Laser Peening Vs. Shot Peening: Engineering Of Residual .
Laser Peening vs. Shot Peening: engineering of residual stresses,surface roughness and cold working0. Higounenc11 Metal Improvement Company I Curtiss Wright Surface Technologies, Bayonne, FranceAbstractIn Laser Peening (LP), compared to Shot Peening (SP), the magnitude of residual compressivestress at the surface is the same: about 60% of elastic limit; but the depth is much higher. InLow Cycle Fatigue (LCF) depth of residual compressive stress is beneficial because it doesnot only delay crack initiation, but also crack propagation. Depth of residual stress is also critical in Foreign Object Damage (FOO) and Crack Grow Rate (CGR) applications.Changes in surface roughness, as soon as cold work, are much more significant in shot SPcompared to LP. Those changes can have a positive, or a negative influence depending onapplication and condition: HCF or LCF, deteriorating or non-deteriorating environment such ascorrosion or FOO, fretting, surface contact fatigue.Thus, LP is complementary to SP in some specific applications such as LCF, FOO, CGR. Theother advantage is the extremely high process control that allow more easily to take into account the credit of LP in the design .Keywords: shot peening, laser peening, fatigue life, crack nucleation, crack propagationIntroductionLaser Peening (LP) is a process in which an iintense beam of laser light (irradiance 2 to1OGW/cm2) is directed on to a sacrificial ablating material placed on the surface of the component to be treated. The light rapidly vaporizes a thin portion of the ablative layer, producinga plasma that is confined by the inertia of a thin laminar layer of water (-1 mm thick) flowingover the surface. In response to the rapidly expanding plasma, a shock wave (peak pressure-100 kbars) is generated in the part, and run into the material, creating a plastic strain thatresult in residual stress field (Fig.1 ).The spot size for an individual laser peening impact is a square in the range of 3 mm to asmuch 10 mm on a side scaled so that the shock wave intensity is one to two times the dynamicyield strength of the material being peened. This pressure wave generated by this large footprint laser beam can propagate into the metal to a depth roughly equivalent to its width withnearly constant yielding intensity thereby plastically compressing deeply into the material. Theresulting transverse expansion of the compressed material reacts within the component togenerate a compressive stress field and/or strain (shape change) depending on the component's geometry and material properties. The plastic strain depth results in residual compressive stress induced to depths up to 8 mm.Shot peening (SP) [1] is process in which the surface of a part is bombarded with small spherical media called shot (material can be steel, glass or ceramic, from 50µm to 3mm in diameter).Each piece of shot striking the metal acts as a tiny peening harmer imparting a small indentation or dimple on the surface. In order for the dimple to be created, the surface layer of the partmust be yeld in tension. Below the surface the compressed grains try to restore the surface toits original shape producing a hemisphere of cold-work material highly stressed in compression. Overlapping dimples develop a uniform layer of residual compressive stress (Fig.2).226
Fig. 1 & 2: laser peening (left) and shot peening (right) basic concepts1- Effects of Laser Peening and Shot Peening on materialThe effects of laser peening and shot peening on the processed material can be grouped inthree categories: Redistribution of residual stresses Surface topography modification Cold work1-1 Residual stressResidual stress profile resulting from shot peening and laser peening can typically be described by two values: The magnitude of compressive stress at the surface (or just below the surface):nearly the same in LP or SP, about 60% of the elastic limit. The depth of compressive stress, which is the depth where residual compressivecurve cross the residual stress in the bulk material: it is up to 10 times deeper in LPcompared to SP.An example on carburized 9310 is shown on figure 3. 100"iu0. --350,!( I)-;; ,::,-700 -- ----F--------------l"iii(I) I-1050, ·-····-·-----------,I-1400 ; - - - - - , - - - - - , - - - - - - - - , - - - - . , - - '00.51.01.52.02.5Depth (mm)Figure 3: residual stress profiles in carburized 9310 after heat treating, SP and LP1-2 Surface topographyIn LP there is almost no surface topography modification because spots are very big (3 to1Omm) and depth of indent is very small (a couple of tenth of microns).227
