Stresstech Bulletin 14
Text: Murat Deveci, Figures: Stresstech
Shot peening is a cold working process in which the surface of a component is peened with small spherical balls called shots. The process induces plastic deformation on the surface, relieves surface tensile stresses and introduces beneficial compressive residual stresses. Shot peening is a similar process with surface rolling, both deform the surface plastically to develop a compressive residual stress depth profile.
Inducing compressive residual stresses on a component’s surface, increases the resistance to fatigue failures and stress corrosion cracking (SCC) thus lengthening the lifetime of the component.
It is easy to apply shot peening on steel, aluminum, titanium, nickel base alloys and some ceramics. For the components which need high cyclic fatigue and strength, such as springs, gears, camshafts, crankshafts and turbine blades shot peening is a crucial step in the production.
Barkhausen noise analysis could be used to show the difference between a shot peened and not peened residual stress depth profile. Barkhausen noise method could be utilized as an automated inspection method to separate shot peened and not shot peened parts non-destructively. In relatively soft steels, the Barkhausen noise signal level consistently increases, in hard martensitic (carburized or surface hardened) materials, Barkhausen noise signal level decreases.
Barkhausen noise can quantify the subsurface stress without the need of removing the surface layer. However, Barkhausen noise analysis requires a calibration study to produce MPa values. When Barkhausen noise is used, evaluation of shot peened components is non-destructive and fast.
Shot peening affects various properties of the component such as residual stress distribution, surface roughness, structural integrity (distortion), hardness, crack initiation and propagation.
In his paper, Prof.Schulze has divided the parameters which influence the results of shot peening treatments into three categories as device related, shot related and workpiece related. The paper describes the equipment related parameters as coverage, impact angle, peening time, shot velocity workpiece parameters as geometry, hardness, temperature and shot related parameters as shape, size, mass, and so forth. 
Another example of shot peening stresses is given below. In this example, the residual stress depth profiles are created by Prism equipment which works with a technology based on a combination of traditional hole drilling and ESPI methods.
In traditional hole drilling, by removing a volume of material, the stress equilibrium is changed. Moreover, the remaining material re-balances its stress fields, and this relaxation and surface distortions are measured as changes in electrical resistance.
ESPI – Electronic Speckle Pattern Interferometry is a non-contact technique capable of measuring and monitoring non-uniform strain fields at high resolution.
To sum up, it can be said that shot peening induced residual stresses are quite beneficial. However, there is a need to confirm their values and distribution through the depth. There are several methods to create stress depth profiles as X-ray diffraction, Hole drilling with ESPI or Barkhausen noise.
Stresstech is a research oriented company with more than 30 years’ experience in residual stress engineering and shot peening stress evaluation. Feel free to contact us to learn more about residual stresses and shot peening stress determination.
 Characteristics of Surface Layers Produced by Shot Peening, Volker Schulze, Institut für Werkstoffkunde I, Universität Karlsruhe (TH), Karlsruhe, Germany
 Non-destructive Evaluation of Residual Stress Depth-profiles by Barkhausen Noise Analysis and their Validation by XRD Method Combined with Electrochemical Surface Removal, P. Jacob, S. Marrone