Mats L. Ericson

Transverse single-fibre test for interfacial debonding in composites: 1. Experimental observations

Abstract Micromechanical methods for studies of interfacial debonding in fibre composites provide the possibility of comparing different materials. Since most previous composite studies focus on shear loading of the fibre matrix interface, the debonding process in single-fibre glass fibre/epoxy composites was studied in situ by optical microscopy during transverse tensile loading. Specimens had cylindrical debond cracks of known dimensions created in fragmentation tests. Mechanisms for debond growth were described for two materials where the difference was in glass fibre surface treatment. As the debonds reached a critical size in the arc (circumferential) direction, unstable debond growth occurred in the fibre direction. The debond angle at instability was fairly similar for both materials, although the material based on glass fibre treated with a coupling agent reached twice as high stress before instability.

Transverse single-fibre test for interfacial debonding in composites : 2. Modelling

A method was developed for determination of interfacial toughness parameters from a new transverse single-fibre test. In previous work, experimental procedures were developed. Interfacial debond growth in the arc and fibre direction was measured for transparent matrix polymer composites. In the present study, an earlier stress analysis by Toya was modified and combined with a critical energy release rate criterion, taking mode mixity effects into account. The developed model was used to determine interfacial toughness parameters. The model was able to describe and explain the main experimental observations of the debonding process. Treatment of the contact problem present for large debond angles is of interest for future work.

Deformation and fracture of glass-mat-reinforced polypropylene

Abstract The mechanical properties of glass-mat-reinforced thermoplastics, GMT, can be varied within wide limits through the choice of glass mat, fibre content, and thermoplastic matrix. A great variety of fibre configurations and fibre contents is commercially available. In order to understand how the fibre structure of GMT materials controls the mechanical properties, two structurally different GMT materials were studied. Several grades with different fibre contents were used for each material. One material had short fibres (12 mm long), discretely dispersed with in-plane random orientation. The other material had in-plane continuous looped bundles of fibres. The deformation and fracture of these materials have been studied as a function of fibre content. The present work shows that the material with discretely dispersed fibres has higher values of tensile creep modulus, tensile strength, elongation at fracture, and work of fracture (area under the stress/strain curve). The results are discussed and related to the structural differences between the two materials.

Processing and mechanical properties of orientated preformed glass-mat-reinforced thermoplastics

Abstract The stiffness and strength of moulded, glass-mat-reinforced thermoplastics (GMT) components would be increased by the presence of highly orientated fibres at critical locations. A previously described method to produce preformed GMT materials was therefore further developed to make orientation of the fibres in the preform possible. Two ‘orientation plates’ were used to orientate the fibres during spray-up of a glass-fibre/polyethylene preform. The preform was then heated by hot gas and compression moulded. The ratio of the highest and lowest stiffnesses of a given plate was in the range of 2·7 to 3·8. Micromechanics equations were used with classical lamination theory to design a model laminate with stiffness properties in close agreement with experimental data. Reasons for the relatively wide fibre orientation distribution and low fibre length efficiency factor obtained for the model laminate are discussed and improvements suggested.

Specimen Size Effects on Modulus of GMT and Other Inhomogeneous Composites

Mean values and standard deviations are often used to characterize certain mechanical properties. It is widely known that some properties, e.g., tensile strength of brittle solids and their standard deviations, are affected by the volume studied. In the pres ent study, the mean value and the standard deviation for the longitudinal elastic modulus of a cuboid specimen are treated. A model to predict the mean values and standard devia tions for varying lengths and widths is presented. Experimental data from the literature for glass mat reinforced polypropylene is used to show that the model shows excellent ac curacy with experimental data.

Microscopy of the morphology in low styrene emission glass fiber/unsaturated polyester laminates

Low styrene emission (LSE) unsaturated polyester resins are of interest in the context of increasing environmental concerns in the society. LSE resins have been developed to decrease styrene emission during the processing of composites based on unsaturated polyesters. In this article we applied a microscopy methodology to study morphology effects in laminates based on LSE polyesters. The study connects to the longer term objective to improve the understanding of how additives reduce styrene emission without imparting delamination resistance in composite laminates based on LSE polyesters. The major morphology differences between laminates made from different polyesters are discussed, including birefringent layers present as an interphase between different layers.

Design and potential of instrumented ultramicrotomy

Ultramicrotomes are generally used for preparation of very thin sections for transmission electron microscopy. Recently it has been shown that when the sample holder of the ultramicrotome is instrumented with a force transducer, it is possible to measure the very small sectioning force during sectioning, and calculate the energy dissipated. In the present work, the instrumentation is further improved. The new sample holder, which uses two piezo-electric force transducers can measure two force components simultaneously. It is not only robust and stiff, but it also shows high sensitivity and reproducibility. It is possible to detect sectioning forces lower than 0.1 mN. The method is demonstrated on two amorphous polymers, poly(methyl methacrylate) and epoxy. Fracture energies in the same order of magnitude as theoretical predictions from chemical bond fracture only are recorded. It is therefore suggested that the method of instrumented ultramicrotomy is a useful tool when information on covalent bond density is needed. Potential future applications are identified including research on nano-scale fracture, characterization of molecular anisotropy and developments of the ultramicrotome.

A method of measuring energy dissipation during crack propagation in polymers with an instrumented ultramicrotome

In order to characterize very local energy dissipation during crack propagation in polymers, an ultramicrotome was instrumented to measure the energy dissipated during sectioning. The work to section per unit area, Ws, was measured for five different amorphous polymers [polymethyl methacrylate (PMMA), polystyerene (PS), polycarbonate (PC) and two epoxy resins] in the glassy state. When the section thickness was varied between 60 and 250 nm, Ws varied between 15 and 100 Jm−2, depending on the material and section thickness. The method and the results are compared with other methods used for determining the energy dissipation at a local level as well as at a macroscopic level in polymers. The differences between different polymers were found to be contradictory to macroscopic fracture toughness, Glc, measurements. The material that showed the highest Ws had the lowest Glc values reported. Possible mechanisms for energy dissipation during sectioning are also discussed.