 |
 |
 |
 |
 |
 |
|
 |
 |
 |
|
 |
|
|
Thermal Stress Model Predicts Die Life
Results Confirm Experience with Heated Dies (continued) The transient, nonlinear thermal analysis took into account copper's material properties for the casting and the superalloy's properties for the die. Certain assumptions were necessary, among them an estimate of the thermal contact conductance, also called the gap conductance. Simulations assumed a worst-case scenario with a high and constant value of contact conductance (10,000 W/m2K). The copper temperature was assumed to be 1200 C (2192 F). The researchers next calculated thermally induced strains arising from the respective temperature profiles. Calculations were made using finite element analysis (FEA) — also within ANSYS — in which the complex shape of the die is partitioned into a grid of small elements for which local strains can be determined using iterative computer calculations that take into account the die materials' physical and mechanical properties. Only the die was considered in this part of the investigation. The FEA was conducted using two-dimensional elements. It quickly became apparent that the thermally induced stresses — all of them compressive — exceeded the yield strength of the die materials; therefore, both elastic and plastic properties of the material had to be taken into account. The resulting strain maps were then evaluated to identify nodes at which stresses and strains are highest, i.e., where thermal fatigue failure would most likely begin. It remained to calculate the fatigue life (reported as the number of die-casting shots) based on those strains and known or estimated material properties under those conditions. This calculation was made using a technique known as the method of universal slopes, which was originally developed for NASA. Its fundamental equation of strain is: 
where  is the sum of the plastic and elastic strain ranges. Calculated strains are the effective or von Mises strains, determined from the calculated principal strains, 1, 2, 3,
In this case, calculation of the number of cycles to failure considers only the plastic portion of the strain equation, since plastic strains predominate, or, more to the point, the die operates in a low-cycle fatigue regime. 
MORE >> |
|