Quality Control and Thermal Shock Damage Characterization of High-Temperature Ceramics by Ultrasonic Pulse Velocity Testing

Ultrasonic pulse velocity testing was carried out to perform nondestructive quality control of refractory plates used as substrates in fast firing of porcelain whiteware. Two cordierite-mullite refractory compositions characterized by different microstructure morphologies and crack propagation behavior were investigated after a number of industrial thermal cycles. Ultrasonic velocity measurements were used to determine the presence of internal voids/cracks in the samples, originating from the manufacturing procedure. A brief discussion about the correlation between microstructure, crack propagation behavior, and thermal shock resistance is presented. Empirical models were developed to predict the service life of refractory plates from measured values of ultrasonic velocities on as-received samples.     

基于近场动力学方法的含缺陷板Ⅰ型应力强度因子计算

针对含多缺陷板脆性断裂问题,引入非局部近场动力学理论,结合J积分计算含缺陷板Ⅰ型应力强度因子。通过含Ⅰ型单裂纹脆性板以及系列含等长双裂纹板的应力强度因子计算,验证该方法的可行性和计算精度。进一步应用于含不等长双裂纹、含孔及孔边裂纹脆性板的应力强度因子计算,验证了该方法对于计算含复杂缺陷板Ⅰ型应力强度因子的适用性,并分析了裂纹位置和长度及孔径等多缺陷板裂尖应力强度因子的影响。     

Crack Patterns in Thin Polymer Layers

Summary: During the solidification of thin polymer layers different crack patterns can occur. There are several mechanisms of the development of regular crack defects and layer fractures. In case of self‐organization caused by Marangoni instability at the fluid layer surface the substrate can be periodically uncovered by spreading motions when dewetting hinders a back flow from the higher spots of the layer. Another type of crack patterns is generated from shrinkage processes and stress differences in the drying layer. Mostly these patterns are characterized by intersecting straight cracks. In this paper some examples of unusual shrinkage‐crack patterns in polymer layers are presented. Their propagation is independent on surface flow and surface deformations caused by the Marangoni effect, although the strength of polymer layers is impaired by the interfacial instability. Especially at layer edges or spots with thickness differences one can observe periodic wavy or circularly bend shrinkage‐crack structures. As a third type ramified surface defects are studied in thin layers. Often they only propagate at the layer surface.

Wavy shrinkage‐cracks in a PMMA layer with longish surface elevations.     

Ultrasonic Guided Waves Scattering Effects From Defects in Adhesively Bonded Lap Joints Using Pitch and Catch and Pulse-Echo Techniques

In this article a method to evaluate defect dimensions in adhesively bonded lap joints based on the measurement of scattering effects of ultrasonic guided waves is presented. A simplified theoretical model is proposed which was initially tested in plates with through holes. The experimental results obtained using both pitch-and-catch and pulse-echo techniques for 500 kHz and 1 MHz frequencies confirm the validity of this model. To evaluate the lap joint defects, a set of samples with artificial defects were manufactured and the form and dimensions were confirmed using C-scan ultrasonic images. With the same methodology used in through-hole analysis, scattering effects of defects were measured. The results obtained with the pitch-and-catch technique with 1 MHz transducers allow us to say that an estimate of defect dimensions could be done by using the proposed model with reasonable accuracy and according with the predictions.     

粉煤灰的超细粉磨及其对水泥性能的 影响实验研究

本试验选用了内蒙古鄂尔多斯电力集团棋盘井电厂的粗粉煤灰,其活性指数为67.1%,达不到粉煤灰用于作水泥混合材料的国家标准(大于70%)。采用机械活化的措施——超细粉磨,将粉煤灰磨细到不同的细度和比表面积,这时粉煤灰的活性指数不仅合格,而且当将它掺入到P·II52.5等级的纯硅酸盐水泥中时,即使掺入量高达40%~50%,仍可得到P·F42.5等级或42.5R等级的粉煤灰硅酸盐水泥。这为扩大粉煤灰的利用提供了有效的措施。     

