Martin Riedler

Failure Mechanism of Pure Nickel (Ni 200/201) under Thermo-Mechanical Loading

Cyclic loading of metallic engineering components at constant elevated or fluctuating temperature causes a complex evolution of damage which be can hardly be described in a unique and straightforward manner. Often the thermal behaviour of the base metals is to weak, so thermal barrier coatings were needed. Nickel is generally used for such thermal barrier coatings. Therefore it is necessary to study the thermo-mechanical fatigue (TMF) of this material. The lifetime of these coatings is very strong affected by the temperature loading in general, both described by nodal temperatures and their local gradient. The thermal cyclic loading takes place as thermo-mechanical and low cycle fatigue (LCF) damage regime. To classify the thermo-mechanical failure mechanism of pure nickel, OP (out of phase) and IP-TMF (in phase) test series were examined. The use of damage parameters like the unified energy approach make sense, a more detailed life time calculation for pure Nickel can be done by using the Neu-Sehitoglu model. Summary, thermomechanical loadings activate multiple damage mechanism. Surface embrittlement by oxidation is the major distinctive mechanism in addition to pure fatigue damage. Different lifetime approaches were tested and analysed to fulfil the requirements for the fatigue analysis of nickel made components.

Liftime Optimization of Hot Forged Aerospace Components by Linking Microstructural Evolution and Fatigue Behaviour

Heavy-duty aerospace components are frequently hot forged to satisfy the high requirements concerning their mechanical behaviour. Only the usage of high-performance materials together with a near-optimum manufacturing process enables the production of parts that are at the same time lightweight and mechanically extremely durable. Not only the static properties, but also the fatigue behaviour of Inconel718 is strongly influenced by the material’s microstructure resulting from the forging and heat treatment processes. Therefore, the static and fatigue properties may be controlled via the microstructural properties by suitably adjusting the parameters of the manufacturing processes. The present work links the complete forging and heat treatment process to the local distribution of the material’s fatigue strength within a component; the effect of the operating temperature is also considered. To this purpose, an empirical model is derived from fatigue tests on specimens with different microstructures at different temperatures. The resulting fatigue strength model is implemented, along with a microstructural evolution model from earlier work [1], into a finite element code in order to predict the local fatigue strength distribution in a component after being subjected to an arbitrary forging process. In a further step, the finite element code is linked to an optimization tool for determining the optimum set of manufacturing process parameters such that the component lifetime is maximized while taking process constraints into consideration.

Thermomechanische Ermüdung von Aluminiumlegierungen

Kurzfassung In umfangreichen Untersuchungen wurden die relevanten Einflüsse auf das zyklische Verformungs- und Lebensdauerverhalten ermittelt. Dabei kamen zwea unterschiedliche Prüfverfahren zur Bestimmung der Lebensdauer unter thermomechanischer Beanspruchung zur Anwendung. Diese werden im vorliegenden Beitrag sowohl mittels Versuchen als auch mittels FE-Simulationen verglichen. Dabei zeigt sich, dass Versuche mit starr eingespannten, zyklisch beheizten Proben unter Berücksichtigung der lokalen Effekte mit Versuchen des servohydraulischen TMF-Prüfstandes mit Dehnungs- und Temperaturregelung vergleichbar sind. Weiters werden zur Modellierung des Lebensdauerverhaltens unterschiedlich komplexe Ansätze gegenübergestellt. Die verwendeten Verfahren reichen von einparametrigen Ansätzen bis zum Schädigungsmodell nach Sehitoglu mit mehr als 20 Parametern. Abhängig von Material und Anwendung zeigt sich jeweils ein anderes Kriterium als besser geeignet. Energiekriterien scheinen den besten Kompromiss aus Genauigkeit und Aufwand im Einsatz darzustellen. Größere Abweichungen treten auf, wenn sich die beteiligten Schädigungsmechanismen ändern. Liegen beispielsweise lokal In-Phase-TMF-Belastungen mit einer zusätzlichen Kriechschädigung vor, so kann dies bei der Verwendung von Energiekriterien, die auf Basis von LCF- und OP-TMF-Versuchen kalibriert wurden, zu nichtkonservativen Aussagen führen.

Four Point Bending Fatigue Tests of Forged Ti 6Al 4V

Abstract The paper derives from a long-term research program that is aimed at developing qualitative and quantitative design guidelines to influence mechanical surface treatments in order to improve the fatigue life of structural components. A four point bending test rig was developed using finite element analysis. High cycle fatigue tests were performed on plane specimens taken from Ti 6Al 4V forgings with mill-annealed or bimodal microstructure. The high cycle fatigue behaviour of specimens with two different surface conditions (as-forged and machined) was compared. In order to assess the fatigue failure mechanisms, detailed investigations of the surface layer were carried out. Residual stresses were shown to play an important role in fatigue.

Thermo-Mechanical Fatigue Lifetime Assessment with Damage-Parameters, Energy-Criterions and Cyclic-J-Integral Concepts

The simulation of the thermo-mechanical fatigue (TMF) behaviour of cylinder heads is an important design step in the automotive industry. The steady rise of engine power and the demand of lightweight construction with a concurrent enhanced reliability require an optimised dimensioning process. The goal of this paper is to apply classical damage parameters, plastic and total energy criterions and cyclic J-integral concepts for a thermo-mechanical lifetime assessment of aluminium and cast iron alloys.

Fatigue Analysis of Forged Aerospace Components Based on Micro Structural Parameters

With the objective of creating a simulation model for the lifetime calculation of forged aerospace components it is necessary to clarify the damage mechanisms in the materials used. This has been researched for the Ni-base alloy Inconel 718 by varying the forging parameter effective plastic strain rate, which is realised by using three types of equipment: hydraulic press, screw press and hammer. Specimens processed at the screw press show the highest lifetime by keeping all other forging parameters unvaried. Micro structural investigations show that the amount and morphology of dominant as-large-as grains play a important role. This methodology is currently investigated for Ti-6Al-4V. Lifetime tests show that besides effective strain and anisotropy the influence of morphology is important. As soon as the model status allows lifetime analyses the thermo-mechanical process (forging and heat treatment) can be developed depending on the desired lifetime specifications in order to realise an interdisciplinary lifetime optimisation of forgings. A further aim is the use of basic coherences of safe-life and fail-safe approaches in the low and high cycle fatigue region in order to reasonable handle with flaws and defects at the edge layer.