Introduction
Shot peening is a critically important metalworking process that can effectively enhance the fatigue life of components. In today's society, there is a wealth of information available on shot peening, but sometimes it can be overwhelming and obscure the bigger picture. As the proverb goes, "Not seeing the forest for the trees" means that focusing too much on minor details may cause one to lose sight of the overall direction. This article explains the content of Figure 1 through six key elements.
Components, such as trailer leaf springs, are subjected to cyclic loads during operation.
Cyclic loads result in corresponding cyclic stresses.
If the stress is sufficiently high and the number of cycles is large enough, cyclic stress can lead to fatigue failure. The stress responsible for crack growth must be tensile stress.
Shot peening can effectively mitigate the effects of fatigue failure.
Shot peening introduces a "magical skin" on the surface of the component, characterized by compressive stress and cold working effects. The thickness of this "skin" depends on the intensity of shot peening.
Coverage is a parameter that describes the effect of shot peening on the component's surface. It is defined as the ratio of the area covered by dimples to the total surface area of the component.
The suitability of the shot peening process is determined by several factors.
Generally, the customer is responsible for specifying the shot peening requirements—intensity and coverage—which are established by design and process experts. The operators at the shot peening company are responsible for achieving the required intensity and coverage levels.
1 Cyclic Loads
Most components are subjected to cyclic loads, especially those used in automotive and aerospace applications. Loads have four significant characteristics:
Frequency
Variability
Magnitude
Type
1.1 Frequency
In some cases, such as with coil springs and leaf springs, loads are clearly cyclic, though they may be less obvious compared to aerospace components (where pressure and tension loads alternate during flight). Within the same timeframe, a spring might endure trillions of load cycles, while an aerospace component (e.g., a landing gear) may only experience thousands. The frequency of loads can vary significantly and is a critical consideration in design. Some components must be designed to withstand trillions of cycles, while others may only need to endure thousands.
1.2 Variability
The loads on a single component can vary widely. For example, an unloaded truck experiences lower cyclic stress than a fully loaded one; a trailer requires less traction on a smooth asphalt road than on a rough mountain path; and an aircraft's landing gear undergoes higher loads during an emergency landing compared to a normal landing. Both component design and shot peening processes must fully account for load variability.
1.3 Magnitude
The magnitude of the load determines how long a component can operate before failure. Larger loads result in shorter operational cycles. Estimating the magnitude of cyclic loads is a topic worthy of in-depth study.
1.4 Type
The "black or white" extremes of cyclic loads are "tension-compression" and "repeated bending." All components experience various types of loads in certain proportions. Shot peening is most effective when the cyclic load is primarily repeated bending.
2 Cyclic Stress
Cyclic loads on any component can induce corresponding cyclic stresses. Stress can alternate between tensile and compressive, as shown in the figure above. It is the tensile stress (represented by the red curve in the figure) that leads to fatigue failure.
Stress in components is generally "elastic stress," meaning it disappears once the load is removed. Applying a tensile load to a component causes it to stretch, while a compressive load causes it to contract. The amount of stretching or compression is called "strain." For elastic stress, stress and strain have a linear relationship, which is a fundamental principle of mechanics known as "Hooke's Law" (published in 1660). The ratio of stress to strain is the modulus of elasticity, E, which is also a material property.
The number of stress cycles a component endures during service varies significantly depending on its type. Components can be designed with a target number of stress cycles (the number of cycles before fatigue failure) in mind. In the 19th century, British design led to a trend of "Victorian engineering," where components were designed to be large with low stress levels. Some of these components are still in operation today! However, such over-design is no longer acceptable for modern transportation vehicles due to weight penalties that reduce economic efficiency.
An important aspect of cyclic stress is "stress concentration." If a component has a notch, the derived stress will concentrate. The level of stress concentration depends on the sharpness of the notch. This is why transitions, such as those from an axle to a flange, are designed to be smooth. Shot peening helps reduce the effects of stress concentration caused by notches.
