The reliability of an engineering product is greatly influenced by its operating environment. During manufacture, transport, storage and service varying adverse climatic conditions may be encountered affecting both product performance and survival capability. Simulation of these conditions under laboratory conditions can both assist the design and development process and demonstrate that a product is fit-for-purpose.

Climatic conditions may include extremes of temperature, altitude, humidity, salt mist, solar radiation, mould growth, sand and dust, etc. Equipment is frequently expected to operate in a global marketplace, and must therefore take account of world climatic conditions. These can be severe, ranging from desert to arctic temperatures, 10 to 90% relative humidity, and pressure variations of 200 to 1000 mBar. In space and military applications the climatic environment will be even more hostile.

It is unlikely that any product will see a perfectly benign environment. Solar radiation on enclosed products (especially stored behind glass), diurnal temperature variations, and ingress of moisture are frequent conditions which are ignored during the design process. The range of examples is vast, but may include:

MATERIAL PROPERTIES

• Materials change state (gas to liquid to solid - and vice versa).
• Organic materials discolour, crack or craze
• Outgassing of component materials
• Many plastics give off poisonous fumes
• Hardening & embrittlement (loss of elasticity) of materials
• Cracking and crazing of materials
• Binding of parts from different contraction of dissimilar materials
• Stiffening of shock mounts
• Many plastics shatter below -20^oC
• Changes in material strength
• Static fatigue of restrained glass

ELECTRONIC COMPONENTS

• Fixed resistors & capacitors change in value
• Electronic circuit stability varies with differences in temp gradients
• Changes in performance of transformers & electromechanical components
• Relays and magnetic/thermally activated devices alter their operating/release margins
• Shortened operating lifetime

EXPLOSIVES / PROPELLANTS

• Solid pellets/grains separate or crack
• High pressure created within sealed cases
• Change of burning rates
• Expansion of cast explosives within cases
• Explosives melt and exude

MECHANICAL / LUBRICATION

• Parts bind from different expansion of dissimilar materials
• Packing, gaskets, and seals deteriorate and fail
• Bearings and shafts become distorted
• Hydraulic oil and vaporises when too hot and freezes when too cold
• Lubricating oils may decompose or oxidize into sludges and carbonaceous residues
• Loss in lubrication and lubricant flow due to increased viscosity when hot, and less viscous when cold

HUMAN

• Decrease in dexterity, hearing and vision of personnel wearing protective clothing