Introduction

Abstract

This volume on the results of the research of the Collaborative Research Center 871 (CRC 871) contains selected papers presented at the Final Symposium of the CRC 871, which took place from March 31 through April 1, 2022 at Leibniz University Hannover, Germany.

1 Preface

This volume on the results of the research of the Collaborative Research Center 871 (CRC 871) contains selected papers presented at the Final Symposium of the CRC 871, which took place from March 31 through April 1, 2022 at Leibniz University Hannover, Germany. The chapters herein are a compilation of articles presented by the sub-projects at the symposium. The CRC 871 “Regeneration of Complex Capital Goods” was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—SFB 871/3—119193472. All participants of the CRC 871 would like to express their gratitude to the DFG for the funding. Overall, the book provides readers with the state of the art of Maintenance, Repair, and Overhaul (MRO) in different engineering and economic disciplines with a special focus on the regeneration of civil aircraft engines.

2 Introduction Collaborative Research Centre 871: Regeneration of Complex Capital Goods

From 2010 through 2022, the CRC 871 contributed to developing a scientific basis for maintaining complex capital goods. The main goal of the CRC 871 was to develop new and innovative methods for restoring or even improving the functional properties of capital goods in order to reuse as many of the worn components as possible. The functional benefit of regenerated components and of the entire capital good are assessed through model-based simulations. Rule-based decisions are used to select the optimum regeneration path in order to achieve the maximum benefit for the customer.

In the CRC 871, jet engines were chosen as an application example because of the great challenge provided by their high complexity. The methods and procedures developed can also be transferred to other complex capital goods such as wind turbines, railway vehicles, and heavy-duty gas turbines. The objective of the CRC 871 required a cooperation of scientists from engineering, including the areas of product development and product design, industrial and production engineering, as well as business administration. Overall, twelve institutes from three faculties of Gottfried Wilhelm Leibniz University Hannover (LUH), the Laser Zentrum Hannover (LZH), and one institute of the Technical University of Braunschweig participated in the CRC 871. One researcher was associated as he moved to the Technical University Dresden.

2.1 Motivation and Objectives

The maintenance and servicing of capital goods such as aircraft engines, wind turbines, or rail vehicles contributes significantly to the operating cost. To reduce this share and save expensive resources, maintenance processes and repair procedures need to become more efficient. In state of the art regeneration processes, when the CRC 871 started in 2010, skilled workers carried out maintenance work and repairs on the basis of prescribed guidelines, see Fig. 1. The individual experience played an essential role, which can lead to poor reproducibility of decisions and a great variability of repair results.

Fig. 1

Example of the current repair process of a civil aircraft engine

The objective of the CRC 871 is to maintain and, if possible, improve the functionality of complex capital goods. A breakdown of the main objectives of the SFB 871 is presented below:

• Early identification of the components to be regenerated and the possible regeneration steps

• Continuous consideration of successive findings in the planning of the ongoing regeneration process

• Improved processes of regeneration and their integration in regeneration paths

• Model-based prediction of the functional properties of complex capital goods and regeneration-related expenses

• Based on the model-based predictions, rule-based decisions about the regeneration steps in order to maximize the benefit for the customer

• Integration of the model-based prediction of production-related expenses and functional benefits at the level of the entire capital goods

To achieve these subobjectives, the CRC 871 developed the scientific basis for an innovative approach: The example of aircraft engines was used to develop a combined virtual and real repair process. This novel approach used classic methods formerly reserved for product and manufacturing development and transferred them to maintenance (Fig. 2): Before selecting a regeneration path and the subsequent repair in a virtual process with the aid of a digital twin for all regeneration paths, the production-related cost and the functional benefit was evaluated and a decision for the most efficient regeneration path was derived from this rule-based assessment. The selection of the most efficient regeneration path depends on the customer business model and thus may differ by the customer.

Fig. 2

New repair process for civil aircraft engines based on the CRC 871

2.2 Research Program

The aircraft engine was chosen directly at the beginning of the CRC 871 in the year 2010 as the central research object for the joint research. Aircraft engines are very complex machines where all involved disciplines reach the limits of physical understanding and design methods. This applies to both, the individual components and sub-assemblies as well as to the overall system behavior. Therefore, aircraft engines are perfect for the interdisciplinary research of regeneration in the CRC 871. In the first funding period, the CRC focused on a turbine blade and developed an innovative regeneration process. In the second funding period, the developed methods were transferred to an assembly using the example of a compressor blisk. In the third funding period, the research was focused on the overall system, i.e. the aircraft engine, reaching maximum complexity (Fig. 3).

Fig. 3

Funding periods and its research focus

The sub-projects of the CRC 871 are grouped into four project areas. These project areas and sub-projects are depicted in Fig. 4 and were derived from the need to conduct research on the regeneration of civil aircraft engines.

Fig. 4

Project areas of the CRC 871

Project area A “Inspection and Diagnostics” aimed to improve the volume and quality of information from the inspection, which is necessary for planning the regeneration process. The sub-projects of this project area developed methods for an earlier, cheaper, more comprhensive, and faster detection of the condition of the engine or of single components.

In project area B “Interaction of the Production Process with Functional Product Properties”, the objective was to determine and to consider the complex interaction between production processes and functional properties of the components. To this end, the sub-projects simulated effects of the production process on the components and the overall system. The results of these simulations are used to determine the influence on the functional properties like the component life expectancy, fuel consumption, or vibration behavior.

The focus of project area C “Variance of Production and Material Properties in Regeneration” was on the variances which occur due to wear during operation and due to repair. The objective was to handle these variances and to estimate the influence on the functional properties.

In project area D “Integral Control of the Regeneration Process”, the results from the other project areas were used for the integral control of each regeneration step in order to build an integrated process. An additional objective was to integrate the simulations of the functional properties of the components, which made it possible to evaluate the functional properties of the overall system, such that the complete regeneration process could be controlled.

In addition to the project areas, research was also carried out on bringing the CRC’s subjects into the curricula of primary and secondary schools by an additional, dedicated sub-project during the last funding period. Furthermore, the practical applicability of the research results was shown by a system demonstrator.