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Mekanika Teknik III (Strength of Materials)

Mekanika Teknik III (Strength of Materials). Compression and Tension (axially loaded member). Elastisitas. Badan material terdiri atas partikel kecil atau molekul Akibat gaya luar  partikel-partikel menahan perubahan bentuk tampang badan  gaya partikel

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Mekanika Teknik III (Strength of Materials)

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  1. MekanikaTeknik III(Strength of Materials) Compression and Tension (axially loaded member)

  2. Elastisitas • Badan material terdiriataspartikelkecilataumolekul • Akibatgayaluar partikel-partikelmenahanperubahanbentuktampangbadan  gayapartikel • Gaya partikeltetapbekerjaselamagayaluarbekerja kondisistate of strain. • Kerja yang dilakukangayaluardiubahmenjadienergipotensialregangan

  3. Elastisitas • Batangmengalamipertambahanpanjangsebesar dx • Titikbekerjanyagayamengalamipergeserankebawah (searahdgngaya)  kerjapositif • Jikabeban P dihilangkan, maka dx berangsurhilangdantitikbekerjanyagayabergerakkeatas.

  4. Elastisitas • Kemungkinan yang terjadi : • Kondisibadankembaliseutuhnyasepertisebelumdiberibeban  DinamakanbadanElastiksempurna • Kondisibadankembalisebagaian  Dinamakanbadanelastiksebagian. Sebagiankerja yang dilakukangayaluaruntukberdeformasidiubahdalambentukpanas. • Kondisibadantidakkembalisamasekali  DinamakanbadanPlastis

  5. Berdasarkanujitarikbatangprismatis diketahuielongasisebandingdgngayatarik • Dimana : • P : gaya yang bekerja • L : panjangbatang • A : luaspenampangbatang • E : konstantaelastik material •  : pertambahanpanjangbatang Hooke’s Law

  6. Gaya tarikpadabatangprismatikakanterdistribusimeratapadaseluruhpenampangbatangnya. • Penambahanpanjangbatang per unit panjangdinotasikandalamregangantarik. • Sehinggahukum Hooke dapatdituliskan : Hooke’s Law Unit gayatarikdibagidenganluasanpenampang unit penambahanpanjangdibagidenganpanjangawalbatang

  7. Diagram UjiTarik KesetimbanganHubungan Antaragayatarikdanbesarnyaelongasi Hanyadapatterpenuhipadabatas-batastegangantariktertentu Batas-batastersebut : BATAS PROPORSIONAL Tergantungpada: SIFAT MATERIAL

  8. Diagram UjiTarik

  9. Diagram UjiTarik for small values of the strain the relationship is linear (straight line) and the stress is proportional to the strain. This behaviour is valid until the stress reaches the proportional limit σP. If the stress exceeds the proportional limit the strain begins to increase more rapidly and the slope of the curve decreases. This continues until the stress reaches the yield stress σY. From this point of the stress-strain diagram the strain increases at a practically constant stress: the material begins to yield. At the end of the yielding the slope of the curve increases again which shows that the material can sustain an additional load. This phenomenon is called strain hardening.

  10. Diagram UjiTarik Experiments show that an elongation of the bar leads to a reduction of the cross-sectional area A. This phenomenon is referred to as lateral contraction. Whereas the cross-sectional area decreases uniformly over the entire length of the bar in the case of small stresses, it begins to decrease locally at very high stresses. This phenomenon is called necking. Since the actual cross section Aamay then be considerably smaller than the original cross section A, the stress σ = F/A does not describe the real stress any more. It is therefore appropriate to introduce the stress σt= F/Aawhich is called true stressor physical stress. It represents the true stress in the region where necking takes place. The stress σ = F/A is referred to as nominalor conventionalor engineering stress.

  11. Working Stress • InformasisifatmekanikBahan : • Batas proporsional • Titikleleh • Titik ultimate • Titikhancur Diagram UjiTarik Tegangan Kerja Digunakansebagaidasarpenyelesaianengineering problems. Mencarinilaitegangan yang termasukdalamkategorisafe stress

  12. Working Stress MenentukanTeganganKerja Tegangan di bawahbatasproporsional Masihdalamkategorielastiksempurna Bilateganganberada di atasbatasproporsional, umumnyabeberapabagiantetapmeregangmeskibebantelahdilepas permanent set !! Bagaimana??? Hindaritegangan di atasbatas Proporsional  Menghindari permanent set Supayaaman

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