Fatigue Crack Initiation Mode Research Articles

Strengthening mechanism of abnormally enhanced fatigue ductility of gradient nanostructured 316L stainless steel

Structural materials with a good combination of high fatigue strength and high fatigue ductility are highly desirable in engineering applications. However, it remains a challenge to “break” the exclusive relationship between fatigue strength and fatigue ductility. The cylindrical 316L stainless steel specimens with a gradient nanostructured surface layer were prepared using surface mechanical rolling treatment (SMRT). Fully reversed strain-controlled tension–compression fatigue experiments indicated that the SMRT processed 316L specimens achieved simultaneous improvement of fatigue strength and fatigue ductility. The abnormal increase in fatigue ductility is attributed to the significant secondary cyclic hardening caused by the martensitic phase transformation and the transition in fatigue crack initiation mode. The martensitic phase transformation enhances the fatigue strength while reducing the proportion of plastic deformation in the controlled strain amplitude. The competition between the driving force and resistance of fatigue nucleation in different structural units results in the migration of fatigue crack initiation sites from the surface to subsurface, thereby prolonging the fatigue crack initiation life. This work reveals the synergistic strengthening mechanism of fatigue strength and fatigue ductility of a gradient nanostructured 316L stainless steel.

The Low-Cycle Fatigue Behavior, Microstructure Evolution, and Life Prediction of SS304: Influence of Temperature.

To study the fatigue failure and microstructure evolution behavior of SS304, low-cycle fatigue tests are conducted at room temperature (RT), 300 °C, and 650 °C. The results indicate that, because of the influence of the dislocation walls, carbon-containing precipitates, and deformation twins, the cyclic hardening behavior is presented at RT. However, different from the cyclic hardening behavior at RT, the cyclic softening behavior of SS304 can be observed due to the dynamic recovery and recrystallization containing dislocation rearrangement and annihilation at 300 °C and 650 °C. In addition, two fatigue crack initiation modes are observed. At RT, the single fatigue crack initiation mode is observed. At high temperatures, multiple crack initiation modes are presented, resulting from the degradation of material properties. Furthermore, a new fatigue life prediction model considering the temperature is conducted as a reference for industrial applications.

Microstructural effects on fatigue crack initiation mechanisms in a near-alpha titanium alloy

The role of different microstructural constituents on crack initiation in two-phase titanium alloys is still an area of great controversy. The present study investigates the effects of primary α volume fraction and concomitant macrozones on two different fatigue crack initiation modes concurrently observed in a near-α titanium alloy. Statistically representative regions were monitored in quasi-in-situ studies by interrupting fatigue testing to detect slip trace formation leading to crack initiation. In addition, high-resolution 2D strain maps were generated to quantify in-plane shear strains of slip traces related to microstructural features. The detailed analysis demonstrates that at 90% of the proof stress basal 〈a〉 slip plays a crucial role in crack initiation, regardless of whether cracking is transgranular or intergranular. In addition, a distinct shift from transgranular to intergranular crack initiation was observed with increasing αp fraction driven by the increased number of αp/αp grain boundaries. The high-resolution 2D strain mapping suggest that these intergranular cracks initiated from a burst of basal 〈a〉 slip starting from (0001) twist grain boundaries but penetrating one side of the grain pair in case of a partial (0001) twist grain boundary. From these observations, a new criterion for intergranular cracking is proposed based on a geometrical grain boundary parameter and the preference of neighbouring αp grain pair well aligned for basal 〈a〉 slip. It was found that while (0001) twist grain boundaries increase with increasing αp fraction, macrozones do not further enhance their frequency. However, macrozones of hard orientated grains did enhance transgranular crack formation by potentially higher local stress and increasing slip length over clustered αp grains with low misorientation reducing the requirement for out-of-plane shear.

Enhanced fatigue performance of aluminum alloy through surface strengthening treatment

Surface gradient distribution of the grain size was achieved on the 7075-T651 aluminum alloy by means of surface mechanical rolling treatment (SMRT). The tensile properties and fatigue resistance have been improved due to the action of SMRT. After SMRT, fatigue crack initiation mode exhibited the transition from surface to interior as the stress decreases. The mechanism of fatigue strengthening was discussed based on the gradient grain layer generated after SMRT combined with the cyclic deformation behavior and the transition of crack initiation mode.

Improving the stress-controlled fatigue life of low solid-solution hardening Ni-Cr alloys by enhancing short range ordering degree

The tension–tension fatigue behavior of low solid-solution hardening Ni-Cr alloys with an approximately constant stacking fault energy was systematically investigated under different stress amplitudes. With an enhancement of short range ordering (SRO) degree, the fatigue crack initiation mode is evolved from intergranular cracking to slip band cracking, and the cyclic deformation mechanism changes from wavy to planar dislocation slip. Both the fatigue strength coefficient (σ′f) and exponent (b) can be simultaneously increased by inducing SRO structure that greatly enhances the cyclic strain hardening capacity, deformation homogeneity and slip reversibility, and the fatigue life is thus significantly improved.

