Volume 48, Issue 3

Kurzfassung Die Arbeitsausschüsse NMP 231 und NMP 232 beschäftigen sich mit der Normung von Prüfverfahren in der Bauakustik. Neben der rein nationalen Normung machen die Ausschüsse ihren Einfluss auf europäischer und internationaler Ebene aktiv geltend. Insgesamt steht dem geringen Interesse der Industrie ein großes Interesse der öffentlichen Hand gegenüber. Der Großteil der Normung der näheren Zukunft wird sich mit der Pflege bzw. Aktualisierung des bestehenden Normenwerks beschäftigen.

The service durability of weld seams is one of the most challenging problems of component fatigue. Hereby, the complexity is due to several peculiarities of welded structures: the detailed weld seam geometry with macroscopic and microscopic notch effects and/or defects, the properties of the welded material, residual stresses and multiaxial proportional or non-proportional loading conditions. Non-experimental approaches to design against fatigue have to account for the difference between weld seams with and without post-weld treatment as well as proportional and non-proportional loading. The application of fracture mechanics parameters allows for a more realistic description of damage progression. This paper is intended to explain possible design approaches based on numerical fracture mechanics and their combination with stress and strain based approaches using clear and classified damage criteria (engineering crack size, fracture). All numerical approaches are presented with special regard to complexity and modeling requirements from the point of view of the practicing design engineer. Non-proportional loading is considered by the identification of a damage critical plane which requires special post-processing routines within the numerical analysis. Again, the question of practicability is raised.

Recent efforts to develop simple unified models of both wear and rolling contact fatigue (Kapoor et al. [1], Franklin et al. [2]) are discussed, in view of previous theoretical and experimental results on ratchetting in rolling contact. It is shown that ratchetting in rolling contact is a combination of “structural ratchetting” (that modelled with the perfect plasticity model) and “material ratchetting”, and the latter is very sensitive to the hardening behaviour of the material. Also, rolling contact fatigue at large number of cycles in the Clayton and Su experiments [3, 4] seems not well correlated with shakedown theory, and accordingly, simple ratchetting equations based on excess of shakedown such as that of Tyfoor et al. [5], do not seem well suited as a Wohler SN life curve. However, these conclusions are only very qualitative as the materials in the two tests are different, and at present empirical separate models for wear and rolling contact fatigue based on hardness of materials and a posteriori data fitting seem the only quantitative way forward for engineering purposes.

This paper presents a new model that accounts, on a local scale, for the coupling between plasticity due to gliding in shear bands and damage occurring when the accumulated plastic strain has reached a threshold value. The irreversible thermodynamics with internal state variables is employed to keep a middle way between extensive description of plastic and damage flow and application of accessibility requirements. Plasticity and damage are governed by their proper complementary rules (yield functions and potentials). At the same time, a coupling occurs between the damage variable and the hardening parameters. A large experimental database relative to the fatigue behavior of a mild steel C36 submitted to different loading modes (tension, torsion, combined proportional tension and torsion) proves the efficiency of such a model. The prediction of Wöhler curves for cyclic complex stress states can be readily done, but the main feature of this approach is to ensure a clear link between mesoscopic parameters like the hardening behavior of individual grains and the subsequent local damage.

In mechanical engineering, fatigue assessment based on Finite Element analysis of components, where complex non-proportional loads are acting on, plays an important role in identifying critical spots, and predicting the lifetime, till crack initiation. In this contribution the life time analysis of a dynamically loaded crankshaft of a V6 diesel engine is demonstrated. Finite Element method, multi-body system simulation and an elasto-hydrodynamic simulation of the oil film have been combined for a modal analysis in the frequency domain. Based on these results a fatigue analysis follows to identify the critical engine speed range. In notches rotating principal stresses can be found, which reduce the lifetime significantly. To quantify the non-proportionality of stresses and the resulting decrease of the fatigue limit, a statistical measure originally proposed by Chu et al. is applied. The critical plane criterion has been used in conjunction with a stress based method (concept of synthetic S/N-curves, influence parameter concept) and linear damage accumulation according to Miner's rule.

In the article a proposal of loading non-proportionality measure H for sinusoidal biaxial loading has been presented. On the basis of non-proportionality measure, there has been proposed a bending and torsional fatigue strength, with a phase shift of components and the mean value has been proposed. Moreover, a high-cycle fatigue life criterion based on the same non-proportionality measure has been proposed. The predictive capability of both criteria were demonstrated by analysing experimental results from the literature. The predicted results are generally in good agreement with the experimental ones.

This paper presents calculations of fatigue life of aluminium welded joints (tube-tube and flange-tube) under pure bending, pure torsion, and combined in- and out-of-phase bending with torsion, compared with against experimental results. In order to reduce a multiaxial loading state to an equivalent uniaxial state, an energy-based criteria is applied in the critical plane where cracking occurs. The best results were obtained using the criterion of maximum shear strain energy density for calculating equivalent local stress amplitude for tube-tube welded joints, for both proportional and non-proportional bending with torsion. For flange-tube joints under proportional and non-proportional bending with torsion, the best results were obtained using the criterion of maximum shear and normal strain energy density.

In order to handle the long living radioactive waste (spent nuclear fuel) SKB is planning to build a deep repository that requires no monitoring by future generations. The spent nuclear fuel will be encapsulated in copper canisters consisting of a graphite cast iron insert shielded by an outer 30–50 mm thick copper cylinder for corrosion protection. The most critical part of the encapsulation process is the sealing of the canister, which is done by welding the copper lid to the cylindrical part of the copper shell using friction stir welding and electron beam welding. The quality of the welding process and the reliability of the NDT system – radiographic and ultrasonic testing – must be satisfactorily determined and combined to derive assumptions regarding the frequency of undetected welding defects for a set of canisters as input for the risk assessment. This is done using the POD method according to the “Reliability Handbook MIL 1823” and its generalization.