Rapid Crack Initiation Research Articles

Failure Analysis of CRDINiCr Butterfly Valves for Nuclear Power Plant Applications

Corrosion-resistant iron with nickel and chromium (CRDINiCr) is often used in butterfly valves for flow control at nuclear power plants, where resistance to corrosion, oxidation, and wear is significant. In this study, a failure analysis of a CRDINiCr alloy butterfly valve was performed by combining morphology characterization and in situ elemental composition analysis of failure of various regions of the valve. Based on the testing and analysis conducted in this study, it was determined that the inspected valve body material exhibited several defects, including poor graphitization, porosity, and the presence of eutectic carbides. These imperfections compromised the required plasticity criteria, resulting in significant embrittlement of the material. Therefore, under the impact stresses applied during the pressure testing, these vulnerabilities facilitated rapid crack initiation and propagation. The presence of such defects significantly compromised the material’s resistance to fracture under dynamic loading conditions, underscoring the critical importance of stringent quality control in the production of such materials to ensure their reliability and performance in operational settings at nuclear power plants.

Analysis on mechanical properties and mesoscopic acoustic emission characteristics of prefabricated fracture cemented paste backfill under different loading rates.

The mechanical characteristics of cemented paste backfill (CPB) are significantly influenced by the loading rate (LR) and initial defects. Therefore, the CPB with prefabricated fracture (PF, PFCPB) was prepared, and uniaxial compressive strength (UCS) tests considering LR and acoustic emission (AE) monitoring were performed. The particle flow code (PFC2D) was introduced to analyze the mesoscopic crack evolution of the filling body, and the moment tensor theory was used to invert the AE signals characteristics. The results show that as the PF angle increased, the UCS and elastic modulus (EM) of PFCPB decreased and then increased, and the 30° PF was the turning point. The mechanical properties of PFCPB were deteriorated by the presence of PF. Meanwhile, the mechanical properties of PFCPB were positively correlated with the LR. The stress-strain curve of PFCB (excluding 90°) showed bimodal curves. After the UCS test, the macro crack of PFCPB sprouted at the tip of PF or converged toward PF. PFCPB mainly suffered from shear failure, accompanied by a few tensile failures. Numerical simulation results showed that the crack initiation stress of PFCPB was reduced by the PF. The number of cracks first dropped and then gradually increased when the PF angle was enhanced, while gradually increased with the LR increased. Meanwhile, the mesoscopic AE characteristics of CPB were strongly in line with the test results. The AE events of 0 ~ 60° PFCPB experienced two slow rising periods and rapid rising periods. The temporal and spatial distribution characteristics of AE corresponded to the crack evolution trend. The PF was prone to stress concentration, especially at the tip and upper and lower surfaces of 0 ~ 45° PF, resulting in rapid crack initiation and reducing the energy storage limit and mechanical behavior of 0 ~ 45° PFCPB. The increasing LR (within a certain range) was in favor of improving the mechanical behavior of the filling body. The research results can provide a basic reference for the stability evaluation of the filling body with initial defects.

New Technology and Experimental Research on Thick-Walled Tube Fatigue Impact Loading Precision Separation

Traditional separation methods for thick-walled metal tubes include turning and sawing, which suffer from wasted raw material and low efficiency. In view of this, this paper proposes a new process of using impact load to promote crack generation and tube separation. Based on the principles of radial repeated impact load, stress concentration effect and fatigue fracture, the rapid initiation and stable expansion of tube fatigue crack are promoted. In addition, the crack initiation mechanism of the tube V-notch root cracks under radial repeated load when the tube is in a restrained state. For the experimental study of the GCr15 steel tube, a multistep decline frequency time tube separation control curve with an initial frequency from 4 Hz to 31 Hz and termination frequency from 1 Hz to 8.5 Hz was designed, and the precision tube separation device is loaded by pneumatic fatigue shock to achieve tube precision separation. In addition, a tube fracture quality evaluation method is proposed. According to the test results, the stress concentration effect of V-notch can significantly reduce the average stress in the process of tube fatigue separation and accelerate the generation of microcracks. Under the continuous action of repeated impact load, the loading method of multistep decline can effectively control the rapid crack initiation and stable expansion of the GCr15 tube V-notch root crack. Moreover, the tube final fracture region has relatively small defects, which can obtain good fracture quality.

