Crack-like Voids Research Articles

Guided elastic wave propagation in elastic metamaterial plate with periodic array of interfacial crack-like voids

Abstract A special kind of elastic metamaterials with periodic unit-cells composed of two layers with internal crack-like voids situated at the interfaces between two neighboring sub-layers is studied. To investigate experimentally guided waves propagation in EMMs with arrays of voids, several specimens with periodic arrays of crack-like voids have been manufactured using additive manufacturing techniques. The elastic waves have been excited by a rectangular piezoelectric transducer bonded at the surface of the specimen, while the wave-fields on the surface of the EMM specimen are measured by a laser Doppler vibrometer. The dispersion characteristics are investigated using the matrix pencil method and band-gaps are observed experimentally.

Elastic Waves Excitation and Focusing by a Piezoelectric Transducer with Intermediate Layered Elastic Metamaterials with and without Periodic Arrays of Interfacial Voids.

Optimization of the structure of piezoelectric transducers such as the proper design of matching layers can increase maximum wave energy transmission to the host structure and transducer sensitivity. A novel configuration of an ultrasonic transducer, where elastic metamaterial insertion is introduced to provide bulk wave mode conversion and to increase wave energy transfer into a substrate, is proposed. Configurations of layered elastic metamaterials with crack-like voids are examined theoretically since they can provide wide band gaps and strong wave localization and trapping. The analysis shows that the proposed metamaterial-based matching layers can sufficiently change wave energy transmission from a piezoelectric active element for various frequency ranges (relatively low frequencies as well as higher ones). The proposed configuration can also be useful for advanced sensing with higher sensitivity in certain frequency ranges or for demultiplexing different kinds of elastic waves.

Design and Manufacturing of the Multi-Layered Metamaterial Plate with Interfacial Crack-like Voids and Experimental-Theoretical Study of the Guided Wave Propagation

A novel kind of acoustic metamaterials (AMMs) with unit cells composed of two layers made of dissimilar materials with a crack-like void situated at the interface between bars is considered. Recently, the authors showed numerically that this novel kind of AMMs can provide unidirectional propagation of guided waves. Several AMM specimens (the finite stack of periodic elastic layers with and without voids) have been manufactured using additive manufacturing techniques and regular gluing. The details of the manufacturing process are discussed. In the experiment, the elastic waves have been excited by a rectangular piezoelectric wafer active transducer bonded at the surface of the specimen. Vibrations of the opposite side of the AMM specimen are measured via a piezoelectric sensor. The band gaps are observed in the experiment and values of their width and location correlate with numerically predicted ones.

Void coalescence mechanism for combined tension and large amplitude cyclic shearing

Void coalescence at severe shear deformation has been studied intensively under monotonic loading conditions, and the sequence of micro-mechanisms that governs failure has been demonstrated to involve collapse, rotation, and elongation of existing voids. Under intense shearing, the voids are flattened, such that the void volume diminishes, whereafter the flattened crack-like voids rotate and elongate until interaction with neighboring micro-voids dominates the material response and coalescence sets in. Eventually, this leads to a complete loss of load carrying capacity. The severe shear loading, imposed at the far boundary, is in an early state of the deformation associated with significant stretching of parts of the void surface, while other parts remain practically un-deformed. A largely uneven distribution of the strain hardening, therefore, evolves along the void circumference and, thus, one cannot expect the void to return to its original shape in the case where the far-field loading is reversed. The present numerical work aims to investigate the evolution of micro-voids subject to constant tension and large amplitude cyclic shearing. The far-field loading, the void shape, and the void growth are monitored, and the calculations are pushed to coalescence and complete loss of load carrying capacity. The initially circular cylindrical voids are predicted to develop protrusions in the shearing plane with normal in the direction of the applied tensile load. These protrusions evolve during repeated cyclic shearing and spread towards neighboring voids - eventually being responsible for void coalescence.

Modeling of Processing-Induced Pore Morphology in an Additively-Manufactured Ti-6Al-4V Alloy.

