Volume 46, Issue 6

The influence of microstructural parameters on low cycle fatigue damage in some single phase and biphasic stainless steels has been studied. In a single phase austenitic steel, the fatigue life increase with decreasing grain size is attributed to the positive effect of microstructural barriers in the bulk. In a metastable austenitic steel, the grain size of the initial microstructure has a combined influence on the volume fraction of martensite formed during cyclic loading and on the fatigue life. In this case, the beneficial effect of grain refinement is indirect because all cracks grow in the martensite. Possible damage patterns in a cluster of grains with different properties are discussed using the example of a duplex austenitic-ferritic steel. It is demonstrated that the propagation behaviour of short cracks related to the resistance of microstructural barriers in the bulk has a decisive influence on the fatigue life. The macroscopic fatigue resistance is thus controlled by damage events at the microstructure size scale.

This paper presents the recent developments performed on an axial-torsional hydraulic test machine to simulate multiaxial thermo-mechanical fatigue tests on hollow single crystal specimens for turbine blade applications. After a detailed description of the testing device and the experimental procedure, the modelling of the tests using a finite element method is discussed for multiaxial loading, principally for torsion loading.

The first stage of fatigue is an important problem because its width is very short but its life is 80 % of the whole life. And it is well known that initial crack growth is very fast under the conditions of existing compressive stress. The reason is that a crack opening arises from the remaining tensile stress. Therefore, in this study the effect of compressive stress was investigated. As a result, it was found that the acceleration of initial crack growth is proportional to compressive stress, and striations were found in the initial crack.

Notched compact tension specimens of type 304 stainless steel (notch tip radius of 1 mm and 4 mm) were low-cycle-fatigued at stress ratio R = −1 and 0.05 in order to investigate the through-thickness fatigue damage accumulation around the notch. The recrystallization-thermal etching technique was used to observe the accumulated plastic zone APZ with the accumulated plastic strain ∊ ac above 0.12(APZ 0.12 ) and APZ with ∊ ac above 0.02 (APZ 0.02 ) at midthickness, quarter thickness and surface. Both APZ 0.12 and APZ 0.02 are formed early during the fatigue (before crack initiation) near midthickness and not at less thickness or at the surface. The reason why the fatigue crack initiates near the midthickness of the notch root surface was found to be due to more fatigue damage near midthickness than at the surface. Magnetic permeability(degree of the martensite formation) at midthickness was also higher than at the surface, supporting the above result.

For the first time metal matrix composites have been investigated by 3D computed tomography combined with enhanced interface contrast due to X-ray refraction. The related techniques of refraction topography and refraction computed tomography have been developed and applied during the last decade to meet the actual demand for improved non-destructive characterization of high performance composites, ceramics and other low-density materials and components. X-ray refraction is an optical effect that can be observed at small scattering angles of a few minutes of arc as the refractive index n of X-rays is nearly unity ( n = 1 − 10 −6 ). Due to the short X-ray wavelength, the technique determines the amount of inner surfaces and interfaces of nanometer dimensions. The technique can solve many problems in understanding micro and sub microstructures in materials science. Applying 3D refraction computed tomography, some questions could be clarified for a better understanding of fatigue failure mechanisms under cyclic loading conditions.

The sensitivity of penetrant and magnetic particle testing (PT and MT) depends, among other things, on the quality and performance of the “detection media” (for PT designated as “product family”). The determination of the sensitivity is carried out on “reference pieces” with natural or artificial gap-like flaws by determining the visibility of the indications as a measure of the performance. European standardisation specifies the requirements of detection media, in accordance with prEN ISO 3452-2 for PT and EN ISO 9934-2 for MT for type and batch as well as for process control testing. This paper describes the evaluation and qualification of performance during the type testing procedure. The proposed methods are validated by experimental investigations using an image processing system for the determination of the visibility. The reliability of the proposed methods is discussed with reference to the qualification of the detection media.

Kurzfassung Die Magnetpulverprüfung ist eine der empfindlichsten zerstörungsfreien Prüfungsmethoden, um Oberflächenfehler, wie beispielsweise Risse, an ferromagnetischen Werkstoffen nachzuweisen. Auf Grund der geringen technischen Anforderungen an die Prüfung, die außerdem vollkommen abhängig von der Qualifikation des Inspektors war, wurde sie jahrelang als die „Aschenputtel“-Methode betrachtet. In den vergangenen sieben bis acht Jahren wurde eine Technologie entwickelt, die die Realisierung von vollautomatischen Risserkennungsgeräten erlaubt. Solche Geräte setzen bereits Automobilteilehersteller zahlreicher Länder ein. Diese Technologie ermöglicht es, mit der Magnetpulverprüfung dasselbe Niveau bei der Qualitätsprüfung, Automatisierung und Reproduzierbarkeit zu erreichen wie mit jedem modernen Wirbelstrom- oder Ultraschallprüfgerät.