Structural analysis of sol-gel derived TiO2 nanoparticles: a critical impact of TiO2 nanoparticles on thermo-mechanical mechanism of glass fiber polymer composites

The incorporation of inorganic nanoparticles with thermosetting epoxy polymer is an emerging field of research over the past few years. It is well analyzed that epoxy matrix is brittle in nature that shows rapid crack initiation and rapid propagation without increasing applied stress value. Therefore, researchers are showing their interest in nanoparticles embedded epoxy composites to improve their fracture resistance (brittleness and toughness). In this investigation, the dispersion of TiO2 nanoparticles at different weight fractions (0-2 %) with glass fiber reinforced epoxy composites is performed to enhance structural and thermo-mechanical properties. The TiO2 nanoparticles are prepared by sol-gel method and structural analysis of TiO2 nanoparticles shows greater interfacial bond with epoxy matrix and glass fibers due to fine dispersion of nanoparticles. From obtained results, a significant enhancement in their tensile strength (38.56 %), flexural strength (30.52 %), inter-laminar shear stress (25.22 %), impact strength (327.10 %), micro-hardness (48.53 %) and fracture energy (40.19 %) with a minimal detrimental effect on toughness was revealed for GFRP-T1.0 compare to GFRP-T0.0 composite laminates. The stiffness and rigidity also improved up to 52.72 % and 34.13 % respectively for GFRP-T1.5 compare to GFRP-T0.0 composite laminates. The effects of nanoparticles contents and clustering size on thermal stability and glass transition temperature of developed composites are observed by thermo-gravimetric analysis. The surface morphology of TiO2 nanoparticles is characterized by transmission electron microscope (TEM) while the dispersion of nanoparticles and failure of developed composites were analyzed by scanning electron microscopy (SEM).

On the effect of internal channels and surface roughness on the high-cycle fatigue performance of Ti-6Al-4V processed by SLM

Selective laser melting (SLM) is an additive manufacturing process allowing for the production of metallic high performance materials and components with complex geometries. Therefore, possible applications are direct production of casting molds, tools or turbine blades with internal cooling channels for an optimized cooling efficiency. A disadvantage of the technology is the process-inherent surface roughness, which is critical especially under fatigue loading conditions. Since internal surfaces often cannot be smoothened due to limited accessibility, the objective of this study is to assess the fatigue properties of Ti-6Al-4V samples designed with internal axial channels featuring a rough as-built surface. Samples with various diameters and numbers of channels have been tested not always exhibiting a deterioration of the fatigue performance compared to solid samples. Subsequent fractography by scanning electron microscopy revealed distinct failure mechanisms. Besides the fatigue crack initiation on features of the unmodified internal surfaces, residual porosity in the bulk, i.e. lack-of-fusion defects, keyhole defects and gas pores, respectively, could be identified as crack origin. Relatively low scatter of fatigue lives found is attributed to rapid crack initiation and, thus, the dominant influence of the (micro-) crack growth regime.

Crack Initiation in Austenitic Stainless Steel Sanicro 25 Subjected to Thermomechanical Fatigue

Thermomechanical fatigue experiments were performed with austenitic stainless Sanicro 25 steel. Several amplitudes of mechanical strain in a wide temperature interval (250-700 °C) were applied to the specimens. Mechanical response was recorded and fatigue lives were obtained. Scanning electron microscopy combined with FIB technique was used to study the mechanism of crack initiation in in-phase and in out-of-phase thermomechanical cycling. Different mechanisms of the crack initiation were found in these two types of loading. During in-phase loading fatigue cracks start in grain boundaries by cracking of the oxide. Cracks grew preferentially along grain boundaries which resulted in rapid crack initiation and low fatigue life. In out-of-phase loading multiple cracks perpendicular to the stress axis developed only after sufficiently thick oxide layer was formed and cracked in low temperature loading half-cycle. The cracks in oxide allowed localized repeated oxidation and finally also cracking. The cracks grow transgranularly and result in longer fatigue life.

