Martin Černý

Resonant frequency study of tensile and shear elasticity moduli of carbon fibre reinforced composites (CFRC)

Abstract The dynamic elastic properties are important characteristics of composite materials. They control the vibrational behaviour of composite structures and are also an ideal tool for monitoring of the development of CFRCs’ mechanical properties during their processing (heat treatment, densification). The present studies have been performed to explore relations between the dynamic tensile and shear moduli and some structural features (viz., fibre fraction, fibre type, porosity, weave pattern of woven reinforcement) of various unidirectional or bi-directional fibre reinforced carbon/carbon composites, made out of PAN- or pitch-based fibres as reinforcements and phenolic resin or coal tar pitch as matrix precursors. The dynamic tensile and in-plane shear moduli were determined from resonant frequencies of a beam with free ends. The longitudinal dynamic Young’s modulus of unidirectional CFRC composites – besides its dependence on the original fibre modulus and fibre volume contents – also reflects changes induced in matrix and fibres by heat treatment. The in-plane shear modulus does not depend on the fibre type but there exists its distinct tendency to increase with increasing fibre fraction. For bi-directionally reinforced composites, the longitudinal tensile modulus is more sensitive to the fabric weave pattern than to the fibre type. Tensile modulus of diagonally cut specimens and in-plane shear modulus of longitudinally cut ones are mutually correlated and, therefore, simultaneously controlled by densification steps and graphitisation heat treatment.

Strength, elasticity and failure of composites with pyrolyzed matrices based on polymethylsiloxane resins with optimized ratio of D and T components

Two mixtures of T and D siloxane monomeric components labelled as TxDy (molecular ratio x:y equal 3:1 or 4:1) were chosen as matrix precursors for manufacturing Nextel720 reinforced unidirectional composites which, after pyrolysis at 1000 or 1100°C, revealed good endurance in an oxidizing environment up to 1500°C. Vickers hardness of the heat treated (1000–1500°C) samples of pyrolyzed matrices T3D1 and T4D1 are mutually similar (1100–1400 HV0.2) and reach their maximum between 1200–1300°C. Flexural strength of the pyrolyzed composites is 150–170 MPa and 170–250 MPa for T3D1 and T4D1, respectively. After annealing 3 h in air at 1200–1300°C, the strength slightly decreases but similar treatment at 1500°C yields strengths exceeding those of the pyrolyzed material. Shear modulus of the pyrolyzed T4D1 composite is roughly twice that of the T3D1 one (15 GPa vs. 8 GPa) and both increase sharply to 22–25 GPa after annealing at 1500°C, which manifests substantial improvement of the matrix properties. Fracture toughness of the composites, as measured by chevron notch test at RT, 550°C, and 1100°C, yields 4–5 MPa.m−1/2 for T3D1 and 3–4 MPa.m−1/2 for T4D1. For both composite types, the fracture toughness drops by 1 MPa.m−1/2 when measured at 550°C, which can be attributed to suppression of fibre pull-out due to stress state changes caused by the coefficient of thermal expansion (CTE) mismatch. Fracture surfaces generated during flexural tests of the annealed samples reveal decreasing occurrence of pullout towards the highest annealing temperature.

Properties of modified polysiloxane based ceramic matrix for long fibre reinforced composite materials

Abstract The main goal of the work was to prepare a cost effective and simple to preform high temperature matrix for composite materials. To fulfil expectations, it was necessary to optimise the design of the composite to have an optimal fibre–matrix interaction. A number of modified polysiloxane resins were studied in various steps of heat treatment. This contribution deals with changes in the behaviour of the matrix as a stay alone material. This knowledge enables the optimisation of composite properties. A fully instrumented indentation technique for the determination of reliable parameters characterising the microstructural changes was used. The fracture behaviour of the prepared composite matrixes was evaluated in terms of indentation cracks. Both optical and scanning electron microscopies were employed in microstructural observations and fracture mechanism qualification.

Role of Pyrolysis Conditions on Fracture Behaviour of Fibre Reinforced Composites

Fracture response of matrix prepared by pyrolysis of polysiloxane resin used for composite reinforced by long fibres was the main goal of this contribution. A set of composites with matrix prepared by partial pyrolysis of polysiloxane resin was studied. An effect of pyrolysis temperature on the composite behaviour and fracture resistance was monitored. An optimal procedure of pyrolysis was established. Heat treatment at 1550°C in air atmosphere was conducted on fully pyrolysed matrix to explore its high temperature potential. Determination of reliable parameters characterising microstructural changes in the matrix by instrumented indentation technique was used. Both optical and scanning electron microscopy was employed in microstructural observations and fracture mechanism qualification. Observation of indents and associated cracking caused by microstructural changes as well as 3D surface reconstruction using confocal microscopy was employed.

Fracture Properties of Basalt Fibre Composites with Cured or Pyrolysed Matrix

Composite materials based on polysiloxane matrix reinforced by basalt fibres were prepared in laboratories of the IRSM ASCR. The composite samples were pyrolysed at 400 ÷ 750 °C after moulding and curing at 250 °C. Measurement of several mechanical characteristics (flexural strength, fracture toughness, impact strength, and measurement of elasticity) demonstrates a favourable influence of pyrolysis in comparison with the cured-only composite material. Fracture toughness was measured by chevron-notch technique and fracture surfaces were investigated using a scanning electron microscopy.

Fracture response of SiOC-based composites on dynamic loading:

The fracture resistance to the dynamic loading of composites based on partially pyrolysed SiOC glass reinforced with basalt fibres was investigated using an instrumented impact test. The three-point bend loading configuration of the specimen bars prepared from the composite plate was used. Evolution of the matrix during the pyrolysis plays a key role in creation of bonding between the matrix and fibres, which was in the form of woven fabrics. Different temperatures of pyrolysis were applied in order to determine their influence on the fracture behaviour during impact tests. The high speed camera was employed to observe fracture response to impact loading during the fracture process. The temperature range for pyrolysis was chosen from 600 to 800℃ with the step of 50℃. The partial melting of basalt fibres occurred at temperatures above 800℃. Observation of fractographic features on the fracture surfaces was performed using scanning electron microscopy.

Fibre-Matrix Interface Development during High Temperature Exposition of Long Fibre Reinforced SiOC Matrix

The fracture behaviour of long fibre reinforced composites is predetermined mainly by properties of fibre-matrix interface. The matrix prepared by pyrolysis of polysiloxane resin possesses ability to resist high temperatures without significant damage under oxidising atmosphere. The application is therefore limited by fibres and possible changes in the fibre matrix interface. The study of development of interface during high temperature exposition is the main aim of this contribution. Application of various techniques as FIB, GIS, TEM, XRD allowed to monitor microstructural changes in the interface of selected places without additional damage caused by preparation. Additionally, it was possible to obtain information about damage, the crack formation, caused by the heat treatment from the fracture mechanics point of view.