Jan Siegl

Young's modulus-and fatigue behavior of plasma-sprayed alumina coatings

The fatigue behavior and Young’s modulus of plasma-sprayed gray alumina on low-carbon steel substrates were investigated. The investigation of the properties of composites that were defined as “coating-substrate” composites included measurements of the microhardness profile, the residual stress on the top of the coating, and the residual stress profile in the substrate. Fatigue samples were periodically loaded as a cantilever beam on a special testing machine. Failed samples were observed with a scanning electron microscope to determine the failure processes in the coating. The Young’s modulus of the coating was measured by the four-point bending method. Samples were tested both in tension and compression under low (300 N) and high (800 N) loads. The authors’ experiments revealed that the average fatigue lives of coated specimens were nearly two times longer than those of the uncoated specimens. The measurements of Young’s modulus of the coating yielded values that varied between 27 and 53 GPa, with an average value of 43 GPa. Loading in tension caused a decrease in the Young’s modulus of the coating, while loading in compression led to an increase in Young’s modulus. The increase in the lifetime of coated samples was likely due to compressive residual stresses in the substrate, originating during the spray process. The failure of the coating was due to several processes, among which the most important were splat cracking, splat debonding, and the coalescence of cracks through the voids in the coating.

Fractographic observations and predictions on fatigue crack growth in an aluminium alloy under miniTWIST flight-simulation loading

Abstract Sheet specimens from the 2024-T3 Al alloy were tested under two miniTWIST load histories and a special load sequence, which combines the most severe flights from miniTWIST and batches of constant-amplitude cycles. Fractographic observations were made to determine the fatigue crack growth increments in the most severe flights. The bands caused by the severe flights could be distinguished on the fracture surface and an accurate reconstitution of the crack growth curve could be made. The crack increments occurring in the most severe flights were measured and compared with the values predicted by the modified CORPUS model.

Fatigue Crack Growth in Bodies with Thermally Sprayed Coating

Many applications of thermally sprayed coatings call for increased fatigue resistance of coated parts. Despite the intensive research in this area, the influence of coating on fatigue is still not completely understood. In this paper, the localization of crack initiation sites and the dynamics of crack propagation are studied. The resonance bending fatigue test was employed to test flat specimens with both sides coated. Hastelloy-X substrates coated with classical thermal barrier coating consisting of yttria stabilized zirconia and NiCoCrAlY layers. The strain distribution on the coating surface was evaluated by the Digital Image Correlation method through the whole duration of the fatigue test. Localization of crack initiation sites and the mode of crack propagation in the coated specimen are related to the observed resonance frequency. The individual phases of specimen degradation, i.e., the changes of material properties, crack initiation, and crack propagation, were identified. The tested coatings strongly influenced the first two phases, and the influence on the crack propagation was less significant. In general, the presented crack detection method can be used as a sensitive nondestructive testing method well suited for coated parts.

Influence of Cold-Sprayed, Warm-Sprayed and Plasma Sprayed Layers Deposition on Fatigue Properties of Steel Specimens

Titanium powder was deposited onto steel specimens using four thermal spray technologies: plasma spray, low-pressure cold spray, portable cold spray, and warm spray. The specimens were then subjected to strain-controlled cyclic bending test in a dedicated in-house built device. The crack propagation was monitored by observing the changes in the resonance frequency of the samples. For each series, the number of cycles corresponding to a pre-defined specimen cross-section damage was used as a performance indicator. It was found that the grit-blasting procedure did not alter the fatigue properties of the steel specimens (1% increase as compared to as-received set), while the deposition of coatings via all four thermal spray technologies significantly increased the measured fatigue lives. The three high-velocity technologies led to an increase of relative lives to 234% (low-pressure cold spray), 210% (portable cold spray), and 355% (warm spray) and the deposition using plasma spray led to an increase of relative lives to 303%. The observed increase of high-velocity technologies (cold and warm spray) could be attributed to a combination of homogeneous fatigue-resistant coatings and induction of peening stresses into the substrates via the impingement of the high-kinetic energy particles. Given the intrinsic character of the plasma jet (low-velocity impact of semi/molten particles) and the mostly ceramic character of the coating (oxides, nitrides), a hypothesis based on non-linear coatings behavior is provided in the paper.

