in Vol. 5 - January Issue - Year 2004
Dr. Chris Rodopoulos (Ph.D)
Interview with Dr. Chris Rodopoulos (Ph.D.), Academic at the Material Research Institute Sheffield Hallam University in England.
It is the first time ever that MFN did not interview a leader from the industry, but an academic working for a university. It was most impressive for MFN to see how exceptionally well Dr.Chris Rodopoulos coordinates projects in which university and industry work together. That is why MFN decided to approach him. The combination of a strong academic background and a good sense for what's doable in the industry is a very important quality.
In the last 7 years he has participated in a variety of research projects around the world and for various research sponsors including NASA-Langley, Rolls-Royce plc, Airbus UK, ISTRAM, the European Union, The Royal Academy of Engineering, Hyundai Motor Company, CLFA, Metal Improvement Inc., EPSRC, etc.
(?) MFN: Chris, what’s your history with surface engineering treatments?
(!) C.R.: Before proceeding with the answers let me take the opportunity to thank the Metal Finishing News for entrusting me with the role of the Scientific Adviser for MFN.
Despite the fact that I have been investigating and teaching fatigue damage for many years, my history with surface engineering treatments is about five years old. At the time I was Faculty member of the Department of Mechanical Engineering at Sheffield University. I remember that first meeting when some very respectful Airbus engineers asked us to provide them with some input towards the optimisation of shot peening for aluminium components subjected to cyclic duty.
(?) MFN: When you say cyclic duty are you referring to fatigue?
(!) C.R.: Not just fatigue. Cyclic duty can involve many things including corrosion damage, wear, fretting, etc.
(?) MFN: Let us go back to the first question. So, Airbus came to Sheffield University looking for answers. Did they get them?
(!) C.R.: Yes and no. At the time I was working with an exceptional researcher Dr. Eduardo de los Rios. Eduardo being a member of the International Scientific Committee on Shot Peening realised the problems ahead and I remember that we immediately put down a research strategy. If my memory serves me well, our first priority was the problem with the Almen Intensity.
(?) MFN: Sorry to interrupt you, but Almen Intensity has been around for many years and is very well respected.
(!) C.R.: I know, but the problem is that Almen I j ntensity has nothing to do with fatigue. As a matter of fact characterising shot peening with Almen Intensity just makes things very complicated. Let me explain a bit more. I can use different shot types, different impact angles and different coverage rates and still being able to achieve the same Almen intensity but when it comes to fatigue I will have different lifelines.
(?) MFN: Why is that?
(!) C.R.: It is well known that fatigue damage is very sensitive to the surface finish. The problem however with Almen is that it does not provide any information regarding the condition of the surface. In addition we need to clarify a few things. First, a surface finish X can have a completely different fatigue life effect on components made of different materials. For example a fine grain material will be more surface finish sensitive than a coarser grain material. Second, the surface finish mostly depends on the type of the shot, the velocity of the flow stream, the angle of impact and finally the coverage rate.
(?) MFN: Is it just the surface finish?
(!) C.R.: No. But we are entering very deep waters here. Let me try to put things into a perspective. Residual stress profile and work hardening profile are very important to fatigue but similarly to the surface finish, they can not be categorised according to a particular Almen Intensity.
(?) MFN: So you are saying that the use of Almen is completely irrelevant?
(!) C.R.: No. Almen strips can continue to characterise the manufacturing process, especially saturation and coverage but they have to be treated with care.
(?) MFN: So, do you see a different future for the Almen Technology?
(!) C.R.: I think that in terms of fatigue, Almen intensity should be linked in some way to work hardening and surface residual stresses. I know it sounds a bit complicated but we do have the science to achieve that within the next 3 to 5 years.
(?) MFN: So you think that there is a market for this?
(!) C.R.: Absolutely.
(?) MFN: Let’s go back to fatigue and shot peening. Do you see any particular future problems?
