Shot Peening: Almen Intensity, Residual Stress & Process Validation Standards.
- Silvio Ruiu
- Feb 27, 2024
- 7 min read
Updated: Jan 26
Mechanical Fatigue: Why Static Limits Aren't Enough.
Shotpeening 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 only cause the brake; it is measured following ISO 6892-1.**
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 changing material or manufacturing methods. Further investigations shown 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 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 have always the same kind of crack, from a beginning point - due to a material defect such as an inclusion for example - where the part (or the specific point of it) is periodically pulled; from there, usually named "origin", the fracture starts moving deeper each loading cycle into the part, making the typical "beach marks" as per the area named "fatigue zone" in the pic, till the section is not enough anymore to hold the stress applied and the part suddenly broke, "final failure". If the crack is related to the fatigue phenomenon, always broke this way; please see pic below.
Shotpeening, what is going to happen to the parts in the process.
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. To measure peening is less simple as it may appear, and it is why sometimes it looks like an alchemy than a real engineering task.
Measuring Intensity: Almen Strips and Process Validation.
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 his 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 personality in the field, establish to use 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 A is the thinnest and C is the thickest). Standard way to mark peening intensity is for example 6 -N, it means the parts were peened to reach the intensity of 6 Almen, using the strip N type and this strip bended for 0.15 millimeters as per chart below
Shotpeening, how the process is built.
To develop a peening process requires first and foremost to understand the failure (fatigue life or not) and where exactly it happens; it is also important to have enough consistency on this failures, it means a significant number of parts that broke in the same conditions and under the same load/stress. Viceversa it is possible to investigate a part to make it lighter/cheaper or with less raw material because the increasing of performance is covering the saving in weight/material, aviation industry has often this approach.
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 costumer and his experience;
Using CM library - 60 years of tests in any field - 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.
Shotpeening, coverage concept explained simple.
Any part has a 3D shape and it is a must to understand how the crack proceed and from which side/zone of the part begins, to target appropriately the Almen strip. Luckily within certain limits too much peening is NOT an issue, we gonna be back on this concept later, because it involves the peening coverage of the part.
Intensity we want to achieve has been chosen, shots (media) has been decided accordingly with its mass and chemical composition for the parts we wanna peen later, and so exposure time and media speed launch. The rounded table with planetary provides the best exposure possible to the strip, the satellite rotates itself by 60 degrees each lap of the main table. First cycle gave the first point on the Almen saturation curve , a diagram where we have peening intensity on the Y axis and exposure time on X axis; same Almen strip goes back into the peening cabinet to run the second cycle; afterwards we measure the arc of the strip getting the peening intensity getting the second point of the curve, so it's for the third point and the fourth one, like:
When doubling the time of exposure the peening intensity grows by less than 10%, it means the saturation of the strips has been reached; we left the elastic field to go into the plastic one, where the stress layer generated by the shots is "inside" the material giving that compression tension that will avoid the fatigue crack to begins.
Job is now to compare the peening reached with the expected one, being over the expected value, the first attempt is to reduce speed launch of the shots - than we can reduce the size (mass) of the media till we reach the expected value with saturation; viceversa we are going to increase speed and media dimension if we land below the expected value.
It may appear as a long job and indeed it is, shortcut is a combo of client past experience and CM's old tests library; it gives the possibility to avoid time wasting.
The concept of coverage is related to making sure the peening intensity has been fully reached and due to the asymptotic behavior of the saturation curves, it has been decided to double the time of exposure to make make sure the target intensity has been reached. Today it is possible to measure the residual stress with proper X-Ray equipments, those machines are really expensive and so the related tests; sometime it is difficult to understand if a part has been peened to not by a visual check and it is really IMPOSSIBLE to understand if the expected intensity has been reached or not. To make it easier 200% coverage makes all on the safer side of this process, because it's commonly used also for critical parts in any mechanical fields.
Shotpeening, how the process is built, peening the parts.
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 improve for the parts that is worth to.
Shotpeening, machine layout and how to make it real 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.
8 - conclusion.
In the end this is an overview about what shotpeening really is and how to set the shotpeening process for your parts; it is not complicated, requires cooperation and a good partner whom knows which is the direction where to go to avoid any time waste and increase properly the benefits, no matter if the goal is increasing fatigue life performance or redesign a part to get the same performance using less raw material. You can contact here.
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2) Shot Peening: Almen Intensity, Residual Stress & Process Validation Standards. this post
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