Thermal fatigue crack growth in stainless steel

A judgment of residual service life of engineering parts exposed to thermal fatigue makes it possible to deal with economic and safety issues in power plants. The aim of this study is to analyze a fatigue crack initiation and propagation in A321 stainless steel bodies subjected to repeated thermal shocks. For this purpose, various methods of crack propagation monitoring were used. The first stage of experiments included mechanical cyclic loading of specimens with the central notch at fixed temperatures ranging from 20 °C to 410 °C. The crack growth rate was only minimally influenced by temperature in this case. Thermal loading of the same specimens with ΔT varying from 150 °C to 340 °C showed very rapid crack initiation in the notches and its asymmetric growth. Metallographic and fractographic analyses of failed specimens were carried out after 1000, 3000 and 6000 thermal cycles. The comparison of the fracture surface micromorphology confirmed the similarity in the mechanism of the thermal and mechanical fatigue crack growth. Stress analysis using the finite element method consisting of transient thermal and mechanical solutions was performed in order to simulate the experiments. Thermal fatigue crack growth assessment was carried out on the basis of the experiments and the computed thermally induced stress intensity factors. This model successfully confirms the discussed analogy of thermal and mechanical stress induced damage.

Measurement of rapid crack propagation in pressure pipes: A static S4 approach

A method for creating rapid crack propagation in pressurized pipes under slow static loading using modified S4 apparatus is described. In the development of the method a complexity involved with dynamic loading in the S4 test (ISO 13477) is eliminated by the use of a displacement controlled static loading machine. The experimental system consisted of an universal testing machine, a low compliance wedge loading device, notch tip quenching apparatus and a pipe specimen where a through thickness hole is drilled to accommodate the wedge loading device. The pipe specimen is made in such a way that a section containing a hole is free from the internal pressure, while the rest of the specimen is made to carry the internal pressure which would eventually drive the unstable crack along the pipe axis. The idea of such rapid crack initiation under static loading was derived from the concept of time-temperature equivalence, where impact loading may in part be simulated by lowering the temperature at the site of rapid crack initiation. The details of the method for rapid crack propagation under static loading are described and the correlation of the results to rapid crack propagation obtained by ISO 13477 is illustrated. The two methods were shown to compare quite well in terms of critical pressure determination and the details regarding normalized rapid crack length versus the internal pressure curve as well as the crack propagation pattern.

Fatigue crack growth behavior of the simulated HAZ of 800 MPa grade high-performance steel

The present study focuses on the fatigue properties in the weld heat-affected zone (HAZ) of 800MPa grade high-performance steel, which is commonly used in bridges and buildings. Single- and multi-pass HAZs were simulated by the Gleeble system. Fatigue properties were estimated using a crack propagation test under a 0.3 stress ratio and 0.1 load frequencies. The microstructures and fracture surfaces were analyzed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results of the crack propagation test showed that the fatigue crack growth rate of coarse-grained HAZ (CGHAZ) was faster than fine-grained HAZ (FGHAZ), although both regions have identical fully martensite microstructures, because FGHAZ has smaller prior austenite grain and martensite packet sizes, which can act as effective barriers to crack propagation. The fatigue crack growth rate of intercritically reheated CGHAZ (ICCGHAZ) was the fastest among local zones in the HAZ, due to rapid crack initiation and propagation via the massive martensite–austenite (M–A) constituent.

