me 370/570 materials science and engineering-i chapter

1Based on Materials Science and Engineering An Introduction, W.D. Callister, Jr. 5th edition, 2000, Wiley

This material is for educational used only within Wright State University

ME 370/570Materials Science and Engineering-I

Chapter VIII

• Instructor: Dr. R. Srinivasan• Mechanical and Materials

Engineering Dept.



Failure in Materials

• If we know what causes failure, we can:– Develop better materials– Design better structures and components to avoid

failure

• Failure of a material can be due to:– Excessive elastic deformation– Excessive plastic deformation– Fracture

• Sections to be covered– 8.1 – 8.4, 8.6 – 8.9, 8.13 – 8.14



•Ductile materials undergo plastic deformation before failure•In a tension test, this means necking before fracture•The fracture surface may be a point•The fracture surface be dimpled

BrittleDuctile



Fracture surface appears dull, fibrous, or dimpled



Brittle metals exhibit a flat failure surface

Ductile metals exhibit a “cup and cone” failure surface

The type of loading that caused failure can be identified from the appearance of the failure surface. – Failure Analysis ME 470/670



Uniaxial Tension Shear loading



Brittle failure



Intergranular

Brittle Failure Transgranular



Ductile and Brittle Failure

• Ductile failure– Extensive plastic

deformation– Large amount of

energy is absorbed prior to failure

– Surface appears dull, fibrous or dimpled

– Occurs after prior warning

• Brittle failure– Little or no plastic

deformation– Energy absorbed is

small– Surface appears shiny,

granular or faceted– Catastrophic/sudden



Charpy

Izod

Brittle FailureContributing factors

•Low Temperature•High deformation rate•Notches

•(triaxial state of stress)

Impact testA heavy pendulum is allowed to drop from a predetermined heightEnergy absorbed by the specimen is measured



More energy is absorbed when there is more plastic deformation (shear deformation)



Brittle at low T Ductile at high T



Fatigue Failure• Fatigue loading is a load that fluctuates with time• Fatigue failure is failure under fatigue loading

– Occurs after a large number of cycles of loading– Occurs at stress levels that may be below the yield

stress when no plastic deformation is expected– Occurs without warning and without gross plastic

deformation; seemingly brittle failure– Characteristic failure surface

• Responsible for 90% of service failures of metallic components



Origin

Beach marks

Final rupture

Ductile failure



During service the load on a component changes with time

Could be cyclic, as in a rotating shaft or random as with a shock-absorber or an airplane wing.



Laboratory tests typically bend a sample back and forth to subject the sample to alternating tension and compression



S-N Curve

N in a log scale

Plain carbon steelsBeta-titanium alloysBCC structure

Aluminum, copper, nickel base alloys

Specimens break after a certain number of cycles (N) at an applied stress level (S)



The SN curve shows scatter

1% fail

99% fail

Depending on the criticality of the component being designed one can choose the appropriate curve



•Cracks start on the surface or at discontinuities where stress is high•The crack initially propagate on a plane of high stress (45° to tensile stress axis (Stage I)•Propagation then switches to a plane of high tensile stress (Stage II)



Beach marks or striations are a result of the Stage II crack propagation during each cycle



A superimposed mean stress shifts SN curve down to lower alternating stress values and shorter life



Designing for fatigue involves•Avoiding stress concentrations

•Notches, machining marks, keyways, sharp corners etc.



Designing for fatigue involves•Hardening the surface by cold working (shot peening)



Designing for fatigue involves•Hardening the surface by case hardening

•Diffusing carbon or nitrogen into the surface of a low carbon steel



Creep Deformation• Creep is time dependent deformation under

constant load.• Occurs at high temperatures (>0.4 Tmp)• The material does not strain harden• Recovery and recrystallization can occur

under load. Therefore continuous deformation

• Often seen in turbine blades, rocket engines, etc.



Need to minimize creep deformation in this direction



Material selection for creep loading

• Materials with a high melting point• Solid solution strengthening• Strengthening by distributing fine particles

– Precipitation strengthening– Dispersion strengthening

• Crystallographic orientation

• The idea is to prevent easy movement of dislocations

me 370/570 materials science and engineering-i chapter

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