Characterisation of Mechanical Properties by Small Punch Test

The specific desired properties for structures and components working in critical environments (e.g. different structure parts of power plants) require current information about degradation processes coming out in materials. Obtaining of this information by the help of the classical tests of mechanical properties (tensile test, Charpy test, fracture toughness test, creep test etc.) is very limited namely in the case of nuclear power plants pressure vessel. Hence, the new innovative techniques based on miniaturized specimens have been developed for evaluation of mechanical properties and their changes. One of very promising techniques is Small Punch Test. Present paper deals with characterization of three different steels (15Ch2MFA, 10GN2MFA with different heat treatment and steel O8Ch18NT10 with various degree of deformation).

European Project “Supercritical Water Reactor – Fuel Qualification Test”: Summary of general corrosion tests

The main target of the EUROATOM FP7 project “Fuel Qualification test for SCWR” is to make significant progress towards the design, analysis and licensing of a fuel assembly cooled with supercritical water in a research reactor. The program of dedicated WP4 - Pre-qualification was focused on evaluation of general corrosion resistance of three pre-selected austenitic stainless steels 08Cr18Ni10Ti, AISI 347H and AISI 316L, which should be pre-qualified for application as a cladding material for fuel qualification tests in supercritical water. Therefore, the experiments in support of WP4 concentrated on 2000 h corrosion exposures in 25 MPa SCW at two different temperatures 550 and 500°C dosed with both 150 and 2000 ppb of dissolved oxygen content. Moreover, water chemistry effect was investigated by conducting tests in 550°C SCW with 1.5 ppm of dissolved hydrogen content. At first, corrosion coupons were exposed for 600, 1400 and 2000 h in JRC IET, VTT and SJTU autoclaves connected to recirculation loop allowing continual water chemistry control during the test. Following examination of the exposed specimens consisted of weight change calculations and detailed macro and microscopic investigation of oxide layers using SEM and EDX. With respect to general corrosion results, all tested steels showed sufficient corrosion resistance in SCW conditions taking into account the conditions foreseen for future fuel qualification test in the research reactor in CVR Rez. When the results of weight change calculations were compared for all three materials, it was found out, that the corrosion resistance increased in the following order: 316L<347H<08Cr18Ni10Ti. Results obtained in hydrogen water chemistry did not indicate any significant beneficial effect compared to tests in SCW with 150 or 2000 ppb dissolved oxygen content. Additional tests were dedicated to investigation of surface finish effect. In these exposures polished, sand-blasted and plane-milled surface finish technique were investigated. Beneficial effect of surface cold work in particular of sand-blasting was clearly demonstrated.

Comparison of Conventional Mechanical Testing with Innovative Techniques for Determination of Mechanical Properties of Nuclear Power Plant Components Materials

Within the nuclear power plant operational life management, components lifetime extension requires information on structural material degradation. Innovative testing methods of Small punch testing and Automated ball indentation test are based on the determination of material properties from sub-sized specimens. Present paper is focused on the employment of these techniques in the NPP irradiated materials testing and evaluation at the accredited hot cell testing laboratory of ÚJV Řež, a. s., Mechanical testing department. Comparison with the testing results from the conventional methods is depicted.

On the Failure Mechanisms in Reactor Pressure Vessel with Austenitic Cladding

The austenitic cladding of the WWER pressure vessel is made from two different layers with different fracture toughness values. Based on the fractographic analysis of the tested specimens in the initial, as well as in the irradiated conditions, it was found that individual failure micromechanisms take place during the crack propagation. The obtained results were used to find the relationship between the failure micromechanism changes and the fracture toughness values, as well as to assess the effect of neutron irradiation on the failure micromechanisms.

Characterization of Local Stress-Strain Behavior in WWER 440 Weld and Base Metal by Instrumented Indentation Technique

15Ch2MFA (base metal) as well as 10ChMFT (weld) steels used for WWER 440 nuclear reactor pressure vessel manufacturing present a gradient in mechanical properties through the wall thickness, which can hardly be assessed by conventional testing such as tensile or Charpy tests. Mechanical properties in the weld and base metal were therefore determined by performing a series of instrumented indentations across the weld at room temperature. The results were treated by so-called automated ball indentation technique. Local stress-strain behavior obtained by instrumented indentation was correlated to the tensile test data and microstructure characterized by metallographic analysis.