FATIGUE BEHAVIOUR OF SCRATCH DAMAGED SHOT PEENED
SPECIMENS AT ELEVATED TEMPERATURE
*R.A. Cláudio, **C.M. Branco, *** J. Byrne and ***A. Burgess
*Dep. of Mechanical Eng., Escola Superior de Tecnologia de Setúbal
Campus do IPS, Estefanilha, 2910-761 Setúbal
**Dep. of Mechanical Eng, Instituto Superior Técnico
Av. Rovisco Pais, 1049-001 Lisboa
***Dep. of Mechanical and Design Eng., University of Portsmouth
Portsmouth, Hants PO1 3DJ, UK
[email protected]; [email protected]; [email protected]
ABSTRACT
An investigation of the fatigue behaviour of scratch damaged shot peened surfaces was undertaken. Experimental tests and
some FE (finite element) work was made in order to evaluate the effect of different scratches in a shot peened surface at
elevated temperature in RR1000 powder metallurgy nickel base superalloy. Several specimens were tested in the conditions
of as machined and shot peened. The geometry is representative of a critical feature of a gas turbine aero engine compressor
disc with an elastic stress concentration factor Kt=1.32 due to a curvature radius of 4.5 mm. In some specimens, a scratch was
created with a cutting tool in the curvature of the specimen after the shot peening treatment. The depth of the scratches lie
within the peak of compressive residual stress due to shot peening and within the end of the compressive layer respectively. It
was observed that shot peening has a strong benefit in early crack propagation, this effect being less pronounced when a
scratch is created. If the scratch size is within the end of the compressive layer, the effect of shot peening was almost
removed, however if the scratch size lies within the peak of compressive residual stress, an improvement of around 40% in
cyclic life remained, compared with the as machined condition. To understand the scratch behaviour under the shot peened
surface some FE results are presented with the stress distribution evolution in front of the scratch. It is shown that for the
bigger scratch sizes the residual stress introduced by shot peening will almost vanish after loading.
Introduction
It is generally accepted that surface condition has a strong effect on fatigue life and that in the absence of defects 80 – 90% of
total high cycle fatigue life is taken up by crack nucleation/initiation at the surface. Shot peening is a quite successful surface
treatment process for extending the service life of a large number of components being and is extensively applied in several
industries. Some studies indicate that it is possible to achieve, with an appropriate use of shot peening, an increase in service
life in the order of 600-1500%, [1]. To change the surface conditions, a stream of metal, glass or silica particles (“shot”) is
animated at high velocity and projected against the surface of the metallic component in a defined and controlled way, Figure
1. The multiple impacts of the shot will cause plastic deformation in the surface that creates residual stress, increases
hardness and changes surface roughness. Kobayashi et al. [2], explains the mechanism of the creation of residual stress by
shot peening. The objective of the compressive layer is to offset the applied stress, resulting in a benefit in terms of fatigue,
corrosion-fatigue and fretting fatigue. The principal benefit of shot peening in fatigue is created by the compressive stress field
at the surface and a limited effect of cold work, which has advantages in reducing the likelihood of crack formation, [3, 4].
However shot peening only introduces a very thin layer of compressive residual stress, in the order of hundreds of
micrometers as exemplified in Figure 2. Below the compressive layer, near the surface, is an elastic region in a tension state to
achieve equilibrium and not cold worked, which may have a detrimental effect on fatigue life if a scratch created in the treated
surface is greater than the compressive layer, Burgess et al. [5, 6]. Some concern arises if the treated surface is damaged in
some way, for example by a scratch. Almost no information is available about the influence of damage in a shot peened
surface. Little research has been focused on FOD (Foreign Object Damage) in service, [7-11] and on the use of shot peening
to repair cracked components as reported by [6, 12] . Hammersley et al. [13] compare experimental results for two shot
peening techniques in the presence of a 6 mm defect. They concluded that for the conventional shot peening technique
residual life of damaged and shot peened components was lower than undamaged and without shot peened components. This
observation creates some concern if a defect of reasonable size is created in the shot peened surface.
Media
Shot nature
Soft & hard steel
glass, ceramic
stainless steel
Impact angle
From 90 degrees
down to 45 degrees
σ
Shot diameter
Magnitude of compressive stress
max
is at least one-half of the material UTS.
From 50 microns
up to 6 mm
σ
σ
The surface stress s and the depth of compression o
will vary with the material properties and the peening parameters.
σs
Shot velocity
From a few m/s
up to 150 m/s
σ max
Compressive residual stresses
are balanced by low magnitude
and deep tensile stress
σ0
Metallic material
Figure 1. Elements of shot peening process
(Metal Improvement Co. Inc.)
