1
Peening effects on fatigue Crack Growth in Friction Stir Welds
Omar Hatamleh1,*, Jed Lyons2 , & Royce Forman3
1) Structures & Dynamics Branch, NASA-Johnson Space Center, Houston, Texas 77058 2) Mechanical Engineering Dept., University of South Carolina, Columbia, South Carolina 29208 3) Materials & Processes Branch, NASA- Johnson Space Center, Houston, Texas 77058 Abstract The surface modification from laser and shot peening was used to introduce compressive
residual stresses into friction stir welded (FSW) Aluminum Alloy (AA) 7075-T7351.
Their influence on the fatigue crack growth of FSW was characterized and evaluated for two
different crack configurations. The results indicate a significant decrease in fatigue crack
growth rates resulting from using laser peening compared to shot peening versus their native
welded specimens.
Keywords: FSW, laser peening, shot peening, fatigue crack growth 1. Introduction
Friction stir welding (FSW) (illustrated in Figure 1) is a relatively new welding
technique invented by the Welding Institute in England in 1991 [1]. Since then, this
technique has emerged as a promising solid state process with encouraging results,
especially when used on high strength aerospace aluminum alloys that are usually
difficult to weld [2]. Using frictional heating from a rotating tool, combined with forging
pressure, high strength bonds are produced at temperatures below the melting
temperature of the material [3]. * Corresponding author. Tel: (281) 483-0286; Fax: (281) 244-5918 Email: [email protected]
2
An adequate spindle tilt toward the trailing direction ensures that the tool shoulder
holds the stirred material by the threaded pin, and the material is moved efficiently from
the front of the pin to the back.
Figure 1 Principle of the friction stir welding process.
Residual stresses in FSW may be considerably less than those in fusion welds since
FSW takes place at a lower temperature than fusion welding. However the rigid
clamping arrangement used in FSW, along with the heating cycle the material
experiences during welding, can still significantly affect residual stresses in the FSW
weld [4, 5]. These residual stresses can significantly affect the service performance of
welded materials by facilitating the fatigue crack growth process [6]. Residual tensile
stresses in welds can also lead to faster crack initiation.
The use of FSW is becoming more popular due to the advantages it offers compared
to other conventional fusion welding techniques. The capability of FSW to weld high
strength aluminum alloys like AA 7000 series has resulted in welded joints being used in
Advancing Side
Retreating Side
Rotating Tool
3
critical load bearing structures, and is being used by modern industries for structurally
demanding applications [7]. This has instigated the need for techniques and methods that
can alleviate the tensile residual stresses in welded components.
Several studies [8-19] have investigated the fatigue behavior of FSW aluminum
alloys, but none of these studies has focused on the effects of laser peening on fatigue
crack growth. Laser peening (LP) (shown in Figure 2) is a technique with the capability
to introduce a state of residual compressive stresses that can significantly increases
fatigue properties [20, 21].
Figure 2 Laser peening process
The LP process utilizes high energy laser pulses (several GW/cm2) fired at the
surface of a metal coated with an ablative film, and covered with a transparent layer
(usually water). As the laser beam passes through the transparent layer and hits the
surface of the material, a thin layer of the ablative layer is vaporized. The vapor
continues to absorb the remaining laser energy and is heated and ionized intoa plasma.
The rapidly expanding plasma is trapped between the sample and the transparent layer,
creating a high surface pressure, which propagates into the material as a shock wave [22].
4
When the peak pressure of the shock wave is greater than the dynamic yield
strength of the material, it produces extensive plastic deformation in the metal. The actual
depths of the LP induced stresses will vary depending on the type of material, the laser
peening processing conditions chosen, and the material properties [23]. Compressive
stresses produced by laser peening can also be generated deeper below the surface by
using successive shocks. Previous research [24, 25] has shown that the residual stress
resulting from laser peening can be significantly higher and deeper than for conventional
shot peening.
In this study, the surface modification from laser and shot peening was used to
introduce compressive residual stresses into FSW AA 7075-T7351. Changes in fatigue
crack growth from differences in residual stresses resulting from different peening
techniques was assessed and evaluated. The effects of crack orientation with respect to
the weld have also been explored.
2. Experimental Procedure
The aluminum alloy 7075-T651 was used in this investigation. AA 7075 is a
precipitation-hardened aluminum alloy widely used in aerospace applications due to its
high strength. The 7075-T651 was supplied as a 6.35mm plate with an ultimate and yield
strength of 561 MPa, and 536 MPa, respectively, and an elongation of 11%.
