Jaroslav Valach

Hybrid Testing of Historic Materials

The paper describes hybrid approaches to testing of mechanical properties of historic materials which are mostly available in very small amounts and bulks unsuitable for standard testing procedures and techniques. Three case studies are presented. In all three, simple mechanical testing procedures were combined with special optical measurements and tailored software tools.

Inspection of Post Impact Fatigue Damage in Carbon Fibre Composite Using Modulus Mapping Technique

This study is focused on inspection of damage extent induced into C/PPS composite material by fatigue and impact loading. Initial damage to specimens was induced by drop-weight out-of-plane impact damage. Several levels of damage states (intact specimen, fatigued and impacted specimen, ruptured specimen) were inspected using modulus mapping (MM) technique. Quantification of the damage level was based on comparison of results from MM obtained in distinct locations on the specimens. Regions of interest were selected in order to determine magnitude of damage after impact and to assess remaining loading capabilities of the material. For this purpose, material maps provided information about location where matrix had been inflicted by the damage. Results show that impact loading has no measurable influence on mechanical properties of the matrix. However, gradient in mechanical properties was detected in the vicinity of crack. Results were validated using quasi-static nanoindentation and constant strain rate continuous measurement that showed depth profile of mechanical properties.

Evaluation of Material Parameters of Multiphase Materials Using Drift Distortion Corrected SEM Imaging

In contrast to imaging in visible spectrum temporally-varying distortion (drift distortion) caused by positional errors of electron beam during the scanning process occurs in SEM devices. This effect is always present in the data from SEM and its magnitude depends on acquisition time, magnification and conductivity of the sample (where lower conductivity causes higher drift distortion). In this paper 2D digital image correlation based on Lucas-Kanade algorithm was used to assess drift distortion characteristics of MIRA II LMU SEM device during imaging of materials with different conductivity and microstructural properties. Using the DIC technique deformations in the micrographs were evaluated and tool for correction of positional errors was developed. As shown on a set of selected multiphase mixtures this tool enables qualitative backscattered electron analysis independently on the material type and imaging parameters including acquisition time. This enables reliable evaluation of material parameters that have influence on effective mechanical properties.

Evaluation of Local Plastic Flow in the Vicinity of Indentation by the Means of DIC Applied on SEM Micrographs

The paper presents application on digital image correlation (DIC) and microindentation for investigation of plastic flow under Brinell ball indenter applied on steel specimen made of two parts screwed together. Specimens in two different material states (a) as delivered (b) annealed were investigated. This approach enables internal surface to act as the external one and to be examined by aforementioned methods. Results obtained by application of DIC on scanning electron microscope (SEM) images are compared to microhardness maps and agreement is demonstrated.

Material Testing of Natural Stones Used in Historical Buildings Based on Scanning Electron Microscopy and Nanoindentation

In the case of material parameters required for evaluation of built heritage preservation state, knowledge of the least possible volume of removed sample is essential in order to minimize damage accumulation to the buildings. These requirements lead to determination of representative volume element (RVE) that was in this paper calculated using combination of image and signal processing techniques. Then, a detailed map of material properties was created using nanoindentation to evaluate local characteristics of the material. Furthermore, atomic composition of samples was quantified by energy dispersive spectrometry (EDX) detector for scanning electron microscopy (SEM) device. Presented method based on combination of SEM, EDX and nanoindentation techniques demonstrates possibility of effective testing in the field of historical buildings preservation.