Objectives and state of the art

1. Industrial objectives and expected achievements

The purpose of this study is to take prestress surface treatment into account in the design phase of development of a product. Today, within the framework of a continuous effort to improve the quality/price ratio of products, a large number of industries, such as the aeronautical, automobile, energy and mechanical engineering sectors, are seeking to produce mechanical and structural components with higher performance characteristics in return for a minimum overcost.

The manufacturers of aeronautical parts such as British Aerospace (AIRBUS) and Aerospatiale, are putting increasing emphasis on the residual stress parameter in order to obtain better quality control of products. This is the case, for example, of helicopter gear parts, to ensure reliability of the thermal-chemical treatment process, or of aluminum alloy parts to control deformation and secure compatibility of the metallurgical surface quality with their current applications.

A number of entities are involved in these actions designed to control industrial processes:

(1) Within the firm, the residual stress concept is primarily used in the R&D and quality departments; however, the concept is also starting to find its way into manufacturing entities and engineering and design offices.

(2) Outside the industrial firm, the residual stress concept is beginning to be used in the acceptance inspection of semi-finished products, e.g. laminated and drop forging and industrial parts such as gear parts, commercial aircraft wheels, and so on.

For the producer of aero gas turbine engines, such as SNECMA, the aim of having better control over surface treatments is to optimise the use of materials in aero gas turbine engines. From a technical point of view, aircraft turbine development is spurred on by the need for higher engine thrust / weight ratios and improved reliability. This calls for deeper insight into the behaviour under stress of a growing number of components. Increasing utilisation of the inherent potential of materials requires familiarity with the residual stress state as well as the effects of heat treatment, operating conditions and high temperatures on machined surfaces, especially parts subject to low cycle fatigue loads such as disks and on welded structures such as rotors and casings. The task is made more difficult by the great diversity of materials in use. The major challenge is to develop measuring techniques for in-depth residual stress evaluation in specific materials such as nickel based alloys and plasma spray coatings. Both kinds of materials present difficulties for the use of conventional techniques such as the X-ray diffraction method, due to the microstructure (single crystal, large grain size etc.) and difficulties in removing successive layers from ceramic coatings. Another issue is design tool development. SNECMA has developed a number of design approaches for fatigue life prediction. The next development will be the integration of residual stress into the calculation of fatigue life.

For automobile and truck manufacturers, such as Volvo and Scania, and automobile suppliers such as Bosch, surface modification is very important in improving the fatigue and wear behaviour of components, especially chassis, engine and transmission parts in cars and trucks.

In the car industry, residual stresses are taken into account in correcting design defects resulting in problems which come to light during endurance trials. These problems are often the concern of the Manufacturing Department. In the case of connecting rods, the weight of a mechanical part is almost always a handicap. This is especially true for parts with fast reciprocating movements, particularly in conventional engines - pistons, piston rings, piston pins and connecting rods. The knock-on effect of making these components, and therefore the connecting rod, lighter, is multiple - reduction in engine weight, reduction in forces on bearings, reduction in excitation due to moving masses.

For a optimised geometry of the connecting rod, progress can be made by the application of prestressing through the use of shot peening. The results of studies conducted by Renault show that the push-pull fatigue limit improves from 15kN to 25 kN.

Some of the highest stressed springs in automotive engineering can be found in modern diesel injection pumps for heavy duty trucks. At Bosch, the quality and output of those high-end-springs is realised by shot peening under prestress. All springs used in Bosch injection nozzle holders, and mainly all in fuel injection pumps are machined by centrifugal wheel shot peening. Other broad-used examples for shot peening can be found in transmission parts of distributor-type fuel-injection pumps, as cam plate and cross coupling, and in power tools for industry.

In the future, at Bosch residual stresses should be considered consequently already in the design phase of a component. Therefore, we are working on different topics concerning origin, simulation and assessment of residual stresses caused by machining or heat treatment or cold working processes. So, devices as FEM-postprocessing for material damage assessment by lifetime calculation with residual stresses taken into account would be very helpful to achieve this goal.

There are other examples which show the advantage of being able to predict the mechanical behaviour of components. One example is calculating the fatigue limit of a crankshaft. The major problem lies in predicting the residual stress induced by cold rolling, rather than predicting the 3D relaxation of residual stress and multiaxial fatigue behaviour. Another example concerns sheet metal forming and the effect of spring back. The springback effect is directly related to the residual stress level and they can be simulated either by a 2D semi-analytical model or by a 2D finite element model. But the case of a real component, such as a side member, is more complicated. In this case, a 3D simulation needs to be used, applied during the different phases of part forming, tool removal, and part ejection. However, we are not up to that stage yet and work in this respect should prove to be very interesting in the future.

