Journal of Materials Processing Technology 172 (2006) 159–162 A process model for shot peen forming T. Wang a,∗ , M.J. Platts a , A. Levers b a Institute for Manufacturing, Department of Engineering, University of Cambridge, Mill Lane, Cambridge CB2 1RX, UK b Airbus UK Ltd., Broughton, Chester CH4 0DR, UK Received 16 October 2002; received in revised form 15 May 2003; accepted 5 September 2005 Abstract An equivalent loading unit that can produce a plastic layer was used to model the macroscopic forming effects of shot peening. Because a specific plastic coverage exists in which the individual peen impacts can be regarded as acting independently, their cumulative effects can be assumed to be distributed in the plastic layer as the result of a static load. The loading unit corresponds to the macroscopic stretching strain and can be conveniently calibrated from the peening parameters, such as shot radius, mass flow rate and air pressure. For more intensive peening, the number of loading cycles is in direct proportion to the peening time, which makes the model applicable to practical applications. Compared with experiments, this model gives a good prediction of the peen-formed deflection. © 2005 Elsevier B.V. All rights reserved. Keywords: Shot peening; (Shot) peen forming; Process model; Finite element analysis (FEA) 1. Introduction It is well known that shot peening the surface of a metal sheet by small hard shot with sufficient kinetic energy can form a specific shape. This is especially used for shaping aircraft wing skin panels. The (shot) peen forming process generally has lower manufacturing costs because there is no need for dies and presses or subsequent thermal processes, and once the process param- eters have been determined, the process is easily reproducible. It also has better adaptability to modern aircraft designs, for example, the capability to form tapered and sculptured integral structures, single and double curved shapes and virtually any size of parts. In addition, peening can provide beneficial per- formance to the peen-formed component like the general shot peen hardening process. In the practial applications, the design of peening parameters for a specific shape has been through trial and error or some numerical methods that are heavily dependent upon tests and experiences. Homer and VanLuchene [1], VanLuchele et al. [2] and Van- Luchene and Cramer [3] presented a numerical method for predicting peening intensity patterns by assuming the peen form- ing process as a linear finite element system responding to the peening induced stresses. Empirical equations determined by ∗ Corresponding author. E-mail address: tw225@eng.cam.ac.uk (T. Wang). physical experiments in Boeing Commercial Airplane Group were relied upon to set up the relationship between the induced stresses and the peening intensity. As recognised in their paper, Homer and VanLuchene [1] stated that the experimental data are based on peening of unconstrained specimens but assumed to be applicable for constrained specimens, which may introduce error because in reality various constraints, such as geometry, jigs (pre-stresses) and peening sequences are possible. Other methods or models [4,5] were also based on similar assump- tions, which not only require a large number of tests but may also have problems if constraints are considered. It has been believed that if the fundamental principle, such as the creation of plasticity by shot peening, can be included in the model, it can simulate the reality more accurately. This can reduce the experimental costs and furthermore, provide a more effective process design. For example, ‘the squeezed layer model’ [6,7] and ‘the thermal stressing model’ [8,9] were developed. The squeezed layer method models the peen form- ing process by assuming squeezing of the surface layer of a solid-element model, which requires high computation cost. The thermal stressing model was developed to adopt shell ele- ments based on a stress analysis using the stresses under a single impact and assuming it is equivalent to a full coverage situation (saturation peening). However, this model is difficult to calibrate for realistic peening applications and the assump- tion of employing the stresses produced by a single impact is disputable. 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.09.006