on compact tension specimens where crack closure effects were accounted for by monitoring crack opening load using automated compliance measurements. The third method was performed on a cantilever bending specimen that was designed to minimize closure effects. Crack opening load was not monitored for the bend specimens. All tests were performed at 500°F (260°C) in a laboratory air environment. It was observed that the fatigue crack growth rate data for each of the first two methods collapsed to one curve when crack closure was incorporated into the analysis, with a unique value of threshold stress intensity range, ΔK th,eff . However, the value of threshold stress intensity range, ΔK th , determined from the third method differed slightly from the values obtained from the other two procedures. This suggests that a unique value of ΔK th may not exist for this material under these test conditions even when crack closure is accounted for, and that it may be a function of the test method and the specimen geometry that are used in the investigation.

Fatigue crack growth behavior in inconel 706 at 297 K and 4.2 K

The near-threshold fatigue crack growth rate (FCGR) behavior of Inconel 706 was investigated at ambient (297K) and liquid helium (4.2 K) temperatures, respectively. Specimen orientation did not affect the FCGR properties of Inconel 706. At 297 K, a significant influence of R-ratio on the rates of crack propagation was observed while at 4.2 K, the R -ratio effect was minimal. The extent of oxide-induced crack closure was shown to be insignificant in influencing near-threshold crack growth kinetics of Inconel 706 at both temperatures of 297 and 4.2 K. Roughness-induced crack closure was believed to be the dominant mechanism responsible for the influence of R -ratio on the FCGR properties of Inconel 706; this was proved quantitatively by direct crack closure measurements conducted at 297 and 4.2 K. A greater degree of roughness-induced crack closure was observed at 297 K than at 4.2 K; this correlated with the more pronounced R-ratio effect at 297 K. Decreasing the temperature from 297 to 4.2 K decreased the growth rates of fatigue cracks in Inconel 706. The effect of temperature on crack propagation behavior increased with increasing R-ratio. Crack closure could not rationalize this temperature effect. Moreover, the increase in material strength or Young's modulus on cooling from 297 to 4.2 K could not totally account for the influence of temperature on the near-threshold FCGR properties. Dislocation dynamics appears to offer a qualitative explanation for this temperature effect.

Fatigue crack propagation in unplasticized poly(vinyl chloride) (uPVC): 2. Near-threshold fatigue crack growth

Abstract A new computer-aided test method was developed to measure near-threshold fatigue crack growth rates for a 150 mm class 12 uPVC pipe. A computer program was written to drive the fatigue testing machine and an optical microscope of 0.01 mm resolution was used to determine crack growth in the single-edge-notched specimen. The efficiency and advantages of this new technique are discussed and compared to the current ASTM proposed method. For crack growth rates of the order of 10 −9 m cycle −1 the present computerized method was approximately seven times more efficient and therefore greatly minimized the testing times required to collect near-threshold fatigue data. Using this method the effects of stress ratio and level of processing on the near-threshold fatigue crack growth were investigated. It is observed that multiple crazes were predominant at stress ratios larger than 0.6 but they apparently disappeared at lower stress ratio values. This phenomenon was explained in terms of a critical stress criterion for craze formation and the elastic stress field near the crack tip. The fatigue threshold ΔK th at a crack growth rate of 5 × 10 −10 m cycle −1 was found to decrease linearly with stress ratio from 0 to 0.5 and to remain constant thereafter. The two levels of processing studied for this uPVC pipe material did not produce any effect on ΔK th .

Automatic near-threshold fatigue crack growth rate measurements at liquid helium temperature

The development of a fully automated test apparatus for near-threshold fatigue crack growth rate measurements in a liquid helium environment is described, and some initial results for AISI 300 series stainless steels are presented. The experimental apparatus consists of a servohydraulic test machine and a cryostat, complete with a minicomputer, a programmable arbituary waveform generator, a programmable digital oscilloscope and a fully automatic liquid helium refill system. The technique uses 6.4 mm thick compact specimens subjected to systematically decreasing loads, with 24 h operation at 40 Hz, the crack growth being continuously monitored by specimen compliance measurements. The results presented in this study include da/ dN vs ΔK curves and threshold fatigue stress intensity factors, ΔK th , at 4 K for AISI 304L, 304LN and 316 stainless steels. The near-threshold fatigue behaviours of these materials are similar, and the fatigue crack growth rate trends at intermediate ΔK levels nearly agree with published results.

The Influence of Load Ratio and Temperature on the Near-Threshold Fatigue Crack Growth Rate Properties of Pressure Vessel Steels

Near-threshold fatigue crack propagation behavior was examined in four pressure vessel steels (SA508 Cl 2a, SA533 Gr A Cl 2, SA508 Cl 3a, and SA533 Gr B Cl 2) and two submerged arc weldments (SA508 Cl 2a and SA533 Gr A Cl 2). At a given temperature and load ratio, near-threshold crack growth rates in the four base materials and two submerged arc weldments were essentially identical. Increasing the load ratio from 0.20 to 0.50 increased the rate of near-threshold crack propagation at a given temperature. At a fixed load ratio, increasing the temperature from 24 to 288°C (75 to 500°F) increased the rate of near-threshold crack growth. The effects of load ratio and temperature on the near-threshold crack growth behavior of pressure vessel steels can be explained based on a crack closure concept.