Search

Silicon (Si) is one of the most abundant elements on Earth, and it is the most widely used semiconductor. Despite extensive study, some properties of Si, such as its behaviour under dynamic compression, remain elusive. A detailed understanding of Si deformation is crucial for various fields, ranging from planetary science to materials design. Simulations suggest that in Si the shear stress generated during shock compression is released via a high-pressure phase transition, challenging the classical picture of relaxation via defect-mediated plasticity. However, direct evidence supporting either deformation mechanism remains elusive. Here, we use sub-picosecond, highly-monochromatic x-ray diffraction to study (100)-oriented single-crystal Si under laser-driven shock compression. We provide the first unambiguous, time-resolved picture of Si deformation at ultra-high strain rates, demonstrating the predicted shear release via phase transition. Our results resolve the longstanding controversy on silicon deformation and provide direct proof of strain rate-dependent deformation mechanisms in a non-metallic system. Understanding the how silicon deforms under pressure is important for several fields, including planetary science and materials design. Laser-driven shock compression experiments now confirm that shear stress generated during compression is released via a high-pressure phase transition. [ABSTRACT FROM AUTHOR]

Deformation and molecular chain evolution under combined electrical and mechanical loads are characterized for poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer films with 55 and 68 mol % VDF content [termed P(VDF-TrFE) 55/45 and P(VDF-TrFE) 68/32, respectively]. In both copolymer films, the direction of the applied electric field with respect to the stress field direction has significant influence on the stress-strain behavior and the evolution of molecular chain structure. When the electric field is applied parallel to the tension direction, the molecular chains have the tendency to convolute, which prevents the films from acquiring a more oriented fibrillar structure compared with film structures under pure mechanical tensile stress. Consequently, the yield strength is enhanced while the fracture strain is reduced. When the electric field is applied along the film thickness direction, it causes randomly oriented dipoles in the films to orient preferentially along the direction of the external electric field. The combination of the mechanical tensile load and the electric field in the film thickness direction contributes to a well aligned and closely packed molecular chain structure, and consequently both UTS and fracture strain are increased. The VDF content dependence on the stress-strain behavior is discussed. [ABSTRACT FROM AUTHOR]

This study explores the evolution of nanoscale islands and wire structures during deposition and surface ripening. Ultraviolet photoelectron emission microscopy has been employed to study the real time growth process of individual erbium silicide nanostructures on Si(001) surfaces at temperatures up to 1050°C. During the initial island formation process compact islands form and some undergo a shape transition to elongated islands oriented along the 〈110〉 directions of the Si substrate. The initial island formation is driven by the surface and interface energies of the silicide/ Si structure. The widths of the growing islands remain essentially constant while the lengths increase. The observed elongated islands are ∼150 nm wide, which is larger than the width of prior reported erbium silicide nanowire structures. We propose that the ∼150 nm elongated islands are partially relaxed, possibly through the formation of misfit dislocations. The results indicate a temperature regime where island growth is mainly governed by surface diffusion of the deposited Er adatoms and a higher temperature regime where Ostwald ripening contributes to the island morphology. [ABSTRACT FROM AUTHOR]

Tool path ripple error (TPR_error) is one of the main reasons due to the medium-high spatial frequency error on the surface of aspheric optics. The purpose of this paper is to analyze the effect of the tool step size to the TPR_error in sub-aperture deterministic polishing (SDP) and study a method which can optimize the tool step size to restrain this error. Three groups of simulation experiments were conducted using three different tool influence functions to simulate the uniform removal of the material. As the TPR_error is influenced by three factors, which are full width at half maximum (FWHM) of tool influence function (TIF), tool step size, and depth of material removed, each group of the experiments was conducted under the fixed TIF and depth of material removed. It turns out that both peak-to-valley (PV) and root-mean-square (RMS) values of the TPR_error become larger with the increase of the tool step size, and the variation tendency likes a reversed 'L' shape curve. And, the method adopted in the simulation was further validated by the experiment. Therefore, the tool step size at the inflection point would be optimal to restrain the TPR_error together with saving the polishing time to a certain extent. This method could be used to determine the best-suited tool step size in SDP whose typical TIF is a Gaussian or Gaussian-like shape. [ABSTRACT FROM AUTHOR]

In this paper, a nonlocal phase field method is presented to solve anisotropic diffusion-driven morphological evolution and migration of void defects in finite metallic film interconnects by utilizing a nonlocal phase field model considering a small scale effect. The nonlocal elastic theory is used to describe a small scale effect on the morphological evolution and migration of the void. In example calculations, the effects of the stress field, the electric field, and the anisotropic diffusion characteristic on the evolution of void defects in finite metallic film interconnects are described and discussed. The result in comparison with literature shows that the small scale effect based on nonlocal elastic model induces the migration diffusion of the crack tip to decrease. [ABSTRACT FROM AUTHOR]

