From a Sheet of Paper to Shot Peening in Manufacturing: Intensity, Saturation and Coverage
Updated: Sep 30
Shot peening has a vocabulary of its own, and the engineering principle behind it is simple. Here it is, in the most basic way possible.
What is shot peening? Explained with a sheet of paper in 4 steps
Take an A4 sheet from the printer round the corner and lay it flat on the table.
Put one index finger on each short edge and make it oscillate: stretch it, shorten it, stretch it again. You can keep going as long as you like, the sheet will not break.
Now cut a small notch into one of the long edges and start again. This time the notch opens, and it keeps opening until the sheet gives way.
Do it again with a fresh sheet and the same notch. Before you start, slide both fingers slightly towards the centre — one millimeter, two at most, over almost thirty centimeters of paper. The sheet stays flat and does not even look any different. Hold it like that and oscillate. The notch does not open.
The sheet is your part. The notch is the defect every fatigue failure starts from — an inclusion, a machining mark, a sharp corner. The oscillating fingers are the load your part takes every turn, every cycle, every stroke. And that tiny compression you put in with your hands is what shot peening does: it puts it into the skin of the part by hammering it with media, and there it stays. The part works exactly as before. The crack simply has no way to open.
Mechanical Fatigue: Why Static Limits Aren't Enough
Shot peening is a way to increase the life of parts against mechanical fatigue failures, to better understand the problem we should first investigate and properly name the issue:
Mechanical Fatigue is happening when a part is loaded/unloaded periodically and value of the load(s) applied is below the static limit of the material itself; a load/unload is defined cycle, total amount of cycles before failure is the fatigue lifetime.
Static limit of the material is that amount of stress that applied to the material itself one time, causes it to break; it is measured following ISO 6892-1.**
Where the fatigue problem was first met
Two centuries ago during the first development of railways industry, one of the issues was related to the broken axes of trains carriages under loads; while stress due to the load was consistently far from static limit of the material used, still those shafts were breaking regardless of the materials or method. Further investigations showed that the failures were all of the same kind.
Conclusion was that a part loaded periodically can fail even under a stress far away from the static limit of its material. Figuring it out:
Point A is loaded by pushing/pulling stress each lap the shaft makes, the failure happens as per conclusion above.
Preventing Crack Propagation: The Role of Compressive Residual Stress
Fatigue failures always show the same kind of crack, starting from a beginning point — due to a material defect such as an inclusion, for example — where the part, or that specific point of it, is periodically pulled. From there, usually named "origin", the fracture moves deeper into the part at each loading cycle, making the typical "beach marks" you can see in the area named "fatigue zone" in the picture, until the remaining section is not enough to hold the stress applied and the part suddenly breaks: "final failure". A crack related to the fatigue phenomenon always breaks this way; please see the picture below.
What happens to a part during shot peening?
The main idea to avoid the origin of the crack is to create in the peripheral of the part a compressive residual stress layer by a plastic deformation to avoid that the crack just begins; one of the most efficient way to get this job done is spraying a consistent amount of shots onto the part to build this compression layer; with the right process settings (mass and speed of the shots, as well exposing time) benefit is remarkable, with parts lasting even 3 or more times longer compared with those not peened. Measuring peening is less immediate than it may appear, and that is why it sometimes looks more like alchemy than engineering.
Measuring shot peening intensity: how the Almen strip works
From the purely engineering point of view, being the peening a compressive residual stress inside the skin of the material it should be measured by MegaPascal, it is finally like any other load; today technology allows it using a specific Xray equipment, pretty rare to find it due to its very high cost. Coming back to the period when this phenomenon was properly approached, there was no technology able to measure peening; instead Mr. Almen, one of the most eminent figures in the field, established the use of specific specimens (A.K.A. Almen strips) and to measure the residual bend on them after the exposure under the shots; the more the bend, the more intensively the parts were peened.
Key value is the intensity of the peening, and this number is related to the specimen used to do the measure (N,A or C - while the N is the thinnest and C is the thickest). The standard way to mark peening intensity is for example 6N: an arc height of 0.006 inches measured on an N type strip, which is 0.15 millimeters as per the chart below.
Almen intensity and coverage in shot peening
Intensity and coverage are the two conditions a successful peening process has to meet, and they are not the same condition. Intensity is how hard the surface is being hit, and it is verified on the Almen strips. Coverage is what guarantees the parts have actually been peened, and it is bought with exposure time.
Almen strip saturation
Saturation is the point where more time almost stops adding intensity. It is read from the saturation curve — arc height against exposure time — and defined as the earliest point at which doubling the exposure produces no more than a 10% further increase in arc height. The arc height at that point is the peening intensity, the number that goes on the drawing. Saturation of the Almen strip is what turns the result into a number you can check and repeat tomorrow, and therefore into a part lifetime performance (fatigue life) you can promise and guarantee.
