13th International Conference on Fracture June 16–21, 2013, Beijing, China 2 2. Materials and experimental methods The material examined in this study was Ti-6Al-4V and the cast cylinders were as shown in Figure 1, including one as-cast cylinder and the other which was post HIPped. The as-cast material have a yield stress averaged at 750MPa，the ultimate tensile stress is about 840 MPa and the elongation is about 4%. After HIPping, the yield stress is increased to 735MPa，the ultimate tensile stress is about 870MPa and the elongation is up to 8%. The Microstructures in thin sections before and after HIPping were examined by optical microscopy after polishing and etching with 1% HF + 3%HNO3+ H2O solution. The fracture toughness tests using compact tension were carried out on a MTS servo-hydraulic testing system in accordance with ASTM standard E399 [4] for damage tolerance testing. Samples with the dimension of 36 mm ×37.5 mm × 15 mm were cut by electrodischarge machining (EDM) and all surfaces of each slice were ground and polished. In this testing method, a fatigue pre-cracked sample was loaded to induce crack extension while continuously measuring force versus displacement. A 3D image of the fracture surface was obtained by VHX-1000. The fracture surfaces of specific samples were examined using SEM. The fatigue crack-growth (FCG) experiments were performed using the compact-tension samples prepared according to ASTM Standard E647-99 [5]. Fatigue crack growth testing was carried out using a RUMUL fatigue machine. Samples were cut from the thin section of the structure for FCG examination. A fatigue pre-crack was first introduced at the root of the centre notch of the sample. The crack length was measured using a crack-opening-displacement (COD) gauge using the compliance method. The fatigue crack growth tests were performed in laboratory air at a stress ratio, R=Kmin:Kmax, of 0.5. Optical microscope was used to observe the relationships between the crack path and the microstructure. Fatigue surfaces of specific samples after failure were examined by scanning electron microscopy (SEM). 3. Results and Discussion Figure 1. Ti64 component for COLTS project. thin section thick section
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