Fatigue 3

Question 1

What are the stages of crack formation?

Answer 1

• Stage-I (Nucleation): Back and forth slip on crystallographic planes, slip band
formation. Within slip bands, pores will nucleate and coalesce. Extrusion and intrusions may appear, much faster micro crack formation.
• Stage-II (Crack Growth): Micro cracks join, macro crack. Crack escapes the shearing stress control and is driven by normal stress, continuous crack growth during each cycle along a plane normal to external loads.
• Stage-III (Catastrophic Failure): Final failure with separation of inclusions and pore formation.

Question 2

How are extrusions and intrusions formed?

Answer 2

• Plastic deformation (slip) occurs in high stress concentration regions.
• Due to the cyclic stress, continuous plastic ‘oscillation’ of metal layers in and out of the surface occurs and produces ridges.
• Work hardening prevents these metal layers to slip back and an extrusion is created.
• Intrusions are created in a similar manner. Several intrusions may interconnect and initiate a crack

Question 3

How are striations formed? (Lairds Model)

Answer 3

• When load is applied, the crack opens and high shear stresses develop in the ± 69º directions  plastic deformation at the lobes in front of crack tip (Fig. b).
• The material ahead of the last striation rounds out in order to
reduce the stress concentration to stop plastic deformation (Fig. c).
• Crack length increases such that equilibrium is reached between applied load and stresses ahead of crack tip.
• During the compressive loading, plastic deformation is not reversed. The bulge at the nose is collapsed (Fig. d) because of reverse shear stresses in the ± 69º directions. A new striation is
created.

Question 4

how are residual stresses formed, and which are preferred?

Answer 4

Residual stresses: Materials processing methods induce stresses in the products.
• These residual stresses are “locked into” the component in the absence of external loading.
• If the residual stresses are opposite in sign to the applied stress: the applied stress has to first overcome the residual stresses and then overcome the yield stress of the material in order to deform the material.
• If the residual stresses are of the same sign as the applied stress: it is easy to deform the material.
• Tensile residual stresses are detrimental to fatigue properties of the material.
• In general, compressive residual stresses are beneficial and improve fatigue properties.
• Since fatigue cracks originate from the surface, inducing compressive residual stresses on the surface improves fatigue properties of the material.

Question 5

What is shot peening?

Answer 5

a surface treatment where zones of compressive residual stresses are induced just under the surface of the material

Question 6

What is carburizing?

Answer 6

Carbon is diffused into the surface of steel (austenitic) at elevated temperatures and then
quenched rapidly, which forms hard martensite layer on the surface.

Question 7

What is nitriding?

Answer 7

Nitrogen is introduced into the surface of steel (ferritic).
• No martensite formation in the surface layer.
• Hard nitrides are precipitated on the surface.
• Both carburizing and nitriding can induce compressive residual stresses into the surface.

Question 8

What is Autofrettage?

Answer 8

Internal pressure is applied on a tube whose inner diameter is slightly lower
than the desired one.
• Pressure is applied such that the inner layers of the tube are plastically
deformed whereas the outer layers remain elastic.
• When internal pressure is released, outer layers try to come back to their
original position, which is resisted by the inner layers.
• Result: Inner layers are under residual compressive stresses. Fatigue
properties of the product can be increased.