THE EFFECT OF OVERLOADS AND UNDERLOADS ON FATIGUE CRACK GROWTH

F. Romeiro  M. de Freitas       

Anales de la Mecánica de la Fractura, nº 18 . 2001 . Pág. -
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Resumen: The effects of variable amplitude loading have been summarized as leading to retardation of crack growth or acceleration of crack growth due to respectively overloads and underloads, but these effects remain very load and material sensitive. Analysis carried out through the crack closure calculations have been performed but are not fully satisfactory on the description of the related phenomena. This presentation relates to a study where fatigue crack growth tests were carried out on specimens made of a mild steel, C45 (DIN), using M(T) specimens according to ASTM E647. In order to fully understand the effect of variable amplitude loading within a block loading, a wide range of fatigue tests were carried out, where not only the variations of overloads and underloads were used (N=1) but also the variation of the number of constant amplitude loading cycles (n=9, 99, 999 and 9 999). Results show the amount of retardation or acceleration related to each block loading where the effect of load variation is observed. Analysis of the results is carried out with models based on crack closure effects and the dimension of crack tip plasticity and allow to assess the effect of variable amplitude loading on crack growth.

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Dep. Engenharia Mecânica, Esc. Sup. Tecnologia, Inst. Politecnico de Leiria, Morro do Lena, Alto do Vieiro, 2410 Leiria, Portugal
Dep. Engenharia Mecânica, Secção de Projecto Mecânico, Instituto Superior Técnico Av. Rovisco Pais, 1049-001 Lisboa, Portugal