Mechanics and micro-mechanisms of LCF and dwell fatigue in Ti-6Al-4V
Creators: name: Tympel, Peter
Publisher: name: Imperial College London
Year: 2016

description: All manufacturers of gas turbines, and indeed, of engineering systems, suffer from both managed and unexpected cracking issues, where components are retired from service. This PhD project was sponsored by a group in Rolls-Royce plc concerned with fractog- raphy and failure investigation, with the purpose of asking whether dwell, high cycle and low cycle fatigue, stress ratio or block loading can be distinguished from fracture sur- faces. Therefore, the central theme of this thesis is the examination of fracture surfaces of a Ti-6Al-4V alloy and relating them to the fatigue loading regimes. The as-received material was unidirectional rolled and exhibited a nearly equiaxed microstructure with a strong {11 ̄20}⟨0001⟩ texture. A series of LCF and dwell experiments were performed on unnotched cylindrical samples to investigate initiation behaviour under both loading con- ditions. The formation of fatigue striations was investigated with corner crack specimens. The fatigue initiation behaviour was substantially different between continuously cy- cled samples and specimens that experienced long dwell periods at high stress. Cracks in continuously cycled samples typically initiated at the surface by facet formation when cycled at 92% of the yield stress. At the same cyclic stress but with a 2 minute dwell pe- riod, the cycles to failure reduce and multiple subsurface crack initiation occurred. Areas with cleavage-like failure each with a single initiation facet would form across the gauge length. The cleavage like areas were identified as regions where grains were preferentially oriented with the c-axis along with the loading direction (macrozones). The crack prop- agates faster through these grains, causing the crack to grow along the rolling direction. The initiation facets were typically tilted about 32◦ towards the loading direction. The facets tilted around the normal direction that suggested that initiation facets are formed outside the macrozone. EBSD measurements of the initiating area confirmed that dwell initation occurred at the edges of macrozones. In dwell initiation facets the basal slip systems were most likely to be active under tension, while LCF initiation facets showed highest shear stresses on ⟨a⟩ type pyramidal planes. The Schmid factors of dwell facets were generally lower than those of neighbouring grains. This supports the load shedding mechanism based on the Stroh model. Propagation facets under continuous cycling were only seen around the initiation point, with increased tilt angles of about 38◦. Cracks propagate in α titanium by the formation of fatigue striations. Above a prop- agation rate of approximately 100 nm per cycle each striation was formed by a single load cycle. Due to the fact that they form in stage II of crack propagation they are referred to as Paris striations. The 1:1 ratio broke down when the crack propagated at lower rates and then a striation was the result of up to several hundred load cycles. These were ￼referred to as non-Paris striations. The dislocation analysis below the striations showed slip bands on prismatic planes. Outside the Paris region slip bands were only seen along one plane. The slip bands were tilted at nearly 30◦ towards the crack growth direction and intersect with the surface. In Paris striations slip bands were seen along all prismatic planes. The observed slip band spacing correlated with the striation separation. A model for formation was proposed where striations form by extrusion of material onto the sur- face due to localised glide along the slip bands. Slip activity can not be fully reversed when the crack closes due to an instant oxide layer forming on new created surfaces. For striations to form only one slip band needs to be active. Additional slip bands form in the space between to allow for a homogeneous deformation. The effect of load ratio R, frequency and waveform on the striation profile was ex- amined using AFM. The ratio of striation height to separation (H) was about 0.12 and s constant for any ∆K values. The shape of the striations was investigated by compar- ing the slope and flank length on either side of the striations. On average the striations showed a steeper rise and longer fall. A change in R ratio, frequency or waveform did not influence the H value but did influence the striation shape. The flank length was s thought to be proportional to the distance that dislocations glide per cycle. The relative length of the striation during load removal increased with (i) higher R ratios or (ii) longer time under stress by either a lower test frequency or a trapezoidal waveform. The ratio of slopes associated with the striation rising and falling was influenced by the applied waveform.