Stress intensity factors for tunnelling corner cracks under mode I loading

The influence of stress state on fatigue crack growth in nickel‐base superalloys at high temperature is considered, based on studies in corner crack specimens of Inconel 718 at 600 °C. At high frequency and low R, cycle‐dependent trans‐granular crack growth occurs along the whole crack front, and growth rates are similar at the surface and within the interior of specimens, maintaining the original quarter‐circular shape. For conditions of low frequency and high R, increased crack growth rate per cycle is observed with the crack tunnelling ahead at the centre. A time‐dependent intergranular crack propagation mode occurs in the plane strain interior, attributed to an oxidation mechanism, whereas near the surfaces under plane stress, a trans‐granular cyclic plasticity mechanism is observed. It is proposed that in addition to frequency and R, that stress state influences the competition between the mechanisms controlling crack growth and the transition between them: plane strain in the interior favouring an oxidation‐controlled intergranular cracking mechanism as compared with the plane stress surfaces where cyclic plasticity dominates. An FEM study suggests that this influence of stress state is not associated with variation of ΔK along the crack front.

Section: Discussionmentioning

confidence: 99%

Influence of stress state on high temperature fatigue crack growth in Inconel 718

Antunes

Ferreira

Branco

et al. 2001

“…[5] are only exact at the crack tip. Difficulties associated with the use of quarter point elements in the vicinity of a corner point 16 are avoided.…”

Section: Finite Element Modellingmentioning

confidence: 99%

A finite element analysis of cracked square plates and bars under antiplane loading

Pook

2003

Finite element analyses were carried out on cracked 20 mm square plates and bars ranging in thickness from 2.5 mm to a length of 60 mm. The crack extended from the middle of one side of the square to its centre, and was modelled as a narrow, parallel‐sided notch with a semicircular tip. An antiplane loading was applied to the side containing the crack. An infinitely long bar under the antiplane loading used is in pure Mode III. It was found that the central portions of 40, 56 mm and 60 mm long bars were in pure Mode III, and also that KIII was approximately constant. These central portions were therefore representative of an infinitely long bar. Towards the ends of a bar KIII decreased. At the ends of a bar corner point effects meant that Mode II stress intensity factors and displacements were induced in the corner region. The size of the corner region was independent of bar length. In the 2.5, 5 and 10 mm thick plates out of plane bending means that the antiplane loading became a mixed Mode II and Mode III loading. At a centre line KII is zero by symmetry. Behaviour in the corner region was a function of plate thickness. For both plates and bars, as has been predicted theoretically, the ratio KII/KIII tends to a constant value as a surface is approached. For a thickness of 20 mm, that is a 20‐mm cube, behaviour represents a transition between plate and bar behaviour.

“…On a notch surface the equations given in Creager and Paris 5 are only exact at the notch tip. Difficulties associated with the use of quarter point elements in the vicinity of a corner point 25 are avoided.…”

Section: Finite Element Modellingmentioning

confidence: 99%

Finite element analysis of corner point displacements and stress intensity factors for narrow notches in square sheets and plates

Pook

2000

It is possible to model a crack as a narrow parallel‐sided notch with a semicircular tip. Surface displacements of narrow notches in the vicinity of a corner point where the notch tip intersects a free surface were investigated, using finite element analysis, for each of the three modes of crack tip surface displacement. Some reasonably accurate relevant stress intensity factors were determined by analysis of notch tip stresses. The extent of corner regions in which stress intensity measures, rather than stress intensity factors, dominate crack tip stresses was determined by analysis of the notch surface displacements and of relevant stress intensity factors. Under nominal Mode III loading corner point effects are local, but under nominal Mode I and nominal Mode II loadings they are a combination of local and global effects. Volterra distorsioni are useful in the description of crack, and narrow notch, surface displacements under load.