Yielding and Failure Criteria - PowerPoint PPT Presentation

Loading...

PPT – Yielding and Failure Criteria PowerPoint presentation | free to download - id: 68a653-NDhlM



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Yielding and Failure Criteria

Description:

Mechanics of Materials Lab Yielding and Failure Criteria Plasticity Fracture Fatigue Jiangyu Li University of Washington – PowerPoint PPT presentation

Number of Views:4
Avg rating:3.0/5.0
Date added: 3 July 2019
Slides: 49
Provided by: jiangyu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Yielding and Failure Criteria


1
Mechanics of Materials Lab
  • Yielding and Failure Criteria
  • Plasticity
  • Fracture
  • Fatigue
  • Jiangyu Li
  • University of Washington

2
Failure Criteria
  • Materials have flaw or crack in them
  • Linear Elastic Fracture Mechanics (LEFM)
  • Stress intensity factor (K) describes the
    severity of the existing crack condition
  • If K exceeds the Critical stress intensity (Kc),
    then failure will occur
  • Materials Assumed to be perfect
  • Brittle Materials
  • Max Normal Stress
  • Ductile Materials
  • Max Shear Stress
  • Octahedral Shear Stress

3
Maximum Normal Stress Fracture Criterion
4
Octahedral Shear Stress Criterion
5
Safety Factor and Load Factor
  • 7. 32 A circular bar must support a axial loading
    of 200 kN and a torque of 1.5 kN.m. Its yield
    strength is 260 MPa.
  • What diameter is needed if load factors YP1.6
    and YT2.5 are required.

6
Stress Strain Curve
Bauschinger Effect
7
Elastic-Perfect Plastic and Linear Hardening
8
Power Hardening and Ramberg-Osgood Relation
9
Secant Modulus
10
Stress-Strain Curve
11
Displacement Mode
Sliding mode
Opening mode
Tearing mode
12
Stress Concentration
13
Stress Intensity Factor Tension
14
Stress Intensity Factor Bending
15
Stress Intensity Factor Circumferential Crack
-
16
Stress Intensity Factor
17
Superposition
18
Brittle vs. Ductile Behavior
19
Plastic Zone
20
Limitation of LEFM
21
Effect of Thickness
22
Correlation with Strength
23
(No Transcript)
24
Energy Release Rate
25
Strain Energy
Increasing the strain rate increase strength, but
decrease ductility
Modulus of toughness modulus of resilience
26
Impact Test
  • Charpy V-notch Izod tests most common
  • Energy calculated by pendulum height difference
  • Charpy metals, Izod - plastics

27
Trend in Impact Behavior
  • Toughness is generally proportional to ductility
  • Also dependent on strength, but not so strongly
  • Brittle Fractures
  • Lower energy
  • Generally smooth in appearance
  • Ductile Fracture
  • Higher energy
  • Rougher appearance on interior with 45 shear
    lips

28
Effect of Temperature
Decrease temperature increase strength, but
decrease ductility
29
Ductile-Brittle Transition
30
Static Failure
  • Load is applied gradually
  • Stress is applied only once
  • Visible warning before failure

31
Cyclic Load and Fatigue Failure
  • Stress varies or fluctuates, and is repeated many
    times
  • Structure members fail under the repeated
    stresses
  • Actual maximum stress is well below the ultimate
    strength of material, often even below yield
    strength
  • Fatigue failure gives no visible warning, unlike
    static failure. It is sudden and catastrophic!

32
Characteristics
  • Primary design criterion in rotating parts.
  • Fatigue as a name for the phenomenon based on the
    notion of a material becoming tired, i.e.
    failing at less than its nominal strength.
  • Cyclical strain (stress) leads to fatigue
    failure.
  • Occurs in metals and polymers but rarely in
    ceramics.
  • Also an issue for static parts, e.g. bridges.
  • Cyclic loading stress limitltstatic stress
    capability.

33
Characteristics
  • Most applications of structural materials involve
    cyclic loading any net tensile stress leads to
    fatigue.
  • Fatigue failure surfaces have three
    characteristic features
  • A (near-)surface defect as the origin of the
    crack
  • Striations corresponding to slow, intermittent
    crack growth
  • Dull, fibrous brittle fracture surface (rapid
    growth).
  • Life of structural components generally limited
    by cyclic loading, not static strength.
  • Most environmental factors shorten life.

34
Fatigue Failure Feature
  • Flat facture surface, normal to stress axis, no
    necking
  • Stage one initiation of microcracks
  • Stage two progress from microcracks to
    macrocracks, forming parallel plateau-like
    facture feature (beach marks) separated by
    longitudinal ridge
  • Stage three final cycle, sudden, fast fracture.

Bolt, unidirectional bending
35
Fatigue-Life Method
  • Stress-life method
  • Facture mechanics method

36
Alternating Stress
?a (?max-?min)/2 ?m (?max?min)/2
37
S-N Diagram
sa
The greater the number ofcycles in the loading
history,the smaller the stress thatthe material
can withstandwithout failure.
smean 3 gt smean 2 gt smean 1
smean 1
smean 2
smean 3
log Nf
Note the presence of afatigue limit in
manysteels and its absencein aluminum alloys.
38
S-N Diagram
Endurance limit
39
Safety Factor
40
Facture Mechanics Method of Fatigue
41
Crack Growth
gt
gt
42
Fatigue Life
43
Crack Growth Rate
44
Fatigue Failure Criteria
45
Effect of Mean Stress
46
Fatigue Failure Criteria
Multiply the stress By safety factor n
47
Example Gerber Line
AISI 1050 cold-drawn bar, withstand a fluctuating
axial load varying from 0 to16 kip. Kf1.85 Find
Sa and Sm and the safety factor using Gerber
relation Sut100kpsi Sy84kpsi Se0.504Sut
kpsi
Table 7-10
2
Change over
1
3
48
Safety Factor with Mean Stress
About PowerShow.com