Roland Hessert

Influence of process and geometry parameters on the surface layer state after roller burnishing of IN718

Highly stressed components of modern aircraft engines, like fan and compressor blades, have to satisfy stringent requirements regarding durability and reliability. The induction of compressive residual stresses and strain hardening in the surface layer of these components has proven as a very promising method to significantly increase their fatigue resistance. The required surface layer properties can be achieved by the roller burnishing process, which is characterised by high and deeply reaching compressive residual stresses, high strain hardening and excellent surface quality. In order to achieve a defined state of the surface layer, the determination of optimal process parameters for a given task still requires an elaborate experimental set-up and subsequent time-consuming and cost-extensive measurements. The development of well funded process knowledge about the correlation of the process parameters, the processed geometry and the surface layer state is the subject of this article.

Cooperating twin robots form a new X-ray diffractometer for stress analysis

Abstract X-ray diffraction measurements on large components with complex geometry are needed in industrial applications requiring the non-destructive characterization of the near-surface material condition in terms of residual stresses, work hardening, phase transformation and formation of reaction compounds. Because many regions of interest on these components are not accessible with conventional laboratory or even mobile X-ray diffractometers, a novel center-free diffractometer with two cooperating robots named “Charon XRD” has been developed at MTU Aero Engines. Two six-axis robots are synchronized using a special optical measuring system to achieve the highest positioning accuracies. This paper describes the actual design and operating mode of Charon XRD, its current and potential functionality, and presents calibration and reference measurements, along with results on aero-engine high-technology components like bladed integrated disks.

Third Order Elastic Constants and Rayleigh Wave Dispersion of Shot Peened Aero-Engine Materials

Aero-engine components exposed to high mechanical stresses are made of high-strength alloys and additionally, they are surface treated by shot peening. This process introduces compressive residual stress into the material making it less sensitive to stress corrosion cracking and fatigue and therefore benefits the components performance and lifetime. Moreover cold work is induced in an amount depending on the peening parameters. To approximate the remaining lifetime, a quantitative, non-destructive method for stress assessment is required. It was shown that surface treatment of such alloys can be characterized by broadband Rayleigh wave dispersion measurements. However, the relative contributions of residual stress and cold work, respectively, remained an open point. This paper presents the determination of third order elastic constants (TOEC) for IN718 and Ti6246, providing, together with a model for the inversion of dispersion data, a quantitative access to the acoustoelastic effect. Finally, some measurements of differently treated samples are given.

Ultrasonic evaluation of residual stresses in aero engine materials using bulk and Rayleigh surface waves

The evaluation of residual stresses using ultrasound can be a very complex issue, because different material properties may effect the propagation of ultrasonic waves. Nevertheless, in the manufacturing of modern aero engines it is essential to benefit from the full potential of the employed materials. In this context, it is indispensable to test whether ultrasonic stress measurement is applicable for the highly developed nickel- and titanium-based alloys. This contribution contains basic investigations on the achievable measurement effect in samples made of Inconel IN718 and the Titanium alloy Ti 6-2-4-6. Furthermore, we give an overview over the principles of ultrasonic stress measurement using bulk and Rayleigh waves and present first results which are discussed with respect to texture effects and future work.