Cohesive Zone Modeling of Propellant and Insulation Interface Debonding

The reliability of the bonding of propellant to insulation is a key part of the analysis of rocket motor structural integrity. In this study, the debonding of the propellant/insulation interface was investigated by combining experiment and simulation. The improved exponential cohesive zone model and the bilinear cohesive zone model were used to predict the fracture properties of the adhesive interface. Double cantilever sandwich experiments and uniaxial tensile tests were performed to determine the corresponding model parameters. Furthermore, cohesive parameters were calibrated by applying an inverse analysis based on Hooke-Jeeves optimization algorithm. Good agreement was observed between the numerical simulation of double cantilever sandwich beam tests and the experimental curves. These results demonstrate that cohesive zone models can simulate the crack initiation and propagation of propellant/insulator interface in mode I. The bilinear law was shown to be more suitable for simulating fracture of the propellant/insulation interface in a strict sense than the exponential law. The numerical load-displacement curve was found to be sensitive to all cohesive parameters.     

Thermal Properties,Fracture Toughness and Water Absorption of Epoxy-Palm Oil Blends

Epoxidized palm oil (EPO) was blended with cycloaliphatic epoxide, epoxy novolac and diglycidyl ethers of bisphenol-A. The fracture toughness and thermal properties of epoxy/EPO blends were characterized using single-edge notched bending tests and differential scanning calorimetry. Increased EPO loading improved the fracture toughness (K _IC ) of the epoxy blends. The epoxy blends with higher EPO loading exhibited higher degree of conversion. The glass transition temperature (T _g ) of the epoxy blends shifted to higher temperature as the increasing of DSC heating rate. Water absorption caused T _g reduction of epoxy blends but it was determined that the water molecules absorbed were totally reversible.     

Effect of Sol-Gel-Derived Nano-silica on the Properties of Natural Rubber-Poly Butadiene Rubber-Reclaim Rubber Ternary Blends/Silica Nanocomposites

Silica incorporation into natural rubber (NR)-polybutadiene rubber (PBR)-reclaim rubber (RR) ternary blend system was carried out by sol-gel technique at different temperatures. The effect of RR on silica reinforcement was studied for NR-PBR-RR blend systems. The physicochemical properties of sol-gel vulcanizates indicates that the reinforcing efficiency of the nanocomposites increases with increasing RR content. Sol-gel vulcanizates prepared at 50°C shows superior mechanical properties than others. The amount of silica incorporated by sol-gel technique was determined through thermogravimetry analysis, which indicates the increasing trend of thermal stability with silica content. SEM studies indicate the coherency and homogeneity in the NR-PBR-RR/SiO ₂ nanocomposites.     

Use of building rubbles as recycled aggregates

The application of building rubble collected from damaged and demolished structures is an important issue in every country. After crushing and screening, this material could serve as recycled aggregate in concrete. A series of experiments using recycled aggregate of various compositions from building rubble was conducted. The test results show that the building rubble could be transformed into useful recycled aggregate through proper processing. Using unwashed recycled aggregate in concrete will affect its strength. The effect will be more obvious at lower water/cement ratios. When the recycled aggregate was washed, these negative effects were greatly improved. This is especially true for the flexural strength of the recycled concrete. The recycled coarse aggregate is the weakest phase at a low water/cement ratio. This effect will dominate the strength of recycled concrete. This mechanism does not occur in recycled mortar. The quantity of recycled fine aggregate will govern the mortar strength.     

Reaction sintering of alumina/mullite nanocomposites from nano-sized starting powders

Alumina/mullite ceramic nanocomposites were prepared by the mixtures of nano-sized starting powders of alumina with silica and alumina with silicon carbide. Silica from deliberate addition and as the product of silicon carbide oxidation reacted completely with alumina to form mullite. Silica from direct addition segregated at the grain boundary and intergranular mullite was formed whereas silica from oxidation was surrounded by alumina matrix and intragranular mullite was formed after reaction sintering. The most significant difference was fracture behaviour where intragranular mullite nanoparticles promoted transgranular fracture in alumina matrix due to thermal mismatch around nanoparticles and intergranular mullite nanoparticles gave rise to intergranular fracture similar to pure alumina. Wear resistance of the nanocomposites was better than that of alumina. Pull-out formation in the nanocomposites was less and pull-out size was also smaller. Fracture toughness of the nanocomposites was significantly higher than that of alumina.     

