Fracture toughness of interfaces between glassy polymers in a trilayer geometry

We studied the fracture behavior of trilayer A/B/A assemblies based on polystyrene (PS) and poly(methylmethacrylate) (PMMA) where the central layer of the A polymer was confined (0.5–200 μm) between two thick plates of the B polymer (1– 3 mm). Diblock and random P(S-MMA) copolymers were used to provide a good stress transfer across the interfaces. Fracture experiments were performed with the double-cantilever beam method and the fracture mechanisms were observed by optical microscopy on microtomed slices of the damaged zone. The measured _c of the A/B interface fractured during the test was dependent on the molecular structure at the interface (random copolymer, diblock copolymer or no copolymer), on the crazing stress of the bulk materials and on the interfacial shear stresses. When the phase angle of the loading was even slightly positive, oblique crazes were observed in the PS increasing greatly _c. If PS was the central layer, this resulted in a very marked dependence of _c on the thickness of the central layer for a thickness range 10–200 μm which was not observed when the PMMA was the central layer. Thermal treatments modifying the interfacial shear stresses were also found to have a very strong effect on _c.     

Role of wollastonite additive on density, microstructure and mechanical properties of alumina

Wollastonite is a low fusing material and has a considerable effect on the densification of alumina bodies at sintering temperatures ranging from 1300 to 1450 °C. The wollastonite acts as a flux and accelerates the liquid forming process leading to lower temperature densification. The addition of 1.0 wt.% wollastonite alters the microstructure to form well-defined grains with an average grain size in the range of 3.0–5.0 μm. The grains are distributed uniformly and the fracture occurs along the grain boundaries and the microstructure reveals the presence of highly dense and sintered grains. The hardness and bend strength for 0–5.0 wt.% wollastonite increases from 8.5 to 14 GPa and from 119 to 249 MPa respectively at a sintering temperature of 1400 °C.     

Numerical simulation analysis and experimental validation on wear/fracture mechanisms of polycrystalline cubic boron nitride superabrasives in high-speed grinding

In this study, a two-dimensional finite element model is proposed to investigate the wear/fracture mechanisms of polycrystalline cubic boron nitride (PCBN) superabrasives in high-speed grinding process. The special geometric microstructures of PCBN grains are constructed by using the classic Voronoi tessellation technique, and cohesive elements are embedded into the geometric model of PCBN grains as the potential crack propagation paths for simulating the wear/fracture behaviours of PCBN grains under grinding loads. The effects of uncut chip thickness per grain (a_gmax) on the stress distribution characteristics and wear/fracture behaviours of PCBN grains during grinding are discussed in detail. Results show that the wear behaviour of PCBN grains during grinding mainly occurs around the grain vertex region; however, the fracture behaviour, leading to the quick failure of PCBN grains, is prone to appear around the grain–filler bonding interface, which is usually on the opposite side of the in-feed direction. Moreover, to separate the PCBN grains from the macro-fracture during grinding, the uncut chip thickness per grain should be kept smaller than 1.0?µm to prevent the unfavourable fracture behaviour from appearing around the grain–filler bonding interface. Furthermore, the corresponding single-grain grinding trials are performed to validate the numerical simulation results by evaluating the wear/fracture morphologies of the PCBN superabrasives in the actual grinding operation.     

Sintering and densification mechanisms of tantalum carbide ceramics

Dense tantalum carbide (TaC) ceramics were prepared using TaC nanopowder via spark plasma sintering (SPS). The effects of the sintering temperature and applied pressure on the densification and grain growth behaviour of TaC ceramics were investigated. The results showed that high temperature and pressure promoted sintering densification, while their increase caused an increase in the grain size of TaC ceramics. A highly dense TaC ceramic (∼97.19%) with a fine grain size of 2.67 μm was obtained by sintering at 1800 °C for 10 min under 80 MPa. The Vickers hardness, Young's modulus and fracture toughness were 15.60 GPa, 512.66 GPa and 3.59 MPa·m^1/2, respectively. The densification kinetics were investigated using a creep deformation model. Diffusion and grain boundary sliding were proven to be the dominant densification mechanisms based on the stress and grain size exponents combined with the microstructural characteristics. The apparent activation energy of the mechanism controlling densification was 252.94 kJ/mol.     

