應力比對TC4鈦合金超高周疲勞失效機理的影響

摘要: 采用頻率為100 Hz的電磁諧振疲勞試驗機進行疲勞拉伸試驗, 研究了兩種應力比(R=0. 1和-1) 對TC4鈦合金的超高周疲勞失效機理的影響.結果表明, 兩種應力比下的S-N曲線都呈現"雙線"型, 但各自表示的意義及失效機理不同.當R=0. 1時, TC4鈦合金的疲勞失效形式有兩種, 即由加工缺陷誘發的表面失效和內部魚眼失效, 這兩種失效形式都伴隨著顆粒平面(Facet) 出現; 而當R=-1時, 僅存在表面失效, 且無Facet的出現.基于斷裂力學的討論可知, 在正應力比及真空環境下, 對應小裂紋擴展的門檻值更低, 更有利于裂紋擴展及Facet的形成. TC4鈦合金的整個內部疲勞失效過程及機理可解釋為: (1) 滑移線或滑移帶在部分α晶粒上的出現; (2) 微裂紋的萌生和接合; (3) 顆粒亮區(GBF) 的形成; (4) 魚眼的形成; (5) 魚眼外的失穩裂紋擴展; (6) 最終的瞬時斷裂.
- TC4鈦合金 /
- 應力比 /
- 失效機理 /
- 魚眼 /
- 應力強度因子
Abstract: As technology has developed along with the increasing mechanical demands placed upon it, the need for fatigue exceeding 107 cycles or even longer for machines and components is necessary, not only for safety and reliability, but also for minimizing the economic and human costs brought about by failure. Titanium alloys have been one of the most widely used and most important materials in the aerospace domain owing to their superior properties of high strength-weight ratio and good temperature resistance. Studies have shown that S-N curves of TC4 alloy exhibit a linearly decreasing tendency and no fatigue limit around 107 cycles under very highcycle fatigue. Thus, fatigue strength design according to the traditional standard is adventurous to some extent. In this study, an electromagnetic resonant fatigue testing machine at a frequency of 100 Hz was employed to carry out fatigue tests and investigate the influence of two stress ratios (R = 0. 1 and-1) on TC4 titanium alloy under a very high-cycle fatigue regime. The results show that S-N curves under each of the two stress ratios present the so called"duplex characteristic"while their respective failure mechanisms are different. The fracture of specimens under R = 0. 1 corresponds to two modes, i. e., surface failure induced by machining defects, and interior fisheye failure, accompanied by the appearance of facets. The horizontal part of the S-N curve at stress ratio of 0. 1 represents the transition stress between the surface failure and the interior failure, beyond which the surface failure can take place, while surface failure without facets only occurs under R =-1. Based on fracture mechanics, the threshold value of small crack growth is lower under a positive stress ratio and in a vacuum, which is more conducive to crack propagation and formation of facets. From these results, the interior fatigue failure process and mechanism of TC4 titanium can be explained as follows: (1) the appearance of slip lines or bands inpartial α grain; (2) initiation and coalescence of micro-crack; (3) formation of granular bright facets (GBF); (4) formation of fisheye; (5) unstable crack propagation outside the fisheye; (6) instantaneous fracture.
- TC4 titanium alloy /
- stress ratio /
- failure mechanism /
- fisheye /
- stress intensity factor

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圖 1 試樣形狀及尺寸(單位: mm)

Figure 1. Shape and dimensions of specimen (unit: mm)

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圖 2 TC4鈦合金的S-N特性

Figure 2. S-N property of TC4 titanium alloy

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圖 3 失效斷口形貌圖. (a) R=0.1時的表面失效; (b) R=0.1時的魚眼失效; (c) GBF內的小平面; (d) R=-1時的表面失效

Figure 3. Morphology of the fracture surface: (a) surface failure under R=0.1; (b) fisheye failure under R=0.1; (c) facets within GBF; (d) sur-face failure under R=-1

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圖 4 小平面上的滑移線. (a) 表面失效; (b) 內部失效

Figure 4. Slip lines on facets: (a) surface failure; (b) interior failure

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圖 5 缺陷或裂紋尺寸與應力幅值的關系. (a) 表面失效; (b) 內部失效

Figure 5. Relationships between defect or crack size and stress amplitude: (a) surface failure; (b) interior failure

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圖 6 ΔK與N_f的關系. (a) 小裂紋擴展; (b) 長裂紋擴展

Figure 6. Relationships between ΔK and N_f: (a) small crack growth; (b) long crack growth

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圖 7 超高周疲勞的內部失效機制. (a) TC4鈦合金的微觀組織; (b) α晶粒內的滑移線或滑移帶; (c) 微裂紋的萌生與接合; (d) GBF的形成; (e) 裂紋穩定擴展及魚眼形成; (f) 裂紋失穩擴展直至最終斷裂

Figure 7. Interior failure mechanism in a very high-cycle fatigue regime: (a) microstructure of TC4 titanium; (b) slip lines or bands inαgrains; (c) mirco-crack initiation and coalescence; (d) formation of GBF; (e) stable crack growth and formation of fisheye; (f) unstable crack growth until final fracture

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表 1 TC4鈦合金的化學組成(質量分數)

Table 1. Chemical composition of TC4?%

Fe	C	N	H	O	Al	V	Ti
0.3	0.1	0.05	0.015	0.2	6.1	4.0	余量

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