Water presence detection in a concrete crack using smart aggregates

Liquid migrating into existing concrete cracks is a serious problem for the reliability of concrete structures and can sometimes induce full concrete structural failures. In this paper, the authors present recent research on water presence detection in concrete cracks using piezoceramic-based smart aggregate (SA) transducers. The active sensing approach, in which one piezoceramic transducer is used to generate stress waves and others are used to detect the stress wave responses, is adopted in this research. Cracks formed in concrete structures act as stress reliefs, which attenuate the energy of the signals received by the SAs. In case of a crack being filled with liquid, which changes the wave impedance, the piezoceramic transducers will report higher received energy levels. A wavelet packet-based approach is developed to provide calculated energy values of the received signal. These different values can help detect the liquid presence in a concrete crack. A concrete beam specimen with three embedded SAs was fabricated and tested. Experimental results verified that the SA-based active sensing approach can detect a concrete crack and further detect the liquid presence in the concrete crack.     

Spherical Indentation and Fracture of Glass Plates

Spherical indentation of thick and thin glass plates was investigated numerically and experimentally. The energy release rate at the tip of a cone crack was calculated by using finite element techniques and used to investigate the applicability in thick plates of Roesler's law relating the cone crack radius to the indentation load. Indentations of thin glass specimens resting on different substrates were also studied numerically and experimentally. The stresses in the thin specimens were calculated and correlated with the observed failures. On the basis of these results, a crack initiation mechanism map was developed for glass specimens on different substrates.     

Failure Modes in Ceramic-Based Layer Structures: A Basis for Materials Design of Dental Crowns

A research program on failure modes induced by spherical indenters in brittle layer structures bonded to polymeric substrates, in simulation of occlusal function in all-ceramic dental crowns, is surveyed. Tests are made on model flat and curved layers bonded onto a dentin-like polymer base, in bilayer (ceramic/polymer) and trilayer (ceramic/ceramic/polymer) configurations. All-transparent systems using glass as a porcelain-like outer or veneer layer and sapphire as a stiff and strong core support layer enable in situ observation of the entire evolution of fracture modes in the brittle layers, from initiation through to failure. With the fracture modes identified, tests are readily extended to systems with opaque polycrystalline dental core ceramics, notably alumina and zirconia. A variety of principal failure modes is identified: outer and inner cone cracks developing in the near-contact region at the top surface; radial cracks developing at the bottom surface along the loading axis; margin cracks from the edges of dome-like structures. All of these modes are exacerbated in cyclic loading by time-cumulative slow crack growth, but inner cones are subject to especially severe mechanical fatigue from hydraulic pumping of water into the crack fissures. Conditions under which each mode may be expected to dominate, particularly in relation to geometrical variables (layer thickness, contact radius) and relative material properties, are outlined. Clinical issues such as crown geometry, overload versus fatigue failure, role of residual stresses in fabrication, etc. are addressed.     

Influence of shear stress state on dynamic fracture of a glass ceramic Macor

The effect of shear stress on the dynamic failure of ceramic materials is not sufficiently investigated in the published literature. With the use of a bespoke split Hopkinson pressure bar, this paper presents an effort to investigate the dynamic shear compressive response of Macor, a model ceramic material with zero porosity and light weight characteristics. A cone specimen and cylindrical specimens with varying inclined angles are used to introduce the shear stress to the Macor ceramic. The dynamic failure initiation and crack propagation are monitored by the high speed photography and Digital Image Correlation techniques. It is found that the equivalent stress of Macor at the initiation of failure decreases nonlinearly with the increase of shear stress. The high speed images show that the crack originates from the minimum cross-section of the cone specimen and the obtuse angle corner of the inclined cylindrical specimens. The cracks propagate parallel to the inclined plane instead of the axial loading direction. The fractographic analysis shows the compacted zone in the shear fracture surfaces of the cone specimen and the inclined cylindrical specimens. This indicates a significant role of shear loading in the dynamic failure process of Macor.     

Green Route for the Synthesis of Alanine-based Poly(amide-imide) Nanocomposites Reinforced with the Modified ZnO by Poly(vinyl alcohol) as a Biocompatible Coupling Agent

The objective of this article was fabrication and characterization of novel optically active poly(amide-imide)/zinc oxide nanocomposites with different modified ZnO nanoparticle contents under ultrasonic irradiation technique. For better dispersion of ZnO nanoparticles in a polymer matrix, their surface was modified with poly(vinyl alcohol) as a coupling agent. Effects of poly(vinyl alcohol) modifier on dispersity of nanoparticles, morphological structures, and thermal stability of the obtained nanoparticles were studied by several techniques. According to the transmission electron microscopy images of the nanocomposite, the average diameter of particles was around 7–15 nm in the poly(amide-imide).     

