ΔK Regime Research Articles

Comparison of fatigue crack growth behaviour in electron-beam and laser powder-bed-fusion Inconel 718

A comparative study on fatigue crack growth behaviour was made between the electron-beam powder-bed-fusion (PBF-EB) and laser powder-bed-fusion (PBF-LB) Inconel 718. The crack followed a transgranular path with a faster propagation rate for the PBF-EB Inconel 718, whereas a combination of intergranular and transgranular path was observed for the PBF-LB, and its slower rate being comparable to the wrought counterpart. The main fatigue crack in the PBF-EB Inconel 718 exhibited a sawtooth shaped path at the micro-scale, with intensive slip traces close to the crack surfaces owing to the very low work hardening rate. Based on the digital image correlation (DIC) analysis of the crack tip field, fatigue-crack sawtooth path in the PBF-EB Inconel 718 can be successfully predicted by using a strain energy density criterion, which dictates the crack growth follows the direction of minimum distance from the crack tip to elasto-plastic boundary. For the PBF-LB Inconel 718, the predominant fatigue crack was straight at low ΔK, but severe deflections occurred at the medium and high ΔK regimes. A clear correlation exists between the incipient intergranular cracks and the main crack path deflection. This suggests that once the accumulated damage in the plastic zone around the crack tip reaches the critical value, intergranular cracks can form the new front of the fatigue crack, causing the main crack path deflection. Elasto-plastic fracture mechanics parameters of rp and ΔCTOD, derived from the DIC-based crack tip field analysis, can qualitatively predict the lower crack growth rate of the PBF-LB Inconel 718.

Fatigue crack growth behavior of 42CrMo high-strength steel with tempered sorbite/bainite microstructures: Roles of grain and constituent in microstructures

Microstructural differences through the thickness have always been an attention focus for large-scale steel structures, since they could significantly influence fatigue crack propagation (FCP) characteristics. Here, the effects of microstructure in FCP behaviors of 42CrMo steel were analyzed with emphasis on two different tempered sorbite/bainite (TS/B) microstructures: (i) fine TS/B structures (M42); (ii) coarse TS/B structures (M52). The results show that the FCP rate is relatively slower for M52 than for M42 in the regime through the later stage of near-threshold (I-L) to the early stage of Paris-regime (Ⅱ-E) with low ΔK, but is fairly similar for the two cases in the other ΔK regimes. Under relatively low ΔK, the FCP path of M52 is more tortuous with numerous micro-cracks in B and larger-sized crack deflections at the carbides and boundaries compared to M42, especially in the I-L where fatigue micro-cracks can deflect at the single coarse carbide, thereby resulting in roughness-induced crack closure and crack retardation behavior. Theoretical analysis shows that FCP behaviors are closely related to the cyclic plastic region size (Δrp) at the crack-tip. In the early stage of near-threshold region, Δrp is restricted within a grain-size scale for both steels, thus leading to a similar FCP rate. However, from the I-L to the Ⅱ-E, coarse B/ferrite-grains in M52 are always larger than the Δrp of fatigue crack-tip, and thereby lead to numerous micro-cracks formation and crack deflections at the carbides and B boundaries, inducing crack closure. In the later stage of Paris region, B/ferrite-grains in the two cases are much smaller than Δrp, so that FCP behaviors depend on the microstructure group effects, or overall strength and fracture toughness of materials, which cause the close FCP rate.

Room and elevated temperature fatigue crack propagation behavior of Inconel 718 alloy fabricated by laser powder bed fusion

The fatigue crack propagation (FCP) behavior of Inconel 718 alloy manufactured by laser powder bed fusion (L-PBF) was studied at 25 and 650 °C, and compared to those of conventionally processed counterpart. The FCP rates of L-PBF Inconel 718 alloy were found to be significantly higher than those of conventionally processed counterpart at 25 °C in low and intermediate ΔK regimes. The FCP rates decreased in near-threshold ΔK regime at 650 °C, while the reversal occurred above ΔK of 7 MPa√m, as compared to those at 25 °C. The FCP behavior of L-PBF Inconel 718 alloy was discussed based on micrographic and fractographic analyses.