In SP on contrary, each dimple is much more deep (figure 4 ), depending of course on SP parameters.Figure 4: surface topography on aluminum blocks 8.5 x8.5mmafter shot peening, partial coverage on left and full coverage on right1-3 Cold workSP induce a lot of cold work, while LP almost not. Reasons are: Each dimple from SP creates more plastic deformation than LP shock wave and indent. SP is stochastic process: each zone needs in average to be stroked 13 times toachieve 100% coverage while one or two shots are enough in laser peening (assuming square shots).Most of metallic materials will harden during cold working, figure 5.aMonotonica-f. curvea.,.1 cycle CT-f. curvea.,.n cycles 0-f. curves0Figure 5: Cyclic elasto-plastic deformations can result in hardening,or softening the material2- Different phases of the fatigue lifeUnderstanding of the fatigue mechanism is essential for engineering of residual stress, surface roughness, and cold work.Crack nuclei start as invisible microcracks in slip bands. Nucleation of microcracks generallyoccurs very early in the fatigue life, almost immediately if a cyclic stress above the fatiguelimit is applied. Microcracks remain invisible for a considerable part of the fatigue life.After a microcrack has been nucleated, crack growth can still be a slow and erratic processdue to effect of microstructure. After some microcracks has occurred away from the nucleation site, a more regular crack grow is observed. This is the beginning of the real crack growperiod [2].Thus, fatigue life until failure consists in two periods: the crack initiation period the crack propagation period.Different phases of the fatigue life are indicated in figure 6.228
Dislocation activities. at low stress amplitude {below yeld stress) Preferability at material surface because: the surface grain ls less contained by neighbouring material surface stress concentrations: holes, roughness. corrosion pit, fretting .Initiation period Grain size Invisible Still a surface phenomenon Penetrate into the material Depend on the material as bulk property No longer a surface phenomenonPropagation periodFigure 6: different phases of the fatigue life.S-N, or Wohler curve, represent the end of the propagation period, when the part fails.Now, let's imagine we could detect, and plot the end of the initiation period: it's the Frenchcurves (figure 7).On logarithmic chart the gap between two curves tend to zero as long as the number of cycles:in High Cycle Fatigue domain (HCF) the percentage of the crack propagation period in the totallife is very small, and this percentage increase in Low Cycle Fatigue (LCF).Crack initiation curve (French curve)/Fatigue curve (Wohler ··."··············!1 o I 05IJI10 6Endurante limitI!107Fatigue life N (cycles)Figure 7: French curve, representing the crack initiation area, and Wohler curve, representing the crack propagation time; assuming non deteriorating environment.It's important to remember that: Crack initiation period represent a big percentage of the total fatigue life of a structure. Crack initiation is a material surface phenomenon229
Crack propagation period is material bulk properties dependent.Now, let's see how residual stress profile, surface roughness and cold work influence the fatigue properties.3- Engineering of residual stresses, surface roughness and cold work3-1 Engineering of residual compressive stressAssuming non deteriorating environment such as Foreign Object Damage (FOO) or corrosion,big depth of residual compressive stress is beneficial in LCF only, where crack propagationperiod is a significant percentage of the total fatigue life (figure 8) .,,.- -,- -,- -r- "'.(I)V)Fatigue life N (cycles)Figure 8: comparison of SP and LP: effect of residual stresses only innon-deteriorating environmentAssuming deteriorating environment, the low depth of residual compressive stress from SPcan be insufficient to increase significantly the fatigue properties (figure 9)."'(I)VI.V)\\\\' ' ',,J.,I- -Base- -material- -. - - - - - !1l- - - - - - - - !iFatigue life N (cycles)Figure 9: comparison of SP and LP: effect of residual stresses only indeteriorating environment3-2 Engineering of surface topographySurface roughness from shot peening is characterized by peaks and valleys.In HCF application, the roughness increase resulting from SP has a negative effect because itwill most often increase the surface Kt, and increase cracks nucleation; this effect is even moredetrimental when surface roughness before SP is smooth, and when material is notch sensitive.In LCF, this effect is less significant because crack propagation is not a surface phenomenonIn fretting-fatigue and fretting-corrosion applications, peaks from shot peening have a positiveeffect because they reduce the friction factor; valleys have also a positive effect because the230