As we know, the oxide film on the surface of Al-Li alloys has to be cleaned before welding in order to achieve welds without pores. However, doing so is difficult and may result in undesirable pollution with chemical cleaning. In this study as a precondition of laser welding the plate of Al-Li alloy was not treated in any way before welding. A CO2; laser system was employed in the study on porosity control of 1420 Al- Li alloy by dual-beam laser welding. The experimental results show that the influence on the right surface oxide film to the porosity in the weld is bigger than that from the backside when the laser welding is in the form of a beam on the surface of the plate. However, the sensibility of porosity from the back-side oxide film has a distinct connection with the formation of an inverse weld when butt-joint laser welding is carried out. Optimized laser welding techniques have been achieved from experiments in order to decrease the porosity in the weld and increase weld quality. The tensile strength of the laser welding joint was also tested in the experiment. The data indicate that the average value of tensile strength about the joints can be up to 336.1 MPa, which is equal to 87.5% of the value of the substrate alloy. [ABSTRACT FROM AUTHOR]

Abstract: This work mainly focuses on the development of germanium-based thin-film piezoresistive strain gauges on thin NiTi and stainless steel cylinders. These microgauges enable the real-time measurement of the cylinder’s strain distribution from which its deflection status can be deduced, based on an analytical mechanical model of thin cantilever beams. The adaptation of thin cylinders to typical micromachining equipment requires several adjustments in preparation works. The fabrication process of germanium microgauges also has to be adjusted according to the particularities of metal substrates. A number of polycrystalline germanium strain microgauges of various dimensions are implemented on thin cylinders. The results of the fabrication process are presented. The existing constraints of the current process and the potential improvement measures are then discussed. [Copyright &y& Elsevier]

This paper proposes a hybrid approach to determine the fracture resistance for mode I and mixed-mode I and II fracture specimens, combining both numerically computed and experimentally measured load ( P) versus load-line displacement (LLD or Δ) relationships for metallic fracture specimens. The hybrid approach predicates on the same principle as the conventional, multiple-specimen experimental method in determining the energy release rate. The hybrid method computes the P-Δ curves from multiple finite element (FE) models, each with a different crack depth. The experimental procedure measures the P-Δ curve from a standard fracture specimen with a growing crack. The intersections between the experimental P-Δ curve and the numerical P-Δ curves from multiple FE models dictate the LLD levels to compute the strain energy ( U) using the area under the numerical P-Δ curves. This method provides accurate estimates of the J resistance data for both SE(B) specimen under mode I loading and single-edge notched specimens under mixed-mode I and II loading. [ABSTRACT FROM AUTHOR]

Abstract: The transient thermal stress problem of a semi-infinite plate containing an infinite row of periodically distributed cracks normal to its edge is investigated in this paper. The elastic medium is assumed to be cooled suddenly on the crack-containing edge. By the superposition principle, the formulation leads to a mixed boundary value problem, with the negating tractions arisen from the thermal stresses for a crack-free semi-infinite plate. The resulting singular integral equation is solved numerically. The effects on the stress intensity factors due to the presence of periodically distributed cracks in a semi-infinite plate are illustrated. For both the edge crack and the embedded crack arrays, the stress intensity factors increase, due to the reduction of the shielding effect, as the stacking cracks are more separated. For the case of embedded crack array, one has the further conclusion that the stress intensity factors decline as the crack array shifts from the plate edge. [Copyright &y& Elsevier]

Micromechanics analysis for metal matrix composites (MMC) reinforced by particles of large volume fraction is pursued in this paper. Particle reinforced Al2O3/Al MMC is made under a pressure-less filtration technique. Samples are tested in uniaxial tension, compression and three-point bending. The composite is modeled as identical elastic spheres linked by columns of elastic perfectly plastic matrix. Damage evolves in three mechanisms: the particle/matrix interface may debond if the interfacial energy release rate reaches a critical value; the ductile matrix may fail if voids expand to a certain degree; and the particle may break if the largest principal stress exceeds its strength. An averaging scheme is used to derive the macroscopic constitutive relation. Two random variables are adopted for the fluctuations of local stresses in particles and along particle/matrix interfaces. The predicted stress–strain curves, crack growth paths, and the composition of failure surfaces compare favorably with the testing data. [Copyright &y& Elsevier]

