Design and Development of an Accessible Clip Gauge Extensometer for CMOD Measurement in Fracture Mechanics

ABSTRACT This work reports on the design, construction, calibration, and validation of a low‐cost, clip‐on extensometer for crack mouth opening displacement (CMOD) measurements in fracture mechanics specimens. The device employs readily available components and integrates a full Wheatstone bridge with instrumentation‐amplifier conditioning to achieve high measurement resolution at minimal cost. A dedicated calibration protocol, combined with the compliance‐based framework specified in the standard, enables direct conversion of bridge output to CMOD. The device's performance was verified on compact tension specimens through a monotonic test, and cross‐validated against a finite element simulation and analytical displacement prediction. The results demonstrated linear‐elastic agreement within experimental uncertainty across a verification window, as well as a reproducible CMOD–time histories suitable for both toughness evaluation and crack‐growth assessments. By providing full documentation of the hardware and manufacture procedures, this study facilitates rapid replication in contexts where commercial instruments may be cost‐prohibitive. Furthermore, it reaffirms that CMOD measurement using clip‐on extensometers remains a robust and essential technique, complementing modern optical methods.