The protocol is organised in five stages, each closed by a check, as shown in Figure 1. Table 1 gives suggested starting values for the settings discussed. They are starting points drawn from the guidance cited and should be confirmed for each set-up through the checks; they are not validated optima.
Settings of the proposed protocol, suggested starting points and the reasoning behind each
| Setting | Suggested starting point | Reasoning |
|---|---|---|
| Field of view | Full shear span plus a margin at each plate | Position of the critical crack is not known beforehand |
| Camera arrangement | Stereo pair on the measured face | Removes false strain from out-of-plane movement Sutton et al., 2008 |
| Speckle size | Three to five pixels | Avoids aliasing while keeping subsets small Jones and Iadicola, 2018 |
| Subset size | Large enough to hold at least three speckles | Unique matching; larger subsets blur the crack |
| Step size | About a third to a half of the subset | Dense displacement data along the crack path |
| Static images | A series before loading, repeated at holds | Gives the noise floor and reveals drift |
| Gauge length across crack | A little more than one subset on each side | Displacement jump is taken from undisturbed data |
| Independent check | Displacement transducers or crack gauge at two points | Validates the extracted openings |
The five stages of the proposed protocol and the check that closes each stage
Planning starts from the smallest crack opening that the test needs to resolve and works backward. A well-conducted correlation resolves displacement to a small fraction of a pixel, so the physical size of a pixel on the specimen sets the displacement resolution. The field of view is chosen to cover the whole shear span from load plate to support plate, with a margin, because the position of the critical crack is not known in advance. If that field of view gives a pixel too coarse for the opening of interest, the choice is between a higher-resolution sensor, a second camera pair covering the expected crack region at closer range, or acceptance of a higher threshold. The decision should be made and recorded before the beam is cast.
The second planning decision is between one camera and two. A deep beam under load moves and may rotate slightly out of its plane at the supports, and the web can bulge as the strut approaches crushing. For this reason a stereo system is recommended for the measured face. Where only one camera is available, a long stand-off distance reduces the false strain from out-of-plane movement, and a displacement transducer normal to the face should record that movement so that its effect can be bounded.
The measured face is cleaned, coated with a thin matt white paint and given a random black speckle pattern. The pattern should have high contrast, no preferred direction and roughly equal areas of black and white, with individual speckles some three to five pixels across in the image; smaller speckles alias and larger ones force larger subsets (Jones and Iadicola, 2018). For a shear span of the order of a metre this implies speckles of about a millimetre, applied by stencil, roller or spray and checked in a trial image. Lighting must be even and steady. Diffuse light-emitting diode panels are preferable to laboratory daylight, and the exposure should use the available grey-level range without saturating. The lens aperture is set for a depth of field that keeps the whole face sharp, and focus, aperture and camera position are then locked.
A stereo system is calibrated with a target of known geometry moved through the measurement volume, which determines the intrinsic parameters of each camera and their relative position; the underlying technique is that of Zhang (2000). A single camera needs at least a scale and a distortion correction. With the beam in the rig and unloaded, a series of images is then taken and correlated against the first. Since nothing has moved, the scatter of the computed displacements is the noise floor of the whole measurement chain. The check for this stage is that the noise, expressed as an apparent crack opening using the same extraction settings that will be used later, is clearly smaller than the smallest opening of interest. If it is not, lighting, pattern or resolution must be improved before loading.
Loading should be under displacement control so that the post-cracking response can be followed. Images are triggered together with the readings of load and of the conventional transducers, so that every image carries a load value. A modest frame rate is adequate while the beam is uncracked. Because failure of a deep beam without stirrups is sudden, the rate should be raised once diagonal cracking is seen, and a camera with a rolling buffer is useful for capturing the last images before the strut fails. At each load hold a short burst of images allows the noise to be rechecked and the influence of creep during the hold to be seen. The opposite face of the beam remains available for conventional crack mapping by eye, which gives an independent record of the crack pattern.
Correlation settings govern how sharply a crack is rendered. The subset must be large enough to contain several speckles for a unique match, and no larger, since a subset that straddles a crack returns a displacement that belongs to neither side. Strains computed by differentiating the displacement field are useful for locating cracks, which appear as bands of high apparent principal strain, but they are not a physical strain once the concrete has cracked, and their magnitude depends on the gauge length of the strain calculation. Open-source and commercial correlation codes both allow these settings to be controlled (Blaber et al., 2015), and every one of them should be reported.
Crack kinematics are obtained from displacements, not strains. Once the crack path has been traced from the strain map, pairs of points are placed on either side of it at regular intervals along its length, far enough from the crack that their subsets lie wholly in uncracked material. The relative displacement of each pair, resolved normal and tangential to the local crack direction, gives the opening and the sliding at that location. This is the procedure used on slender members by Campana et al. (2013) and Cavagnis et al. (2015), and automated versions have been published (Gehri et al., 2020). The crack tip at a given load stage is taken as the point where the opening falls to the noise floor recorded in Stage 3, which makes the reported tip position explicitly dependent on the resolution of the measurement.
The closing check compares openings extracted from the images with those from the independent gauges at the same locations and load stages. Agreement within the noise floor validates the chain from image to opening for that test. Disagreement is investigated before any of the kinematic data are used.