1. Registered Electrical & Mechanical Engineer BMayer@ChabotCollege.edu
Engineering 45
Material Failure (2)
Bruce Mayer, PE
Registered Electrical & Mechanical Engineer

2. 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

3. 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

4. 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

5. Fatigue Failure Caused by Load-Cycling at <y
Brittle-Like Fracture with Little Warning by Plastic Deformation
May take Millions of Cycles to Failure
Crack Initiation Site(s)
“Beach Marks” Indicate of Crack Growth
Distinct Final Fracture Region
Show Dean Severud Failure Specimen
Fatigue Failure Time-Stages

6. Fatigue Parameters Recall Fatigue Testing (RR Moore Tester)
tension on bottom
compression on top
counter
motor
flex coupling
bearing
specimen
Stress Varies with Time; Key Parameters
m  Mean Stress (MPa)
S  Stress Amplitude (MPa) s
max
min
time m S
Failure Even though max < c
Cause of ~90% of Mech Failures

7. 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

8. Fatigue Design Parameter
case for
steel
(typ.)
N = Cycles to failure 10 3 5 7 9
unsafe
safe
S = stress amplitude
Fatigue (Endurance) Limit, Sfat in MPa
Unlimited Cycles if S < Sfat
Some Materials will NOT permit Limitless Cycling
i.e.; Sfat = ZERO
case for Al
(typ.)
N = Cycles to failure 10 3 5 7 9
unsafe
safe
S = stress amplitude

9. Factigue Crack Growth
Fatigue Cracks Grow INCREMENTALLY during the TENSION part of the Cycle
Math Model for Incremental Crack Extension
typ. 1 to 6
Opening-Mode (Mode-I) Stress Intensity Factor
increase in crack length per loading cycle
Example: Austenitic Stainless Steel

10. Improving Fatigue Performance
N = Cycles to failure
moderate tensile, m
larger tensile, m
S = stress amplitude
near zero or compressive, m
Impose a Compressive Surface Stress (to Suppress Surface cracks from growing)
Method 1: shot peening
Method 2: carburizing (interstitial)
Sigma,m = Mean Stress
Carbon is an interstitial that puts compressive strain into the lattice
Remove Stress-Concentrating sharp corners
bad
better
bad
better

11. 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)

12. Creep: ε 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
Stage-1 → Primary
a period of primarily transient creep. During this period deformation takes place, and Strain Hardening Occurs

13. Creep: ε 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
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”

14. 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
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
A Diffusion-Like Mechanism

15. Creep Failure Estimate Rupture Time Occurs Along Grain Boundaries
S590 Iron, T = 800 °C, σ = 20 Ksi
Occurs Along Grain Boundaries
L(10 3
K-log hr)
Stress, ksi
100 10 1 12 20 24 28 16
data for
S-590 Iron 2
applied
stress
g.b. cavities
24x103 K-log hr
The Time-to-Rupture Power-Law Model
G.B. creep is called “Coble” creep; see also Nabarro-Herring creep
time to failure (rupture)
function of
applied stress
temperature
1073K
Ans: tr = 233hr

16. WhiteBoard Work P Problem 8.17 Al 2014-T6 Find Loads: Pmax, Pmin 0.60”
σm = 5 ksi
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

17. S-N Data for 2014-T6 Al
19.5 ksi

20. Creep Test Instrument