ME 370/570 Materials Science and Engineering-I Chapter VIII ...

ME 370/570 Materials Science and Engineering-I Chapter VIII • Instructor: Dr. R. Srinivasan • Mechanical and Materials Engineering Dept.

This material is for educational used only within Wright State University

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

1

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 This material is for educational used only within Wright State University


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Ductile

Brittle

•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



3

Fracture surface appears dull, fibrous, or dimpled



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Ductile metals exhibit a “cup and cone” failure surface

Brittle metals exhibit a flat failure surface

The type of loading that caused failure can be identified from the appearance of the failure surface. – Failure Analysis ME 470/670 This material is for educational used only within Wright State University


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Uniaxial Tension


Shear loading


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Brittle failure



7

Brittle Failure

Transgranular

Intergranular



8

Ductile and Brittle Failure • Ductile failure

• Brittle failure

– Extensive plastic deformation – Large amount of energy is absorbed prior to failure – Surface appears dull, fibrous or dimpled – Occurs after prior warning This material is for educational used only within Wright State University

– Little or no plastic deformation – Energy absorbed is small – Surface appears shiny, granular or faceted – Catastrophic/sudden


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Brittle Failure Contributing factors •Low Temperature •High deformation rate •Notches •(triaxial state of stress) Charpy Izod

Impact test A heavy pendulum is allowed to drop from a predetermined height Energy absorbed by the specimen is measured This material is for educational used only within Wright State University


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



12

Brittle at low T


Ductile at high T


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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 This material is for educational used only within Wright State University


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Origin

Beach marks

Ductile failure

Final rupture



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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.



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



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Plain carbon steels Beta-titanium alloys BCC structure

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

S-N Curve N in a log scale Aluminum, copper, nickel base alloys



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The SN curve shows scatter

99% fail

1% fail

Depending on the criticality of the component being designed one can choose the appropriate curve This material is for educational used only within Wright State University


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•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)



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



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



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Designing for fatigue involves •Avoiding stress concentrations •Notches, machining marks, keyways, sharp corners etc.



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



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Designing for fatigue involves •Hardening the surface by case hardening •Diffusing carbon or nitrogen into the surface of a low carbon steel This material is for educational used only within Wright State University


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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. This material is for educational used only within Wright State University


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Need to minimize creep deformation in this direction



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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 This material is for educational used only within Wright State University


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