In this work, the effect of adding short deformable polyvinylalcohol fibers into dimethacrylate resins on mechanical properties of composite materials was investigated. Furthermore, micro- or nano-fillers were added into the matrix. The effect of filler/short fibers combination on mechanical properties of so-called hybrid composites was studied. Composites were characterized by differential compensation photocalorimetry (DPC), termogravimetric (TGA) and dynamic mechanical (DMA) analysis. The fracture surfaces were examined using scanning electron (SEM) and confocal laser (CLSM) microscopy. Elastic modulus and strength, the critical value of stress intensity factor and strain energy release rate and the viscoelastic properties of composite materials were measured. The mechanical properties of composite materials are affected not only by the type of filler/reinforcement, but also by properties of the matrix. These properties also depend on the way of network formation during photo-initiated polymerization. For this reason, heat of polymerization, degree of conversion and the maximum polymerization rate of mixture of dimethacrylate monomers were determined.

This diploma work deals with relationship between composition and mechanical properties of polymeric composite with polypropylene matrix and magnesia hydroxide as a filler. The influence of content of filler (20,40,60 w.%) and temperature (-30°C +60°C) on dynamic fracture toughness is observed. The regresion function describing the dependence of studied mechanical properti on temperature for all experimental materials where found. Experimental data were confront with data gained on the same materials then 10 years ago and influence of time on mechanical properties of follow - up composites is evaluate.

The standards of fracture toughness determination prescribe size requirements for size of test specimens. In cases of limited amount of test material miniature test specimens offer one from the possibilities of fracture toughness evaluation. Because of small loaded volumes in these specimens at the crack tip the loss of constraint occur affecting measured values of fracture toughness. In such cases the size requirements for valid fracture toughness characteristics determination are not fulfilled. These specimens can be even on limits of load range of test devices and handle manipulation by their small dimensions. The important task related to these specimens is, apart from methodology of their preparation and measurement of deformations, the interpretation of measured values of fracture toughness and their possible correction to standard test specimens. Moreover, in the upper shelf region of fracture toughness quantification and interpretation of size effects is still not resolved sufficiently. This thesis is by its aims experimentally computational study focused on evaluation of size effect on fracture toughness in the upper shelf region. The size effect was quantified by testing of miniature and large specimens’ sizes in order to determine J R curves. Two geometries of miniature test specimens, there point bend specimen and CT specimen, were used. The experimental materials were advanced steels developed for applications in nuclear and power industry, Eurofer97 steel and ODS steel MA956. Finite elements analyses of realized tests together with application of micromechanical model of ductile fracture were carried out in order to evaluate stress strain fields at the crack tip in tested specimens from Eurofer97 steel. By comparison of experimental results and numerical simulations of J R curves the mutual dependencies between geometry of specimens and element sizes at the crack tip were derived. On the basis of acquired relationships, the methodology of J R curve prediction for standard specimen size from limited amount of test material was proposed. Main contribution of thesis is description of effect of material’s fracture toughness level on resistance against ductile crack propagation in miniature specimens. For material where significant crack growth occurs after exceeding the limit values of J integral (Eurofer97), the loss of constraint is considerable and highly decreases resistance against tearing. Miniature specimens then show significantly lower J R curves in comparison with standard size specimens. This effect is the opposite to the behaviour of miniature specimens in transition region. In case of material with low toughness, in which significant crack growth occurs in the region of J integral validity (ODS MA956), the effect of constraint loss is small without large impact on resistance against tearing. In such case miniature specimens demonstrate comparable J R curves as specimens of larger sizes. Next important contribution is proposed methodology for prediction of J R curve from small amount of test material using micromechanical modeling.

A ceramic vortex finder (CVF) is the central pipe (gas outlet) of a gas cyclone, which is an important structural component within gas cyclone separation. Its main application is within high temperature processes, within the energy and cement industry where the atmosphere is highly abrasive and corrosive. The CVF has the purpose of inducing well-defined flow field resulting in optimal performance of the cyclone.  Ceramics and refractories are superior when it comes to high temperature stability, resistance against chemical attacks along with abrasion resistance. The aim for a longer lifetime of the CVF creates an importance of further investigation into factors influencing the strength of the material. Fracture toughness is an important parameter to focus on in the CVF application. This is because it’s known that fracture is the major cause of failure for brittle materials. This creates an interest into further investigation of the parameters influencing the casting process including water addition and porosity.  This project work aimed to investigate the impact of water addition on the CVF material. The focus was on identifying how much the casting process influenced the fracture toughness. The results showed that 4.5% water addition is the best amount to use for the CVF application. This water amount gave the minimum porosity with the highest fracture toughness. The water amount ensured a good flowability of the castable along with a curing time above 50 minutes, making it possible to cast in the production.

