Reliability of Materials and Structures
Dimensioning of structural parts for strength and reliability is an essential element of product development in mechanical and plant engineering. The general methods to achieve this purpose are being developed in the competence field “Fitness-for-Purpose and Damage Tolerance”, and applied to specific industrial problems in the competence field “Material Efficiency and Reliability in Transportation, Mechanical and Plant Engineering”.
Fitness-for-Purpose and Damage Tolerance
Increasing time and cost pressure in product development as well as the quest for cost and weight optimized products call for an integrated approach towards product design. On the part of strength of materials and structures, this is met by so-called Fitness-for-Service or Fitness-for-Purpose concepts. These concepts account simultaneously for the four main factors affecting safe operation of a structural part:
- external loading
- required lifetime
- local properties and state of the material
- eventual damage during manufacturing and/or operation
The prediction accuracy depends crucially on knowing the loading of the structural component during operation as exactly as possible. This comprises static and cyclic mechanical loading including occasional or periodic overloads as well as environmental influences such as elevated or frequently changing operational temperatures, corrosive media, hydrogen embrittlement etc.
The lifetime of a component depends on the product lifetime and the decision whether the component can or should be replaced in regular service intervals – leading to the classical dilemma of balancing initial costs, maintenance costs and product reliability.
The material selection is then performed according to loading, lifetime and eventual additional requirements with respect to component dimensions and weight. Special attention has to be paid to manufacturing and post-treatment processes, which may significantly enhance the local material properties in highly loaded regions. For example, heat treatment and thermo-mechanical post-treatments such as hammer peening, shot peening, roller burnishing or deep rolling may induce compressive residual stresses with an enormous potential for strength and/or lifetime enhancement.
Accounting for eventual damage during manufacturing and/or operation leads to the concept of damage tolerance. Using methods from fracture mechanics, the influence of production induced flaws (casting, forging, rolling flaws) as well as operation induced flaws (Foreign Object Damage, e.g., bird strike in aerospace applications or ballast impact in railway axles and bogies). This approach naturally leads to optimized maintenance concepts, where the inspection intervals are determined according to the detected flaw size. This means, of course, that the presence of any flaws exceeding a certain size must be excluded by means of appropriate non-destructive testing methods.
Material Efficiency and Reliability in Transportation, Mechanical and Plant Engineering
All influencing factors described above exhibit some statistical scatter: loading, material strength, flaw size, detection limit of non-destructive testing. To account for the respective uncertainties, suitable safety factors have to be chosen. However, it must be noted that the trend of downsizing component dimensions as well as every safety factor to the lowest possible allowed value will lead to an increasing number of component failures.
Methods of robust optimization aim at considering simultaneously all these influence factors and find that combination of values that leads to an optimal product excelling by efficient material usage as well as high reliability, tolerating the unavoidable slight changes in component dimensions, material properties and external loading.
Especially in safety-critical applications, it is useful to assess the diverse sources of scatter separately in detail. By means of probabilistic methods one can get more precise information about the dependence of the failure probabilities on the various influences from material, manufacturing and operation; the allowable failure probability must be supplied from a risk assessment by the product manager, customer or (as in aeronautics and railway transport) by the responsible public authorities.
- H.-P. Gänser, A. Leitgeb, K. Glinsner, W. Eichlseder: Computation of a modified Haigh-Goodman diagram for damage tolerant design for infinite fatigue life. Proc. Inst. Mech. Engrs Part C, J. Mech. Engng Sci. 221 (2007) 619-623
- H. Maderbacher, B. Oberwinkler, H.-P. Gänser, W. Tan, M. Rollett, M. Stoschka: The influence of microstructure and operating temperature on the fatigue endurance of hot forged Inconel® 718 components. Mat. Sci. Engng A 585 (2013) 123-131
- J. Maierhofer, R. Pippan, H.-P. Gänser: Modified NASGRO equation for physically short cracks. Int. J. Fatigue 59 (2014) 200-207
- J. Maierhofer, H.-P. Gänser, R. Pippan: Modified Kitagawa-Takahashi diagram accounting for finite notch depths. International Journal of Fatigue 70 (2015) 503-509
- J. Maierhofer, R. Pippan, H.-P. Gänser: Modified NASGRO Equation for Short Cracks and Application to the Fitness-for-purpose Assessment of Surface-treated Components. Procedia Materials Science 3 (2014) 930-935
- H.-P. Gänser, J. Maierhofer, R. Tichy, I. Zivkovic, R. Pippan, M. Luke, I. Varfolomeev: Damage tolerance of railway axles – The issue of transferability revisited. International Journal of Fatigue 86 (2016) 52-57
- U. Zerbst, M. Vormwald, R. Pippan, H.-P. Gänser, C. Sarrazin-Baudoux, M. Madia: About the fatigue crack propagation threshold of metals as a design criterion – a review. Engineering Fracture Mechanics 153 (2016) 190-243
- S. Kolitsch, H.-P. Gänser, J. Maierhofer, R. Pippan: Fatigue crack growth threshold as a design criterion – statistical scatter and load ratio in the Kitagawa-Takahashi diagram. International Conference on Materials, Processing and Product Engineering 2015 (MPPE 2015). IOP Conf. Series: Materials Science and Engineering 119 (2016) 012015 doi:10.1088/1757-899X/119/1/012015
- H.-P. Gänser, J. Maierhofer, T. Christiner: Statistical correction for reinserted runouts in fatigue testing. International Journal of Fatigue 80 (2015) 76–80