help in neutralizing fretting fragments at the bottom of craters. So, surface roughness from SPis beneficial.In contact fatigue applications, peaks have negative influence because they increase stressconcentration at the surface; on the opposite valleys have positive influence because theyimprove lubrication. So, increase of surface roughness can be beneficial or detrimental depending on surface topography and application (micro or macro-pitting).3-3 Cold workCold working, when hardening the material (typically changing austenite into martensite inaustenitic stainless steels, or in gears after carburizing), will be very positive in HCF applications because it will: reduce the grain size and delay micro-crack propagation, improve the mechanical properties of the material, increase the magnitude of residual compressive stress.But on the other side is has been demonstrated that work hardening increase the residualstress relaxation. So, work hardening can be detrimental in LCF application, and thermal fatigue applications.ConclusionThe fatigue life consists of two periods, the crack initiation and the crack propagation. In mostof the applications the crack initiation period, which is a surface phenomenon, is a big percentage of the total fatigue life. There is no need in deep residual compressive stress but surfaceintegrity is important to preserve.LP is complementary to shot peening in some specific applications such as LCF, FOO, CGRwhere SP is inefficient because crack propagation period is a significant percentage of the totalfatigue life.Also, even if control and repeatability of SP becomes more and more precise when using newgeneration of robotic machines, the process control of LP is unequalled and allow more securely designers to take the credit of fatigue improvement in the design.References[1] Shot Peening Applications, Eight Edition, Metal Improvement Company[2] Jaap Schijve, Fatigue of Structures and Materials, Second Edition, Springer, pp 14-15231
Fig. 1 & 2: laser peening (left) and shot peening (right) basic concepts 1-Effects of Laser Peening and Shot Peening on material The effects of laser peening and shot peening on the processed material can be grouped in three categories: Redistribution of residual stresses Surface topography modification Cold work 1-1 Residual stress
Shock wave Fig.1 Schematic of laser peening Fig. 2 Comparison of residual stress profiles in each condition Table2 Laser peening parameters the compressive residual stress layer built by laser peening is deeper than that of shot peening(3). (2) Material Some aluminum forgings have been used in numerous fatigue critical parts of aircrafts with
Jan 18, 2001 · H:\Docs\Training\SPO_01_18_2001 REVISED.wpd 3of37 1. INTRODUCTION 1.1 Shot Peening Described Shot peening is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple.
shot peening. It is seen that the area fraction and thickness of the nanocrystalline layer increased with peening time. In the Fig. 3 SEM micrographs of Fe-0.05C-1.29Mn high tensile strength steel after shot peened for 10s. (a) as shot peened and (b) annealed at 873K for 3.6ks after shot peening.
development of high peak power short pulse from Nd:YAG laser along with its peening application. It presented the design scheme of laser and the characteristic of laser beam transmission. Zhu [15] et al. discussed the influence of laser shock peening on surface morphology and mechanical property of Zr-based bulk metallic glass.
The net result of shot peening is often a noticeable improvement in fatigue properties (Ref. 5). Shot peening under a prestress can produce an even higher level of compressive stresses (Ref. 3). Laser-shock peening (LSP) was first used by Battelle Columbus Laboratories in 1974 (Ref. 6). In this process, the
Table 2 gives the parameters used for shot peening the dog bone samples. Shot peening was done using a Westward blast cabinet equipment with a pressure regulator. To produce consistent results, the jig in Fig. 9 was used to secure the sample. Fig. 8 Pseduo-Almen strip deflection after shot peening. ABS 1.75 mm filament X Y Z
using fine shot peening. These rollers are herein denoted as SP rollers. The sur-face roughness was varied by approximately 0.2 to 2.1 using shot peening. The test roller without shot peening was also conducted for comparison. These roll-ers are herein denotedas NP rollers. The surface roughness of the non-peened
including ANSI A300. A good practice in mixed planting areas is to plant trees first followed by the larger shrubs, low shrubs and finally with ground cover plants. This prevents damage to the smaller plants; however the Contractor is responsible for sequencing. Check that plants are moist at the time of planting. Verify that trees or shrubs if marked with compass orientation are planted in .
