• Orell, Olli Aleksi

Abstract

Determination of the mechanical properties of materials is largely based on established test methods that aim to generate simple, and typically uniaxial, stress state to the test specimen. If the test specimen behaves homogeneously, the deformation caused by the loading can be measured reliably by common methods. However, as the complexity of the tested material or the geometry of the test specimen increases, the general discrete measuring methods do not necessarily give a true picture of the behaviour of the specimen. This is particularly true for heterogeneous and anisotropic materials whose mechanical response is strongly dependent on the observation point, direction and scale. In these cases, reliable measurements require a more comprehensive examination of deformations, which is typically challenging with traditional discrete measuring methods.<br/><br/>Numerical analysis techniques (such as the finite element method) are increasingly taken advantage of in the identification of material properties, especially when non-uniform stress state is known to occur in the specimen during testing. However, detailed data of the deformed specimen is required in the simulation of the experiments. The measured data must be reliable, as the results of the models are as good as the used raw data.<br/><br/>Digital image correlation (DIC) is an optical method for determining the deformation field from the surface being studied. The objective of this thesis is to improve the reliability and precision of the results produced by the selected experimental test methods using DIC, especially when testing heterogeneous and non-linear–behaving materials. The work is focused on compression tests and methods for determining fracture mechanics properties of polymer coatings and adhesives. The thesis is based on five original scientific publications in which the selected test methods are used to characterise and identify properties of very different materials, including soft hydrogel, fibrous polymer composites, thin polymer coating and adhesive.<br/><br/>In the thesis, the developed method is presented for determining elastic material constants for orthotropic polymer composite using the continuous deformation field provided by DIC with high spatial resolution. DIC was also used to measure the realistic deformation of the soft and transparent compressed hydrogel specimen, enabling more valid characterisation of the stress-strain relationship of the material. The DIC experiments revealed hidden factors that affected the behaviour of the test specimens in the compression tests. The precision of the test methods was thus improved by the systematic use of DIC, since without the full-field measurements, the factors are difficult to recognise and consider in the analysis of the results.<br/><br/>Generally, the analysis of the fracture mechanics tests is based on the monitoring of the progressive crack growth in the test specimen, which is known to be a major source of uncertainty in the methods. The determination of the crack length is performed typically by visual means, which is challenging, if not impossible, especially in mode II tests where the crack faces do not separate per definition. This thesis presents the developed methods to quantitatively evaluate the crack propagation for the investigated fracture mechanics tests based on the deformation fields provided by the DIC. The presented methodology significantly reduces the uncertainty resulting from the subjective interpretation performed by the operator. Major effort is made to evaluate fracture testing with cyclic loading, i.e. fatigue.

Topics

• impedance spectroscopy
• surface
• polymer
• experiment
• simulation
• crack
• anisotropic
• fatigue
• composite
• compression test