heä so� poisson tuøy thuoäc vaät lieäu 0 05 G modulus tröôït löïcbình phöông

Heä so? poisson tuøy thuoäc vaät lieäu 0 05 g

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: heä soá poisson, tuøy thuoäc vaät lieäu, = 0 0,5. G: modulus tröôït, [löïc/bình phöông chieàu daøi) ] ) 1 ( 2 [ E G (2.101) G ; ) ( E 1 G ; ) ( E 1 G ; ) ( E 1 yz yz y x z z xz xz z x y y xy xy z y x x (2.102) 4.4. Theá naêng bieán daïng ñaøn hoài: Xeùt moät phaân toá coù tensor öùng suaát nhö hình 2.15. Giaû söû trong khoaûng thôøi gian t phaân toá coù caùc bieán daïng: yz xz xy z y x , , , , ,       . Boû qua löôïng voâ cuøng beù baäc cao ta coù theå xem coâng ngoaïi löïc taùc ñoäng leân phaân toá nhö toång coâng cuûa caùc thaønh phaàn öùng löïc rieâng reû trong caùc ñoä dôøi töông öùng: dxdydz dxdydz dxdydz dxdydz dxdydz dxdydz dA yz yz xz xz xy xy z z y y x x       (2.103) y yx yz z x xy y dy x dx dz z zy zx xz Hình 2.15
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Giaùo Trình CÔ CHÖÔNG 2: ÖÙNG SUAÁT VAØ BIEÁN DAÏNG 94 TS. Vuõ Coâng Hoøa Neáu khoâng coù söï toån thaát naêng löôïng thì coâng naøy ñöôïc chuyeån hoùa hoaøn toaøn thaønh theá naêng bieán daïng ñaøn hoài tích luõy trong phaân toá: dA ) dU ( (2.104) Ñaët : , dxdydz dU dv dU u (2.105) Ta coù:       yz yz xz xz xy xy z z y y x x u u (2.106) (do quan heä giöõa öùng suaát vaø bieán daïng laø baäc nhaát): nguyeân lyù coäng taùc duïng. yz yz xz xz xy xy z z y y x x 2 1 u (2.107) 2 yz 2 xz 2 xy z y z x y x 2 z 2 y 2 x G 2 1 ) ( 2 E 2 1 u (2.108) ) ( 2 E 2 1 u u u 1 3 3 2 2 1 2 3 2 2 2 1 tt hd (2.109) Vôùi: 2 0 2 1 3 2 3 2 2 2 1 hd E 1 2 3 ) ( ) ( ) ( E 6 1 u (2.110) 2 0 2 3 2 1 tt E 2 3 ) 2 1 ( ) ( E 6 2 1 u (2.111) 5. THUYEÁT BEÀN: 5.1. Thí nghieäm keùo vaø neùn vaät lieäu: Ñeå tìm ñieàu kieän beàn cho traïng thaùi öùng suaát ñôn, ngöôøi ta tieán haønh thí nghieäm keùo (ñoái vôùi vaät lieäu deûo) vaø thí nghieäm neùn (ñoái vôùi vaät lieäu doøn).
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Giaùo Trình CÔ CHÖÔNG 2: ÖÙNG SUAÁT VAØ BIEÁN DAÏNG 95 TS. Vuõ Coâng Hoøa 5.1.1. Thí nghieäm keùo: Maãu thí nghieäm keùo baèng theùp. l 0 : chieàu daøi phaàn thí nghieäm d 0 : ñöôøng kính ban ñaàu cuûa maãu. Hình 2.16 Trong quaù trình maãu chòu keùo, ñoaïn l 0 coù moät ñoä giaõn l. Duøng buùt ghi treân maùy ta tìm thaáy töông quan giöõa l vaø P nhö ñoà thò ôû hình . Söï laøm vieäc cuûa vaät lieäu coù 3 giai ñoaïn: (hình 2.17).
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