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The influence of porosity on the fatigue life for sand and permanent mould cast aluminium
2006 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 28, no 12, 1752-1758 p.Article in journal (Refereed) Published
Abstract [en]

The automotive industry always strives to achieve light weight components to reduce fuel consumption and to meet environmental requirements. One way to obtain weight reduction is to replace steel components with components made of aluminium or other light weight materials. Aluminium has good corrosion properties and a high strength to weight ratio which makes it favourable in many applications. The increased use of aluminium castings in the automotive industry does also imply that the need for design data for aluminium, increases. Especially for castings, the influence of casting defects are always an issue. For this reason fatigue properties for as-cast sand and permanent mould specimens with different contents of porosity have been studied. Sand cast and permanent mould cast aluminium specimens of two different geometries were fatigue tested in cyclic bending at R= -1. Prior to fatigue test specimens were examined by X-ray and sorted into three quality groups depending on the porosity level. The aim of this work was to investigate the fatigue life for sand cast and permanent mould cast AlSi10Mg with different amounts of porosity. An additional aim was to predict the largest defect contained in a specified volume of a component, by using a statistical analysis of extreme values, and relate it to the fatigue life. The results showed that fatigue strength for a smooth specimen geometry decreases by up to 15% with increased porosity. For specimens with a notched geometry, no influence of porosity on the fatigue strength was found. This is believed to be due to a much smaller volume subject to high stress than for specimens with low stress concentration.

Place, publisher, year, edition, pages
2006. Vol. 28, no 12, 1752-1758 p.
Keyword [en]
fatigue, alummium, cast, fracture mechanics, porosity
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-24411DOI: 10.1016/j.ijfatigue.2006.01.001ISI: 000241601900008OAI: oai:DiVA.org:kth-24411DiVA: diva2:349520
Note
QC 20100907Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Fatigue strength of engineering materials: the influence of environment and porosity
Open this publication in new window or tab >>Fatigue strength of engineering materials: the influence of environment and porosity
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

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

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 29 p.
Series
Trita-IIP, ISSN 1650-1888 ; 06:09
Keyword
Stainless steel, cast aluminium, hardened steels, fatigue, corrosion, crack propagation, spot weld, porosity
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-4195 (URN)
Public defence
2006-12-08, Brinellsalen M311, KTH, Brinellvägen 68, Stockhollm, 10:00
Opponent
Supervisors
Note
QC 20100907Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2010-09-07Bibliographically approved

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