PEEN FORMING

Peen forming offers the following advantages:

• Increased resistance to flexural bending fatigue and stress corrosion cracking
• All forming is accomplished by inducing beneficial residual compressive stresses
• Precisely engineered and controlled process
• The peen forming procedure can accommodate varying curvature requirements, skin thickness, cutouts, reinforcements and pre-existing distortion
• Peen formed wingskins exhibit residual compressive stresses on the top and bottom surfaces
• Design changes are easily accomplished by altering the peen forming procedure
• No forming dies are required
• Performed at room temperature
• Peen forming can be performed with or without an external load applied on the component
• Corrects distortion caused by manufacturing processes

Applications include:

• Wingskins
• Airframe components
• Transmissions
• Large connecting rods
• Crankshafts
• Shafts (generally)
• Rocket structure

Go to: Compressive stresses | Correction of Distortion | The process | Design and Quality controls

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Peen Forming uses the compressive stresses induced by controlled shot peening to alter the stress pattern, magnitude and depth within a structure to deliberately create a change in the component shape. Gentle curves within the elastic range of the material are regularly formed to consistent tolerances.

Laser peening is an alternative technique for the introduction of residual compressive stresses. It has the advantage over conventional shot peening in that it induces these stresses to greater depths, consequently a tighter radius of curvature can be achieved or performed on thicker materials.

Peen forming is the preferred method of forming aerodynamic contours into aircraft wingskins. It is a dieless forming process that is performed at room temperature and it is best suited for forming curvatures where the radii are within the elastic range of the metal without abrupt changes in contour.

Parts formed by peen forming exhibit increased resistance to flexural bending fatigue and where peen formed pieces are processed on one side only, the result causes both sides to have compressive stress. These compressive stresses serve to inhibit stress corrosion cracking and to improve fatigue resistance.

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The peen forming procedure can accommodate varying curvature requirements, varying skin thickness, cutouts, reinforcements and pre-existing distortion.

Peen forming can also be used to create flat material from a component that has been distorted during manufacture or heat treatment. The raw material will have stresses of varying and generally unknown magnitude from casting, rolling, forging, heat treatment and other manufacturing processes. If this material is then machined with the majority of the metal removed from one side only there will be dramatic alteration to the stress pattern in the final machined part and subsequent distortion could result. A very successful and acceptable method of correcting this distortion is by using the process of controlled shot peening to stretch the material on the concave side of the distorted component.

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During the process, the surface of the component is impacted under controlled pressure from small, round steel shot acting as tiny peening hammers to induce a beneficial residual compressive stress which elastically stretches the surface as shown in the diagram on the right. The surface will bend or ‘arc’ towards the peened side and the resulting curvature will force the lower surface into a compressive state. This precisely engineered and controlled process enables the surface to be manipulated into the desired and sometimes complex contours.

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MIC is able to advise and assist aircraft wing designers in the early stages of design through the development of computer modelling techniques that allow feasibility studies of particular designs. The program takes three-dimensional engineering data and, based on the degree of compound curvature, calculates and illustrates the degree of peen forming required. It also exports numerical data to define the peening that is required to obtain the curvatures. These techniques ensure that the designed aerodynamic curvatures are met with economically beneficial manufacturing processes.

Go to: Advantages and applications | Compressive stresses | Correction of Distortion | The process

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