Laser shock peening (LSP) is a promising surface treatment for fatigue life extension of metallic materials. The benefits include deeper residual stresses (>1 mm) and a smoother surface finish than conventional glass bead peening. However, fatigue tests on 7050 aluminium alloy specimens have shown that LSP may cause a reduction in life due to internal cracking generated during the process. A series of tests were carried out to identify any technological risks such as internal cracking associated with the use of LSP. Numerical modelling was also carried out and the results of this numerical prediction agreed well with the experimental results

Barter, S. A.

Clark, G.

Ding, K.

Liu, Q.

Rey, C.

Sharp, P. K.

Ye, L.

Swinburne Research Bank

Spallation-like phenomenon induced by laser shock peening surface treatment on 7050 aluminum alloy

University of Queensland eSpace

SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
Spallation-Like Phenomenon Induced By Laser Shock Peening Surface 
Treatment On 7050 Aluminum Alloy 
Q. Liu1, K. Ding2, L. Ye3, C. Rey1, S.A. Barter1, P.K Sharp1 and G. Clark1 
1Air Vehicles Division, Defence Science and Technology Organisation (DSTO), Australia 
2Centre for Railway Engineering (CRE), Central Queensland University, Australia 
3Centre for Advanced Materials and Technology (CAMT), University of Sydney, Australia 
 
 
 
ABSTRACT Laser shock peening (LSP) is a promising surface treatment for fatigue life extension of 
metallic materials. The benefits include deeper residual stresses (>1 mm) and a smoother surface finish than 
conventional glass bead peening. However, fatigue tests on 7050 aluminium alloy specimens have shown 
that LSP may cause a reduction in life due to internal cracking generated during the process. A series of tests 
were carried out to identify any technological risks such as internal cracking associated with the use of LSP. 
Numerical modelling was also carried out and the results of this numerical prediction agreed well with the 
experimental results. 
 
1 INTRODUCTION 
Laser Shock Peening (LSP) is an innovative surface treatment method, which has been proven to 
greatly improve the fatigue life of many metallic components [Thompson, et al, 1997; Liu, et al, 
2002a]. Compared to conventional Shot Peening (SP), the LSP process introduces a deeper layer of 
compressive residual stresses - up to a millimetre deep [Clauer and Lahrman, 2000] - whereas SP 
provides a compressive residual stress layer of about 250 µm deep. LSP produces very little or no 
modification of the original surface roughness or dimensions of the component and has been 
successfully applied to increase fatigue life, reduce fretting fatigue, enhance resistance to corrosion 
and increase compressor blade foreign object damage resistance [Peyre, et al, 1995; Clauer, 1997; 
Dane, et al, 1997; Ruschau, et al, 1999]. The results of previous testing by the Defence Science and 
Technology Organisation (DSTO), Australia, have demonstrated LSP to be effective in extending 
the life of extruded 7075-T6 Al alloy coupons [Liu, et al, 2002a]. However, experimental results 
[Liu, 2002b] also revealed that some problems might arise under particular conditions, which will 
prevent the achievement of the full potential life extension. This paper reports the results of an 
investigation into the effect of laser power density on the fatigue life of 7050-T7451 aluminium 
alloy and the internal cracking phenomenon.    
 
2  MATERIAL AND EXPERIMENTAL PROCEDURE 
2.1 Material and Specimen Geometry 
The material under investigation in this paper is a 7050-T7451 aluminium alloy which is mainly 
composed of ~2.07Cu, 2.05Mg, 6.05Zn and bal. Al in wt.%. The test specimens were manufactured 
from the 150 mm thick plate. The specimens were cylindrical with a reduced central area with a 
continuous radius in the form of an ‘hour glass’. The diameter at the middle section was 10 mm. 
2.2 Surface Treatment by LSP and SP 
The laser system was a dual Q-switched high energy-pulsed neodymium (Nd)-glass phosphate laser 
with a wavelength of 1.054 µm and pulse duration of 15 to 25 ns (nanoseconds). Its repetition rate 
was 0.125 to 0.25 Hz. The averaged laser beam diameter was about 5.5 mm. The system was 
configured to ‘shot’ simultaneously from two opposing sides and was used in this configuration to 
treat the specimens here. For comparison, a number of specimens received glass bead peened 
surface treatment with a coverage rate of 200% and Almen intensity of about 8A. Some of the 
specimens were tested in un-peened (as-machined surface) condition as benchmark. 
 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
2.3 Testing Procedure and Metallographic Analysis 
The fatigue specimens used for this work were surface treated by LSP by LSP Technology, Inc, 
USA. The fatigue tests were conducted in a MTS 100kN, digitally controlled test machine, under 
load control mode, using a wing root bending moment load spectrum representative of about 300 
hours of service for a military fighter aircraft. The applied stress was 390 MPa.  The test frequency 
was 10 Hz. The fracture surface of the specimens treated by LSP was analysed using an optical 
microscope.  
 
