Effect of superimposed monotonic fracture modes on the ΔK and Kmax

Acta Materialia 51(2003)

3399–3414

Effect of superimposed monotonic fracture modes on the ⌬K

and K max parameters of fatigue crack propagation

K.Sadananda a,∗,A.K.Vasudevan b ,I.W.Kang c,

1

a

Materials Science and Technology Division,Code 6323,Naval Research Laboratory,Washington,DC 20375,USA

b

Office of Naval Research,Code 332,Arlington,VA 22217,USA c

Defense Quality Assurance Agency,Seoul,South Korea

Received 23August 2002;received in revised form 28February 2003;accepted 5March 2003

Abstract

Fatigue crack growth is represented using the Unified Approach in terms of two crack-tip driving forces,⌬K and K max .In this approach,crack growth is related solely to fracture mechanics ing this approach,crack-growth trajectory path can be defined in terms of ⌬K ∗–K ∗max ,which are two limiting values (they are the same as the thresholds

when d a /d N =0),for a given crack-growth rate.Pure fatigue is represented by a line ⌬K ∗=K ∗

max

(a 45°line in the ⌬K ∗–K max plot).Normally,fatigue crack growth occurs along with superimposed environmental effects at low crack-growth rates (or low ⌬K values)and/or with monotonic fracture modes at high crack-growth rates (or high ⌬K values).Both of these superimposed effects are functions of the applied K max .They cause shifts in the ⌬K ∗–K ∗max crack-growth

trajectory maps.In the Unified Approach,the ⌬K ∗=K ∗

max line provides a reference that can be used to quantify the superimposed effects of environment and overload fracture.This paper presents a detailed analysis of these effects,taking as an example an idealized superimposition model.The model predictions are compared with the actual materials behavior of several aluminum alloys and their composites.In SiC particulate-reinforced Al composites,the coarse particulates ahead of the crack-tip fracture readily,causing rapid growth of fatigue crack.The fracture process is K max -dependent and hence this superimposed monotonic fracture process get reflected in the crack-growth trajectory map represented in terms of

⌬K ∗–K ∗

max

.The analysis also shows that the relative ratio of monotonic to fatigue modes varies with load ratio,R .At low load ratios,due to low K max value,the crack-growth process is predominantly fatigue-dominated,while at high R -ratios due to high K max value crack growth is fracture-dominated.These regimes are reflected differently in the trajectory map.It is shown that crack-growth trajectory maps provide a powerful tool to investigate the changing fatigue mechanisms as a function of load ratio,crack-growth rate and environment.Without proper understanding and quantification of these mechanisms as function of crack-tip driving forces,any life-prediction methodology can only be empirical at best.Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

Keywords:Fatigue crack growth;Unified approach;Two parameter analysis;Overload fracture;Load ratio effects;Fatigue mech-

anisms

∗

Corresponding author.Tel.:+1-202-767-2117;fax:+1-202-767-2623.

E-mail address:sadananda@ (K.Sadananda).

1359-6454/03/$30.00Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.doi:10.1016/S1359-6454(03)00159-9

1

Visiting scientist at Naval Research Laboratory,Code 6323,Washington,DC 20375,USA.