高速船推進軸系疲勞斷口分析之探討

Fatigue Fracture Surface Analysis for Propulsion Shafting System of High Speed Craft

高速船舶推進軸系為減輕重量，採用高強度合金材料，目前常使用之材料為不銹鋼SUS630。本文就此材料進行孔蝕試驗及疲勞試驗，利用電子掃描顯微鏡（Scanning Electron Microscope，SEM）就其疲勞斷口進行照相，並針對電子掃描照相所獲取的圖像進行分析探討，依據圖像顯示，當無孔蝕發生時，其疲勞斷裂原則上遵循疲勞裂紋初始期、疲勞裂紋成長期及疲勞瞬間斷裂期三個階段；但當有孔蝕發生時，其疲勞斷裂階段即無疲勞裂紋初始期，而直接進入疲勞裂紋成長期及疲勞瞬間斷裂期兩個階段。因此，當以斷口觀察來判斷斷裂原因時，應特別注意，不能以沒有疲勞裂紋而斷定其為非疲勞破壞。另由電子掃描照相所獲取的圖像進行分析推算，得知金屬材料常數n、C、與疲勞裂紋擴展常數（da/dN）並非固定的常數，隨著試驗應力大小而變動，針對不銹鋼SUS630材質，當△K值介於26至46之間時，對於應力幅值不確定時，建議金屬材料常數n值取為3及金屬材料常數C值取為360×10^（-15）。 Usually, high tensile strength alloy is used for propulsion shafting system of a high-speed craft for the purpose of reducing shaft weight and the diameter. In general, the stainless SUS 630 has been selected. In this paper, pitting corrosion test and fatigue test have been carried out for the sampling piece of such SUS630 shaft. Meanwhile, metallurgical photographing over the fatigue fracture surface by a Scanning Electron Microscope (SEM) has performed. By the survey of these graphs, it shows that in the case of samples without pitting corrosion holes the fatigue fracture occurred in three stages, i.e., fatigue initialization, fatigue propagation and rapidly abrupt fracture. While, in the case of samples with pitting corrosion holes, the fatigue initialization stage disappear. Thus, owing to this phenomenon, the identification of fatigue fracture should be carefully used the metallurgical graphic of the fracture surface. In view of stainless steel SUS630 material, the material constant n value takes is 3 and the metal material constant C value takes is 3.60E-15 when the ∆K value is situated between 26 to 46.