Komposit • Komposit adl gab antara 2 atau lbh material yg pd permukaan antara ke2 benda tsb tdk bercampur atau tdk bereaksi scr kimia. Jd ke 2 benda campuran tsb msh bs dilihat bedanya (beda dg fasa fasa logam yaitu fasa perlit yg mrpkn gab antara ferit dan sementit dg kandungn Fe dan C shg tdk bisa dikatakan sbg komposit) • Gab dr ke2 material pd komposit adlh: – – – – –
Memp kekuatan tinggi Memp brt jns rendah Kuat dan cukup kaku Tahan thdp beban kejut atau impact Memp ketangguhan tinggi
• Bag komposit yg lbh byk (%-nya) berfgsi sbg pengikat dsbt matriks, utk bahan pengisinya atau yg diikat oleh matriks dsbt fasa terdispersi (dispersed phase)/ dispersan/ serat (fiber)/ fillers (pengisi) • Ukuran/ % serta penataan fillers sgt berpengaruh thdp sifat mekanik komposit yg dihasilkan. • Secara visual komposit digolongkan macroscopic composite (kasat mata) dan microscopic composite (dg bantuan mikroskop utk lihat). • Macroscopic composite : beton, dinding, structural laminates, tripleks dll • Microscopic composite : dispersion strengthened (komposit dispersi), particel reinforced (komposit partikel), fiber reinforced (komposit serat, arah kontinyu, serat acak, hybrid)
Komposit terdiri dari matriks dan fasa terdisphersi
Microscopic composite 1. Komposit dispersi, material yg diikat matriks memp dimensi 0,01 – 0,1 µm (10 – 100 nm), dispersi partikel scr merata biasanya sampai 15% volume. 2. Komposit partikel, mempunyai ukuran partikel > 1 µm diameter, dg konsentrasi 20 -40% volume 3. Serat komposit, ukuran serat mulai 0,001 inch dg konsentrasi vol serat sampai 70% volume
• Pada komposit bahan dispersi, partikel atau serat biasanya lbh keras dp bahan matriksnya. • Bahan dispersan, partikel atau serat bisa berasal dr bahan metalik, non metal, bhn oksida (keramik) dll. Contoh bhn dispersan: ThO2, pelapisan Al powder dll
Particle reinforced composite
• Ikatan matriks terhadap partikel maksimal dsbt upper bound (ikatan terkuat), yg minimum dsbt lower bound • Kekuatan atau modulus young (E) sbb: – Upper bound: – Lower bound:
• • • • •
E = modulus young V = fraksi volume C = komposit M = matriks P = partikel/ pengisi
Komposit partikel • Termasuk komposit partikel adalah cermet (ceramics – metal) dmn carbida sementit diikat dg logam WC dan TiC, misalnya= diikat dg cobalt dan nikel. Material ini dgnkan sbg alat potong mesin perkakas. Dibuat dg teknologi metalurgi serbuk, dicampur, dipres dan dipanasi (sintering) • Pada material ban, dicampurkan serbuk carbon C (carbon black) kedalam karet, sebesar 15-30% volume, dg diameter partikel carbon black 20-50 mm
Komposit serat (Fiber Reinforced Composites) • Komposit serat (fiber) mempunyai kekuatan dan kekakuan tinggi • Fungsi matriks pd komposit serat adl utk mengikat antara serat satu dg lainnya, jg utk meneruskan beban tegangan dr gaya luar. • Kekuatan komposit adl kekuatan diantara matriks dan seratnya. • Biasanya matriks komposit memp modulus elastisitas lbh bsr dp seratnya. • Matriks komposit jg melindungi serat2nya dr kerusakan akibat abrasive (aus) dan kerusakanpd permuk serat akibat reaksi dg lingkungan • Kerusakan pd permuk serat memicu retak/sobek. Kerusakan komposit akibat terpisahnya serat dr matriksnya dsbt pullout. Perpatahan serat ke serat pd kondisi getas dsbt dg cotastropic failure. • Gaya ikat (bounding forces) antara matriks dan serat sgt penting utk mengurangi gejala rusak fiber pullout dan mempertinggi tegangan yg bs diteruskan
• Bahan matriks logam : aluminium , tembaga • Bahan matrik polimer : poliester, epoxi, phenol, silicon, nilon • Bahan serat : whiskers, fibers, wires • Whiskers, bahan dr single crystals yg lebar dibanding diameter/tebalnya sangat besar. Terbuat dari graphite, SiC, SiNO2, Al2O3 • Fibers, bahan polikristal ataou amorphi dg diameter yg kecil dibanding panjang, biasanya dr ceramic atau polimer seperti: nilon, kaca, asbes, Al2O3 • Wires, kawat berdiameter lebih besar dibuat dr Mo, W, baja (misal pd ban)
KOMPOSIT MAKROSKOPIK • • • •
Komposit structures Dinding struktural laminates Composites sandwich Dinding & beton (concrete)
Structural Composites A structural composite is normally composed of both homogeneous and composite materials, the properties of which depend not only on the properties of the constituent materials but also on the geometrical design of the various structural elements. Laminar composites and sandwich panels are two of the most common structural composites; only a relatively superficial examination is offered here for them. LAMINAR COMPOSITES A laminar composite is composed of two-dimensional sheets or panels that have a preferred high-strength direction such as is found in wood and continuous and aligned fiber-reinforced plastics.The layers are stacked and subsequently cemented together such that the orientation of the high-strength direction varies with each successive layer (Figure 16.16). For example, adjacent wood sheets in plywood are aligned with the grain direction at right angles to each other. Laminations may also be constructed using fabric material such as cotton, paper, or woven glass fibers embedded in a plastic matrix.Thus a laminar composite has relatively high strength in a number of directions in the two-dimensional plane; however, the strength in any given direction is, of course, lower than it would be if all the fibers were oriented in that direction. One example of a relatively complex laminated structure is the modern ski (see the chapter-opening illustration for this chapter).
Sandwich panels, considered to be a class of structural composites, are designed to be light-weight beams or panels having relatively high stiffnesses and strengths. A sandwich panel consists of two outer sheets, or faces, that are separated by and adhesively bonded to a thicker core (Figure 16.17). The outer sheets are made of a relatively stiff and strong material, typically aluminum alloys, fiber-reinforced plastics, titanium, steel, or plywood; they impart high stiffness and strength to the structure, and must be thick enough to withstand tensile and compressive stresses that result from loading.The core material is lightweight, and normally has a low modulus of elasticity. Core materials typically fall within three categories: rigid polymeric foams (i.e., phenolics, epoxy, polyurethanes), wood (i.e., balsa wood), and honeycombs (see below). Structurally, the core serves several functions. First of all, it provides continuous support for the faces. In addition, it must have sufficient shear strength to withstand transverse shear stresses, and also be thick enough to provide high shear stiffness (to resist buckling of the panel). (It should be noted that tensile and compressive stresses on the core are much lower than on the faces.)
