Practical Reliability Engineering, 5th Edition

5

Load–Strength Interference

5.1 Introduction

In Chapter 1 we set out the premise that a common cause of failure results from the situation when the applied load exceeds the strength. Load and strength are considered in the widest sense. ‘Load’ might refer to a mechanical stress, a voltage, a cyclical load, or internally generated stresses such as temperature. ‘Strength’ might refer to any resisting physical property, such as hardness, strength, melting point or adhesion. Please note that the Load-Strength concept is often referred in the literature as ‘Stress-Strength’.

Examples are:

1. A bearing fails when the internally generated loads (due perhaps to roughness, loss of lubricity, etc.) exceed the local strength, causing fracture, overheating or seizure.
2. A transistor gate in an integrated circuit fails when the voltage applied causes a local current density, and hence temperature rise, above the melting point of the conductor or semiconductor material.
3. A hydraulic valve fails when the seal cannot withstand the applied pressure without leaking excessively.
4. A shaft fractures when torque exceeds strength.
5. Solder joints inside a vehicle radio develop cracks before the intended service life due to temperature cycling fatigue caused by the internal heating.

Therefore, if we design so that strength exceeds load, we should not have failures. This is the normal approach to design, in which the designer considers the likely extreme values of load and strength, and ensures that an adequate ...