Papers by Author: Pavol Mikula

Abstract: Focusing 3-axis diffractometer set-up equipped with bent perfect crystal (BPC) monochromator and analyzer offers the sensitivity in determination of strains Dd/d < 10^-4 in polycrystalline materials which is about one order of magnitude higher with respect to that of conventional 2-axis neutron scanners. It also offers possibility of line profile analysis for reasonable sample volumes and counting times. In this paper, the feasibility of using the 3-axis set-up even for measurements of rather large bulk polycrystalline samples with an acceptable resolution is presented. As the 3-axis set-up exploits focusing in real and momentum space, by a proper adjustment of the curvature of the analyzer, a high-resolution determination of the lattice changes can also be achieved even on large irradiated gauge volumes, though with a slightly relaxed resolution. It can be successfully exploited namely, in the strain/stress measurements on samples exposed to an external load, e.g. in tension/compression rig, in aging machine etc. In addition to the original performance where the analysis is carried out by rocking the BPC analyzer and the neutron signal registered by a point detector, a new alternative is offered which uses a fixed rocking angle position of the analyzer and the detector signal is registered by a one-dimensional position sensitive detector (PSD). This trick makes possible in some cases the elastic strain/stress measurements considerably faster and thus reduces the drawback of the time consuming step-by-step analysis.

Abstract: 17NiCrMo7, 19MnCr5 and 27MnCr5 low-alloy case-hardening steels samples have been investigated by SANS (Small-Angle Neutron Scattering), to achieve data on bulk nanoscale structure characterisation and complementary analytical and crystallographic information. The reported results are related in particular to the size distribution of nanosized pores which can help to comprehend the structural basis for the physico-chemical properties and thus to improve quality and durability of the considered materials. A complementary PIXE investigation has been also carried out, with the aim of a non-destructive assessment of the elemental composition of the considered samples.

Abstract: In this contribution the results of residual strain/stress measurements performed on several single pass fillet steel welds which were carried out at different welding conditions, namely, with different filler materials are presented.

Abstract: Residual stress distributions in fillet welds in 8 mm 900 MPa steel have been mapped perpendicular and parallel to the weld line and also through the thickness in the vicinity of weld toe position. Measurements were carried out on four welds when two of them were performed with conventional and two with the so called LTT (low transformation temperature) filler materials. Both neutron and X-ray diffractions were used for determination of the residual stress distribution. Fatigue properties have also been evaluated for all test welds. Neutron diffraction measurements showed that the stress profiles perpendicular to the weld toe qualitatively did not depend on filler material type although the absolute stress levels differed. Trends were similar for positions 2, 4 and 6 millimetres below the surface for all three stress components; σ_x (direction perpendicular to the weld), σ_y (parallel to the weld) and σ_z (through the thickness). X-ray diffraction showed difference in residual stress level at the weld toe. Lower residual stress levels have been identified for LTT filler material when compared to the conventional consumable compositions. The effect of residual stress is discussed in relation to fatigue properties of all four welds. Remarkable higher fatigue strength has been measured for welds prepared by the LTT filler materials.

Abstract: This paper deals with the residual strain/stress measurements in the vicinity of electron beam welds of Charpy-V notched specimen by neutron diffraction. Specimen with welds on one side as well as with welds on two opposite sides were prepared by reconstitution method. Welding was performed with electron beam in a vacuum and proceeded in accordance with the ASTM E 1253 standard.

Abstract: Thermal neutrons are very useful probe in a nondestructive determination of internal stress/strains, due to their high penetration into most materials. In comparison with conventional X-ray techniques, real bulk information on both macro-and micro-strains in materials can be obtained by neutron diffraction (ND) techniques. Knowledge of the spatial and directional distribution of internal residual stresses (RS) is increasingly considered fundamental to determine their influence on properties of engineering materials and a consequent material behaviour. As the assesment of stresses is always related to the stress free material state, an accurate evaluation of the unstressed lattice parameters (e.g., the interplanar distance), in order to determine RS by ND is one of the key tasks. It helps to avoid inacceptable errors in the course of the real material strain and stress evaluation. The availability of carefully measured zero-strain standards is also essential to confirm the absence of systematic instrumental effects determining the diffraction profile at a chosen scattering angle. In this paper, the state of the art of the main analytical and experimental procedures currently established or adoptable to determine these critical parameters, particularly regarding industrial applications, is presented.

Abstract: It is well known that fatigue strength of welded joints does not depend on steel strength. Better fatigue strength of welded joints, e.g. longer life time of fatigue loaded weld structures, can be achieved with a smooth transition between the weld and the base material to minimize stress concentration. It has also been recognized that residual stresses play a critical role in the fatigue behaviour of welds. In the last decade an extensive research has been performed in order to increase the fatigue strength of high strength steel weldments. The martensite and bainite transformation start temperatures of weld metals have been shown to have a large effect on fatigue life time of high strength steel welds. This is of particular importance if the full potential of high strength steels is to be used in fatigue loaded constructions. A detailed investigation of the effect of phase transformation temperature on residual stress distribution in the vicinity high strength steel welds and its effect on fatigue life time has been performed. The transformation temperature of the weld metal was varied by changing the chemical composition of the filler material. Residual stress distributions have been measured by neutron as well as by X-ray diffraction and fatigue tests have been performed on the fillet welds. A strong effect of weld metal phase transformation temperature on residual stress level was observed. Fatigue strength increased approximately three times when an optimised low transformation temperature filler material was used in comparison to the application of conventional filler material.

Abstract: In this paper properties of the dedicated neutron strain/stress diffractometer installed at the beam port ST-1 of HANARO reactor in KAERI which has been optimized at a small monochromator take-off angle, are described. Thanks to the employment of the horizontally focusing bent perfect crystal monochromator and the optimization procedure good resolution can be achieved and namely, the luminosity of the instrument can be considerably increased which permits us to enlarge the range of the material depth for residual strain/stress mapping. Moreover, new alternative of the diffractometer permits also to investigate some kinetic processes in polycrystalline materials running within few seconds.

Abstract: The residual stress (RS) status induced in the substrate of coated materials by the coating process plays frequently a major role in lending the component’s characteristics. RS assessment can give, thus, a substantial contribution in justifying different cases of failure or else bad performance of coated components due to e.g. the coating delamination or to other occurrences which are not simply interpretable via the conventional mechanical tests or microstructure analyses. The adoption of both neutron diffraction (ND) and X-ray diffraction (XRD) techniques has revealed its usefulness in assessing the RS values in proximity of the coating interface area, respectively, without any layer removal or hole drilling at the extreme surface. In this paper, some real cases of RS determination in coated materials by using these techniques and exploiting their complementarity are described. ND, in particular, is very suitable for crucial applications, where a much different stress situation than that assessed by XRD could be present at some depth below the surface. The results achieved can yield trends adoptable in monitoring of the coating features.

Abstract: The performance of an unconventional high-resolution neutron diffractometer using Bragg diffraction optics that can be efficiently employed in strain/stress diffractometers are reported. Presented results demonstrate their experimental abilities for powder diffraction, namely for residual strain/stress measurements. In addition to the macrostrain scanning capability, the device can also be used for microstrain/stress studies by suitable analysis of the diffraction profiles. Two examples of the strain measurements are presented.