Matti Isakov

Barkhausen noise characterisation during elastic bending and tensile-compression loading of case-hardened and tempered samples

This study examined the Barkhausen noise (BN) response of carburising case-hardened steel with varying tempering stages. The test material was loaded in bending and in alternating loading. The aim of the study was to obtain relevant multiparameter BN data from different loading conditions and to investigate the effect of applied stress on the BN response. The test bar series was made from case-hardened steel. Different tempering parameters were used to vary the surface hardness and the surface residual stresses of the studied series of test bars. In the bending tests, the samples were subjected to incrementally applied loading in the purely elastic deformation region. In addition, uniaxial stepwise loading with tensile and compressive stress was applied to selected samples simultaneously with the BN measurements. The BN measurements were performed under different loading conditions along with X-ray diffraction strain measurements in bending. The results revealed linear behaviour between the reciprocal root mean square value and the stress values obtained with strain gages and X-ray diffraction for the tempered samples.

Iterative Determination of the Orientation Relationship Between Austenite and Martensite from a Large Amount of Grain Pair Misorientations

An automatic, iterative method to determine the orientation relationship between parent austenite and martensite is described. The algorithm generates the orientation relationship from grain boundary misorientations through an iterative procedure based on correct symmetry operator assignment. The automatic method is demonstrated to work on both martensitic and bainitic steels and to provide comparable results to a manual grain selection method.

Sedimentation stability and rheological properties of ionic liquid–based bidisperse magnetorheological fluids

The sedimentation stability and rheological properties of ionic liquid–based magnetorheological fluids comprising a mixture of micron- and nano-sized particles were experimentally studied. Three different fluids with the same total particle concentration of 15 vol% were prepared for testing: one containing only microparticles and two others in which 5 or 10 wt% of the microparticles were replaced by nanoparticles. The nanoparticles were surface stabilized against oxidation. For comparison purposes, silicon oil–based magnetorheological fluids with similar solid fractions were also prepared and tested. The results indicate that, with ionic liquid as a carrier fluid, the addition of nanoparticles at 10 wt% reduces the sedimentation rate almost by an order of magnitude from that without nanoparticles, while the reduction in the dynamic yield stress is only marginal. The ionic liquid–based fluids also had a better dispersion of particles.

Characterization of a Ferritic Stainless Sheet Steel in Simple Shear and Uniaxial Tension at Different Strain Rates

In this study the mechanical properties of ferritic stainless steel EN 1.4521 (AISI 444) were characterized in uniaxial tension and simple shear. The specimen geometries were designed so that tests could be carried out both with a conventional uniaxial materials testing machine and at high strain rates with the Tensile Hopkinson Split Bar method. During the tests, specimen surface deformation was measured using a three dimensional digital image correlation technique based on a two-camera stereovision setup. This technique allowed direct measurement of the specimen gauge section deformation during the test. Test results indicate that the selected approach is suitable for large strain plastic deformation characterization of ductile metals. The stress-strain data obtained from the simple shear tests shows a correlation with the tensile test results according to the von Mises effective stress-strain criterion. Since necking is absent in shear, test data can be obtained at considerably higher plastic strains than in tension. However, the final fracture occurs under a complex loading mode due to the distortion of the specimen geometry and multiaxial loading introduced by the simple shear arrangement. Test results also show that reliable material data can be obtained at high strain rates.Copyright

Improved specimen recovery in tensile split Hopkinson bar

This paper presents an improved specimen recovery method for the tensile split Hopkinson bar (TSHB) technique. The method is based on the trapping of residual stress waves with the use of momentum trap bars. As is well known, successful momentum trapping in TSHB is highly sensitive to experimental uncertainties, especially on the incident bar side of the set-up. However, as is demonstrated in this paper, significant improvement in the reliability of specimen recovery is obtained by using two momentum trap bars in contact with the incident bar. This makes the trapping of the reflected wave insensitive to striker speed and removes the need for a precision set gap between the incident bar and the momentum trap.

Characterization of Sheet Metals in Shear over a Wide Range of Strain Rates

A new specimen geometry, based on ASTM B831, for characterization of sheet metals in shear is introduced. The specimen can be tested in both quasi-static and dynamic tests using a load frame and a tensile split-Hopkinson bar (SHB) apparatus, respectively. Specimens with this new geometry and spool-shaped torsion specimens were fabricated from the same 0.5″ rolled 2024-T351 aluminum plate. Static and dynamic tests at strain rates up to 2,000 1/s on both samples were conducted using an axial-torsional servo-hydraulic load frame, a tension SHB, and a torsion SHB. Three-dimensional digital image correlation was used to directly measure deformation of the surface of the sample’s gage section in both static and dynamic tests. Stress–strain curves obtained from tests with both types of specimens agree, indicating that the new sample geometry is suitable for the characterization of ductile sheet metals in shear.

