Multiaxial fatigue crack path prediction using critical plane concept

Prediction of fatigue crack orientation can be an essential step for estimating fatigue crack path. Critical plane concept is widely used due to its physical basis that fatigue failure is associated with certain plane(s). However, recent investigations suggest that critical plane concept might need revision. In this paper, fatigue experiments that involve careful measurement of fatigue crack were reviewed. Predictions of fatigue crack orientation using critical plane concept were examined. Projected length and angle were used to characterize fatigue crack. Considering the entire fatigue life, this average representation suggests that it is more reasonable to assume the plane of maximum normal strain as the critical plane even though fundamentally the plane of maximum shear strain is more likely to be the critical one at early initiation stage

Uso de modelos multiaxiais para estimativa de caminho de trincas sob condições de fadiga por fretting na liga Al7050 T7451

Trabalho de Conclusão de Curso (graduação)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Mecânica, 2018.O objetivo deste trabalho é avaliar modelos para a previsão de direção de iniciação de trincas em condições de Fadiga por Fretting. Os modelos de fadiga multiaxial em questão são baseados em abordagens de Plano Crítico e associados a Teorias de Distância Crítica para incorporar o efeito de gradiente de tensão nestas estimativas de caminho de trinca. Mais especificamente, considerou-se na análise os modelos multiaxiais de Carpinteri e Spagnoli, Smith-Watson-Topper, Curvas Modificadas deWöhler e Fatemi-Socie. Propôsse também uma metodologia denominada Direção Crítica para definir o plano crítico ao longo de um comprimento e não apenas em um único ponto material mais solicitado. Para a validação destes modelos, são utilizados dados de ensaios realizados no Laboratório de Fadiga, Fratura e Materiais da Universidade de Brasília. O material analisado é o Al 7050 T7451 e a configuração geométrica de contato é do tipo cilindro-plano. Para os carregamentos adotados, as melhores estimativas de caminho inicial de trinca foram obtidas pelo modelo de Carpinteri-Spagnoli associado a metodologia da Direção Crítica.The aim of this work is to evaluate a variety of models to predict the initial crack direction under fretting fatigue conditions. The models here discussed are based on Critical Distance Theory and were applied in conjunction with the Critical Direction Method. In order to validate these models, routines were made in MatLab for numerical simulation and their results were compared with data from the literature and experiments done at the Fatige, Fracture and Materials Laboratory of the University of Brasília. The flat dog bone especimens and pad here analized are made of Al 7050 T7451 and the criticalplane multiaxial theories are Carpinteri and Spagnoli, Smith-Watson-Topper, Modified Curves of Wohler and Fatemi-Socie. For the adopted loads, the results that obtained the least error were those of Carpinteri-Spagnoli, applying the methodology of the Critical Direction

Multiaxial fatigue crack path prediction using critical plane concept

Prediction of fatigue crack orientation can be an essential step for estimating fatigue crack path. Critical plane concept is widely used due to its physical basis that fatigue failure is associated with certain plane(s). However, recent investigations suggest that critical plane concept might need revision. In this paper, fatigue experiments that involve careful measurement of fatigue crack were reviewed. Predictions of fatigue crack orientation using critical plane concept were examined. Projected length and angle were used to characterize fatigue crack. Considering the entire fatigue life, this average representation suggests that it is more reasonable to assume the plane of maximum normal strain as the critical plane even though fundamentally the plane of maximum shear strain is more likely to be the critical one at early initiation stage

Fatigue of Forged AZ80 Magnesium Alloy

The majority of research surrounding the fatigue of Mg alloys generally exhibits a rigid dichotomy between theoretical and applied contributions. This research work addresses both of these domains of the field from a more holistic sense, yet still remains highly detail oriented. Automotive suspension components generally have complex geometries and undergo highly multiaxial loading. This is partly due to the packaging constraints imposed by the many dynamic systems within a vehicle, and the impetuous towards lightweighting to improve efficiency and reduce greenhouse gas emissions. As such, the current optimal solution for such a component typically a complex shape made from a material with high specific strength. Both forging and casting lend themselves to facilitating large scale production of such components using industrially compatible processes. Forging however produces a product with attributes which are more optimally suited for advanced vehicle lightweighting applications. Of the commercially available Mg alloys, the AZ80 alloy is a Mg alloy with good forgeability, a high aluminium content, and superior strength. However the fatigue properties of this alloy are largely unknown, especially in complex multiaxial loading paths such as which automotive suspension components undergo. This thesis acts to fill this gap in knowledge, by providing the foundation for the understanding of the complex cyclic behavior of forged AZ80 Mg, as well as predicting its fatigue life to ensure the satisfaction and safety of the end consumer. Various small scale forging methods were investigated and characterized in such a way that it one can connect them to the larger scale component in the engineering application. Two varieties of base material were selected to be forged into these small scale forgings, cast and extruded. Furthermore, an understanding was developed on the influence of material texture on the cyclic deformation mechanism and resulting fatigue life. The implications of multiaxial loading on the fatigue behaviour was also characterized as well as the effect of nonproportional loading. A variety of different models were utilized to reliably predict the fatigue life of forged AZ80 Mg in both simple uniaxial and complex non-proportional bi-axial loading paths. The culmination of all of these research objectives enabled effective utilization of forged AZ80 Mg as a lightweight material for a variety of different fatigue critical engineering applications. It was concluded that the thermomechanical history imparted to the material via forging resulted in a texture intensification and a rotation of the crystallographic cells to align with the loading direction during forging. Secondly, following forging, both the cast-forged and extruded-forged material exhibited an significant increase in fatigue life. It was also discovered that the style of closed-die forging being investigated had spatially varying properties with texture orientations which varied based on the local forging directions and intensities which were dependent on the starting texture as well as thevi thermomechanical history. Furthermore, following characterization of the materials behaviour over a variety of different loading paths, the biaxial fatigue response is somewhat dominated by the axial component and the non-proportional effect to be detrimental to the fatigue life. Finally, it was concluded that the optimal forging condition tends towards the coldest temperature and fastest strain rate which are pragmatically possible (within the context of warm forging) that produce a forging free of defects and of high quality. This optimal condition corresponded to extruded AZ80 Mg forged at a temperature of 250°C and 20 mm/sec