PSI - Issue 31

I. Kožar et al. / Procedia Structural Integrity 31 (2021) 134 – 139

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I. Kozˇar et al. / Structural Integrity Procedia 00 (2019) 000–000

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For beams without fibers, one is typically interested in displacement up to the peak load as is shown in Fig. 2.(a). Including the post-peak regime into the diagram full force-displacement behavior is recorded as shown in Fig. 2.(b). Fig. 2.(b) shows results for flexure of all the nine specimens and their mean that has been calculated (thick green line). The mean curve has been used as a reference for numerical analysis. It is interesting to observe how a sharp bi-linear load – displacement curves from individual flexure experiments trans-form into a well-known softening curve for the mean result.

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Fig. 2. Experimental results for beam without fibers: a) from testing machine up to the peak load; b) statistics after the peak load.

3. Numerical model

Numerical model is based on an already known moment – curvature (m- κ ) relationship that has rather long tradition in engineering analysis of beams. This analysis is usually limited to the pre-peak load intensity level and the cross section is not discretized, it is described with a relation, instead. In our work cross-section is discretized into layers where the number of layers determines accuracy and smoothness of the solution. Optimization of the number of layers is not in the focus of this work and it has been determined by experience, in our case 12. Each layer has equal force – displacement relation described with the equation f c ( x, a, b c , b t ) =   a x E exp − ax b c if x < 0 a x E exp − ax b t if x ≥ 0 (1) where a and b are shape parameters ( b c for the compression part of material deformation and b t for the tension part), E is modulus of elasticity. The material model determines formulation of the function; we have taken fiber bundle model to represent our material behavior (see e.g., Mishnaevsky (2011)); also, the function is similar to one in the microplane material model Ozˇbolt et al. (2001). Fig. 3.(a) presents the influence of parameters a and b on shape of the function and Fig. 3.(b) demonstrates different behavior of the function in tension and compression (since b c and b t are generally different). This different behavior of the function in different sections of domain requires a special solution procedure that takes into account that the function is in different domain for each layer. Our model is built in Wolfram Mathematica (2020) where the function is described as a ‘piecewise’ function so that even more complicated domain segmentation could be taken into account. Bending model is based on two equilibrium equations, force balance

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(2)

f c [( h i − h ) tan ( κ )] = 0

F ( , κ ) = ∆ h

and moment balance

layers i =1

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( h i − h ) · f c [( h i − h ) tan ( κ )] = m

M ( , κ ) = ∆ h