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Flashcards in Mechanical Deck (65)
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1

Deform

change shape

2

Fail

break

3

Mechanical Properties

Stiffness
Strength
Toughness

4

Materials can gradually degrade and fail over time as a result of

Wear
Creep
Fatigue

5

The Tensile Test

-Take sample of material
-Pull ends to stretch it
-Measure force F
-Measure stretch L-Lₒ

-know diagram

6

Tensile Test practicalities

-any size, but parallel sides
-cross section same throughout
-can make ends of specimen bigger so its easy to grip in testing machine

7

Stress/Strain curve

-to see how sample stretches for given applied force
-x-axis: stress
-y-axis: strain

-X on curve indicates point which sample breaks

8

Stress formula

σ = Force/Area

N/m² or Pa

9

Strain formula

ε = (L-Lₒ)/Lₒ

No units

10

Cross sectional area

For rectangular A = width x thickness

For circular, A = πr²

11

Types of stress

Tensile stress (pulling)
Compression (pushing)
Shear
Pressure

12

Compression and Tensile stress

Pushing is still known as tensile stress, but has negative value

13

Shear

A type of stress that causes sliding

14

Shear stress formula

τ = Force/Area

15

Shear Strain formula

γ = L/Lₒ = Tanθ

16

Pressure

created by having same force acting in all directions eg. hydrostatic pressure underwater

17

Pressure formula

P = F/A, same as tension but written as positive when compressive

18

Strain due to pressure

A change in volume, called dilatation

19

Dilatation formula

△=-(V-Vₒ)/Vₒ

20

Any type of stress

can be expressed as a mixture of these three:
tension, shear, pressure

21

Stress/Strain curve: First Stage

-Elastic Deformation
-When line is straight at start
-material behaves like spring
-remove stress, strain goes back to zero
-stress ∝ strain

22

Stiffness

-Slope of stress/strain curve in elastic region
-called Young's Modulus (or elastic modulus), E
-if line is straight E=stress/strain at any point in line ie E = σ/ε

-we measure E when applied stress is tensile, most materials have same E value in compression as in tension

23

Yield Strength and Plastic Deformation

Above certain stress, σᵧ, line becomes flatter and curved,

24

σᵧ

This point where line stops being straight is called yield stress or yield strength of the material

25

Deformation can be

Temporary (elasticity)
Permanent (plasticity)

26

Bending

Creates tension on one surface, compression on the other, no stress or strain in middle

27

Common types of bending

-Cantilever Bending
-Three-point bending
-Four-point bending

diagrams in lecture 2 slides

28

Testing for bending

We test for deflection (d) of the loading point as a function of the applied force F

29

Bending Equations

will be given on exam, know what the letters stand for

30

Other Types of Stiffness

-If material loaded in shear or with a pressure, different elastic modulii can be calculated:
Shear Modulus (G)
Bulk Modulus (k)