The fatigue behaviour of metastable (AISI-304) austenitic stainless steel wires

The fatigue behaviour of AISI 304 stainless steel has been investigated as a function of drawing strain. Both smooth and notched wire samples were subjected to three-point fatigue testing carried out under load control at room temperature. This study has established that approximately 20% of prior deformation-induced martensite is a critical amount which determines the subsequent fatigue response of this steel. In steel with less than 20% of deformation-induced martensite, any martensite formed during the fatigue process will act beneficially by retarding fatigue cracking, raising the fatigue limit and resulting in a ductile fatigue fracture. However, in the presence of more than 20% of deformation-induced martensite, any martensite induced by cyclic strain will encourage more rapid crack initiation which leads to more brittle fracture surface characteristics. A scenario for predicting the fatigue behaviour of cold drawn AISI 304 stainless steel wire has been proposed in this study.

Stress corrosion monitoring of 7000 series aluminium alloy welded specimens using acoustic emission

AbstractAcoustic emission (AE) was continually monitored during stress corrosion tests on weldable aluminium alloys (7000 series) supplied by the Defence Research Agency of the UK Ministry of Defence to progress the investigation and understanding of stress corrosion in differently treated aluminium alloys. The welded structures were exposed under known stress conditions to two corrosive environments (marine and humid). The monitored AE activity enabled the crack initiation and propagation associated with stress corrosion cracking to be detected. The different nature of the two environmental conditions to which all of the aluminium alloy specimens were exposed under load greatly influenced the stress corrosion cracking propensity of the alloys. The humid environment was found to be more aggressive than the marine environment, resulting in more rapid crack initiation and failure within the alloys tested. It was concluded that differences in alloy composition and surface treatment had a profound effect on t...

The influence of the sulfur/oxygen ratio in the environment on the creep and creep damage behavior of a heat-resistant steel

Investigations have been carried out on the creep behavior of a 32Ni-27Cr-0.07Ce heat-resistant steel in several S-O bearing environments at 700 °C. It was found that the creep strength is reduced if severe corrosion, as was found in the case of the highest S/O ratio, causes a reduction of load-bearing cross section. Deformation-induced fingerlike corrosion paths along grain boundaries, which form an alternate corrosion pattern, were less damaging, even when the penetration was deep. Creep ductility was strongly reduced in all environments regardless of the S/O ratio. The formation of sulfide-oxide corrosion products causes rapid crack initiation at the surface and promotes crack propagation.

A finite element model of persistent slip band interaction with strengthened surface films during low cycle fatigue

Hardened surface layers such as those produced by ion implantation can inhibit the emergence of persistent slip bands (PSBs) at the free surface during low cycle fatigue, thereby extending crack initiation life. They have little effect, however, on bulk PSB formation. PSBs nucleated in the interior eventually impinge on the underside of the surface layer, producing localized cyclic shear strains in the film which lead to film rupture and rapid crack initiation. As a result, increases in fatigue life are modest. The present finite element model analysis shows that PSBs carrying plastic strains characteristic of fatigued f.c.c. metals produce stresses in an elastic surface layer which may exceed 1% of the surface film shear modulus. The detailed results are consistent with phenomenological data on surface damage accumulation during low cycle fatigue of ion-beam-modified nickel. Implications for the design of surface microalloys for inhibition of fatigue crack initiation are discussed.

The cyclic stress-strain behavior of polycrystalline Al-5wt.%Mg

Push-pull fatigue tests have been carried out on binary Al-5wt.%Mg specimens in the annealed and cold-worked conditions at constant plastic strain amplitudes between 4 × 10 −5 and 2 × 10 −2 to characterize the cyclic stress-strain response and the strain-fatigue life behaviour. The annealed material hardens substantially at all strain levels and, at saturation, exhibits a constant plateau stress of 270 MPa for plastic strain amplitudes between approximately 10 −4 and 2 × 10 −3. The high degree of cyclic hardening is related to the formation of a high density of fine dislocation loops. Intense slip bands are also observed to penetrate the bulk of the specimen. The strain-fatigue life plots of the annealed and cold-worked material reveal a two-slope behaviour. The high fatigue life exponent at low plastic strain amplitudes is thought to be due to rapid crack initiation in isolated surface slip bands.