Materials Engineering – Day 2
• Deriving Information from Stress Strain curve of a
ductile metal. We will do an exercise.
• Review ductility and brittleness. Contrast in
terms of fracture appearance.
• Remove a common source of confusion. Stiffness
is one thing and strength is another.
• The Concept of Hardness. Discussion of Hardness
testing.
• A sad fact. Performance on the tension test is
one thing. But sometimes surprising things
happen in the service environment.
Exercise
• The exercise is a sample quiz question. Please
plan to participate.
• We will be comparing answers as we go.
Ductile vs. Brittle Behavior
• Let us look at some pictures of failure in metal.
Here they are.
• Notes on Brittle
1. Gross deformation is not great
2. Failure is by cleavage mechanism
3. Fracture surface is faceted and shiny
• Notes on Ductile
1. Gross deformation is visible
2. Failure is by microvoid coalescence
3. Fracture surface is dimpled and dull
Test: What type of fracture is this?
Difference between Stiffness and
Strength
• Material A has E = 10x106 psi and UTS = 50 ksi
• Material B has E = 30x106 psi and UTS = 35 ksi
WHICH IS STRONGER?
WHICH IS STIFFER?
Summary
• Stress and strain: These are size-independent
measures of load and displacement, respectively.
• Elastic behavior: This reversible behavior often
shows a linear relation between stress and strain.
To minimize deformation, select a material with a
large elastic modulus (E or G).
• Plastic behavior: This permanent deformation
behavior occurs when the tensile (or compressive)
uniaxial stress reaches sy.
• Toughness: The energy needed to break a unit
volume of material.
• Ductility: The plastic strain at failure.
6
Hardness and Hardness Testing
• What’s the problem with the tension test?
1. Expensive & complex
2. Destructive
3. Has to be done in the lab
4. Takes a while
• This leads to a more simple characterization of
material behavior, through the test for hardness.
Hardness is resistance to indentation by a hard
object like a needle or ball.
Hardness
• Resistance to permanently indenting the surface.
• Large hardness means:
--resistance to plastic deformation or cracking in
compression.
--better wear properties.
apply known force
measure size
of indent after
removing load
e.g.,
10 mm sphere
d
D
most
plastics
brasses
Al alloys
easy to machine
steels
file hard
Smaller indents
mean larger
hardness.
cutting
tools
nitrided
steels
diamond
increasing hardness
8
Hardness Testing Methods
• Rockwell. 1) Apply preload. 2) Add main load and
measure depth. 3) Remove main load and
measure depth.
Difference in depths is read out as a hardness.
Machine for
Testing Hardness
(specimen placed
On circular stage)
Hardness: Resistance to Penetration/
Indentation/ Scratching
Advantages




Disadvantages
can predict strength, wear

resistance
inexpensive, easy

relatively nondestructive
common designer specification
strength prediction only
quantitative for hard steel
predictions are qualitative (no
design numbers)
Kinds of Hardness Tests
• Rockwell A, B, and C. B is soft metals, C is for
hard metals. Eg. Steel knife blade RC about 60.
Crankshaft RC about 45. Values below 20 and
above 100 RC are not reliable. Also superficial.
• Brinell Hardness. Oldest measure. A steel (HBS)
or tungsten (HBW) balls are used as indenters.
You measure the diameter of the indentation and
use a formula to get HB. UTS (psi) = 500 x BHN
• Vickers. A diamond indenter is used. A very wide
ranging scale.
• Here is a comparison or more
Hardness: Measurement
Table 6.5
13
Hardness vs. Strength
Correlate depending on metal
UTS
Cast Iron
Steel: UTS = 500 BHN
BHN