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The influence of surrounding environment on the fatigue properties for a high pressure die cast AlSi9Cu3 alloy
2007 (English)In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 30, no 8, 759-765 p.Article in journal (Refereed) Published
Abstract [en]

The influence of mechanical strength for aluminium castings is often correlated in the literature to the occurrence of cast defects like porosity. However, because aluminium castings in the automotive industry are often used without surface treatment such as painting etc., the influence of corrosion and its effect on fatigue properties are required. Basically a surrounding environment can affect fatigue strength either by enhancing initiation or by increasing fatigue crack propagation properties. In this study, the influence of pre-exposure prior to fatigue testing has been evaluated. This implies that environmental enhancement of fatigue initiation due to corrosion has been studied. Two different environments [seawater acidified test (SWAAT) and Volvo Indoor Corrosion Test (VICT)] often employed for assessment of corrosive properties in the automotive industry have been used for pre-exposure of specimens. Based on experimental results, it is shown that fatigue strength is reduced by approximately 50% for specimens pre-exposed to SWAAT environment, while insignificant influence was found for specimens pre-exposed to VICT environment. The degradation of fatigue strength was found to be due to localized corrosive attacks. Propagation of these corrosive attacks takes place preferably in the eutectic phase and especially at the borderline between primary aluminium dendrites and the cutectic phase.

Place, publisher, year, edition, pages
2007. Vol. 30, no 8, 759-765 p.
Keyword [en]
aluminium castings, corrosion defect, fatigue
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-24414DOI: 10.1111/j.1460-2695.2007.01104.xISI: 000248685700009OAI: oai:DiVA.org:kth-24414DiVA: diva2:349541
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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