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Projects / Programmes source: ARIS

Modeling of fatigue strength of spring steels and life-time prediction of leaf springs

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Research activity

Code Science Field Subfield
2.04.02  Engineering sciences and technologies  Materials science and technology  Metallic materials 

Code Science Field
T450  Technological sciences  Metal technology, metallurgy, metal products 
Keywords
Si-Mn-Cr-Mo-V based high-strength spring steels, fatigue strength, the influence of microstructure and geometry, mono-leaf parabolic springs, finite element based modelling, probabilistic approach, extreme value statistics, damage accumulation, life time prediction
Evaluation (rules)
source: COBISS
Evaluation of bibliographic research performance indicators according to ARIS methodology
Citations Citations for bibliographic records in COBIB.SI that are linked to records in citation databases
source: WoS source: Scopus source: COBISS
Researchers (16)
no. Code Name and surname Research area Role Period No. of publications
1.  19191  Tomaž Ahačič    Technical associate  2007 - 2010  24 
2.  08236  Boris Arzenšek  Materials science and technology  Researcher  2007 - 2010  225 
3.  09434  PhD Mirko Doberšek  Materials science and technology  Researcher  2007 - 2009  162 
4.  10842  PhD Matjaž Godec  Materials science and technology  Researcher  2007 - 2010  888 
5.  15528  PhD Anton Jaklič  Materials science and technology  Researcher  2007  154 
6.  22454  PhD Tadej Kokalj  Interdisciplinary research  Researcher  2007 - 2010  76 
7.  21381  PhD Miha Kovačič  Manufacturing technologies and systems  Researcher  2007 - 2010  248 
8.  18783  Boris Kumer  Manufacturing technologies and systems  Researcher  2007 - 2010  46 
9.  07642  PhD Vojteh Leskovšek  Materials science and technology  Researcher  2007 - 2010  359 
10.  17199  Nataša Lipovšek    Technical associate  2007 - 2010  81 
11.  17190  Joško Mislej    Technical associate  2007 - 2009  3 
12.  21523  PhD Iztok Naglič  Materials science and technology  Researcher  2007 - 2010  179 
13.  18782  PhD Bojan Senčič  Manufacturing technologies and systems  Researcher  2007 - 2010  92 
14.  26192  PhD Irena Škulj  Materials science and technology  Researcher  2007 - 2008  47 
15.  08195  PhD Borivoj Šuštaršič  Materials science and technology  Head  2007 - 2010  412 
16.  05438  PhD Matjaž Torkar  Materials science and technology  Researcher  2007 - 2010  469 
Organisations (2)
no. Code Research organisation City Registration number No. of publications
1.  0206  Institute of Metals and Technology  Ljubljana  5051622000  6,152 
2.  1610  ŠTORE STEEL podjetje za proizvodnjo jekel, d.o.o. (Slovene)  Štore  1197967  738 
Abstract
Producers of steel springs evaluate their quality on the basis of results of structural testing of randomly selected springs in individual batches. The information provided by such evaluation comes too late and is frequently inadequate for the steel producer. This is due to the fact that the manufacture of steel springs consists of many working operations which can significantly deteriorate steel quality if they are not carried out correctly. On the other hand, steel producers usually evaluate steel quality on the basis of results of standard tensile testing, hardness measurements and the Charpy impact test, which is not sufficient in the case of dynamically loaded steels. Standardized dynamic testing of materials; i.e. determination of fatigue strength, is a time consuming and expensive task. It is usually applied only in the steel development stage, but not as a standard control procedure during regular production. Therefore, in the frame of the present project a faster method will be used and adopted for the estimation of ˝unnotched˝ fatigue strength and life-time prediction of springs. The method is based on the high-frequency (HF) pulsator using standard V-notched Charpy samples. Woehler’s (S-N) curves will be made and notched fatigue strength of selected spring steel, depending on basic steel structure (inclusions, segregations) and heat-treatment conditions will be determined. Finite element methods based modelling of dynamic testing of CVN samples with HF pulsator will be performed. Stress/strain distribution near the notch tip depending or the real dynamic loading conditions during fatigue testing will be determined and theoretical elastic/plastic stress concentration factor will be calculated. S-N curves of investigated spring steel on smooth and notched cylindrical specimens will be also determined and on the basis of the calculation of the fatigue strength reduction factor, the appropriateness of the modelling procedures will be evaluated. For experimental work and investigations, a typical spring steel 51CrV4Mo type is selected. Its basic micro structure will be analysed and optimisation of the heat-treatment process will be performed. The effects of inclusions content and size, rate of segregations