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ME203: Materials and Materials Processing

Unit 1: Ways That Materials Can Fail – What Can Go Wrong?   When a material is subjected to forces, it responds by changing its shape. In the worst case, it breaks apart. Engineers identify two types of shape change: elastic deformation (recoverable) and plastic deformation (permanent). Elastic deformation is when we stretch a rubber band and it snaps back to its original length when we let go. Plastic deformation is the dent in an automobile fender.

When applied forces are small, we observe only elastic deformation. For brittle materials, that is all we observe. The definition of a brittle material might be: (elastic deformation) → (fracture).

Other materials are ductile materials. For these materials, increasing the applied forces results in plastic deformation being superimposed onto the initial elastic deformation. With ductile materials: (elastic deformation) → (plastic deformation) → (fracture).

Engineers measure shape change by defining a property called mechanical strain. Strain is a dimensionless number most often expressed as a percent. (Note: Actually, the engineer does not include the qualifier mechanical, but if we type just strain into a search engine, we will get mostly medical results.) To isolate just the material properties, the engineer expresses forces as mechanical stress,which is force per unit area like with atmospheric pressure.

When a material suddenly snaps, the situation is referred to as fast fracture. This may occur even when initial force loads appear to be within safe limits. Contributing conditions can be studied by impact testing (particularly at low temperatures), fatigue testing (often simulating vibration), and creep testing (particularly at high temperatures). Also, considered material failure is interaction with the environment­ – corrosion by oxidation and/or wet corrosion.

Unit 1 Time Advisory
This unit should take you approximately 31 hours to complete.

☐    Subunit 1.1: 4 hours

☐    Subunit 1.2: 5 hours

☐    Subunit 1.3: 4 hours

☐    Subunit 1.4: 4 hours

☐    Subunit 1.5: 4 hours

☐    Subunit 1.6: 4 hours

☐    Subunit 1.7: 6 hours

Unit1 Learning Outcomes
Upon successful completion of this unit, you will be able to:
- describe the common mechanisms by which engineering materials fail. - associate common descriptive words like strong, tough, andbrittle with engineering handbook values; and - describe the laboratory tests that measure these handbook values.

1.1 Elastic Deformation   Please note the following information on elastic deformation:

  • Materials of high elastic modulus are referred to as stiff; materials of low elastic modulus are referred to as flexible.
  • The elastic modulus of metals is relatively insensitive to alloy content.
  • Elastic deformation depends on material property (elastic modulus) and geometry (how thick, for example). Generally, materials are not chosen based on modulus, because deformation can be restrained by changing geometry. We shall see example calculations of this in Unit 3.

1.1.1 Mechanical Stress   - Reading: The Saylor Foundation’s “Mechanical Stress” Link: The Saylor Foundation’s “Mechanical Stress” (PDF)

 Instructions: Please read this short article, which introduces
mechanical stress.  

 *Note: This reading needs some introduction as to not confuse
new-comers to this field of study. The stress tensor does* not *have
dimension of pressure, rather it has pressure per unit length. It is
a type (0,2) tensor that when given a unit vector returns a
pressure.*

1.1.2 Mechanical Strain   - Reading: The Saylor Foundation’s “Mechanical Strain” Link: The Saylor Foundation’s “Mechanical Strain” (PDF)

 Instructions: Please read this short article, which introduces
mechanical strain.

1.1.3 Elastic Constants   - Reading: The Saylor Foundation’s “Elastic Constants” Link: The Saylor Foundation’s “Elastic Constants” (PDF)
 
Instructions: Please read this short article, which introduces elastic constants.

  • Web Media: YouTube: Brightstorm’s “Elastic Modulus” Link: YouTube: Brightstorm’s “Elastic Modulus” (YouTube)

    Instructions: Watch this video lecture on the definition of E.

    Watching this lecture 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.

1.2 Plastic Yielding   Please note the following information on plastic yielding:

  • Materials of high yield strength are referred to as strong; materials of low yield strength are referred to as weak.
  • Materials of high percent elongation are referred to as ductile; materials of low percent elongation are referred to as brittle.
  • Yield strength is sensitive to processing; many metals can be processed to be weak during fabrication and subsequently be processed to be strong.
  • Hardness numbers can be calibrated to be a measure of yield strength.

1.2.1 Yield Strength   - Reading: The Saylor Foundation’s “Yield Strength” Link: The Saylor Foundation’s “Yield Strength” (PDF)

 Instructions: Please read this short article, which introduces
yield strength.

