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Fatigue strength of engineering materials: the influence of environment and porosity
KTH, School of Industrial Engineering and Management (ITM), Industrial Economics and Management (Dept.).
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 [en]
Stainless steel, cast aluminium, hardened steels, fatigue, corrosion, crack propagation, spot weld, porosity
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4195OAI: oai:DiVA.org:kth-4195DiVA: diva2:11162
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
List of papers
1. Corrosion fatigue of austenitic and duplex stainless steels in 3% NaCl
Open this publication in new window or tab >>Corrosion fatigue of austenitic and duplex stainless steels in 3% NaCl
1993 (English)In: Proc. Fatigue under spectrum loading and in corrosive environments , A.F. Blom Ed. 26-27 August 1993, Ljungby, Denmark, 1993Conference paper, Published paper (Refereed)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-24402 (URN)
Note
QC 20100907Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2010-09-07Bibliographically approved
2. Corrosion fatigue of duplex and austenitic stainless steels in 3% NaCl at 80 degrees C and room temperatur
Open this publication in new window or tab >>Corrosion fatigue of duplex and austenitic stainless steels in 3% NaCl at 80 degrees C and room temperatur
1994 (English)In: Proc. Duplex Stainless Steels, Glasgow, Scotland, 13-16 November 1994, 1994Conference paper, Published paper (Refereed)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-24403 (URN)
Note
QC 20100907Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2010-09-07Bibliographically approved
3. Fatigue strength of spot welded stainless sheet steels exposed to 3% NaCl solution
Open this publication in new window or tab >>Fatigue strength of spot welded stainless sheet steels exposed to 3% NaCl solution
1998 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 20, no 5, 383-388 p.Article in journal (Refereed) Published
Abstract [en]

Fatigue properties of spot welded stainless sheets steels have been investigated in a 3% NaCl solution and, for comparison, also in air at ambient temperature. Corrosion fatigue tests have been conducted both for one austenitic Stainless steel type AISI304, and for one duplex (similar to 50% austenite, 50% ferrite) stainless steel SAF2304. For the duplex steel, the effect of preexposure of unloaded specimens to a 3%NaCl solution before corrosion fatigue testing was also investigated. Corrosion fatigue strength in 3% NaCl for the stainless steels investigated was found to be reduced by approximately 30-40% compared to fatigue strength in air. Preexposure of unloaded specimens prior to fatigue testing was shown not to reduce fatigue strength further.

Keyword
corrosion fatigue, stainless steel, spot weld
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-24405 (URN)10.1016/S0142-1123(98)00009-7 (DOI)000074060400007 ()
Note
QC 20100907Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2017-12-12Bibliographically approved
4. Development of a method for corrosion fatigue life prediction of structurally loaded bearing steels
Open this publication in new window or tab >>Development of a method for corrosion fatigue life prediction of structurally loaded bearing steels
2001 (English)In: Corrosion, ISSN 0010-9312, E-ISSN 1938-159X, Vol. 57, no 5, 404-412 p.Article in journal (Refereed) Published
Abstract [en]

In this study a simplified method to predict corrosion fatigue properties for the Cr-alloyed bearing steel 100CrMnMo8 is proposed. With this method corrosion pits are assumed to act as defects, which could initiate fatigue cracks. A linear elastic fracture mechanics (LEFM) model was used to evaluate the critical corrosion pit size when mechanical fatigue crack propagation occurred. The unknown parameters in the prediction model were corrosion pit growth rate and the threshold stress intensity factor (DeltaK(th)) for fatigue crack growth. In this study corrosion pit growth rate was determined for 100CrMnMo8 in a chloride-containing water solution by aid of a statistical method. Corrosion pit growth rate was found to be approximately proportional to the square root of time. The time dependence of corrosion pit growth Zed also to the conclusion that characterization of corrosion fatigue strength must be made with respect to both exposure time in a corrosive environment and the number of cycles to a runout level. DeltaK(th) fatigue crack growth was estimated from interrupted rotating bending fatigue tests in the corrosive environment in question. Since only the corrosion pit growth rate and DeltaK(th), are needed as input in the proposed corrosion fatigue prediction model, much effort can be saved since this can reduce the amount of fatigue testing. Corrosion pit growth rate can be determined on unloaded test coupons In the desired environment. Hence, for materials selection, exposure tests followed by a corrosion pit size analysis can indicate the corrosion fatigue performance in a given environment.

Keyword
corrosion fatigue, life prediction, localized corrosion, steel
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-24408 (URN)000176356300003 ()
Note
QC 20100907Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2017-12-12Bibliographically approved
5. The influence of porosity on the fatigue life for sand and permanent mould cast aluminium
Open this publication in new window or tab >>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.

Keyword
fatigue, alummium, cast, fracture mechanics, porosity
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-24411 (URN)10.1016/j.ijfatigue.2006.01.001 (DOI)000241601900008 ()
Note
QC 20100907Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2017-12-12Bibliographically approved
6. The influence of porosity on the fatigue strength of high-pressure die cast aluminium
Open this publication in new window or tab >>The influence of porosity on the fatigue strength of high-pressure die cast aluminium
2006 (English)In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 29, no 5, 357-363 p.Article in journal (Refereed) Published
Abstract [en]

Aluminium is a lightweight material with high strength and good corrosion resistance among other beneficial properties. Thanks to these properties, aluminium is more extensively used in the vehicle industry. High-pressure die casting of aluminium is a manufacturing process that makes it possible to attain complex, multi-functional components with near-net shape. However, there is one disadvantage of such castings, that is, the presence of various defects such as porosity and its effect on mechanical properties. The aim of this work was to investigate the influence of porosity on the fatigue strength of high-pressure die cast aluminium. The objective was to derive the influence of defect size with respect to the fatigue load, and to generate a model for fatigue life in terms of a Kitagawa diagram. The aluminium alloy used in this study is comparable to AlSi9Cu3. Specimens were examined in X-ray prior to fatigue loading and classified with respect to porosity level and eventually fatigue tested in bending at the load ratio, R, equal to -1. Two different specimen types with a stress concentration factor of 1.05 and 2.25 have been tested. It has been shown that the fatigue strength decreases by up to 25% as the amount of porosity of the specimen is increased. The results further showed that the influence of defects was less for the specimen type with the higher stress concentration. This is believed to be an effect of a smaller volume being exposed to the maximum stress for this specimen type. A Kitagawa diagram was constructed on the basis of the test results and fracture mechanics calculations. A value of 1.4 Mpa m(1/2) was used for the so-called stress intensity threshold range. This analysis predicts that defects larger than 0.06 mm(2) will reduce the fatigue strength at 5 x 10(6) cycles for the aluminium AlSi9Cu3 material tested.

Keyword
aluminium, cast, fatigue, fracture mechanics, porosity
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-24412 (URN)10.1111/j.1460-2695.2006.00997.x (DOI)000236394700002 ()
Note
QC 20100907Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2017-12-12Bibliographically approved
7. The influence of surrounding environment on the fatigue properties for a high pressure die cast AlSi9Cu3 alloy
Open this publication in new window or tab >>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.

Keyword
aluminium castings, corrosion defect, fatigue
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-24414 (URN)10.1111/j.1460-2695.2007.01104.x (DOI)000248685700009 ()
Note
QC 20100907Available from: 2010-09-07 Created: 2010-09-07 Last updated: 2017-12-12Bibliographically approved

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Citation style
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