Jakub Antoš

Use of Open Source DIC Tools for Analysis of Multiple Cracking in Fiber-Reinforced Concrete

Digital image correlation (DIC) became indispensable when monitoring and analyzing adevelopment of displacement or strain fields. The method is capable of capturing strain localization, itis not limited to a relative measurement of discrete points as conventional methods and appears to bemore accurate than measurements by means of extensometers or strain-gauges that often suffer fromimperfect attachment to the measured surface. As open source DIC tools appear, the method becomesfeasible and the development is supported by the growing computational power of modern computers.The presented paper introduces open source 2D DIC tools and simple rules to follow when employingthe method. The presented case study on behavior of fiber reinforced high-performance concretedemonstrates a computational feasibility, accuracy and sensitivity of the method for a relatively lowfinancial cost.

Influence of Aggregate Stiffness on Fracture-Mechanical Properties of Lime-Based Mortars

Addition of relatively stiff aggregates into lime-based mortars is responsible for an increase of the effective mortar stiffness and stress concentrations around aggregates during mechanical loading. To observe the damage development during the three-point bending and splitting tests a 2D plane-stress nonlinear finite element analysis utilizing isotropic damage model was carried out and the results were validated against experimentally obtained data. The study revealed that the finite element model is able to capture the trends observed during the experimental investigation. The results of the numerical modeling and experimental investigation show the advantages of the use of relatively compliant crushed brick aggregates in ancient structures.

An integrated experimental-numerical study of the performance of lime-based mortars in masonry piers under eccentric loading

Abstract The paper focuses on the performance of various lime-based materials, alternative to Portland cement mortars, intended for application in repairing historic structures when subjected to mechanical loading. Results of basic material tests indicate that the use of metakaolin as a pozzolanic additive produces mortars with superior strength and sufficiently low shrinkage. Moreover, mortar strength can be further enhanced by the addition of crushed brick fragments, which explains the longevity of Roman concretes rich in pozzolans and aggregates from crushed clay products such as tiles, pottery, or bricks. An integrated experimental-numerical approach was used to identify key mortar parameters influencing the load-bearing capacity of masonry piers subjected to a combination of compression and bending. The simulations indicate increased load-bearing capacities for masonry piers containing metakaolin-rich mortars with crushed brick fragments, as a result of their superior compressive strength.

A jigsaw puzzle metamaterial concept

Abstract A concept of a planar modular mechanical metamaterial inspired by the principle of local adaptivity is proposed. The metamaterial consists of identical pieces similar to jigsaw puzzle tiles. Their rotation within assembly provides a substantial flexibility in terms of structural behavior, whereas mechanical interlocks enable reassembly. The tile design with a diagonal elliptical opening allows us to vary elastic properties from stiff to compliant, with positive, zero, or negative Poisson’s ratio. The outcomes of experimental testing on additively manufactured specimens confirm that the assembly properties can be accurately designed using optimization approaches with finite element analysis at heart.

PERFORMANCE OF CEMENT COMPOSITES REINFORCED WITH SURFACE-MODIFIED POLYPROPYLENE MICRO- AND MACRO-FIBERS

This paper focuses on the mechanical properties investigation of cement pastes reinforced with surface treated polymer fibers. The cement matrix was composed of Portland cement (CEM I 42.5 R, w/c ratio equal to 0.4). Two polypropylene fiber types (micro- and macro-fibers) were used as randomly distributed and oriented reinforcement in volume amount of 2 %. The fibers were modified in the low-pressure inductively coupled cold oxygen plasma in order to enhance their surface interaction with the cement matrix. The investigated composite mechanical properties (load bearing capacity and response during loading) were examined indirectly by means of four-point bending mechanical destructive tests. A response of loaded samples containing treated fibers were compared to samples with reference fibers. Moreover, cracking behavior development was monitored using digital image correlation (DIC). This method enabled to record the micro-cracks system evaluation of both fiber reinforced samples.

ASSESSMENT OF 2D-DIC STOCHASTIC PATTERNS

Cost of experimental testing of materials and structures subjected to mechanical loading often constitutes a significant portion of a project budget. Therefore a collection of data in a maximum possible amount is desirable. Extensometers and strain-gauges attached to the specimen surface often fail and cannot provide full-field information about the development of displacements and strains. Digital Image Correlation (DIC) is capable of providing such information. Unsuitable texture or artificially applied pattern, essential for DIC analysis, can spoil the DIC outcomes completely. An additional investment into preparation of new experimental testing can be avoided with the use of tools employing algorithms for stochastic pattern assessment. The development of such algorithms and their implementation into an open-source DIC software is the goal of the presented research.

Effect of Curing Humidity on Stiffness of Cement Based Mortars with Recycled Fillers

Values of the dynamic Young’s modulus were monitored on cement-based samples containing recycled concrete. In particular, the study was focused on investigation of the relationship between the elastic stiffness and amount of recycled microfiller to replace Portland cement. The amount of the microfiller was ranging between 30 and 70 % per weight of the samples. The samples were cured in common laboratory conditions for 50 days, followed by their placement into a water bath for 2 days and consequent natural drying period for 6 days. The development of elastic stiffness was monitored using non-destructive impulse excitation method in order to demonstrate the influence of hydration when the specimens are stored in high humidity. The increase of elastic stiffness proves a secondary hydration of both – Portland cement and cement grains from the recycled cementitious material.

Use of Digital Image Correlation to Track Strain Evolution in Compressed Masonry Piers

In order to develop a constitutive material model and to verify its consistency when implemented in a computational code, it is necessary to understand the material and to carry out a comprehensive experimental analysis. This can be a challenging task in the case of composite materials and structures, such as masonry, when using conventional measurements. Strain gauges and allow recording strains at a limited number of discrete points and do not provide sufficient amount of data, thus increasing the cost of the analysis. From that reason a full-field non-contact measurements, such as Digital Image Correlation (DIC), became very popular and valuable for analysis of structures subjected to mechanical loading and precise detection of the onset of strain localization. The presented study deals with tracking the strain localization using DIC in the case of masonry piers loaded by the combination of bending and compression. In such case the strain localizes into more compliant mortar joints while the complete collapse occurs when the masonry blocks fail to transfer tensile stress due to transversal expansion. The obtained data will be used for the validation of a finite element model to predict the behavior of masonry structures.