Testing data indicated that T stress significantly alters R-curves of ferroelectrics. The model of stress-induced polarization switching is adopted to evaluate the fracture toughness of ferroelectrics under K field and T stress. Analytical solution is obtained to estimate the steady state fracture resistance of mono-domain ferroelectrics. The result is generalized to multi-domain ferroelectric ceramics via Reuss approximation, which enables us to explain quantitatively R-curves under two testing configurations. [Copyright &y& Elsevier]

ABSTRACT Cracks often initiate from the mechanical joints which are widely used in structural components. It has been reported that cracks in mechanical joints are under mixed-mode condition and there is a critical angle at which mode I stress intensity factor becomes maximum. The crack propagates in an arbitrary direction and the prediction of fatigue crack growth path is needed to provide against crack propagation and examine safety. In this study, mixed-mode fatigue crack growth tests are performed for horizontal and critical inclined cracks in mechanical joints. Fatigue crack growth paths are predicted using a weight function approach and maximum tangential stress criterion. [ABSTRACT FROM AUTHOR]

The effects of irregular ligaments among a row of infinite circular holes are analyzed. For rigorous calculations of simplicity, an efficient alternating method is used to study interactions among multiple holes with irregular distributions. Numerical results for stress concentration factors are presented for various patterns and hole spacing. Based on these analyses, acceptance crierion irregularities in a row of circular holes is also evaluated. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical study of a gas-particle flow in three inclined rectangular jets in crossflow. The predicted gas phase velocities and particle phase velocities are validated against previously reported experimental data. Two turbulence models, the standard k–ɛ model and Shear Stress Transfer (SST) model, are used to model the gas phase turbulence. This work shows that both models provide acceptable predictions of the gas flow and mixing generated by the three jets. Neither model could accurately reproduce the jet core and the flow near bottom wall. The particle phase in this flow comprises a large number of small particles. Thus particles follow the gas phase flow closely and any errors in the turbulence model and gas flow predictions are passed on to the particle phase simulation. This paper also includes a literature review on rectangular jets in crossflow and gas-particle laden jets in crossflow. [Copyright &y& Elsevier]

Abstract: A new experimental approach for determining the sheet metal flow stress curve for diffuse necking deformation through the hydraulic bulge expansion (HBE) test is introduced. For this approach, a flat sheet of metal is bulged under the load of hydraulic pressure within the confines of a rigid binder/die tool set of circular die opening. The hydraulic pressure and volume flow rate of the pressurized water entering the die cavity are recorded in real time. Operating under the assumption of thin-walled pressure vessel behavior, the real time data are used to compute the flow stress curve for the material type considered. The novel aspect of the proposed technique is the use of water volume measurement for determining the radius of curvature of the expanding dome. An example flow stress curve for 1.0mm thick DDQ cold rolled sheet steel is shown to be in good agreement with the flow stress curve obtained through the standard uniaxial tension test. One key advantage of this proposed method is that the analyst may determine the diffuse necking portion of the flow stress curve without employing cumbersome techniques associated with uniaxial tensile test methods. Two disadvantages of the proposed method are the uncertainty associated with having to assume a yield surface, and the error associated with having to assume membrane behavior in the work piece. The introduction of this new method will increase researchers’ access to practical methods for obtaining sheet metal flow stress curves. [Copyright &y& Elsevier]

Fatigue specimens of A508-3 steel were irradiated in the swimming-pool test reactor in China Institute of Atomic Energy, the fluence was 3 × 10 19 n/cm 2 at 300 °C, then low-cycle fatigue tests were carried out at ambient temperature, with the fatigue strain range is 0.32–1.8%. The results indicate that, irradiated A508-3 specimens exhibit cyclic softening and instability behavior during the test, and the cyclic softening rate increased with strain range increased; fatigue life decreased from 1.7 × 10 5 to about 5 × 10 2 , as the strain range increased from 0.32% to 1.8%, the fatigue life of A508-3 steel increased after the neutron irradiation; fatigue fracture initiated at the surface of specimen, and more individual cracks formed on the specimens of higher strain range compared with the specimens of lower strain range. [ABSTRACT FROM AUTHOR]

Recent molecular dynamics simulations of dynamic crack propagation have shown that there is a finite crack opening for a shear crack propagating at a sub-Rayleigh speed, but the crack opening becomes significantly smaller once the crack tip velocity exceeds the shear wave speed. To understand this difference between the crack opening for sub-Rayleigh and intersonic shear cracks, we develop in this paper a finite deformation continuum theory incorporating the linear harmonic potential to describe the deformation of a crack in a solid with triangular lattice structure. Using the asymptotic method developed by Knowles [Eng. Fract. Mech. 15 (1981) 469], we show that even after the geometric nonlinearity of finite deformation is accounted for, the intersonic shear cracks have a vanishing crack opening displacement. [Copyright &y& Elsevier]