1.2.2 Tensile Test   - Reading: The Saylor Foundation’s “Tensile Test” Link: The Saylor Foundation’s “Tensile Test” (PDF)

 Instructions: Please read this short article, which introduces the
tensile test.
  • Reading: Middle East Technical University: Dr. Riza Gürbüz’s “Tension Test” Link: Middle East Technical University: Dr. Riza Gürbüz’s “Tension Test” (PDF)

    Instructions: Please click on the link above and download and read the “Tension Test PDF.” Some material in the document is a repeat of the material in this section, while some material expands on the material presented in this sub-subunit. You may skip over the details of performing an experiment, but note especially the diagrams in the Appendix.

    Reading this document should take approximately 30 minutes.

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

  • Web Media: YouTube: Learn ChemE’s “Stress-Strain Diagrams” Link: YouTube: Learn ChemE’s “Stress-Strain Diagrams” (YouTube)

    Instructions: Please click on the link above and watch this brief video, which presents some of the same information.

    The reference to dislocation motion at the yield stress is a topic we will visit later. The hand-sketched stress-strain curve shows nicely that plastic deformation occursin addition to elastic deformation. Wherever we unload, whether at the yield point or at fracture, we recover the elastic deformation, with the sample following a line parallel to the initial loading line. Note that the curve is not representative of what is seen in the laboratory. For a sample of 20% elongation and an elastic limit of about 0.2%, the totally elastic region would represent only 1/100th of the graph – just about the width of the vertical axis line.

    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.

  • Web Media: YouTube: Laboratory Testing Inc.’s “Tensile Test Stainless Steel Specimen” Link: YouTube: Laboratory Testing Inc.’s “Tensile Test Stainless Steel Specimen” (YouTube)

    Instructions: Please click on the link above and watch this brief video to see a tensile sample necking before fracture.

    At the start of the video, the speaker mentions having a 500-diameter tensile specimen. He means that the sample has a diameter of 0.500 inches. Actually, the standard diameter is 0.505 inches, as this diameter computes a cross-sectional area of 0.200 square inches, and makes the engineering stress in psi (pounds per square inch) numerically five times the force in pounds. If you can resolve the graph being plotted by the test machine, you will note that this sample exhibits a sharp change in slope (yield point) as plastic deformation begins.

    Watching this video (several times as needed) 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.

1.2.3 Hardness   - Reading: The Saylor Foundation’s “Hardness” Link: The Saylor Foundation’s “Hardness” (PDF)

 Instructions: Please read this short article, which introduces
hardness.
  • Reading: Gordon England’s “Hardness Testing” Link: Gordon England’s “Hardness Testing” (HTML)

    Instructions: This is a nice reference page that provides the mathematical definitions of the scales mentioned in this section, plus identification of some other hardness measurements. Read the definition of hardness, explore some of the links on different types of hardness test methods, and explore some of the links on conversion charts and tables. It may be helpful to use this website as a reference throughout the course.

    Reading this webpage should take approximately 1 hour.

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

1.3 Fast Fracture   Please note the following information on fast fracture:

  • A critical combination of stress and crack length can result in cracks becoming unstable, propagating with the speed of sound to snap the product to failure. This occurs even though the nominal level of stress is less than the yield stress.
  • Micro cracks may be a product of processing, especially in large welded structures.
  • Micro cracks may also nucleate during service, especially during repetitive loading situations or over extended periods of time.
  • Materials resistant to fast fracture are referred to as tough.
  • Fast fracture is particularly a concern at low temperatures. Materials that are ductile at room temperature may become brittle at cold temperatures.

1.3.1 Energy Criterion   A sample with an interior crack has a higher internal energy than a sample without a crack, because of the surface energy of the crack surfaces. If this were the only concern, then cracks should be self-healing to lower the total internal energy. In practice, cracks in unstressed materials do not heal, but remain at a stable length. However, if a crack grows into material that is elastically stressed, the crack lowers the stored elastic energy in the surrounding material. This favors crack growth. Thus, we have a competition of two effects. The result of this competition can have two alternative interpretations:

  • At a given stress (stored elastic energy level), there is a critical crack length. Shorter cracks are stable; longer cracks propagate to fracture.
  • For a given crack length, there is a critical stress. At lower stresses, the crack is stable; at higher stresses, the crack propagates to fracture.

1.3.2 Stress Intensity Factor   - Reading: The Saylor Foundation’s “Stress Intensity Factor” Link: The Saylor Foundation’s “Stress Intensity Factor” (PDF)
 
Instructions: Please read this short article, which introduces the stress intensity factor.

1.3.3 Impact Test   - Reading: The Saylor Foundation’s “Impact Tests” Link: The Saylor Foundation’s “Impact Tests” (PDF)
 
Instructions: Please read this short article, which introduces the impact test.

  • Web Media: YouTube: CORE-Materials’ “Cracking and Fracture in Paper” Link: YouTube: CORE-Materials’ “Cracking and Fracture in Paper” (YouTube)

    Instructions: Watch this brief video on critical crack length. The silent video shows fast fracture in a sheet of paper placed in tension.

