Reduction Of Fatigue Crack Growth Rate Research Articles

Effects of Al content and α2 precipitation on the fatigue crack growth behaviors of binary Ti–Al alloys

The influence of Al content and α2 precipitation on the fatigue crack growth (FCG) behaviors of binary Ti - xAl (x = 2, 4, 6, 8 wt %) alloys was investigated. Ti–Al binary alloys with different Al contents were solution-treated and followed by aging treatment to obtain Widmanstatten microstructure and promote α2 precipitation at higher Al content. Results show that the increase of Al content dramatically reduces the FCG rate. The enhanced resistance to FCG propagation is related to the increase of the critical resolved shear stress (CRSS) for dislocation slipping that acts as crack initiation sites, basing on solution strengthening with the addition of Al. Furthermore, roughness-induced crack closure with increasing the Al content also contributes to the decrease of FCG rate. With the increment of the Al content, the early fatigue crack propagation stage, in which the crack propagation significantly affected by microstructural factors, will be enlarged due to the decrease of crack tip plastic zone, leading to crack deflection and the reduction of FCG rate. In Ti–8Al alloy, the presence of α2 precipitates and the increment of its size and volume fraction after aging treatment will result in significant increase of FCG rate. Moreover, the activation of large-scale slipping in relatively larger colonies promotes the crack to propagate along the large-scale slipping that induces smoother crack path and higher FCG rate.

Effect of microstructure on the fatigue crack growth behavior of Cu–Be–Co–Ni alloy

The fatigue crack growth (FCG) behaviors of Cu–Be–Co–Ni alloy subjected to interrupted aging (IA), normal aging (NA) and overaging (OA) treatment were discussed. The distinctions in microstructure of the alloys after various aging treatments led to different FCG behaviors. In order to understand the different FCG behaviors, a model based on the reversible plastic zone (RPZ) size and microstructural factors was applied in this study. At the relatively low stress intensity factor range (ΔK < 17 MPa m1/2), the RPZ size of each sample was smaller than the corresponding grain size, so the fatigue crack propagated inside the grain and adjacent to the grain boundaries (GBs). For the IA alloy, fine and coherent γ″ precipitates which could be sheared by dislocations promoted the fatigue crack resistance, leading to the decrease of FCG rate. When the value of stress intensity factor range becomes higher (ΔK > 21 MPa m1/2), the RPZ size of each sample was 1–2 times of the corresponding grain size. Consequently, the crack trended to grow along GBs. For the OA alloy, the crack propagated along the interfaces between the discontinuous precipitation (DP) cells and parent phase. The tortuous interfaces hindered the propagation of the fatigue cracks, resulting in the reduction of FCG rate.

Fatigue crack growth behavior in powder-metallurgy 6061 aluminum alloy reinforced with submicron Al2O3 particulates

Fatigue crack growth (FCG) behavior of aluminum alloy 6061 particulate composites reinforced with submicron Al2O3 has been investigated in two volume fractions of 10% and 20%. The composites were fabricated by powder processing and consolidated by hot extrusion into 17mm round bars. FCG rates were measured at a fixed load ratio of R=0.1 using three-point bending specimens. Both composites showed higher FCG resistance than that reported for a monolithic AA6061 over the threshold and Paris regimes. Increasing the reinforcement content from 10% to 20% resulted in slight improvement in the near-threshold fatigue response but caused marked reduction in FCG rates over the intermediate-ΔK Paris regime. FCG rates in the 10% composite were adequately described by Paris law with a single slope for the entire range of ΔK tested. The 20% composite, however, exhibited an abrupt transition in Paris slope at Kmax≈13MPam. Examination of the fatigue crack-surfaces revealed that this transition was accompanied with a change in the growth mechanism from FCG via matrix cracking to accelerated FCG due to concurrent damage introduced by micro-void nucleation and growth at the crack-tip process zone. Variations in FCG rates through the matrix with reinforcement content correlated proportionally with 1/(Eσy2).

Comparison of the fatigue propagation behaviour of polystyrene and 95/5 polystyrene/polyethylene blends

The fatigue crack propagation (FCP) behaviour of polystyrene (PS) and 95/5 PS/high density polyethylene (HDPE) was studied at cycling frequencies (f) of 0.2, 2 and 20 Hz. At the latter two frequencies, the fatigue crack growth rates (FCGRs) in the blends were lower than in PS, especially when a styrene/ethylene–butylene/styrene (SEBS) triblock copolymer was added as a compatibilizer. The fractographic features observed were analysed in detail. Discontinuous growth bands (DGBs), associated with the fracture of crazes, formed at low FCGRs in PS and at low and high FCGRs in the blends. Large dimple-like features formed at intermediate FCGRs and fatigue striations at high FCGRs were observed in PS. The observations indicated that the reduction in FCGR when HDPE was added to PS was associated with the presence of stretched HDPE particles which fractured behind the crack front, with increased particle–matrix adhesion favoured by the compatibilizer. An increase in cycling frequency decreased the FCGRs, with the fractographic observations indicating that this effect was associated with a decrease in the time-dependent deformation in the fracture process zone slightly ahead of the crack tip.

Crack tip oxidation of a superalloy in molten nitrate salt

Alloy 800 has been proposed for use in the receiver tube panel arrays of advanced solar central receiver (SCR) designs. In this application the alloy will be exposed to a molten mixture of sodium and potassium nitrate salts at temperatures ranging up to approximately 600/sup 0/C While these salts are routinely used in a variety of applications including metal heat treating and process heat transfer, common industrial experience has been limited to maximum temperatures of 400/sup 0/C - 450/sup 0/C. There is, therefore, considerable interest in the compatibility of these salts with containment alloys at the higher temperatures associated with SCR designs. Additionally, the containment alloy may be subject to thermally induced fatigue damage resulting from intermittent cloud cover and diurnal cycling. Previous work has found that slower near-threshold fatigue crack growth rates (FCGR) in Alloy 800 result when it is tested in molten nitrate salt at 600/sup 0/C (relative to air). This reduction in FCGR was attributed to an environmentally induced increase in oxidation resulting in oxide-induced crack closure. The purpose of the present work was to examine the influence of a typical nitrate salt composition on crack tip oxidation in Alloy 800 in order to determine if inmore » fact the proposed corrosion phenomena could explain the fatigue crack growth characteristics of the alloy.« less