Effects of Adiabatic Heating Estimated from Tensile Tests with Continuous Heating

The mechanical behavior of metastable austenitic stainless steels is strongly influenced by the strain induced phase transformation of austenite into martensite. The phase transformation rate is significantly affected by the strain rate and by the adiabatic heating at higher strain rates. Uncoupling of the effects of strain rate and adiabatic heating can lead to a better understanding of the strain-induced martensitic transformation and allow more accurate material modeling. This paper presents a preliminary analysis of the effects of adiabatic heating during a tensile test. The adiabatic heating as a function of strain was calculated from the stress-strain curves obtained in adiabatic conditions. Then the tensile tests were carried out at a lower strain rate while continuously heating the specimen at the same rate as obtained in the adiabatic conditions. With this method, the thermal conditions of the adiabatic tests were reproduced in the low rate conditions, which would normally be isothermal without the external heating. The martensite fraction was evaluated using the magnetic balance method. In this paper, we present a detailed description of the experimental procedure and discuss the observed changes in the mechanical behavior and microstructure of the studied steel.

Characterization of Flame Cut Heavy Steel: Modeling of Temperature History and Residual Stress Formation

Heavy steel plates are used in demanding applications that require both high strength and hardness. An important step in the production of such components is cutting the plates with a cost-effective thermal cutting method such as flame cutting. Flame cutting is performed with a controlled flame and oxygen jet, which burns the steel and forms a cutting edge. However, the thermal cutting of heavy steel plates causes several problems. A heat-affected zone (HAZ) is generated at the cut edge due to the steep temperature gradient. Consequently, volume changes, hardness variations, and microstructural changes occur in the HAZ. In addition, residual stresses are formed at the cut edge during the process. In the worst case, unsuitable flame cutting practices generate cracks at the cut edge. The flame cutting of thick steel plate was modeled using the commercial finite element software ABAQUS. The results of modeling were verified by X-ray diffraction-based residual stress measurements and microstructural analysis. The model provides several outcomes, such as obtaining more information related to the formation of residual stresses and the temperature history during the flame cutting process. In addition, an extensive series of flame cut samples was designed with the assistance of the model.

Temperature and Strain Rate Effects on the Mechanical Behavior of Ferritic Stainless Steels

To gain knowledge about the applicability of ferritic stainless steels in exhaust pipes and other high temperature applications, mechanical testing of EN 1.4509 (ASTM S43932) and EN 1.4521 (ASTM 444) was conducted at 600 °C and 800 °C. Tensile tests with short high temperature exposure were carried out to determine the material properties in the as-received condition. To study the high temperature service performance and the effects of possible microstructural changes during long-term high temperature exposure, tensile tests were performed for samples that had undergone a 120 h furnace heat treatment at 600 °C. As an example of the effect of exposure time, serrated flow was observed in the tests for as-received EN 1.4509, which indicates dynamic strain aging. The effect, however, disappeared after the 120 h heat treatment, suggesting that notable microstructural changes take place at high temperatures. Also fatigue and high strain rate tensile tests were conducted on the test materials to reveal the effects of high temperature exposure on their properties, microstructure and service performance.

Metastable Austenitic Steels and Strain Rate History Dependence

This paper addresses a previously relatively little discussed topic related to the plasticity of metastable austenitic steels, namely the strain rate history dependence. In this concept, the mechanical response of a material is not necessarily determined only by the current deformation conditions, such as temperature and strain rate, but also by the previous values of these variables. From the microstructural point of view, strain rate history effects are a direct manifestation of the variations in the microstructural evolution during plastic deformation. For metastable austenitic steels, which can undergo strain-induced phase transformation from austenite to martensite, strain-rate history effects can be notably large. The purpose of this paper is to present and discuss the experimental methods and test procedures the authors have found applicable for the studies of the strain rate history dependence of a metastable austenitic stainless steel EN 1.4318. Special emphasis is put on studying the strain rate history dependence at high strain rates, which is complicated by the dynamic nature of the tests and the lack of closed loop control. The presentation is concluded with examples of test results that demonstrate the relevance of the research topic.