The FSW specimens for this investigation were made at the NASA Johnson Space
Center. The rotational speed used to weld the plates was 350 RPM in the
counterclockwise direction, and the translation speed was 2.54 cm/min. The welding
direction was aligned with the rolling direction. The FSW panels produced by NASA
5
were 122 cm x 40 cm x 0.65 cm. Following the welding process, the welded plates were
aged from the T651 condition to the T7351 condition. The FSW puts the weld nugget
microstructure in a supersaturated solid solution condition; therefore, heat treatment is
necessary to prevent the welded material from continuing to age at room temperature [26,
27].
Following the heat treatment, the welded plates were inspected using radiographic
and penetrant inspections. The inspections results did not reveal any indication of voids
or defects in the weld. After that, bending tests using strip specimens with dimensions of
17.8 cm x 2.54 cm were done. Both the root and the crown sides of the weld were tested
to evaluate the quality of the weld. The samples were inspected visually afterward with
no crack indications revealed.
The mechanical properties of the welded plates were verified by a tension test.
The tensile specimens used in the test consisted of conventional dog bone coupons in
accordance with ASTM 08 using specimens with a width of 12.7 mm. The orientation of
the specimens was with the weld in the center of the specimen and the load was applied
perpendicular to the weld direction. The ultimate and yield strength of the tested coupons
were 339 MPa and 226 MPa respectively with an elongation of 5.5%.
Optical micrographs were taken of the transverse cross section of a weld. The
specimen was cut and sectioned and then subjected to several successive steps of
grinding and polishing until proper surface finish was achieved for a metallographic
analysis. After that, the specimen was etched using a Keller’s reagent.
6
Before the shot and laser peening were applied, the specimens were milled on the
top side of the weld removing about 0.4 mm of material. The dimensions of the 6mm
thick M(T) specimen and the crack locations are shown in Figure 3. The specimens
dimensions were in accordance with ATMS E 647. Some fatigue crack growth
specimens were shot-peened using 0.0234” glass beads with an Almen intensity of 0.008-
0.02A and a 100% coverage rate. Other specimens were peened using single and triple
layers of laser peening.
The laser peening was performed at the Metal Improvement Company in
Livermore California, and was applied using a square laser spot size of 4.72 x 4.72 mm2
with a laser power density of 4 GW/cm2 and 18ns in duration. The spots within a layer
were overlapped 3%. Peening between layers had an off-set of 50% in the two in plane
directions. A peening frequency of 2.7 Hz and a 1 micron wavelength laser was
employed. Both sides of the specimen were shocked using the same conditions.
After the peening process, a 0.25 mm thick through thickness notch was
introduced to the testing specimens using an electric discharge machine (EDM). The
notch was 12.5 mm in length and was introduced at two different locations. One was at
the plate weld centerline, and the other was 7.5 mm from the edge of the weld at a
location corresponding to the Heat Affected Zone (HAZ).
7
(a) Configuration I (b) Configuration II
Figure 3 Fatigue test coupons configurations (Suggestion to label the samples as
Configuration I and Configuration II to make the later reading easier to follow)
3 Results and discussion
3.1. Weld microstructure and hardness
A cross section of the weld is illustrated in Figure 4. The cross section revealed
the classical formation of the elliptical onion rings structure in the center of the weld.
The FSW samples investigated revealed no visible porosity or defects.
8
Figure 4 A Cross section of the welded specimen
A 50x magnification of the cross section of the weld showing the transition from
the nugget- Thermo Mechanical Affected Zone (TMAZ)-HAZ microstructure on the
retreating area of the weld is illustrated in Figure 5. Relative to the nugget, the TMAZ
experiences a lesser degree of plastic deformation and is exposed to lower temperatures;
therefore, recrystallization is not evident in this region. The grain structure in this region
is elongated, with some considerable distortions that may be attributed to mechanical
action from the welding tool. The HAZ is unaffected by mechanical effects and has a
grain structure that resembles the parent material grain structure. Previous work by Jata
et al [28] has indicated that strengthening precipitates in this region have grown in size
and were several times larger than in the parent material. Transitions from TMAZ to the
HAZ and from the HAZ to the base material are gradual and not distinguished by any
abrupt change in microstructure.