多裂纹压力容器剩余疲劳寿命计算方法研究

在考虑容器局部互扰裂纹（短裂纹群）对主裂纹（主导裂纹）扩展速率的影响后,精确计算了容器剩余疲劳寿命。通过实例验算,该方法适合工程要求。     

Characterization of Ballistic Impact Damage in Wired Glass

The ballistic impact behavior of wired glass has been studied. Three types of commercially available samples, with thin and thick wire mesh, were impacted with steel, glass, and tungsten carbide spheres of various sizes using a gas gun. Impact energies varied from 0.5 to 40 J. A system of cracks was found to develop that comprised primarily a major cone crack. At higher energies, additional radial and lateral cracking developed along with increased damage to the front face of the sample. The extent of damage was assessed in terms of basal cone crack area, number of cracks, mass loss, and fracture load of impacted samples. It was found that damage, at a given impact energy, increased with increasing projectile diameter. Moreover, 10 mm diameter glass spheres caused more damage than smaller (3–4.74 mm diameter) steel spheres. No significant differences were observed with regard to the ballistic behavior of the thin- and thick-wired glass types.     

A modified indentor technique for generating cone cracks in diamond and new results obtained from the technique

The traditional Hertzian method of generating cone cracks in diamond by using a spherical diamond indentor is costly in terms of preparation time of the indentor and the fact that very few indents can be made before the indentor fails. A new technique of indentor strength testing was developed which utilises a polycrystalline diamond (PCD) tip shaped into a flattened cone of included angle 120° with a flat tip of diameter 90 μm. To reduce the stress intensifying-effect of asperities on the PCD tip, a 50 μm thick, 316 L annealed stainless steel shim was inserted between the indentor tip and the diamond sample. Atomic force microscopy and micro Raman spectroscopy of cone cracks on synthetic 1b {100} polished diamond surfaces have provided additional information on the deformation and fracture mechanisms of diamond. Deformation of material near the inner edge of the ring crack appears to be larger due to friction along the crack interface preventing complete relaxation of the indented cone. Depressions within the ring crack are believed to be evidence for the microplasticity of diamond at room temperature without associated fracture.     

Evaluation of the sensitivity of ultrasonic detection of disbonds in graphite/epoxy-to-metal joints

A study was conducted to evaluate, improve and optimize ultrasonic techniques for the detection of disbonds in composite laminate-to-metal joints. Four graphite/epoxy laminates joined to metal plates were investigated using matched pairs of compression and shear wave ultrasonic transducers of three frequencies (1,2.25, and MHz). Pulse-echo and through transmission modes were used and A-scans, C-scans and frequency spectra were analysed. It was found that the higher frequency (5 MHz) compression wave transducer operated in the through transmission mode gave the best results in most cases. Shear wave transducers were found to be cumbersome to use and unsuitable for the cases studied.     

裂纹形态对丁腈橡胶力学性能和耐疲劳性能的影响

丁腈橡胶为石化行业应用最为广泛的一类橡胶材料。在受到周期应力的作用下，丁腈橡胶内已存在的裂纹会不断扩展而最终导致橡胶断裂。本文考察了预制缺陷的种类及大小对丁腈橡胶疲劳过程中裂纹扩展情况的影响，并对断面形貌进行了观察。结果表明，在圆形、纵向和横向三种缺陷中，纵向缺陷对丁腈橡胶的力学性能和耐疲劳性能影响最小；对圆形缺陷来讲，随孔直径的增大，丁腈橡胶的耐疲劳性能和力学性能逐渐下降。     