Mechanical properties and recrystallization of alumina ceramics reinforced by silicon carbide whisker

The mechanical properties and recrystallization of a material consisting of Al₂O₃ and SiC whisker are considered for different hot pressing regimes. The best properties are obtained at an SiC whisker content of 20% in hot pressing in an argon medium at 1600°C (the ultimate bending strength is 600 MPa, the critical coefficient of stress intensityK _Ic=5.5 MPa · m^1/2). The bending strength measured at 1000°C is at least 400 MPa for the material pressed at 1700°C in air. It is established that the presence of SiC whisker on grain boundaries of Al₂O₃ noticeably impedes the growth of alumina grains in recrystallization for all chosen regimes of hot pressing (1650 – 1900°C, 15 – 45 min). The dominant grain size is 1 –5 µm. A mathematical model is suggested for evaluating the maximum grain size in uniform grain growth, which isD*=0.571 [1++(1+16.151/f)^1/2]d, whered, f are the diameter and the volume fraction of SiC whisker. The material can be recommended for use under conditions of thermomechanical stress and abrasive wear.Translated from Ogneupory, No. 5, pp. 8 – 12, May, 1995.     

Creep behaviour of alumina–mullite–zirconia nanocomposites obtained by a colloidal processing route

The high-temperature creep behaviour of high-purity alumina (A) and an alumina–mullite–zirconia nanocomposite (AZS) has been studied. The alumina–mullite–zirconia nanocomposite was prepared by using a colloidal processing route (powder–alkoxide mixtures). Creep tests were carried out in air in a 4-point-bending-fixture from 1200 to 1400 °C under constant stresses ranging from 30 to 220 MPa. Creep parameters (stress exponent n and activation energy Q) were correlated with microstructural features in order to determine the dominant creep mechanisms for both materials. The slow crack growth region (SCG), given by pairs of critical stress and the corresponding critical strain rate at the temperatures 1200, 1300 and 1400 °C of both materials was studied.

It was found that the creep rate of AZS was two orders of magnitude lower than the creep rate of undoped alumina A. The dominant creep mechanism of A is assumed to be a combination of grain boundary and lattice diffusion controlled creep. The creep mechanism for AZS is different and depends on the temperature. It is supposed that lattice diffusion controlled creep (Nabarro–Herring) is the dominant creep mechanism at 1200 °C, whereas at 1300 °C it is supposed to be grain boundary sliding accommodated by grain boundary diffusion. Comparing the slow crack growth region of both materials, a dramatic improvement was observed. The slow crack growth region of alumina is shifted nearly twice concerning the applied stresses for AZS at the temperatures 1200, 1300 and 1400 °C.     

Dynamic behaviour and visco-elastic damage model of ultra-high performance cementitious composite

The dynamic behaviour of ultra-high performance cementitious composite (UHPCC) with compressive strength of 200 MPa with different steel fiber volume fractions was studied under impact using the split Hopkinson pressure bar. Three aspects of the testing: a gimbal device, wave shaping and direct strain measurement, were used to increase experimental accuracy. Results indicate that UHPCC has obvious strain rate effects. The peak stress, peak strain, elastic modulus and the area under the stress–strain curve increase with increasing strain rate. When the strain rate exceeds a threshold value, specimens with and without fibers begin to fracture. At high strain rate the unreinforced specimens fracture into small parts while fiber reinforced ones only have fine cracks on the edges. A visco-elastic damage model of UHPCC is proposed based on a nonlinear visco-elastic model (the ZWT model) and the material damage measured by the ultrasonic wave velocity method.     

Elaboration and characterization of a refractory based on Algerian kaolin

A refractory material was elaborated from kaolin extracted from the region of Djebel Debbagh (Algeria). Kaolin grog was obtained by calcination at a temperature of 1350 °C during 1 h. It was used as aggregates with granulometric distribution composed of fine fraction (mean grain size: 100–250 μm) and coarse fraction (mean grain size: 1000–2500 μm). Crude kaolin (size < 75 μm) was also used as a binder with an amount representing 15% of the dry material. After a 9.28% moistening and a rotting of 1 day, cylindrical samples were shaped by uniaxial pressure at 80 MPa. The samples were submitted to a natural drying during 24 h, a stoving at 100 °C and a calcination at 600 °C during 1 h. They were fired at high temperatures between 1250 and 1450 °C.

