對接銲之殘留應力與其對疲勞裂縫進展壽命的影響分析

Residual Stress of Butt-Welded Joint and Corresponding Effects on the Fatigue Crack Propagation

本文應用有限元素模擬與實驗量測潛伏對接銲之殘留應力，並檢討該殘留應力對於貫穿型疲勞裂縫進展壽命的影響。在模擬計算部份，設定移動熱源，透過熱傳計算，解出試片溫度場後，依據熱彈塑性理論的二維分析，以熱結構偶合（thermal-structure couple）元素，模擬板材對接銲之後的殘留應力。在實驗部份，分為殘留應力量測與疲勞裂縫進展兩項；前者以鑽孔法（Hole-Drilling Strain-Gage Method），藉由三軸應變規的輸出變化，求出殘留應力；而後者則分別使用殘留應力試片與經退火沒有殘留應力試片，進行疲勞裂縫進展實驗，比較其進展壽命。另外銲冠（reinforcement）造成應力集中的影響，也一併列入檢討。殘留應力的模擬與實驗結果顯示：模擬計算得到的殘留應力與實驗數據相當符合。疲勞裂縫進展壽命方面，沿著銲趾（weld toe）進展的裂縫，因為受到拉伸殘留應力作用，其壽命變短。銲冠削除後的試片、因為應力集中消失的因素，壽命因而變短。至於對接銲接頭，因為殘留應力與應力集中的效應互相抵銷，所以其進展壽命與母材幾乎相同。 Residual stress of submerged butt-welded joint by finite element simulation and experiments are carried out, and its effects on the fatigue crack propagation life are discussed. A traveling heat source is set as heat input during welding process, and temperature distribution of the specimen is then calculated by heat transfer model, By using thermal-structure couple elements and setting this element as elastic-completely plastic material, the residual stress after the heating process is estimated. The real residual stress distribution is measured by Hole-Drilling Strain-Gage Method. With the use of several specimens having various residual stress and stress concentration, fatigue crack propagation tests are carried out, and how would it affects the crack propagation life are discussed. The results show that the simulated residual stress is consistent to the outcome of measurement. When a crack propagates along the weld toe where residual tensile stress existing, its propagating crack life is shorter then that of base metal. For those reinforcement-removed specimens without stress concentration, it has a longer life then that of base metal. Because the effect of residual stress and stress concentration cancel each other, the butt-welded specimens and base specimens have identical propagating life.