Enhanced mechanical properties of yttrium doped ZnO nanoparticles as determined by instrumented indentation technique

Yttrium doped (1, 3 and 5?wt%) zinc oxide nanoparticles were synthesized via sol-gel process. The phase, structural and mechanical properties were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and micro hardness based on indentation technique. The lattice parameters and grain sizes of the samples were calculated from the XRD data. As the lattice parameters increased, the grain sizes decreased dramatically, resulting in more grain boundaries and strong grain connectivity in the ZnO microstructure. Load-depth curves were obtained by applying indentation loads in the range from 400 to 2000?mN at room temperature. As the Y concentration increased, a significant increase was observed in the hardness values computed from loading-unloading curves using the Oliver and Pharr method. The indentation modulus of the samples reached a saturation value for 3% Y and then decreased as the doping rate increased. Moreover, the crack formation around the indent on the sample surface was examined by electron microscopy and was identified as radial/median type. The fracture toughness of the samples was calculated using the Vickers indentation fracture method. Increased fracture toughness values confirm that ZnO nanoparticles are mechanically strengthened by Y doping.     

Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics

Damage resistance and R-curve behavior of multilayer Al₂O₃/SiC ceramics were evaluated in bending by the indentation-strength and the single-edge-notched-beam methods. Due to the crack deflection at the Al₂O₃/SiC interfaces, a plateau indentation strength response was achieved, suggesting an exceptional resistance to contact-induced damage. Moreover, fracture toughness was observed to increase from 8.0 to 15.5 MPa m^1/2 with increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior.     

Ternary Gels of a Polymer Blend and a Recycled Oil: Their Use as Bitumen Modifiers

A blend of random ethylene‐vinyl acetate copolymer (EVA) and triblock styrene‐butadiene‐styrene copolymer (SBS) was dissolved in a recycled engine oil to obtain ternary thermoreversible gels. As the temperature was increased, first a network associated with EVA disappeared, and a second one associated with SBS dominated, maintaining the elastic response of the system. The principal advantage of these ternary systems is that their mechanical properties and thermal stability are better than that of binary gels. These gels, made from waste, can be used as bitumen modifiers to obtain binders of improved properties and good stability, which are useful for road surfacing.

Temperature sweeps of elastic modulus performed at a frequency of 1 Hz.     

Influence of Submicron TiO2 Particles on the Mechanical Properties and Fracture Characteristics of Cured Epoxy Resin

Submicron titanium dioxide (TiO₂) was used in different weight fractions as a toughening agent for amine-cured epoxy resin. After the use of X-ray photoelectron spectroscopy (XPS), which confirmed that the TiO₂ particles were evenly distributed in the cross-linked epoxy resin matrix, the composites were characterized by tensile and impact testing, followed by scanning electron microscopy of the fracture surfaces. The results indicated that the submicron TiO₂ toughening particles markedly improved the mechanical properties of the cured epoxy resin compared to the untoughened epoxy resin. The optimal properties were achieved at a TiO₂ concentration of 4 wt. %, at which point the toughness and the impact resistance values increased by 65% and 60%, respectively. The results also indicated that an increase in the amount of TiO₂ causes a decrease in toughness. Stress whitening, out-of-plane flaking, and thumbnail markings were the major visible features of the toughening mechanisms.

It is suggested that, at 4 wt. % of the submicron TiO₂ particles, microvoids are developed in the epoxy matrix. These microvoids are able to absorb some of the deformation work applied to the material, and thus enhance the toughness of the material. On increasing the TiO₂ content in the matrix (> 4 wt. %), the submicron particles got closer to each other and the microvoids were converted to macrovoids, which may act as stress concentrating flaws, leading to the deterioration of the mechanical properties of the epoxy resin.     

