Issue 41

J. Klon et alii, Frattura ed Integrità Strutturale, 41 (2017) 183-190; DOI: 10.3221/IGF-ESIS.41.25 185 Specimen Width W [mm] Initial crack length a 0 [mm] Rel. crack length  0 = a 0 /W Length L [mm] Span S [mm] Breadth B [mm] Crack length a − a 0 [mm] D 040 D 40 3 0.075 96 87.04 40 25 6 0.15 22 12 0.3 16 D 093 C 93 6.98 0.075 223.2 209 40 58.12 13.95 0.15 51.15 27.9 0.3 37.20 D 215 B 215 16.13 0.075 516 467.84 40 134.37 32.25 0.15 118.25 64.5 0.3 86 D 500 A 500 37.5 0.075 1200 1088 40 312.5 75 0.15 275 150 0.3 200 Table 1 : Nominal specimen dimensions tested in [14, 15]; the estimated crack length (width of ligament) are shown in the right column. T HEORY BACKGROUND ATENA software TENA 2D finite-element method (FEM) computational software [12,13] was used – the tool for modelling of structures damaged by cracks. Not only the crack formation, but also their further propagation in dependence on the loading process (increase of force/deflection) can be investigated. Nonlinear material models – such as fracture, plasticity and damage – are included in this sw. to simulate ‘real’ behavior of the studied material. Fracture-plastic material model for simulation of quasi-brittle materials (concrete) – which is used for all configurations – combines the constitutive models for tensile (fracture) and compressive (plastic) behavior. The model of fracture is based on orthotropic formulation of smeared cracks with crack band model implementation (cohesive crack model). Evaluation of loading diagrams The total amount of the dissipated energy (work of fracture W f,b ) can be determined from the recorded P−d curves, or from the recalculated R−a curves (both methods give the same results). Heron's formula enables to evaluate the dissipated energy from recorded P−d curves. This formula works with the area of a triangle defined by its corners in Cartesian coordinate system. In this case, triangles are formed by origin of coordinate system and two consecutive points of loading diagrams [15]. The last point corresponds to the relative crack length 0.7 (α = 0.7), see the schema in Fig. 2. Then, the areas of all triangles are summed. Recorded P−d curve can be also transformed into the R−a curve (or possibly R−  , where  = a/W is the relative crack length). In this transformation, the equivalent elastic crack model [16] is employed for estimation of the current (effective) crack length a at an arbitrary stage of the fracture process (this is the way how to identify the point, when the crack reached the relative length 0.7, as mentioned above). The crack length a is determined from the difference between the initial compliance of the specimen with the crack of length a 0 and the specimen compliance at the current point of the P−d diagram. Then, the value of fracture resistance R is calculated from the current load and effective crack length (for each point of the loading diagram), most conveniently as       2 2 Ic 1 ( ) ( ) K R P a Y E E [J/m 2 ], (1) where  ( P ) is the nominal stress in the line of the crack in the specimen due to the load P and Y (  ) is the corresponding geometry function. Thus, the corresponding point of R−a curve is calculated for each point of P−d curve. The value of W f,b calculated from area under P−d curve is equal to the area under the R−a curve. Transformation of the P−d diagram into the R−  curve with indication of meanings of W f,b , is shown in Fig. 2 and Fig. 3. A