Fundamentals of the Deep Rolling of Compressor Blades for Turbo Aircraft Engines

The weakening of the fatigue strength of turbine blades due to local impa caused by foreign objects (Foreign Object Damage FOD) represents i3 safety risk in modern aviation. It is possible to employ deep rolling to courat component weakening in a particularly effective way. In order to derive a possible process parameter-set in advance, fundamental knowledge regarding the elastic and plastic deformation was researched. For this, FE analysis and practical tests were performed and compared. They are showing the complex deformation mechanisms occurring during the rolling contact. Subject Index deep-rolling, elastic and plastic deformation, fan and compressor blades, foreign object damage (FOD), Introduction The safety of turbo aircraft engines is influenced to a large extent by their tolerance to the damaging effects of foreign objects. Foreign object damage (FOD) occur above all within the fan, the first compressor stages and on the blade edges in particular. Notches, tensile residual stresses and sometimes even micro cracks are induced by FOD. Sometimes these damages lead to crack initiation and rapid crack growth upon subsequent application of alternating loads. This causes much earlier fracturing than is the case with undamaged blades [Martinez 2002, Bache 20021. Strain hardening of the rim zone of the component is one promising way to suppress the crack formation resulting from alternating loads, as well as to stop or slow down crack growth [Altenberger 2003, Nalla 20031. Furthermore, deep-rolled testpieces reveal the capacity to withstand almost double as many cycles of stress compared to samples strain-hardened by shot peening or laser shot peening [Nalla 2003, Shepard 20031. This is largely attributable to the greater plastic strain that may be achieved. Deep rolling of thin-walled fan and compressor blades is possible by using special rolling tool produced by the company ECOROLL (Fig. I). This tool uses a hydrostatically supported ball as the rolling element. This is mounted in a suitable holder and supported by a hydraulic piston which, in turn, may be moved axially in a sleeve mounted on the tool shank and which ensures a constant rolling force corresponding to the hydraulic pressure. In order to prevent the thin-walled blades from bending during the deep-rolling process, the rolling forces are compensated for by two opposed rolling tools. The design of such deep-rolling processes is, however, associated with timeand cost-intensive test efforts as no suitable methods exist for deriving the necessary process variables from the desired rim zone properties. l C S P 9 : SHOT PEENING hydrostatically bedded deep rolling of opposed rolling tools rolling ball turbine blade deep-rolled leading edge Fig. 1: Principle of a hydraulically driven and hydrostatically supported rolling tool and machining tests One needed aspect is the knowledge regarding the elastic and plastic process taking place during the rolling contact. In spite of several research papers in which deeprolling in particular [Achmus 1999, Schaal 2002, Black 1997, Skalski 19951, or the contact in general between a sliding or rolling element and a workpiece that deforms plastically [Johnson 1985, loannides 19991 is examined, there is, to date, no fundamental understanding of the process. In order to generate this fundamental knowledge and to be able to derive a possible process parameter-set in advance, the entire process was researched. Methods, Results and Discussion The experiments demonstrated that the deep-rolling process is largely based on a compression operation. The surface of the rolling ball is pressed orthogonally with the rolling force onto the surface. Should the workpiece have a rough surface, the first thing to occur is that micro compressions form between the ball and the roughness peaks. This leads to the generation of maximum equivalent stresses within the roughness peaks which soon assume high values and reach the yield point. This leads to a vertical plastic compression of the roughness peaks. At the same time, the roughness valleys in between are raised and their notches reduced. 3D-finite-element simulations show that, along with the levelling of the roughness peaks, a maximum equivalent stress is formed beneath the surface of the workpiece. As long as these stresses do not reach the yield point of the material, the stress field along the contact normals corresponds to the theory of elastic stresses from Hertz and Foppl. As can be seen in Fig. 2, area "a", this maximum equivalent stress extends on the left and right of the contact normals (c.n.) towards the surface and ends directly next to the outer contact area (c.a.) between the ball and the workpiece. If the rolling force is further increased, the equivalent stress reaches the yield point and plastic material deformation occurs beneath the ball, stretching laterally up to the surface of the workpiece. A detailed analysis of the stress sites reveals a large area of vertical compressive stress directly beneath the rolling ball. This causes vertical plastic compression, leading to a depression in the surface of the workpiece, as well as to a horizontal MODELLING AND SIMULATION 127 extension of the material volume perpendicular to the direction of the compression (Fig. 2, area "b"). FE-Modelof mlinq force Deep Rolling