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A21 — symmetric 6×6 stiffness matrix gives 21 = 6·7/2 unique entries
B9
C36
D27
Answer & Solution
Correct answer: A. 21 — symmetric 6×6 stiffness matrix gives 21 = 6·7/2 unique entries
Stiffness tensor C_ijkl has minor symmetries (from σ and ε symmetry) reducing 81 → 36 components.
Major symmetry (from strain-energy existence) reduces 36 → 21 independent.
The 6×6 contracted matrix has upper-triangle 6+5+4+3+2+1 = 21 entries.
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