VOL. 12 January ISSUE YEAR 2011
in Vol. 12 - January Issue - Year 2011
FE Simulations and Experiments for Better Shot Peening
Mario Guagliano, Associate Professor at Politecnico di Milano
Mario believes in the role of simulation for shot peening development. Here he is discussing with Sara Bagherifard, PhD student, about the result of an analysis.
Mario in the Labs with technician Piero Pellin preparing a XRD residual stress measurement on a shaft
A view of the Mechanical Engineering Department Labs at Politecnico di Milano
This month MFN meets Mario Guagliano, Associate Professor at the Department of Mechanical Engineering in Politecnico di Milano, the oldest and largest technical university in Italy. We met him in his office and then we went to the Department Labs where we had the opportunity to see the powerful testing facilities and some experimental tests performed on shot peened parts under the projects directed by Mario. He collaborates with different industries in projects aimed at characterizing the role of shot peening in improving mechanical behaviour of structural and mechanical components, especially to enhance their fatigue strength.
(?) MFN: Mario, when did you start your studies on shot peening?
(!) M. G.: Before answering your question, I would like to thank Metal Finishing News for entrusting me with the role of Scientific Adviser and MFN Trainer. MFN is a magazine that, in my opinion, plays an important role in spreading shot peening knowledge all over the world and strongly contributes to the diffusion of this process for improving mechanical behaviour of materials.
About my experience in shot peening, I would say my interest in mechanical surface treatments dates back to 1988, when I did my Master of Science thesis on the effect of deep rolling on fatigue strength of crankshafts in diesel engines for marine propulsion. At that time I learned how important residual stresses are for improving the fatigue behavior of notched components and I recognized the significance of correctly choosing treatment parameters to obtain the desired results. Afterwards, I became research assistant in Politecnico di Milano and started my research on shot peening, investigating different materials, components, in-service conditions and fields of application. In all the mentioned cases my interest was mainly devoted to finding optimal treatment parameters for improving the results in terms of fatigue strength.
(?) MFN: It must be a very difficult task!
(!) M. G.: For sure it is. Moreover the problem is not fully solved, namely a general solution is not yet available. Indeed there are many factors influencing treatment results. The way shot peening is usually set up, by using Almen intensity and coverage, is not sufficient to completely characterize results in terms of induced residual stresses, surface work hardening and roughness variations, which are the three main changes introduced by shot peening in the surface layer of the material. There is even more: once these parameters are set, a more difficult task is to relate them to the fatigue strength and life time of the peened components. Do not forget that residual stresses relax. Many approaches have been proposed in the past but they are all applicable almost only under the conditions for which they were tested, thus are not general at all.
(?) MFN: What do you suggest to solve this problem?
(!) M. G.: Our approach considers two steps. The first one aims at determining residual stress distribution and at determining its relationship to peening parameters (shot size, shot velocity, coverage, target material...), as well as assessing surface work hardening and quantifying surface roughness. To achieve this goal we use finite element simulations. We have gained in the past years great expertise in this field, which provides results very close to experimental tests for a great range of materials and a wide range of treatment parameters. Ten years ago I published a paper that included a method, based on FE analysis, to relate Almen intensity to treatment parameters and to induced residual stress, the results of which were satisfactory; however now we are able to make more and more refined analyses, also thanks to the increased potential offered by the new generation of FE codes.
(?) MFN: And what about the second step?
(!) M. G.: Before passing to the second step we make some experimental measurements to validate the numerical model. Having assessed the numerical results, we relate the applied loading condition to the residual stress, the roughness and the surface hardening with some criterion for fatigue life and/or strength assessment. The criterion choice varies as a function of the material, the amount of the applied load (that can be categorized as low-cycle or high-cycle fatigue), stress gradient and so on. Sometimes it is better to adopt a criterion based on fracture mechanics; in this case shot peening is able to stop propagating cracks; in other cases concepts like the "local fatigue limit" are more suitable for obtaining accurate results. In addition we have developed some original approaches for strength analysis of nitrided steels with surface defects and also for quenched and tempered steels, the results of which have been very satisfying.
(?) MFN: If I am not wrong you believe that the finite element method is a helpful tool for predicting the results of shot peening.