A selective laser melting (SLM)-based, additively-manufactured Ti-6Al-4V alloy is prone to the accumulation of undesirable defects during layer-by-layer material build-up. Defects in the form of complex-shaped pores are one of the critical issues that need to be considered during the processing of this alloy. Depending on the process parameters, pores with concave or convex boundaries may occur. To exploit the full potential of additively-manufactured Ti-6Al-4V, the interdependency between the process parameters, pore morphology, and resultant mechanical properties, needs to be understood. By incorporating morphological details into numerical models for micromechanical analyses, an in-depth understanding of how these pores interact with the Ti-6Al-4V microstructure can be gained. However, available models for pore analysis lack a realistic description of both the Ti-6Al-4V grain microstructure, and the pore geometry. To overcome this, we propose a comprehensive approach for modeling and discretizing pores with complex geometry, situated in a polycrystalline microstructure. In this approach, the polycrystalline microstructure is modeled by means of Voronoi tessellations, and the complex pore geometry is approximated by strategically combining overlapping spheres of varied sizes. The proposed approach provides an elegant way to model the microstructure of SLM-processed Ti-6Al-4V containing pores or crack-like voids, and makes it possible to investigate the relationship between process parameters, pore morphology, and resultant mechanical properties in a finite-element-based simulation framework.

Study on degradation mechanism of perovskite solar cell and their recovering effects by introducing CH3NH3I layers

We demonstrate the degradation mechanism of CH3NH3PbI3 perovskite solar cells (PSCs) and their recovery effects by introduction of CH3NH3I layers. After degradation by exposure to air under a 1-sun condition for 18 h, the absorption of the CH3NH3PbI3 perovskite between 530 and 800 nm decreases, and thereby the energy band gap increases. X-ray diffraction (XRD) patterns indicate that moisture with sunlight corrodes the CH3NH3PbI3 perovskite into PbI2, which generates cracklike voids on the surface of illumination-degraded CH3NH3PbI3 perovskite thin films. As a result, the PSC photovoltaic performance is degraded and the power conversion efficiency (PCE) is reduced from 16.8 to 0.7%. However, the PCE is recovered to 7.6% by spin-coating a CH3NH3I layer into the illumination-degraded CH3NH3PbI3 perovskite thin films. The recovery of the absorption between 530 and 800 nm and the CH3NH3PbI3-perovskite-assigned peaks in the XRD patterns verifies that the illumination-induced PbI2 is recombined with spin-coated CH3NH3I and that the CH3NH3PbI3 perovskite structure is recrystallized.

Finite element analysis of oxidation induced metal depletion at oxide–metal interface

In many situations, chromium depletion and voiding below oxide/alloy interface accompany with the selective oxidation of Cr–contained alloys. At the same time, stresses are induced at the oxide/alloy interface, in the metal and in the oxide. The combination of depletion and stress generation initiates interfacial crack-like voids which eventually lead to spallation of the oxide scale. In this paper, a continuum thermodynamic model which accounts for stress-diffusion interaction in the oxidation of Cr–Fe alloys is carried out via a two-dimension finite element implementation. For the purpose of generating depletion areas, grain areas and grain boundary areas in the alloy are defined via random Voronoi tessellation. Periodic boundary condition is also employed. The model predicts that the depletion of chromium mostly occurs at the fast diffusion area (grain boundary), and large normal tensile stresses are generated at both metal-scale interface and alloy grain boundaries. Once the depletion-induced voids reaches a certain level at the interface, concentration of stress could induce further propagation of interfacial crack.

Creep Void Growth Simulation of Local Area in a Steam Turbine Rotor

In order to reduce maintenance cost of high temperature components in aged thermal power plants, it is necessary to improve accuracy of remaining life assessment methods. High temperature components such as steam turbine rotors are operated under creep loading condition where creep voids initiate and grow on grain boundaries. Development of a void growth prediction method is important for reliable maintenance of these components. Purposes of this study are to clarify void growth behavior of a turbine rotor material under creep loading condition, and to develop void growth simulation program that can predict damage extension process in the components under actual operating condition quantitatively. Creep tests have been conducted on a 1Cr-Mo-V forging material and creep damaged specimens were produced by interrupting the tests. From observation of the creep damaged specimen, spherical shape voids initiate and grow up to their length of 2μm on the grain boundary at initial stage of damage, and then these voids change their shape to crack-like to grow until their length reaches around 10μm. Finally, crack-like voids coalesce each other to form one micro crack along grain boundary. It can be concluded that void growth rate is controlled by diffusion and power law creep under constrained condition based on theoretical consideration of void growth mechanism. Through these discussions, a new void growth model was proposed by modifying conventional models. A void growth simulation program was developed by incorporating the void growth model into a personal computer. It was confirmed that void growth process under the creep condition in the experiment was well predicted by the simulation program. The simulation program was also applied to predict creep void growth behavior in an actual steam turbine rotor during operation.

