Moiré Tell Tales produce a fringe pattern that changes shape as the two parts move. You can explore the effect using the tool below (requires Java, we're working on a version that doesn't).
Concentric circular fringes indicate zero displacement. The Moiré Tell Tale should be adjusted to as close to this state as possible at installation. This tell tale is very nearly centred. A slight displacement is detectable by the fact that the fringes are slightly off centre within the circular region:
When the crack has moved, the fringes will no longer be circular. An "arrow" appears in the fringe pattern, which indicates the direction of movement. The number and shape of fringes indicates the amount of movement:
Moiré Tell Tales are read by comparing the fringe pattern with test patterns, either using the tool below, or using a printed read out sheet. The most reliable method is to count the number of broken fringes (those that reach the edge rather than looping back to the centre). Displacements can be measured to an accuracy of 0.1mm this way.
This excerpt from the readout sheet (full sheet linked below) indicates that the displacement in the image above is 2.5mm:
The fringe pattern photographs well. Photographs can be taken on site, and displacements determined later in the office.
Moiré Tell Tales can be recorded in video easily. This provides a very quick overview of movement which might provide useful information. The video can be reviewed at reduced (or accelerated) playback rate and archived for future reference.
Moiré Tell Tales can be left in place for long periods. Long term movement can then be monitored simply by occasional recording using a camera or mobile phone.
Compared with other means of long term crack monitoring, Moiré Tell Tales have some significant advantages.
Read movements of as little as 0.1mm in two directions.
No additional equipment required to read the tell tale. Only a standard camera is needed to make a record.
Readings of a correctly installed Moiré Tell Tale are not sensitive to the technique of the person recording.
Records can be made from considerable distances.
Dynamic response can be monitored using the same simple device as long term trend.
Each Moiré Tell Tale has a unique ID, encoded in a QR Code (a type of 2D bar code). Photographs of Moiré Tell Tales can be filed automatically by searching images for these codes.
Because Moiré Tell Tales indicate movement by means of a bold pattern, they can be photographed from considerable distances. A long telephoto lens will allow recording from some tens of metres. Even greater distances can be achieved by attaching a camera to a telescope using a digiscoping adaptor.
