corrosion Fatigue
introduction: corrosion fatigue is a big and widely section.
What this term paper is going to represent , First the definition
of fatigue corrosion and what the most effective process on it ,
After that the process of corrosion fatigue in generally and how
can the engineer know if there is a corrosion fatigue . Also,
present the most important to understanding the fatigue life which
is S-N diagram. Then presenting how the corrosion fatigue initiates
and propagates . what happen to fatigue in corrosive environment
will be discuss and showing some graphs about that. After that the
evidence of corrosion fatigue and the difference between the
corrosion fatigue and just fatigue . Also, presented some
Characteristics of corrosion fatigue with a graph and the Variables
affecting corrosion fatigue.
corrosion Fatigue: Corrosion Fatigue is process in which the
failure of a metal by cracking under repeated cyclic loading at
lower stress level and in a corrosive environment . A corrosive
environment plays a significant role in the fatigue of
high-strength structural materials like steel, aluminium and
titanium alloys. Also The corrosive environment can cause a faster
crack growth. fatigue can be in ships, aircraft, heat exchangers
tubes , pump shaft and steel equipment. Fig(1): Corrosion Fatigue
Cracks in Brass
Process of corrosion fatigue : This is a process where the
fractures will happened under condition of corrosion. The cracking
begins at the surface defect. Hole and irregularities and propagate
trans granularly . Failure occurs by crack propagation . The crack
are unbranched and propagate perpendicular to the surface of the
metal. But before all that We have two stages first one Is about
the initiation of corrosion fatigue crack then the propagation of
corrosion fatigue crack.
Fig(2): trans granular cracks in corrosion fatigue diagram N-S:
A fatigue curve is commonly known as S-N diagram . is obtained by
plotting the number of cyclic required to Couse failure against the
maximum applied cyclic stress . Fatigue occurs when a material
fails under a cyclical stress which is below the yield stress of
the material and on each repeated stress cycle, the crack grows.
The repeated cyclic will continue repeating until the cracking is
happened . in the fig there is two lines the first one is about the
fatigue curve in the air as you can see the endurance limit .
endurance limit is the maximum stress at which a material will
never exhibit fatigue. Some metals dont have endurance limit. As
long as the cyclic increase and the stress decrease the fatigue
happened. the second one is about the fatigue corrosion which is
about the aluminium alloys in tap water. As you can see aluminium
dont have endurance limit. Fig(4): fatigue and corrosion fatigue
curves for an aluminium alloy
In figure (4) there are three conditions for the same metal . As
shown in the figure fatigue behaviour of the metal in air stops at
almost 300 N/mm*2 this is the endurance limit for the metal in air.
The second and third lines the metal is in corrosive environment
For water and sodium chloride solution as the graph represented
they are under the endurance limit for the metal . Fig(4): fatigue
and fatigue corrosion for some metal in air-water and sodium
chloride solution.
fig(5):low cyclic fatigue
Initiation of corrosion fatigue cracks: The a fatigue crack has
two parts, initiation and propagation . There are several possible
for starting the initiation of fatigue such as scratch on the metal
,twin boundaries, and slip band. The cracks are initiated start
often at the surface of the metal. But it could also start from
under surface in the presence of surface defect . during the
loading part of the cycle , slip bands will occurs . the surface
created by slip may oxidize during the unloading period . After
that the first step cyclic make the intrusion or extrusion . Then
by continued deformation in the next cyclic the intrusion may grow
and form a crack.
Fig(6):slip band intrusion and extrusion
Propagation of corrosion fatigue: After the crack is happened
now the dissection the phenomena of propagation of corrosion
fatigue . The crack start to propagate in the direction of the
stress axis. The stress concentration cusses local deformation in a
zone at the crack tip . because of the increasing in plastic zone
the crack will growth. The zone continue growing until it reach the
thickness of the sample. After the zone became as the thickness of
the sample there is no stress plain. The crack propagates in a
direction perpendicular to the tensile stress. The crack undergoes
rotation. After that on each cyclic the crack start to growth.
Corrosion fatigue evidence: The difference between the fatigue
and the corrosion fatigue is that in the fatigue rarely there is
any evidence that conform that in the fatigue there is more than
one crack while in the corrosion fatigue have more than one crack
its usually easy to determine whether the failure was because of
the corrosion fatigue or by fatigue . two things that special the
corrosion fatigue the surface of the metal is pitted and the cracks
are often are in the bottom of the pits. Fig(7): corrosion fatigue
failure from the origin
Corrosive Environment : Fatigue crack rate depends very strongly
on the environments as well as on the metal . In an Aggressive
environment the rate of the metal cracking is higher than in the
inert environment and the two environment have the same condition.
The lowest crack rate was observed in the inert environment . there
is two type of environment liquids and gaseous , generally the most
aggressive rate the cracking could increase in is in the liquid.
figure(8): influence of corrosive environment on fatigue crack rate
This figure is about corrosive and the inert environment . As the
graph presented the in the y-axis there is the log dl/dn which is
about the time and I the x-axis log ka which is threshold value. As
you can see the the the inert environment are less than corrosive
environment in the kc (fracture toughness) , So based on the graph
you can know that rate of fatigue cracking in corrosive
environmental more than the inert environment.
Characteristics of corrosion fatigue:1- The main thing that
unique the fatigue corrosion is the cracks near the fracture , in a
normal fatigue usually there is one crack near the fracture.2- The
corrosion fatigue cracks in certain steels are trans granular .
figure (2).3- The crack generally initiated at the surface where
the stress is maximum. 4- The striations in corrosion fatigue is
less clearer than in fatigue failure. Figure (9).
Fig(9): fatigue striations
Variable affecting corrosion fatigue: 1- Mechanical : Stress
intensity . Cyclic load frequency. Stress ratio 2- Metallurgical :
Microstructure. Composition . Heat treatment. Fracture toughness.3-
Environmental : Temperature. Species. Ph. Potential.
Conclusion: What I came up to after this report is knowing what
is the corrosion fatigue and the most affective on it is the
corrosive environment and it reduce the metal life , the difference
between the corrosion fatigue and fatigue, learning and studying
the most important diagram for corrosion fatigue which is S-N
diagram and knowing the endurance limit , the process of fatigue
and there is to things that the corrosion fatigue depend on them
before the failure of the metal which is initiation and propagation
of corrosion fatigue , the evidence of whiter or not there is
corrosion fatigue or just fatigue by knowing that there is in
corrosion fatigue more than one cracking and pits in it.
Reference:1- Philippe Marcus, corrosion mechanisms in theory and
practice .2- Herbert H. Uhlig , the corrosion handbook. 3- Zaki
ahmed , principles of corrosion engineering and corrosion
control.4- Mirko Klesnil and Petr Lukas, Fatigue for metallic
material.5- Pierre R.roberge, corrosion inspection and
monitoring.