Влияние высокотемпературного отжига на сопротивление высокоскоростному деформированию и разрушению тантала при температуре 20 и 500-=SUP=-o-=/SUP=-C

Two series of shock-wave experiments have been conducted in order to measure the Hugoniot elastic limit and determine the strain rate dependence of critical fracture stress for tantalum experiencing spall fracture. Tantalum specimens have been preannealed in vacuum at 1000°C. The evolution of elastoplastic compression shock waves at room and elevated up to 500°C temperatures has been presented from complete wave profiles recorded by a VISAR laser Doppler velocimeter. The spall strength dependence on the strain rate during the expansion of the material in a rarefaction wave has been determined.

Modeling of Mechanical Behavior of Ceramic Nanocomposites

Deformation and damage occurring at the meso-scale level in structured representative volumes (RVE) of modern nanocomposites in wide loading conditions were simulated. The computational models of a structured RVE of ceramic nanocomposites were developed using the data of structure researches on meso-, micro -, and nanoscale levels. The critical fracture stress on meso-scale level depends not only on relative volumes of voids and inclusions, but also on the parameters of inclusion clusters. The critical fracture stress at the meso-scale level depends not only on relative volumes of voids and strengthened phases, but also on sizes of corresponding structure elements. In the studied ceramic composites the critical failure stress is changed non-monotonically with growth of the volume concentration of strengthening phase particles. At identical porosity, concentration of nanovoids in the vicinity of grain boundaries causes the decrease in the shear strength of nanostructured and ultrafine-grained ceramics. It is revealed that the occurrence of bimodal distributions of the local particle velocity at the meso-scale level precedes the nucleation of microcracks. At mesoscale level of ceramic nanocomposites the pressure and particle velocity distribution don’t display a resonance behavior under submicrosecond single shock pulse loading or repeated pulse loadings.

Numerical Model of Microstructure and Fracture of Coated Aluminum Alloys: A Novel Design Approach

Al/Si alloys are considered to be one of the most promising light weight alloys that can be used extensively in aerospace and automotive industry except for the poor tribological behaviour. However, with advancement and precision of the surface coating depositing techniques, new coating design which significantly enhances the tribological properties of the light weight alloys becomes attainable. In this paper, an innovative coating design is presented and thoroughly analyzed using finite elements method. The proposed model consists of Al/Si 319 as a matrix within which the geometrically defined hard Si particles are dispersed on the surface, and a hard coating layer then deposited in between the Si particles so that the lateral movement of the Si particles is constrained. ABAQUS is utilized to model and address the effects of different parameters, such as coating material, the hard coating thickness, and geometrical shape of the Si particles on the fracture and deboning of the entire structure. Two Si particles shapes are studied: circular and elliptical. Three coating materials are investigated: DLC, CrN and Al2O3. Besides, four coating thicknesses of 4 µm , 8µm, 15µm and 20µm are tested. It is found out that there is no single significant parameter which affects the fracture and deboning of Si particles, yet it is the combination of different parameters. The Si particle geometry plays a major role in determining the critical fracture stress with a circular shape outperforms the elliptical shape. The combination the circular Si particles and the CrN as coating material gives the highest critical fracture stress. Finally, DLC does not perform well with the circular Si Particle and it show the highest possible fracture stress with elliptical Si particle

Evaluation of Critical Fracture Stress in Low Alloy Steels by Finite Element Analysis of Small Punch Test

The critical fracture stresses (σ* f(sp)) in various low alloy steels for a reactor pressure vessel(RPV) were evaluated by a small punch test (SP test) and a finite element analysis (FEA) in the cleavage temperature region(-150~-196 °C). The load-displacement curves and distances from the center to the fracture surface (Df) of the FEA results are in good agreement with the experimental results. The maximum principal stresses (SP fracture stresses, σf(SP)) were determined from the FE analysis, when the maximum load was applied to the SP test. The SP critical fracture stress, σ* f(sp) in various reactor pressure vessel (RPV) steels was found to have a linear relationship with the values obtained from the precracked specimens (σ* f(PCVN)). The σ* f(sp) shows a lower value than σ* f(PCVN) because the SP specimen had a lower triaxial stress condition. However, this result indicates that a small punch test could be a useful method to evaluate the cleavage fracture behavior of low alloy steels.

Fragmentation of Kidney Stone by Shock Wave-Experimental Approach

To understand fundamentals of kidney stone™s fragmentation by shock wave impingement, experimental approach was carried out. Air gun system was used to transfer a well-controlled stress wave into a thin disk specimen. The specimen was fabricated with plaster and its dimensions are 10 mm in diameter and 1.2 mm in thickness. The obtained results are summarized as follows: As the stress pulse duration is decreased from 12 microseconds to 5 microseconds, the critical stress for fracture increased tremendously. The critical fracture stress for a specimen with a small hole in the specimen center is about one fifth of that for the plain specimen. The fracture takes place along the diameter perpendicular to the stress wave incident direction. Numerical stress analysis suggests that the principal stress criterion can explain the fracture mode.

Numerical Modeling of Fracture Toughness in RPV Steel Containing Segregated Zones

Segregation zones, called ghost lines, may be present near the inner side of PWR vessel issued from plain ingots of A508 class 3 steel. They are enriched in alloying elements and impurities and may have a significant influence on mechanical properties, in particular on fracture toughness. Toughness values of a compact tension specimen containing segregation zones have been evaluated using a bimaterial model with a 3 D finite element simulation. Two fracture criteria have been tested and the results compared with the French database of RPV steel toughness values. The first model assumes that fracture occurs when the maximum tensile stress ahead of the crack tip reaches a critical value over a critical distance. The second model, which is statistical, is based on Beremin’s theory. The results obtained with both models are compared. It is shown that the critical fracture stress model reproduces with a good accuracy the lower bound of toughness values which were determined in RPV steel. The Beremin model gives an estimate of the scatter in fracture toughness measurements. This model takes also into account a size effect related to the ghost line density present along the crack front.