Material Analysis Using an Elasto-Plastic FEA Model

Summary

The objective of our project was to enhance our understanding of material properties by generating and utilizing stress-strain curves. These curves were obtained through testing specimens and imported into SolidWorks for simulation analysis. By evaluating and comparing the performance of different material models—linear and elasto-plastic—we aimed to understand their impact on simulation results. To achieve this, we conducted material tests using the Instron machine with dog bone test specimens, extracting key mechanical properties. Using a cantilever beam as a case study, we analyzed the performance of each material model across simulations, highlighting the differences in behavior and accuracy. This approach allowed us to explore the practical applications and limitations of these models in engineering design.

Dog Bone Dimensioning and Instron Testing

We used two different standards to determine the required dimensions for the dog bones, ASTM-E8/EGM-24 for the metals and ASTM D638-14 for polypropylene. The specific dimensions were then chosen based on the thickness of the stock material and modeled in SolidWorks to be cut out on the water jet in Bray. These dimensions differ depending on factors like the thickness and material to keep a consistent standard for data with some recommendations for border conditions and stock material that may fall on the extreme of a certain set of dimensions. 

We then used the Instron machine in Bray to test each of our samples, starting by setting up the machine with the specific materials and test in mind. We set up the Brass 360 with a static extensometer to get accurate measurements due to it being significantly more rigid than either the polypropylene or aluminum. The crosshead speed was set relative to each material; 2.25 mm/min for brass and aluminum, 5 mm/min for polypropylene. We then set up the machine with each sample and designated the Young’s modulus and the stress vs. strain curve to be collected for each run. 

Results and Analysis

Linear and nonlinear statistics studies were conducted on 1x1x10 inch cantilever beams to compare the results of the two models. A linear model is automatically utilized in a static analysis, but several different models can be used for a nonlinear study, of which we chose the Plasticity von Mises Model. Material properties such as elastic modulus and yield strength were determined from the stress-strain curve we obtained from Instron testing.  Copies were made of the materials in the SolidWorks material library to implement the Instron test data as a property. The properties of the materials were preset using the existing files, as well as MatWeb, to fill in any gaps in the information. A Stress-Strain Curve was selected under the Tables and Curves tab, and the Instron data was imported. Certain parameters were kept constant to make the results comparable, including fixing one end, creating the mesh, and using the same loads. 

The nonlinear model showed more displacement because, unlike the linear model, it accounts for material yielding and plastic deformation. It uses the full stress-strain curve from Instron testing, reflecting elastic and plastic behavior. The material deforms plastically as stress exceeds the elastic limit, leading to greater displacement. The linear model assumes a proportional relationship between stress and strain, underestimating deformation under high stress. Overall, the analysis was a success and we are able to conclude that all three materials benefit from using an elasto-plastic model rather than the standard linear model used in SolidWorks. Aluminium 6061 showed a difference in maximum deflection of 0.6 inches from linear to plasticity von Mises, Brass 360 saw approximately 0.25 in, and polypropylene was 0.04 in. The higher values seen in the metals could be due to the fact that they are better suited for a plasticity von Mises study, but further analysis would need to be conducted to confirm this assumption.