Consolidation of a Cu–2.5 vol.% Al 2O 3 powder using high energy mechanical milling

In situ consolidation of a Cu–2.5 vol.% Al 2O 3 powder by high energy ball milling has been studied by examining changes of size, morphology, macrostructure and microstructure of the powder particles, lumps, and balls formed with increasing milling time under different conditions. This study clearly demonstrates that the consolidation of the Cu–2.5 vol.% Al 2O 3 powder by high energy ball milling is accomplished initially by coalescence of powder particles through cold welding which leads to formation of lumps, then by cold welding and re-shaping of the lumps into small balls, and finally by cold welding of small balls into larger balls. The maximum diameter of the large balls is comparable with the diameter of the milling balls used as a milling medium. The two types of defects in the consolidated lumps and balls in the large balls, concentrated voids and crack-like voids, show that the material undergoes a large amount of plastic flow during the process of consolidation. The consolidation of the Cu–2.5 vol.% Al 2O 3 powder can also been achieved by a combination of cryomilling and room temperature milling, but the consolidation process is slower due to increased hardness of the powder particles. The study also shows that the Al 2O 3 fine particles are incorporated into the Cu matrix forming a composite structure when the lumps are formed, and the large balls exhibit a nanostructure with grain sizes smaller than 100 nm.

Comparison of densification and distortion behaviors of W-Ni-Cu and W-Ni-Fe heavy alloys in liquid-phase sintering

This paper aims to correlate the densification and distortion behaviors of liquid-phase sintered 80W-14Ni-6Cu and 80W-14Ni-6Fe heavy alloys with the melting characteristics of the Ni-Cu and Ni-Fe matrices. Differential thermal analysis (DTA) of die-pressed compacts reveals that the melting range of the Ni-Cu matrix is extended from 1235°C to 1453°C by the in situ alloying between elemental Cu and Ni powders, whereas the melting of the Ni-Fe matrix is limited to a narrow temperature range between 1464°C and 1480°C. Dilatometry and furnace sintering tests show that densification due to liquid-phase sintering of 80W-14Ni-6Cu starts at 1287°C and proceeds at a low rate to 1450°C, where full densification without distortion is achieved. In contrast, densification due to liquid-phase sintering of 80W-14Ni-6Fe occurs at a very high rate above 1475°C, and full density can be obtained at 1500°C. For both alloy compacts, distortion is induced by prolonging the sintering time or elevating the sintering temperature after full densification. Crack-like voids develop in the 80W-14Ni-6Cu compact to accommodate the gravity-induced distortion, while spherical pores are dominantly formed in the 80W-14Ni-6Fe compact as a result of water vapor entrapment.

Net-shape forming of ZrO2-based ceramics and the effect of shaping process on superplastic deformation

In this study, a slip casting method was used for the production of tensile specimens for superplastic deformation from zirconia powders containing 3 and 8 mol% yttria. It was seen that the slip casting process allowed the production of complex, net-shape and economical production of a relatively small number of individual shapes. The factors affecting the casting characteristic of the slip system, namely, solid content, slip viscosity, dispersing agent concentration and moulding conditions were investigated and optimum values for the above were determined. The effect of shaping process on superplastic deformation was also investigated for slip-cast and die-pressed specimens. It was seen that superplastic ductility obtained from slip-cast specimen was higher than that of die-pressed one. The reason for ductility enhancement was due to the homogeneous dispersion of the powder and elimination of agglomerates in slip-cast specimen, compared to die-pressed specimen in which agglomeration caused non-homogeneous sintering leaving crack-like voids responsible for early fracture.