Effect of Microstructure on High Cycle Fatigue and Fatigue Crack Propagation Behaviors of β-Annealed Ti–6Al–4V Alloy

High cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of β-annealed Ti64 alloys having a microstructure of either α colonies or basketweave were studied. It was found that the resistance to HCF of β-annealed Ti64 alloys with a microstructure of α colonies was similar but slightly inferior to those with a microstructure of basketweave. On the other hand, the FCP rates of β-annealed Ti64 alloys having a microstructure of α colonies were lower than those of basketweave, particularly in low ΔK regime. Crack path observation on the surfaces of β-annealed Ti64 alloy specimens suggested that crystallographic crack growth prevailed in the microstructure of α colonies, leaving cleavage facets on the fracture surface and less fatigue damage at the tip of crack. Based on the fractographic and micrographic examination, the effects of microstructure on the HCF and FCP behaviors of β-annealed Ti64 alloys were discussed.

Effect of Pre-Straining on High Cycle Fatigue and Fatigue Crack Propagation Behaviors of Precipitation Hardened Steel

Steel sheet is often shaped by straining to form a final product, and deformation process can influence the fatigue resistance. In this study, high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of precipitation hardened (PH) high strength steel with pre-straining levels of 0, 5 and 9% were investigated. Regardless of pre-straining level, fatigue cracks were found to be initiated at surface pits formed during pickling. The effect of pre-straining was not significant on the HCF resistance of PH steel, while the FCP rates increased with increasing level of pre-straining only in low and intermediate ΔK regimes. It was suggested that marginal increase on HCF resistance of PH steel was due to the combined effect of enhanced resistance to crack initiation and reduced resistance to crack propagation with pre-straining. The effect of pre-straining on HCF and FCP behavior of PH steel was discussed based on micrographic and fractographic observations.

Initiation and growth behaviour of small internal fatigue cracks in Ti‐6Al‐4V via synchrotron radiation microcomputed tomography

AbstractSmall internal fatigue cracks initiated in Ti‐6Al‐4V in the very high cycle regime were detected by synchrotron radiation microcomputed tomography (SR‐μCT) at SPring‐8 in Japan. The initiation and growth behaviours of the cracks were nondestructively observed, and the da/dN‐ΔK relationship was measured and compared with that obtained in a high vacuum environment. SR‐μCT revealed that more than 20 cracks were initiated in one specimen. The crack initiation life varied widely from 20% to 70% of the average fatigue life and had little influence on the growth behaviour that followed. The initiation site size of each internal crack detected in one specimen was comparable with the size of the fracture origins obtained in ordinary fatigue tests. These results suggest that the surrounding microstructures around the initiation site are likely a dominant factor on the internal fracture rather than the potential initiation site itself. The internal crack growth rates were lower than 10−10 m/cycle, and extremely slow rates ranging from 10−13 to 10−11 m/cycle were measured in a lower ΔK regime below 5 MPa√m. The internal crack growth rate closely matched that of surface cracks in a high vacuum, and the reason for the very long life of internal fatigue fractures was believed to result from the vacuum‐like environment inside the internal cracks.

The HCF behavior and life variability of a Ti-6Al-4V alloy with transverse texture

In this study, the microtexture effect on HCF behavior of a hot-rolled Ti-64 plate was investigated. The fatigue life scatter was relatively small and the life variability exhibited a single distribution mode due to microstructural and textural effects. All samples exhibited the Stroth-type crack initiation and the number of pile-up locations between the initiation facet and surrounding propagation facets was related to the life variability. Cracks tended to initiate at the boundary of two microtextured regions (MTRs) located in sample surface area and this is likely caused by the preferred dislocation pile-up between “hard” and “soft” α grains at MTR boundary. During early facet propagation period, crack front was deflected when crossing MTR boundaries. There was no crack front deflection when striation propagation mode became dominant in the high ΔK regime. The closely orientated grains within MTRs provided an easy path for microcracks to propagate and caused the overall deterioration of fatigue properties.

