Hình 217 5111 Giai ñoaïn töông quan baäc nhaát ñaøn hoài ñoaïn 0A Trong giai

Hình 217 5111 giai ñoaïn töông quan baäc nhaát

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Hình 2.17 5.1.1.1. Giai ñoaïn töông quan baäc nhaát (ñaøn hoài): ñoaïn 0A. Trong giai ñoaïn naøy töông quan giöõa P vaø l laø ñoaïn thaúng 0A, (baäc 1). Neáu ta keùo maãu vôùi löïc P < P tl thì maãu seõ coù moät ñoä giaõn daøi töông öùng, sau ñoù ta giaûm daàn löïc ñi thì ta thaáy buùt ghi treân bieåu ñoà seõ theo ñöôøng A0 veà laïi goác 0. Töùc laø thanh khoâng coøn bieán daïng luùc P = 0. Ñònh luaät Hooke hoaøn toaøn ñuùng trong giai ñoaïn naøy. Ñoaïn naøy goïi laø giai ñoaïn ñaøn hoài tæ leä. Giôùi haïn tæ leä qui öôùc: ) cm / N ( A P 2 0 tl tl (2.112) 5.1.1.2. Giai ñoaïn chaûy: ñoaïn BC. Do toác ñoä thí nghieäm khoâng ñuû chaäm, buùt ghi seõ vaïch ñöôøng thaúng 0A leân quaù ñieåm B roài haï xuoáng vaø taïo neân ñöôøng gaãy khuùc trong ñoaïn BC. Töông öùng vôùi B ta coù löïc P ch vaø giôùi haïn chaûy qui öôùc: 0 ch ch A P (2.113) , P , l de ûo lie äu Vaät 0 ' 0 H D K B P A B C ch p tl p p p 0 l 0 d
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Giaùo Trình CÔ CHÖÔNG 2: ÖÙNG SUAÁT VAØ BIEÁN DAÏNG 96 TS. Vuõ Coâng Hoøa Trong ñoaïn naøy ta thaáy löïc khoâng taêng nhöng bieán daïng tieáp tuïc taêng. Vaät lieäu caøng deûo thì ñoaïn BC caøng daøi. 5.1.1.3. Giai ñoaïn cuûng coá (taùi beàn): ñoaïn CDH Quan heä giöõa P vaø l laø moät ñöôøng cong, löïc coù taêng thì bieán daïng môùi taêng. Neáu taêng löïc ñeán ñieåm K trong ñoaïn CD vaø giaûm löïc coù taêng thì bieán daïng môùi taêng. Neáu taêng löïc ñeán ñieåm K trong ñoaïn CD vaø giaûm löïc töø töø thì buùt ghi seõ vaïch 1 ñöôøng song song vôùi 0A ñeán 0 . Ñieàu naøy chöùng toû treân thanh coøn 1 bieán daïng dö khi ñaõ boû heát löïc. Neáu laïi taêng löïc thì buùt seõ vaïch theo ñöôøng 0 KDH, 0 K xem nhö 1 giai ñoaïn tæ leä. Hieän töôïng giôùi haïn tæ leä taêng goïi laø bieán cöùng nguoäi. Khi P =P B , treân maãu xuaát hieän 1 choã thaét vaø thanh baét ñaàu bò phaù hoûng. Giôùi haïn beàn qui öôùc: ) cm / N ( A P 2 0 B B (2.14) Ba giôùi haïn: tæ leä, chaûy, beàn theå hieän tính chòu löïc cuûa vaät lieäu. Trong cheá taïo maùy, do yeâu caàu caùc chi tieát laøm vieäc khoâng coù bieán daïng dö neân ta chæ cho pheùp öùng suaát thöïc trong chi tieát < ch . Ñeå coù moät ñoä an toaøn chaéc chaén ta thöôøng laáy: ] [ n ch Z (2.115) N: heä soá an toaøn, n>1 : öùng suaát cho pheùp Trong xaây döïng vaø vaät lieäu doøn: n ] [ B (2.116) Hình 2.18 ] [ p B p doøn lie äu Vaät l
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Giaùo Trình CÔ CHÖÔNG 2: ÖÙNG SUAÁT VAØ BIEÁN DAÏNG 97 TS. Vuõ Coâng Hoøa 5.1.2. Thí nghieäm neùn: Maãu thí nghieäm laø hình truï troøn. Chieàu cao maãu neùn thöôøng laáy töø (1 1,5) ñöôøng kính. Quan heä giöõa P vaø l ñoái vôùi vaät lieäu deûo nhö hình.
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