Abstract: In many cases, shot peening is used to increase the compressive residual stress in the surface layers of a working piece to get a better durability of this part. To increase the induced compressive residual stress a significant amount, stress peening is used. This article deals with the different methods of stress peening and the resulting residual stresses and durability obtained.

1. Introduction and Definition

Shot peening is a technology, which is a standard procedure. Peening (in the technical sense of the word) is the interaction between a particle (with the necessary hardness) and the surface of a working piece. If the particles have a round shape, you call it shot peening [Mü 93 a]. In the surface layer (up to 0.5 mm depth) compressive residual stresses are induced. At lower hardness of the working piece an additional hardening is achieved. In order to achieve better results by the peening process stress peening is used. Here, the working piece or component is stressed in the direction of the later loading. After this step, the original peening procedure is done and afterwards the unloading. The compressive residual stress profile, which is now achieved, is significantly higher than that gained by normal peening.

2. Different Methods

There are different methods suitable to achieve the preload on the working piece. You can distinguish three different ways and the corresponding stresses (only tensile stress makes any sense to increase the compressive residual stresses):
-Pure tensile stress over a whole cross section
-Bending stress, which gives in some surface layers tensile stress
-Torsional stress, which gives tensile stress over the whole surface in 45° direction to the torsional axis
For all three methods, there are applications in the automotive industry, which are described below.
The amount of the compressive residual stress depends on the (torsional-) preload (?ks) ?ks during peening [Mü 90, Mü 92, Bo 94, Mü 93a, Mü 93b, Mü 96, Mü 97a, Mü 97b, Ze 91, Ze 93].
In figure 1 the different steps of residual stresses during the whole stress peening process are shown. By loading (first step) tensile stresses in the surface layer are obtained. Then you peen the surface. The amount of residual stress induced by peening during the loaded state is independent of the preload and it looks like the residual stress profile without loading [Mü 90, Mü 92, So 00]. The last step is to unload the working piece. Now an additional amount of the compressive residual stress is achieved.

3.0 Examples of Performance Increase

3.1 Pure tensile stress as prelaod

Pure tensile stress as preload is used for connecting rods, which are mostly used in engines for racing cars. The distribution of the crack initiation is significantly narrower compared to the corresponding distribution of normal peened connecting rods. An increase in fatigue strength of 20 % is achieved [EN 91].
In a wide range stress peening is done in the following described applications.

3.2  Bending stress as preload

Especially in the motor vehicle industry leaf springs are used as a main part for the suspension of nearly every type of truck. The classic leaf spring was manufactured with leaves, which had a constant thickness along the single leaf. Under load a linear increasing stress along the leaf is obtained. The first step for reducing the weight of the spring was the parabolic shape of the leaves to get the same amount of stress along the whole leaf. So the number of leaves could be reduced by more than 50 % and the rate of the spring was the same. The fatigue strength of the springs increases around 50% because now there is a gap between the different springs. In both cases, the springs were normal shot peened.
The next step to increase the durability was the stress peening of the parabolic springs. They were preloaded so that at the surface the stress is at the yield strength or sometimes higher. After unloading to a depth of 0.15 mm the amount of compressive residual stress is at a maximum, which is shown in figure 2.
In S/N-diagram (figure 3) you can see a big increase in the durability between normal peened and stress peened parabolic spring. Another smaller increase can be detected between the two different preloads (750 N/mm²; 1500 N/mm²).

3.3 Torsional stress as preload

In normal passenger cars the suspension is dominated by coil springs. The main stress in coil springs is a torsional stress. For nearly ten years, coil springs for car suspensions have also been stress peened for some car models
If the spring is loaded too much, the shadowing of the different coils may give a negative effect. An optimum of the compressive residual stress distribution is reached around half of the maximum stress possible. Figure 4 shows no great endurance increase for high preloads.
A residual stress profile of stress peened coil springs is shown in figure 5. The load is around 40 % of the maximum possible load. At this load a minimum increase in the durability of a factor of two to three reached. At this load the influence from the shape of the coil spring is very small, which causes a different local increase of residual stress and the durability along the wire.

4. Recent Developments

At the Technical University of Karlsruhe in Germany investigations are made by combining stress peening with warm peening. Two effects could be achieved. The compressive residual stress increases another significant amount. At  high dynamic loads the stability of the compressive residual stress is increased over the first 104 cycles. [ see e.g.: Wi 99, Me 02, Me 03a, Me 03b]

5. Conclusions

Stress peening has been implemented in the automotive industry especially for springs as a method to increase the fatigue strength in order to reduce the weight. Recent developments in combination with warm peening show a further potential for another increase in durability of stress peened pieces.

6. References

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