Vratislav Mareš

Analysis of the Failure of the Engine Piston of the AlSi Alloy

The aim of the article is piston damage evaluation of a highly exposed combustion engine. The analysed piston was made of an AlSi-based alloy. Atypical damage, which occurred relatively early in the lifetime of the component, was evaluated by metallographic and fractographic analyses. The analysis took into account influences of mechanical and thermal fatigue processes in relation to the microstructure of the material. The metallographic observations of the microstructure revealed the occurrence of cracks extending over the secondary phases and precipitates. Cracks were initiated on the coarser Si phase particles. The crack initiation site is located at the root of the bridge between the sealing piston rings. The damage of the piston was metallographically documented in wide range.

The Effect of Applied Load on Hardness of Steels

Hardness measurements can simple, but very useful provide materials mechanical properties. This is atest that is extremely localized in volume and does not affect significantly the structural properties. Theaim of this article is to highlight on influence of the applied load forces to the hardness measurementresults. In engineering practice it is generally the most widely used method of measuring the Vickershardness. This method has a variability of the loading forces in a wide range from nano/micro up tohigh loads in hundreds N. There is a well-known plastic material response, therefore, were taken intoaccount several steels commonly used in technical practice S355 NL+N, 316LVM, 34CrMo-4. Wheninterpreting the results should always point out to the value of the load. In common engineering practiceleads to confusion, or to comparing the hardness of the material obtained at different loads. Thisproblem is quite often occurs at nowadays and needs to be solved. Focus should be not only in termsof results depending on the load, but also from a statistical perspective of scattering measurements atvarious loads. Generally, in the technical practice is discussed independence measurement results ofVickers hardness on applied load, due geometric similarity of indents. When is used different forcesthere is a changed of the hardness results. This phenomenon is known as a size effect. This effect isgenerally related to each mechanical testing of materials. However, engineering practice getting intoconflict with this effect.

Dynamic Tensile Testing of High Strength Armor Steel Plates

In this paper was studied dynamic behavior of the armor steels Armox 500T and Secure 500 by testing specimens in quasi-static tensile test with strain rate 1∙10-3 s-1 and high-speed tensile test within range of intermediate strain rates from 100 s-1 to 400s-1 at the room temperature. Hardness test and quasi-static tensile test confirmed material properties specified by the manufacturer. Stress-strain diagrams showed very low strain-rate hardening effect at investigated strain rates. Total elongation at fracture was larger in case of Armox 500T for the whole strain rate range. Deformation energy density was calculated from the stress-strain curve and temperature rise due to adiabatic heating was estimated. Because of higher total elongation, Armox 500T was able to withstand higher deformation energy.

Combination Criterion for Multiple Laminar Flaws in Steel Components

A laminar flaw is a planar subsurface flaw parallel to the rolling direction of the plate, where the applied stress is typically parallel to the rolling direction. The laminar flaw oriented within 10 degree of a plane parallel to the component surface is defined as a laminar flaw, in accordance with the definition of the American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel (B&PV) Code Section XI. The ASME Code provides combination criterion for multiple laminar flaws. If there are two or more laminations, these laminations are projected to a single plane and, if the separation distance of the projected laminations is less than or equal to 25.4 mm, the laminations shall be combined into a single large laminar flaw in the assessment. The combination criterion was established on the basis of the non-destructive examination capabilities in the 1970’s. However, this methodology did not consider the offset distance of the laminations nor the mechanical interaction between the flaws. Therefore that combination methodology is not suited in case of a large number of laminar flaws. This may occur e.g. in case of hydrogen flaking in steel forging components. Actually, when multiple discrete laminar flaws are close to each other, interaction between the flaws has to be taken into account and these flaws shall be combined to a single laminar flaw for assessment. Stress intensity factor interactions for inclined laminar flaws were analyzed in the frame of hydrogen flaking issue in reactor pressure vessels of Doel 3 and Tihange 2 Belgian nuclear power plants. Based on the mechanical interaction between flaws, new combination criterion was developed and was presented in this paper.

Evaluation of Materials Degradation Extent after their Long Term Exposition in Power Plants Equipments

Load processes and their interaction over long-term service exposition influence on degradation of construction materials in the equipment of conventional power plants. A wide range of destructive testing methods and number of test for each of them were used to clarify of the extent and mechanism of the damage steel components of the power plants pressure circuit. Changes of material properties were by those methods evaluated in state after operational exposition - till 190 000 hours and after heat treatment reprocessing, for all observed components in the entire profile of the wall thickness. Great attention was paid to metallographic evaluation of the creep damage also. The results of test were compared with the supposed properties of the undegraded material in origin state.

Process of Identifying Stress Fields from Strain Fields in the Specimen with a Hole

The paper is focused on the process of identifying stress fields from strain fields in the specimen with a hole. The experiment was realized on the specimen with a hole made from anisotropic material. The main attention is paid to the analysis of deformation in the areas of stress (near the hole). That geometry generates a heterogeneous strain field which has been measured during the test using a digital image correlation system. The advantage of using heterogeneous strain fields in the identification procedure is that a complex state of stress-strain can be analyzed at the same time. On the other hand the stress field cannot be directly computed from the test and a suitable identification procedure has to be developed. Here, the virtual fields method (VFM) adapted for plastic strain has been used to identify the hardening behaviour and the anisotropy of the material. The values obtained by the VFM have been compared with the results coming from a standard identification made with uniaxial tensile tests.