An X-ray diffraction (XRD) analysis showed that the refractory samples are composed of mullite and silica. Silica is a mixture of a vitreous phase and cristobalite at 1300, 1350 and 1400 °C and becomes completely amorphous when the samples are fired at higher temperature (1450 °C). The sample porosity is about 30%. The mechanical tests carried out as a function of temperature revealed different behaviours of the material. From the ambient up to 600 °C, the refractory behaviour is pseudo-plastic caused by micro-cracking. Between 700 and 900 °C, the samples become more rigid. At 1000 °C, the material exhibits a visco-plastic behaviour. The amorphous phase governs the sample properties variation with temperature increasing. Its content varies between 28% and 34% according to the firing temperature. Thermal shock tests realized in water showed that the refractory samples present good thermal shock resistance.     

Microstructure and mechanical properties of gas pressure sintered Al2O3/TiCN composite

Al₂O₃–30 wt.%TiCN composites have been fabricated successfully by a two-stage gas pressure sintering schedule. The gas pressure sintered Al₂O₃–30 wt.%TiCN composite achieved a relative density of 99.5%, a bending strength of 772 MPa, a hardness of 19.6 GPa, and a fracture toughness of 5.82 MPa m^1/2. The fabrication procedure involves solid state sintering of two phases without solubility to prepare Al₂O₃–TiCN composite. Little grain growth occurred for TiCN during sintering while Al₂O₃ grains grew about three times to an average size of 3–5 μm. The interface microstress arising during cooling from the processing temperature because of the thermal and/or mechanical properties mismatch between the Al₂O₃ and TiCN phase is about 50 MPa. Such a compressive microstress is not high enough to cause grain boundary cracking that may weaken the composite but it can introduce dislocations within grains, which is very good to enhance the composite properties.     

A micromechanics study of competing mechanisms for creep fracture of zirconium diboride polycrystals

A micromechanics model was developed to simulate creep fracture of ceramics at high temperatures and material properties pertinent to zirconium diboride (ZrB₂) were adopted in the simulation. Creep fracture is a process of nucleation, growth, and coalescence of cavities along the grain boundaries in a localized and inhomogeneous manner. Based on the grain boundary cavitation process, creep fracture can be categorized into cavity nucleation-controlled and cavity growth-controlled processes. On the other hand, based on the deformation mechanism, the separation between two adjacent grain boundaries can be categorized into diffusion-controlled and creep-controlled mechanisms. In this study, a parametric study was performed to examine the effects of applied stress, cavity nucleation parameter, grain boundary diffusivity, and applied strain rate on cavity nucleation-controlled versus growth-controlled process as well as diffusion-controlled vs. creep-controlled mechanism during creep fracture of ZrB₂.     

Effect of Bond Thickness on Fracture Behaviour in Adhesive Joints

To study the effects of bond thickness on the fracture behaviour of adhesive joints, experimental investigation and finite element analysis have been carried out for compact tension (CT) and double-cantilever-beam (DCB) specimens with different bond thickness. Fractography and fracture toughness exhibited apparent variations with bond thickness. Numerical results indicate that the crack tip stress fields are affected by bond thickness due to the restriction of plastic deformation by the adherends. At the same J level, a higher opening stress was observed in the joint with a smaller bond thickness (h). Beyond the crack tip region, a self-similar stress field can be described by the normalized loading parameter, J/hσ₀. The relationship between J and crack tip opening displacement, δ, is dependent on the bond thickness. The strong dependence of toughness upon bond thickness is a result of the competition between two different fracture mechanisms. For small bond thickness, toughness is linearly proportional to bond thickness due to the high constraint. After reaching a critical bond thickness, the toughness decreases with further increase of bond thickness due to the rapid opening (blunting) of the crack tip with loading. A simple model has been proposed to predict the variation of toughness with bond thickness.     