Effect of spherical inclusions on fatigue anisotropy of HSLA-100 steel

The fatigue limit of the transverse direction was obtained remarkably (125 MPa) lower than that of the longitudinal direction. The fatigue anisotropy was related to different characteristics of inclusions on longitudinal and transverse planes as well as the crystallographic texture. These parameters change the fatigue crack initiation mode from the metal matrix to spherical inclusions. Exaggeratedly conservative results were obtained utilising the Murakami’s model in predicting the lower bound fatigue limit and largest size of inclusion.

Correlation of Microstructure and Fatigue Properties of Hot-Rolled Steel Strips for Automobile Structural Application

In this work, five different hot-rolled steel strips with different microstructures were produced through different microalloying design and adjusting final coiling temperature. The relationship between microstructure and mechanical properties, especially fatigue limit, was analyzed in detail. The results showed that the steels with mainly quasi-polygonal ferrite and pearlite have a similar yield strength as the steels with acicular ferrite and martensite, but they are higher than the DP (ferrite + martensite) steels that are strengthened by the second martensite phase. This is probably because the yield strengths of fine ferritic steels are further enhanced by the precipitation strengthening mechanism of nano-sized particles. Under the present experimental conditions, the fatigue limit is proportional to yield strength rather than tensile strength. It is also found that the precipitation strengthening mechanism is more beneficial to improving the fatigue stress than the strengthening mechanism by introducing martensite, although all these steels have the same fatigue crack initiation mode.

Effects of surface gradient nanostructuring on the fatigue behavior of the friction stir welded AlZnMgCu alloy

Surface gradient nanostructuring was performed on the friction stir welded joints of high strength AlZnMgCu alloy by means of surface mechanical rolling treatment (SMRT). Uniaxial symmetric tension-tension fatigue tests (R = 0.1) showed that two orders of magnitude extension in fatigue lifetime was achieved on the FSW joint of the AlZnMgCu alloy, exhibiting the transition of fatigue crack initiation mode from surface to interior. The enhanced fatigue mechanism was discussed based on the measured in-depth distribution hardness and compress residual stress, as well as the observed microstructure modifications.

Some influencing variables on internal fatigue crack initiation in structural materials

AbstractThis paper emphasizes the important effect of Kt and corrosive environment on internal fatigue crack initiation of structural materials in very high cycle regime. The conventional and ultrasonic fatigue testing results on notched specimens with Kt of 1.5 to 2.5 for YXR3 steel with HRC 60.7 showed subsurface fatigue crack initiation on all notched specimens at Nf higher than 106. On the contrary, fatigue crack initiated at notched surface for HAP72 with HRC 67 with Kt of 2.0. The fatigue crack initiation mode in notched specimens at high cycle regime is governed by surface hardness of high strength steels. The contrasting effect of refrigeration compressor gaseous environment on fatigue crack initiation mode is also demonstrated in the plate bending fatigue testing results of 1% carbon steel.

Microstructure damage evolution associated with cyclic deformation for extruded AZ31B magnesium alloy

Fatigue damage evolution of extruded AZ31B magnesium (Mg) alloy is investigated under strain-controlled tension-compression loading along the extrusion direction at various strain amplitudes, and the different cyclic deformation behaviors are observed. At the strain amplitude of 2%, the tensile peak stress displays significant cyclic softening, whereas the compressive peak stress shows consistent cyclic hardening. At 1%, moderate cyclic hardening is observed at both the tensile peak and compressive peak stresses. At 0.5%, the tensile peak stress presents stable cyclic hardening, whereas the compressive peak stress almost keeps constant. The microstructure morphologies associated with the cyclic deformation are analyzed by scanning electronic microscope (SEM). The degree of deformation twins is evaluated by analyzing X-ray diffraction (XRD) using a normalized parameter λ. The results show the fatigue crack initiation modes and its propagation modes are dependent on the strain amplitude. At 2%, grain boundary (GB) cracking and triple joint cracking are detected after 1st loading cycle. At 1%, fatigue crack initiates at grain boundary (GB cracking), twin boundary (TB cracking) and triple joint of three neighboring grains. Both grain boundary induced (GB-induced) intergranular and persistent slip band induced (PSB-induced) transgranular propagation modes play an important role in the early-stage crack growth. At 0.5%, crack initiation modes are similar to that at 1%, but GB-induced intergranular propagation mode dominates the early-stage crack growth. The effects of the microstructure (texture, grain size and uniformity) on the fatigue damage behavior are discussed.

