Non-closure of cracks and fatigue crack growth in ? heat treated Ti-6A1-4V

Abstract

The influence of microstructure on fatigue crack growth continues to be a subject of controversy [I]. For the alloy Ti-6AI-4V this is illustrated by the conflicting results concerning the effect of B heat treatment on fatigue crack growth rate [2][3][4][5][6][7][8]. The ranges of growth rate examined in these works were not the same but there was some overlap, and a comparison can be made at a AK corresponding to a common rate of 10-4mm/cycle for the mill annealed condition. Most of the results showed that B heat treatment was associated with a reduced growth rate compared with that obtained after mill anneal [2,3,6-8], but in two instances no effect was found [4,5]. When observed, increased resistance to crack growth was considered to be related largely to crack branching and tortuosity of crack path promoted by the microstructure.Recent work on 20 mm thick compact tension specimens supports the view that B heat treatment of Ti-6AI-4V can produce enhanced fatigue crack growth resistance at intermediate growth rates [9] and this is illustrated by the data in Fig. i. An obvious difference between the two material conditions examined was the tortuous fracture path and crack branching associated with the ~ heat treatment compared with the relatively smooth, planar fracture face associated with the mill anneal. The crack mouth opening was monitored during crack growth and it was found that for the B heat treated material the opening was not proportional to the applied load throughout the fatigue cycle, as shown for example in Fig. 2. A response of this type in aluminium alloy specimens was previously attributed by Elber [i0] to a closing of the crack from the tip backwards due to tensile plastic deformation left behind the propagating crack tip. The decrease in specimen compliance at the lower end of the load cycle displayed in Fig. 2 is consistent with load bearing contact across some parts of the crack faces. Hence, AK at the crack tip was less than that expected from the applied load range. The mill mlhealed material did not exhibit such effects and therefore the reduced growth rates after ~ heat treatment are considered to be linked to the reduction in AK by load transfer across the crack faces.An explanation for load transmission across the fracture faces of the ~ heat treated Ti-6AI-4V specimen during the fatigue cycle may be obtained by analogy to the behaviour found in B heat treated Ti-6AI-5Zr-0.5Mo-0.25Si which showed a similar tortuosity of fatigue crack path and non-linearity of the crack mouth opening -load response [9]. The profile of a fatigue crack in a part-through-cracked specimen of this material was examined.It was found that the crack was open, even when unloaded, over a large part of the area behind the crack tip across which the slope of the load-crack mouth opening record at minimum load indicated that load bearing contact was being made. Discrete regions were evident where the crack was apparently held open due to local mismatch of the fracture faces. An example is given in Fig. 3 which show...

Mixed-mode, high-cycle fatigue-crack-growth thresholds in Ti-6Al-4V: Role of bimodal and lamellar microstructures

“…[39][40][41][42] This behavior results from a strong preference for crack growth along the basal [25,27] and prismatic planes [28,29] in the hexagonal ␣ phase across the entire or significant portions of lamellar colonies, relatively easy decohesion of ␣/␤ interfaces, [30] and difficulty in traversing colony boundaries and prior ␤ grain boundaries. [25,26] The present results (Figure 3) confirm that the lamellar Ti-6Al-4V alloy, with its microstructurally heterogeneous crackgrowth resistance, exhibits superior (large-crack) fatiguecrack growth properties relative to the more homogeneous bimodal microstructure; specifically, the lamellar microstructure has higher ⌬G TH thresholds, or combinations of ⌬K I,TH and ⌬K II,TH (Figures 3(a) and (b)), for all mixedmode loading conditions investigated (⌬K II /⌬K I ϭ 0 to 1.9).…”

“…Since these dimensions pertain to the lamellar colony size, which is have been reported for mode I fatigue-crack growth in other quite large (ϳ500 m), such behavior is anticipated for titanium alloys. [39][40][41]55] This reversal has been attributed crack sizes that are of practical interest for turbine engine largely to the prominant role of crack-wake shielding on the high-cycle fatigue. large-crack fatigue resistance of lamellar microstructures; such effects are necessarily limited for small cracks, due to their restricted wake.…”

A theoretical model for roughness induced crack closure

“…5. The maximum CTOD at Kmax is given by [23] 0.49K~ax ~max -- (9) EO'y During unloading, closure corresponding to the deviation of the linear K vs CTOD record would start immediately when the asperities behind the crack tip start to interfere to produce disregistry. In similar lines with relationship between Kma x and ~ .....…”

“…The OICC can be considered to synergistically arise from RICC. The development of RICC has been found in a variety of high strength planar deforming microstructures such as Ni-base superalloys [7,8], titanium alloys [9,10], precipitation strengthened Al-alloys [11] and to some extent in low strength steels. In low strength microstructures [12], PICC together with RICC operate to lead to crack arrest at threshold.…”

CRACK CLOSURE IN SMALL FATIGUE CRACKS—A COMPARISON OF FINITE ELEMENT PREDICTIONS WITH IN‐SITU SCANNING ELECTRON MICROSCOPE MEASUREMENTS

“…Oxide-induced crack closure is often due to the presence of a moist atmosphere or to elevated temperature testing in air which leads to oxidation of the freshly formed fracture surfaces during the fatigue crack growth process [7]. Roughness-induced fatigue crack closure was first noted by Purushothaman and Tien [S] and Beevers et al [9,10]. It was observed that discrete points of contact between fracture surface asperities can cause fatigue crack closure.…”