Figure 5 A section of the weld nugget-TMAZ-HAZ interface at 50x
9
Figure 6 reveals the fine and equiaxed grains typical of a recrystallised structure
at different regions of the nugget and fairly uniform grains with no apparent defects. The
grain sizes in this region are of the order of 5-10 μm, and are significantly smaller than
the parent material grain due to the higher temperature and extensive plastic deformation.
It is also noted that the average grain size was different for different regions of the
nugget. For example, the grains at the bottom side of the plate were relatively smaller
than those at the top. This may be attributed to the fact that the bottom surface of the
FSW plate is in contact with the backing plate, which may act as a heat sink. Therefore,
lower temperatures and shorter thermal cycle at the bottom of the welded plate
effectively retards the grain growth and results in smaller grains. These findings confirm
the results obtained by Mahoney et al [29] showing similar results for AA 7050.
Figure 6 Nugget structure at 1000x at different regions of weld nugget
10
A micro-hardness test was also performed on a cross section perpendicular to the
welding direction. The test was conducted on a Struers micro-hardness machine, and the
measurements were taken using a 300g for 3 seconds. The results of the test are
illustrated in Figure 7. The figure shows a softened region corresponding to the weld
nugget. The variations in hardness can be correlated to the microstructure developed after
the welding process. The center of the nugget was significantly harder than the TMAZ
immediately outside the weld nugget boundary. The soft region is probably caused by
coarsening and dissolution of strengthening precipitates during the thermal cycle of the
FSW. Softening was produced throughout the weld zone. The lowest hardness did not lie
in the center of the weld, but around 12 mm away from the weld centerline,
corresponding to the weld interface. The hardness levels increased as precipitation
hardening became more effective with increasing distance from the weld.
Microhardness Profile
0
20
40
60
80
100
120
140
160
180
-29 -27 -25 -23 -21 -19 -17 -15 -13 -11 -9 -7 -5 -3 -1 1 3 5 6 8 10 11 13 15 16 18 20 21 23Millimeters Across Weld
Har
dnes
s R
eadi
ng (K
noop
-300
gf)
Figure 7 Micro-hardness test across the weld of FSW 7075-T7351
Nugget TMAZ TMAZ HAZ HAZ Base Base
11
3.2. Fatigue Crack Propagation
The fatigue testing was performed under axial loading at constant
amplitude using a servo-hydraulic machine. The crack length was measured using the
direct current potential difference method. This technique utilizes a constant DC current
passed through a gage attached to the specimen while the voltage difference across the
notch is monitored. All fatigue crack growth (FCG) specimens measurements were
carried out at constant amplitude loads in laboratory air at a frequency of 22 Hz, and at a
stress ratio R=0.1. Before the FCG tests were conducted, the specimens were cycled
until a pre-crack measuring 0.5 mm was obtained from the EDM notch. The fatigue pre-
crack provided a sharpened fatigue crack and symmetry for the M(T) specimen, which
has an important effect on subsequent crack growth data. Upon fatigue cycling, the crack
propagated almost instantaneously from the notch.
The fatigue behavior in the tests was determined by measuring the fatigue crack
length (a) versus the number of cycles (N) for the base non-welded material, the non-
shocked FSW, shot peened, and two laser peened conditions. Figure 8 shows the crack
propagation test data for configuration one (T-L) in AA 7075-T7351 at R=0.1. If we
consider the fatigue life to be the point where the a-N curves become almost vertical, the
specimen processed with three layers of laser peening had substantial fatigue life
improvements with a fatigue life about 200% longer that the non-peened FSW, and
around 40% higher than the base un-welded material. The shot peened specimens tested
did not result in any measurable decrease to the fatigue crack growth rate. The fatigue
life improvement from laser peening was due to the higher and deeper compressive
residual stress resulting from laser compared to conventional shot peening [24, 25].
12
Figure 8 Results for crack length vs. number of cycles for FSW 7075-T7351
Figure 9 illustrates the fatigue crack growth rates using different peening
techniques. The crack growth rates are monotonically increasing, which suggest that the
applied stress intensity factor is increasing as a function of crack length for a given
applied load. The behavior of the baseline specimen is typical of most conventional
metallic samples, where cracking initiates at the machined notch tip and grows to failure
under continual fatigue cycling [30]. The crack growth rates for the three layer laser
peening were around 51-69% less than the crack growth rates for the unpeened and the
shot peened FSW specimens at ΔK=8.5-11.5 mmMPa .
