VOL. 22 September ISSUE YEAR 2021

Standards Forum

in Vol. 22 - September Issue - Year 2021
Coverage Control and the Difference to Saturation
Christian Tyroll

Christian Tyroll

The most misunderstood area in the application of the shot peening process is the difference between two of the probably most-used terms in shot peening: these are ‘Coverage’ and ‘Saturation’. For this article to be properly understood, it is necessary to examine the difference.
Coverage explicitly relates to the ratio of indentations in the peened surface, to the areas of the peened surface that do not exhibit indentations, and is a function of exposure or, in other words, time. As the surface to be peened is exposed to the shot stream and the shot strikes the surface, so does the number of indentations in the peened surface increase, until such time that no part of the original surface remains untouched by the shot. It is theorised that it is impossible to achieve true 100% coverage, therefore most specifications will state that 100% coverage is achieved when 98% of the surface is covered by indentations. Coverage rate is best described as logarithmic, since the probability of a shot impact not occurring where a previous impact has occurred reduces, as the surface becomes more and more covered.
Saturation is a term that relates almost explicitly to the Almen strip. The point at which the strip almost stops bending regardless of further exposure to the shot stream, is considered to be the point of saturation. This point is determined by the statement in most specifications that the strip shall not increase in bend or arc, by more than 10%, when the exposure time to the shot stream is doubled. It is only by coincidence that the coverage of the strip reaches 100% at almost the same time as saturation is achieved. 
The only time that saturation becomes relevant when shot peening a part is when shot peen forming or shot peen correction is being performed. ‘Forming’ is when the induced residual compressive stresses are used to form a shape, for example a curve in a part such as an aircraft wing skin. ‘Correction’ is employed when a machined component that exhibits some form of distortion after release from manufacture is corrected by inducing residual compressive stresses in specific areas to counteract the distortion. The reason the term saturation is relevant under these two applications is because there is a point in exposure, or time, at which the process does no more useful work, just as with the Almen strip. The shot peening effect in terms of forming or correction reduces logarithmically as it does with the Almen strip, and the only way to increase the effect is to increase the energy in the shot particles. This can be achieved by increasing velocity (pressure for example) or mass (larger shot).
200% coverage is achieved by doubling the exposure time that was required to achieve 100% (98%). And this leads us to a difficult area of discussion. If a specification calls for 100% coverage, then it is logical to say that 200% coverage would be non-compliant. However, it is common for many components to exhibit complex geometries, this at times makes it exceedingly difficult, if not impossible to achieve 100% in a specific area without causing coverage in excess of 100% in another area. This phenomenon has been largely accepted in the industry as unavoidable, but where does this leave the shot-peening shop with regards to compliance with the specification? Nadcap for example, has brought sharp focus to precise compliance to specifications. The OEM specifications and the national specifications are becoming ever more detailed and explicit in the control of the shot-peening process, and so the question of coverage control is now becoming a more detailed subject. 
At least two significant OEM specifications cover the subject of coverage consistency across the part. The generally adopted method is the production of a coverage map that is produced by gradually increasing the exposure time of the component and recording the amount of coverage achieved over each feature of the component. This then forms part of the process control sheet as a declaration that the specified coverage has been achieved in all areas. What this doesn’t do though, is provide any latitude for areas that are unavoidably peened beyond the specified 100%. Even with the most advanced robot, there will be an area of a part with complex geometry that will unavoidably receive more than the specified coverage. 
Fortunately, there appears to be a move towards an understanding of this situation with some specifications defining acceptable over-coverage limits. This condition can be complicated by section thickness for example, where distortion of the part can be caused by over-exposure to the process. Such complications make accommodating over-coverage within a specification a complex task. At the other end of the spectrum, most specifications define maximum coverage limits that are often (and must be) material-specific; these do tend to vary from specification to specification and often range from 300% to 500%. However, when accommodated by the specification on a production basis, over-exposure is limited to lower values.
The subject of coverage is as important as it is vast, but the one that possibly varies the most from specification to specification. Perhaps there will be more detail on this subject in future renditions of AMS2430 and AMS2432. J2277, which is referenced by AMS2430 defines methods for the determination of coverage.

For questions contact Christian.Tyroll@noricangroup.com

Standards Forum
by Christian Tyroll, 
MFN Contributing Editor
more information at www.mfn.li/trainers