(!) C.R.: Yes. In my opinion there are three basic problems. The first and probably the most important is the relaxation of the residual stresses. I know that several people are now working on the fundamentals of this problem and they have started suggesting alternatives. A good example is the warm peening. However, the problem is a bit more complicated because it involves apart from the specific material in question, the type of loading and the stress levels. For example, multi-axial loading could be more detrimental in terms of relaxing residual stresses. In addition, when you have variable amplitude loading, as in the case of wing skins, or even impact loading, as in the case of landing gears, the residual stresses could relax very fast. The other problem is what happens when part of the residual stress profile is gone and you still have to compensate for your increased roughness. Here I can see two issues, either you continue getting some minimum improvement or you start loosing life. The third problem which is probably the future of shot peening is the re-peening scheduling. If for example we know that the residual stresses will relax to a critical level, meaning that I now start damaging the life of the component, we will need to have a re-peening schedule. The problem here is very complicated. For starters we need to think about peening a surface which has a previous history of work hardening and which, depending very simply on the tolerances of the material, could end up with a tremendous loss of ductility.
(?) MFN: Are there any other important problems?
(!) C.R.: There most definitely are. Here I would like to divert a bit from the shot peening technology and enter a relative field. That of peen forming. As a structural integrity engineer one of the problems that I have and I know that many aerospace companies are sharing similar views, is the effect of peen forming on corrosion fatigue. By definition peen forming uses low levels of coverage and that could change the locality and density of corrosion pits or hydrogen effects. As a result I might end up having conditions of multiple site damage away from where I would normally expect to see them, for example joint lines, etc.
(?) MFN: Looking at the future of surface engineering treatments, what’re your views, is there a market?
(!) C.R.: Normally such an answer is an interview in itself, but I will try to squeeze things a bit. I would be positive here but I will put a life span of 20 to 25 years. The engineering market is moving very fast towards composite materials where surface engineering treatments as they are at the moment do not apply. However there are two possible very big future markets. One is the metal matrix composite market, where we can use surface engineering treatments to compensate for some of the residual stresses and the other is the area of Titanium intermetallics for the aeroengine market. For the next 20-25 years I would probably say that the market will be like that. The automotive industry will decline unless we start treating inside engine blocks and I think the aerospace industry will continue to dominate. A few guesses here are the use of surface engineering treatments to fight corrosion fatigue and to reduce the residual stresses from new manufacturing technologies like high speed machining, laser beam welding and friction stir welding. Another potential area is medical implants, railway components and possibly the area of long weld lines in ships. Of course, forming will have its prime time and possibly new technologies will emerge.
(?) MFN: What do you mean by new technologies?
(!) C.R.: I think that laser shock peening and ultrasonic impact treatment are coming in very strong. Both have tremendous potential and they will most definitely grab a large part of the traditional shot peening market.
(?) MFN: Is there any advice that you want to give to our readers?
(!) C.R.: Indeed. I most strongly believe that the market is in the process of realising the problems behind surface engineering treatments and day by day they are becoming more cautious. I believe that companies who want to continue to survive in this business should start investing in the research of such problems and change their policy to be 100% open to the customer. As an academic I would be very happy to see all these companies spending money on research and development rather than relying on 100% marketing. I believe that strong and unbiased scientific evidence represents the best marketing in the world.
(?) MFN: And one last question. We know that you recently moved to the Materials Engineering Research Centre at Sheffield Hallam University. What’re your new plans there?
(!) C.R.: At the Materials Engineering Research Centre we probably have one of the most powerful structural integrity research groups in the world with a tremendous amount of expertise and testing facilities. Our target as a group is to provide our research sponsors with excellent advice, specialised data and market advice-and believe me when I say that being an academic we can see a much wider picture of the industry and therefore point them towards areas where their investment can generate profit. I personally like to see my research and my ideas maximising profit for my research partners.
We at MFN would like to thank Dr. Chris Rodopoulos for this interview.
Dr. Chris A Rodopoulos (Ph.D.)
Materials Research Institute
Sheffield Hallam University
City Campus, Sheffield
S1 1RW, England
Tel.: +44.114 225 4257