應力集中對表面型疲勞裂縫進展行為之影響

The Stress Concentration Effect on the Propagating Behavior of Surface Cracks

銲道表面銲趾部之應力集中處出現表面型疲勞裂縫的初期，該微小裂縫的進展速度受到應力集中的影響；但是裂縫成長至某深度後，裂縫前緣離開應力集中處已有一段距離，因此不再受表面應力集中的影響；然而裂縫兩端一直處於應力集中處，因此應力集中的效應始終影響裂縫兩端的進展速度。為了釐清表面應力集中對表面型疲勞裂縫進展速度的影響，本研究利用沒有應力集中的平板試片與兩種表面應力集中程度不同的凹槽試片進行裂縫進展實驗，並根據實驗結果對應力集中的影響程度，進行量化的評估。實驗結果顯示，出現在平板試片之表面裂縫呈現半圓形的形狀，而在凹槽試片上的則呈半橢圓形狀；而且應力集中係數（Kt）愈大，扁平橢圓的長短軸差異也愈大，將不具應力集中效應的應力強度因子範圍乘上[（Kt-1）×05]+1後，利用Paris' law可以估算表面型疲勞裂縫的進展速度；而且這個結果套用於對接銲的實驗數據，得到良好的結果。 When a surface crack occurs from the root of welded joint where high stress concentration exists, the propagation rate of the crack is affected by the stress concentration. As the crack grows and becomes to have a larger size, the crack propagation rate around the crack tip that far away from the stress concentration region would not be affected by the stress concentration, however, at the surface of the crack, it is always affected by the stress concentration. In order to figure out how the stress concentration would affects the propagation rate, flat specimens and two kinds of round-grooved specimens that have distinct size of round grooves on the surface are used to carry out fatigue propagating tests The test results show that in comparison with the flat specimens, the surface cracks occurring from the round- grooved specimens have a more flat shape and shorter propagation life. Referring to the tested results, a correction factor of [(K(subscript t))×0.5]+1 to calculate the stress intensity factor range is proposed. By introducing this factor into Paris' law, the growth rate of surface crack can be estimated. Moreover, this method is verified its capability of calculating the growth rate of butt-welded joints as well.