Influence of net section damage on creep crack growth

Abstract

Fracture mechanics concepts for describing creep crack growth in terms of ductility exhaustion in a process zone at the crack tip are reviewed and extended to include damage accumulation in the ligament ahead of a crack. Applications are considered which show that net section damage has most influence for short cracks and plane stress conditions where significant damage can develop in the uncracked ligament. It is shown that, under plane strain loading, insufficient ligament damage occurs during the crack growth phase for it t Q have an appreciable effect on failure times. A method is also presented for accounting for the influence of an incubation period prior to the onset of cracking and for making residual life estimates.

Prediction of the safety margin in the creep crack growth process using damage mechanics and fracture mechanics

“…( 2.3)) can be found experimentally or determined by some analytical formulas (Webster and Ainsworth 1994). There is also an approximate solution which describes the creep crack growth quite well for most of the cases (Nishida 1989):…”

“…Many other models of damage development exist, such as the stressbased Murakami-Liu model (Liu and Murakami 1998), strain-based models (e.g. Nishida et al 1989), the micromechanical model (e.g. Tvergaard 1986).…”

Fracture mechanics in the creep range

“…3. This can be included (19) Consequently, equation (33) can be used to estimate creep crack growth into progressively deteriorating material. It can also be employed for making residual life assessments of plant provided the damage incurred to the present time in service-exposed material we can be established.…”

Estimating the creep-fracture mechanics parameter C [t] from uniaxial creep strain and rupture data

“…and (17) (b) Ainsworth's estimate To determine cA[t] it is required to provide an estimate of the reference stress from equation (22) using limit analysis methods, and the characteristic distance R from equation (21). For the CT specimen the limit load P, is …”

“…iiz, for a von Mises yield criteria and plane stress conditions, is m = 1.467(1.155a: + 1)ll2 -1 -1.155a1 m = 2.131 + 5.316: -distance R, with K : and u:ef substituted into equation(21), isR = {F[~/Wliiz}~W(54)Substitution of equations (53) and (54) into (20), and assuming a time hardening description of creep gives 3 iizP c*[brcf] =-{F[u/W]}~W 1 Ak(&rtL (55) BW k = l The time function z in equation (24) is Substitution of equations (55) and (56) into (24) yields (c) Webster's estimate In the Webster method of estimating C,[t] is given by equation (27) C*[b,] = bOmuo, ((wpa)uJ+1aglra/w9 sl (58) where g1 is given by equation (46) with mk replaced by s. The transition time tl, is determined from equation (26) with J,, given by equation (44), and C*[b,] from equation (58).…”