A Survey of Computational Models for Adhesion

This work presents a survey of computational methods for adhesive contact focusing on general continuum mechanical models for attractive interactions between solids that are suitable for describing bonding and debonding of arbitrary bodies. The most general approaches are local models that can be applied irrespective of the geometry of the bodies. Two cases can be distinguished: local material models governing the constitutive behavior of adhesives, and local interface models governing adhesion and cohesion at interfaces in the form of traction–separation laws. For both models various sub-categories are identified and described, and used to organize the available literature that has contributed to their advancement. Due to their popularity and importance, this survey also gives an overview of effective adhesion models that have been formulated to characterize the global behavior of specific adhesion problems.     

Deriving the primary cumulative distribution function of fracture stress for brittle materials from 3- and 4-point bending tests

In this article, the primary three-parameter Weibull cumulative distribution function (cdf) of the critical stress provoking failure in a brittle material for a uni-axially and uniformly tensioned area ΔA is derived from 3- and 4-point bending test data. The model proposed finds application in the characterization of ceramics and glasses, and is intended as an initial step to be extended to different practical cases in future applications, as for instance, element design and local models in fracture mechanics, with previous consideration of the randomly distributed crack orientations. A comparison of the results provided by the model proposed with those found using another one referred to in the literature, demonstrates good agreement between both, whereas the former simplifies the convergence procedure and can be applied for the assessment of data obtained from different test geometries and types. Thus, the suitability of the new approach is confirmed.     

Growing more ductile epoxies: an essential work of fracture study

Ambient, amine-cured epoxy compositions exist within the dual constraints of VOC regulation and the vitrification effect, which limits the ultimate T _g of these materials. These combined constraints result in epoxy products that are densely crosslinked and which contain appreciable quantities of nonfugitive diluents or plasticizers. Such materials tend to be more brittle than traditional solvent-based epoxy coatings based on solid epoxy resin and polyamide hardeners. There is thus a need for a practical method to measure their fracture toughness. This work introduces the method of essential work as a useful way to determine the fracture toughness of thermoset systems. This method is then used to relate fracture toughness to the crosslink network structures of amine-cured epoxy compositions. The test compositions are varied systematically with a view to structure/properties interpretation, and employ an in situ chain extension approach to “grow” more ductile networks. Solvent uptake of selected compositions is also determined, and the relative trade-off between ductility vs. solvent uptake is examined. This article was awarded the Outstanding Paper Award in New Coatings Technology at the 33rd Annual International Waterborne, High-Solids, and Powder Coatings Symposium in New Orleans, LA, February 2006, and was presented at the Thermoset Resins Formulators Association Conference in Montreal, Quebec, Canada, September 2006.     

In situ synthesis,mechanical properties,and microstructure of reactively hot pressed WB4 ceramic with Ni as a sintering additive

In this study, tungsten tetraboride (WB₄) ceramics were synthesized in situ from powder mixtures of W and amorphous B with Ni as a sintering aid by reactive hot pressing method. The as-synthesized ceramics exhibited porosity as low as 0.375% and ultra-high Vickers hardness (H_v), as much as 49.808?±?1.683?GPa (for the low load of 0.49?N). It was seen that the addition of Ni greatly improved the sinterability of WB₄ ceramic. Besides, the flexural strength and fracture toughness of WB₄ ceramic were measured for the first time to be 332.857?±?36.763?MPa and 4.136?±?0.259?MPa?m^1/2, respectively, suggesting that the ceramic has good mechanical properties. The effects of sintering temperature and holding time on the densification, Vickers hardness, and mechanical properties of WB₄ ceramics were also investigated systematically as part of our study. The results indicated that increasing the sintering temperature can obviously improve the densification and mechanical properties of the ceramics. The bulk density and Vickers hardness of WB₄ ceramic sintered at 1650?°C for 60?min under 30?MPa revealed the highest values of 6.366?g?cm^?3 and 27.948?±?0.686?GPa (for the high load of 9.8?N), respectively. The flexural strength increased to the highest value of 332.857?±?36.763?MPa for sintering temperature up to 1550?°C, but decreased slightly as the sintering temperature further increased to 1650?°C. On the other hand, the fracture toughness increased gradually with increasing temperature. It was also found that Vickers hardness showed a similar trend as the densification of the samples with increasing temperature and holding time. Besides, no obvious improvements in the densification, mechanical properties, and Vickers hardness of the samples with sintering time were observed in this study. The microstructure and fracture behaviours of the as-synthesized WB₄ ceramic were also revealed, and the toughening mechanism has been discussed.     

