ABSTRACT. The current energy policy envisages extended lifetime for the current nuclear power plants (GEN II NPP). This policy imposes a large research effort to understand the ageing of power plant components. In this goal, it is necessary to improve knowledge about safety, reliability and components' integrity for more than forty years of operation. In Central and Eastern Europe, the majority of NPPs are VVER types, where some of the components are produced from austenitic steel 08Ch18N10T. Irradiated 08Ch18N10T may exhibit brittle behavior, namely delamination cracks are found in some cases on the fracture surface of irradiated 08Ch18N10T with elongated δ-ferrite. Delamination cracks have also been observed on the fracture surface of high-strength steels or aluminum-lithium alloys. This article presents a state-of-the art review to provide a detailed analysis of the influence of delamination cracks on the toughness of metal alloys. In general, the delamination cracks are present in metal alloys having a high texture and microstructure anisotropy. Three types of delamination cracks have been observed and are classified as crack arrester delamination, crack divider delamination and crack splitting delamination. The microscopy characterization, 3D fracture theories and computational studies explaining possible causes and effects of delamination cracks on the mechanical properties of metal alloys are presented.
The hardening behavior of AISI 304 steel is investigated at various strain rates, from the quasi-static state to ultra-high strain rates, because it is necessary for numerical simulation of high-speed deformation problems. This kind of testing at a wide range of strain rates has not been yet reported in the literature although it is indispensable for accurate numerical analyses where deformation takes place with a wide spectrum of strain rates. AISI 304 steel is a kind of austenitic stainless steel used in various engineering fields, which does not harden by heat treatment, but by cold working such as shot peening. In order to obtain hardening properties at each strain rate, tensile tests were carried out using a universal testing machine of the INSTRON 5583 for the quasi-static state, a high speed material testing machine of a servo-hydraulic type for intermediate strain rates, and a tensile split-Hopkinson bar for high strain rates with a digital image correlation method. The hardening properties at the ultra-high strain-rate region were obtained from Taylor impact test results by calibration with an experimental–numerical hybrid inverse optimization for reliable extrapolation results. Finally, the hardening flow stress curves were obtained at various strain rates from the quasi-static state to ultra-high strain rates by interpolating the data with the extended Lim–Huh model. The result shows that the yield stress of 759 MPa at the quasi-static state increased to 1429 MPa at a strain rate of 106 s−1, which is about 1.9 times of the quasi-static yield stress. As a demonstration example, the dynamic hardening properties obtained were applied to a shot peening simulation that required hardening curves at a wide range of strain rates from the static state to 106 s−1. The simulation result with the dynamic hardening properties is compared to that with the quasi-static properties. The comparison shows a notable difference in the maximum compressive residual stress by 32%, demonstrating that it is important to consider the dynamic hardening properties in such high strain rate simulation as shot peening for accurate simulation results.
Laser Shock Peening (LSP) is a surface treatment technique for metallic materials. It induces plastic deformation at the surface of up to around 1 mm in depth. This process introduces residual stresses that lead to strain hardening, and potentially improvements in fatigue, stress corrosion cracking (SCC) and general corrosion behaviour in many, but not all, corrosive media. In this paper, two specimens made of AISI 304L stainless steel, one LSP-treated and one un-treated, were tested at 280 °C and 8 MPa in VVER (or PWR) primary circuit water chemistry using in situ Electrochemical Impedance Spectroscopy (EIS). This experiment serves to qualify the influence of LSP on the changes in corrosion behaviour in high-temperature, high-density water. The residual stress (RS) measurement of the surface showed a compression RS. Before LSP treatment, RS at the surface was 52.2 MPa in the rolling direction 0°RD and 10.42 MPa in the transverse rolling direction 90°RD. After the treatment, surface RS was −175.27 MPa and −183.51 MPa for Scan and TScan directions, respectively. The effect of compressive RS at the surface was studied and showed an increase in corrosion rate. The analysis of oxide layer by SEM revealed differences between LSP-treated and untreated AISI 304L specimens and their connection to corrosion rates.
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