服役后API 5L X56钢腐蚀疲劳裂纹扩展速率试验研究

Experimental study on the corrosion fatigue crack growth rate of X56 steel after service

为研究服役后X56双层海底管道钢的疲劳裂纹扩展速率,对外层管上截取的标准紧凑拉伸试样分别在空气和海水环境下进行不同最大疲劳载荷(P max=9 kN,10 kN,12 kN)的疲劳裂纹扩展试验。与空气环境中CT试样的疲劳裂纹扩展速率相比,裂纹生长至15.38 mm时,9、10、12 kN载荷下海水环境中CT试样的疲劳裂纹扩展速率分别提高了1.82倍、1.54倍、1.43倍;随着裂纹扩展长度增大,外界载荷起主导作用,海水腐蚀的影响越来越小。综合分析最大疲劳载荷对疲劳裂纹扩展的影响,结果发现相同裂纹长度的前提下,疲劳载荷增加,应力强度因子幅值(ΔK)和疲劳裂纹扩展速率(d a/d N)增加,海水腐蚀影响逐渐减小。疲劳裂纹扩展试验中所施加的最大疲劳载荷对CT试样的疲劳寿命具有较大影响,对Paris常数的影响较小。扫描电镜下CT试样的疲劳断口均为穿晶型断裂。海水环境中CT试样的疲劳断口表现出更多的二次裂纹和更高的撕裂脊。随着最大疲劳载荷增加,解理断裂形成的解理台阶晶面面积和高度差逐渐增大,疲劳断口越粗糙,解理特征越明显。

To study the fatigue crack growth rate(FCGR)of X56 pipe-in-pipe submarine pipeline steel after service,the fatigue crack growth tests under different maximum fatigue loads(P max=9 kN,10 kN,12 kN)were carried out on standard compact tensile(CT)specimens taken from the outer pipe of pipe-in-pipe in air and seawater,respectively.Compared with the fatigue crack growth rates of the CT specimens in air,when the crack grows to 15.38 mm,the fatigue crack growth rates of the CT specimens in seawater under the load of 9 kN,10 kN and 12 kN are increased by 1.82 times,1.54 times and 1.43 times,respectively.As the crack growth length increases,fatigue load plays a leading role,and the influence of seawater decreases gradually.A comprehensive analysis of the influence of the maximum fatigue load on fatigue crack growth is conducted.It is found that under the premise of the same crack length,the fatigue load increases,the stress intensity factor amplitude(ΔK)and the fatigue crack growth rate(d a/d N)increase,and the influence of seawater corrosion gradually decreases.The maximum fatigue load applied in the fatigue crack growth test has a more significant impact on the fatigue life of the CT specimens,but has a negligible effect on the Paris constant.The fatigue fracture faces of different CT specimens observed by scanning electron microscope are all transcrystalline fractures.The fatigue fracture faces of CT specimens in seawater show more secondary cracks and higher tear ridges.With the increase of the maximum load,the crystal face area and the height difference of the cleavage steps formed by cleavage fracture gradually increase,and the rougher the fatigue fracture,the more pronounced the cleavage characteristics.