Study On The Creep Crack Growth Behavior Of PE Pipes

Polyethylene?PE?pipes has been widely applied into gas and water supply,mines mortar conveying,drainage,irrigation and chemical piping system.The lifetime requirements for water and gas pipes is roughly 50 years,an investigation reveals that many PE pipes failed during survice time.Creep crack growth?CCG?is one of the most important factors that contribute to the failures of PE pipes.In practical,the connections of PE pipes often use fused or hot melt welding,which makes the welding joints degradated.During installation procedure,many defects and scratches produced in the PE pipe surface.Welding joints as well as surface defects is the sensitive area for crack initiation and slow crack growth,which affected the PE pipe lifetime during service.Therefore,it is very important to study the crack growth behavior of PE materials as well as its welding area to ensure the safity use of PE pipes.In according with GB/T 8804-2003,tensile tests were carried out on PE100 material and its welded area at different tensile rate to determine the yield strength?_y,elongation at break?_f and elastic modulus E.The results shows that with the increase of the tensile rate,?y increases gradually,?f decreases gradually and E increases gradually.The yield strength,elongation at break and elasticity of the welded specimen is smaller than the parent material specimen,which indicated that the mechanical properties of the welded area are worse than the parent material.According to ISO 18489-2015,the fatigue test of PE100 and its welded area was carried out at different stress ratios R and different initial stress intensity factor K_I,max.The crack growth rate at different R and K_I,max was calculated,and the law of crack growth rate when R=1 was obtained by extrapolation method.For the axial and circumferential cracks at the inner wall of PE pipe,the crack shape was determined based on a special algorithm,which optimized shape of the crack for a constant value of the stress intensity factor along the crack front.The limit crack size under certain load was calculated based on the plastic collapse of PE pipe.Based on the creep crack growth rate of PE pipe material and welded area,the residual lifetime of PE pipe under different initial crack depths was calculated,and the maximum initial crack depth of PE pipe to meet the service life of 50 years was determined.The results shows that the relative depth of the maximum allowable inner wall axial crack was 0.016 for 50 years service life under 1.6 MPa internal pressure,and the corresponding SIF value is 0.20MPa·m^0.5;the relative depth of the maximum allowable inner wall circumferential crack is0.124,the corresponding SIF value is 0.26 MPa·m^0.5;the relative depth of the maximum allowable inner wall circumferential crack of PE100 welded area is 0.105,and the corresponding SIF value is 0.24 MPa·m^0.5.