0
5
10
15
20
25
30
35
40
0 20000 40000 60000 80000 100000 120000 140000Number of Cycles
Cra
ck L
engt
h (m
m)
Laser (3 layers)Laser (1 layer)Shot PeeningNo PeeningBase
13
On the other hand, FCG was around 28-42% less for specimens processed with a
single layer of laser peening. The differences in crack growth rates started to decrease
and results started to converge at ΔK >20 mmMPa . These results are similar to the
one reported by Bussu et al [8] for FSW welds using AA 2024-T351. This trend occurs
because as cracks increase in length, the stresses are relaxed and crack growth rates
reduced. This was further confirmed by Galatolo et al [31] where measured residual
stresses acting on the crack tip decreased when the crack size got larger, eventually
disappearing when the crack length was 40mm. Similar trends were also reported by
Donne et al [17], where base and FSW FCG results were converging at higher stress
ratios. This was attributed to the fact that residual stresses were leveled off by the big
plastic zone ahead of the crack.
It was also noted from Figure 9, that the unpeened FSW specimens had a higher
fatigue crack growth rate when compared to the base material. One possible explanation
could be that the microstructure in the HAZ corresponds to one of an overaged structure.
As discussed previously, while grains in the HAZ are similar to the base material; the
strengthening precipitates in this region have grown in size. The resistance to fatigue
crack growth should decrease in microstructures that are overaged [32]. Therefore,
microstructures in which strengthening precipitates are coarsened usually have higher
FCG rates compared to microstructures that contain fine precipitates.
Nevertheless, John et al [26] offered a different explanation for behavior by
investigating the FCG in the HAZ for specimens with different dimensions, and then
compared the results to the base unwelded material. To ensure that the microstructure in
the vicinity of the crack growth region was similar in all specimens, the crack growth
14
occurred in the same region with respect to the weld. The results indicated that the
difference in FCG between the FSW and the base material was due to the contribution
from the residual stresses, and not the microstructure. The results obtained here are
different than those obtained by Ignat’eva et al [33] for fusion welds. In fusion welding,
crack growth rates in the HAZ were reduced compared to the parent material. This could
be due to different residual stress distributions in different welding processes.
1.00E -05
1.00E -04
1.00E -03
1.00E -02
1 10 100
da/d
n (m
m/c
ycle
)
Laser (3 layers)Laser (1 layer)S hot P eeningNo PeeningBase
Figure 9 Crack growth rates for configuration one in FSW 7075-T7351
The number of cycles to grow a 25mm crack from one side of the EDM notch for
configuration one is shown in Figure 10. While improvement from shot peened over the
unpeened specimens was negligible, laser peening resulted in an improvement of 27%,
and 74% in the number of cycles for one and three laser layers respectively. It is also
noted that specimens processed with one layer of laser peening did not surpass the base
15
material, while specimens processed with three layers still had an improvement of 20%
over the base unwelded material.
0
20000
40000
60000
80000
100000
120000
Base No Peening S hot Peening Laser Peening (1 layer)
Laser Peening (3 layers)
Cyc
less
Figure 10 Number of cycles to grow a 25mm crack from on side of the EDM Notch for Configuration I in FSW 7075-T7351
The FCG data for Configuration II (L-T) Al 7075-T7351 under R=0.1 constant
amplitude loading are shown in Figure 11. The data in the graph compares the base un-
welded material to FSW welded specimen both with no peening and peened using
different techniques and parameters. The shot peened specimen in this study did not
improve the fatigue crack growth. Similar results were identified by Honda et al [34]
were it was revealed that overall, shot peening had negligible or only minor effect on
crack growth rates in AA 7075-T7351.
Specimens processed with three layers of laser peening exhibited the highest
increase in fatigue life with an increase of around 280% over the unpeened FSW
specimens, and around 190% over the base un-welded material. Yang [20] attributed the
reduction in FCG rates in laser peened specimens to the compressive residual stress
resulting from the peening process. As a result, the effective stress intensity factor that
controls the FCG in the peened specimen is lower than that of the unpeened ones.