Near-threshold fatigue crack propagation behavior of austenitic high-Mn steels

High-Mn austenitic steels utilizing TWIP (twinning induced plasticity) effect have excellent combination of tensile strength and ductility. The near-threshold fatigue crack propagation (FCP) behavior, as represented by the ΔKth value, of high-Mn steels was examined with the emphasis on the effect of stacking fault energy (SFE), grain size, twinning and tensile properties. Even though no predominant parameter determining the near-threshold FCP behavior of high-Mn steels was found, the SFE showed the most reasonable correlation to the ΔKth values among the variables examined. It was also suggested that the slip reversibility as determined by SFE could not solely explain the near-threshold FCP behavior of high-Mn steels. The presence of twin boundaries appeared to be not beneficial in improving the resistance to FCP of high-Mn steels in low ΔK regime. The near-threshold FCP characteristics of high-Mn austenitic steels were discussed and correlated with the well-known parameters presumably affecting FCP.

Fatigue crack propagation behavior of Fe25Mn and Fe16Mn2Al steels at room and cryogenic temperatures

The fatigue crack propagation (FCP) behavior of Fe25Mn and Fe16Mn2Al austenitic steels was investigated at 298 and 110 K, and the results were compared with the reported results of Fe24Mn2Cr steel. It was found that the FCP behavior of high-Mn, austenitic steels was largely influenced by the stacking fault energy (SFE) and the grain size. The resistance to FCP of high-Mn steels in this study was enhanced in the near-threshold ΔK regime with decreasing temperature from 298 to 110 K. The improvement for the Fe25Mn and the Fe16Mn2Al specimen was, however, marginal as compared to that of the Fe24Mn2Cr specimen. Other than the change in SFE, the secondary cracking at cryogenic temperature appeared to affect the FCP behavior of high-Mn steels, since the secondary cracks perpendicular to the crack propagating direction could reduce the effective stress intensity factor, decreasing the FCP rates. Sufficiently high stress concentration at grain boundary tended to occur at low temperature for relatively large grain sized Fe24Mn2Cr specimen and cause the secondary cracking, but not for the Fe25Mn and the Fe16Mn2Al specimen.

Fatigue crack propagation behavior of Fe24Mn steel weld at 298 and 110 K

The fatigue crack propagation (FCP) tests were conducted on Fe24Mn steel in the region of base metal (BM), weld metal (WM) and fusion line (FL) at 298 and 110 K. The FCP rates of Fe24Mn specimens in the region of BM, WM and FL were greatly decreased, while no notable difference in the fracture mode was observed, with decreasing temperature from 298 to 110 K. The FCP rates of Fe24Mn WM and FL specimens were slightly lower than those of BM specimen at both room and cryogenic temperatures. The SEM fractographic analyses suggested that each specimen showed the transgranular facets at both temperatures in low and intermediate ΔK regimes. However, the morphological details varied depending on the region of weld and testing temperature. Relatively large sized facets were observed for the WM specimen with the columnar grain boundary playing the same role as the grain boundary in the BM and the FL specimens in the near-threshold ΔK regime. The FCP behavior of Fe24Mn steel in the region of BM, WM and FL is discussed at 298 and 110 K based on the fractographic and micrographic observation.

Effect of service exposure on fatigue crack propagation of Inconel 718 turbine disc material at elevated temperatures

In this study, the fatigue crack propagation behavior of Inconel 718 turbine disc with different service times from 0 to 4229h was investigated at 738 and 823K. No notable change in microstructural features, other than the increase in grain size, was observed with increasing service time. With increasing service time from 0 to 4229h, the fatigue crack propagation rates tended to increase, while the ΔKth value decreased, in low ΔK regime and lower Paris' regime at both testing temperatures. The fractographic observation using a scanning electron microscope suggested that the elevated temperature fatigue crack propagation mechanism of Inconel 718 changed from crystallographic cleavage mechanism to striation mechanism in the low ΔK regime, depending on the grain size. The fatigue crack propagation mechanism is proposed for the crack propagating through small and large grains in the low ΔK regime, and the fatigue crack propagation behavior of Inconel 718 with different service times at elevated temperatures is discussed.

