Fracture, Toughness, Fatigue, and Creep

1. Fracture, Toughness, Fatigue, and Creep Materials Science & Manufacturing PROCESSES

2. Why Study Failure In order to know the reasons behind the occurrence of failure so that we can prevent failure of products by improving design in the light of failure reasons

3. Mechanical Failure ISSUES TO ADDRESS... • How do flaws in a material initiate failure? • How is fracture resistance quantified; how do different material classes compare? • How do we estimate the stress to fracture? • How do loading rate, loading history, and temperature affect the failure stress? Computer chip-cyclic thermal loading. Hip implant-cyclic loading from walking. Ship-cyclic loading from waves.

4. What is a Fracture? • Fracture is the separation of a body into two or more pieces in response to an imposed stress that is static and at temperatures that are low relative to the melting temperature of the material. • The applied stress may be tensile, compressive, shear, or torsional • Any fracture process involves two steps—crack formation and propagation—in response to an imposed stress.

5. Fracture Modes • Ductile fracture • Occurs with plastic deformation • Material absorbs energy before fracture • Crack is called stable crack: plastic deformation occurs with crack growth. Also, increasing stress is required for crack propagation. • Brittle fracture • Little or no plastic deformation • Material absorb low energy before fracture • Crack is called unstable crack. • Catastrophic

6. Very Moderately Fracture Brittle Ductile Ductile behavior: (%EL)=100% Large Moderate Small Ductile vs Brittle Failure • Classification: • Ductile fracture is usually desirable! Ductile: warning before fracture, as increasing is required for crack growth Brittle: No warning

7. Example: Failure of a Pipe • Brittle failure: --many pieces --small deformation • Ductile failure: --one/two piece(s) --large deformation

8. Moderately Ductile Failure- Cup & Cone Fracture void growth shearing void necking fracture and linkage at surface nucleation s 50 mm 50 mm • Resulting fracture surfaces (steel) 100 mm particles serve as void nucleation sites. • Evolution to failure: crack occurs perpendicular to tensile force applied

9. Ductile vs. Brittle Failure cup-and-cone fracture brittle fracture

10. Transgranular vs Intergranular Fracture Intergranular Fracture Transgranular Fracture

11. Brittle Fracture Surfaces • Transgranular (within grains) • Intergranular (between grains) 304 S. Steel (metal) 316 S. Steel (metal) 160mm 4mm Polypropylene (polymer) Al Oxide (ceramic) 3mm 1mm

12. Stress Concentration- Stress Raisers σm›σo Suppose an internal flaw (crack) already exits in a material and it is assumed to have a shape like a elliptical hole: The maximum stress (σm) occurs at crack tip: where t = radius of curvature so = applied stress sm = stress at crack tip Kt = Stress concentration factor t Theoretical fracture strength is higher than practical one; Why?

13. Concentration of Stress at Crack Tip

14. s o m Stress Conc. Factor, K t = s o w s 2.5 max h r , w/h 2.0 increasing fillet radius 1.5 r/h 1.0 0 0.5 1.0 sharper fillet radius Engineering Fracture Design • Avoid sharp corners! s Kt

15. Crack Propagation Cracks propagate due to sharpness of crack tip • A plastic material deforms at the tip, “blunting” the crack. deformed region brittle Effect of stress raiser is more significant in brittle materials than in ductile materials. When σm exceeds σy , plastic deformation of metal in the region of crack occurs thus blunting crack. However, in brittle material, it does not happen. plastic When σm›σy

16. Fracture Toughness: Design Against Crack Growth Kc= --Result 2: Design stress dictates max. allowable flaw size. --Result 1: Max. flaw size dictates design stress (max allowable stress). amax fracture fracture no no amax fracture fracture • Crack growth condition: • Largest, most stressed cracks grow first! σc σc

17. Fracture Toughness • Brittle materials do not undergo large plastic deformation, so they posses low KICthan ductile ones. • KIC increases with increase in temp and with reduction in grain size if other elements are held constant • KIC reduces with increase in strain rate

18. Design Example: Aircraft Wing • Use... 9 mm --Result: 112 MPa 4 mm Answer: • Material has Kc = 26 MPa-m0.5 • Two designs to consider... Design B --use same material --largest flaw is 4 mm --failure stress = ? Design A --largest flaw is 9 mm --failure stress = 112 MPa • Key point: Y and Kc are the same in both designs. • Reducing flaw size pays off!

19. Impact Tests • A material may have a high tensile strength and yet be unsuitable for shock loading conditions • Impact testing is testing an object's ability to resist high-rate loading. • An impact test is a test for determining the energy absorbed in fracturing a test piece at high velocity • Types of Impact Tests -> Izod test and Charpy Impact test • In these tests a load swings from a given height to strike the specimen, and the energy dissipated in the fracture is measured

20. (Charpy) final height initial height A. Charpy Test Impact energy= Kinetic energy + energy absorbed by specimen Energy absorbed during test is determined from difference of pendulum height

21. b. Izod Test • Izod test varies from charpy in respect of holding of specimen

22. Effect of Temperature on Toughness • Increasing temperature... --increases %EL and Kc • Ductile-to-Brittle Transition Temperature (DBTT)... Low strength FCC metals (e.g., Cu, Ni) Low strength BCC metals (e.g., iron at T < 914°C) polymers Impact Energy Brittle More Ductile s High strength materials ( > E/150) y Temperature Ductile-to-brittle transition temperature

23. Fatigue Test • Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating loads (e.g. bridges, aircrafts, ships and m/c components) • The term Fatigue is used because this type of failure occurs after a lengthy period of repeated stress of strain cycling. • Failure stress in fatigue is normally lower than yield stress under static loading. • Fatigue failure is brittle in nature even in ductile metals • The failure begins with initiation and propagation of cracks

24. Types of Cyclic Stresses

25. Types of Cyclic Stresses Random Stress Cycle

26. Terms Related to Cyclic Stresses Mean stress: Range of stress: Stress Amplitude: Stress Ratio:

27. 4. Creep • Creep is defined as time dependent plastic deformation under constant static load/stress (steam turbines blades under centrifugal force, pipes under steam pressure) at elevated temperatures • At relatively high temperatures creep appears to occur at all stress levels, • But the creep rate increases with increasing stress at a given temperature.

28. 4. Creep Test • A creep test involves a tensile specimen under a Constant Load OR Constant Stress maintained at a constant temperature. • Temperature: Greater than 0.4Tm

29. Stress & Temp Effects on Creep • Time to rupture decreases as imposed stress or temperature increases • Steady creep rate increases with increase of stress and temperature

30. Good Luck