467

CRACK BLUNTING AND ARREST IN CORROSION FATIGUE OF MILD STEEL

J. C. Radon, C. M. Branco, and L. E. Culver Department of Mechanical Engineering, Imperial College Exhibition Road, London SW7 2BX, UK tel: 01/589 5111 Some experimental work investigating corrosion fatigue crack propagation in mild steel has previously been reported [1,2]. Oxidation at the crack tip leading to a slowing down or eventually stopping of the crack growth may occur under specific conditions [i]. Since the mechanics of crack propagation in corrosion fatigue of mild steel is not fully understood, an extensive experimental programme was initiated by the present authors. This work was a continuation of fatigue tests performed on the same material in air using a constant stress intensity specimen (contoured DCB test piece) [3], and it involved conducting tensile fatigue tests in a 3.5% NaCI solution~ Two different testing frequencies were investigated, namely 35 and 0.25 Hz. At 35 Hz no significant difference was obtained in the crack growth rate when compared with tests in air under the same range of stress intensity factor AK. (AK = Kma x - Kmi n where K is stress intensity factor.) However when testing at the lower frequency the phenomenon of crack blunting was observed for certain va ues of AK. Crack blunting and subsequent arrest are known to occur in stress corrosion cracking [4], but in neither stress corrosion nor in corrosion fatigue has the influence of external variables been fully investigated. A detailed analysis is now under way. The effect of crack blunting and consequent arrest on the crack growth pattern is shown in Fig. I. All the tests started with a sharp crack obtained by fatigue pre-cracking in air at low AK levels. During the test the crack growth rate was constant and increasing with AK. At a later stage blunting occurred and finally the crack arrested. Fig. 1 also shows that the time for crack arrest (or the total number of cycles) increases with increasing values of AK or with the crack growth rate for the initially sharp crack. Tests performed to date at three different mean values of the stress intensity Km showed a similar trend, Fig. 2. It will be seen that each curve has a critical value of AK at which the crack does not arrest. Furthermore the time for crack arrest increases with Km. The crack arrest phenomenon seems to be dependent on da/dN, (Fig. 3). Crack blunting and subsequent arrest do not occur above a certain value (da/dN) b. The above process may be explained by the combination of the dissolution and the mechanical effect of fatigue. The dissolution process contributes mainly by increasing the radius of curvature at the crack tip, therefore blunting the crack and finally leading to its arrest. Under these conditions it will be expected that the crack growth rate decreases when blunting is occurring, eventually becoming zero. The above explanation gives a qualitative justification of the shape of the curves presented in Fig. I. With increasing crack growth rate a longer time will be necessary for the dissolution process to increase the radius of curvature at the crack tip since a larger volume of material

Int Journ of Fracture 12 (1976)

468

w i l l h a v e t o be r e m o v e d . Fig. 3 supports this conclusion. The d i s s o l u t i o n p r o c e s s c o n t r i b u t e s a l s o t o an i n c r e a s e i n c r a c k g r o w t h r a t e as l o n g a s t h e g e o m e t r y o f t h e c r a c k t i p i s n o t s i g n i f i c a n t l y altered. T h i s h a p p e n s a t da/dN > ( d a / d N ) b .

The stress conditions at the crack tip leading to the slowing down and arrest of blunt cracks are discussed in [4]. The tangential stress a t at a crack tip having a radius of curvature p may be qualitatively expressed by the equation derived in [5] s t

~

AKIIp

which shows t h a t as p i n c r e a s e s a t d e c r e a s e s . I t i s a l s o assumed t h a t t h e d e c r e a s i n g v a l u e o f g t r e d u c e s t h e l o c a l v a l u e o f AK. Then t h e e f f e c t i v e v a l u e o f AK, o r AKe? , may c h a r a c t e r i z e t h e s t r e s s d i s t r i b u tion at the crack tip. AKef - d e c r e a s e s when p i s i n c r e a s i n g and a t a r r e s t AKef m i g h t be e q u a l t o AKth , t h e t h r e s h o l d v a l u e o f AK i n t h e same e n v i r o n m e n t and f o r a s h a r p c r a c k . F u r t h e r a n a l y t i c a l work i s n e c e s s a r y t o e s t a b l i s h t h e a p p r o p r i a t e stress distribution a t t h e c r a c k t i p f o r d i f f e r e n t g e o m e t r i e s and 0 values. The d a t a p r e s e n t e d i n F i g s . 2 and 3 may be o f i n t e r e s t f o r d e s i g n p u r p o s e s as a t some s u b c r i t i c a l AK v a l u e s a c o r r o s i v e e n v i r o n m e n t may be b e n e f i c i a l and u l t i m a t e l y l e a d t o t h e a r r e s t o f a c r a c k . In such a s i t u a t i o n an i n i t i a l c o r r o s i v e c r a c k p r o p a g a t i o n may be t o l e r a ted.

REFERENCES

[1]

K. J e r r a m and E. K. P r i d d l e , The D e v e l o p m e n t and E v a l u a t i o n o f a F a t i g u e Crack L e n g t h M o n i t o r i n g S y s t e m f o r L i q u i d E n v i r o n m e n t s , CEGB R e p o r t RD/B/N2703 ( 1 9 7 3 ) .

[2I

L. P. Pook and A. F. Greenan, Fatigue Crack Growth Threshold for Mild Steel, a Low Alloy Steel and a Grey Cast Iron, NEL Report, No. 571 (1974).

[3]

C. M. B r a n c o , J .

C. Radon, and L. E. C u l v e r , Journal of Testing

and Evaluation 3 (1975) 407-412. [4]

M. Creager and P. C. Paris, International Journal of Fracture

Mechanics 3 (1967) 247-252. [5]

K. Heckel and R. Wagner, International Journal of Fracture II (1975) 135-140.

28 January 1976

Int Journ of Fracture 12 {1976)

469

AK (Nm~/2 ) 550

-r

500

__I

i

(.D

Y

5

400

c:,

300 250

I

I

50

100

CYCLES (xl03)

Figure i.

Crack growth vs number of cycles for a range of AK.

J I

175

•

I I

I

175

-

R = 0

I

-

I

,J

Km(Nmm 2 ..c 1500

~150

I000 ]50

500/

DA

IR=0

DA

<

rr" U n-' O IJ-

o

R:0

a

500

<

K m ( N mm 3/2 )

Y C)

125

125 -(w

~000

1500

r,-

D i0

,,o

IJA

U.t 100

10O

-

o

[D /

75,, loo

?5

I

I

I

200

400

600 Z~K ( N m ~ 3/2)

Figure 2. Time for crack arrest vs AK

>"

AO 0

(do/dN)b.

j

]

I 800

,',IO0

Olo-e

i0-s

do/dN

I

xj

10 .4

10 -3

(ram/cycle)

Figure 3. Time for crack arrest vs da/dN for an initially sharp crack

Int Journ of Fracture

12

(1976)