Papers by Author: Dagmar Mikulová

Abstract: Laser surface hardening is an advanced method of surface treatment of structural steels with a great potential for wide industrial applications. According to the recent literature results and knowledge about laser hardening, fatigue resistance can be either reduced or increased, even considerably, depending on numerous parameters of basic material, the technology parameters etc. This contribution contains results of a partial study of effect of laser hardening of relatively small specimens on fatigue resistance of 42CrMo4 steel. Two different parameters of laser hardening were used, one of them resulted in considerable longitudinal residual stresses surface speed of laser beam 4 mm/s. Results of fatigue tests of basic reference material had a surprisingly high, atypical scatter, particularly in the region near fatigue limit. Fractographical analyses indicated that this scatter was connected with presence of single inclusions, even quite large, which in some cases caused fatigue crack initiation. Compressive residual stresses after the laser treatment improved fatigue strength and reduced the scatter, likely due to short crack retardation in the compressive residuals tress field. Further analyses and discussion are provided using Murakami method of fatigue life evaluation of materials containing defects.

Abstract: Al 7075 alloy is a high strength material usually used for highly stressed components in lightweight structures, typically in aircraft, aerospace and defence applications. It can be applied in different heat treatment conditions, but the T7351 temper state is most widely used because of improved stress-corrosion cracking resistance. An investigation of effects of overloads on fatigue crack growth (FCG) and retardation in Al 7075-T7351 alloy was carried out. FCG rates were measured at load asymmetry R = F_min / F_max = 0.1, in quite wide region of growth between 10^-8 and 10^-5 m/cycle (stress intensity factor range ΔK between 6 and 40 MPa m^1/2). Retardation effects of overloads of the magnitudes 2.7-times and 3.0-times of the maximum load in the constant range fatigue loading were significant. Crack mouth opening displacement was evaluated at numerous stages of crack growth including pre-cracking with so called load shedding method. The overloads resulted in substantial crack closure effects, which, however, did not occur immediately after the overloading, but after further fatigue crack extension. Results are discussed considering both theoretically and experimentally estimated plastic zone size and considering crack closure issues recently published in the literature

Abstract: Surface treatment processes are usually applied to improve surface characteristics and resistance. Particularly duplex processes like nitriding combined with nanostructured based coatings are considered as the most advanced ones, with a large application potential in automotive industry. An example is an application in gear wheels, where fatigue loading in gear teeth roots exists besides contact fatigue on teeth sides. Contact fatigue, has to be therefore well balanced with pure fatigue resistance. Some surface treatments leading to excellent contact properties, wear resistance and extremely low friction coefficient may reduce resistance to pure fatigue crack initiation and subsequent growth to premature failure. In the paper, results of an experimental programme aimed at evaluation of fatigue resistance at repeated loading of nitrided steel after application of two types of coating, (i) nanostructured C-layer deposited by a modified cathodic arc technique according to Microcoat patent and (ii) Cr-WC:H-DLC layer produced according to Hauzer patent [1], are presented and discussed. Fatigue tests were carried out using small samples with 2.1 mm diameter, with nitrided layer of 0.35 mm thickness. Tests were completed with fractographical analyses of initiation areas. Results and analyses showed that coating effects depended on the specific technology and parameters. The Cr-WC:H-DLC layer affected fatigue limit favourably unlike the Microcoat C-layer, which resulted in reduction of fatigue strength. A detailed analysis of recorded data during static tensile tests indicated an occurrence of significant surface and subsurface residual stresses with a strong possible effect on fatigue crack initiation mechanisms. A significant, unfavourable effect of inclusions in the subsurface layer was demonstrated.

Abstract: An evaluation of threshold values and fatigue crack growth (FCG) rates is a fundamental condition for an application of damage tolerance philosophy in numerous machinery components. This philosophy starts to be applied even in such problematic cases like aircraft engine blades. The paper contains results of an evaluation of FCG rates including threshold values in a heat resistant Inconel 718 / René 41 welds used in advanced manufacture processes of jet engine rotor blades. Measurement was performed at load asymmetry R = 0.05, temperature 700 oC, at low load frequency f = 0.3 Hz. Methodological approaches are mentioned. Results of FCG and particularly threshold values are discussed in connection with fractographical analyses. In addition, crack closure phenomenon and closure values are considered and discussed.

Abstract: This work analyses an influence of case hardening on fatigue resistance of models of hollow shafts which have a variable diameter ranging from 40 mm to 67 mm, wall thickness of 7 mm and production surface defects on an inner surface layer of the hollow shafts. Case hardened (on both sides) and non case hardened shafts were tested. Both types of shafts were made out of 16MnCrS5 (DIN) steel by cold reduction rolling and were subjected to a flat bending. Tests indicated that the fatigue resistance of the case hardened shafts was significantly higher. The main reason for the lower resistance of the non case hardened shafts was initiation of magistral cracks from the defects on inner surfaces. In case of the case hardened shafts the magistral cracks initiated from the notch on outer surfaces where macroscopic stress was maximal. Different mechanisms of the cracking are discussed from the point of view of compressive residual stresses in the case hardened layer.

Abstract: Results of an experimental investigation of effects of two advanced technologies for materials cutting, namely water-jet and electroerosive cutting, respectively, are described. Water jet cutting, with abrasive particles and specific parameters, and electroerosive cutting were applied to an aircraft Al-alloy sheet Al 2124 T851 of a considerable thickness, namely 50.8 mm. Reference batch of specimens was manufactured using fine finishing milling. Surface quality of each of the cutting and machining technologies was evaluated and fatigue strength was investigated. Three point bend fatigue tests performed at constant stress amplitude showed a considerable deterioration effect of both technologies on fatigue strength. The damaging effect of both technologies was comparable. Fractographical analysis using scanning electron microscopy (SEM) showed fatigue crack initiation in numerous surface micro-notches occurring as a result of the cutting. In addition, the crack initiation was frequently accelerated by surface or subsurface fairly large particles of intermetallic phases. This mechanism also occurred with milling specimens.

Abstract: Water jet cutting is an advanced technology of separating and machining materials. It is well known, however, that fatigue properties are particularly strongly affected by surface conditions and quality of surface layer. Water jet cutting was applied to two types of aircraft sheets, namely Al-alloy sheet of fairly high thickness 50.8 mm and steel sheet of thickness 25 mm. Fatigue resistance and crack initiation mechanism of the materials after the cutting were compared with those studied using a reference batch of specimens manufactured by fine milling and grinding, respectively. Character of surface damage caused by water jet cutting resulted in a considerable reduction of fatigue strength in comparison with milled or ground surface, respectively. The results are evaluated considering effects of microscopic character of surface conditions, like numerous micronotches caused by the technology used, on fatigue initiation process, as well as effects of subsurface inclusions on fatigue crack initiation and early growth.