Performance of acoustic emission and digital image correlation for fracture analysis in cementitious mortars under fatigue loading

Abstract

Fatigue loading in brittle materials introduces damage at a micro-scale. These micro-fractures can accumulate and cause a significant reduction in material stiffness or even lead to structural failure. Deformation-based monitoring techniques can be inadequate when detecting damage at a micro-level. Hence, here is where advanced nondestructive testing (NDT) methods such as acoustic emission (AE) and digital image correlation (DIC) can play a great role. The paper aims to combine AE and DIC for advanced fatigue damage analysis on a cylindrical sample subjected to a fatigue Brazilian splitting test. The damage progress quantified from cumulative AE event count, and horizontal displacement measured with DIC showed a very good correlation. The damaged region was identified with an AE localization plot and with a DIC displacement field plot, the damaged area was well represented by both techniques. When a material goes through an irreversible change, part of the released energy will generate an elastic wave. The wave will propagate through the material and the whole phenomenon is termed an acoustic emission. By using AE sensors on the surface of the sample, these waves are captured and analyzed to better understand the source of the emission. Its high sensitivity to micro-changes makes it an ideal technique to use for fatigue damage investigation. Digital image correlation relates sequentially taken images to determine displacement and strain measurements. While DIC analyzes material evolution using surface deformation measurements, AE on the other hand is capable of monitoring damage inside the volume of material. Hence, their combination provides an efficient test setup for monitoring fatigue micro-fracture. The paper aims to combine AE and DIC for advanced fatigue damage analysis on cylindrical samples that are subjected to monotonic and fatigue Brazilian splitting tests. The analysis will use damage progress under monotonic loading as a reference to further understand fatigue fracture. Methods of AE data filtering and AE source localization are discussed and evaluated for their use in fracture analysis. The utilization of DIC to track the material stiffness evolution is investigated. In addition, the paper compares fracture process zones quantified by AE and DIC. From the experimental analysis, it was found that both methods captured the fatigue damage patterns, yet provided complementary information for damage evaluation.

Micromechanics-based variational phase-field modeling of Brazilian splitting tests

“…Another property of this formulation is that the degradation function is zero for α = 1, which means that the kinematic hardening modulus goes to zero and consequently, the plastic free energy is fully degraded. Furthermore, an initial damage value (α 0 ) has to be defined to prevent division by zero and infinite hardening parameter in equation (11). The initial damage represents the uniformly distributed penny-shaped cracks or initial flaws present in the material.…”

“…Typically, the length scale is related to the characteristic length of the microstructure being modeled, such as the size of a grain. Therefore, the maximum grain size of the sample, i.e., 2 mm, is chosen as the value of length scale in this study, in agreement with [53]. For the sake of brevity, the mesh-converging results are not included in this paper, but it is worth noting that setting l 0 /h = 4 resulted in such convergence.…”

“…However, the difficulties in determining the point of crack initiation in experimental setups have led to debate over the validity of the measured tensile strength [10]. Recent research has examined fracture patterns in the Brazilian splitting test, both experimentally and numerically [11,12], and found that the crack initiation point can vary [13]. Some studies have reported that the crack initiates and propagates from the area under the loading points [14], while others have found that it starts at the center of the disc [15].…”

“…Calibration is an important step towards the more practical application of state-of-the-art phase-field fracture models. To accomplish this, Brazilian splitting tests are conducted utilizing the Digital Image Correlation (DIC) technique to measure displacement and monitor the failure behavior [53]. The simulation of this test is done by using a micromechanics-based phase-field model enhanced with viscosity.…”