The major challenge is to consider process parameter and selection during the design phase, in order to develop on-line process control techniques and test specimens with complex shapes such as gears and springs. The processes concerned here include case hardening, nitrocarburizing, plasma nitriding, machining of soft and hardened steel, shot peening, induction hardening, quenching of aluminum components, sheet forming etc.

For the industrial partners in the energy sector, such as SIEMENS, ABB and Wartsila NSD, the main objective is to optimise manufacturing processes and predict fatigue behaviour taking residual stress into consideration. Measuring techniques need to be improved and standardised for a better definition of quality control and maintenance procedures.

Two examples of stress corrosion cracking of nuclear power plant components in which residual stress might play a role with regard to in-service behaviour and fatigue life are given below

- crack formation in welds on stainless steel piping found world-wide in boiling water reactors. To reduce tensile residual stresses (or introduce pressure residual stresses) in the root area, a number of optimisations are possible, e.g. narrow gap welding, last pass heat sinking, additional cover layer melting, etc.

- crack formation in the welding of stainless steel core shrouds (welded thickness of up to 45mm, together with flanges up to 140 mm thick) in boiling water reactors were detected. FEM-calculations and residual stress measurements were carried out.

Another example are highly stressed diesel engine components such as camshaft cams and cam followers. The surface modification process used is case hardening. The lifetime of these components is sometimes limited by sudden removal of the material from the surface as a result of decreased fatigue life of the material. Optimising the fatigue properties for such heavily loaded surfaces calls for deeper understanding of surface finishing methods and the ability to optimise the surface finish e.g. grinding parameters in order to introduce a beneficial residual stress state. From a quality control point of view, new residual stress measurement methods, such as x-ray diffraction and the magnetoelastic method, provide interesting possibilities for detecting surface defects, which can not be detected using present methods.

The last kind of application for the energy sector is the residual stress introduced by machining. By investigating how residual stresses are affected by machining parameters and how these stresses influence fatigue properties, components with beneficial surface residual stresses can be manufactured more efficiently. Examples of applications include gas turbine disks, diesel engine connecting rods and crankshafts for marine transport and power generation.

For companies such as Stresstech, this network provides a unique opportunity to discover new applications and develop existing ones in the field of stress measurement. These firms produce stress measurement equipment based on X-ray diffraction and micromagnetism. Participation in the network will strengthen their competence and provide many useful inter-European contacts.

This initial analysis shows that the problem of residual stress is an important topic for the different sectors of industry. The network can contribute to the exchange of methodologies used by different partners and to the transfer of research results from universities and research centres to industry. Specific seminars and short courses will be organised to train industrial users in operating this advanced tool. Remarks from industrial users can also contribute to determining the orientation of research in the future. The problems actually encountered in industry are often more complex than academic cases. Tools need to be improved. The approaches used by research laboratories sometimes need to be simplified for industrial applications.

2. State of the art and degree of innovation

The residual stress consideration is becoming increasingly important today for two reasons[1] i.e. the necessity for the designer to reduce the weight of the structure while maintaining the same safety level in order to remain competitive and the introduction of multimaterials, which induces residual stress. With the development of different experimental and numerical techniques, it is now possible to introduce residual stress into the design office for the integrated design of mechanical components.

Many research projects have been conducted recently in the field of residual stress. But very few projects are relative to design with residual stress problems. This network will promote a global approach (material, processing and mechanical design) to prestress engineering.

More and more, industries tend to use concurrent engineering methods to avoid these problems. With these methods, technical capabilities are organised around a project instead of a sequential structure. Problems often appear, however, during development of the project because employees lack the necessary unified technical and numerical tools to communicate. In response to this, a concurrent engineering platform will be created by the network in order to model the design (CAD and structure computation), the technical feasibility and the life cycle possibilities of a mechanical system. The main objective of this scientific project is the design and optimisation of prestress processes (forging, surface treatment and quenching, welding).

[1]Handbook of measurement of residual stresses, SEM (society for Experimental Mechanics, USA), 1996, Prentice Hall, Edited by J.LU