Processing and properties of an all-oxide composite with a porous matrix

Processing and mechanical properties of an all-oxide fiber composite with a porous matrix are presented here. The processing approach for an all-oxide composite was developed to be simple and involve one sintering process. The composite uses a porous matrix instead of riser coatings to deflect cracks from the fibers. A processing method involving recently developed methods for reshaping and forming saturated high-volume fraction (>50 vol%) particle bodies was used to form the composite. Good infiltration of the woven fiber tows was obtained. Sintering in a pure HCl gas atmosphere was used to produce a porous matrix without shrinkage during processing. The sintering process also produced coarsening which makes the microstructure stable against densification during use and thereby prevents forming cracklike voids and retains sufficient porosity for crack deflection. Measurements of interlaminar shear strength and strength of the composite show that composite produced by this processing method is comparable to previous all-oxide materials produced using the oxide fibers used here. The mechanical properties are rationalized in terms of the features on the fracture surfaces. Disintegration of the matrix to allow energy dissipation during fracture was apparent and correlates with the measurements of the fracture toughness of the material. Moderate notch insensitivity was demonstrated with a net section strength in the presence of a notch being 700% of the unnotched strength.

Adsorption of polyvinylpyrrolidone (PVP) and its effect on the consolidation of suspensions of nanocrystalline CeO2 particles

CeO2 particles with an average size of ∼9 nm were synthesized under hydrothermal conditions. The adsorption of polyvinylpyrrolidone (PVP) on to the particle surfaces was measured in aqueous suspensions in the pH range of ∼3.7 to 11.5. The amount of adsorbed PVP decreased significantly with increasing pH value. For suspensions prepared at a pH value of ∼3.7, complete adsorption occurred for ∼2.5 wt % of PVP added to the suspension. Further additions of PVP produced a gradual increase in the adsorption until a limiting value was reached when the total amount of PVP added to the suspension was 10 wt %. At this PVP concentration, ∼6 wt % of the PVP was adsorbed and ∼4 wt % remained free in solution. The effect of the adsorbed PVP on the microstructural homogeneity of films deposited by spin coating of suspensions was investigated. With no addition of PVP, crack-like voids were prevalent in the dried and sintered films. Crack-free films were obtained from suspensions containing 10 wt % of PVP. Higher PVP additions (25 wt %) produced an increase in the viscosity of the suspension but no observable change in the microstructural homogeneity of the films. The use of adsorbed polymers for steric stabilization coupled with data from the adsorption isotherms is shown to provide a rational approach to the deposition of homogeneous films from suspensions of nanocrystalline particles.

Effect of Matrix Grain Growth Kinetics on Composite Densification

The effect of matrix grain growth kinetics on the densification behavior of a particulate composite has been investigated. Large alumina inclusions were added to two types of zirconia matrices having different grain growth kinetics, i.e., Zr(8Y)O2 and Zr(3Y)O2. Despite the evolution of different grain sizes, both matrices reached an end‐point relative density of 0.92 and 0.94, respectively, even though they contained 0.25 volume fraction of inclusions (based on total solid volume). However, the morphology and spatial distribution of the residual pores were distinct. The Zr(8Y)O2 matrix, which developed very large grains, had a dual microstructure with porous regions surrounded by a connective, denser region, as previously reported for an alumina matrix containing zirconia inclusions. Conversely, the Zr(3Y)O2 matrix was fully dense and uniform, but the composite contained large cracklike voids at inclusion/matrix interfaces. The large opening displacement of these cracklike voids locally relaxed the constraint to matrix shrinkage imposed by the inclusion network.

Mechanical Properties and Failure Analysis of Alumina‐Glass Dental Composites

Strength measurements and fractography were used to investigate the failure of alumina‐glass dental composites containing 75 vol% alumina and 25 vol% glass. Alumina compacts were prepared by slip casting and sintering at 1100°C for 2 h. Dense composites were made by infiltrating partially sintered alumina with glass at 1150°C for 8 h. Young's modulus and the hardness of the composites were 270 GPa and 12 GPa, respectively. The mean strength (460 MPa) and fracture toughness (4.0 MPa·m1/2) of the composites were insensitive to the glass thermal expansion coefficient (αglass= 5.9 × 10−6 to 7.8 × 10−6°C−1). Typical flaws were pores and cracklike voids formed by poor particle packing and differential sintering near agglomerates of alumina in the composite. Crack deflection and crack bridging were observed in indentation cracks. Fracture toughness was single‐valued because the alumina particle size was small (∼3 μm). Alumina‐glass composites are promising new ceramics for dental crown and bridge applications, because their strength and fracture toughness are ∼2 times greater than those of current dental ceramics.

Generation of Cracklike Voids during Sintering of Al2O3‐10ZrO2 Ceramics and Their Prevention by Presintering with Low Pressure (2 MPa)

During sintering of Al2O3‐10ZrO2 ceramics, cracklike voids were formed due to the differential in densification between agglomerates and the matrix. The small external pressure (2 MPa) applied during presintering at 1200°C prevented the formation of these cracklike voids. The results are explained in relation to the microstructural development during the initial stage of sintering.

