Issue 50

A. Marinelli et alii, Frattura ed Integrità Strutturale, 50 (2019) 438-450; DOI: 10.3221/IGF-ESIS.50.37 442 Figure 5 : Configuration of specimens for (a) 3PB and (b) 4PB tests. It is assumed that the longitudinal axis of the specimens coincides with the strong anisotropy direction of the material. Testing then took place using a stiff Instron Universal testing machine calibrated to EN12390 and the bending load was applied uniformly along the thickness of the specimens with the aid of one or two identical steel rollers. Appropriately positioned Linear Variable Differential Transformers (LVDTs) were used to capture deflections at mid-span (Fig.6a) and CMOD was measured with a special clip gauge supported by a pair of machined knife edges glued to either side (Fig.6b). Both 3PB and 4PB tests were carried out for comparison purposes, to check the independence of the critical COD from the method used for its determination. Specimens of the same dimensions were tested with notches of various lengths to study the influence of the size (notch length) on the magnitude of the critical COD. Using the same testing machine and loading controlled by the displacement of the hydraulic jack, the compressive strength for both materials was determined following BS EN 1926: 2006 (Natural stone test methods – Determination of uniaxial compressive strength) (Fig.6c). Figure 6 : (a) A typical 3PB test with measuring equipment; (b) support for the clip gauge measuring CMOD; (c) typical compression test. Investigating the size-and shape-effects for Portland limestone: the specimens and the experimental set-up Within the scope of this paper, the second stage of experimental investigation aiming at investigating the size- and shape- effects for Portland limestone, comprised 3PB tests on specimens with span/depth ratios of 5/2, 4 and 6, bearing a 4 mm wide machined notch at their mid-span for 1/3 of their depth. For each span/depth ratio and a constant breadth of 40 mm, three different sizes of specimen (span length = 200 mm, 400 mm and 800 mm) were tested, with four repetitions each, in order to observe the influence of size as well as shape on flexural strength, deflection at mid-span, CMOD and fracture energy. The type of test and specimens’ dimensions adopted (Table 1) were based on considerations of appropriate standards and publications [20,21] and limitations regarding laboratory facilities and costs. The experimental protocol implemented for this stage followed the principles described in BS EN 12372: 2006 (Natural stone test methods – Determination of flexural strength under concentrated load) and by RILEM Technical Committee 50-FMC [20]. The specimens were left to dry in a ventilated oven at 70±5 o C until a constant mass was achieved and then were stored at 20±5 o C to reach thermal equilibrium. Testing took place within 24 hours, using the same stiff Instron Universal testing machine calibrated to EN12390 and in configuration as per Fig.7, having the specimen’s strong plane of anisotropy perpendicular to the direction of the applied load. A clip gauge was again positioned across the notch on the specimen, to record the CMOD. Appropriately positioned LVDTs were used to capture deflections at mid-span. Adopting a displacement-control procedure at testing, the loading rate applied was 0.1 mm/min. Following this, the peak load was reached within approximately 2-3 minutes and, given our interest in the post-peak behaviour of the material till the specimen can bear no load, the duration of each test was 8-10 minutes. (a) (b) (a) (b) (c)