3 MODELLING OF LASER SHOCK PEENING PROCESS 
A 3-D finite element model was developed to simulate the LSP process, where two laser pulses 
simultaneously impact both sides of the target surface. For simplicity, a square laser spot replaced 
the circular one. Since the model was symmetrical in geometry and subjected to a symmetric 
impulse pressure [Ding, et al, 2002], a “quarter model” was used instead of a full model to speed up 
analysis. A schematic of the configuration of the finite element (FE) model with finite and infinite 
elements as well as boundary conditions is shown in Figure 1, where the radius (R) is 6 mm.  To 
simulate the two-sided LSP process, some assumptions were made for the FE calculation. These 
were: (1) Infinite elements being adopted for non-reflecting boundaries; (2) The specimen material 
would obey a Von Mises plasticity criterion; (3) The pressure pulse induced by the plasma was 
uniform over the entire surface of the laser spot; and (4) The dynamic yield strength (σydyn) under 
uniaxial stress conditions defined in terms of Hugoniot elastic limit (HEL) [Johnson and Rohde, 
1971]. In the FE analyses, the ABAQUS codes were used to calculate the propagation of the short 
duration shock wave and the dynamic stresses in the target [ABAQUS, 2001] The detail of the 
finite element analysis can be seen in [Ding, et al, 2002].  
 
4 EXPERIMENTAL RESULTS AND DISCUSSION 
4.1 Fatigue Testing Result 
A previous study found a spallation-like phenomenon (i.e. internal cracking) [Liu, et al, 2002b]. In 
order to further understand the effect of laser power density on the fatigue life, a series of tests with 
a range of laser power densities, from 2 GW/cm
2
 to 7 GW/cm
2 , were performed on the 7050 Al 
specimens, with the same geometry as in the previous study. Figure 2 shows the experimental 
results of the fatigue life as a function of laser power density. This result clearly shows that when 
the laser power density is 5 GW/cm2 or higher, the fatigue life is dramatically reduced. The reason 
for this reduction of the fatigue life is the formation of internal cracks in the specimens during the 
laser treatment, which act as large crack initiators, larger than any surface discontinuities. Below 
this 5 GW/cm2 value, internal crack was only found at 3 GW/cm2 level. The causes of the internal 
crack in some 3 GW/cm2 specimens are still being analysed. As Figure 2 clearly shows when LSP is 
optimal, a 3 times increase in fatigue life can be achieved over un-peened specimens compared to 
1.5 – 2 times for glass bead peened specimens at this high peak fatigue stress of 390 MPa. 
However, under some conditions the fatigue life is equal to or less than the as-machined mean 
value. 
 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
Finite Element
Infinite Element
Impact Zone
Symmetric 
Boundary 
Surface
Symmetric 
Boundary 
Surface
y
x
z
0
Zp
R
R
A
D
0 B
C
 
Fig.1 Three-dimensional (3D) Finite element model with symmetric boundary conditions 
 
 
0 1 2 3 4 5 6 7 8
0
20
40
60
80
100
120
140
160
180
200
 without internal crack
 with internal crack
 un-peened 
(mean value)
Glass bead peened
 (mean value)
7050-T7451 Al 
Fa
tig
ue
 L
ife
, N
o 
of
 B
lo
ck
s
Laser Power Density, GW/cm2  
Fig.2 Fatigue life as a function of laser power density on LSP 7050 Al specimens under spectrum loading at applied 
peak stress of 390MPa.   
 
4.2 Fracture Surfaces 
A typical macroscopic fracture appearance of a LSP fatigue specimen is shown in Figure 3. In this 
figure, (a) shows the overall appearance of a fracture and (b) shows the detail of internal cracking 
near the central axis of the specimen. The bright area shown by dashed line in Figure 3 was the 
fatigue zone. Except for the central internal cracking, there existed multiple surface cracks in new 
7050 Al series specimens. The main reason for this was the presence of surface black ‘burnt’ spots 
(defects)The large in-plane cracks initiated perpendicular to the fatigue cracks. The overall fracture 
profiles in the LSP specimens were typically extremely uneven since many fatigue cracks on 
different planes were initiated by these large laser induced cracks. More details of fractographic 
analysis can be found in a DSTO report [Liu, et al, 2004]. 
 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
 Surface cracks 
 Internal cracking
(a) (b)
 Fatigue zone
 
Fig.3 Macroscopic appearance of 7050-T7451 aluminium alloy (new series) induced by LSP at the applied peak stress 
of 390MPa laser power density of 7GW/cm
2
: (a) the overall appearance of the fracture and (b) the macroscopic cracks 
are clearly shown across the centre of the specimen. 
 