The essential work of fracture of a thermoplastic elastomer

This paper presents the fracture behaviour of a thermoplastic elastomer, HYTREL 5556. Since with this material it is not possible to successfully apply the LEFM nor the EPFM, it has been studied by following the ESIS protocol for determining the essential work of fracture in plane stress and extended for mixed-mode conditions, which should give a material constant, independent of the sample geometry. DDENT specimens were used in two different thickness, and results showed that the essential works of fracture in plane stress and mixed-mode were the same for both thickness for this material. Received: 24 March 1997/Revised: 30 May 1997/Accepted: 30 May 1997     

Fatigue fracture behaviour of PEEK: 2. Effects of thickness and temperature

Experimental results on the effects of specimen thickness and environmental temperatures on fatigue fracture behaviour of poly(ether ether ketone) (PEEK) are reported. Low cycle fatigue experiments are conducted on injection moulded single-edge notched specimens of 1.57, 2.70 and 5.42 mm in thickness at ambient temperatures, and on specimens 2.70 mm thick at environmental temperatures of 39, 50, 63, 75 and 100°C. In all the thickness experiments and in the experiments with temperatures of 39 and 50°C, the crack tip profile is initially round. At long crack lengths the crack tip profile changes to a triangular shape. When the test temperature is 63, 75 and 100°C, the crack tip remains round throughout the fracture process. The crack tip angle is primarily dependent upon the test temperature. Examinations of the fracture surfaces and transverse sections indicate that in the thickest specimen, relatively rough fracture surfaces are observed and a few discontinuities (crazes or cracks) underneath the main crack path. Thus, crack propagates in a ‘brittle’ manner. In all other experiments both ‘brittle’ and ‘ductile’ modes of fracture are observed. The point of transition from ‘brittle’ to ‘ductile’ fracture is dependent upon the specimen thickness and test temperature. Fatigue striations are seen throughout the fracture surfaces. Correlation of the striations and the number of cycles indicates a one-cycle crack growth mode. Hysteretic losses during fatigue crack growth are negligible until a few cycles prior to unstable fracture. Crack opening displacements are independent of the specimen thickness and increase with rise in temperature. When crack growth rates are correlated with the elastic energy release rate, they are independent of specimen thickness and increase with increase in temperature.     

Microstructural development of liquid-phase-sintered silicon carbide during annealing with uniaxial pressure

β-SiC powders containing 1.1 wt.% α-SiC particles as seeds were hot-pressed at 1800 °C and then annealed at 2000 °C under 25 MPa uniaxial pressure to enhance grain growth. Microstructural development during annealing with pressure was investigated quantitatively and statistically using image analysis. The bimodal grain-thickness distribution in samples annealed with pressure was obtained due to abnormal grain growth of some grains. In situ-toughened microstructure has been developed after 3-h annealing. The grain-thickness and aspect ratio of large grains increase with annealing time, but grain growth comes mainly from increases in thickness after 3-h annealing, owing to the impingement of large gains. Typical flexural strength and fracture toughness of 4-h annealed sample were ∼500 MPa and ∼7.5 MPa m^1/2, respectively.     

High strength TiC matrix Fe28Al toughened composites prepared by spontaneous melt infiltration

Fe28Al bound TiC matrix composites with TiC content of 75–90% in volume (vol.%) were successfully fabricated by spontaneous melt infiltration. Amounts of Fe28Al in excess and below the pore volume of the TiC preform were used for optimization of fabrication techniques. Young's modulus, hardness, flexural strength and fracture toughness of the composites were measured. Four-point bending strength of Fe28Al/90–75 vol.% TiC ranges to 990–1260 MPa. The high strength is attributed to the good infiltration ability of molten Fe28Al in the porous TiC preform and to processing refinements. TiC preform pre-sintering and indirect infiltration all lead to fully dense and defect-free composites. The relationship between Vickers hardness and indentation fracture toughness and the dependence of mechanical properties on microstructure of the composites were also studied. Results of SEM and XRD analysis show TiC and Fe28Al as the only crystalline phases of the composite. Fe28Al ligaments have ductile behaviour and greatly toughen the composites. Crack front deviation during fracture also increased the fracture resistance of the composites.     