Cryogenic S–N Fatigue and Fatigue Crack Propagation Behaviors of High Manganese Austenitic Steels

In the current study, the S–N fatigue and the fatigue crack propagation (FCP) behaviors of high manganese austenitic steels, including Fe24Mn and Fe22Mn, were studied, and the results were compared with STS304 (Fe-1Si-2Mn-20Cr-10Ni). The S–N fatigue tests were conducted at 298 K and 110 K (25 °C and −163 °C), respectively, and at an R ratio of 0.1 under a uniaxial loading condition. The FCP tests were conducted at 298 K and 110 K (25 °C and −163°C), respectively, and at R ratios of 0.1 and 0.5, respectively, using compact tension specimens. The resistance to S–N fatigue of each specimen increased greatly with decreasing temperature from 298 K to 110 K (25 °C to −163 °C) and showed a strong dependency on the flow stress. The FCP behaviors of the austenitic steels currently studied substantially varied depending on testing temperature, applied ΔK (stress intensity factor range), and R ratio. The enhanced FCP resistance was observed for the Fe24Mn and the Fe22Mn specimens particularly in the near-threshold ΔK regime, while the enhancement was significant over the entire ΔK regimes for the STS304 specimen, with decreasing temperature from 298 K to 110 K (25 °C to −163 °C). The S–N fatigue and the FCP behaviors of high manganese austenitic steels are compared with STS304 and discussed based on the fractographic and the micrographic observations.

Transition of crack propagation from a transgranular to an intergranular path in an overaged Al-Zn-Mg-Cu alloy during cyclic loading

The fatigue crack propagation behavior in the overaged Al-Zn-Mg-Cu alloy was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results revealed that a fatigue crack tended to transgranularly propagate in the near-threshold regime, whereas intergranular crack propagation was dominant at the high ΔK regime. The transition of crack propagation from a transgranular to an intergranular path that occurred in the Paris regime was strongly influenced by the misorientation of adjacent grains and precipitate free zones. In addition, a crystallographic model of crack propagation was proposed to interpret the transition. The fatigue short crack propagation on a single slip plane was responsible for the formation of a transgranular propagation path in the near-threshold regime. The fatigue long crack propagation, which was conducted by a duplex slip mechanism in the Paris regime, led to the formation of fatigue striations. The formation of a zigzag crack in the near-threshold regime was ascribed to the high misorientation of adjacent grains.

Influence of Temperature and Long Term Ageing on the Fatigue Crack Growth in a Precipitation Hardened Martensitic Stainless Steel

The high-temperature Fatigue Crack Growth (FCG) behaviour of a 15-5 PH (precipitation-hardened) martensitic stainless steel was investigated in three different metallurgical conditions, i.e., as-received, aged at 300°C for 1000h and 5000h. FCG tests were conducted at room temperature and 300°C under constant amplitude loading with two load ratios (R = 0.1 and 0.7) and under variable amplitude loading. The results indicate that the fatigue crack growth rates (FCGRs) increase with R-ratio at the lower ΔK regime and merge together in the high ΔK regime, regardless of ageing conditions and temperature. At 300°C, the FCGRs are higher than at room temperature for all ageing conditions. Under variable amplitude loading, the same retardation effect was observed for all the given ageing conditions at room temperature

Effect of Microstructure on Fatigue Crack Propagation and S–N Fatigue Behaviors of TMCP Steels with Yield Strengths of Approximately 450 MPa

In the present study, stress (S) – number of cycles to failure (N) (S–N) fatigue and fatigue crack propagation behaviors of three thermomechanical control process steels with different microstructures but similar yield strengths of approximately 450 MPa were investigated. The P + F steel was predominately pearlite plus ferrite, whereas B1 and B2 steels were both bainitic steels with martensite-austenite and pearlitic islands. Despite the significant difference in microstructural features, the resulting fatigue crack propagation rates and near-threshold ΔK values were comparable with each other. The hard phases, such as pearlite colonies in the P + F specimen, tended to affect fatigue crack propagation behavior in a similar manner, and severe crack branching was observed in intermediate and high ΔK regimes. Despite similar fatigue crack propagation rates and near-threshold ΔK values, the resistance to S–N fatigue was substantially different for each steel specimen. Depending on fatigue crack initiators, such as the ferrite/pearlite phase boundaries for the P + F specimens and the cracked martensite-austenite and/or small pearlitic islands for the bainitic specimens, the cycles for crack initiation varied greatly.

