LSP to Prevent FOD And Fatigue
Foreign object damage (FOD) and fatigue cracking are two of the primary failure mechanisms for metals and alloys. Laser peening is regularly applied to fan and compressor blades in aircraft gas turbine engines to protect against failure from foreign object damage.
The United States Air Force performed a study comparing damage tolerance enhancement for laser peened and shot peened fan blades.1 These blades are fabricated from the Ti-8Al-1V-1Mo titanium alloy and used in the General Electric F101 engine.
The laser shock peened blades were treated with one LSP intensity. The shot peened blades were treated at two different intensity levels; one a typical dual-intensity peening procedure using glass beads followed by shot at an intensity of 0.014A, and the other a high intensity peening procedure consisting of peening a narrow, 0.25-inch (6.3-mm) wide strip at the leading edge of the blade to 0.012C and a 0.5-inch (12.7-mm) wide strip behind it to 0.018C. These are shown in Figure 1. The latter shot peening condition was intended to provide a deep residual stress distribution comparable to that of LSP. With both methods, the blades were not treated over the entire surface, but rather over a patch 1.5 inches (38 mm) long by 0.75 inches (19 mm) wide along the leading edge of the airfoil in the region where the blades are most sensitive to FOD damage.
To simulate FOD, after the LSP and shot peening treatments, either a 0.25-inch (6.3-mm) deep notch made with a chisel, or a 0.125-inch (3.1-mm) deep notch made by electrical discharge machining was introduced into the leading edge of the blades. These notches were located in the center of the treated region as shown on the left in Figure 1.
Figure 1 - The configuration of the treated areas on the leading edge of F101 First Stage fan blades.1
Before fatigue testing, the blades were heated to 400 F and held for 24 hours to accommodate any stress relief which might occur under this condition in service. After the stress relief, the blades were tested in a siren testing device. The test procedure was to begin testing at a low stress amplitude level, then increase the stress amplitude by 10 ksi after every 106 fatigue cycles. The results are shown in Figure 2. It is striking that LSP was able to retain a fatigue life at least equivalent to that of the undamaged, peened blade, even in the presence of the severe damage. Shot peening also increased the fatigue life of the damaged blades compared to blades that received no peening treatment at all.
Figure 2 - Comparison of laser peening and shot peening for foreign object damage resistance in F101 fan blades.1
Several different aspects of laser shock processing and fatigue conditions have been investigated in aluminum alloys. One comparison made in 2024-T351 aluminum alloy plate concerned fatigue crack propagation from a fastener hole, Figure 3. The fatigue specimens were large: 0.25-inch (6.3-mm) thick, with a gauge section 4 inches (101.6 mm) wide by 10 inches (254 mm) long. The fatigue behavior was determined by measuring the fatigue crack length, a, vs. the number of cycles, N, for the non-shocked condition and two laser shock peened conditions. For one of the LSP conditions, the hole was in the center of a solid treated spot. In the other condition, the hole was surrounded by an annular-shaped spot. The specimens were tested in tension-tension, R = 0.1, at 15 ksi maximum stress amplitude. Considering the fatigue life to be nominally the point where the crack length curves become nearly vertical, the LSP condition with the solid spot had a fatigue life about 40 times longer than the non-shocked condition, whereas the condition with the annular spot had a life about 3 times longer than the non-shocked condition.
Figure 3 - Fatigue crack propagation in non-shocked and laser shock peened 2024-T351 aluminum. The crack length, a, is shown vs. the number of cycles, N.
When the areas around holes in sheet materials are laser shock peened with a solid spot around a hole, the crack tends to initiate on the hole surface at mid-thickness, and then tunnels down the mid-thickness of the sheet between the compressive surface layers before it breaks through to the surface beyond the laser shocked region. While the crack propagation rate is slowed considerably by this behavior and fatigue life is significantly increased, the crack itself is not easily detected. This is sometimes a concern in failure-sensitive applications.
The annular spot was chosen to demonstrate that a crack could be initiated and detected at a fastener hole, and then be significantly slowed when it reaches the laser shocked region. This would allow ample time for the detection and monitoring of cracks originating from holes before failure.
LSP is also effective in arresting pre-existing cracks. Large specimens similar to that described above were pre-fatigued to create cracks 0.020 inches (0.5 mm) long, coming out each side of the fastener hole. The region ahead of each crack was then laser shock peened, and the specimens re-tested. The results are shown in Figure 4. The unshocked condition had a fatigue life of about 145,000 cycles, and the laser shocked condition without a pre-crack had a lives of 700,000 to 1,000,000 cycles. After pre-cracking and laser shocking, the fatigue lives were in the same range as those of the laser shocked material tested without a precrack.
Figure 4 - The effect of laser shock peening on pre-cracked specimens of 2024-T351 aluminum.
The fatigue life of weldments can also be extended by LSP. Plates of 5456 aluminum alloy 0.25 inches (6.3 mm) thick were butt-welded together, the weld bead machined off, and the weld and heat-affected-zones laser shocked with overlapping spots. Test specimens machined from this plate were then tested in tensile fatigue, R=0, in both the as-welded and laser shock peened conditions. Results from the testing are presented in Figure 5. At a stress amplitude of 20 ksi, the fatigue life was increased by at least an order of magnitude. More significantly, at 23 ksi the fatigue life was increased from less than 50,000 cycles to more than 3 to 6 million cycles without failure after LSP.