Use of tuffs from central Turkey as admixture in pozzolanic cements: Assessment of their petrographical properties

Tuffs from Galatean Volcanic Province were studied for their use as admixtures in pozzolanic cements. The effects of petrographical properties on the pozzolanic activity of mortar specimens were investigated by optical microscopy, X-ray powder diffraction (XRD), scanning electron microscope equipped with an energy dispersive X-ray system (SEM-EDX), and chemical analysis. The chemical compositions of tuffs conform well to the requirements of ASTM C 618 and the Turkish Standard TS 25, and SiO₂+Al₂O₃+total Fe₂O₃ exceeds 70%. Pozzolanic activities were determined according to their 7th day flexural and compressive strengths and vary between 1.7 and 3.0 MPa and 7.4 and 16.0 MPa, respectively. The mechanical strength of mortars is affected by alteration of tuffs used in the mixture. Clay minerals and zeolites form by the alteration of volcanic glass, which is the most reactive phase and has a reducing effect on mechanical strength. The alteration also causes the enrichment of tuffs with respect to K₂O+Na₂O. The methods used provided rapid evaluation of tuffs as potential admixtures in cements.     

Use of Functional (Meth)acrylic Cross‐Linked Polymer Microparticles as Toughening Agents for Epoxy/Diamine Thermosets

(Meth)acrylic cross‐linked polymer microparticles (CPM, also named microgels) were used as toughening agent for an epoxy/amine network. CPM were mainly based on butyl acrylate and consequently they were rubbery at ambient temperature. Various types of reactive groups were introduced onto the CPM: epoxy, carboxy (meth)acrylic double bonds, and epoxy + acrylic double bonds, carboxyl + methacrylic double bonds. Non functional microparticles were also used. Before any reaction, most of the CPM were soluble in the thermoset precursors. Nevertheless, the CPM functionality strongly influenced their initial miscibility in the epoxy‐amine monomers and their final dispersion in the cross‐linked matrix, as well as the mechanical properties of the network. Non‐functional CPM did not lead to a high increase of fracture toughness because of the low adhesion between microparticles and epoxy matrix. However, fracture toughness was increased with reactive CPM because of better adhesion between the microparticles and the matrix. The best toughness was obtained with microparticles containing two types of reactive groups, allowing at the same time cross‐linking reactions between CPM and chemical bonding between CPM and the epoxy matrix. In this case, fracture toughness can be greatly improved, up to 3‐times if the chemical composition of the microparticles was wisely chosen, without significantly reducing the thermal properties.

Viscoelastic properties of toughened DGEBA/MCDEA networks.     

Cure Characteristics and Mechanical Properties of Short Nylon-6 Fiber Neoprene Rubber Composite Containing Epoxy Resin as Bonding Agent

ABSTRACT

Cure characteristics and mechanical properties of the short nylon fiber reinforced neoprene rubber with and without epoxy bonding agent at various fiber loadings were studied. The fiber loading was varied from 0 to 30 phr and the resin content was in the range 0 to 5 phr. Minimum torque and cure time were increased in the presence of resin. Mechanical properties like tensile strength and abrasion resistance showed an increase with resin concentration. It was found that epoxy based bonding agents enhanced the properties of short nylon fiber reinforced neoprene rubber.     

