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Sökning: swepub > Karlstads universitet > Bergström Jens

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1.
  • Ekengren, Jens, 1976- (författare)
  • Estimating inclusion content in high performance steels
  • 2008
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Non-metallic inclusions in steel pose a major problem for the fatigue resistance, especially regarding fatigue at very long lives corresponding to low cyclic stress levels, as well as being detrimental to material toughness and polishability.The largest inclusions are quite rare, which makes conventional detection methods timeconsuming if reliable results are to be obtained. Based on surface scanning using light or electron microscopes, these methods provide results that have to be converted to reflect the statistical volume distribution of inclusions.Very high cycle fatigue (in the order of 109 cycles or more) using ultrasonic fatigue at 20 kHz has been found efficient at finding the largest inclusions in volumes of about 300 mm3 per specimen. The inclusions found at the fatigue initiation site can then been used to estimate the distribution of large inclusions using extreme value statistics.In this work, a new method for estimating the volume distribution of large inclusions is presented as well as a suggested ranking variable based on the volume distribution.Results from fatigue fractography and area scanning methods are compared to the endurance limit at 109 cycles for a number of batches from two high performance steels.In addition, the extreme value distributions of fatigue initiating inclusions in six high performace steels, produced by different routes, are presented. It is shown that all modes of the Generalized Extreme Values distribution can be found in different materials. This result shows that the assumption of mode I distribution, also known as Gumbel or Largest Extreme Value distribution, must be substantiated.
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2.
  • Ekengren, Jens, 1976-, et al. (författare)
  • Relating gigacycle fatigue to other methods in evaluating the inclusion distribution of a H13 tool steel
  • 2007
  • Ingår i: Fourth International Conference on Very High Cycle Fatigue (VHCF-4). - : TMS (The Minerals, Metals & Materials Society). - 9780873397049 ; , s. 45-50
  • Konferensbidrag (refereegranskat)abstract
    • Inclusions play a crucial role for the fatigue properties of high strength steel, but to find the largest inclusions by microscopy measurements large areas have to be examined.In this study ultrasonic gigacycle staircase fatigue testing has been used to find large inclusions in an H13 tool steel. The inclusions have been examined in SEM and their size distribution modeled using methods from extreme value statistics. The inclusion distribution obtained from the fatigue crack surfaces is compared to distributions acquired by microscopy study of cross sections as well as ultrasound immersion tank measurements and to the corresponding staircase fatigue data via the Murakami √Area model.It is shown that the fatigue method more effectively finds large inclusions than the other methods. It is also shown that the correlation between predictions of inclusion sizes by the √Area model from stress levels and fatigue initiating inclusions is weak forthis material.
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3.
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4.
  • Bergström, Jens, 1954-, et al. (författare)
  • Galling in sheet metal forming
  • 2008
  • Ingår i: Proceedings of the IDDRG 2008 Conference. - Olofström : Industriellt utvecklingscentrum i Olofström AB. - 9789163329487
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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5.
  • Chai, Guocai, et al. (författare)
  • Crack Initiation in Bulk Matrix of Austenitic Stainless Steel during Very High Cycle Fatigue
  • 2023
  • Ingår i: Materials Performance and Characterization. - : American Society for Testing Materials. - 2379-1365 .- 2165-3992. ; 12:2
  • Tidskriftsartikel (refereegranskat)abstract
    • In the very high cycle fatigue regime, fatigue crack initiation in high-strength steels is usually correlated to a subsurface inclusion with a fine granular area (FGA). Localized stress-strain concentration at the subsurface inclusion is a critical factor. Fatigue crack initiation with an FGA in the bulk matrix without any defect has rarely been reported. In this paper, a fundamental study on the formation of FGAs in the bulk matrix of an austenitic stainless steel has been carried out using a progressive stepwise load-increasing test with a cycle step of about 108 cycles. FGA formation in the subsurface bulk matrix has been observed. The micro structural damage in the fatigue-tested specimens has been studied using the electron channeling contrast imaging electron microscopy technique. Strain localization and grain fragmentation are the main processes for the formation of FGAs. Local plasticity exhaustion leads to crack initiation due to local stress concentrations. This method can also be used to predict the fatigue damage process, especially the damage rate in individual specimens.
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6.
  • Chai, Guocai, et al. (författare)
  • Formation of fine granular area in a non-defect matrix of austenitic stainless steel during very high cycle fatigue
  • 2023
  • Ingår i: Fatigue & Fracture of Engineering Materials & Structures. - : John Wiley & Sons. - 8756-758X .- 1460-2695. ; 46:6, s. 2364-2373
  • Tidskriftsartikel (refereegranskat)abstract
    • A fine granular area, FGA, is a typical phenomenon observed at the very high cycle fatigue fracture crack origin with a subsurface defect in the material. The FGA has been widely investigated, and different mechanisms have been proposed. In this paper, the formation of FGA in a non-defect matrix of one austenitic steel during very high cycle fatigue was studied using a progressive stepwise load-increasing method and electron scanning microscopy/electron channeling contrast imaging (ECCI) technique. A nano rough surface area or FGA at the fatigue crack origin has been observed in the subsurface matrix without any defect. It is a new phenomenon. A mechanism was proposed using the dislocation plasticity theory. The formation of FGA in a non-defect matrix is a localized plasticity exhausting process by strain localization, grain fragmentation, stress concentration and nano crack initiation and propagation along low-angle grain boundaries. 
