Chapter 3 – Mechanical Properties of Material Recall, for design and analysis, material properties are needed!! Why?? Design: sallow = sfailure/F.S. - select geometry to satisfy this! Analysis: Want s < sallow – analyze structure to make sure this is true!! Both cases need sfailure – what is sfailure and how is it determined??? Also, how much does it deform and strain under load?? MATERIAL PROPERTIES!!

Both cases need sfailure – what is sfailure and how is it determined??? First, sfailure can be either: sY = Yield strength – stress level at which material yields (i.e. permanent deformation. OR su = Ultimate strength – stress level at which material fails (i.e. catastrophic failure) Next, where do they come from??

ANSWER: The Tension Test: Figure: 03-02

Round material specimen: Figure: 03-01-01UN

Figure: 03-04

Ductile Fracture: Steps to fracture: Linear, yield, strain hardening, necking then fracture!! Figure: 03-04-01UN

Example: find yield strength and ultimate Figure: 03-06

Example: find yield strength and ultimate for material that does not exhibit knee behavior Figure: 03-07

Hooke’s Law (key material property = Young’s Modulus: Slope of linear portion of stress-strain curve. Measure of material stiffness E = s/e s = E * e What is E for the above curves?? How is it different for the steel curve vs the aluminum?? Known as Hooke’s Law

All these curves are steels. What do they have in common? What is different? Figure: 03-13

All these curves are steels. What do they have in common? What is different? Figure: 03-17

Other properties determined from stress strain curve: Figure: 03-16a

Strain Hardening (i.e. cold working): Figure: 03-14a

More Examples of Stress-Strain Curves

Rubber – VERY ELASTIC!!!! Figure: 03-08

Cast Iron – Ductile or Brittle?? Figure: 03-09

Figure: 03-10a

Concrete: Ductile or Brittle?? Figure: 03-11

Plastic – highly non-linear behavior, why?? Figure: 03-12

Example: Find yield, tensile, E: Figure: 03-18Ex3.01

Figure: 03-20-05P3.8

EX: Su = ultimate Strength = 47,820 psi Sy = Yield Strength = 44,200 psi E = Young’s Modulus = (34,640 – 10,597)/(.0036 - .0011) = 9.6 E6 % Elongation = 11.5% .002 = .2% offset

Final Concept: Poisson’s Ratio, n: Figure: 03-21a