A Sacrificial Crack-and-Strain Witness Strip for Small Bridges and Culverts: Design and Computed Threshold Budget

SIMULATED, NOT MEASURED. A design and computation study of a passive, unpowered witness strip for small bridges and culverts: a strip bonded across a crack carries eight sacrificial ligaments that break in a fixed order as the opening grows, uncovering contrast stripes that are read by counting. Computed budget: trigger thresholds 0.20 to 1.25 mm in steps of 0.15 mm (2,000 to 12,500 microstrain over a 100 mm gauge), reading uncertainty plus/minus 0.075 mm, worst-case temperature-driven apparent opening 0.0235 mm (11.7 percent of the first threshold, margin factor 8.51, residual 0.00705 mm with a reference strip), probability 0.987 or better of a correct level at a band centre under the stated manufacturing scatter, governing computed service life 8.63 years with replacement capped at 5.0 years, and parts cost $9.45 per crossing point. No hardware was built, deployed, or tested; nothing in the paper is a safety certification.