Figure 2. Typical residual stress profile of a shot peened
surface. (Metal Improvement Co. Inc.)
Very little research is published about the influence of notches in a shot peened surface [14-16] . From this little research it is
possible to conclude that in some circumstances part of the shot peening effect is lost by the presence of scratches bigger
than the compressive depth of residual stress. The main reason proposed for the loss of shot peening effect by all workers is
stress relaxation. Dingquan et al. [14] observed experimentally, for a notch with ρ = 1 mm that the shot peening effect will relax
rapidly for soft materials, but for hard materials compressive residual stress remains. They also concluded that if relaxation
occurs, it stabilizes after a few cycles of fatigue loading. Work by Bergstrom [15], showed no difference in fatigue limit for
notched, notch radius ρ = 2.5 mm, and smooth specimens. But this work can not be compared with others because specimens
were tempered at high temperature after shot peening. From all this above is possible to conclude that almost no information is
available about the damaging effect of small scratches in the fatigue behaviour of shot peened components at high
temperature. This work is a contribution to the understanding of the behaviour of scratches in a compressive stress field.
Specimens
The geometry of the specimens tested experimentally and analysed by FE in this work is called “washer”, Figure 3. The
washer specimen is representative of a critical region of a gas turbine aero engine compressor disc.
Figure 3. Washer specimen, typical disc and aero engine.
2
The specimen has in the critical section a cross sectional area of 11 x 5.08 = 55.88 mm with an elastic stress concentration
factor Kt=1.32 due to a curvature radius of 4.5 mm. Several specimens were tested in the conditions as machined and shot
peened (110H 6-8A 100%). In some specimens, a scratch was created with a cutting tool in the curvature of the specimen
(R=4.5mm), after the shot peeing treatment. The scratch was 3 mm long with a 90º V groove with a root radius of 50 µm and
two different depth sizes, 50 µm and 100 µm. These depths lie within the peak of compressive residual stress and within the
end of the compressive layer respectively. Details of the 50 µm and 100 µm scratch sizes and a photo of a 50 µm scratch with
potential drop wires welded on are presented in Figure 4. In total about 50 specimens were tested representing six different
conditions: as machined, shot peened, as machined with a 50 µm scratch, shot peened with a 50 µm scratch, as machined
with a 100 µm scratch and shot peened with a 100 µm scratch. More details about the specimen and the scratch geometry can
be found in, [17].
11mm
a) 50 µm scratch.
b) 100 µm scratch.
c) Photo of a 50 µm scratch.
Figure 4. Scratch geometry (units in mm).
The material used in this work was a new generation nickel base superalloy developed using a powder metallurgy (PM)
technique, whose commercial name is RR1000. This alloy was specially designed for gas turbine discs, having as main
features good fatigue and creep resistance in addition to resistance to corrosion and oxidation under severe operating
conditions. The material was processed through a powder route, which involved argon atomisation to a 75 µm powder size
then HIPed and extruded to 12 inch diameter billet. The microstructure of RR1000 is fundamentally formed by an austenitic
matrix phase (γ), and by an intermetallic phase (γ’) (Ni3(Al, Ti)). Grain structure is quite fine, with an average size of 7 µm.
More details about RR1000 can be found in references, [17-21] . The montonic properties of RR1000 at 650ºC are:
E = 188.6GPa; ν = 0.255; UTS = 1448MPa and σ02 = 1034MPa, [22].
Experimental testing parameters
A servo-hydraulic testing machine, INSTRON® 1382 with a controller Fast Track 8800® connected to a computer, was used to
load the specimens, in load control, with a trapezoidal waveform 1-1-1-1s and load ratio of R=0.1. Loads were initially selected
so that failure could occur in the range of 1000 to 100000 cycles. A furnace, controlled in 3 regions, was used to keep the
specimens at 650ºC during the fatigue test. Crack propagation rate was measured with a DCPD (direct current potential drop)
system developed for these tests, [23]. The current specified was 40Amp and the resulting signals, measured close to the
crack and in the reference position, were acquired with a 16 bit DAQ board and 800x grain, [22, 23]. The potential drop wires
had to be welded at a distance of 1.6mm from the scratch, Figure 4 c), to avoid the risk of failure at the welds. This caused an
inevitable lack of sensitivity to measure small cracks, [24]. In order to calibrate the potential drop and to observe crack shape
as it was growing, several beach marks were made on the fracture surface by changing the time at maximum load from 1s to
97s. The whole system was controlled by a single computer with software specifically made in LABVIEW®. The software
continuously records the testing information, and does automatic beach marking for DCPD calibration.
