Characterization Of The Creep Crack-tip Constraint Effects Based On The C*-Q* Two-Parameter Theory

Crack-tip constraint effects have a great influence on the creep crack growth rate and remaining life of structures with defects.In this paper,a new type of heat-resistant steel,P92 steel was used as the research object,and the influence factors of creep crack-tip constraint effects were studied systematically using the constraint parameter Q by numerical method.At the same time,a load-independent creep constraint parameter Q*was proposed,and it was used to characterize the constraint effects on the surface crack front of high temperature pipes.The main work in this paper can be summarized as follows:?1?Crack depths,specimen geometries and loading configurations have a great influence on the creep crack-tip constraint effects.Under the condition of keeping the load level?C*?and specimen thickness and width remain constant,the values of the creep crack-tip constraint increase with increasing the crack depths a/W and specimen thicknesses and decrease with increasing specimen widths.Furthermore,the CT specimens have higher crack-tip constraint than the CCT specimens while the constraint of the SENB,SENT and DENT specimens is in the middle between them.The order of specimens in terms of crack-tip constraint from high to low is CT>CST>SENB>SENT>DENT>CCT.?2?The creep constraint parameter Q is dependent on the load C*,and it decreases with increasing the C*.Analogous to the definition of the load-independent constraint parameter under elastic-plastic condition,a new load-independent creep constraint parameter Q*was introduced,and its load-independence was confirmed by finite element analyses results.At the same time,the effects of the distance r from crack tip and the creep time t/t_red on the parameter Q*were also analyzed,and a fixed distance r=0.2 mm was used to define the Q*.?3?Based on the load-independent creep constraint parameter Q*,the pressure pipelines with axial surface cracks served at elevated temperature were studied.The creep crack-tip constraint effects were characterized by 3-D finite element method for pipelines with internal and external surface cracks with different geometrical sizes.And the empirical equations that represent the relationships between the commonly used typical values of constraint and crack sizes were obtained by fitting.The results reveal that the maximum constraint level occurs near the deepest crack front part for long cracks?smaller a/c?,while it occurs near the free surface for shorter cracks?larger a/c?.The constraint values increase with the increase of the crack depth for a specific ratio a/c.The position with higher creep crack-tip constraint level is always accompanied by higher C*value ahead of crack front which is generally regarded as the parameter of the crack driving force under creep conditions.