Competing fatigue failure behaviors of Ni-based superalloy FGH96 at elevated temperature

Fatigue experiments were performed on a polycrystalline P/M processed nickel-based superalloy, FGH96 at 600°C to investigate competing fatigue failure behaviors of the alloy. The experiments were performed at four levels of stress (from high cycle fatigue to low cycle fatigue) at stress ratio of 0.05. There was large variability in fatigue life at both high and low stresses. Scanning electron microscopy (SEM) was used to analyze the failure surfaces. Three types of competing failure modes were observed (surface, sub-surface and internal initiated failures). Crack initiation sites were gradually changed from the surface to the interior with the decreasing of stress level. Roles of microstructures in competing failure mechanism were analyzed. There were six kinds of fatigue crack initiation modes: (1) surface inclusion initiated; (2) surface facet initiated; (3) sub-surface inclusion initiated; (4) sub-surface facet initiated; (5) internal inclusion initiated; (6) internal facet initiated. Inclusions at surface were the life-limiting microstructures at higher stress levels. The probability of occurrence of inclusions initiated is gradually reduced with decreasing of stress level, simultaneously the probability of occurrence of facets initiated is increasing. The existence of the inclusions resulted in large life variability at higher stress levels, while heterogeneity of material caused by random combinations of grains was the main cause of fatigue variability at lower stress levels.

Some Influencing Variables on Internal Fatigue Crack Initiation in Structural Materials

Major variables which cause internal fatigue crack initiation are nonmetallic inclusions in high strength steels, surface roughness and notch in structural components and corrosive environments. This paper focused on the effect of stress concentration factor and corrosive environment for internal fatigue crack initiation of structural materials mainly based upon author's experimental results.In the results of conventional and ultrasonic fatigue testing on notched specimens with stress concentration factor of 1.5, 2.0 and 2.5 for 0.65 mass % carbon matrix high speed steel with HRC 60.7, subsurface fatigue crack initiation was observed on all notched specimens at failed number of cycles over than 106. On the contrary fatigue crack initiated from notched specimen surface for powdered high speed steel with HRC 67 with stress concentration factor of 2.0. The fatigue crack initiation mode in notched specimens at high cycle region may be governed by surface hardness of high speed steels.The different effect of corrosive environment on fatigue crack initiation is demonstrated in the plate bending fatigue testing results of 1% carbon steel and13Cr-1Mo steel in refrigeration compressor gaseous environment. For 1% carbon steel fatigue crack initiated from subsurface at lower fatigue life of 5x106 cycles,while fatigue crack initiated from surface corrosion pit at higher fatigue life of 5x106 cycles. For 13Cr-1Mo steel subsurface fatigue crack initiation was predominantly observed in air and in refrigeration compressor gaseous environment.

Fretting fatigue behaviour of Ti–6Al–4V alloy under plane bending stress and contact stress

Fretting fatigue tests for Ti–6Al–4 V alloy were conducted by use of the plate fatigue specimen with bolt-tightened shoe on both sides of the plate. It was clarified that the repeated bending stress at the contact area where fretting fatigue failure starts linearly decreased as stress over the contact area increased. Fretting fatigue crack starts from the pit where stress concentrate. The pit initiates when fretting debris were removed from the surface striation formed due to the contact slip movement. The fretting fatigue crack initiation mode was transgranular, while the fretting fatigue crack propagation mode was striation.

Fatigue crack growth from stage I to stage II in a corrosive environment

How a corrosive environment such as a 3.5% NaCl aqueous solution would affect the transition of fatigue crack initiation mode from stage I (shear mode) cracking to stage II (normal mode) cracking was investigated, using notched specimens of HT80 and SNCM439 steels (corresponding to AISI 4340 steel).The experimental result that the transition of fatigue crack growth from stage I to stage II was accelerated in a hydrogen-related environment was discussed in terms of hydrogen induced dislocation multiplication near a crack tip. The microstructures immune to environmental damage were examined, using a precipitate-hardened steel.

The influence of hydrogen on atomic binding energy, critical slip shear stress, and fatigue crack propagation rate of aluminium single crystals: Tong, Z.-X., Lin, S. and Hsiao, C.-M. Metall. Trans. A May 1989 20A, (5), 921–924

How a corrosive environment such as a 3.5% NaCl aqueous solution would affect the transition of fatigue crack initiation mode from stage I (shear mode) cracking to stage II (normal mode) cracking was investigated, using notched specimens of HT80 and SNCM439 steels (corresponding to AISI 4340 steel).The experimental result that the transition of fatigue crack growth from stage I to stage II was accelerated in a hydrogen-related environment was discussed in terms of hydrogen induced dislocation multiplication near a crack tip. The microstructures immune to environmental damage were examined, using a precipitate-hardened steel.

Control of fatigue crack initiation mode and fatigue life of α-iron: Hong, S.I. and Laird, C. Mater. Sci. Eng. A Mar. 1989 110, (1–2), L23–L26

Control of fatigue crack initiation mode and fatigue life of α-iron: Hong, S.I. and Laird, C. Mater. Sci. Eng. A Mar. 1989 110, (1–2), L23–L26