(!) M. G.: Yes, it is a very powerful tool but also really dangerous. Simulations of shot peening are multifaceted and quite complex: the problem is highly non-linear (the material constitutive law, the surface contact...) with considerable strain rate. This complexity necessitates having reliable data for material modelling and defining the contact algorithm. I would suggest, always, implementation of some experimental tests (especially residual stress measurements) to validate the FEM results; therefore, I may say performing FEM analysis is not easy, but it works and is really helpful!
(?) MFN: Mario, you are an academic. During your lectures do you talk about shot peening to your students?
(!) M. G.: Certainly! I believe that shot peening is a useful and important treatment to improve the mechanical strength of metal parts. It also has a limited environmental impact, and we know how crucial this problem is today. Introducing the important aspects of shot peening in courses held in universities, to my idea, is the best way to increase the use of shot peening in the future. I hope that by hearing about shot peening in class, students will remember and consider it during their professional career. I would find it worthwhile to mention that, normally during the Master of Science courses in Mechanical Engineering, I invite engineers from industries dealing with shot peening to give lectures, thus letting students know shot peening also from a practical point of view.
(?) MFN: And what about your relations with industries?
(!) M. G.: At the time I started my studies on shot peeling, it was not a popular treatment in Italy; it was considered a good solution only for very particular applications. In other cases it was considered only an additional cost. However, the situation has now changed a lot. Shot peening is being applied more and more in many industrial sectors and by different companies, both large groups and small to medium enterprises. This generated a great need for knowledge. Once the industries were mainly interested in just knowing the values of the residual stresses since their customers asked for this information. Now, however, they want suggestions on parameters, plants, assessment of expected life, and so on. Accordingly, I increased my cooperation on shot peening with companies in many sectors: aeronautics, helicopters, oil-drilling systems, automotive, mechanical...; and not only for fatigue strength assessment, but also for studying surface damage due to contact fatigue or fretting. Now my research group collaborates also with companies outside Italy.
(?) MFN: Which are the topics about shot peening that, in your opinion, will be of interest in the future?
(!) M. G.: I think that interest in shot peening will not decrease. Shot peening has different advantageous points compared to other types of surface treatment. It is important to underline that it little environmental impact, for sure much less than thermo-chemical treatments. Besides there is a continuously-increasing demand for lightness, reliability and performance of systems and components. Shot peening is an important ingredient which helps to meet all these requirements. Probably shot peening will be more used in non traditional fields, like bio-med implants, and at the same time it will have to compete with other technologies in traditional sectors of applications.
(?) MFN: Could you be more precise?
(!) M. G.: Traditional shot peening is now in competition with laser peening or ultrasonic treatments but the result of the game is still not clear. Laser peening is really promising but it is very expensive. I listened to a lecture by Prof. Stephens, who wrote the famous book on fatigue together with Fuchs, at the last International Conference on Fracture in Ottawa, where he described the results of tests carried out on shot peened and laser peened welded joints used in the automotive industry. Well, a clear advantage for laser peening was not evidenced and the conclusion was that due to its high cost it is still not a good option to shift to laser peening.
On the other hand, shot peening will have to change and to ensure better results to compete.
(?) MFN: What do you mean by this?
(!) M. G.: I mean that new solutions will have to be found and further research is necessary to investigate how shot peening can affect, in positive, the behaviour of non traditional materials, like metal matrix composites, or to investigate if re-peening could increase the residual life of old plants.
Another promising field of research is the application of shot peening to obtain a nano-structured surface layer. This could lead to a considerable increase in performance of shot peened components, but a lot of work has to be done to find the suitable treatment parameters and to assess if satisfactory results can be obtained with traditional plants.
(?) MFN: And are you involved in these innovations?
(!) M. G.: Yes, we have managed to obtain surface nanostructured materials by using extreme shot peening parameters both on steels and on aluminium alloys and the first fatigue tests show a remarkable improvement with respect to parts peened with conventional parameters, especially if notched parts are considered. We have also developed a finite element approach to assess the treatment condition and parameters that allow generating a nanostructured surface layer of material, obtaining a good correspondence with respect to TEM experimental observations.
(?) MFN: Finally, a message for our readers.
(!) M. G.: Thanks for reading my column every issue. I encourage readers to send me comments and questions and, if they are interested in the activity of my group, they can contact me to have more details. In Politecnico di Milano we have very well-equipped laboratories and we can do experimental and numerical research on shot peening to improve the competitiveness of our research sponsor and to increase the performance of the system they produce. We would be happy to discuss with them about possible cooperation. A great big "CIAO" to all readers.
We would like to thank Mario Guagliano for this interview!