Fatigue Crack Paths 2003

Figure 2a presents a structure, whose cracks generally are exposed to a plane Mixed

Mode loading. The local loading conditions at the crack tip are characterised by the

stress intensity factors KI and KII, which show in case of quasi-static loading the same

time-dependency the applied stress σ(t).

b)

K

K

K

II

I

Imax

K

IImax

σ(t)σ(t)

a)

Δ K I

Δ K II

K

= R K *

K

= R K *

Imin

Imax

IImin

IImax

t

t

c)

K

II

K

IIC

Fracture curve

Range of fatigue crack growth

Threshold curve

K

K

I

IC

Figure 2. a) Mixed Modecracks in a structure

b) Stress intensity factors KI(t) and KII(t) at a mixed modecrack

c) KI/KII-diagram with the range of fatigue crack growth.

The fatigue crack growth is governed the cyclic stress intensity factors ΔKI und ΔKII

(Figure 2b) respectively the cyclic comparative stress intensity factor ΔKV, which is

determined by [1, 2]

Δ + Δ +

= Δ Δ

(1).

K

2 I I 2 I K 6 K 2 1 2 K

V

Fatigue crack growth is possible, if ΔKVexceeds the threshold value ΔKth for Mode

I. Stable and controlled crack growth occurs until ΔKVreaches the fracture toughness

value ΔKC= (1-R)KIC [2]. Figure 2c shows the range of fatigue crack growth for a plane

Mixed Modesituation. Crack paths, which arise under these complex loadings will be

described in the following sections.

E X P E R I M E N T AILNVESTIGATIONSO F C R A C KG R O W T UHN D E R

C O M P L EL XO A D I N G

For the pupose of experimental investigations of plane Mixed Modeproblems several

specimen types have already been proposed in the past. A summary is given in [3, 4].

Besides other ones in particular the CTSspecimen [5, 6] with the appropriate loading

device (Figure 3) has been established for Mixed Mode experiments. The loading