NEW METHODS FOR PRESTRESS ESTIMATION

Alexander Zilbershtein

Institute of Precambrian Geology & Geochronology , Russ. Ac. Sci., Makarova emb., 2, St. Petersburg, 199034, Russia, E-mail: gleb@ad.iggp.ras.spb.ru

Keywords: prestress, induced internal stress, optical anomalies, twin density.

Known methods for prestress estimation used the density of free dislocations, the size of subgrains and/or the size of recrystallized grains cannot provide the single and correct result sometimes. It is necessary to design the different independent methods for prestress estimation.

The optical anomalies induced by the prestress may be used for this aim. It was obtained experimentally that the absolute value of the optical anomalies (change of refractive index and/or reflectance, induced birefringence, change of the angle between optical axes, etc.) is proportional to the value of the prestress. The united expression for various crystals was obtained in the form:

|X_ij| = |P_ij| mbK / (2p (1 - n)) (1),

where {X_ij} is the internal residual stress induced by the {P_ij} prestress, K = const (6 +/- 3) 10^-3 m/N,

the m, b and n are the constants of a crystal (shear modulus, Burgers vector and Poisson coefficient). The value of the |X_ij| may be determined as from the value of the optical anomaly (using the theory of piezooptic effect and/or piezoreflection phenomenon), as by the other way (using the broadening of X-ray lines induced by the prestress for example).

The Eq.(1) permits to estimate the prestress value by use of the optical anomalies value.

The twin density may be used for the |P_dif| differential prestress estimation too. The density are measured in grains of polycrystalline aggregate for crystals deformed plastically by twinning. The expression for the |P_dif| estimation from the D average (for aggregate or the thin section) twin density was obtained in the form (by use of the experimental data for calcite by Rowe & Rutter, 1990):

|P_dif| = m K₀ ln (1 + K₁D/S - K₂a²/S) (2),

where m, a and S are the constants of a crystal (shear modulus, average lattice parameter and shear),

K₀ = const (0.475 10^-2), K₁ = const (0.345 mm), K₂ = const (1.716 nm^-2).

The Eqs.(1),(2) may be used for the prestress estimation by new way.