4.3 Numerical Analysis 
As stated previously, laser power density has a significant influence on fatigue life of LSP 7050 
specimens through the occurrence of internal cracking. These cracks are closely related to the 
dynamic tensile stresses and material properties. The former is dependent on the plasma pressure 
generated on the surface of the specimen. If the laser power density is increased, the plasma 
pressure is also increased. Consequently, the amplitude of the dynamic tensile stress wave is 
increased.   
Figure 4 shows the distribution of the predicted dynamic stresses along the y-direction (refer to 
Figure 1) at different times with different pressures applied to the surface of the specimen. The 
results show that at the beginning, the compressive stresses occur near the treated surfaces. The 
peak compressive stress is about -2000 MPa. However, the peak compressive stress is reduced with 
time. Increasing the peak pressure leads to an increase in amplitude of both compressive and tensile 
stresses. At the same time (i.e. 1200 ns after the peening) near the centre of a specimen, the 
dynamic tensile stress can reach a very high level, which may exceed the critical stress of the 
material.  
Once the tensile stress exceeds the critical stress, a crack can form. For example, Figure 4 shows the 
numerical results under the peak surface pressure of 2.7 GPa (corresponding to a laser power 
density of 7 GW/cm2). At the solution times1 of 800, 1000 and 1200 ns, the peak tensile stress 
reaches 1200, 1060 and 1210 MPa at 2.27, 1.01 and 0 mm from the centre of the specimen, 
respectively. With decreasing surface pressure (i.e. laser power density), the corresponding stresses 
are reduced. The dynamic fracture strength of the 7050 Al specimens is ~900 MPa. [Liu, et al, 
2004]. Figure 5 shows the relationship between the predicted dynamic tensile stress and peak 
surface pressure during the LSP process. Clearly, when the pressure is higher than about 2.2GPa 
(corresponding to a laser power density of 4.65 GW/cm2), the local tensile stress can easily reach 
the critical tensile stress (~900 MPa), leading to the formation of a crack.   
 
                                                 
1 The solution time means the time that is simulated by the analysis. The analysis can simulate the stress states that 
would exist at specific time after the peening. 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
  
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
0 1 2 3 4 5 6
Radial from centre of specimen, y [mm]
D
yn
am
ic
 S
tr
es
s,
 σx
x [
M
Pa
]
200 ns 400 ns
800 ns 1000 ns
1200 ns 2000 ns
                                               
Fig. 4 Dynamic stress distributions, σxx, in the radial direction (y) at the end of six increments at the surface pressure 
of P =  2.7 GPa (corresponding to laser power density of 7 GW/cm2). 
  
At the centre 
 (1200 ns) 
 Critical power density
 Critical fracture stress 
 Critical peak pressure
 
Fig. 5 Peak tensile stress as a function of peak surface pressure during the LSP process on 7050 Al specimens 
predicted by the 3D finite element analysis 
 
5 CONCLUSIONS 
The phenomenon of the internal cracking in 7050-T7451 aluminium alloy induced by LSP was 
investigated. One of the main factors causing the internal cracking was studied by experimental and 
numerical analysis. Conclusions can be drawn as follows: 
(1) The internal cracking caused by LSP in 7050 Al specimens was responsible for their shorter-
than-expected fatigue life. 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
(2) The numerical model developed to predict what power density levels could cause internal 
cracks was verified by the experimental results. 
(3) The laser power density is one of main factors causing the internal cracking in LSP specimens 
of 7050 aluminium alloy. When the laser power density is higher than a critical value, internal 
cracking can take place within the material, which shortens its fatigue life. In this investigation, 
the critical value was found to be about 4 ~ 5 GW/cm
2
 obtained from both experimental and 
numerical results, although unaccounted for geometry effects may make the safe critical power 
density lower.   
(4) The causes of anomalous results from the 3 GW/cm2 power density level are being 
investigated. 
(5) If LSP process is optimised for the material/configuration, fatigue life extension can be 
achieved.  
 
ACKNOWLEDGEMENTS 
The authors sincerely appreciate the support for this work from both the Air Vehicles Division, 
DSTO and Australian Defence Force. Authors express their sincere thanks Mr C. Rey and Mr. B. 
Jones for conducting the fatigue testing.  The authors also gratefully acknowledge LSP 
Technologies Inc, USA, for providing the laser shock peening treatment. 
 
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Spallation-Like Phenomenon Induced By Laser Shock Peening Surface Treatment On 7050 Aluminum Alloy

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Spallation-Like Phenomenon Induced By Laser Shock Peening Surface Treatment On 7050 Aluminum Alloy

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