Experimental results
Figure 5 shows the SN curves for all the specimens tested with the testing parameters defined above. The maximum stress is
the nominal stress across the critical section of the specimen which does not include concentration factors due to the 4.5 mm
notch root neither due to the scratch. Each point represents an experimental test and the lines the general trend of the points.
Points with an arrow pointing to the right “→” identify when the test was stopped for specimens that did not break. Each point
type represents a particular set of tests. Closed in points are the shot peened results while open symbols are the as-machined
results. The square identifies the results for the specimens without scratch, the diamond for the 50 µm scratch depth and the
triangles for the 100 µm scratch depth. As can be expected for the specimens without scratch it is perfectly clear that shot
peening has a strong beneficial effect, when compared with the as-machined condition. This means that for this alloy, even at
high temperature and LCF (low cycle fatigue) conditions, shot peening is quite effective.
As expected, the fatigue life of the specimens reduces with the presence of a scratch, being lower for deeper scratch sizes.
For the 50 µm scratch depth shot peening retains a small effect but almost no influence for the 100 µm scratch depth. For the
scratched specimens it is possible to notice a threshold in fatigue life especially for the shot peened condition. For example for
the 50 µm scratch when the maximum nominal stress is less than 800 MPa, the number of cycles to failure is quite high (for
the results available no specimen broke). But if the stress level is higher than 800 MPa fatigue life is reduced drastically to
some thousands of cycles. The same behaviour can be observed for the 100 µm scratch with a stress level of about 680 MPa.
For the deepest scratch size it seems that the shot peening effect is completely lost for LCF conditions, if it exists is quite
small; however for HCF (high cycle fatigue) conditions some doubt remains, and more tests are required.
An estimation of crack initiation was performed using the experimental PD (Potential Drop) measurements. Only specimens
with scratches could be monitored with this technique. For the as machined specimens, these broke by the PD welds,
invalidating the test. Crack initiation was defined by the number of cycles when the PD measurements increased 0.2% relative
to the initial PD mean value (without any propagation). Because of the risk of welding the PD wires close to the scratch, the
resolution to measure so small cracks is quite low. Taking the number of cycles according to this criterion, some early crack
propagation could be included. However, as Burgess [6] points out, it is important to note that, in practical engineering,
initiation is merely an arbitrary defined point.
1200
Shot Peened
No Scratch
Maximun Stress [MPa]
1100
1000
As machined
No Scratch
900
As Machined
50µm Scratch
Shot Peened
50µm Scratch
800
As Machined
100µm Scratch
Shot Peened
100µm Scratch
700
600
1000
10000
100000
1000000
Cycles to failure, Nf
Figure 5. S/N curves obtained experimentally.
Figure 6 is a graphical representation of the number of cycles to a 0.2% increase in PD normalized by the total number of
cycles to failure for all the specimens where it was possible to use the DCPD technique. The label Sc identifies the scratch
depth (50µm or 100 µm), and the AM or SP identifies surface condition (as machined or Shop Peened). From these results is
possible to notice a dependency of crack initiation with the applied load for the 50 µm scratch depth. For lower stress almost
all of the life of the specimen is spent in crack initiation. As the stress increases the life spent in crack initiation reduces almost
linearly. At high stress, close to 950 MPa, it seems that life spent in crack initiation is very short, only about 10%. For the
100 µm scratch depth, the proportion of life spent in early crack propagation seems to be independent of stress. For this
situation, the life spent in initiation only represents a very small proportion of total fatigue life, being in the order of 10 - 20%.
The influence of shot peening in crack initiation, for both scratches sizes, is not clearly noticed. From the present results it
seams that shot peening has no influence on crack propagation in the presence of such size scratches.
Crack propagation results are presented in Figure 7. The da/dN vs ∆K curves are plotted, in a log-log scale, for all the
specimens tested with a 50 µm scratch depth that broke. The crack propagation rate is for the deepest point of the crack. ∆K
solutions for this geometry were obtained numerically, using FE analysis, for the crack shape measured experimentally, [17].
As said earlier, because of the sensitivity of the DCPD, the results are only plotted for crack sizes bigger than 1 mm. In the
range of ∆K between 36 and 46 MPa. m the experimental points follow a linear relationship between da/dN vs ∆K on a log-log
scale. Eliminating the last points from all curves, the remaining points from all the as machined (AM) and all the shot peened
(SP) specimens were interpolated by a power fit, whose parameters are in the graph. As can be seen shot peened specimens
have a small crack propagation rate (about 5% less) when compared with as machined ones and the slope of both curves is
approximately the same.
