CAE Analysis & Application

• Failure theories, within the context of FEA and material mechanics, provide criteria by which a material's susceptibility to failure under various states of stress can be evaluated. Different materials and loading conditions may require different failure theories for accurate predictions. Here's an overview:

• Types of Failure Theories:

  • Von Mises Criterion (Distortion Energy Theory): This is one of the most commonly used theories for ductile materials. It's based on the energy of distortion (deviatoric stress) and predicts failure when this energy exceeds a certain threshold.

  • Maximum Normal Stress Theory (Rankine's Theory): This theory predicts failure will occur when the maximum principal stress in a material exceeds the uniaxial tensile or compressive strength. It's often used for brittle materials.

  • Maximum Shear Stress Theory (Tresca's Criterion): According to this theory, failure occurs when the maximum shear stress in a material exceeds the shear strength of the material. This theory is often applicable to ductile metals.

  • Mohr's Theory: This is a combination of shear and normal stresses and provides an envelope of failure criteria. It's more general and can be tailored to fit various materials.

  • Maximum Strain Energy Theory (Beltrami's Criterion): This theory is based on the total strain energy and is used less frequently than the distortion energy theory.

  • Mohr-Coulomb Criterion: Often used for predicting failure in soils and some brittle materials, it incorporates both cohesive and frictional properties of the material.

• Which Should Be Used for What:

  • Brittle Materials: For these materials, which tend to fracture without significant plastic deformation, the Maximum Normal Stress Theory or Mohr-Coulomb Criterion might be more applicable.

  • Ductile Metals: For metals that can undergo significant plastic deformation before failure, the Maximum Shear Stress Theory or the Von Mises Criterion are often chosen.

  • Soils and Some Brittle Materials: The Mohr-Coulomb Criterion is frequently employed.

• In conclusion, the selection of a failure theory in FEA should be based on the material in question, the expected loading conditions, and the desired level of conservatism in the design. Furthermore, comparing predictions from multiple failure theories, when possible, can provide a more comprehensive understanding of potential failure scenarios. It's also beneficial to validate FEA predictions with experimental tests whenever possible.

Factor of Safety

• Using information from an FEA study to redesign objects in CAD involves a holistic approach that combines computational data with engineering judgment. The goal is to make design improvements based on simulation results while considering real-world applicability and constraints. Here's a breakdown of methods and considerations:

• Manual Interpretation and Iteration:

  • Stress Distribution: Identify regions of high stress or "hot spots" and consider increasing material or adding fillets in those areas to better distribute the stress.

  • Deformation Analysis: Check areas with excessive deformation and consider reinforcing or modifying them to reduce unwanted movements.

  • Factor of Safety Analysis: Adjust material or geometry in regions where the safety factor is below the desired value.

• Topology Optimization:

  • This advanced method uses algorithms to evolve a design iteratively based on the applied loads and boundary conditions. The resulting design typically minimizes material use while maintaining performance.

  • It often results in organic-looking structures that can be challenging to manufacture using conventional methods but are suitable for additive manufacturing or 3D printing.

• Redesign Considerations:

  • Manufacturability: Always ensure that any design changes made based on FEA results are still manufacturable using the intended processes.

  • Cost: Consider the cost implications of any design changes, including the addition of material or the potential increase in manufacturing complexity.

  • Functional Requirements: Ensure that redesigns don't compromise the primary function or performance requirements of the component or system.

  • Real-World Constraints: Remember that FEA is an approximation, and factors not accounted for in the simulation might exist in real-world applications. Always cross-check FEA-based design changes with practical constraints and expert judgment.

• Based on insights gleaned from FEW studies and engineering knowledge, necessary design modifications are made in the CAD system. Subsequently, FEA is re-run on this adjusted model to gauge the impact of the modifications. Through a process of comparison and repeated iteration—altering the design and reanalyzing it—engineers refine the model until they obtain a design that adeptly meets all stipulated requirements and constraints.