Stress and Strain
The maximum permissible design stress for a component usually depends on the need to avoid either catastrophic failure or excessive elastic or plastic strain under anticipated service conditions. Examples of critical service conditions include operating loads, be they static or fluctuating, operating temperature and expected service life. However the loads that the casting will be subjected to during any secondary production operations should not be forgotten as these could have detrimental affects on the casting, such as causing cracking or distortion.
Up to recent times relatively little “stressing” of components designed for manufacture by casting or moulding has occurred outside of the aerospace and automotive industries. It is still something of a rarity but perhaps not for much longer. The reduced cost and increased power of computers and the increasing user friendliness of software may soon lead to more routine use of Engineering Analysis. It offers the potential to reduce the unit cost of the product whilst maintaining confidence in its durability.
The mechanical properties of zinc alloys change with temperature. As temperature rises the tensile strength and creep resistance fall and the ductility increases. The impact strength as measured by the unnotched Charpy technique shows a sudden increase at around zero Celsius for most alloys. This is best not interpreted as a severe embrittlement at low temperatures because the impact strengths of zinc alloys at far below zero are comparable to those of aluminium alloy diecastings at room temperature and above.
Historically an upper service temperature limit of 100 degrees Celsius was applied to zinc alloy diecastings made from ZP3 or ZP5 in stressed applications and 150 degrees in “unstressed situations”. For ZP2 and ZP8 this rule of thumb can be increased to 130 degrees for the stressed situation. These figures are still a useful guide to the potential usability of a zinc diecasting but they are of no great help when trying to optimise its design. The design stress applicable for these situations depends whether the loading is continuous, cyclical or intermittent and upon the distortion that can be accommodated without the product failing to meet its specified performance targets.
For castings subject to significant continuous loads the design stress applicable will depend upon the creep strength of the material at the service temperature, and the desired service life of the component. If the casting is similarly loaded both in use and in storage and the application is at ambient temperatures then the design life will be the full life expectancy of the assembly. If however the part is subject to increased loads or temperatures during operation then it is likely that the design life assumed should be the total operating time required. Only when the loading or temperature in storage is a significant proportion of that in operation need both factors be taken into account.
The maximum design stress for parts subject to continuous loads is probably best calculated using the creep equation shown on page 45 of Engineering Properties of Zinc Alloys published by ILZRO (see the section on creep in the Zinc Diecasting Alloys section). This requires decisions on the maximum allowable percentage creep strain, service temperature, and life.
For components subject to fully reversing fluctuating loads the maximum design stress should be the fatigue strength of the alloy divided by a reasonable safety factor. Where the fluctuating element of the load is only a proportion of the full load both creep strength and fatigue strength factors should be considered to see which is the most critical.
The loads that a component will be subjected to by any anticipated abuse are obviously not to be forgotten. The ability of the material to withstand such abuse is best judged using its instantaneous properties ie tensile, yield and impact strengths.