Standard 4-point bending test on 11 inch long composite spar sample. ------------------------------------------------------------------- Mark Drela 8 Feb 00 Test description ---------------- The spar sample was designed to produce material stresses in the spar caps and shear skin comparable to those in an RC glider on a hard winch launch. ===== CF spar cap | | | | endgrain balsa core | | | | ===== CF spar cap Glass skin on the bias is wrapped twice all the way around, with the seam along the top of one spar cap. 4.5 lb endgrain balsa core: 0.35 in wide x 0.48 in high Precured prepreg CF caps : 0.35 in wide x 0.021 in thick (3 plies) 1.5 oz glass skin (+/-45) : 2 layers, 0.005 in thick total Small endgrain basswood inserts replace balsa core at load points. Sample weight = 11.2 g Caps were first bonded to balsa core with slow epoxy. Glass skin was wrapped completely around spar with West 105/206 epoxy, under 15 in-Hg vacuum (intentionally reduced vacuum to avoid crushing the soft balsa core). Sample was cured for several days before test. Four load points L = 3.7" apart: F/2 F/2 | | v v ==================X=== ^ ^ | | F/2 F/2 Skin failed in shear at F = 180 lb at location X, with diagonal buckle lines and tears. Failure appears to have started along the edge or the spar cap, where the skin goes over the 90 degree bend. Rounding off the cap edge to reduce this kink curvature is likely a good idea if the skin is marginal for the shear load. Stress calculations -------------------- Cap thickness : Tc = 0.021 in Spar width : w = 0.35 in Net spar height: h = 0.50 in 1 spar cap area: Ac = 0.00735 in^2 = w Tc Bending inertia: I = 0.00092 in^4 = h^2 Ac / 2 Shear skin thk : Ts = 0.005 in Shear skin area: As = 0.005 in^2 = 2 h Ts Max bending moment: M = L F/2 = 330 lb-in Spar cap load : P = M/h = +/-660 lb Cap axial stress : sigma = P/Ac = +/-90000 psi (no sign of failure) Max skin shear load: S = F/2 = 90 lb Skin shear stress : tau = S/As = 18000 psi (at failure)