Towards the measurement of two independent viscoelastic functions using spherical indentation

Instrumented indentation is a promising testing technique for obtaining the time dependent viscoelastic properties of bitumen and bitumen-based mastics. It offers several advantages: controlling the length scale of measurement and thus could be used in multi-scale modelling of asphalt, the possibility of in-situ tests and the minimal requirements of sample preparation. The measured viscoelastic properties are of high importance to the established models of asphalt deformation and fracture degradation with a direct e ect on the cost and service life of roads. Most of the viscoelastic indentation experiments and solutions in the published literature assume a constant and known Poisson's ratio and use the relaxation indentation method, a method that applies a Heaviside step function of indentation depth, this method introduces diculties for low stiffness materials with high relaxation rates such as bitumen. The goal of this report is to develop a testing method capable of measuring two independent viscoelastic functions while using a loading scheme, ramped load/depth, that would alleviate the problems that plague the viscoelastic indentation of low stiffness materials and specically bitumen. The method of functional integrals is used on the Hertz solution of the contact problem to obtain two independent equations, the equations requires measurement of the load, indentation depth and circumferential strain during indentation. non-linear regression is performed on the two equations simultaneously to obtain the viscoelastic parameters. Numerical simulations are used to validate the obtained analytical solutions and the e ects of nite specimen size and strain measurement errors are investigated. Polyoxymethylene (POM) is used as a model material to test the developed technique. Indentation relaxation tests and indentation ramped tests are performed on the material and the results are validated by uniaxial tensile creep tests. The ramped indentation test results proved successful in measuring the viscoelastic shear relaxation function, but with higher error at the beginning of the test. The diculties encountered during the tests are presented and analysed and their e ects are projected and discussed upon using the developed method on bitumen and bitumen{based mastics