Fatigue strength of engineering materials the influence of environment and porosity

University dissertation from Stockholm : KTH

Abstract: The objective of this work was to use LEFM in order to assess the detrimental influence of surrounding chloride-containing environments for stainless steels, hardened steel as well as for a cast aluminium alloy. An additional aim was also to use LEFM to assess the influence of porosity on the fatigue properties for different commercial cast aluminium alloys and manufacturing methods. The environmental influence on fatigue performance was mainly evaluated from fatigue crack growth measurements using compact tension (CT) specimens. In addition, fatigue performance in the high cycle regime was studied using spot welded specimens and smooth specimens.Corrosion fatigue tests for stainless steels were performed in different chloride-containing aqueous solutions and compared to the behaviour in air. Variables, which have been investigated, included temperature, redox potential and fatigue test frequency. The environmental influence on fatigue performance has also been compared to localised corrosion properties. Fatigue crack propagation rates were found to be higher in 3% NaCl than in air for all stainless steels investigated. The highest alloyed austenitic steel, 654SMO, showed the least influence of the environment. For duplex stainless steels the environment enhanced fatigue crack propagation rate to a higher degree than for austenitic stainless steels. This is explained by a material-dependent corrosion fatigue mechanism.In the high cycle regime, fatigue properties for spot welded stainless steels specimens were found to be decreased between 30%-40% due to the presence of 3% NaCl. For the hardened steel 100CrMnMo8 a fracture mechanics approach was employed for prediction of corrosion fatigue properties. In this model corrosion pit growth rate and the threshold stress intensity factor for fatigue crack propagation are needed as input parameters.For the high pressure die cast aluminium alloy the environmental influence of fatigue initiation through pre-exposure of smooth specimens was studied. Depending on environment used for pre-exposure, fatigue strength was found to be reduced by up to 50 % compared to the fatigue strength in air. Fatigue strength reduction was clearly associated to corrosion pits in the aluminium material. A fracture mechanics model was further successfully used to predict the environmental influence.The influence of porosity on the fatigue strength for the cast aluminium alloys tested has been described by a Kitagawa diagram. In design, the Kitagawa diagram can be used to predict the largest allowable pore size if the load situation in the component is known. The size of the porosity could either be evaluated directly from x-ray images or from metallographic prepared cross-sections using a method of extreme value analysis