The Effect of Shot Peening and Residual Stresses on Cumulative Fatigue Damage

A series of constant and cumulative fatigue tests under the effect of shot peening were conducted for two aluminum alloys 2024 and 5052. Three unpeed specimens were tested for each alloy under low-high stress levels (120-280 MPa) for 2024 aluminum alloy and (40-90 MPa) for 5052 aluminum alloy at room temperature and stress ratio R= -1. Other specimens were exposed to shot peening with different blasting time before the cumulative fatigue testing. It is found for 2024 Al alloy that as the shot peening time increases the cumulative fatigue life is improved but above 10 min. the life is reduced. For 5052 Al alloy the cumulative fatigue life is reduced as shot peening time increases.


1-Introduction
Components are regularly subjected to dynamic loads, which make them prone to fatigue failure.It is a well known fact that almost all fatigue cracks form at the surface due to a variety of surface stress concentration failures.Evidently, the control of surface initiation and growth of cracks is an effective means of enhancing the fatigue endurance of metallic components.[1][2][3] The shot peening process is widely utilised for this purpose as it produces plastic deformation of the surface [3] leading to the creation of both surface work hardening and high residual compressive stresses at, or just below the surface layer [4].The magnitude of the residual compressive https://doi.org/10.30684/etj.28.15.14 2412-0758/University of Technology-Iraq, Baghdad, Iraq This is an open access article under the CC BY 4.0 license http://creativecommons.org/licenses/by/4.0stresses and the depth to which it extends beneath the surface of the component must be controlled, it becomes immediately apparent that, for shot peening to be consistently effective and reproducible, a number of parameters must be closely controlled, among those, the most important are the exposure time to the shot peening , shot speed , projection angle , dimensions, shape, nature and hardness of the shot and coverage area The objective of the current work is to study the effect of shot peening on two aluminum alloys (2024 and 5052) with their cumulative fatigue behaviour Shot peening is a cold working process in which the surface of a part is bombarded with small spherical media called shots.Each piece of shot striking the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple.In order for the dimple to be created, the surface fibers of the material must yield in tension.Below the surface, the fibers try to restore the surface to its original shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in compression.Overlapping dimples develop an even layer of metal in residual compressive stress.It is well known that cracks will not initiate or propagate in a compressively stressed zone [5].Since nearly all fatigue and stress corrosion failures originate at the surface of a part, compressive stresses induced by shot peening provide considerable increases in part life.The maximum compressive residual stress produced at or under the surface of a part by shot peening is at least as great as half the yield strength of the material being peened.Many materials will also increase in surface hardness due to the cold working effect of shot peening.Benefits obtained by shot peening are the result of the effect of the compressive stress and the cold working induced.Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress by corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion caused by cavitations.Benefits obtained due to cold working include work hardening, inter granular corrosion resistance, surface texturing, closing of porosity and testing the bond of coatings [1].
The surface finish is a parameter that has a considerable effect on the fatigue strength of a part.All the shot peening parameters modify the surface finish of the machine element depending on the improvement in the fatigue strength required and the surface finish tolerances that have to adhere to one or other of the parameters [5].
Residual stresses are those stresses remaining in part when all manufacturing operations are completed, and with no external load applied.These residual stresses can be either tensile or compressive.In most applications for shot peening, the benefits obtained is the direct result of the residual compressive stress produced.
Residual compressive stress has four important characteristics: PDF created with pdfFactory Pro trial version www.pdffactory.comThe Aluminum alloy 5052 provides good resistance to stress corrosion in marine atmospheres and has good welding characteristics.
Notably, this class of alloy have been widely used in low temperature applications, which satisfy the most severe requirements of liquefied fuel storage in aircrafts and transportation at cryogenic temperature.[7] Chemical analysis of the alloys was carried out at Scanning Center of geological survey and mining using X-rays method.The result are summarized in table (1)

2-2 Mechanical Properties
Mechanical tests of the alloys were conducted at the center of standardization and quality control.The tensile test was done using instron 225 testing machine that has a maximum capacity of 150 kN.Four specimens have been taken from the received round bar of diameter ( φ = 16 mm).Shapes and dimensions were taken according to German engineering standard (DIM 50123).The obtained values are shown in table (2)

