User-defined material model for steel fibre reinforced concrete (SFRC)

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User-defined material model for steel fibre reinforced concrete (SFRC)

A new custom material model for steel fibre reinforced concrete has been developed as a user material law in ANSYS simulation software. The material model has the form of the hyperbolic Drucker–Prager yield criterion. Representation of the steel fibre reinforced concrete behaviour in tension is provided by trilinear softening function. The material model takes into account thermal dependency.

Results of the development process including real data calibration has been published in:

European Journal of Wood and Wood Products
Holz als Roh- und Werkstoff
ISSN 0018-3768

Article Citation: Ekr, J., Caldova, E., Vymlatil, P. et al. Timber steel-fibre-reinforced concrete floor slabs subjected to fire. Eur. J. Wood Prod. 76, 201–212 (2018). https://doi.org/10.1007/s00107-017-1221-8

Considered measurements have provided in cooperation with Czech Technical University in Prague (CVUT)

Motivation

In certain cases, the usual reinforced concrete may be replaced by a Steel Fibre Reinforced concrete (SFRC) mixture. This innovative structural mixture with specific hardened concrete properties was developed to reduce slab thickness, and it offers several advantages. For example, experimental and theoretical studies show that the presence of steel fibres may increase the ultimate strain and improve the ductility of fibre-reinforced concrete elements.

In order to be able to simulate the structures utilizing SFRC concrete mixture a special material model was developed as a user material law in ANSYS simulation software.

Theory

New material model for SFRC was developed as a user material law in ANSYS simulation software. The material model has the form of the hyperbolic Drucker–Prager yield criterion.

where σ is a stress tensor, ­α is the pressure sensitivity coefficient, I1 is the first invariant of the stress tensor, a is a material parameter, J2 is the second invariant of the stress deviator tensor, and ­σy is the uniaxial yield stress. Material properties a­ and ­sy can be computed from the uniaxial compressive strength fc and from the tensile strength ft, as follows

For the integration of the constitutive relations, the Closest Point Projection numerical method is used as a return mapping algorithm. Improvements have been performed for the numerical stability. The algorithm has been written in the C++ programming language and integrated using ANSYS User Programmable Features (UPFs).

The softening function is trilinear, and can be determined with three uniaxial yield stresses σy,0, σ­y,1, σy,2 and three equivalent plastic strains ε­pl,eqv,1, εpl,eqv,2, εpl,eqv,3

An advantage of the material model proposed here, in comparison with the ANSYS Microplane material model (ANSYS simulation software 2017) used in the preliminary study, lies in the advanced representation of the behaviour of SFRC in tension.

Calibration

Material tests were performed to provide the necessary data for calibrating the SFRC material model. The mean material properties for the Drucker-Prager material model were determined on the basis of the results from the four-point bending tests at ambient temperature (20 ◦C) and at elevated temperatures (500 and 600 ◦C). Three four-point bending tests were performed at ambient temperature and six four-point bending tests were performed at elevated temperatures. The experimental program was later extended by three further tests at ambient temperature. Force-deflection curves were fitted as the average of the curves received from the measurements.

Reference & Application

The material model was applied for the analysis of the timber steel-fibre-reinforced concrete floor slabs subjected to fire. Real scale experiments were performed as well as numerical simulations using newly developed SFRC material model for ANSYS.


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