Band 48, Heft 1-2

Kurzfassung Dieses Porträt bildet den Auftakt einer Aufsatzreihe in der Zeitschrift “Materialprüfung”, in der einzelne Fachbereiche bzw. Sachgebiete des Normenausschusses Materialprüfung vorgestellt werden.*

Kurzfassung Der vorliegende Beitrag vermittelt eine Übersicht der Normungsaktivitäten des Normenausschusses Materialprüfung im Bereich der zerstörungsfreien Prüfung.

Deformation of most metals and alloys is time-dependent and this dependency becomes more pronounced at temperatures exceeding a third of the material's melting point. Furthermore, the deformation response of a material to cyclic loading involves transients prior to stabilization. Some manifestations of the transient responses are: strain hardening or softening, loading sequence effect, strain-rate and strain-path history dependency, creep-plasticity interaction, strain ratcheting, among others. In addition, a constitutive model formulated in terms of macroscopic parameters must have a certain underlying microscopic rationalization. In this paper rate-dependent constitutive relations for an inelastic material are presented. This constitutive model is of a coupled nature, in the sense that the effect of prior creep on the subsequent plastic deformation and vice versa, are taken into account. Thus, the framework for these constitutive relations is based on the concept that any loading sequence can be predicted by two separate (elastoplastic and creep) but coupled models. A number of examples covering a wide range of cyclic loading types are presented.

The use of ceramic materials in machine parts with multiaxial and non-proportional stresses requires near-application testing to gather parameters for proper strength and lifetime prediction and to reduce the costly testing of prototypes. Therefore, experimental procedures have been developed for testing ceramic specimens with natural flaws under proportional and non-proportional stresses. Testing rings of different diameter ratios under internal pressure allows the estimation of short-term strength, crack growth parameters and of multiaxial failure criteria even with surfaces in “as-fired”-conditions. Assessment numbers, which quantify the influence of failure criteria on test results, are used to compare different experimental setups regarding their ability to reveal failure criteria. An additional setup allows testing with non-proportional stresses and different load sequences. A cylindrical specimen with a necked-down portion is loaded by alternating torque and compressive force to achieve variable mixed-mode-loading on natural cracks in the necking. Test series with alumina and zirconia are in progress and preliminary results are presented.

This paper takes a closer look at the use of the Modified Wöhler Curve Method (re-interpreted in terms of the Theory of Critical Distances) to predict the high-cycle fatigue strength of notched components subjected to fatigue loading. This method takes as its starting point the assumption that fatigue damage depends on all the physical phenomena taking place within the fatigue process zone (the so-called structural volume). The material volume controlling all the fatigue processes has a size which does not depend on the degree of multiaxiality of the stress field damaging critical sites, but its size is different for different materials. Assuming that crack initiation in the high-cycle fatigue regime is Mode II governed, the Modified Wöhler Curve Method must be applied by considering the linear-elastic stress state at the centre of the structural volume. The main advantage of this method is that it is based on the use of linear-elastic stresses: this aspect makes it suitable for being used to post-processing results from linear-elastic Finite-Element models. Finally, accuracy and reliability of the proposed method was checked by using data taken from technical literature: the Modified Wöhler Curve Method proved to be a useful tool for assessing mechanical components in situations of practical interest.

Both the fatigue crack path and fatigue life of CK45 steel and 42CrMo4 steel under various multiaxial loading paths are studied in this paper. The replica method was applied to monitor the crack initiation and small crack growth, the fractographic analyses were carried out on the fracture surface and the crack initiation angle was measured. The effects of non-proportional loading on both the crack path and fatigue life were studied, and the flattening of asperities on the crack surface due to compressive normal stress was also observed. An improved model is proposed based on correcting the strain range parameter of the ASME code approach, taking into account the additional hardening caused by the non-proportional loading path, which can improve the predictions of the fatigue lives for various non-proportional loading paths and provide an easy way to overcome the drawbacks of the current ASME code approach for non-proportional fatigue. Based on these corrected strain range parameters, a strain intensity factor range is used to correlate with the experimental results of small crack growth rates. It is concluded that the orientation of the early crack growth can be predicted well by the critical damage plane, but the fatigue life can not be predicted accurately using only the parameters on the critical plane, since the damage on all the planes contributes to fatigue damage as stated by the integral approaches.

Various components of nuclear power plants are submitted to very sharp multiaxial thermomechanical loadings, due for instance to the incomplete mixing of flows at different temperatures. As an example, thermal fatigue damage has been detected in auxiliary loops of the primary cooling circuits of Pressurized Water Reactors. In particular, crack networks were observed in in-service pipes submitted to thermomechanical loading resulting from cyclic temperature gradients across the wall-thickness of components in 304 L type austenitic stainless steel. The thermal fatigue behaviour of AISI 304 L type steel has been studied using a specific thermal fatigue test, called Splash, developed in order to reproduce experimentally such thermomechanical biaxial loading in the thickness of parallelepipedic specimens. All tests have been performed at a maximum temperature of 320°C, but with different minimum temperatures. First, the morphological characteristics of the growing networks were analysed, in surface and in depth. Crack initiation is multiple and occurs on sliding lines or at material defects. Crack network stabilization is observed after 400 000 cycles at a temperature of 150°C. The maximum depth is 2.5 mm. Secondly, the stability of the thermal-fatigue cracknetworks previously obtained was investigated under additional isothermal mechanical loading (four-point bend tests). Selection mechanisms of a dominating crack are observed, showing a great influence of shielding effects, branching and tortuous path. Comparison of the dominating crack behaviour with one having a single crack initiated at a notch tip reveals a significant delay effect.

Generally, most machine parts are loaded with a combination of different variable forces and moments which often causes a state of multiaxial stress in the fatigue-critical areas of the parts. In most cases, a nonproportional cyclic multiaxial state of stress occurs. Compared to in-phase loading, multiaxial loading with a phase shift between the stress components and a load ratio of τ a /τ a ≈ 0.5 between tension/compression and torsion leads to a significant influence on the fatigue lifetime. The reason is the changing direction and rotation of the principal stresses during one cycle. In this paper, a model is designed to simulate the damage process based on the growth of microcracks under the influence of cyclic loading. Crack growth is initially dominated by shear stresses leading to microstructurally short cracks (stage I) and continues to grow under the influence of normal stresses (physically short cracks). The results of the lifetime estimation generated by means of the new concept based on microcrack growth are compared and verified with those experiences obtained from multiaxial fatigue testing.