SAM-Consult - Techniques for Failure Prevention and Diagnosis

Solid Mechanics: Failure Criteria
Techniques for Failure Prevention and Diagnosis

There exist a set of basic techniques for preventing failure in the design stage, and for diagnosing failure in manufacturing and later stages.

In the Design Stage

It is quite commonplace today for design engineers to verify design stresses with finite element (FEA) packages. This is fine and good when FEA is applied appropriately. However, the popularity of finite element analysis can condition engineers to look just for red spots in simulation output, without really understanding the essence or funda at play.

By following basic rules of thumb, such danger points can often be anticipated and avoided without total reliance on computer simulation.

Loading Points	Maximum stresses are often located at loading points, supports, joints, or maximum deflection points.
Stress Concentrations	Stress concentrations are usually located near corners, holes, crack tips, boundaries, between layers, and where cross-section areas change rapidly. Sound design avoids rapid changes in material or geometrical properties. For example, when a large hole is removed from a structure, a reinforcement composed of generally no less than the material removed should be added around the opening.
Safety Factors	The addition of safety factors to designs allow engineers to reduce sensitivity to manufacturing defects and to compensate for stress prediction limitations.

In Post-Manufacturing Stages

Despite the best efforts of design and manufacturing engineers, unanticipated failure may occur in parts after design and manufacturing. In order for projects to succeed, these failures must be diagnosed and resolved quickly and effectively. Often, the failure is caused by a singular factor, rather than an involved collection of factors.

Such failures may be caught early in initial quality assurance testing, or later after the part is delivered to the customer.

Induced Stress Concentrations	Stress concentrations may be induced by inadequate manufacturing processes. For example, initial surface imperfections can result from sloppy machining processes. Manufacturing defects such as size mismatches and improper fastener application can lead to residual stresses and even cracks, both strong stress concentrations.
Damage and Exposure	Damages during service life can lead a part to failure. Damages such as cracks, debonding, and delamination can result from unanticipated resonant vibrations and impacts that exceed the design loads. Reduction in strength can result from exposure to UV lights and chemical corrosion.
Fatigue and Creep	Fatigue or creep can lead a part to failure. For example, unanticipated fatigue can result from repeated mechanical or thermal loading.