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7.
  • Ekengren, Jens, 1976-, et al. (författare)
  • Assessment of strength and inclusions of tool steels in very high cycle fatigue
  • 2009
  • Ingår i: Proceedings of the 8th International Tooling Conference, Vol 1. - Aachen : Verlag Mainz, Wissenschaftsverlag. - 9783810793058
  • Konferensbidrag (refereegranskat)abstract
    • Fatigue strength is an important material property for many tooling applications, particularly in high performance applications. The research in Very High Cycle Fatigue (VHCF) has demonstrated that the traditional fatigue limit may not be valid for many materials subjected to 107 or more load cycles. Presently, both materials data and mechanism knowledge is missing on VHCF applications, even though many components are run at these life lengths. The fatigue strength is commonly controlled by different defects initiating failure, as in well controlled laboratory experiments may be internal inclusions. In this paper VHCF experimental testing was accomplished by the use of ultrasonic fatigue testing run at 20 kHz allowing long life evaluation within reasonably short test time. Fatigue strength, failure mechanisms and inclusion content were accordingly assessed. Fatigue strength data on H13 tool steel are presented, as well as a statistical approach considering available defect distribution and load distribution in the critically stressed volume, important to both steel supplier and end-user.
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8.
  • Ekengren, Jens, 1976-, et al. (författare)
  • Estimating the volume distribution of large defects using Generalized Extreme Values
  • 2011
  • Ingår i: Extremes. - 1386-1999 .- 1572-915X.
  • Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
    • The presence of defects, such as oxides and other non-metallic inclusions, is an important factor in determining the properties of steels. Due to improvements in the manufacturing of high-quality steels, the amount of large defects has decreased and therefore it has become increasingly difficult to accurately determine their distribution using conventional methods. Previously, a method for estimating the distribution of large defects using a conversion from the Gumbel distribution has been presented. However, it has been shown that the Gumbel distribution is not always appropriate for modelling the sizes of the largest defects and that the Generalized Extreme Values (GEV) distribution should be used instead. In this work a more general method for the estimation of the total volume distribution of large defects isproposed, showing how the volume distribution may be calculated from the estimated parameters for the GEV distribution. The new method is applied to the results of a series of specimens made from high-quality tool steel tested in ultrasonic resonance fatigue. Possible methods for obtaining the confidence limits of thevolume distribution are also discussed.
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9.
  • Ekengren, Jens, 1976-, et al. (författare)
  • Extreme value distributions of inclusions in six steels
  • 2012
  • Ingår i: Extremes. - : Springer Science and Business Media LLC. - 1386-1999 .- 1572-915X. ; 15:2, s. 257-265
  • Tidskriftsartikel (refereegranskat)abstract
    • There is a prevailing assumption that the largest inclusions in steel volumes follows mode I of the Generalized Extreme Values (GEV) distribution. In this work, the GEV distributions of non-metallic inclusions in six different high performance steels, of different grades and processing routes, were investigated by means of fractography of inclusions causing failure in ultrasonic fatigue testing to one billion cycles and all three modes of the GEV were found for the different steel grades. Values of the shape parameter ξ of the GEV distribution as high as 0.51 (standard deviation 0.11) were found in one steel grade. Thus, the present results show that the assumption of GEV-I (Gumbel, LEVD) distribution has to be substantiated before being used to estimate the size of the largest inclusions.
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10.
  • Ekengren, Jens, 1976-, et al. (författare)
  • Influence of life length on estimated defect distribution in a low defect steel material
  • 2011
  • Ingår i: Proceedings of the Fifth International Conference on Very High Cycle Fatigue. - Berlin : Deutscher Verband für Materialforschung und -prüfung. - 9783981451603 ; , s. 177-182
  • Konferensbidrag (refereegranskat)abstract
    • This paper reports on results of very high cycle fatigue tests on a steel with a relatively low number of defects per unit volume. Two series of newly designed dog-bone specimens, with low stress gradients, were tested at two constant stress levels, aiming at providing fatigue life lengths of 106–108 cycles and 107–109 cycles, respectively. Fatigue-initiating defects on the fracture surfaces were measured and the corresponding size distributions were analysed to investigate the influence of the elevated stress level needed to achieve shorter life lengths. The presented results indicate a difference between the distribution of initiating defects in the long and shorter life length test series. The main conclusion is that fixed stress level fatigue testing to life lengths around 107 cycles may be a suitable method to estimate the content of large defects in low defect steel materials, provided a suitable test specimen geometry with low stress gradients is used.
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