Investigation of Thermal Shock in a High-Temperature Refractory Ceramic: A Fracture Mechanics Approach

The results of a study on the thermal shock behavior of a high-temperature refractory ceramic that is used as a furnace liner in the melting of steels are presented in this paper. The experimental studies show that thermal shock damage initiates by edge cracking after the first shock cycle. Subsequent subcritical crack growth occurs by the incremental extension of dominant cracks until catastrophic failure occurs. The observations of the crack profiles also reveal the formation of viscoelastic bridges that promote crack-tip shielding/toughening via crack bridging. Following a brief discussion of the respective mechanisms of fracture and thermal shock damage at different temperatures and temperature ranges, the implications of the results are discussed for refractory ceramics that are toughened by viscoelastic crack bridging.     

撞击加载下炸药晶体的破碎特征

分别对AP、HMX、RDX晶体进行落锤撞击加载,用环境扫描电镜观察了3种晶体的破碎特点。实验结果显示:3种晶体虽然都出现剪切带,但在破碎特征方面有明显不同,其中AP以晶体的局部脆性断裂为主,晶体的破碎程度最小,且加载后仍呈颗粒状;HMX和RDX以晶体的塑性变形和破碎为主,且加载后多呈现"薄片"结构;加载后RDX晶体比HMX易分散,但HMX晶体的破碎程度更严重。研究认为:晶体的撞击加载是力-热转化过程,晶体内部热点形成与晶体的力学强度有关,认为这一转换过程与晶体的分子组成和热反应机理有关。在低速撞击加载下,具有较强分子稳定性的AP晶体破碎程度最小,且不易形成热点。对于RDX和HMX晶体,"薄片"结构的形成原因除晶体破碎外,内部还可能存在相变、转晶等变化。     

球罐用B610CF钢板冲击与断裂性能试验研究

针对宝山钢铁股份有限公司开发的B610CF钢板及其焊接接头的冲击及断裂性能进行了系统的试验研究.结果表明:钢板调质状态和调质+消除应力热处理状态在- 20℃时都具有较高的冲击性能,钢板和焊接接头在- 20℃时都有着良好的断裂韧度,该钢用于- 20℃以上工况的球罐是完全可行的,所用焊接材料与母材匹配且焊接工艺合理.     

Time Dependent Crack Growth and Loading Rate Effects on Interfacial and Cohesive Fracture of Adhesive Joints

Double cantilever beam fracture specimens were used to investigate rate dependent failures of model epoxy/steel adhesively bonded systems. Quasi-static tests exhibited time dependent crack growth and the maximum fracture energies consistently decreased with debond length for constant crosshead rate loading. It was also possible to cause debonding to switch between interfacial and cohesive failure modes by simply altering the loading rate. These rate dependent observations were characterized using the concepts of fracture mechanics. The time rate of change of the strain energy release rate, dG/dt, is introduced to model and predict failure properties of different adhesive systems over a range of testing rates. An emphasis is placed on the interfacial failure process and how rate dependent interfacial properties can lead to cohesive failures in the same adhesive system. Specific applications of the resulting model are presented and found to be in good agreement when compared with the experimental data. Finally, a failure envelope is identified which may be useful in predicting whether failures will be interfacial or cohesive depending on the rate of testing for the model adhesive systems.     

Impact Modification and Fracture Mechanisms of Core–Shell Particle Reinforced Thermoplastic Protein

Mechanical properties and fracture mechanisms of Novatein thermoplastic protein and blends with core–shell particles (CSPs) have been examined. Novatein is brittle with low impact strength and energy‐to‐break. Epoxy‐modified CSPs increase notched and unnotched impact strength, tensile strain‐at‐break, and energy‐to‐break, while tensile strength and modulus decrease as CSP content increases. T_g increases slightly with increasing CSP content attributed to physical crosslinking. Changes to mechanical properties are related to the critical matrix ligament thickness and rate of loading. Novatein control samples display brittle fracture characterized by large‐scale crazing. At high CSP content a large plastic zone and a slow crack propagation zone in unnotched and tensile samples are observed suggesting increased energy absorption. Notched impact samples reach critical craze stresses easily regardless of CSP content reducing impact strength. It is concluded that the impact strength of thermoplastic protein can be modified in a similar manner to traditional thermoplastics.