and heat-treatment on fatigue strength will be experimentally evaluated. Local-fatigue strength of spring steel on the samples cut out (high-pressure water jet cutting) will be determined directly from the characteristic sites of finally manufactured springs and the differences between the input and the output quality of steel will be evaluated. The existing models for the life-time prediction of springs during fatigue will be analysed and our own knowledge and results of this project will be used for the upgrading of FEM based simulation. The correlation between fracture toughness KIC and fatigue strength of the investigated spring steel is still to a great extent unknown and will therefore be determined and analysed. For this, precracked CVN samples will be also used. Precracking will be performed on a HF pulsator. We expect that this approach is simpler than the existing method of KIC determination. In the frame of the present project the usability of the results of fracture toughness determination for the estimation of dynamic behaviour of the investigated steel will also be evaluated.
Significance for science
The research and investigation results of present project enable a new approach to the treatment of fatigue of metallic materials and fatigue life prediction with the help of critical inclusion size based on genetic programming. The determined critical inclusion size is approximately 140 microns. On the other side, standard metallographic investigation under light microscope does not give an adequately large enough investigated volume and therefore the result is much smaller maximal inclusion size (approx. 50 microns) determined with the help of extreme value statistics. The results of project have also shown that relatively accurate prediction of fatigue life of leaf springs with definite geometry and loading (fatigue) conditions is possible with FEM modeling based on linear accumulation damage concept if experimentally determined S-N curves of investigated material are known which strongly depend on heat-treatment conditions (the obtained strength level), segregation orientation and surface condition. The results of calculated fatigue life based on FEM modeling for the selected mono and two-leaf spring with the selected geometry and fatigue conditions are in a good agreement with the real fatigue life of leaf springs tested by the standard technological test. Both approaches are based on the good theoretical and practical knowledge and understanding of behaviour of high-quality spring steel, which is the result of this project. The experimental results of project have shown strong (dominant) influence of segregation and rolling direction, respectively, on fatigue strength of steel. With the appropriate heat-treatment one can vary strength level of investigated spring steel between 1350 and 1800 MPa and proportionally it change its fatigue strength, ductility and fracture toughness.
Significance for the country
The Štore Steel plant is one of the biggest EU producers of high-quality steels for leaf springs of heavy trucks. The quality of steel and its competitiveness can be assured only by the investment into a new knowledge. In the frame of the present project has been shown that newer, cheaper and faster method can assure better assessment of fatigue strength of produced steel, as well as the influence of metallurgical factors (microstructure, cleanness) on it. It is also possible to assess the fatigue life of certain leaf spring on the basis of FEM modeling if the appropriate geometry of springs is known, as well as its loading conditions independently on leaf spring producer and its results of technological testing of finally produced leaf springs. This is invaluable in the case of quality complaint of spring producer. The introduction of the determination of fatigue strength and S-N curves of steel batch to batch and final control enables constant quality control of produced steel, increase of its quality and competitive ability of steel producer. The method is not useful only for the determination of resistance against dynamic loading of steels but also other metallic and composite materials. The new knowledge as a result of the present project is also important for the other Slovenian producer of metallic materials (for example Al alloys) and user of dynamically loaded elements and structures. The effect of the present project is not only progress in testing and modeling methodology but also new insight into the influence of metallurgical defects (segregation, inclusions, surface effects etc.) on fatigue strength of steel and fatigue life of leaf springs. This has already leaded to the modification of existent production procedure of spring steel, the increase of its cleanness resulting also in a significantly increased fatigue life of certain leaf springs tested by the spring producer.
Most important scientific results Annual report 2008, 2009, final report, complete report on dLib.si
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