Ladislav Pešek

Changes in Mechanical Properties and Microstructure after Quasi-Static and Dynamic Tensile Loading

Dual phase (DP), interstitial free (IF) and advanced high strength low alloy steel (HSLA)sheets have been successfully used for different components of car body. DP and HSLA are used ascrash resistant and IF as cover or “skin” of car body. The development of new vehicles nowadays isbeing driven by the need to simultaneously reduce mass and increase of passenger and pedestriansafety as well as costs saving through cold forming instead of hot forming. Limited publishedinformation is available on the changes in microstructure of these steel grades at different highstrain rates [1-3].This paper deals with changes of mechanical properties, microstructure and fractography of threesteel grades, which were tested at quasi static (10-3 s-1) and high strain rate (3000 s-1). Themicrostructures were characterized in terms of ferrite grain size, aspect ratio of ferrite andelongation of constituent phases. Deformed and undeformed specimens were compared to assess thechanges in the microstructure. The fracture appearance analysis indicates that the fracture patternunder high strain rates is mainly ductile, regardless of steel grades. The microstructure changessignificantly during the deformation process under both quasi-static and dynamic tension in allinvestigated steels. The plastic deformation in ferrite dominates in this process.

Analysis of Mechanical Properties of Individual Phases in Steels by Depth Sensing Indentation

The macroscopic mechanical properties of steel are highly dependent upon microstructure, morphology, and distribution of each phase present. Nanomechanical testing (Depth sensing indentation) provides a straightforward solution for quantitatively characterizing each of these phases because it is very powerful technique for characterization of materials in small volumes. Measuring the intrinsic properties of each phase separately in multiphase materials gives information that is valuable for the development of new materials and for modelling [1]. In this work, depth sensing indentation has been used to reveal mechanical properties of different phases in steel sheets.

Mechanical properties of individual phases, deformation and fracture in mechanically alloyed Al-Al4C3 composite

Using a Depth Sensing Indentation technique (DSI), the Martens hardness HM, indentation modulus EIT and deformation work W for Al matrix and Al4C3 particles have been measured. It has been shown that the hardness of Al4C3 particles is 57 times higher than the hardness of matrix. The change in fracture for the Al-12Al4C3 system was investigated and analysed at different temperatures and strain rates. With increasing tensile load, local cracks are formed by rupture of large particles and by decohesion of smaller particles from the matrix. Further increase of load leads to the crack growth by coalescence of cavities.

Effect of Strain History of Steel Sheets on the Mechanical Characteristics of Individual Microstructural Components by Depth Sensing Indentation

The macroscopic mechanical properties of steel are highly dependent upon microstructure, crystallographic orientation of grains and distribution of each phase present, etc. Nanomechanical testing using depth sensing indentation (DSI) provides a straightforward solution for quantitatively characterizing each of phases in microstructure because it is very powerful technique for characterization of materials in small volumes. Measuring the local properties of each microstructure component separately in multiphase materials gives information that is valuable for the development of new materials and for modelling. The work experimentally analyses the effect of strain history on the mechanical properties of individual components in steel sheets by depth sensing indentation. The measurements were carried out on broken tensile specimens.

Correlation between Hardness and Fatigue Properties

The contribution deals with estimation of tensile properties and fatigue behavior based on hardness measurement. First of all the database of tensile and fatigue properties vs. hardness data was created for a group of steels, from literature survey and performed experiments. Tensile strength, yield strength, ductility and parameters of Ludwig-Hollomon equation in static and cyclic loading were extracted and fitted in relation to the hardness HB. The experimental materials were API 5 L grade steels – X60 and X70 after different deformation exposition. Measured tensile curve (SC) and cyclic deformation curve (CDC) were compared with predicted curves. Hardness was measured in-situ during cyclic loading. The maximum possible hardness values were experimentally determined. The results give a good agreement between estimated and measured data of both static tensile test and fatigue properties.

Pile-Up Correction of Mechanical Characteristics of Individual Phases in Various Steel by Depth Sensing Indentation

The Oliver–Pharr method has extensively been adopted for measuring hardness and elastic modulus by indentation techniques. However, the method assumes that the contact periphery sinks in, which limits the applicability to the materials pile-up [1]. This study proposes an improved methodology to calculate the real mechanical characteristics of individual phases in various steels with significant pile-up. Pile-up correction of mechanical characteristics is based on ratio of pile-up height and contact depth. Pile-up height was measured by atomic force microscopy (AFM). The effect of grain boundaries on the shape and size of the pile-up lobes was also discussed.

Effect of Experimental Factors on Hardness Measurement Using the UCI Technique

The work experimentally analyses the effect of various factors on hardness measured values on thin steel sheets using Ultrasonic Contact Impedance (UCI) technique. The conditions experimentally used are compared with that according the ASTM A 1038-08 standard. UCI is an experimental technique for indirect hardness measurement. The equipment uses a Vickers indenter and the hardness measurement is based on the change of the resonance frequency during indenter ́s penetration [.The UCI hardness may depend on some factors, therefore optimal measurement conditions must be determined. The effect of distance between indents, zinc coating, sample weight, sample mounting and adhesive material for sample fixation were determined.

Toughness of Ultra High Strength Steel Sheets

Currently only few methods exist for thin steel sheet testing, especially based on fracture mechanics concept. Charpy impact test is one of the most used method for testing notch toughness and fracture behaviors because of the simplicity and the other advantages [. This article deals with toughness testing of automotive ultra high strength steel sheets (UHSS). Several standard types of toughness test that generate data for specific loading conditions and/or component design approaches exist. Two definition of toughness will be discussed: i) Charpy V-notch toughness, method includes joining of thin steel sheets to one compact unit and ii) material (tensile) toughness [. Two steels were used, DP1000 and 1400M of 1,8 mm thickness and two joining techniques: bonding with adhesives and joining with holders. Effect of material, joining technology, structural adhesives, and number of joined plates on the toughness values was quantified at the room temperature. Toughness of steels by the tensile test was added for comparison. Fracture surface was observed using scanning electron microscope analysis.

Structure and Properties of Selected Natural Materials

This paper deals with description of properties of selected natural materials mainly theirs shelter function. The investigated materials are horsetail and walnut shell. Both natural materials have porous shell/tubular structure. Walnut shell provides natural shield cover for fruit with gradient distribution of porosity and membrane function. In case of horse tail, except the gradient distribution of porosity there is also the gradient change of chemical composition along the cross-section.

Prediction of Tensile Properties Based on Hardness Measurement

The contribution deals with prediction of tensile properties based on measurement of microhardness. First of all, the database of stress strain, s-e vs. hardness data was created. Tensile strength, yield strength, ductility and parameters of Ludwig-Hollomon equation σ = σ0+kεn ; k, n were correlated with hardness. Various hardness values found in literature were recalculated to Brinell hardness. In tensile testing measured s-e curves were compared with that obtained from the correlation. The investigated materials were API 5 L grade steels X70 after different deformation exposition. The results give good agreement between compared data. The most difference between estimated and measured curve is in area of yield strength, because of Lüders deformation on investigated steel.