A lifetime prediction method based on Cumulative Flaw Length Theory

Ultrasonic pulse velocity testing (UPVT) was carried out to perform non-destructive quality control of refractory plates. Used in conjunction with fracture mechanics, ultrasonic velocity measurements have proved a powerful technique for detecting, positioning and sizing internal voids and cracks in the samples, originated from the manufacturing process. Two cordierite-mullite refractory compositions exhibiting different microstructure and crack propagation behaviour were characterized through their lifetime during which they were subjected to thermal shock loading. In this paper, a new statistical method is proposed which allows to estimate the lifetime when the stress state that will be applied in service (loading) and the scattering of the ultrasonic velocity data in the as-received state are known. Since this lifetime prediction method is based on a non-destructive technique, it could be implemented into a code in an automatic quality control device for continuous lifetime estimation. The correlation between crack propagation behaviour and thermal shock resistance is discussed and semi-empirical models were developed to predict the service life of refractory plates from the measured values of ultrasonic velocities on as-received samples.     

Crack Propagation in Polymer Adhesive/Glass Sandwich Specimens

Moisture-assisted crack growth in polymer adhesive/glass interfaces was measured as a function of the applied energy release rate, G, using a four-point flexure test coupled with an inverted microscope. The specimens consisted of two glass plates bonded together with an epoxy or an epoxy-acrylate adhesive. It was found that cracks formed and grew on both interfaces if the glass surfaces were both smooth; however, roughening the surface of one of the glass plates increased the fracture resistance of the interface sufficiently so that crack growth occurred only on the remaining “smooth” interface (top or bottom). Finite element analysis was used to determine the G and ψ (phase angle) appropriate for the different crack geometries. It was found experimentally that crack growth rates for all crack geometries depended on the applied G via a power law relationship and that for a given applied G, crack growth rates were sensitive to the crack geometry. The results indicate that the primary driving force for moisture-assisted crack growth at a polymer/glass interface is the applied G at the crack tip and that the effect of the phase angle for the different crack geometries (13° to 54°) is negligible.     

Characterisation of subcritical crack growth in ceramics using indentation cracks

Abstract

The introduction of indentation cracks into brittle materials has proved to be a useful tool in characterising subcritical crack growth, notably in efficiently measuring the kinetic growth parameters and in defining whether the material exhibits a fatigue threshold. The accuracy of the subcritical crack growth parameters obtained using indentation mechanics can be excellent, provided the stress intensity factor associated with the indentation cracks is well characterised. Indentation cracks can also be used to measure crack velocity as a function of the applied stress intensity factor by direct observation. In such testing, it is critical that the changes in crack shape as the crack extends are known or accurately predicted. Numerical simulations suggest that the shape changes can be influenced not only by the testing geometry but also the growth kinetics. Finally, it is shown that the fatigue threshold can be determined by allowing median cracks to extend subcritically during indentation.     

Fatigue Damage in Ceramic Coatings From Cyclic Contact Loading With a Tangential Component

The role of a tangential load component in cyclic contact-induced failure of a brittle coating layer is investigated. Tests are conducted on model bilayer systems consisting of glass plates bonded to polymeric substrates, using a spherical indenter in periodic off-axis loading, in a water environment. The principal damage is that of a partial cone crack which forms in the wake of the contact and propagates steeply through the coating layer with continued loading. The critical number of cycles required to propagate the cone cracks through the coating is substantially reduced in off-axis relative to axial loading, and diminishes rapidly with increasing peak load. It is confirmed that the superposition of sliding tractions at the contact can have a profoundly deleterious effect on coating lifetime.     

Surface machining of fine-grain Y-TZP

The effects of diamond wheel grinding on dense (99.7% theoretical density), fine grain (0.38 μm) 3Y-TZP were investigated under various grinding conditions and after annealing temperatures at 1100 to 1300°C for the surface refinement. Scratching test was also used to simulate the interactions of the diamond grit with the TZP. The scratching tracks on the ground surface and the resulted cracks in the subsurface were observed by scanning and transmission electron microscopy (SEM and TEM). The results illustrated the features of plastic deformation, radial crack, median crack, and processing zone. The surfaces were also examined by X-ray diffraction (XRD) which indicated the formation of monoclinic phase, rhombohedral phase (r-phase), and ferroelastic domain switching (FDS). The heat treatment contributed the refinement of surface grains resulted in a reduction of average size from 0.38 to 0.15 μm. Microstructural observation, grinding force measurement, and flexural strength measurement indicate that appropriate grinding parameters are capable of preventing the formation of large defects and retaining the mechanical properties of TZP.