    Watching this video (several times as needed) 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.

1.4 Fatigue Failure   Please note the following information on fatigue failure:

  • Fatigue failure is fast fracture occurring after many cycles of repetitive stress, often resulting from vibrations during service, such as with an airplane wing.
  • Tensile stresses nucleate, grow, and eventually cause fast fracture of micro cracks.
  • Fatigue failure measurements show considerable scatter.

1.4.1 Fatigue Failure Mechanism   - Reading: The Saylor Foundation’s “Fatigue Failure Mechanism” Link: The Saylor Foundation’s “Fatigue Failure Mechanism” (PDF)

 Instructions: Please read this short article, which introduces
fatigue failure mechanisms.

1.4.2 Rotating Beam Fatigue Test and S-N Charts   A standard documentation of high cycle fatigue failure is the rotating beam fatigue test. A rotating sample is loaded with a torque that causes a surface element to undergo alternating tension and compression as the sample rotates. If a crack has nucleated, the tension portion of the loading causes the crack to grow. The results of many such tests to failure are summarized on a S-N diagram (Stress versus Cycles to Failure). This is a statistical summary. Each test defines a single point. There is typically considerable scatter in the results.

  • Web Media: YouTube: I.Get.It’s “Fatigue Analysis Overview” The Saylor Foundation does not yet have materials for this portion of the course. If you are interested in contributing your content to fill this gap or aware of a resource that could be used here, please submit it here.

    Submit Materials

1.5 Creep Deformation   Please note the following information on creep deformation:

  • Creep is plastic deformation that continues to occur over time.
  • Creep deformation is very slow. Strain rates of 1% in 1000 hours, or even 1% over 10 years, are typical.
  • For most materials, creep does not happen at room temperature, only at elevated temperatures.
  • What constitutes an elevated temperature is determined by a material’s melting or softening temperature.

1.5.1 Conditions for Creep   - Reading: The Saylor Foundation’s “Conditions for Creep” Link: The Saylor Foundation’s “Conditions for Creep” (PDF)

 Instructions: Please read this short article, which introduces the
conditions for creep.

1.5.2 Measurement and Prediction of Creep   - Reading: The Saylor Foundation’s “Measurement and Prediction of Creep” Link: The Saylor Foundation’s “Measurement and Prediction of Creep” (PDF)

 Instructions: Please read this short article, which introduces the
measurement and prediction of creep.
  • Web Media: YouTube: ChuffChuffWoo’s “Creep Failure in Steel” Link: YouTube: ChuffChuffWoo’s “Creep Failure in Steel” (YouTube)

    Instructions: Watch this video, which demonstrates that creep is distinct from melting. The video not only demonstrates creep deformation of steel, but also that valuable experimentation does not always require a laboratory of expensive instrumentation. It is presented as an explanation of what caused the Twin Towers to collapse on 9/11.

    Watching this video and pausing to take notes should take approximately 30 minutes.

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

1.6 Corrosion   Please note the following information on corrosion:

  • Corrosion is primarily a concern with metals. Plastics and ceramics are essentially inert in our common atmosphere.
  • Virtually all metals oxidize. However, some oxides are protective, while others are not. Our concern, then, is the nature of the oxide layer that forms.
  • Wet corrosion, also known as electrochemical attack, requires two dissimilar materials. These may be two different chemistries, but many other common situations can establish differences.

1.6.1 Oxidation   - Reading: The Saylor Foundation’s “Oxidation” Link: The Saylor Foundation’s “Oxidation” (PDF)

 Instructions: Please read this short article, which introduces
oxidation.

1.6.2 Wet Corrosion   - Reading: The Saylor Foundation’s “Wet Corrosion” Link: The Saylor Foundation’s “Wet Corrosion” (PDF)

 Instructions: Please read this article, which introduces wet
corrosion.
  • Web Media: YouTube: J Co Review’s “MCAT Gen Chem 7.5: Galvanic Cells” Link: YouTube: J Co Review’s “MCAT Gen Chem 7.5: Galvanic Cells” (YouTube)

    Instructions: Watch this video on electrochemical cells.

    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.

  • Web Media: NDT Resource Center’s “Mechanical Properties” Link: NDT Resource Center’s “Mechanical Properties” (HTML)

    Instructions: Please use this resource as a reference for the topics in this unit. This is a fairly comprehensive resource, not only for the material of this unit but for advanced topics that materials science majors would study. It is a site under development. At the moment, not all of the table of content headings link to their intended webpage. NDT stands for non-destructive testing. Use this website to look up additional information as needed.

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

1.7 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.