Britta Schramm

Fatigue crack paths under the influence of changes in stiffness

An important topic of the Collaborative Research Centre TRR 30 of the Deutsche Forschungsgemeinschaft (DFG) is the crack growth behaviour in graded materials. In addition, the growth of cracks in the neighbourhood of regions and through regions with different material properties belongs under this topic. Due to the different material properties, regions with differing stiffness compared to the base material may arise. Regions with differing stiffness also arise from ribs, grooves or boreholes. Since secure findings on the propagation behaviour of fatigue cracks are essential for the evaluation of the safety of components and structures, the growth of cracks near changes in stiffness has to be considered, too. Depending on the way a crack penetrates the zone of influence of such a change in stiffness and depending on whether this region is more compliant or stiffer than the surrounding area the crack may grow towards or away from this region. Both cases result in curved crack paths that cannot be explained only by the global loading situation. To evaluate the influence of regions with differing stiffness on the path of fatigue cracks the paths and the stress intensity factors of cracks growing near and through regions with differing stiffness are numerically determined with the program system ADAPCRACK3D. Therefore, arrangements of changes in stiffness modelled as material inclusions with stiffness properties different from the base material or modelled as ribs and grooves are systematically varied to develop basic conclusions about the crack growth behaviour near and through changes in stiffness.

Medizintechnische Anwendungen der additiven Fertigung

Aufgrund des hohen erreichbaren Individualisierungsgrads ist die additive Fertigung, d. h. die materialzufuhrende, schichtweise auf 3D-CAD-Daten basierende Herstellung von Bauteilen, geradezu pradestiniert fur medizintechnische Anwendungen. Auf diese Weise lassen sich patientenspezifische, geometrisch komplexe und an die gegebene Beanspruchungssituation angepasste Medizinprodukte (wie z. B. Orthesen, Prothesen und Implantate sowie medizinische Hilfsmittel) fertigen. Der vorliegende Beitrag gibt einen Uberblick uber aktuelle Anwendungen der additiven Fertigung in der Medizintechnik und diskutiert die mit diesem innovativen Fertigungsverfahren fur den medizintechnischen Bereich verbundenen Potenziale. Um Medizinprodukte lebensdauerorientiert, beanspruchungsgerecht und patientenspezifisch unter Berucksichtigung werkstoff- und prozessseitiger Einflussfaktoren sowie der identifizierten Potenziale der additiven Fertigung zu gestalten, mussen verschiedene ingenieurwissenschaftliche und medizinische Fachbereiche interdisziplinar zusammenwirken. Daher werden auch die Vorgehensweise zur Entwicklung und Herstellung additiv gefertigter Medizinprodukte sowie die dafur erforderlichen Fachdisziplinen betrachtet. Die grundlegende Vorgehensweise wird daruber hinaus fur verschiedene medizinische Anwendungsbeispiele verdeutlicht. Im Fokus steht dabei die Gestaltung individueller Esshilfen fur korperbehinderte Personen, um ihnen ein selbststandiges und selbstbestimmtes Essen zu ermoglichen. Daruber hinaus werden numerische Rissausbreitungssimulationen einer Huftendoprothese vorgestellt, um den Einfluss thermischer Nachbehandlungsverfahren auf das bruchmechanische Materialverhalten zu diskutieren. Des Weiteren werden verschiedene Masnahmen zur Strukturoptimierung einer Kurzschaft-Huftendoprothese und einer patientenspezifischen Fusorthese erlautert.

Crack propagation in fracture mechanical graded structures

The focus of manufacturing is more and more on innovative and application-oriented products considering lightweight construction. Hence, especially functional graded materials come to the fore. Due to the application-matched functional material gradation different local demands such as absorbability, abrasion and fatigue of structures are met. However, the material gradation can also have a remarkable influence on the crack propagation behavior. Therefore, this paper examines how the crack propagation behavior changes when a crack grows through regions which are characterized by different fracture mechanical material properties (e.g. different threshold values ?KI,th, different fracture toughness ?KIC). In particular, the emphasis of this paper is on the beginning of stable crack propagation, the crack velocity, the crack propagation direction as well as on the occurrence of unstable crack growth under static as well as cyclic loading. In this context, the developed TSSR-concept is presented which allows the prediction of crack propagation in fracture mechanical graded structures considering the loading situation (Mode I, Mode II and plane Mixed Mode) and the material gradation. In addition, results of experimental investigations for a mode I loading situation and numerical simulations of crack growth in such graded structures confirm the theoretical findings and clarify the influence of the material gradation on the crack propagation behavior. KEYWORDS. Functional fracture mechanical gradation; Crack propagation direction; TSSR-concept; Experimental investigations; Numerical simulations.

Numeric Simulation of Fatigue Crack Growth in a Material Graded Structure

This article deals with the development of functional graded materials which is pursued by the collaborative research centre Transregio 30. The functional gradation is produced by a thermo-mechanical production process and is characterized by different fracture mechanical specific values. Because cracks are by all means responsible for failure of structural elements crack growth has to be considered during the development of these materials. In this contribution simulations of crack growth which were carried out with the system ADAPCRACK3D are presented to illustrate the influence of a material gradation on crack growth.

Schadensvermeidung und Lebensdauerverlängerung in technischen Komponenten

Kurzfassung Maßnahmen zur Schadensuntersuchung, Schadensvermeidung und Lebensdauerverlängerung von technischen Bauteilen und Strukturen stehen im Mittelpunkt dieses Beitrags. Behandelt werden Schäden durch mechanische Beanspruchungen, wobei zwischen Festigkeitsversagen und bruchmechanischem Versagen unterschieden wird. Lebensdauerverlängernde Maßnahmen werden anhand von Praxisbeispielen aufgezeigt.