Strain Gage Measurement Errors in the Transient Heating of Structural Components

Significant strain-gage errors may exist in measurements acquired in transient thermal environments if conventional correction methods are applied. Conventional correction theory was modified and a new experimental method was developed to correct indicated strain data for errors created in radiant heating environments ranging from 0.6 °C/sec (1 °F/sec) to over 56 °C/sec (100 °F/sec). In some cases the new and conventional methods differed by as much as 30 percent. Experimental and analytical results were compared to demonstrate the new technique. For heating conditions greater than 6 °C/sec (10 °F/sec), the indicated strain data corrected with the developed technique compared much better to analysis than the same data corrected with the conventional technique. NOMENCLATURE a coefficient CTE coefficient of thermal expansion DACS data acquisition and control system GF gage factor T temperature, °C z coordinate through the thickness of the coupon α coefficient of thermal expansion, ppm/°C γ thermal coefficient of resistance, ppm/°C ∆T temperature change from initial temperature, °C εapp apparent strain, μstrain εind indicated strain, μstrain εth transient heating error, μstrain εΣT total strain error due to temperature, μstrain εσ stress-induced strain, μstrain Subscripts g gage g – s difference between gage and substrate s substrate x,y rectangular coordinates in the plane of the coupon