Recording-2025-01-24T14:17:55.098Z

Introduction to Mechanical Properties

  • Begin the introduction of mechanical properties in the upcoming week.

  • There will be preliminary material to cover before diving into specific properties.

  • As the term progresses, more design problems will be introduced related to mechanical properties.

Using Edge Pack for Material Selection

  • Familiarization with Edge Pack for selecting materials.

  • Selection Tools:

    • Click on the box to capture materials based on specific properties.

    • A line can be drawn on a graph to represent the chosen properties.

Graphical Representation of Properties

  • After selecting a slope (e.g., negative one), click on the graph to place the line.

  • Determine whether to maximize or minimize the selection based on the line's position.

  • Adjust the line to reduce choices to a manageable number of materials.

Key Concepts in Mechanical Behavior

  • Normal and Shear Stress:

    • Introduction to normal shear and hydrostatic stress and strain.

    • Emphasis on the tensile test to characterize material properties.

Basics of Tensile Testing

  • Builds upon previous knowledge from high school physics, specifically:

    • Free body diagrams

    • Basic trigonometry (resolving forces)

    • Logarithms, useful in calculating material properties.

Understanding Forces and Bending in Materials

  • Consideration of a larger diameter beam (eyeball analogy) leading to greater cross-sectional area, affecting force support.

  • Normal Forces:

    • Includes tension and compression forces acting on materials.

  • Discussion of the neutral axis concept in bending:

    • The midplane remains unchanged during bending, serving as a reference point.

    • Tension and compression are distributed across the beam:

      • Top half experiences highest tension.

      • No tension or compression at the neutral axis.

      • Bottom or concave side is under compression.

Shear Stress Distribution

  • Shear stress increases toward the center of the material, building up to maxima internally while being zero at the surfaces due to free movement.

  • The structural orientation of beams is critical for their stiffness:

    • Standing a 2"x4" on its edge increases rigidity compared to laying it flat.

    • Stacking beams can enhance stability.

Final Remarks on Shear and Material Failure

  • Definition of Shear:

    • A force causing sliding or attempting to create sliding within materials.

  • Example involving submarines:

    • Deeper water increases pressure due to the weight of the water column above.

    • Reference to past events (Titanic) linked to material failure due to compressive forces.

Stress and Strain Equations

  • Introduction of three equations that describe material properties: stress vs. strain.

  • Strain Definition:

    • Normalized change in length (change in length/original length).

    • Often expressed as a percentage, though unitless.

  • Stress Definition:

    • Calculated as force divided by cross-sectional area.

Detailed Examination of Tensile Testing

  • Tensile Test Purpose:

    • Designed to stretch materials at a controlled rate to measure the force required until failure.

  • The testing machine mechanics:

    • Outlined structure of a tensile testing machine indicating rotating screws and the forces applied.

    • Intentionally designed specimen with a smaller middle section to localize stress and induce breaking at that point.

  • Transition from starting conditions to stress-strain graphs detailing the material behavior upon loading.