Semiconductor

Engineering 45





Material

Failure (2)

Bruce Mayer, PE

Registered Electrical & Mechanical Engineer

BMayer@ChabotCollege.edu



Engineering-45: Materials of Engineering Bruce Mayer, PE

1 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

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



Engineering-45: Materials of Engineering Bruce Mayer, PE

2 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

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







Engineering-45: Materials of Engineering Bruce Mayer, PE

3 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

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

Engineering-45: Materials of Engineering Bruce Mayer, PE

4 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Fatigue Failure

 Caused by Load-

Cycling at <y

 Brittle-Like Fracture

with Little Warning

by Plastic

Deformation

• May take Millions of

Cycles to Failure 1. Crack Initiation Site(s)

2. “Beach Marks” Indicate of

 Fatigue Failure Crack Growth

Time-Stages 3. Distinct Final

Fracture Region

Engineering-45: Materials of Engineering Bruce Mayer, PE

5 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Fatigue Parameters

 Recall Fatigue Testing (RR Moore Tester)

specimen compression on top



motor counter

flex coupling

 m   max   min  2

tension on bottom

 S   max   min  2

 Stress Varies with Time; max

Key Parameters m S

• m  Mean Stress (MPa)  time

min

• S  Stress Amplitude (MPa)

 Failure Even though  Cause of ~90% of

max < c Mech Failures

Engineering-45: Materials of Engineering Bruce Mayer, PE

6 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

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

Engineering-45: Materials of Engineering Bruce Mayer, PE

7 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Fatigue Design Parameter

S = stress amplitude

 Fatigue (Endurance) case for

unsafe steel (typ.)

Limit, Sfat in MPa

• Unlimited Cycles if Sfat

safe

S < Sfat

103 105 107 109

N = Cycles to failure



 Some Materials will S = stress amplitude

case for

NOT permit unsafe Al (typ.)

Limitless Cycling

• i.e.; Sfat = ZERO safe



103 105 107 109

N = Cycles to failure

Engineering-45: Materials of Engineering Bruce Mayer, PE

8 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Factigue Crack Growth

 Fatigue Cracks Grow INCREMENTALLY

during the TENSION part of the Cycle

 Math Model for Incremental Crack Extension



typ. 1 to 6

da

 K

m

  K I ~   a

dN Opening-Mode (Mode-I) Stress Intensity Factor



increase in crack length per loading cycle



 Example: Austenitic Stainless Steel

da

dN

m / cyc  5.6 10  K MPa m

12

   3.25





Engineering-45: Materials of Engineering Bruce Mayer, PE

9 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Improving Fatigue Performance

S = stress amplitude



1. Impose a

Compressive

moderate compressive, m

near zero ortensile,

Surface Stress (to

larger tensile, m

m

Suppress Surface

N = Cycles to failure

cracks from growing)

• Method 1: shot peening • Method 2: carburizing (interstitial)

shot

C-rich gas

put

surface

into

compression







2. Remove bad better

Stress-Concentrating

sharp corners bad better

Engineering-45: Materials of Engineering Bruce Mayer, PE

10 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

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)

Engineering-45: Materials of Engineering Bruce Mayer, PE

11 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

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  Stage-1 → Primary

measured over a • a period of primarily

period of time transient creep. During

this period deformation

• Typical Metallic takes place, and Strain

Dynamic Strain at Hardening Occurs

Upper-Right

Engineering-45: Materials of Engineering Bruce Mayer, PE

12 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Creep: ε vs t Behavior cont.1

 Stage-II → Steady

State Creep

• a.k.a. Secondary

Creep

• Creep Rate, dε/dt is

approximately

Constant • a reduction in cross

• Strain-Hardening sectional area due to

and RECOVERY necking, or effective

Roughly Balance reduction in area due to

internal void formation

 Stage-III → • Creep Fracture is often

Tertiary Creep called “Rupture”

Engineering-45: Materials of Engineering Bruce Mayer, PE

13 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

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

d  Qc 

Math Model   s  K 2 exp  

 n



• Where

dt s  RT 

– K2  A Material- – Qc  The Activation

Dependent Constant Energy for Creep

– σ  The Applied Stress – R  The Gas Constant

– n  A Material – T  The Absolute

Dependent Constant Temperature



Engineering-45: Materials of Engineering Bruce Mayer, PE

14 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Creep Failure  Estimate Rupture Time

• S590 Iron, T = 800 °C,

 Occurs Along Grain σ = 20 Ksi

Boundaries

100

g.b. cavities









Stress, ksi

20

10

applied

data for

stress S-590 Iron

1

 The Time-to-Rupture 12 16 20 24 28

L(10 3K-log hr) 24x103 K-log hr

Power-Law Model

T(20  log t r )  L T(20  log t r )  L

temperature function of 1073K

applied stress

time to failure (rupture) Ans: tr = 233hr

Engineering-45: Materials of Engineering Bruce Mayer, PE

15 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

P

WhiteBoard Work

 Problem 8.17

• Ø 0.60” 2014-T6 Al Round bar

• Cyclic Axial Loading in Al σm =

2014-T6 5 ksi

Tension-Compression

• Design Life, N = 108 Cycles 0.60”

• σmean = 5 ksi

• S-N per Fig 8.34

 Find Loads: Pmax, Pmin

• See NEXT Slide P

Engineering-45: Materials of Engineering Bruce Mayer, PE

16 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

S-N Data for 2014-T6 Al









19.5 ksi









Engineering-45: Materials of Engineering Bruce Mayer, PE

17 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Engineering-45: Materials of Engineering Bruce Mayer, PE

18 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Engineering-45: Materials of Engineering Bruce Mayer, PE

19 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

Creep Test Instrument

Engineering-45: Materials of Engineering Bruce Mayer, PE

20 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt