Papers by Keyword: Microcracks

Abstract: In this work instrumented impact Charpy tests were performed to analyze the ductile-brittle transition of Grade A ship plate steels, which are composed of ferrite matrix with pearlite bands in the rolling direction (RD) and pearlite lands in the transverse direction (TD). Observations on fracture surfaces of Charpy specimens showed regions of cleavage from room to down temperatures, therefore blunt four-point double–notch bend specimens (4PBT), were also tested at 25°C, 0°C, –60°C and –196°C in RD and TD, to study cleavage nucleation and propagation mechanisms. In the RD the following four different cleavage microfeatures nucleated microcracks in the notch region of 4PBT in all test temperatures: lamellar-pearlite microstructure, pearlite-boundary, ferrite-grains boundary and grains ferrite-inclusions. However, in the TD at –60°C, close to the lower shelf, only lamellar-pearlite and pearlite-boundary microfeatures were found nucleating microcracks. Nevertheless, the higher density of microcracks was found in the RD. The biggest microcracks were developed in TD in pearlite lands of critical size, in all test temperature, which are the link for the development of microcracks of critical size, which can propagate and fracture steel plates. In RD most microcracks nucleated in a critical region of about 1mm from notch root for all test temperatures, however for the TD the critical region decreases with test temperature, from about 450 µm (+25°C), 370 µm (0°C) and 225µm (–60°C). The analysis also showed the effect of microstructure orientation on the number and size of microcracks, and microcracks arrest.

Abstract: The paper presents the results of the study of the stage of accumulation of damage and fatigue rupture of titanium alloys (using the method of acoustic emission). The main object of research was the development of a method for designing a generalized fatigue diagram characterizing the stage of fatigue damage accumulation. The studies aimed at experimental verification of the hypothesis of the stage of damage accumulation, which can be established only by the registered parameters of acoustic emission with separate analysis by types of acoustic emission sources. In contrast to the method of research, which is carried out fractographic analysis, the use of acoustic emission method can significantly reduce the amount of testing. The types of acoustic emission sources on the distribution plane of two-parameter “AE signal energy E_AE vs. frequency parameter K_f” are considered. Fatigue stages in the tests of trial alloys were determined by the activity of the AE signals emitted by different types of AE sources (dislocation, micro - and macro-cracks). A generalized diagram of fatigue developed according to the specified stages. The developed method significantly reduces the volume of fatigue tests and fractographic studies.

Abstract: Crack propagation after low-cycle fatigue (LCF) deformation has been studied in the chromium martensitic structural steel. Although the study of a fundamental mechanism of fatigue crack growth has received much attention over the last decade, it still remains a sufficiently complex problem and needs full understanding. Moreover, the recent studies show that the cracks propagate discontinuously even on the millisecond timescale, and their growth rate significantly depends on a microstructure of the material. In the present work the boundaries of the former austenitic grains were revealed on the polished surfaces of the thermally treated samples, which subsequently were undergone low-cycle fatigue tests. The experimental studies show that fatigue macrocracks mainly grow along the boundaries of the former austenitic grains, and changetheir propagation direction when crossing the grain boundary, however, remain within 45 ̊ interval with regard the cycling axis. In particular cases, when the boundaries of a martensite packets and those of the former austenite grains lay along the length of a packet, the macrocrack is better formed and with regular borders. After a macrocrack reaches a definite length ~30-50μ, a microcrack is nucleated ahead of the macrocrack tip, and is oriented along the substructure element of the steel. Further deformation tests provide an increase in the length of the main crack via aggregation of microcracks initiated ahead of it during the LCF. In the cases when the macrocrack is deviated, slip bands are formed in martensitic structures along the boundaries of martensite packets (laths). A correlation is revealed between the microcrack components and the substructure elements of the steel as well. The same results were obtained by fractography of the tested and fractured samples. However, in the latter case correlation was established between the cleavage facets and the dimensions of packets.

Abstract: This article provides the reader with results of the study of mesostructure ordered plasma ceramic coatings derived from narrow-fraction powders under optimum modes for this fraction. It is noted that it is succeeded to implement mesostructure ordering of coatings eliminating the formation of continuous columnar structures by means of spraying mode control. It is proven that it is possible to get formation of nanocolumnar structures in a near surface layer, and these structures provide bonds damping between clusters under alternating force and thermal loads of coated parts. To evaluate the quality of formed ordered structure, a quantitative criterion—mesostructure ordering coefficient—is used. The Methods of quantitative evaluation of mesoordering of coating structure has been developed.

Abstract: In this study, the effects of cutting parameter on surface quality was investigated experimentally in wire electrical discharge machining (WEDM) process of ASSAB XW-42 and ASSAB 8407 2M tool steels. The surface quality of WEDMachined (WEDMed) was usually characterized by its roughness, recast layer and cracks. The experiments were conducted under different setting of pulse on time and arc on time on. It can be concluded that surface roughness, recast layer thickness and density of microcracks increased proportionally with pulse on time and arc on time. Thin recast layer and low crack density in the surface of ASSAB XW-42 tool steel was due to the low thermal conductivity of work piece material.

Abstract: Endeavours were undertaken to assess metallographically AlMgSi alloys, (containing 0.58 wt% of Mg, and 0.58 wt% or 6.5 wt% of Si) in their primary states as well as after applying external loads in the Static Compression Test. During the first stage of the study the primary microstructures of the investigated samples were assessed. Samples originated from two technological processes: after strain hardening, in a form of rods (φ 9.5 mm and φ 4.0 mm) gravity casting, rolling, homogenisation and precipitation hardening; and after gravity casting in sand moulds of shaped plates 100 mm x 100 mm x 4 mm of a primary structure and after the precipitation hardening. During the second stage of investigations, microstructures of analysed samples (of various technological history) were compared - after applied external loads - in order to obtain information in what way such diversified primary structure and which of its stereological parameters (amount, shape, size and placement) are essential in initiation of microcracks under static loads.

Abstract: The FIB/SEM investigations of the microstructure changes in the hard brittle W-C based coating deposited on softer steel substrate after nanoindentation tests revealed that a set of approximately equidistant circular cracks forms in the coating in a sink-in zone around the indent and single cracks appear under the indenter tip. Finite element modeling (FEM) indicated development and concentration of the highest principal tensile stresses in the sink-in zone and in the zone below the indenter, which are considered to be the reason for the experimentally observed cracking. The distance from the indenter tip to the first circular crack combined with the calibration curve obtained from the FEM of the location of tensile stress maxima in sink-in zone can be used as a simple method for the determination of the strength of the studied coatings.

Abstract: The failure of a discrete elastic-damage axial system is investigated using both a discrete and anequivalent continuum approach. The discrete damage mechanics (DDM) approach is based on amicrostructured model composed of a series of periodic elastic-damage springs (axial DDM latticesystem). Such a damage discrete system can be associated with the finite difference formulation of aContinuum Damage Mechanics (CDM) evolution problem.The nonlocal CDM models considered in this paper are mainly built from a continualizationprocedure applied to centered finite difference schemes. A comparison of the discrete and thecontinuous problems for the chains shows the effectiveness of the new micromechanics-basednonlocal Continuum Damage modeling, especially for capturing scale effects.

Abstract: This paper is concerned with the evaluation of the parameters of plain weave textile from Kevlar 49 fibers. They were determined following the basic parameters of the textiles e.g. crimp length, crimp amplitude, thickness of the woven fabric, dimensions of the cross-sectional tow (tow width, tow height) and crimp angle. The number of fibers in the warp and tow strands was also determined.The work is also concerned with the analysis of microcracks in the material, that is, microcracks that appeared after the tensile test. Microscopic observation of cracks occurred on the test specimen strip according to DIN EN ISO 14129 for tensile testing. Strips of the material were cut, the metallurgical methods used being kept perpendicular to the thickness of the test specimen. Microcracks occurred in the individual planes located in the chosen reference axis. Microcracks were detected in the entire width of the specimen. The types of microcracks were determined by the length and traits of the microcracks.

Abstract: The aim of the research was to test basic properties of fiber reinforced concrete with PP fibers and cement matrix subjected to heat load of various intensity. Behavior of fiber reinforced concrete was observed in particular from the point of view of change of physico-mechanical properties and microstructure after exposure of test samples to different level of heat load. Volume weight, compressive strength, tensile bending strength, splitting tensile strength and strength in tension of surface layers of concrete were observed.