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Engineering 45. Material Failure (2). Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege.edu. Learning Goals.1 – Failure. How Flaws In A Material Initiate Failure How Fracture Resistance is Quantified How Different Material Classes Compare

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Presentation Transcript
slide1

Engineering 45

MaterialFailure (2)

Bruce Mayer, PE

Registered Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

learning goals 1 failure
Learning Goals.1 – Failure
  • How Flaws In A Material Initiate Failure
  • How Fracture Resistance is Quantified
    • How Different Material Classes Compare
  • How to Estimate The Stress To Fracture
  • Factors that Change the Failure Stress
    • Loading Rate
    • Loading History
    • Temperature

Last Time

learning goals 2 failure
Learning Goals.2 – Failure
  • FATIGUE Failure
    • Fatigue Limit
    • Fatigue Strength
    • Fatigue Life
  • CREEP at Elevated Temperatures
    • Incremental Yielding at <y Over a Long Time Period at High Temperatures
fatigue defined
Fatigue Defined
  • ASTM E206-72 Definition

The Process of PROGRESSIVE LOCALIZED PERMANENT Structural Change Occurring in a Material Subjected to Conditions Which Produce FLUCTUATING Stresses and Strains at Some Point or Points Which May Culminate in CRACKS or Complete FRACTURE After a Sufficient Number of Fluctuations

fatigue failure
Caused by Load-Cycling at <y

Brittle-Like Fracture with Little Warning by Plastic Deformation

May take Millions of Cycles to Failure

Fatigue Failure
  • Crack Initiation Site(s)
  • “Beach Marks” Indicate of Crack Growth
  • Distinct Final Fracture Region
  • Fatigue Failure Time-Stages
fatigue parameters
Recall Fatigue Testing (RR Moore Tester)

compression on top

specimen

counter

motor

bearing

bearing

flex coupling

tension on bottom

s

s

max

S

s

m

s

time

min

Fatigue Parameters
  • Stress Varies with Time; Key Parameters
    • m  Mean Stress (MPa)
    • S  Stress Amplitude (MPa)
  • Failure Even thoughmax < c
  • Cause of ~90% of Mech Failures
more fatigue parameters
More Fatigue Parameters
  • σmax = maximum stress in the cycle
  • σmin = minimum stress in the cycle
  • σm = mean stress in the cycle = (σmax + σmin)/2
  • σa = stress amplitude = (σmax - σmin)/2
  • Δσ = stress range = σmax - σmin = 2σa
  • R = stress ratio = σmax/σmin
fatigue design parameter
Fatigue (Endurance) Limit, Sfat in MPa

Unlimited Cycles if S < Sfat

S = stress amplitude

case for

unsafe

steel

(typ.)

S

fat

safe

3

5

7

9

10

10

10

10

N = Cycles to failure

S = stress amplitude

case for

Al

(typ.)

unsafe

safe

3

5

7

9

10

10

10

10

N = Cycles to failure

Fatigue Design Parameter
  • Some Materials will NOT permit Limitless Cycling
    • i.e.; Sfat = ZERO
factigue crack growth
Fatigue Cracks Grow INCREMENTALLY during the TENSION part of the Cycle

Math Model for Incremental Crack Extension

typ. 1 to 6

increase in crack length per loading cycle

Factigue Crack Growth

Opening-Mode (Mode-I) Stress Intensity Factor

  • Example: Austenitic Stainless Steel
improving fatigue performance
Impose a Compressive Surface Stress (to Suppress Surface cracks from growing)

S = stress amplitude

near zero or compressive, m

moderate tensile, m

larger tensile, m

N = Cycles to failure

Improving Fatigue Performance
  • Method 1: shot peening
  • Method 2: carburizing (interstitial)
  • Remove Stress-Concentrating sharp corners

better

bad

bad

better

creep deformation
Creep Deformation
  • Creep Defined

HIGH TEMPERATURE PROGRESSIVE DEFORMATION of a material at constant stress.  High temperature is a relative term that is dependent on the material(s) being evaluated.

  • For Metals, Creep Becomes important at Temperatures of About 40% of the Absolute Melting Temperature (0.4Tm)
creep vs t behavior
In a creep test a constant load is applied to a tensile specimen maintained at a constant temp. Strain is then measured over a period of time

Typical Metallic Dynamic Strain at Upper-Right

Creep: ε vs t Behavior
  • Stage-1 → Primary
    • a period of primarily transient creep. During this period deformation takes place, and StrainHardening Occurs
creep vs t behavior cont 1
Stage-II → Steady State Creep

a.k.a. Secondary Creep

Creep Rate, dε/dt is approximately Constant

Strain-Hardening and RECOVERY Roughly Balance

Stage-III → Tertiary Creep

Creep: εvs t Behavior cont.1
  • a reduction in cross sectional area due to necking, or effective reduction in area due to internal void formation
  • Creep Fracture is often called “Rupture”
secondary creep
Most of Material Life Occurs in this Stage

Strain-Rate is about Constant for Given T & σ

Work-Hardening Balanced by Recovery

The Math Model

Secondary Creep
  • Where
    • K2  A Material-Dependent Constant
    • σ The Applied Stress
    • n  A Material Dependent Constant
  • Qc  The Activation Energy for Creep
  • R  The Gas Constant
  • T  The Absolute Temperature
creep failure
Occurs Along Grain Boundaries

g.b. cavities

100

2

0

applied

stress

Stress, ksi

10

data for

S-590 Iron

1

12

16

20

24

28

3

24x103 K-log hr

L(10

K-log hr)

temperature

function of

applied stress

1073K

Ans: tr = 233hr

time to failure (rupture)

Creep Failure
  • Estimate Rupture Time
    • S590 Iron, T = 800 °C, σ= 20 Ksi
  • The Time-to-Rupture Power-Law Model
whiteboard work

P

Al2014-T6

σm =5 ksi

0.60”

P

WhiteBoard Work
  • Problem 8.17
    • Ø 0.60” 2014-T6 Al Round bar
    • Cyclic Axial Loading in Tension-Compression
    • Design Life, N = 108 Cycles
    • σmean = 5 ksi
    • S-N per Fig 8.34
  • Find Loads: Pmax, Pmin
    • See NEXT Slide