Liquid Precursor Infiltration Processing of Powder Compacts: I, Kinetic Studies and Microstructure Development

The kinetics of infiltrating a solution precursor into Si3N4 powder compacts were studied using either water or an aqueous solution of Zr‐nitrate and Y‐nitrate that formed a crystalline Zr(Y)O2 (3 mol% Y2O3) solid solution during pyrolysis. When the powder compact contained air, the infiltration involved two steps: (1) relatively rapid intrusion of liquid via flow due to capillary pressure and (2) diffusion of entrapped gas to the surface as its pressure became equal to the capillary pressure. The kinetics of both processes are described with different parabolic rate laws–Darcy's law and Fick's law, respectively. When the intruded precursor is converted to an inorganic during heat treatment, the void space is partially filled with pyrolyzed precursor without shrinkage of the Si3N4 powder. The kinetics of subsequent cycles depend on the permeability of the pyrolyzed precursor, which depends on its microstructural development during heat treatment subsequent to pyrolysis. Surface cracks can form within the powder compact during either precursor drying or pyrolysis; they can be avoided by strengthening the powder compact by forming small necks between touching particles via evaporation–condensation. Precursor molecules were found to concentrate near the surface as the precursor solvent was removed by drying. The movement of precursor molecules was prevented by gelling prior to drying, viz., by soaking the infiltrated bodies in an aqueous NH4OH solution. Microstructures developed during cyclic precursor infiltration and pyrolysis were characterized to show that cracklike voids are produced within the pyrolyzed precursor due to its large volume change during pyrolysis and densification; the size distribution of the cracklike voids is proportional to the size distribution of the voids within the initial powder compact.

Processing of damage-tolerant, oxidation-resistant ceramic matrix composites by a precursor infiltration and pyrolysis method

Damage-tolerant, continuous fiber ceramic matrix composites have been produced by an inexpensive method. According to this method, the space between the fibers is filled with a powder. The powder particles are heat treated to form a porous framework without shrinkage, which is then strengthened with an inorganic synthesized from a precursor. High particle packing densities can be achieved within the fiber preform provided that the particle-to-fiber diameter ratio is small. Filling the interstices with a powder increased the composite density and also limits the size of the crack-like voids within the matrix. In this review we describe the mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. We demonstrate that the packing density of particles around fibers is highest when the particle-to-fiber diameter ratio is small. The kinetics and mechanical behavior of composite systems is summarized to demonstrate the requirements of damage-tolerant properties. An all-oxide ceramic matrix composite produced by this method is discussed.

Microstructure evolution and mechanical strength of silicon nitride ceramics

Microstructure evolution was studied in silicon nitride ceramics by a novel characterization method, and its relevance to the strength was discussed. The characterization method involves an immersion liquid for making green and partially sintered bodies transparent, and a subsequent direct optical microscopic examination. Granules for compaction process were prepared with the spray-drying process and were found to contain pores or deep dimples. Green bodies formed by CIP with these granules contain regularly arrayed pores at the center of granules and also crack-like voids at the boundaries of granules. These pores were preserved in the sintering process and resulted in large pores in the sintered body. They behave as fracture origin in ceramics and reduce the fracture strength. The Weibull modulus was high due to the presence of uniformly distributed pores.

Stress Migration in Aluminum Lines in Integrated Circuits

ABSTRACTThis paper reports on two types of voiding which occur in Al lines in integrated circuits.The first type of voiding occurs in high-temperature processes during chip fabrication, in which the thermal stress is expected to be compressive. Ultrahigh voltage electron microscopy (UHVEM) observations of the voids and voiding process show interrelations between the evolution and morphology of voids and grain-boundaries.The other type of voiding occurs at low temperatures where highly tensile thermal stress develops in Al lines. This type of voiding occurs in accelerated life tests and actual operation. The results of accelerated life tests of intentionally oxygen-contaminated samples are described. Voids observed in low-oxygen samples are mostly large-angle-wedge-shaped and induce essentially no open failures in the early-failure period. In high-oxygen samples, crack-like voids occur, and parallel-slit-like voids also occur. These narrow voids result in high open-failure rates in the early-failure period.