Semiconductor die attach materials for high voltage, high reliability analog devices require high thermal conductivity and retention of dielectric strength. A comparative study of effective thermal conductivity and dielectric strength of selected thermoset/ceramic composites was conducted to determine the effect of ceramic particle size and ceramic particle loading on thermoset polymers. The polymer chosen for this study is bismaleimide, a common aerospace material chosen for its strength and thermal stability. The reinforcing material chosen for this study is a ceramic, hexagonal boron nitride. Thermal conductivity and dielectric breakdown strength are measured in low and high concentrations of hexagonal boron nitride. Adhesive fracture toughness of the composite is evaluated on copper to determine the composite’s adhesive qualities. SEM imaging of composite cross-sections is used to visualize particle orientation within the matrix. Micro-indentation is used to measure mechanical properties of the composites which display increased mechanical performance in loading beyond the percolation threshold of the material. Thermal conductivity of the base polymer increases by a factor of 50 in 80%wt loading of 50µm hBN accompanied by a 10% increase in composite dielectric strength. A relationship between particle size and effective thermal conductivity is established through comparison of experimental data with an empirical model of effective thermal conductivity of composite materials.

No description available.

MetPreg, a composite developed by Touchstone Research Laboratories (Tridelphia, WV), is an aluminum metal matrix composite reinforced by continuous NextelTM 610 alumina fibers. The question is, after processing, are the NextelTM fibers affected in any way that their strengthening contribution to the composite is reduced? From experimentation and statistical analysis, a strength distribution of pre-processed NextelTM 610 fibers is formed and an empirical correlation is developed relating strength to the observed flaw size on the failed single fibers. This correlation is then independently applied to flaw size information gathered from fibers on the fracture surface of MetPreg samples to develop a separate strength distribution of post-processed NextelTM 610 fibers. The pre- and post-processed distributions are compared to one another to determine the effect, if any, that composite processing has on the strength of NextelTM 610 fibers. The results indicate that the in-situ strength distribution of fibers was increased by composite processing. / Master of Science

This thesis is focused on the development of multiple experimental frameworks to characterize the material properties of composite materials for the LS-DYNA MAT213 model. The main objective is to characterize these properties based on the full-field capture of the evolution of strain and stress fields in coupon-level tests at multiple scales (i.e. macroscopic and microscopic). The experimental work characterized the full-field stress-strain curves and subsequently derived the material properties of T700G/LM-PAEK thermoplastic composites. The data was later successfully utilized to generate the deformation and damage sub-models in the LS-DYNA MAT213 model for the material. Additionally, a three-point bending test methodology was created using a size effect study and geometrically scaled coupons to investigate the Mode-II interlaminar fracture toughness of the material. The experimental frameworks developed herein were also extended to characterize other composite materials, such as those produced via additive manufacturing techniques. Future experimental work will investigate fatigue failure methods for three-point bending in T700G/LM-PAEK. The experimental methods described herein will also continue to support analytical efforts that seek to develop a simulation tool based on the LS-DYNA MAT213 model for modeling the temperature and strain rate-dependent impact damages in composites under multi-axial loading.

The thesis is focused on the prediction of the fracture toughness temperature dependence through a universal curve of fracture toughness (also known as the master curve). To determine the parameters of the universal curve of fracture toughness, values acquired from the measurement results of fracture toughness and tensile tests of structural steel P91 are used. The theoretical part is based on a summary of the relevant information from the field of fracture mechanics and brittle-ductile fracture behavior of steels that are important for the understanding of fracture-strain response of materials depending on load conditions. The experimental part of the thesis contains the results from practical measurements and analyses, which were used for determining the parameters of a universal curve of fracture toughness as well as for the evaluation of fracture behavior and description of the impact of structural parameters on this behavior in case of steel P91.

The goal of this research is to investigate the combined effects of glass fibre reinforcement and molecular orientation in polypropylene-short glass fibre composites. Specimens have been fabricated using the injection moulding process and drawn using a small die drawing rig. The effects of die drawing on the fibre composites are complex, with the drawing process orienting both the polymer molecules and the glass fibres. This may be accompanied by the creation of voids in the polymer matrix and their destruction in the compressive stress field thus restoring the interfacial contact area between fibre and matrix. Unfilled and short glass fibre filled polypropylene specimens, with fibre content 7% wt, 13%wt, 27%wt, and 55%wt, were injection moulded prior to the die drawing process. An experimental program of die drawing within an oven at elevated temperature was conducted for polypropylene filled to various levels and at different strain rates. The specimens drew to draw ratios in the range γ=1.41 to γ=5.6. Mechanical characterization of the test materials has been conducted by examining the tensile stress strain and fracture behaviour under uniaxial conditions. The influence of glass fibre content and drawing conditions (draw ratio) on the fracture toughness and crack propagation was investigated using the double edge notched fracture test. The notch lengths ranged from 1.5 to 2.5 mm for 10 mm wide specimens. The critical stress intensity factor increased as the fibre content increased up to a limiting filler level. The fracture toughness of both unfilled and fibre filled polypropylene were found to be highly dependent on draw ratio. The results were analysed to find out the optimal draw ratio and fibre content that yielded the maximum modulus, strength and fracture toughness. Data showed that, at a given draw ratio, modulus, strength and fracture toughness increased with increasing fibre content to a maximum and then decreased. The optimum material was obtained at a draw ratio of 2.5 and filler loading 13wt%.