Case of study: polymers used in push-in of a backpack buckle
Material modelling is a key point for a realistic simulation. A good material model has to predict accurately material behaviours under different loading conditions.
Most products can be dimensioned based on linear elastic models with a stress limit, and the optimization of such products can be performed with basic simulation tools; However, these tools are not relevant when the mechanical test case involves large deformations, contacts between the different parts of the product, and materials that exhibit non-linear behaviours.
For instance, polymers can be highly non-linear and predicting their behaviour through simulation guides the designer in choosing the right grade according to the specifications:
Let’s consider the push-in of a backpack buckle: this is a complex case to model because of the sudden energy release at the end of the clipping phase that can cause non-convergence of the solution. Let’s say that the designer has the choice between two POM (Polyoxymethylene) and wants the push-in force to be below 50N.
Here below are the main properties of the two POM materials.
POM A | POM B | |
| Tensile Modulus [MPa] | 2850 | 1950 |
| Yield Stress [MPa] | 64 | 44 |
| Yield strain [%] | 9 | 9 |
| Nominal strain at break [%] | 30 | 40 |
At DAES, we use our material modelling skills to choose the adequate non-linear model that will fit to experimental material tests data.
Thanks to simulation, we can evaluate the push-in reaction forces, which are 70.6N for POM A and 47.6 for POM B, so we can recommend the designer to choose POM B for this product.
Moreover, the analysis of the Von Mises stress at this peak force shows that the material is below the yield stress for POM B (44MPa).
Virginie Pouzols – Engineer at DAES