16
0
5
10
15
20
25
30
35
40
0 20000 40000 60000 80000 100000 120000 140000 160000Number of Cycles
Cra
ck L
engt
h (m
m)
Laser (3 layers)Laser (1 layer)S hot PeeningNo PeeningBase Material
Figure 11 Results for crack length vs. number of cycles for FSW 7075-T7351
Figure 12 illustrates the fatigue crack growth rates for the same specimen using
different peening techniques. The crack growth rates for the three layers of laser peening
were around 55-82% less than the crack growth rates for the unpeened specimen for
ΔK>10 mmMPa . The results between the different specimens started to converge at
ΔK>20 mmMPa . Although the crack growth rates for cracks initiating at the center of
the weld were faster than the base unwelded material, they did not reach crack growth
rates as reported by Bussu et al [8] where it was shown that crack growth rates for cracks
starting at the FSW centerline were approximately 10 times faster than parent material.
17
1.00E -05
1.00E -04
1.00E -03
1.00E -02
1 10 100
da/d
n (m
m/c
ycle
)
Laser (3 layers)
No Peening
Base
Figure 12 Crack growth rates for configuration two in FSW 7075-T7351
The number of cycles to grow a 25mm crack from one side of the EDM notch for
Configuration II is also shown in Figure 13. In this configuration, shot peened specimens
did not result in any improvement over the unpeened ones. On the other hand, laser
peening resulted in an improvement of 123% for one layer, and 217% for three layers of
laser peening. Specimen processed with three layers also had a 91% increase in the
number of cycles over the base unwelded material.
18
0
20000
40000
60000
80000
100000
120000
140000
160000
Base No Peening S hot Peening Laser Peening (1 layer)
Laser Peening (3 layers)
Cyc
les
Figure 13 Number of cycles to grow a 25mm crack from on side of the EDM Notch for configuration two in FSW 7075-T7351
Comparisons between the fatigue crack growth for configuration one and two are
illustrated in Figures 14, and 15. The results show fatigue crack growth differences
between one and three layers of laser peening and the unpeened FSW specimen. The
differences between the unpeened specimen for both Configurations I and II were small,
while, in the case of laser peening, the results exhibited more profound differences
between both configurations.
For example, in the case of three layers of laser peening, the differences between
Configuration I and II were almost identical until a crack length of 12.5 mm was reached.
At that point the benefits in fatigue life for Configuration II surpassed the fatigue life in
Configuration I by 34%. These differences are mostly attributed to residual stress
distributions in the different crack configurations.
19
0
5
10
15
20
25
30
35
40
0 20000 40000 60000 80000 100000 120000 140000 160000 180000Number of Cycles
Cra
ck L
engt
h (m
m)
Laser 300% Conf2Laser 300% Conf1Unpeened Conf2Unpeened Conf1
Figure 14 Differences in fatigue life between configuration one and two for specimens peened with a triple layer of laser peening on FSW 7075-T7351
0
5
10
15
20
25
30
35
40
0 20000 40000 60000 80000 100000 120000Number of Cycles
Cra
ck L
engt
h (m
m)
Laser 100% Conf2Laser 100% Conf1Unpeened Conf2Unpeened Conf1
Figure 15 Differences in fatigue life between configuration one and two for specimens peened with a single layer of laser peening on FSW 7075-T7351
20
3.3. Fractography
To evaluate the FCG behavior, the specimens were loaded to fracture under static
tensile loading. Fatigue striation patterns were clearly visible on the fractured surfaces of
the tested FSW 7075-T7351 specimens. Nevertheless, the fatigue striations spacing for
the laser peened specimens were smaller when compared to the un-peened, and shot
peened specimens. This reduction in striation spacing indicates a slower FCG rate and is
partially attributed to the deeper compressive residual stresses induced by the laser
peening. The fatigue striations for an un-peened ConfigurationI specimen are shown in
Figure 16. These pictures were taken at different locations across the crack.
Figure 16 Fractographic images for the fracture surface for Configuration I for a FSW AA 7075-T7351 specimen
21
Comparisons between fatigue striations for Configuration I are shown in figure 17
for different peening techniques. The graph represents striation spacing at different
locations across the crack. Under constant stress intensity conditions, striation spacing in
a local region can vary by a factor of two to four [35]. The main reason for this scatter is
attributed primarily to the fact that striations formation is a highly localized event. The
striations spacing is also dependent on both the stress intensity factor and metallurgical
factors such as variations in the grain orientation, and the distribution of inclusions in that
particular area. Therefore, several measurements of striation spacing were taken to get a
meaningful estimate of fatigue striations at a particular crack length.