2-3 Fatigue specimens and testing
The materials 5052 and 2024 were received in the form of rolled rods.To get perfect dimensions of a fatigue specimen and to avoid mistakes, an accurate profile should be attained.
All specimens were manufactured using programmable CNC lathing machine by writing a suitable program from the profile of specimen on an edge of metallic plate.Then, all the specimens were machined, corresponding to that profile by copy machining.During manufacturing of specimens, careful control was taken into consideration to produce a good surface finish and to minimize residual stresses.The test specimen is shown schematically in figure (1).
A rotating bending fatiguetesting machine was used to execute all fatigue tests, with constant and variable amplitude, as illustrated in figure (2) The specimen was subjected to an applied load from the right side of the perpendicular to the axis of specimen, developing a bending moment.Therefore, the surface of the specimen is under tension and compression stresses when it rotates.The bending stress ( σ b ) is calculated using the relation:

2-4 Shot Peening Specifications
The peening operation was performed in a special apparatus (Shot Tumblast control panel model STB-OB).The ball material was cast steel with an average ball size of diameter 0.6 mm and a Rockwell hardness of (48-50 HRC).The pressure is about 12 bars resulting in ball velocities of nearly 40 m /s .The distance of shot peening process is 20 mm .

3-Results and discussions
3-1 2024 Cumulative Fatigue Damage Six unpeened specimens were tested under low-high cumulative fatigue damage (120-280 MPa for 2024 alloy and 40-90MPa for 5052 alloy) at room temperature and stress ratio R = -1.The number of rotating cycles at each stress was 10 5 cycles for the 2024 alloy and 3 ×10 4 cycles for 5052 alloy.The results are given in table (3) Specimens were exposed to shot peening with different blasting time (1,2,3,4,5,6,7,8,9,10 and 15) minutes as shown in Table (4) In the case of 2024 alloy, the effect of shot peening on fatigue property is most pronounced under conditions of cumulative high cycle fatigue.i.e low to high loading as shown in Fig. (4).The reason for this is that probably because the surface layers of 2024-Al-alloy are heavily deformed during shot peening, and this will stop fatigue cracks following the slip planes and hence prevents the faceted fracture surface.The effect of the time of shot peening has a limit value of 10 minutes above which the fatigue life is reduced [8] [9].
For the 5052 alloy, the results are summarized in table, ( 5) and ( 6).
The results of table (6) indicate that increasing shot peening time causes damage in the material structure and reduces the fatigue lives.This can be explained by the high roughness of shot peening surfaces, i-e the effect of shot peening increase the roughness which enhances an important stress concentration in different regions of the surface.As a consequence, it leads to a rapid propagation of cracks in these regions of the surface [3,10].This behaviour is illustrated in Fig.

3-3 Residual Stresses
Shot peening is an effective way of strengthening and prestressing elements for cars , aircrafts , ships , etc.The residual compression stress in the metal layers near the surface increases their lifetime [11] , and as a result the surface state, the stress intensity and the affected depth can vary with respect to the applied shot-peening parameters, i.e time of shot peening, surface, coverage, and shot size [12,13].The residual stresses were estimated experimentally based on the preliminary results of the S-N curve equation.Tables (8) and (9 The variation of residual stresses with time of shot peening is shown in figure (7) According to figure (4) , an increase of fatigue life is noticed , in the range from 1 min to 10 min of shot peening.This improvement is due to the formation of a hard layer on which compressive residual stresses exist.However fatigue life drops suddenly at 15 min of shot peening.At 15 min of shot peening the material becomes brittle.As a result, it can be said that shot peening time has a threshold value beyond which a deleterious effect will appear.[14].For the case of high loading, the maximum value of fatigue life is attained at 6 min of shot peening then the fatigue life decreases.[11] Mitar M., Nebojsa, I, Zoran, C. and Milos, J. "Residual Stresses in shot peening of AlMg 4.5Mn alloy" Material in Technologies, pp. 128-133 ,36 (2002).
[12] R.Fathallah,H.Sidhom,G.Lnglebert,L.Castex "Phenomenological approach to the effects of the exposure time on the shot peening residual stresses"

Table (2) Mechanical Properties Table (3) Low
the load measured in Newton (N),the force arm is equal to 125.7 mm and d is the minimum diameter of the specimen in mm.The test machine is Avery 7305 type which is shown in Fig.(3) (5).PDF created with pdfFactory Pro trial version www.pdffactory.com Fig.(7)  indicates that the compressive residual stresses increase with the peening time until 10 min and at 15 min it becomes tensile stresses.This behaviour agrees with Refs.[15,16].Shot peening caused deleterious effect in the 5052 alloy.Shot peening created high stress concentrations that exists on specimen surface and tensile residual stress increases when the peening time increases.Thus fatigue life of 5052 Al-alloy specimens has decreased due to shot peening by creation of compressed rough surface i-e tensile residual stresses.These results are similar to those found by Mohamed.[17]