PSI - Issue 74

Petr Miarka et al. / Procedia Structural Integrity 74 (2025) 50–55 Petr Miarka / Structural Integrity Procedia 00 (2025) 000 – 000

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3. Experimental results 3.1. Static fracture tests

The maximum loads recorded ranged between 2.5 and 3.2 kN, corresponding to CMOD values at peak between 0.15 and 0.25 mm. The descending branch is characterised by a pronounced loss of load-bearing capacity, which is typical of quasi-brittle fracture behaviour. The shaded envelope in Fig. 3(a) reveals that the variability between specimens is moderate, especially in the pre-peak zones and towards the end of the curve. The widening of the envelope at higher CMOD values reflects the stochastic nature of microcracking in the fracture process zone (FPZ). In total ten load cycles were performed for relative notch depths a / W = 0.3. The longer initial notch depth was chosen to reduce the brittleness of the material and allow test to be performed in full length. The results of low cycle test are presented in Fig. 3(b).

(a)

(b)

Fig. 3. Static fracture tests results (a) and (b) - recorded P-CMOD hysteretic loops.

Using experimental data presented in Fig. 3(a) a mean fracture energy G F = 133 ± 18 N/m was obtained according to RILEM-TCM85 (1985). Such value is within the expected range for conventional concretes of similar compressive strength. The obtained cyclic material response is plotted as Load-CMOD curve, in which the shape of each loading and unloading cycles during a characteristic hysteretic loop behaviour can be observed. A static tests had maximum CMOD value of approx. 1.0 mm, while low-cyclic reached nearly half of this value. The low-cycle fatigue tests exhibit maximum force values comparable to those obtained under static loading conditions, while offering enhanced insight into crack propagation mechanisms during individual loading cycles and the progressive stiffness degradation occurring in the post-peak phase. The shape of the load–CMOD curves characterises the energy dissipation behaviour over the 12-cycle loading steps, capturing the cumulative damage evolution under repeated loading. The results of the low-cycle fatigue test are illustrated by a representative hysteresis loop, where each cycle facilitates fracture characterisation through the analysis of unloading stiffness degradation. This degradation is quantified by the damage parameter ω, defined as follows: =1 − 0 , (1) where E 0 is the initial elastic stiffness of the material, and E i is the stiffness corresponding to i -th hysteresis loop of the load-CMOD curve. A detailed characterisation of the concrete’s flexural fracture response under cyclic loading—focusing on stiffness degradation observed at each load step or within individual hysteresis loops—was carried out following the methodology described in Hori (1992) and Baktheer(2021). This parameter quantifies cyclic stiffness degradation over the loading history, with its evolution across loading cycles shown in Fig. 4.