     

Orientation‐dependent mechanical and thermal properties of plasma‐sprayed ceramics

Due to droplet‐based assembly, microstructure anisotropy is expected in atmospheric plasma‐sprayed coatings (APS), with lamellar separations and interfaces having critical effects on properties. Quantitative determination of these anisotropic properties is difficult due to geometric test constraints. This has been overcome in the literature through a variety of indirect, local, or modeled evaluation, however direct measurement on like‐dimensioned coatings is not available. In this work, 25‐mm thick ceramic coating variants, deposited at two different feed rates, were obtained from industry and macroscopic mechanical and thermal properties were evaluated in both in‐plane and out‐of‐plane orientations using identical specimen geometries. As expected, and confirming select past work, coating anisotropy has a direct influence on measured properties. The response of each property is microstructure‐dependent, highlighting the specific interaction: for instance, the fracture toughness is 120% higher in the through‐thickness orientation versus in‐plane after thermal aging, while the thermal conductivity was 24% lower in the through‐thickness. The former benefits from the lamellar interfaces that provide obstacles to crack propagation while the latter sees these interfaces as efficient phonon scatters. The results provide insights for design through robust property measurements and into operational mechanisms in this class of highly defected ceramics.     

Impact characterization of a carbon fiber‐epoxy laminate using a nonconservative model

The study of polymer and composite behavior under high strain rates is of fundamental relevance to determine the material suitability for a selected application. However, the impact phenomenon is a very complicated event, mainly due to the short duration, large deformation, and high stresses developed in the sample. In this work, we have performed impact tests over a carbon fiber reinforced epoxy using low‐energy in the striker. A nonconservative and nonlineal spring‐dashpot series model has been proposed to reproduce the material behavior. The model considers simultaneously both flexural and indentation phenomena accounting for energy losses by means of the restitution coefficient. Using this model, an excellent fit between the predicted and the experimental force‐time trace has been obtained below the composite failure point, which was recognized by a separation of both mentioned curves. As the epoxy‐fiber laminate has a very low viscoelasticity, the high strain rate Young's modulus obtained from the model was compared with that extracted from a conventional three point bending test, finding a very good match between the values. The study of the dashpot coefficients allows concluding that the dominant mechanism is the composite flexion, while the indentation effects contribution takes on importance at low impact velocities. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2256–2263, 2005     

Impact Fracture and Failure Behavior of in‐situ Polymerized,Ethylene/Butyl Acrylate Rubber Toughened Polyamide‐12

The fracture and failure behavior of in‐situ polymerized polyamide‐12 (PA‐12) blends prepared by reactive extrusion were studied in instrumented high‐speed (v = 1.2 m/s) impact bending tests using the linear elastic fracture mechanics approach. PA‐12 was polymerized in presence (up to 9 wt.‐%) of ethylene/butyl acrylate copolymers (E/BA) of varying BA content and melt viscosity. From the tests performed on injection molded specimens at ambient temperature and –40°C, respectively, the fracture toughness (K_d) and initiation fracture energy (G_d,i) were derived. K_d was less sensitive to either testing temperature or E/BA type and content. G_d,i, on the other hand, went through a maximum at room temperature and monotonously increased at T = –40°C as a function of modifier content. E/BA with higher melt viscosity and lower polarity (lower BA content) performed better than the lower melt viscosity, higher polarity E/BA counterpart. The dominant failure modes and their change both with temperature and modifier content were studied by fractography and discussed.     

Mechanical Behavior and Fracture Toughness of Poly(L‐lactic acid)‐Natural Fiber Composites Modified with Hyperbranched Polymers

Summary: The use of hyperbranched polymers (HBP) with hydroxy functionality as modifiers for poly(L ‐lactic acid) (PLLA)‐flax fiber composites is presented. HBP concentrations were varied from 0 to 50% v/v and the static and dynamic tensile properties were investigated along with interlaminar fracture toughness. Upon addition of HBP, the tensile modulus and dynamic storage modulus (E′) both diminished, although a greater decline was noticed in the static modulus. The elongation of the composites with HBP showed a pronounced increase as large as 314% at 50% v/v HBP. The loss factor (tan δ) indicated a lowering of the glass transition temperature (T_g) due to a change in crystal morphology from large, mixed perfection spherulites to finer, smaller spherulites. The change in T_g could have also resulted from some of the HBP being miscible in the amorphous phase, which caused a plasticizing effect of the PLLA. The interlaminar fracture toughness measured as the critical strain energy release rate (G_IC) was significantly influenced by HBP. At 10% v/v HBP, G_IC was at least double that of the unmodified composite and a rise as great as 250% was achieved with 50% v/v. The main factor contributing to high fracture toughness in this study was better wetting of the fibers by the matrix when the HBP was present. With improved ductility of the matrix, it caused ductile tearing along the fiber‐matrix interface during crack propagation.

ESEM photograph of propagation region of the interlaminar fracture toughness specimens with 30% v/v of HBP.     

