The hybrid double tention test as wide as 3m was carried out on a quenched and tempered 9% Ni Steel plate. The results was analyzed statically with the use of the compliance method and dynamically with the use of the finite-difference method, and an approximate evaluation of the crack arrestability to a extremely long brittle crack was introduced as follows : K _d =√1- v / C _R · P / P ₀ · F (x) ·σ₀√πα Where K _d is the dynamic stress intensity factor, v the crack velocity, C _R the Rayleigh wave velocity in steel, P the load estimated with the use of compliance method, P ₀ the initial load, F (x) the correction factor of the stress intencity factor, σ₀ the initial stress and α the crack length.