IFMASS10

DUCTILE FAILURE CRITERIA UNDER MULTIAXIAL LOADING

Mićunović Milan

Mechanical Engineering Faculty, Kragujevac, Serbia

1. INTRODUCTION

Frequently is necessary to go deeply into plastic range before severe accidents occur

for design of nuclear reactors and some chemical plants. Such a reserve is needed to

sustain the course of hypothetical core melt accidents, potential containment failure

modes and sequences leading to radioactive release. An analogous situation appears in

stamping plants operating on ductile mild ferritic steels where a large plastic strain

without fracture is needed in order to form complicated shell parts of a car body. Ductile

fracture as combination of normal cracking, delamination and fracture along shear-bands

might be divided into two categories: tensile type and shear type failure. It is affected by:

1. history of stress, strain and temperature including their actual values and strain rate,

2. grain size as well as grain boundaries relative strength,

3. size of the considered structure or specimen, and

4. strain distribution (strain gradients).

Concerning history effects it is likely that the comment of McClintock is still valid:

with not even an understanding of the difference between monotones shear and tensile

fracture it is not surprising that the effects of varying stress history are not understood. In

order to clarify at least partly the significance of history diverse stress directions and non

proportional paths are necessary. As a single scalar measure of failure advance equivalent

plastic strain is not convenient. Instead Davis triaxiality factor

(ratio of first stress

invariant and Mises equivalent stress) being for isotropic materials: 0 for shear, 1 for

uniaxial tension and 2 for equibiaxial tension appears to be much more appropriate.

Existing criteria might be classified into:

1. maximum tensile stress (here maximum of largest principal engineering stress

corresponds to maximum load) leading to diffuse instability criterion,

2. hole-growth criterion which is built by means of intrinsically statistical theory with

some inevitable simplification,

3. connection of localization to yield surface corners appearance and development,

4. maximum shear stress criterion useful for crystal grains or grain boundaries but not

poly-crystals, and

5. forming limit diagrams (FLD) being successfully applied to metal sheet forming

where for isotropic materials

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൑ 2ሺ݅. ݁. 0.5 ൏

ଶ

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⁄ ൏ 1ሻ

2. EXISTING MODELS OF DUCTILE FAILURE CRITERIA

2.1. McClintock's model of growing touching holes

As a background for his calculations McClintock had experimental evidence concer-

ning coalescence of holes appearing not only in metals like copper, but also in plastics.

Typically, holes look like “wolf's ear" connecting during growth to each other either