Fractures- Small Crack Problems and Environmental Effects Flashcards
Range of short crack sizes
Fraction of mm to several mm
Problem with relatively short cracks
Low ΔK values can arise with relatively short cracks and higher Δ σ values. Can result in unexpectedly rapid crack growth
Microstructurally small flaws
Cracks of comparable size to a characteristic microstructural dimension, e.g grain size
Mechanically small flaws
Crack size comparable in size to the near-tip plastic zone
Physically small flaws
Significantly larger than microstructually small and mechanically small but no larger than 1-2mm in size
Chemically small flaws
Small cracks for which corrosion effects are of greater importance than would be expected from consideration of large cracks
Fatigue crack development
Initial fatigue crack is a shear crack in a large near-surface grain. Probably formed at a permanent slip band. Shear crack stops at grain boundary due to misorientation between adjacent grains. If fatigue is to occur the crack must propagate across this boundary and re-orientate into a tensile crack. Cracks can become non-propagating if stopped at the grain boundary and this corresponds to a fatigue limit.
Why can progressive removal of surface during fatigue prolong lifetime?
Crack depth is being reduced and predominant grain is being reduced in size.
Crack initiation sites
Permanent slip band sites. Grain boundaries (where PSBs intersect grain boundaries). Near inclusions and pores. At corrosion pits (related to stress corrosion and environmental effects).
Problem with giga-cycle failure
High cycle fatigue due to low amplitude vibrational stresses can cause failures. During most of the fatigue life no crack will be detectable by non-destructive means.
What characterised giga-cycle fatigue?
A low stress range (making it more difficult for surface cracks to overcome microstructural barriers).
Very low crack growth rates.
What does failure from internal defects mean?
May be that the grain boundary barrier to crack growth is reduced in this case. Will still be significant barrier and overcoming it will probably be a significant fraction of total lifetime.
Under which types of loading can chemical interactions modify crack growth?
Can occur under (quasi-) static loading conditions or in conjunction with cyclic fatigue
Graph of crack growth for chemical interactions
Log(da/dt) (crack velocity) vs K1: vertical line up from x-axis, diagonal line up region I, short horizontal region II (not always seen), almost vertical line region III (critical track growth reached).
Log (da/dt) vs log(K1): basically the same shape as above just different x-axis scale.
Region I crack velocity formula from log(da/dt) vs K1 graph
v=da/dt=Cexp(K1)
Region I crack velocity formula from log(da/dt) vs log(K1) graph
v=da/dt=ApK1^np
All ps subscript
Formulae to use for stress corrosion lifetime predictions
K1^2=C^2σ^2πa
Differentiate
2KdK1/dt=C^2σ^2πda/dt
Given the crack velocity formula for region I
Rearrange for dt and integrate to get time to failure between K1c and K1initial
End formula on page 16 lecture 9
Why can lifetime estimates be overestimates from the calculation?
C is assumed to be constant which it isn’t. Critical crack length can be a significant proportion of total plate width changing C.
Why can lifetime estimates be underestimates from the calculation?
If only assumed region I behaviour. Usually there is a region II where the crack moves at a constant velocity despite an increase in length and crack tip stress intensity factor.
True corrosion fatigue graph
Log(da/dn) vs log(ΔK). Similar shape to normal chemically assisted graph but region I is more diagonal line. For aggressive, starts at lower value on x-axis and is always above inert line until K1c.
Synergistic effects of cyclic fatigue and a corrosive environment. Modified Paris law can be used
Mixed corrosion fatigue graph
Log(da/dn) vs log(ΔK). Similar shape to normal chemically assisted graph but region I is more diagonal line for inert. Aggressive line starts vertical from inert line in middle of region II at K1scc. Then diagonal then nearly vertical bit starts above that for inert.
Superposition of cyclic fatigue and a corrosive environment. Increasing R gives a leftward shift of the line Kmax=K1scc.
Examples of environmental effects
Water vapour in fatigue cracks adversely affects crack growth rate (steels, Al, Ti alloys). Presence of H2 accelerates fatigue crack growth in steels, Ti alloys. Thermally activated microstructural changes at high temperatures (may lead to reduction in fatigue resistance, also creep phenomena). Multiple site damage (series of small fatigue cracks at fastener holes adjacent to discrete damage site, may have been exacerbated by chemical corrosion and thermal cycling).