Toxic Species Emissions from Controlled Combustion of Selected Rubber and Plastic Consumer Products

The main objective of this research is to identify the principal toxic species, either airborne or ash phase, expected to be released to the environment when selected plastics or rubberized materials undergo controlled combustion. The results are indicative of what can occur in municipal incinerators, in residential or industrial fires, or in open-air burning of waste materials in areas that are not serviced by trash pickup. The current emphasis is on materials used by the shoe manufacturing industry, especially rubber and plastic-type materials. The results are compared to those obtained during the combustion of a vehicle rubber tire, which was adopted as an arbitrary standard for comparison because of its current importance in recycling efforts. In addition, a comparison is made with the published results on a related topic dealing mainly with polyvinylchloride plastics. Highly toxic gases such as hydrocyanic acid, sulfur dioxide, and hydrogen chloride were among the main substances found during gas colorimetric testing. More than 92% of the particulate mass was found to be in the respirable range (i.e., less than 10 μm in size as based on cascade impactor analysis). In addition, toxic heavy metals, such as lead, chromium, and antimony, were detected in the smoke and ash phases of some of the materials. One of the materials analyzed (USA rubberized sole) was found to generate more hazardous gaseous contaminants (hydrogen chloride in both the smoke and ash phases, in addition to hydrocyanic acid and sulfur dioxide) than the rubber arbitrary standard. This result is suggestive of the need for additional studies with a larger sampling base. Should future studies show a similar trend, then recycling efforts to collect the huge amount of rubber and plastic that is discarded every year as harmless waste in the form of footwear would seem to be in order.     

Use of limestone obtained from waste of the mussel cannery industry for the production of mortars

Various types of cement−SiO₂−CaCO₃ mortar were prepared by replacing quarry limestone aggregate with limestone obtained as a by-product from waste of the mussel cannery industry. The CaCO₃ aggregate consists mainly of elongated prismatic particles less than 4 μm long rather than of the rounded particles of smaller size (2-6 μm) obtained with quarry limestone. The mechanical and structural properties of the mortars were found to be influenced by aggregate morphology. Setting of the different types of mortar after variable curing times was evaluated by scanning electron microscopy (SEM), thermogravimetric analysis (TG) and mercury intrusion porosimetry (MIP) techniques. Mortars with a high content in mussel shell limestone exhibited a more packed microstructure, which facilitates setting of cement and results in improved mortar strength. The enhanced mechanical properties of the new mortars allow the cement content in the final mortar composition to be decreased and production costs to be reduced as a result.     

Synthesis and Characterization of Novel Phosphorous-Coupling Agent for Use as Compatibilizer,Flame Retardant,Hardener and Adhesion Promoter in Making Polymer Networks

A new phosphorous containing coupling agent namely, diethylenetriamino-diethylphosphate (DTDP) was synthesized. The structure was confirmed by means of FT-IR, ¹H-NMR, ¹³C-NMR spectra, and mass spectral analysis. The coupling ability of DTDP was investigated by blending it with DGEBA and PDMS, and thus obtained blends have reflected in an increase in the modulus, glass transition temperature and adhesion strength property between the metal-to-metal interface. The high thermal stability, IPDT temperature and flame retardant properties (LOI) were attributed to the presence of phosphorous atom in the coupling agent.     

Fracture toughness of PAN-based carbon fibers estimated from strength-mirror size relation

Fracture toughness (K_IC) of representative high-strength type PAN (polyacrylonitrile)-based carbon fibers, Torayca™ T300 and T800H, with or without artificial surface defects, were estimated to be ca. 1 MPam^1/2 from the tensile strength vs. fracture mirror size relation, assuming a constant crack-to-mirror size ratio. The corresponding critical energy release rate (Γ) was ca. 7.4 J m⁻², which was close to the value derived from the reported surface energies for a graphite crystal. Similar K_IC values were obtained for the old-type PAN-based carbon fibers from the reported data by the use of the present estimation procedure.