3 Fatigue Curve—Without Shot Peening
Metallic materials inherently possess the ability to resist cyclic stress, typically represented by a fatigue curve. For ferritic materials, the general shape of the fatigue curve is as shown above. First, it is worth noting that the curve is very simple, consisting of two straight lines distinguishing between fatigue failure and no fatigue failure. This is because a "logarithmic scale" is used to display the number of stress cycles. The logarithmic scale is employed because the number of stress cycles can range from tens to hundreds of millions, depending on the application.
For ferritic materials, as the stress level increases, the number of cycles before failure decreases. If the stress level is sufficiently low, fatigue failure will never occur, and this stress is referred to as the "fatigue limit" of the component. In the figure above, the fatigue limit appears at 500 stress cycles, but this value can vary depending on the material and testing conditions. Imagine if a ferritic component were subjected to stress cycles below its fatigue limit indefinitely; fatigue failure would never occur.
4 Fatigue Curve—With Shot Peening
At a specific number of cycles, the cyclic stress level of a shot-peened component is improved. Compared to the fatigue curve without shot peening, the stress level after shot peening is significantly higher.
5 Shot Peening Intensity
"Shot peening intensity" is directly related to the depth of the residual compressive stress layer in the component after shot peening. Higher shot peening intensity results in a deeper residual compressive stress layer and a thicker hardened layer. Shot peening intensity is measured using a set of standard steel test strips (Almen strips) peened for different durations to plot a saturation curve. The intensity is defined as the point where a doubling of the peening time results in a 10% increase in arc height, also known as the saturation point.
The dimples created by the impact of shot media on the component's surface are influenced by the angle of impact. Imagine a machine gun firing at a vertically placed steel plate. If the plate is tilted at an angle, the impact effect of the bullets will be reduced. This phenomenon is also utilized in the design of battle tanks. If the Almen strip is not perpendicular to the shot stream, the peening effect will be similarly reduced, resulting in lower shot peening intensity.
The impact of shot media causes plastic deformation on the component's surface, creating a compressive stress hardened layer that enhances the component's fatigue resistance. Fatigue cracks in components can only initiate and grow under cyclic tensile stress. Applied stress is generally highest on the surface of the component, especially under bending stress conditions. The residual compressive stress on the surface can reduce the tensile stress applied to the component.
The effectiveness of residual compressive stress on the component's surface is not always obvious. As an analogy, consider stretching a rubber sleeve over a shaft, such as a cricket bat or hockey stick handle. The rubber sleeve is under tensile stress. If a cut is made on the surface of the sleeve, the tensile stress will cause the cut to expand. However, if the rubber sleeve is under compressive stress, the cut will close due to the compressive force.
Shot peening intensity directly controls the thickness of the residual compressive stress hardened layer.
6 Shot Peening Coverage
"Shot peening coverage" is defined as the ratio of the area covered by dimples to the total surface area of the component after shot peening. If the amount of shot peening increases, the coverage will correspondingly increase. With a small amount of shot peening, coverage is low; with a large amount, coverage is high, as shown in the figure above.
To understand shot peening coverage, imagine continuously dropping bombs on a target area. If a thrower launches seven bombs, the explosions leave craters, as shown in the left figure above. However, it is likely that the craters from two bombs will overlap, as also illustrated. The total area of the seven craters is actually smaller than seven times the area of a single crater. When the thrower launches another seven bombs, the total crater area increases, but overlapping areas still occur. The rate of increase in the total crater area slows down after a certain point, exemplifying the "law of diminishing marginal returns." Note that even after launching a large number of bombs, some small areas may remain uncovered.
Establishing Shot Peening Standards
The appropriateness of shot peening standards depends on the customer's or the shot peening operator's knowledge of shot peening. If the operator is a single individual, they must take responsibility for understanding the fundamentals of shot peening. If the operator is a team, including design engineers, process engineers, computer experts, and trained employees in the shot peening workshop, the complexity and value of the component require a suitable shot peening process.