Figure 8 shows superposition of beach marks taken from an as-machined and a shot peened specimen. For a 50 µm scratch,
both specimens have identical initial defects due to the scratch. For this particular as-machined specimen, multiple cracks
could be seen easily starting from the scratch root. After the second beach mark, both specimens, as-machined and shot
peened, have approximately the same crack size at the surface, being slightly deeper for the shot peened. Comparing the two
last beach marks of both specimens it is possible to notice a bit more tunnelling effect for the shot peened specimen. The
principal reason for that may be because the shot peened specimen has compressive residual stress near the free surface
decreasing the crack propagation rate at the surface. The result is a small overall decrease in crack propagation rate as seen
previously in Figure 7.
(Number of cycles up to 0.2% increase in
PD)/Nf [%]
100%
Sc50 AM
Sc50 SP
Sc100 AM
Sc100 SP
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
650
700
750
800
850
900
950
Maximum Stress [MPa]
Figure 6. Number of cycles up to 0.2% PD increase normalized by total fatigue life plotted against maximum stress for
scratched washer specimens.
10 -2
da/dN [mm/cycle]
As Machined
da
= 2.431× 10-10 .∆K 4.198
dN
R 2 = 0.9527
10 -3
Shot Peened
da
= 2.553 × 10-10 .∆K 4.154
dN
R 2 = 0.9205
NF113 AM
NF115 AM
NF116 AM
NF118 AM
NF101 SP
NF102 SP
NF103 SP
10 -4
30
40
50
60
∆K [MPa.m1/2]
Figure 7. da/dN curves for all the specimens tested with a 50 µm scratch depth.
70
80
As Machined
Shot Peened
Figure 8. Schematic of beach marks from as machined and shot peened specimens, 50 µm scratch size.
FE Model
A 2D FE model was built to study the scratch behaviour in the presence of residual stress and loading. This model represents
the section of the washer specimen that crosses the middle of the scratch as shown in Figure 9.
Y
Scratch
X
Figure 9. Washer specimen and FE model.
The geometry created in FE has been prepared to model simultaneously all the situations considered i.e. no scratch (only
notched geometry), 50 µm scratch and 100 µm scratch, Figure 10. Because of symmetry, only ½ of the specimen was
analysed. For the models with scratches, symmetry is not totally verified, but as the scratch size is so small that almost no
difference in the global stiffness is noticed. The section was meshed with biquadratic 2D plane strain elements (8 nodes), with
two degrees of freedom per node, whose reference in ABAQUS® (the FE code used) is CPE8. The same mesh was used to
analyse the three geometries studied: no scratch, 50 µm scratch and 100 µm scratch. To analyse a desired geometry, an
ABAQUS® function was used to remove those elements associated with the scratches. This is equivalent to giving zero
stiffness to the elements that are inside the scratch. Figure 10 shows ½ of the washer specimen meshed with the X-Y
coordinate system, whose centre is the notch root. The mesh shown is the coarsest one, used for problems without shot
peening and for validation proposes. A refined mesh (with 4 times more elements), not shown, was used to model all the
situations with shot peening where larger gradients were expected. The use of this more refined mesh was only to have more
points to define stress or strain curves. Solutions were similar for both meshes. In the region of the notch root, near the free
surface (where shot peening is to be introduced), the size of the face of the elements is about 1.8 µm for the refined mesh and
double that for the coarsest one allowing an accurate definition of the residual stress profile due to shot peening.
Loads and boundary conditions are symbolically represented in Figure 10. The left part of the specimen is constrained in
direction X, to support load that is applied in the opposite section. Load was applied in only one node as a concentrated load.
All the nodes in the right of the specimen had a constraint defined to force the same displacement in the X direction. In
practice this load acts as a distributed load. At the bottom a constraint was defined in the Y direction to impose symmetry. A
master-slave surface was defined in the crack faces to prevent overlapping when the crack closes.
The material behaviour was introduced using the Ramberg-Osgood model according to the parameters published in [22].
Reference stress:
Reference strain:
Hardening exponent:
Poisson coefficient:
Young Modulus:
σ0 = 1041 [MPa]
ε0 = 0.545 [%]
n = 24.2
ν = 0.255
E = 188.6 GPa
Y
X
P
Figure 10. 2D Coarsest mesh to model the washer geometry without and with scratches.
The residual stress profile due to shot peening was introduced in the FE model as an initial condition by an external subroutine
programmed in FORTRAN®. The residual stress profile introduced in the model is plotted in Figure 11, identified by the label
NoSc.
