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| 2. |
- Ekengren, Jens, 1976-, et al.
(författare)
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Assessment of strength and inclusions of tool steels in very high cycle fatigue
- 2009
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Ingår i: Proceedings of the 8th International Tooling Conference, Vol 1. - Aachen : Verlag Mainz, Wissenschaftsverlag. - 9783810793058
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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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| 3. |
- Ekengren, Jens, 1976-, et al.
(författare)
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Detecting large inclusions in steels : evaluating methods
- 2009
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Ingår i: Steel Research International. - : Wiley-VCH Verlagsgesellschaft. - 1611-3683 .- 1869-344X. ; 80:11, s. 854-858
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Tidskriftsartikel (refereegranskat)abstract
- The distributions of large non-metallic inclusions in two steel grades have been investigated using light optical microscopy, scanning electron microscopy and ultrasonic fatigue testing in the gigacycle range. The different methods have inherently different capabilities for finding inclusions in different size ranges. A measure of the distribution of large inclusions is proposed as the size S at which half of the fatigue specimens are expected to contain at least one inclusion of size S or larger, corresponding to 50% failure probability. Values of S are obtained using the volume distribution estimated by the three methods. Extrapolation from microscopy measurements on surfaces agree with fatigue fractography results regarding density of large inclusions, as measured by the proposed ranking variable S
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| 4. |
- Ekengren, Jens, 1976-, et al.
(författare)
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Estimating the volume distribution of large defects using Generalized Extreme Values
- 2011
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Ingår i: Extremes. - 1386-1999 .- 1572-915X.
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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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| 5. |
- Ekengren, Jens, 1976-, et al.
(författare)
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Influence of life length on estimated defect distribution in a low defect steel material
- 2011
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Ingår i: Proceedings of the Fifth International Conference on Very High Cycle Fatigue. - Berlin : Deutscher Verband für Materialforschung und -prüfung. - 9783981451603 ; , s. 177-182
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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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| 6. |
- Ekengren, Jens, 1976-, et al.
(författare)
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Relating gigacycle fatigue to other methods in evaluating the inclusion distribution of a H13 tool steel
- 2007
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Ingår i: Fourth International Conference on Very High Cycle Fatigue (VHCF-4). - : TMS (The Minerals, Metals & Materials Society). - 9780873397049 ; , s. 45-50
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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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| 7. |
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| 8. |
- Mussa, Abdulbaset
(författare)
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A study on wear characteristics of high strength steels under sliding contact
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the last decades, significant improvements regarding the design, materials and technology of rock drills have been made. Likewise, in sheet metal forming, forming tools experience very high contact pressures when processing high strength steel sheets. In both applications components operate under extremely tough contact conditions that result in an accelerated component failure. Enhancements on mechanical properties of components material subjected to extreme contact conditions are highly required in order to withstand the application loads and prevent severe wear.The present thesis was focused on understanding of machinery component damage mechanisms under severe contact conditions. A case study of worn components used in rock drilling and sheet metal cold work was carried out. Thread joints from rock drilling and punches from sheet metal pressing were selected for the investigation. For these components, sliding contact under high contact pressure is a common load condition under the components usage. Then, to understand and quantify the influence of contact parameters, load and surface quality on material performance, laboratory simulations were performed. The results were used for a comparative analysis of the typical damage mechanisms observed in the tests and the case study of the components.The case study results showed that the threaded surfaces underwent severe plastic deformation due to the high-pressure sliding contact. The microstructure beneath the worn surface was altered and surface cracks and delamination were frequently observed at the worn surface. The dominant damage mechanism found on the investigated punches was adhesive wear. Material transfer adds friction stresses at the punch surface and ultimately, with repeated punch strokes, it leads to initiation and propagation of fatigue cracks.Wear process in thread joint and punch wear was simulated using the SOFS. The worn specimens tested experimentally showed similar wear mechanisms obtained in the case study. The thread joint wear simulation showed that the total damage at the worn surface was a result of adhesive wear, plastic deformation, and initiation and propagation of fatigue cracks. In addition, the results showed that the type of motion had a significant influence on the worn volume and crack initiation, and more severe wear was observed at reciprocal motion. The punch wear simulation showed that the friction quickly increased as work material from metal sheets transferred to the disc surface. The rate of the material transfer was strongly dependent on the combination of sheet material and tool steel. Further, the present experimental simulations were applicable to characterize and predict wear of components in the application.
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| 9. |
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| 10. |
- AlMotasem, Ahmed Tamer, et al.
(författare)
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Adhesion between ferrite iron-€“iron/cementite countersurfaces : A molecular dynamics study
- 2016
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Ingår i: Tribology International. - : Elsevier. - 0301-679X .- 1879-2464. ; 103, s. 113-120
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Tidskriftsartikel (refereegranskat)abstract
- The adhesive properties of Fe(110)/Fe(110) and Fe3C(001)/Fe(110) countersurfaces have been investigated by using classical molecular dynamics simulations. The simulation results show that Fe3C/Fe exhibits a relatively lower adhesion compared to the Fe/Fe. Additionally, the temperature dependence of the adhesive properties between 300–700 K has been examined. The results demonstrate that, with increasing the temperature, the values of the adhesion force and the work of adhesion continuously decrease in the case of Fe3C/Fe; they initially slightly increase up to 500 K then decrease in the case of Fe/Fe. Furthermore, the effect of lattice coherency between Fe/Fe has been examined and found to slightly reduce the adhesion. These results explain how carbides improve galling resistance of tool steel observed during dry sliding.
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