Toughening of polycarbonate: Effect of particle size and rubber phase contents of the core‐shell impact modifier

The toughening behavior of polycarbonate modified with core‐shell type particles was investigated. The alloys were found to exhibit maximum impact strength upon addition of a modifier with a poly(butyl acrylate) rubbery core of 0.25 μm diameter. The incorporation of particles with diameter greater than 0.25 μm resulted in decreased impact strength. The influence of rubber phase contents on toughness was also studied. It was observed that the alloys exhibited maximum impact strength upon addition of 4 wt % rubber phase. Further increase in the rubber phase content resulted in reduced impact strength. Fractography of the samples showed that, below 4 wt % rubber phase content, the fracture occurs mainly by internal crazing and, from 4 wt % onward, only by shear deformation. When the effect of dual particle size distribution was analyzed, it was found that there was only a moderate increase in toughness compared with alloys containing monosized particles. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 748–755, 2005     

Viscoelastic behavior of formaldehyde and basic chromium sulfate‐crosslinked collagen

Rat tail tendon (RTT) collagen has been crosslinked with 1% basic chromium sulfate (BCS) at pH 3.2, and 2, 4, 8, and 10% formaldehyde (HCHO) at pH 5 and pH 8. The viscoelastic behavior (such as stress relaxation behavior for BCS and HCHO‐tanned RTT) has been studied in water and 6M urea at different temperatures. The total rate of relaxation has been divided into fast and slow components, and computed using the two‐term model and nonlinear least‐squares fit. The rate of relaxation for crosslinked RTT is less than the native one. Activation energy at absolute zero has been computed using k = AT^me/RT and nonlinear least‐squares fit. The activation energy increases for crosslinked RTT than the native one. This is consistent with the observed rate constant values. This may be due to the additional stability imparted to RTT collagen by coordinate covalent and covalent crosslinks through BCS and HCHO, respectively. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2245–2251, 1999     

Preparation and properties of multiwalled carbon nanotube/epoxy‐amine composites

Different amounts of multiwalled carbon tubes (MWCNTs) were incorporated into an epoxy resin based on diglycidyl ether of bisphenol A and both epoxy precursor and composite were cured with 4,4′‐diamino diphenyl sulfone. Transmission and scanning electron microscopy demonstrated that the carbon nanotubes are dispersed well in the epoxy matrix. Differential scanning calorimetry measurements confirmed the decrease in overall cure by the addition of MWCNTs. A decrease in volume shrinkage of the epoxy matrix caused by the addition of MWCNTs was observed by pressure–volume–temperature measurements. Thermomechanical and dynamic mechanical analysis were performed for the MWCNT/epoxy composites, showing that the T_g was slightly affected, whereas the dimensional stability and stiffness are improved by the addition of MWCNTs. Electrical conductivity measurements of the composite samples showed that an insulator to conductor transition takes place between 0.019 and 0.037 wt % MWCNTs. The addition of MWCNTs induces an increase in both impact strength (18%) and fracture toughness (38%) of the epoxy matrix with very low filler content. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Toughness optimization of glass‐fiber reinforced PA 6/PA 66‐based composites: Effect of matrix composition and colorants

Mixing of polyamide 6 (PA 6) and polyamide 66 (PA 66) is integrated in the trend of development of new and improved materials by combination of different polymers and some reinforcing materials to polymer composites. The specific polymer composite PA 6/PA 66 reinforced with short glass‐fibers combines the good coloring of PA 6, and the small moisture absorption of PA 66. Technical applications of PA 6/PA 66 composites are mainly used in the automotive industry. Specific requirements of this industry lead to the necessity to optimize the material resistance against crack propagation of the PA 6/PA 66 composites, using mechanical and fracture mechanical methods. So, the present investigations focus on fracture mechanics toughness optimization of the PA 6/PA 66 composites, including unstable and stable crack growth. The aim of this toughness optimization is to find out the optimal mixing ratio of PA 6/PA 66. Applications of PA 6/PA 66 in the automotive industry and specific client wishes are the main reasons for black‐coloring of the PA materials. The influence of several black‐colorants (carbon black, nigrosine, spinel, iron oxide) on mechanical and fracture mechanical properties of the PA composites is also investigated using fracture mechanical methods. As experimental fracture mechanical method, preferentially, the instrumented Charpy impact test (ICIT) and the new cut method to determine the stable crack growth of glass‐fiber reinforced materials was used. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mechanical and optical properties in precipitated regions of alumina‐rich magnesium aluminate spinel

The toughening and strengthening of transparent ceramics is challenging because microstructural alterations typically lead to light scattering. Here, controlled precipitation of α‐Al₂O₃ from nonstoichiometric spinel is explored to demonstrate unique control over the evolution of second phase Al₂O₃ and how the microstructure might be altered to enhance fracture toughness while minimizing light scatter. Alumina‐rich magnesium aluminate spinel, MgO·nAl₂O₃, where n=2, was hot pressed and HIPed to produce fully dense, single‐phase material. The material was then heat treated in air at 1573 K for up to 20 hours to create a two‐phase spinel‐Al₂O₃ composite. The fracture toughness varies from 0.88 to 2.47 MPa√m depending on the microstructure; enhanced toughness at the surface was due to increased crack tortuosity at phase boundaries, but residual tensile stresses were observed in the interior of the material. Precipitation causes local volume contraction and the formation of porosity, decreasing optical transmission, especially for heat treatment times longer than 5 hours.     