Effect of aging treatments on the fatigue crack growth and threshold behavior of AA 2219 aluminium alloy

The fatigue crack growth and threshold behavior of AA2219 Al-alloy has been examined for naturally aged (NA), under aged (UA), peak aged (PA) and over aged (OA) conditions. Significant differences were observed in the fatigue crack growth at low and high stress intensity ranges. The alloy in the NA condition possesses the highest resistance to fatigue crack initiation which can be viewed from the values of achieved threshold stress intensity range, ΔKTH. In the intermediate ΔK (Paris law) regime, all the four materials fall into a single band. In the high ΔK regime (> 20 MPa√m) PA condition exhibits better crack growth resistance than under aged conditions. The inhomogeneous transcrystalline slip in the UA condition results in the slower crack growth at low ΔK. The fracture morphology changes from crystallographic facets in the threshold region to clearly developed ductile striations in the Paris law regime to microvoid coalescence in the high ΔK region.

Effects of thickness on fatigue properties of investment cast Ti-6Al-4V alloy plates

The present study is concerned with the effects of plate thickness on high-cycle fatigue properties and fatigue crack propagation behavior of investment cast Ti-6Al-4V alloys having Widmanstatten structure. High-cycle fatigue test and fatigue crack propagation test were conducted on three cast plates having different thickness, and then the test data were analyzed in relation with microstructures, tensile properties, and fatigue fracture mode. The high-cycle fatigue results indicated that fatigue strength of the three cast plates was quite similar because of their similar tensile strength. In the case of the fatigue crack propagation, the thicker cast plate composed of thinner α platelets had the slightly faster crack propagation rate than the other plates. The effective microstructural factor determining the fatigue crack propagation rate was found to be the thickness of α platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold ΔK regime.

Fatigue Properties of the Aluminium Alloys 6013 and Al-Mg-Sc

The fatigue properties of the Al-alloys 6013 T6 and Al-Mg-Sc were compared under constant amplitude loading conditions. The fatigue strength as well as the nucleation and propagation behavior of self-initiated microcracks was determined at R = -1 in air and NaCI solution. Variations of the test frequency on the fatigue macrocrack propagation behavior were studied at R = 0.1 in NaCl solution. The results can be explained mainly with the microstructural differences between both alloys. The extremely fine grained AlMgSc alloy exhibited a higher resistance against fatigue crack nucleation despite the lower yield stress in comparison to the coarse grained alloy 6013 T6. The microcrack propagation resistance was also higher for AlMgSc, while the macrocrack propagation resistance in the low ΔK regime was inferior as compared to 6013.

Fracture and fatigue behavior of sintered steel at elevated temperatures: Part II. Fatigue crack propagation

Fatigue cracking resistance of sintered steel as a function of temperature is characterized by crack growth rate vs the stress intensity range, ΔK. The stress ratio effects on fatigue crack propagation (FCP) are investigated from room temperature to 300 °C. The crack closure effects on FCP are evaluated by both theoretical and experimental approaches. We found that the crack closure cannot be fully responsible for the observed increase of fatigue resistance with low stress ratio. Experimental results support that both Kmax and ΔK control near-threshold crack growth. Fatigue crack resistance at high ΔK regime decreases with temperature. The apparent increase of fatigue resistance at the near-threshold regime at elevated temperatures might be attributed to microcrack toughening.

Environment-sensitive closure and fatigue crack propagation behavior of Al 2090

Fatigue crack propagation (FCP) behavior of Al-Li alloys is largely influenced by extrinsic crack closure. The studies on closure have been confined to the relatively benign moist air environment. The synergistic interaction between the aggressive environment and closure on FCP, particularly that in the near-threshold ΔK regime, has often been neglected due to the difficulties in closure measurement in aqueous environments. In the present study, FCP behavior of Al 2090 was examined in moist air, vacuum, and aqueous 3.5 pct NaCl at different R ratios with an emphasis on the understanding of environment-sensitive crack closure mechanisms. The closure in Al 2090 was strongly affected by applied ΔK, R ratio, and environment. The increase in near-threshold da/dN in aqueous 3.5 pct NaCl over that in moist air mostly disappeared after crack closure correction, indicating that environment-sensitive closure was a major determining factor for the FCP rates of high-closure materials. Interestingly, Al 2090 showed a lower closure level in vacuum than in moist air, despite the enhanced fracture surface roughness and reduced FCP rates in low and intermediate ΔK regimes. The increasing R ratio tends to reduce closure contribution and eventually increase the FCP rates of Al 2090 for each environment. The trends observed in the present study show that closure is determined by complex interactions between the environment and R ratio, particularly in a low stress intensity regime.