Figure 5 - Fatigue life increased in welded 5456 aluminum after laser shock peening.
Laser shock peening is also effective against fretting fatigue. Dog-bone specimens and pads of 7075-T6 aluminum were laser treated around a simulated fastener hole in each piece (Figure 6a), then fastened together through the hole with a manufactured (CSK) fastener. This combination was then fatigue tested in tension at R = 0.1. The stress differential between the larger cross-section of the pad and the smaller cross-section of the dog-bone created an elongation differential between the two pieces during each cycle, leading to fretting around the fastener hole. The results are shown in Figure 6b. The tests were initially conducted at 14 ksi. When a long life was reached, the stress was raised in 10 percent increments until failure occurred within few hundred thousand cycles. Even at 16.4 ksi, the fretting fatigue life is increased by LSP.
Figure 6- Increase resistance to fretting fatigue around fastener holes after laser shock peening 7075-T6 aluminum.
Fatigue improvement after laser shock peening thin sections was demonstrated in steel sheet. 4340 steel sheet, 0.060 inches (1.5 mm) thick, heat treated to RC 54 hardness, was laser shock peened and tested in tensile fatigue in the specimen configuration shown in Figure 7a. The roots of both notches were laser shocked with one spot, using a split beam as shown. The fatigue tests were conducted at R = 0.1 with the results shown in Figure 7b. The run-out stress of the unshocked condition was taken from a handbook. The specimen numbers are attached to the individual data points. Specimens 2, 3, and 4 were stepped up in stress after reaching well over a million cycles. Laser shock peening produced a large increase in the run-out stress, from about 85 ksi (586 MPa) to 140 to 150 ksi (965 to 1034 MPa).
Figure 7 - The tensile fatigue strength of 4340 steel sheet increased significantly by laser shock peening.
In thicker pieces, a notch would be treated by laser shock peening directly into the notch. To demonstrate the effectiveness of LSP in this case, a beam specimen of 4340 steel, hardened to 54 RC, 0.25 inches (6.3 mm) thick, 0.75 inches (19 mm) deep and 8 inches (203.2 mm) long, had a notch having a KT 1.7 machined into the upper surface. The notch was shocked from one side only, directly into the notch. The beams were fatigue tested in 4-point bending with tensile loads at the notched surface. Without LSP the specimens failed in the notch after 40,000 cycles. After LSP at intermediate and high intensities, no fatigue failures were observed originating in the notch. Instead, after the bending load was increased to a level about 25 percent higher than the unshocked run-out stress, the specimens began failing under the loading pins. How much higher the notch failure load was raised could not be determined with this test configuration.
A part made from 1026 steel was failing at 30,000 cycles from the formation of cracks at the locations shown in Figure 8a. Just these regions were laser shock peened as shown in Figure 8b, with two overlapping spots. After treatment, the fatigue life was tripled from 33,000 cycles to 92,000 cycles.
Figure 8 - A 1026 steel part, locally laser shock peened to triple its fatigue life.
Fatigue limited metals, including pearlitic and ferritic malleable cast irons, and iron powder metallurgy materials show fatigue life improvements after laser shock peening.
A comparison between LSP and shot peening is shown in Figure9. This was performed on AF 1410 steel cylindrical test bars fatigue tested under flight spectrum loading. The LSP bar’s fatigue life was 56 percent longer than that of the unshocked bars, and 25 percent longer than that of the shot peened bars.
Figure 9 - Fatigue of AF 1410 Steel tested under Flight Spectrum loading conditions comparing shot peening and laser shock peening.
- C. O. Lykins, “Laser Shock Peening vs. Shot Peening, A Damage Tolerance Investigation”, Proceedings Surface Treatment of Titanium Alloys, Cincinnati, OH, October, 1996.
- A. H. Clauer, B. P. Fairand and B. A. Wilcox, “Laser Shock hardening of weld Zones in Aluminum Alloys”, Metallurgical Transactions A, 8A, 1871-1876 (December, 1977).
- A. H. Clauer and B. P. Fairand, “Interaction of Laser-Induced Stress Waves With Metals”, Applications of Lasers in Materials Processing, ed. by E. Metzbower, ASM, Metals Park, OH (1979).
- A. H. Clauer, B. P. Fairand, and J. Holbrook, “Effects of Laser-Induced Shock Waves on Metals”, Shock Waves and High Strain Rate Phenomena in Metals, ed. by M. A. Meyers and L. E. Murr, Plenum Press, New York (1981), pp. 675-702.
- A. H. Clauer, C. T. Walters, and S. C. Ford, “The Effects of Laser Shock Processing on the Fatigue Properties of 2024-T3 Aluminum”, Lasers in Materials Processing, ASM, Metals Park, OH (1983) pp. 7-22.
- T. R. Tucker and A. H. Clauer, Laser Processing of Materials, Report MCIC-83-48, Metals and Ceramics Information Center, Columbus, OH (November, 1983).
- Laser Shock Processing Increases the Fatigue Life of Metal Parts, Materials and Processing Report, 6, (6), 3 (September, 1991).