Introduction

Modes of Failure

    Designer should be aware of the possible modes of failure of a component or structure so that the design process can be carried out with a view to ensuring the avoidance of all possible, relevant failure modes. In some respects, one of the major skills in designing is being able to correctly identify the most probable failure mechanism. A classification of the more common failure modes known for structural components can be made as follows:

• Failure by elastic instability (Buckling);
• Failure by excessively large elastic deformations (Jamming);
• Failure by gross platic deformation (Yielding);
• Failure by tensile instability (Necking);
• Failure by fast fracture (Cracking, Snapping);
• Failure by environmental corrosion (Rusting, Rotting);

    The intention of Fracture Mechanics is to demonstrate and explain the techniques available for designing against fracture.

Yielding

    By yielding, the engineer understands both localised yielding and failure by plastic collapse. A failure by yielding can occur with general yielding or with the onset of limited plastic deformation in the component in question.

    Localised yielding or plastic collapse can be used as the limiting criteria in a certain design situations.

    Plastic deformatino can also be desired and induced in certain situations in order to create beneficial residual stresses or to blunt sharp defects.

Buckling and Jamming

• Buckling - A risk in the design of long slender members in compression. The phenomenon of buckling originates from small misalignments in the application of load when the elastic restoring forces in the slender member are no longer sufficient to keep the system in equilibrium. This usually results in instability with catastrophic deformations until the bent column yields or fractures.
• Jamming - Occur as a result of an oversight in design, excessively large elastic deflection take place.

    Both types of failure can be avoided by ensuring the geometric specifications.

Necking

    A risk for tension members subjected to a load-controlled loading system.

    This can only happen as a result of a gross overload and depends on the interaction of material properties with the structure's geometry and applied stress system.

    In order to design against necking failures, design codes have been developed and the application of safety factors ensures that necking failure is highly unlikely

Cracking

    The economic imperative of the last 50 years has led to attempts to use higher stresses for a given geometrical configuration requiring materials of higher uniaxial strength. The development of these high strength materials and their efficient usage has rendered structures prone to failure by an alternative mode of failure: namely fast fracture or cracking.

    Fast fracture is the unstable propagation of a crack in a structure and is almost invariably produced by applied stresses apparently less than the design stress calculated with the appropriate design code. It is the catastophic nature of these failures that led to the development of Fracture Mechanics. These kind of failures were often described by the term brittle.

    Brittle fracture - one in which the onset of unstable crack propagation is produced by an applied stress less than the general yield stress of the uncracked ligament remaining when instability first occurs.

    These failures are usually associated with gross stress concentrations in large components or structures and with loading systems which do not relax the applied stresses as the crack extends.

    Although in steels these fractures happen at low temperature and/or in thick sections, for both aluminium and steel they can also take place with very thin sheets.

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