| 摘要: | 本研究針對選擇性雷射熔融製造(SLM)的 Inconel 718工件，探討應力比、超載負荷及裂縫閉合等因素對於疲勞裂縫成長之影響、評估不同疲勞裂縫成長預測模式之適用性及準確性，並且分析破壞機構與疲勞裂縫成長之因果關係。
結果顯示，本研究的SLM Inconel 718工件的破壞韌性可達179 MPa√("m" )，比此合金的鍛造件還高。固定負荷振幅疲勞裂縫成長速率在高應力比的環境下裂縫成長速率較快，且起始裂縫成長應力強度因子範圍(ΔK)較低。超載負荷比大於2.0時，疲勞裂縫成長速率才開始有明顯的延遲現象。當負荷歷程的最大應力強度因子小於30 MPa√m時，利用Wheeler模式進行疲勞裂縫成長修正時，材料參數p 值可採用 1.01。大於30 MPa√("m" )後，p值隋著應力強度因子的提高而增大。在裂縫閉合模式中，裂縫閉合因子會隨著應力比或ΔK上升而提高，關係式為U = 0.014ΔK + 0.579R + 0.377。承受變動負荷振幅歷程時，裂縫成長壽命以裂縫閉合模式(包括Elber、Schijve及本研究)的預測結果較佳，平均誤差約25%。當ΔK低於33 MPa MPa√("m" )時，利用本研究所提出的裂縫閉合因子公式所預測的疲勞裂縫成長曲線最佳。在負荷次序效應的實驗中，若負荷歷程的應力比從大到小排列，由於先出現的高應力會降低後續小應力的疲勞損傷，其疲勞壽命提高。若應力比從小到大排列，則疲勞壽命降低。在疲勞破斷面上，可看出固定振幅負荷、變動振幅負荷及超載負荷所產生疲勞紋間距有明顯差異。SLM工件內的冶金缺陷對於裂縫成長速率及破斷面的影響，亦可明顯觀察到。
;This study focuses on the fatigue crack growth properties of Inconel 718 workpieces fabricated by Selective Laser Melting (SLM). It investigates the effects of stress ratio (R), overload load, and crack closure on fatigue crack growth. Additionally, the study evaluates the applicability and accuracy of different fatigue crack growth prediction models. Furthermore, it analyzes the causal relationship between failure mechanisms and fatigue crack growth.
The results demonstrate that the fracture toughness of SLM Inconel 718 workpieces in this study can achieve 179 MPa√("m" ), which is higher than that of the forged Inconel 718 alloy. The fatigue crack growth rate under constant amplitude load increases with stress ratio. The fatigue crack growth rate begins to show a noticeable retardation phenomenon when the overload ratio is greater than 2.0. When the maximum stress intensity factor of the load history is less than 30 MPa√("m" ), the average material coefficient p in Wheeler model equals to 1.01. While maximum stress intensity factor higher than 30 MPa√("m" ), the p value increases with increasing stress intensity factor (ΔK). In the crack closure model, the crack closure factor increases with the rise of stress ratio or ΔK, and the relationship is expressed as U = 0.014ΔK + 0.579R + 0.377. Under variable amplitude loading, the predicted fatigue crack growth life is better by crack closure models (including Elber, Schijve, and the proposed model in this study), with an average error of about 25%. When ΔK is below 33 MPa√("m" ), the fatigue crack growth curve predicted using proposed model shows the best fit with experimental data. With respect to the load sequence effect, the load history with arranging stress ratio in a decreasing order has smaller fatigue damage. The reason for this is that the first high stress will induce compressive residual stress and reduce the fatigue damage of the subsequent low stress. Conversely, arranging the stress ratio in an increasing order leads to a decrease in fatigue life. On the fatigue fracture surfaces, it can be seen that there are obvious differences in the spacing of fatigue striation caused by constant amplitude load, variable amplitude load and overload load. The influence of metallurgical defects within SLM workpieces on crack growth rate and fracture surfaces is also visibly evident. The effect of the metallurgical defects in the SLM workpiece on the crack growth rate and fracture surface can also be clearly observed. |
