Evaluation method of initial fatigue quality of aircraft wing flange fastener holes
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摘要: 為了對飛機機翼緣條緊固孔細節原始疲勞質量進行評估，本文首先對飛機機翼緣條結構中常用的BXXX鋁合金緊固孔試件分別開展了高、中、低3種應力水平下的疲勞試驗，通過斷口判讀和反推得到3組關于裂紋長度a和疲勞壽命t的（a?t）數據，在此基礎上應用當量初始缺陷尺寸（EIFS）控制方程對每個試件的EIFS值進行計算并初步評估，驗證了在不同應力水平下緊固孔結構細節的EIFS無顯著性差異；得到了緊固孔結構細節的裂紋萌生時間（TTCI）分布，在指定應力水平下對緊固孔結構細節95%置信水平下的經濟壽命進行預測，并與設計壽命進行對比，提出了一種不同超越概率P下的結構細節當量初始缺陷尺寸模型，基于給定5%的裂紋超越概率，對結構細節的通用EIFS分布進行評估。通過以上對飛機機翼緣條緊固孔細節原始疲勞質量的三重評估，得到綜合評估結果：飛機機翼緣條緊固孔細節原始疲勞質量滿足要求。Abstract: On analyzing the details of kinetic links of parts and structures of aircrafts, one can find few bad links. But fastener hole is the weakest link where abnormal stress is produced and initiation of crack occurs. The initial fatigue quality of aircraft wing flange fastener is the key parameter, which affects the durability of aircraft structure. The initial fatigue quality of structural details is usually characterized by the equivalent initial defect size (EIFS) and the time to crack initiation (TTCI). To evaluate the initial fatigue quality of aircraft wing flange fastener hole details, this paper first carried out fatigue tests at high-, medium- and low-stress levels on the BXXX aluminum alloy fastener hole specimens generally used in aircraft wing flange structures, and obtained three groups of (a?t) datasets about crack length a and fatigue life t through fracture interpretation and back stepping. On this basis, the EIFS governing equation was used to evaluate the EIFS value of each specimen, and it is found out that there is no significant difference in equivalent initial flaw size under different stress levels; TTCI distribution of structural details is obtained, and the economic life of specified stress level under 95% confidence level of fastener hole structural details was predicted, and compared with the design life; a structural detail equivalent to initial flaw size model under different exceedance probability P was proposed. Based on the given 5% crack exceedance probability, the general EIFS distribution of structural details was evaluated. The comprehensive evaluation results were obtained through the above triple evaluation of the initial fatigue quality of the fastener hole details: the general EIFS distribution and the EIFS value of each test piece are less than the allowable value, and the economic life is greater than the allowable value, so the original fatigue quality of the details of the fastening holes of the aircraft flange meets the stringent requirements.
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圖 6 裂紋擴展（a?t）曲線。（a）A組/低應力水平；（b）B組/中應力水平；（c）C組/高應力水平
Figure 6. Crack growth (a?t) curves: (a) group A—low-stress level; (b) group B—medium-stress level; (c) group C—high-stress level
圖 7 不同參考裂紋尺寸
$ {a}_{\mathrm{r}} $ 下的Ti求解Figure 7. Ti solution under different reference crack sizes
$ {a}_{\mathrm{r}} $ 
圖 8 每個試件的EIFS值求解。（a）Qk求解；（b）EIFS值求解
Figure 8. EIFS value solution of each specimen: (a) Qk solution; (b) EIFS value solution
圖 9 不同超越概率下的結構細節當量初始缺陷尺寸曲線
Figure 9. Equivalent initial defect size curve of structural details under different exceedance probabilities
表 1 方差分析表
Table 1. Analysis of variance
Source Sum of deviation squares Degree of freedom Mean square deviation F value Between groups 0.0000350 2 0.0000175 0.341 Within Groups 0.0008733 17 0.00005137 Total 0.0000908 19 — Note: The degree of freedom refers to the number of variables whose values are not restricted when calculating a certain statistic. Usually the degree of freedom takes df=n-k, where n is the number of samples and k is the number of restricted conditions or variables. 
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表 2 TTCI分布參數
Table 2. TTCI distribution parameters
Group number $ \beta $ $ \varepsilon $ $ \alpha $ A 17956 453 6.529 B 10558 267 C 6832 172
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表 3 經濟壽命
$ {T}_{\mathrm{e}} $ Table 3. Economic life
$ {T}_{\mathrm{e}} $ Group number Economic life, Te/h A 11829 B 6956 C 4501
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表 4 通用EIFS分布參數
Table 4. General EIFS distribution parameters
Group number Qi/10?4 Qβ $ \alpha $ A 3.04 5.446 6.529 B 5.17 C 7.99
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表 5 綜合評估結果
Table 5. Comprehensive assessment results
Evaluation EIFS value of each specimen/mm General EIFS distribution/mm TTCI/h Calculated value 0.00545–0.02599 0.02194 4501 Allowable value 0.125 0.125 4000 Evaluation conclusion IFQ meets requirements IFQ meets requirements IFQ meets requirements
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