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ME203: Materials and Materials Processing

Unit 3: Comparison of Engineering Materials – *Which Is Best?*   This unit draws from the previous two units. How do we select a material to produce a particular part? We need to avoid the possible failures described in Unit 1 by selecting the best material from the classes listed in Unit 2. Most often, there will be several competing candidates. Part of the evaluation will also include consideration of how the materials can be shaped, which we will investigate in Unit 4.

Unit 3 Time Advisory
This unit should take you approximately 33 hours to complete.

☐    Subunit 3.1: 1 hour

☐    Subunit 3.2: 2 hours

☐    Subunit 3.3: 5 hours

☐    Subunit 3.4: 3 hours

☐    Subunit 3.5: 4 hours

☐    Subunit 3.6: 4 hours

☐    Subunit 3.7: 4 hours

☐    Subunit 3.8: 4 hours

☐    Subunit 3.9: 6 hours

Unit3 Learning Outcomes
Upon successful completion of this unit, you will be able to: - select candidate materials for various engineering design scenarios; and - rank competitive materials using handbook data.

3.1 Assessing Engineering Environments   - Reading: The Saylor Foundation’s “Assessing Engineering Environments” Link: The Saylor Foundation’s “Assessing Engineering Environments” (PDF)

 Instructions: Please read this short article, which discusses how
to assess engineering environments.

3.2 Density   - Reading: The Saylor Foundation’s “Density” Link: The Saylor Foundation’s “Density” (PDF)
 
Instructions: Please read this short article, which compares various materials.

3.3 Elastic Stiffness   3.3.1 Comparison of Materials   - Reading: The Saylor Foundation’s “Comparison of Materials” Link: The Saylor Foundation’s “Comparison of Materials” (PDF)

 Instructions: Please read this short article, which compares the
elastic constant of various materials.

3.3.2 Commentary   - Reading: The Saylor Foundation’s “Commentary” Link: The Saylor Foundation’s “Commentary” (PDF)

 Instructions: Please read this short article, which comments on the
elastic stiffness of various materials.

3.3.3 An Example of Materials Selection Based on Elastic Deformation   - Web Media: YouTube: Learn ChemE’s “Elastic Properties of Metals” Link: YouTube: Learn ChemE’s “Elastic Properties of Metals” (YouTube)

 Instructions: Please click on the link above, and view this brief
video. The video presents an application of using handbook data and
calculations to select a material for strength and stiffness. While
identified as elastic design, yield stress is needed to establish an
upper limit for elastic deformation.  

 Note: At one point the narrator misrepresents the units of a
calculation. When calculating the applied stress, the narrator
speaks the units as though they were written N•m and then writes
them N/m. Neither is correct. The units for stress are
N/m<sup>2</sup> or Pa (newtons per meters squared or pascals).  

 Watching this video and pausing to take notes should take
approximately 15 minutes.  

 Terms of Use: Please respect the copyright terms of use displayed
on the webpage above.

3.3.4 An Example of Shape Consideration for Elastic Deformation   - Reading: The Saylor Foundation’s “Example of Shape Consideration for Elastic Deformation” Link: The Saylor Foundation’s “Example of Shape Consideration for Elastic Deformation” (PDF)

 Instructions: Please read this article, which provides various case
studies on shape consideration for elastic deformation.

3.4 Plastic Deformation Strength   3.4.1 Comparison of Materials   - Reading: The Saylor Foundation’s “Comparison of Materials” Link: The Saylor Foundation’s “Comparison of Materials” (PDF)

 Instructions: Please read this short article, which compares the
yield stress of various materials.

3.4.2 Commentary   - Reading: The Saylor Foundation’s “Commentary” Link: The Saylor Foundation’s “Commentary” (PDF)

 Instructions: Please read this short article, which comments on the
yield stress of various materials.

3.4.3 An Example of Using Yield Stress for Materials Selection   - Reading: The Saylor Foundation’s “An Example of Using Yield Stress for Materials Selection” Link: The Saylor Foundation’s “An Example of Using Yield Stress for Materials Selection” (PDF)

 Instructions: Please read this short article, which provides an
example of using yield stress to select materials.

3.5 Fast Fracture Toughness   - Reading: The Saylor Foundation’s “Fast Fracture Toughness” Link: The Saylor Foundation’s “Fast Fracture Toughness” (PDF)

 Instructions: Please read this short article, which discusses fast
fracture toughness.

3.6 Fatigue Failure Resistance   3.6.1 Geometry   Geometry is very important when fatigue loading is a concern. While the overall nominal stress in a part subject to fatigue loading may be kept within calculated safe limits, the local stress at geometric transitions can be much higher. This promotes the nucleation of cracks at these locations, such as at sharp corners. For simple geometries, charts, tables, and equations are available to give values of stress concentration factors. These are multiplying factors between nominal and higher actual stresses. When geometries and loadings are complicated, computer modeling is used. The conclusion is that we should minimize sharp angles with rounded corners to forestall nucleation of fatigue cracks.

  • Web Media: YouTube: NEi Software’s “Nastran Finite Element Analysis Software Engineering Simulation Demo Video” Link: YouTube: NEi Software’s “Nastran Finite Element Analysis Software Engineering Simulation Demo Video” (YouTube)

    Instructions: Please click on the link above and watch this brief video. This is an FEA (Finite Element Analysis) computer animation of several situations, using color to show regions of lower stress (blue and green) and regions of higher stress (yellow and red).

    Watching this video and pausing to take notes should take less than 15 minutes.

    Terms of Use: Please respect the copyright terms of use displayed on the webpage above.

3.6.2 Surface Treatments   Fatigue failure is sensitive to surface finish. A smooth, polished surface is more resistant than a rough surface. Surface scratches and irregularities can be nucleation sites for fatigue cracks. Further, since cracks grow during the tension portion of fatigue, any treatment that imparts residual compressive stresses into the surface will lengthen fatigue life. A fairly simple procedure to treat metals is shot peening. Parts are tumbled in a bath of hardened metal, ceramic, or glass spheres with sufficient force to cause compressive plastic deformation of the surface.

3.7 Creep Failure Resistance   - Reading: The Saylor Foundation’s “Creep Failure Resistance” Link: The Saylor Foundation’s “Creep Failure Resistance” (PDF)

 Instructions: Please read this short article, which discusses creep
failure resistance.

3.8 Corrosion Resistance   - Reading: The Saylor Foundation’s “Corrosion Resistance” Link: The Saylor Foundation’s “Corrosion Resistance” (PDF)

 Instructions: Please read this short article, which discusses
corrosion resistance.

3.9 Computational Exercises   - Activity: The Saylor Foundation’s “Computational Exercises” Link: The Saylor Foundation’s “Computational Exercises” (PDF)

 Instructions: Please complete these computational exercises as a
review of this unit.