FE Results
Before analysing the behaviour of the scratch with residual stress, the stress intensity factors were first calculated for the
geometries: without scratch with the 50 µm scratch and with the 100 µm scratch (all including the root radius of R=4.5mm). For
this analysis a linear-elastic material behaviour was only considered with ν = 0.255. These results are in Table 1.
Table 1. Elastic stress concentration factor for the three models analysed.
Geometry
Washer – No Scratch
Washer – 50 µm Scratch
Washer – 100 µm Scratch
Kt
1.33
3.98
5.03
Analysing Table 1, the elastic stress concentration factor for the washer specimen without scratch is 1.33. The value obtained
is close to the one reported for this specimen which is 1.32. For the specimens with scratch the elastic stress concentration
factor is much bigger, due to the small scratch root radius, giving the values of 3.98 and 5.03 for the 50 µm and 100 µm
scratch sizes respectively.
To understand how the residual stress profile behaves with creation of a scratch, an FE model was built, by removing those
elements associated with the scratch after the residual stress was applied. As observed by Dingquan [14], it is expected that
when a scratch is created in a shot peened surface, the residual stress will redistribute due to the new boundary conditions
that are created. For the condition as-machined nothing was expected because residual stress was not present. Figure 11
shows the residual stress curves in the loading direction plotted together. When a scratch is open, the residual stress tries to
close the scratch causing a large compressive plastic deformation in the scratch root, increasing the compressive stress
further. The depth at which residuals are compressive also increases when the scratch is created. The maximum stress in
direction X is very high. It should be noted that the tensile strength limit of the material was not exceeded because the stresses
in directions Y and Z are also positive, resulting in an equivalent stress state that is less than the tensile strength of the
material.
However even having so high residual stress after scratch creation, it is expected that these may reduce or even vanish due to
creep relaxation and cyclic loading. As an example for a maximum nominal stress of 900 MPa the stresses in the scratch root
calculated by applying the concentration factor are 3582MPa and 4527MPa for the 50 µm and 100 µm scratch sizes
respectively, which is above the material limit. Even with this so high compressive residual stress in the scratch root, plastic
deformation expected due to loading is enough to eliminate part of the shot peening effect, at least close to the surface,
eliminating the capability for shot peening to be effective in resisting crack initiation.
400
200
0
Stress σ xx [MPa]
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
-200
-400
-600
-800
NoSc
Sc50
Sc100
-1000
-1200
-1400
Depth [mm]
Figure 11. FE simulated relaxed residual stress profile when a scratch is created.
Conclusions
A study of the fatigue behaviour of an advanced nickel base superalloy with surface improvement by shot peening, has been
carried out to evaluate the effect on fatigue behaviour of scratches in a residual stress field. Experimental tests and numerical
analysis were carried out on specimens representative of a compressor disc operating in extreme conditions of stress and
temperature. From this work the following conclusions could be drawn:
- It has been demonstrated that for this alloy shot peening is able to increase fatigue life by more than one order of magnitude
compared with the as machined condition, even at high temperature and LCF (low cycle fatigue) conditions.
- From experimental results, for both scratches studied the shot peening effect is reduced in terms of fatigue life. For the
100 µm depth the shot peening benefit almost disappears, however for the 50 µm depth an improvement in cyclic life
compared with the as-machined condition is found of around 40%.
- For all the conditions studied, shot peening never reduces the fatigue life. Even for the biggest scratch, which lies with the
limit of the compressive layer, shot peening does not a negative influence in terms of fatigue life.
- For all the specimens tested with shot peening it seems that an apparent fatigue limit is easily reached for the lower testing
loads. However more experimental tests are needed to fully verify this observation.
- Shot peening has a major benefit in early crack propagation. Without a scratch, the compressive stress field has the ability to
arrest small cracks that initiate at the surface when applied stresses are relatively small. A small benefit was also detected for
long crack propagation, where the compressive layer at the surface creates a tunnelling effect, decreasing overall crack
propagation rate slightly. However, from experimental results, shot peeing seems to have no effect on crack initiation, for the
scratch depths studied. Numerical results reveal that the residual stress will be eliminated after loading due to the so high
stress concentration factor created in the root of the scratch.
Acknowledgments
To Rolls-Royce plc for supplying of specimens and technical information.
This work was supported by Project P131 of RTO/NATO and Project of FCT, Portugal, POCTI/EME 46195/02, “Creep-fatigue
behaviour of powder metallurgy nickel base superalloys”.
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