This article outlines several basic elements of shot peening and introduces the complexity of establishing shot peening standards. The following are the three major elements of shot peening:
Shot Media
Shot Peening Intensity
Shot Peening Coverage
1 Shot Media
The most commonly used shot media include cast iron shot, cast steel shot, cut wire shot, glass beads, ceramic beads, and stainless steel shot. They share the following common characteristics: near-spherical shape, high hardness, good consistency, and reasonable cost.
The choice of shot media is typically based on the component material. Ferritic steel components are usually peened with cast iron shot, cast steel shot, or passivated cut wire shot, as these materials are ferritic. If ferritic shot media are used on stainless steel components, there is a risk of electrochemical corrosion. Different metals and alloys have different electrode potentials, and when two or more metals are in contact, one will act as the cathode and the other as the anode. Aluminum alloy components, which are softer, are often peened with glass or ceramic beads to avoid electrochemical corrosion issues.
Strict specifications govern the process control of shot media quality and size. During use, shot media wear down and change shape due to continuous impact, leading to deterioration. The residual stress in components after shot peening is directly related to the size of the shot media, so vibrating screens are installed in equipment to control shot size. Shot diameters range from 0.2 mm to 3.4 mm, with most shot peening applications using diameters below 1.0 mm.
The shot peening standard setter must decide on the shot media size and type.
2 Shot Peening Intensity
Setting the shot peening intensity standard is critical for the component. The standard setter must specify upper and lower limits for intensity, such as 0.20–0.26 mmA. The intensity is specified as a range because it is difficult to control precisely to a single value. Optimizing shot peening intensity requires extensive testing, considering factors such as material type, load type, and component thickness. Therefore, the selection of shot peening intensity must be carefully considered.
The shot peening intensity is higher when the shot stream is perpendicular to the component surface compared to non-perpendicular angles. Standard setters often specify that the required intensity must be tested at specific locations using Almen strips mounted at appropriate angles (Almen strip holders). For these specified locations, the shot stream's intensification effect must be achieved.
The "magical" residual compressive stress hardened layer formed on the component surface after shot peening can be up to 1 mm thick. Charts are available that describe the relationship between shot peening intensity, component material, and the thickness of the hardened layer. A rough formula is provided here: the thickness of the residual compressive stress hardened layer is approximately two-thirds of the Type A peening intensity.
Setting and optimizing shot peening intensity for components is complex due to the many influencing factors. As early as 1958, Fuchs suggested that for general applications, a shot peening intensity of 0.25–0.35 mmA (0.010–0.014 inches A) is suitable. He also noted that factors such as component thickness, material, and surface condition before peening (e.g., the presence of cracks or notches) affect the fatigue life of shot-peened components. This indicates that optimizing shot peening intensity requires comprehensive consideration, and even with many relevant factors accounted for, it may not guarantee that the process will truly enhance component life. Extensive practical testing is necessary for validation.
3 Shot Peening Coverage
The shot peening standard setter must specify the coverage for the component. Determining the optimal coverage for a component can only be achieved through extensive testing; there is no better method. Currently, there are two schools of thought regarding optimal coverage: one believes that higher coverage is always better, leading to vague requirements such as 300% coverage (essentially three times the time required for "100%" coverage). The other school believes that for most applications, the optimal coverage does not exceed "100%."
It is widely accepted that measuring coverage accurately above 98% is challenging, so a coverage value slightly below 100% is generally considered "full coverage."
Discussion and Conclusion
This article provides a general introduction to shot peening, particularly focusing on the parameters that must be specified by the standard setter. Some necessary simplifications have been made, and this article does not delve deeply into the extensive literature available on the theoretical and practical aspects of parameter variations in shot peening.
Shot peening is widely used for most metal components, primarily to address issues of premature fatigue failure. It is worth reiterating that there is a vast body of literature on how shot peening improves the fatigue life of components. As an "additional" surface treatment process, shot peening is a more economically viable method compared to altering component dimensions or materials.
