Spring Deformation Gauge for Measuring Local Deformations in Triaxial Apparatus
Issues associated with LVDTs and LDTs.
Critical issues (disadvantages/limitations)
Linear variable displacement transformers (LVDT)
(i) Alignment of the transducer inevitably leads to stiff connections that may affect sample behaviour. (ii) Bulky (iii) Potential friction between the moveable iron core (armature) and the internal cage housing the electromagnetic coils, either by misalignment or due to specimen barrelling. (iv) Special arrangements are required to measure radial strain (v) Unless it is of the unguided type, damage may occur if the LVDT goes out of range. (vi) Relatively expensive
Local deformation transducers (LDT)
(i) Nonlinear output (ii) Friction between the gauge strip and the pin connection unto the specimen can change the boundary conditions normally assumed during calibration (iii) Nondurable (iv) Can get damaged during a test when it goes out of range unless a complex mechanism is used to undo it