Maximum Instantaneous Loading
The criteria for failure of a component or assembly may be either yield or fracture or both. In the case of testing prototypes the tensile properties of gravity cast ZL12 alloy lay between those of pressure diecast ZP3 and ZP5, apart from the elongation, which will normally be lower for ZL12. A consequence of this is that combinations of casting design and loading condition which lead to significant redistribution of stress if the casting is plastically deformed can give misleading results when ZL12 prototypes are tested to fracture. Testing to small plastic deformations should give a more indicative result. If this is not permissible then a heat-treated ZL27 prototype should give a more representative result.
Because the strength properties of zinc alloys are sensitive to temperature the tests should be carried out at the maximum temperature the pressure diecasting will experience in service. In the case of higher temperatures, say above about 60ΟCelsius, care should be taken to make sure that the test is performed quickly, say a few seconds only to reach yield. This will permit the simulation of instantaneous loading situations; slower loading would cause the strain rate sensitivity of the alloys to affect the result.
It is recommended that pressure diecast prototypes be used if cyclic load testing is required, although a preliminary test utilising gravity cast ZP8 should give a reasonably indicative result. Again the test should be carried out at the highest service temperature expected.
For Long Term Creep Resistance
Zinc alloy pressure diecastings can be subjected to accelerated creep tests, which give a good correlation with service performance. The tests are carried out either with a higher load than will be used in service or at a temperature higher than that the casting will experience in service, or a combination of both.However two conditions must be satisfied, the stress induced in the casting by the test load must not exceed 50MPa and the test temperature must be less than 150o Celsius.
With complex shape castings it can be difficult to assess the stress levels that will be generated by a given load. In such cases the maximum test load should be related to that which causes yield of the casting at room temperature when loaded in the same manner. For example if a ZP3 diecasting is found to yield at a load of 1000 Newtons then the maximum creep test load would be 1000Newtons multiplied by 50MPa divided by the yield strength of ZP3 (220MPa) ie 181.8 Newtons.It is best to leave some safety margin.
If it is considered necessary to submit a prototype to a creep test, then it is best to use one made of the same alloy as for the proposed pressure diecasting. All the other mechanical properties of such a prototype will be inferior to the diecastings, but the creep properties will probably be superior to some degree. However no other alloy is likely to simulate the pressure diecasting better. Once a pressure diecasting has been produced it can then be submitted to testing to confirm its acceptability.
The relationships between the test and service times temperatures and loads are shown in the equations below. It should be remembered that the longer the test the more accurate will be the result and that preliminary tests should be designed to last 24 hours or so and final tests several weeks at least.
The creep equation for alloys 3, 8 as follows:
ln σ = C’ + Q/RT – ln t
Where σ= stress MPa (50 MPa max)
t = time in seconds
T = temperature in degrees K (423oK, 150oC, maximum)
n = the stress exponent = 3.5
Q= the activation energy for creep kJ/mol
(106 for alloys ZP3 and ZP8)
R = the gas constant = 8.3143 x 10-3 kJ/mol
C’= constant for the given alloy at a given strain
The above equation can be utilised to identify accelerated test conditions that can be used to evaluate the more moderate conditions of longer-term creep performance. This is enabled by the fact that C’ for both the test and in service will be fixed if the same alloy is used and the degree of deformation is equal in each case.
let t1 = service life in seconds
t2= duration of accelerated test in seconds
T1 = service temperature oK
T2 = test temperature oK
σ1 = service stress MPa
σ2 = test stress MPa
Then provided the test temperature does not exceed 150oCelsius and neither stress is in excess of 50 MPa the following equations apply.
ln t1 = n x ln(σ2/σ1)+ ln t2 + Q/RT1 – Q/RT2
ln t2 = n x ln(σ1/σ2)+ ln t1 + Q/RT2 – Q/RT1
(1/T1) – (R (n x ln(σ1/σ2) + ln (t1/t2))/Q)
(1/T2) – (R (n x ln(σ2/σ1) + ln (t2/t1))/Q)
ln σ1 =[ (n x lnσ2)+ ln (t2/t1) + Q/RT1 – Q/RT2 ] / n
ln σ2 = [(n x ln σ1)+ ln (t1/t2)+ Q/RT2 – Q/RT1] / n