Effect of Ag on the microstructure and electrical properties of ZnO

Various amounts of silver particles, 0.08–7.7 mol%, are mixed with zinc oxide powder and subsequently co-fired at 800–1200 °C. The effects of Ag addition on the microstructural evolution and electrical properties of ZnO are investigated. A small Ag doping amount (<0.76 mol%) promotes the grain growth of ZnO; however, a reversed trend in grain growth is observed for a relatively larger Ag addition (>3.8 mol%). It is evident that a tiny amount of Ag (0.08 mol%) may dissolve into the ZnO lattice. High-resolution TEM observations give direct evidences on the segregation of Ag solutes at the ZnO grain boundaries. The grain boundary resistance of ZnO increases 35-fold with the presence of Ag solute segregates. The Ag-doped ZnO system exhibits a nonlinear electric current–voltage characteristic, confirming the presence of an electrostatic barrier at the grain boundaries. The barrier is approximately 2 V for a single grain boundary.     

Wet Erosive Wear of Alumina Densified with Magnesium Silicate Additions

A study was made of the wet erosive wear of polycrystalline alumina of mean grain size >1 μm, containing up to 10 wt% of magnesium silicate sintering aid. For pure polycrystalline alumina, the dominant wear mechanism was grain-boundary microfracture, leading to partial or complete grain removal. In the case of the liquid-phase-sintered materials, wear rates could be as low as 25% of those of pure alumina of the same mean grain size, and the main material removal mechanism was transgranular fracture combined with tribochemical wear. The use of Cr³⁺ photoluminescence line broadening showed much higher levels of local stress in the magnesium silicate-sintered materials (∼450 MPa) than in the pure-alumina materials (∼200 MPa). Grain-boundary compressive hoop stresses, caused by the thermal expansion mismatch between a continuous magnesium silicate film and the alumina grains, provided an explanation for the improved wear resistance of the alumina sintered with magnesium silicate.     

Mode-l Fracture Behaviour of Adhesive Joints. Part II. Stress Analysis and Constraint Parameters

The constraint effect on the fracture behaviour of a rubber-modified epoxy was investigated using compact tension (CT) adhesive joints. An elastic-plastic finite element analysis was conducted to evaluate the stress distribution ahead of the crack tip in the bulk adhesive and adhesive joints of different bond thickness. The models with sharp and finite radius crack tips were evaluated in the analyses. The constraint effect of adherends on the stress triaxiality ahead of the crack tip in the adhesive joints were discussed. The constraint parameters were investigated using the J-Q theory and the J-CTOD relationship. It was found that as the adhesive thickness was increased, the stress triaxiality ahead of the crack tip was relieved by the remarkable deformation of the adhesive material. Similarly, the crack tip constraint was reduced with increasing bond thickness so that the fracture energy increased towards the value of the bulk adhesive. A higher constraint was associated with a lower fracture energy and vice versa. Furthermore, the J-integral did not have a unique relationship with the crack-tip opening displacement (CTOD) for different adhesive bond thickness, as this depends on the constraint around the crack tip. The results of this study will help improve reliability assessment of adhesive joints in engineering applications.     

Thickness-Dependent Mechanical Behaviour of Aluminium-Coated Polyethylene Terephthalate (PET) Films

Mechanical behaviour of aluminium-coated PET films has been investigated through Dynamic Mechanical Analyser. Aluminium is coated on PET substrate by vacuum thermal evaporation method. As thickness of aluminium coating increases from 150 nm to 350 nm, tensile strength decreases from 108.88 MPa to 99.25 MPa. This mechanical behaviour is correlated with microstructure and its evolution with the thickness of aluminium coating. Al-PET film consists of fine globular grains and average grain size increases monotonically with the film thickness. The relative contribution of the grain size to the strength of aluminium thin films is in good agreement with Hall-Patch equation.     

Diffusional Crack Growth and Creep Rupture of Silicon Carbide Doped with Alumina

This study deals with tensile creep and crack growth behavior of silicon carbide doped with alumina at 1400°C. Excellent creep resistance was observed for stresses from 150 MPa to 200 MPa. From the creep exponent of 1.4 and the activation energy of 320 kj/mol, the principal creep mechanism was Coble creep. The creep failure was caused by slow crack growth from a preexisting flaw. The crack was found to grow subcritically along grain boundaries almost in isolation. The relation between the time–to–failure and the applied stress was well treated by a diffusive crack growth model, and the threshold stress of this material at 1400°C was estimated at 165 MPa.