Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer
BMayer@ChabotCollege.edu
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt
Engineering-45: Materials of Engineering
1

Mechanical Testing OutLine
 Yield & Tensile Strength → Tensile Test
 Flexural Strength Bend → Strength Test for BRITTLE Materials
 Hardness → Hardness Test
 Toughness → Impact Test
 Fatigue Life → Rotating Beam Test
Engineering-45: Materials of Engineering
2
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Tensile Test
 A Pull Test Leading to an Engineering
Stress-Strain Plot
• See Last Lecture
Engineering-45: Materials of Engineering
3

 
P
 
L
A o
L o
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Flexural Test – Brittle Materials
 Brittle Materials are Difficult to Grasp without creating CrackInitiation Sites → Hard to
Tensile Test
• Use a flex test
4
 u , flex

3 FL
2 wh
2
Engineering-45: Materials of Engineering
• F 
Load at Fracture
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Hardness Test
 Many Types  Rockwell
Indenters
 Most Common
• Rockwell
• Brinell
Engineering-45: Materials of Engineering
5
 Brinell
Indenter
 Brinell Eqn for HB
HB

 
D

D

P
D
2  d
2

Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt
Brale
Ball

Major Hardness Tests
Engineering-45: Materials of Engineering
6
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Engineering-45: Materials of Engineering
7
Hardness Test
Indenter is pre-loaded (minor
Load) into the component to be measured.
A major load is then applied, and the indenter causes localized plastic deformation.
The size/depth of the indentation (usually 1-2 mm) is measured and is inversely related to the Hardness.
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Rockwell Scales
Engineering-45: Materials of Engineering
8
 As With Most
Hardness
Measurements
Rockwell has NO units
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Steel
Engineering-45: Materials of Engineering
9
Hardness
& Strength
 As Noted Previously
Hardness and
Strength Are Closely
Related
• For STEELS with
 u
 u
HB = 200-350

MPa
 



3 .
45

0 .
5

HB
HB
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Example

Estimate Rc & HB
 Given Naval-Brass
σ-ε Characteristic
Engineering-45: Materials of Engineering
10
 Estimate Rockwell and Brinell Hardness
Values for this
Material
 Plan
• Determine σ u
Material for the
– 450 Mpa Per σ-ε
Behavior
• Consult σ u
Hardness vs
Correlations
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

450
Example

Est. Rc & HB cont.
82
 Consult Fig 6.19 for σ u vs Hardness
1. ID 450 MPa on
Brass Curve
2. Cast Downward for HB
• HB

130
3. Cast Upward for HRB
• HRB

82
130
Engineering-45: Materials of Engineering
11
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Toughness

V-Notch
 Impact Tests are Used to Measure Toughness; the ability to Absorb Energy
Engineering-45: Materials of Engineering
12
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Toughness
 DUCTILE &
STRONG Materials are Toughest
Lo-C Embrittles
IZOD
Engineering-45: Materials of Engineering
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Charpy
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Fatigue Testing
 Fatigue Strength

Ability of a Material to
Resist REPEATED Stress CYCLING
• Destructive Load Cycling is Almost always Tensile
 Rotation of slightly bent
Specimen
Results in
Load Reversals
Engineering-45: Materials of Engineering
14
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Fatigue Behavior
33 ksi load →
2 Mcycle Fatigue-Life
 Endurance Limit

Stress Which allows
Unlimited Cycling
Engineering-45: Materials of Engineering
15
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Variability of Matl Properties
 Material Properties are Statistical Averages
• Actual Values Can Vary 1-2 Standard Deviations
 Use Factor of Safety, N, Based on Working
Stress, σ w and YIELD Strength, σ y
Std Dev, s f
N
  y
Typically
 w
: N

2
 y
(MPa)
Mean , x
Engineering-45: Materials of Engineering
16
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

Factor of Safety Example
• Factor of safety, N
 working

 y
N
Often N is between
1.2 and 4
• Example:
Calculate a diameter, d , to ensure that yield does
NOT occur in the 1045 carbon steel rod below. Use a factor of safety of 5.
 working


N y 1045 plain
 y carbon steel:
= 310 MPa
220 , 000 N

 d
2
/ 4
 5 TS = 565 MPa d = 0.067 m = 6.7 cm F = 220,000N d
Lo
Engineering-45: Materials of Engineering
17
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt

WhiteBoard Work
 Problem 6.22
 Design Criteria
• Round Bar, Ø 15mm x 120mm Long
• Tensile Load = 35 kN
• NO Plastic
Deformation
• Ø o
Ø f
 12 µm
• Available Matls
Material E (Gpa)  y (Mpa) 
(%)
Al Alloy 70 250 33
850 36 Ti Alloy 105
Steel Alloy 205
Mg Alloy 45
550
170
27
20
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-16_Metal_MechProp-3.ppt