Room Temperature Creep And Its Effect On Fatigue Crack Growth Of Structural Steels

With the development of measuring technique and the improvement of required accuracy, it is found that the deformation under constant load increases with time even at low homologous temperature(T/T_m＜0.2) in many structural steels(such as stainless steels, pipeline steels,high strength steels and so on),which is called room temperature creep(RTC). Moreover,such a time dependent deformation has brought about some technical problems and received more attention.Particularly in the case of stress concentration,significant RTC can occur due to stress concentration even under low nominal stress.The occurrence of RTC may influence the original microstructure of materials and hence subsequent properties.Many structural components in service usually experience constant and variable load by turns and may fail due to the crack initiation and growth caused by the combined influence of stress and environment.If the time dependent deformation takes place at crack tips under constant load, it may influence the stress-strain behavior in this region and play an important role on the subsequent fatigue crack growth under variable load.The investigation of effect of RTC on fatigue crack growth is of advantage to developing reliable life prediction models which are capable of handling complex service conditions.This paper investigates RTC,its influencing factors and the effect of RTC and single wave overload(SWOL) on fatigue crack growth with and without stress concentration on a servo-hydraulic fatigue machine using smooth,notched and compact tension(CT) specimens of X70 pipeline steel(BCC) and SUS304 stainless steel (FCC).And based on observation by optical microscopy(OM),scanning electron microscopy (SEM),X-ray diffraction(XRD) and transmitted electron microscopy(TEM),the fundamental mechanisms are discussed.1.At various stress levels,X70 pipeline steel and SUS304 stainless steel with and without stress concentration show RTC behavior.For the smooth and notched specimens of X70 pipeline steel,RTC is strongly dependent on the stress level.RTC is primary at stress levels far lower than the ultimate strength but becomes tertiary as the ultimate strength is approached,during which the fracture following necking occurs.However,the smooth specimens of SUS304 stainless steel only shows the primary RTC at various stress levels,and RTC alone may not cause facture.Similar to X70 pipeline steel,RTC strongly depends on the stress level in the notched specimens of SUS304 stainless steel,and with increasing stress level,it gradually transits from the primary to the tertiary type.RTC at crack tip is significantly influenced by stress intensity factor and the primary creep is the dominant deformation mode at various stress intensity factor levels. 2.At the same percentage of the ultimate strength,RTC in both X70 pipeline steel and SUS304 stainless steel becomes significant with increasing loading stress rate,RTC strain in the normalized X70 pipeline steel is larger than that in the as-received X70 pipeline steel and SUS304 stainless steel show more RTC deformation relative to X70 pipeline steel.Moreover, RTC is also dependent on loading history.Since the deformation in the notched specimen is confined in a localized region,RTC strain markedly decreases compared with the case of the smooth specimen.RTC strain slightly increases with increasing the angle of notch.3.The primary RTC of X70 pipeline steel and SUS304 stainless steel agrees well with logarithmic creep lawε=αlog(βt +1),which includes two regression parametersαandβ.As to the smooth specimens and the notched specimens of X70 pipeline steel and SUS 304 stainless steel,the regression parameterαmonotonously develops with the inverse strain hardening coefficient.Based on the continuity of RTC deformation and the analysis of stress-strain behavior in the tensile test,a model is developed to quantitatively describe RTC strain.Since the evolution ofαis similar to that of d(V_g/α)/dK,this model can also be applied to the case of the CT specimen.In addition,the regression parameterαis strongly dependent on the stress level and is hardly influenced by the loading stress rate,butβis strongly dependent on the loading stress rate and is hardly influenced by the stress level.4.The post-yield RTC markedly increases the flow stress of X70 pipeline steel and SUS304 stainless steel while the pre-yield RTC reduces the elastic limit to some extent. Based on the theory of mobile dislocation density and the theory of stress dependent thermal activation,a theory of localized mobile dislocation density is presented to interpret the aforementioned behaviors,the pre-yield RTC and the transient burst of strain rate-stress curves caused by RTC.In addition,due to strain induced martensite transformation in SUS304 stainless steel,RTC at various stress levels is of primary nature and occurs at continuously falling rate.5.Similar to the SOWL effect,RTC at crack tips can retard the subsequent fatigue crack growth in X70 pipeline steel and SUS304 stainless steel except that the retardation following RTC is more significant at the same overload ratio.Some empirical equations are presented to describe different fatigue crack growth behaviors following RTC and SWOL.Compared with the SOWL effect,the retardation mechanism due to RTC is discussed on the basis of measurement of crack closure and observation of fatigue crack and fracture surface.The serious retardation of fatigue crack growth following RTC is attributed to the plasticity-induced crack closure owing to the time dependent deformation at crack tips.