Observation of fracture process zone by laser speckle technique and governing mechanism in fracture of concrete

Abstract

Fracture tests with a wedge-loading device are carried out on mortar and concrete specimens so as to have stable crack growth. Using laser speckle technique the length of macrocrack and the distribution of crack opening displacement are measured. Results are compared with those obtained by the boundary element method (BEM) analysis for a Dugdale-Barenblatt-type model of a fracture process zone. The governing mechanism in fracture of concrete and the mechanism which is represented by the model are discussed with special attention to the microcracking zone. It is deduced that a Dugdale-Barenblatt-type model does not represent the microcracking zone, thus implying that the microcracking zone and the bridging zone correspond to the pre-peak nonlinear part of the stress-strain curve in a uniaxial tension test and the post-peak tension-softening curve, respectively. It is concluded that the effect of microcracking on the maximum load is less significant than that of bridging. Possible models which include the effect of microcracking in addition to that of bridging are proposed.

Analysis on cohesive crack opening displacement considering the strain softening effect

“…Comparing the listed data with the results in refs. [4] and [21] written in the eighth column of Table 1, we see that the value of is very close to the tension strength 0 σ 0 σ t f tested by the experiment, which shows that the paper's approach is effective and feasible.…”

“…As the FPZ is regarded as a piece of w line, the value of J integral is the surface energy of cohesive crack, which is the same as the expression in refs. [21] and [25]. Because the stress singularity of crack tip is eliminated by the restrict condition eq.…”

“…Consulting the compact tension test data of concrete in refs. [4] and [21], the data of far load and the length of FPZ are listed in Table 1, in which the correct factors are C 3 = 1.1 and 5 for N = 3. Because there is only an unknown constant if N = 2, the selection of factor C 2 has no effect on the distribution of COD.…”

“…Let be the COD value as cohesive stress reaches 0 from , any point of cohesive crack without cohesive stress actually has gone through the total process of stress variation from to 0. As the constitution relationship shows the entire variation process of cohesive crack, the area below the stress curve should be the dissipation work of cohesive crack, or called the fracture energy of each unit crack area 0 u 0 σ 0 σ [6,21] . It is written as…”

Asymptotic analysis of a cohesive crack: 2. Influence of the softening curve

“…This kind of information can, in principle, be extracted from interferometric measurements, as reported for example by the research groups of S.P. Shah [12,13], H. Horii [14] and A.S. Kobayashi [15]. However, the differences are quite small, and it may be extremely difficult in practice to get enough resolution to discriminate experimentally between the profiles shown in Fig.…”