Selection of Strain Gauge is the MAJOR part in Stress Analysis, because accurate strain will be obtained when Grid of the Strain Gauge is suitable with line of Force. In this Gauge Pattern, Grid length & width, STC code Creep Code & Backing material is consider.
Second part of this Technology is related to perfect bond between Strain Gauge & Job to obtain Effective Strain.
A thin foil type strain gage is a sensor whose resistance varies with stretch (Increase Resistance) & Contraction (Decrease Resistance) based on the applied force to strain gauge through specimen (under test job), where Strain Gauge is bonded in line of high strain area.
This Change of Resistance (Delta R) is due to change in length of Strain Gauge (through Specimen), is measured by Electronics Strain Meter.
While static Stress on Test Material is applied, Strain is developing axial as well angular to the direction of applied Force & based on design of structure. When we apply dynamic Force, axial line will change non-predictably. Hence for Strain Gauges are also available with different Pattern (See photos as below) depend on applications.