Improved test method and analytical modelling for fatigue crack growth in coarse‐grain titanium alloy with rough fatigue surfaces

Abstract

Fatigue crack growth rate properties are typically determined by experimental methods in accordance with ASTM Standard E647. These traditional methods use standard notched specimens that are precracked under cyclic tensile loads before the main test. The data that are produced using this approach have been demonstrated elsewhere to be potentially adversely affected by the test method, particularly in the threshold region where load reduction (LR) methods are also required. Coarse‐grained materials that exhibit rough and tortuous fatigue surfaces have been observed to be strongly affected by the tensile precracking and LR, in part because the anomalies caused by crack closure and roughness‐induced closure become more important. The focus of the work reported in this paper was to further develop methods to determine more accurate fatigue crack growth rate properties from threshold through to fracture for coarse‐grained, β‐annealed, titanium alloy Ti‐6Al‐4V extra low interstitial thick plate material. A particular emphasis was put upon the threshold and near threshold region, which is of strong importance in the overall fatigue life of components. New approaches that differ from the ASTM Standard included compression precracking, LR starting from a lower load level and continuing the test beyond rates where crack growth would otherwise be considered below threshold. For the threshold regime, two LR methods were also investigated: the ASTM method and a method where the load is reduced with crack growth such that the crack mouth opening displacement is held constant, in an attempt to avoid remote closure. Constant amplitude fatigue crack growth rate data were produced from threshold to fracture for the titanium alloy at a variety of stress ratios. Spike overload tests were also conducted These data were then used to develop an improved analytical model to predict crack growth under spectrum loading and the predictions were found to correlate well with test results.

Effects of stress ratio on high-cycle and very-high-cycle fatigue behavior of a Ti–6Al–4V alloy

“…Thus, the size of the plastic zone for each rough area is calculated by Eq. (12) and the average value is 3.94 μm, which is close to the average diameter of primary α grains (5.89 μm). This suggests that the size of α grain of the titanium alloy is a characteristic size for crack initiation.…”

“…They proposed a model based on a twodimensional Poisson defect distribution to describe the competition of failure modes. It is known that many factors, such as stress ratio [10][11][12], environment [13] and surface treatment [14][15][16], may affect fatigue crack initiation and propagation behavior of metallic alloys. As an important topic, the effects of stress ratio on fatigue crack initiation and propagation of titanium alloys were investigated in previous papers [17,18].…”

Faceted crack initiation characteristics for high-cycle and very-high-cycle fatigue of a titanium alloy under different stress ratios

“…Titanium alloys are widely used as structural materials in aircraft engineering due to their high strengthto-weight ratio, corrosion resistance and high-temperature resistance [6,7]. Recently, faceted crack initiation was observed in high-cycle fatigue (HCF) and VHCF regimes of titanium alloys, which resulted in the sharp decrease of fatigue strength [8].…”

Evaluation of Fatigue Crack Front Shape for a Specimen with Finite Thickness

“…Many models have been developed in the past [14,15]. Below we present an analytical model [16], which directly incorporates the plate thickness and will be used in conjunction with parametric finite element calculations as outlined above.…”