The Fatigue and Durability Behaviour of Automotive Adhesives. Part I: Fracture Mechanics Tests

A fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The results were plotted in the form of the rate of crack growth per cycle, da/dN, versus the maximum strain-energy release rate, G_max, applied in the fatigue cycle, using logarithmic axes. Of particular interest was the presence of a threshold value of the strain-energy release rate, G_th, applied in the fatigue cycle, below which fatigue crack growth was not observed to occur. The cyclic fatigue tests conducted in a relatively dry environment of 23°C, and 55%; RH were shown to cause crack propagation at far lower values of G_max compared with the value of the adhesive fracture energies, G_c, which were determined from monotonically-loaded fracture tests. Cyclic fatigue tests were also conducted in a “wet” environment, namely immersion in distilled water at 28 C. The “wet” fatigue tests clearly revealed the further significant effect an aggressive, hostile environment may have upon the mechanical performance of adhesive joints, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The development and standardisation of “wet” fatigue tests may provide the basis for a very effective accelerated-ageing test.     

Investigation of optical and thermally stimulated properties of SiO2 nanoparticles‐filled polycarbonate

Polycarbonate nanocomposite containing silicon oxide nanoparticles average size of 5 nm at different weight ratio has been prepared by solution mixing method. The dispersion of nanoparticles in polymer matrix was studied by transmission electron microscopy (TEM). The optical and thermally stimulated behavior of nanocomposites were analyzed by energy dispersive X‐ray spectra (EDX), X‐ray diffraction pattern (XRD), UV–vis spectroscopy, differential scanning calorimetry (DSC), and thermally stimulated discharge current (TSDC). TEM images show the dispersion and size of the nanoparticles, however, EDX indicate the presence of SiO₂ on the surface of the nanocomposite film. An XRD result reveals that the crystallinity increases with increase in concentration of SiO₂ nanoparticles in polymer matrix. The direct and indirect optical energy band gaps decreased and number of carbon atom increased with concentration of SiO₂ nanoparticles. We have observed that the increase of SiO₂ nanoparticles in PC significantly reduces the refractive index. DSC and TSDC show that glass transition temperature increases according to SiO₂ weight ratio. The TSDC of nanocomposites samples could be understand in terms of non‐Debye theory of charge relaxation and co‐tunneling mechanism of charge transport. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fracture toughness of poly(lactic acid)/ ethylene acrylate copolymer/wood‐flour composite ternary blends

A fracture mechanics analysis based on the J‐integral method was adopted to determine the resistance of composites with various concentrations of wood‐flour and ethylene acrylate copolymer (EAC) to crack initiation (J_in) and complete fracture (J_f). The J_in and J_f energies of unmodified poly(lactic acid) (PLA)/wood‐flour composites showed the deleterious effect of incorporating wood fibers into the plastic matrix by significantly decreasing the fracture toughness of PLA as the wood‐flour content increased. The reduced fracture toughness of the matrix induced by adding brittle wood‐flour into PLA was well recovered by impact modification of the composites with EAC. Microscopic morphological studies revealed that the major mechanisms of toughening were through the EAC existing as separate domains in the bulk matrix of the composites which tended to act as stress concentrators that initiated local yielding of the matrix around crack tips and enhanced the toughness of the composites. © 2012 Society of Chemical Industry     

Temperature‐stable dielectric and energy storage properties of La(Ti0.5Mg0.5)O3‐doped (Bi0.5Na0.5)TiO3‐(Sr0.7Bi0.2)TiO3 lead‐free ceramics

A new type of (0.7?x)Bi_0.5Na_0.5TiO₃‐0.3Sr_0.7Bi_0.2TiO₃‐xLaTi_0.5Mg_0.5O₃ (LTM1000x, x = 0.0, 0.005, 0.01, 0.03, 0.05 wt%) lead‐free energy storage ceramic material was prepared by a combining ternary perovskite compounds, and the phase transition, dielectric, and energy storage characteristics were analyzed. It was found that the ceramic materials can achieve a stable dielectric property with a large dielectric constant in a wide temperature range with proper doping. The dielectric constant was stable at 2170 ± 15% in the temperature range of 35‐363°C at LTM05. In addition, the storage energy density was greatly improved to 1.32 J/cm³ with a high‐energy storage efficiency of 75% at the composition. More importantly, the energy storage density exhibited good temperature stability in the measurement range, which was maintained within 5% in the temperature range of 30‐110°C. Particularly, LTM05 show excellent fatigue resistance within 10⁶ fatigue cycles. The results show that the ceramic material is a promising material for temperature‐stable energy storage.