The fatigue striations from the triple layer of laser peening exhibited the least
striation spacing compared to the other investigated specimen. The un-peened condition
resulted in the highest striation spacing in the group. The difference in striation spacing
between the peened and un-peened conditions was reduced at larger crack sizes. This is
attributed to the fact that as the crack increases in size, more residual stresses will be
released; eventually the majority of the residual stresses induced by the peening process
will be diminished at a large crack size, and the peening becomes ineffective in reducing
the FCG process. These results seem to be in agreement with the fatigue crack growth
rates in Figures 9 and 12.
22
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
1mm 5mm 12mmDistance from notch
Stria
tion
Spac
ing
(μm
)UnpeenedS hot P eenedLaser P eened (100% )Laser P eened (300% )
Figure 17 Striation spacing for configuration one at different distances from the
notch
The fatigue striations for Configuration II are also shown in Figure 18. These
pictures were taken at different locations across the weld. The fracture surface
appearance was different according to the location across the weld. In the nugget region,
the fracture surface was flat and smooth. At 15mm from the notch, the surface
morphology was rough. As the fatigue crack extends, the stress intensity factor and the
plastic zone size increase in size. When the plastic zone is large compared to the
thickness of the specimen, plane stress conditions and slant fracture start to take effect.
23
Figure 18 Fractographic images for the fracture surface for configuration two for a FSW AA 7075-T7351
Figure 19 illustrates the fatigue striation spacing for a laser peened FSW specimen
at different distances from the edge of the sample. The striation spacing was higher at
3.0mm compared to the spacing at 0.5 mm from the edge of the specimen. This
difference is attributed to the fact that closer to the edge, the plastic zone is generally
higher because of constraint effects which generate higher resistance to crack growth
rates. The difference may also be attributed to the fact that compressive stresses from the
laser peening are expected to be higher in magnitude at 0.5 mm than at the center of the
crack. Toward the center of the specimen, residual stresses are expected to be tensile to
balance the compressive stresses introduced at the near surface of the specimen.
24
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1mm 2mm 5mm 7mm 12mm 1mm 2mm 5mm 7mm 12mm3 mm from surface
Stria
tion
Spac
ing
(μm
)
=
0.5 mm from surface
Figure 19 Striation spacing for a laser peened specimen at 0.5 and 3 mm from the surface
Fatigue striation spacing measurements along the crack length for specimens
with and without laser peening is presented in Figure 20. Fatigue striations spacing
which can be directly related to the crack growth process were again less than the
unpeened condition. These results seem to agree with the results from [36] where the
effect of compressive stresses for a rivet hole before and after cold working was
investigated in relation to fatigue striation in 2024-T3 aluminum alloy. As in
configuration one, the difference in striation spacing between the peened and un-peened
conditions started to diminish as the crack size increased.
00.10.20.30.40.50.60.70.80.9
1mm 2mm 5mm 7mm 12mmNo Peening
Stria
tion
Spac
ing
(μm
)
=
00.10.20.30.40.50.60.70.80.9
1mm 2mm 5mm 7mm 12mmLaser Peening (3 layers)
Stria
tion
Spac
ing
(μm
)
=
Figure 20 Striation spacing for peened and un-peened FSW 7075-T7351
25
4 Summary
The surface modification from laser and shot peening was used to introduce
compressive residual stresses into friction stir weld (FSW) AA 7075-T7351. The fatigue
behavior in the tests was determined by measuring the fatigue crack length (a) versus the
number of cycles (N) for the base non-welded material, the non-shocked FSW, shot
peened, and two laser peened conditions. Their influence on the fatigue crack growth
rate of FSW was characterized and evaluated for two different crack configurations.
The laser peening using three layers resulted in substantial increase in fatigue life
when compared to the unaltered condition. The improvement ranged between 200-280%
depending on the crack configuration. Fatigue striation spacing for the laser peened
specimen was less than other specimen. This reduction in striation spacing indicates a
slower FCG rate, and was attributed in part to the deeper compressive residual stresses
induced by the laser peening.
5 Acknowledgment
The authors are grateful to Mrs. Irene E. Kaye, Mr. Joseph E. Rogers, and Mr.
Gregory F. Galbreath from the NASA Johnson Space Center for their logistical support
for this project. The authors are also grateful to Mr. Raymond Patin, and John Figert
from the NASA Johnson Space Center, and Dr. Lloyd Hackel from the Metal
Improvement Company for their valuable comments.
26
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