DSGN 215 - Fatigue and Fracture

3 Fracture Mechanics -
3.1 Introduction

In the 1960s when higher strength steels started to be introduced, it was noted that components often failed suddenly at loads that were well below those that would have caused yielding. Investigation these failures led to the development of 'fracture mechanics'.

Changes in component geometry such as those caused by grooves, give rise to stress concentration effects. As long as the geometry is known, these effects can be computed and taken into account during design. However many components contain defects, often cracks, where the end of the crack is very sharp, its radius is not known and can not be measured. For situations like these, the stress concentration can not be determined and an alternative approach is needed. The approach is to use 'fracture mechanics'.

The way in which a cracked component is loaded can be idealised in one of three ways, see diagram on right.

For the vast majority of engineering applications it is 'mode I', the 'opening mode', which is of interest.

The stress near a crack tip can be characterised by a single parameter, the stress intensity factor, where Q is a factor depending of specimen and crack geometry ('geometry correction factor'), a is the crack depth (or half the crack length). Fracture occurs when the value of K reaches Kc the fracture toughness of the material.

Kc can be found from laboratory tests and applied to real structures of different geometries. KIc (the subscript 'I' meaning mode I) is a function of the material thickness and reaches a minimum when the material is thick enough to provide full restraint (plane strain). Extensive values can be found in the literature (see below).

Although strictly the stress intensity applies only to linear elastic fracture mechanics (LEFM), some crack tip plasticity is common in structural materials. Provided this plastic zone is only small <a/15 or < B/15 (where B is the specimen thickness) then the stress intensity is not significantly affected away from the crack tip and LEFM and K can be used.

Structural materials absorb significant energy in the plastic zone near the crack tip which makes them tough, however brittle materials have no, or a very small crack tip plastic zone and have low toughness.

Crack growth rates in components subjected to cyclic loading have been investigated in a number of structural materials and all structural metals have the same 'sigmoidal' shaped crack growth curves when the crack growth is plotted against the stress intensity range (Kmax - Kmin) on log axes.

3.2 How Fracture Mechanics is used in Design and Maintenance Operations

Fracture mechanics may be used to assess if a defect present in a structure will cause failure during the load application or for cyclic loading, how many load cycles will be required to grow the crack to the critical length that causes failure.

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Geometry Correction Factors - Q

One configuration which is often of interest to designers is a surface breaking semi elliptical crack perpendicular to the axis of a finite rectangular component subject to tension, as shown above. Use this applet to calculate the critical (nominal) stress for different dimensions and fracture toughness values.

It should be noted that fully embedded elliptical cracks will grow so that the a/2c ratio tends towards 0.5, a circular crack periphery. This gives a constant stress intensity factor round the periphery.

However in the semi-elliptical surface crack shown above, the equilibrium a/2c ratio is closer to 0.36 because there is an additional stress intensity factor correction arising from the free surface.

For any engineering material it is frequently the case that increasing the UTS or yield strength results in a lowering of the fracture toughness.

 Material UTS Yield strength, MPa KIC, MPa(m)0.5 A533B Alloy steel -- 500 175 4340 steel 2332 1450 51 4340 1827 1503 59 4340 1764 1530 201 52100 -- 2070 14 AISI 403 stainless steel 821 690 77 Maraging steel 18% Ni 1783 -- 174 Ti-6Al-4V -- 910 115 Ti-6Al-4Zr-2Sn-0.5Mo-0.5V 890 836 139 2219-T851 Al 454 340 32 2024 Al -- 455 26 6061-T651 Al 352 299 29 7079-T651 Al 569 502 26 7178 Al -- 490 33

3.3 Geometry Correction Factors - for configurations shown in the right hand panel, above.

These are for mode I loading and normally are only valid when b>>a. For a short single edge through crack of depth a, Q also = 1.12

Data and equations based on these curves are used to predict residual lives of cracked components.

3.4 Residual Life Assessment:

Stage II of the sigmoidal crack growth curve can be represented by a 'power law' relationship giving the fatigue crack growth rate da/dN of a material in terms of the range of the applied stress intensity factor as: where C and m are constants for a particular material, environment and loading condition and is range of the stress intensity factor occurring at the crack tip.

The power law eqn. can be integrated: between the limits of the initial (present) crack length and the final (maximum safe) crack size. Substituting for K:

as Q generally depends upon the crack depth and shape, numerical integration is normally needed.

For very small cracks, which are described by the initial part of the sigmoidal curve, the above results, which are based on the main, middle part of the sigmoidal curve, are conservative.

The effects of temperature may need to be considered. As the temperature rises the yield stress decreases and for a given load the size of the plastic zone will increase, so even though LEFM may be applicable at ambient temperatures, it may not be applicable at elevated temperatures.

It is generally preferable for pressure vessels to leak before they fracture. Fracture mechanics is used to check that when an eliptical crack in the wall has grown so that its depth extends through the wall, allowing leakage, the crack length is still below the critical length that would cause sudden rupture.