TABLE 1 CHEMICAL COMPOSITION Ti Al B Fe Si C N H O Plate 95.57 4.0 0.005 0.23 0.05 0.025 0.024 0.005 0.084 Rolled ring 95.64 3.975 0.005 0.215 <0.05 0.016 0.023 0.073 TABLE 2 MECHANICAL PROPERTIES bs (MPa) 0.2 s (MPa) %d E(MPa) v KU a (J/cm2) Plate 682 605 18.5 1.106´105 0.34 74.9 Rolled ring 712 615 16.3 1.310´105 71.4 Experimental Procedure The J-integral tests were carried out on a Chinese WE-30 material testing machine of 300 KN capacity and in accordance with GB2038-91 standard2. Specimens have been precracked in fatigue. Load P versus load-point displesment D is recorded autographically. After off-loading, specimen crack fronts were marked by re-fatigue and then specimens were broken at low temperture. Crack length a and crack extension aD measurements were made according to the nine-point-average procedure. A 50-fold enlargement optical micrometer was used to measure a and aD . All tests were mdae at room temerature, approximately 22-26o C. The J-integral values were calculated according to the following formula : ) ( 2 ( )] [ 1 2 2 1 2 BW a U W a Y BW P E v J P s R - + - = RESULTS AND DISCUSSIONS The exprimental results of J-integral values of seven groups of specimens are shown in Figure 4-10. From Figure 4 and 5, we can see that the J values of L-T direction specimens are much higher than those of L-S ones. It can also be perceived from Figure 8, 9 and 10 that the J values of rolled ring are as follow : L R R L C R J J J - - - > > This means that in Ti-alloy material, crack orientation has an important effect on fracture toughness. It is generally because of inclusions or intermetallic constituents which assist the crack by nucleating voids. Proper design and selection of crack orientation is therefore important for engineering strcture. Figure 4, 6 and 7 shown that the J values of the welded joint and HAZ are obviously smaller than those of the base metal. it signifies that the welding process can result in a reduced toughness of Ti-alloy. Therefore, the inhomogeneity of the welded joint will bring about a problem in using Fracture Mechanics to assess welded structures. For example, cracks located in the welded joint or HAZ may grow to a critical size and propagate rapidly but then arrest when they enter the parent plate. Likewise, a slow-growing crack in the base metal could suddenly accelerate when entering the
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