Property of Moiré Fringes and Problems Associated with Measurement of Large Strains

The Highest Measurable Limit of Strain and the Way to Eliminate it

Abstract

The authors previously pointed out that when measuring small strains by the moiré method the lowest measurable limit inherent in this method must be taken into consideration. On the contrary, in applying this method to the measurement of large strains, one should consider the highest measurable limit of strain.
This paper deals with the property of the moiré fringes studied from the physiological view point to clarify the highest measurable limit of strain that exists inherently in this method. In our theory, the highest measurable limit of normal strain ε_i(i=x or y) can be expressed as
2cosα_i/3(1+λ)cosθ-1<ε_i<3cosα_i/2(1+λ)cosθ-1
where α_i are shear strain components, λ and θ are values of mismatch and misalignment, respectively.
The highest measurable limit mentioned above can be avoided in the practical measurement by selecting the value of λ properly.