What is coverage in shot peening, and why 200%
A stream of media cannot be aimed. Where each individual shot lands is random, and there is no way to direct the next ones onto the spots not hit yet. What you control is the flow, the speed and the total time of exposure — never the single impact.
Yet the process has to deliver one certainty: that saturation has been reached everywhere on the surface exposed, or everywhere on the area that matters, and not just at the point where the strip was sitting. That gap is real and it has to be closed: how it is done in production is the subject of the last article of this series.
With no instrument able to check that point by point, the answer comes from the saturation rule itself. If doubling the exposure time moves the intensity by less than 10%, then doubling the time again is what makes sure the whole surface has got there. That is coverage: not a measurement of how much surface has been struck, but the exposure time that guarantees every point of it has reached saturation. Hence 200%.
How to set up the peening test
To develop a peening process requires first and foremost to understand the failure — fatigue life or not — and where exactly it happens on the part. It is also important to have enough consistency on those failures: a significant number of parts broken in the same conditions and under the same load. Conversely it is possible to investigate a part to make it lighter or cheaper, using less raw material because the increase in performance covers the saving in weight; the aviation industry often takes this approach.
Every part has a 3D shape, so knowing where the crack starts and from which side is what tells you where to target the Almen strip. The strip has to sit where the part suffers.
Once the decision of peening application is made, next step is to choose the right intensity for the part(s), there are 3 ways to make the first attempts:
Getting peening intensity by the customer and his experience;
Using equipment producer experience to find out which are the common values in that specific field/parts and trying to make them better;
Going to the unknown on parts never peened before: deciding some values and checking with fatigue life test how parts improve their performance in fatigue life tests.
Combination of all the ways above is often used, it is anyway mandatory to run fatigue life tests to get the confirmation of what kind of improvement has been made on the parts. It may be long sometime but it is the only way to validate the process.
How the Almen saturation curve is built
The intensity to achieve has been chosen, the media has been decided by mass and chemical composition for the parts to peen later, and so have exposure time and media launch speed. The rotary table with satellites gives the strip the best exposure possible: the satellite turns 60 degrees on itself at each lap of the main table.
The first cycle gives the first point of the curve — peening intensity on the Y axis, exposure time on the X axis. The same strip goes back into the cabinet for a second cycle, the arc is measured again and gives the second point, then the third, then the fourth.
When doubling the exposure time makes the intensity grow by less than 10%, the strip has reached saturation: the work hardening in the skin of the material is done, the compressive layer is in place, and more time adds almost nothing.
What to do when the intensity is off target
Now compare what the curve gave you with what you wanted. If you landed above the target, the first move is to reduce the launch speed of the media; then, if that is not enough, reduce the size and therefore the mass of the media, until the target value is reached at saturation. If you landed below, you go the other way: more speed, bigger media.
It may look like a long job, and it is. The shortcut is a combination of the client's own past experience and the consultant's, which is what avoids wasting time.
How the parts get peened once the process is set
Knowing timing to achieve the saturation and doubling it to fulfill the coverage obligation we just need to place the parts - usually a small batch - into the cabinet and run to get the parts properly peened.
Next is for our client to run an appropriate fatigue life test to understand the life performance change, unless there was a target peening intensity already clear.
If parts are peened the first time, client has at least two different intensities to check and figure out in which direction to move for further investigations.
If you are wondering if it's a long story...well I can say in my experience it is not short, still the results have such improvement for the parts that it's worth it.
Shot peening machine layout for production
Once the process has been approved, sizes of the parts are established and most of all the quantities in terms of pieces/hour or pieces/minute it is possible to check the option for the peening cabinet, if to use a pass thru tunnel or a rotary table, with or without hoist, if to automate uploading/downloading of client parts. End of this challenge is commissioning the machine at client location and teaching production workforce how to keep the peening performance of the machine itself.
Peening Standards, why are so important and how to approach them
Peening carries a lot of standards because its main field was aviation: getting lighter components that still hold their operational stresses was, and still is, a primary target. It is also mandatory to set procedures that achieve the same benefits wherever in the world the part is produced — "standards and quality" at their highest level, given the safety implications of this field.
Industrial mass production does not need to follow them all; but knowing them is what lets you decide which to follow, as rule or guideline, and which to amend. The detailed list is in the next step, C.
Conclusion
In the end this is an overview of what shot peening really is and how to set the process for your parts. It is not complicated, but it takes cooperation and a partner who knows which direction to take, whether the goal is longer fatigue life or redesigning a part to get the same performance with less raw material. If you want to go into details, you can reach me through the contact page.
Table of content:
2) Shot Peening: Almen Intensity, Residual Stress & Process Validation Standards. this post
