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**Bahan tetulang/Reinforcement**

Whiskers Flake Partikel Gentian

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Tetulang: Whiskers Single crystals grown with nearly zero defects a re termed whiskers They are usually discontinuous and short fibers made from several materials like graphite, silicon carbide, copper, iron, etc. Whiskers differ from particles where whiskers have a definite length to width ratio greater than one Whiskers can have extraordinary strengths upto 7000 MPa

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**Metal-whisker combination, strengthening the system at high temperature**

Ceramic-whisker combinations, have high moduli, useful strength and low density, resist temperature and resistant to mechanical and oxidation more than metallic whiskers

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Tetulang: Flake Often used in place of fibers as they can be densely packed Flakes are not expensive to produce and usually cost less than fibers Metal flakes that are in close contact with each other in polymer matrices can conduct electricity and heat Flakes tend to have notches or cracks around the edges, which weaken the final product. They are also resistant to be lined up parallel to each other in a matrix, causing uneven strength

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Tetulang: Partikel The composite’s strength of particulate reinforced composites depends on the diameter of the particles, the interparticle spacing, volume fraction of the reinforcement, size and shape of the particles.

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**Spherical particle + polymer**

Flaky particle + polymer

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Gentian/Fiber: Continuous and Aligned Fiber Composites a) Stress-strain behavior for fiber and matrix phases Consider the matrix is ductile and the fiber is brittle Fracture strength for fiber is σ*f and for the matrix is σ*m - Fracture strain for fiber is ε*f and for the matrix is ε*m (ε*m > ε*f )

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**b) Stress-strain behavior for a fiber reinforced composites**

-Stage I-the curve is linear, the matrix and resin deform elastically -For the composites, the matrix yield and deform plastically (at ε*ym) -The fiber continue to stretch elastically, the fracture strength of the composite is higher than tensile strength of fiber

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**Elastic Behavior a) Longitudinal Loading**

Consider the elastic behavior of a continuous and oriented fibrous composites and loaded in the direction of fiber alignment Assumption: the interfacial bonding is good, thus deformation of both matrix and fibers is the same (an isostrain condition)

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**From definition of stress, F=σA, thus **

Total load sustained by the composites Fc is equal to the sum of the loads carried by the matrix phase Fm and the fiber phase Ff From definition of stress, F=σA, thus Then dividing through by the total cross-sectional area of the composite, Ac; then we have Am/Ac and Af/Ac are the area fractions of the matrix and fiber phases, respectively. If the composite, matrix and fiber phase lengths are all equal, Am/Ac is equivalent to the volume fraction of the matrix, and likewise for the fibers, Vf=Af/Ac. Eq. 1

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**Hence the equation 1 becomes, **

Based on previous assumption of an isostrain state; Devide Eq. 2 by its respective strain Modulus elasticity of a continuous and aligned fibrous composites in the direction of alignment is or The ratio of the load carried by the fibers to that carried by the matrix is Eq. 2

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Exercise A continuous and aligned glass-reinforced composite consists of 40% of glass fibers having a modulus of elasticity of 69 GPa and 60% vol. of a polyester resin that when hardened, displays a modulus of 3.4 GPa

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**Compute the modulus of elasticity of this composite in the longitudinal direction**

If the cross-sectional area is 250 mm2 and a stress of 50 MPa is applied in this direction, compute the magnitude of the load carried by each of the fiber and matrix phases Determine the strain that is sustained by each phase when the stress in part (b) is applied

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b) Transverse loading A continuous and oriented fiber composites may be loaded in transverse direction, load is applied at a 90º angle to the direction of fiber alignment In this case, the stresses of the composite, matrix and reinforcement are the same.

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**For this situation the stress of the composites and both phases is the same;**

The strain or deformation of entire composites, For isostress condition, the equation becomes which reduce to

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Modulus Elastik vs Vf dibawah keadaan isostress dan isostrain, perhatikan bahan yg dibebankan dlm keadaan isostrain menunjukkan modulus yg tinggi

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Exercise 1 Pertimbangkan komposit epoksi ditetulangkan oleh gentian karbon, gentiannya tersusun selanjar, satu arah dan berisipadu 70%. Modulus Young bagi gentian karbon dan epoksi masing-masing ialah 360 x 103 MPa dan 6.9 x 103 MPa

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**Hitungkan modulus komposit ini di bawah keadaan sama-tegasan dan sama-terikan**

Lakarkan graf tegasan melawan terikan bagi gentian, matriks dan komposit ini di bawah keadaan sama-tegasan dan sama-terikan sebagai contoh pada terikan=0.02. Anda perlu menunjukkan cara kiraan untuk menghasilkan graf tersebut

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iii) Hasilkan lakaran graf kebergantungan modulus komposit, Ec terhadap pecahan isipadu (Vf) gentian karbon di bawah keadaan sama-tegasan dan sama-terikan (Nota: Gunakan sekurang-kurangkan 4 nilai Vf)

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Exercise 2 Komposit yang ditetulangi gentian selanjar dan tersusun telah dihasilkan daripada 30% isipadu gentian aramid dan 70% isipadu matriks polikarbonat. Anggapkan komposit ini mempunyai luas keratan rentas sebanyak 320mm2 dan dikenakan beban pada arah membujur sebanyak N. (Modulus kenyal bagi gentian aramid ialah 131 GPa dan polikarbonat ialah 2.4 GPa). Untuk komposit ini, kira:

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**Modulus kenyal pd arah membujur**

Nisbah beban gentian-matriks Beban sebenar yang ditanggung oleh fasa-fasa gentian dan matriks Magnitud tegasan yg dikenakan ke atas fasa-fasa gentian dan matriks Terikan yang dikenakan ke atas komposit Anggapkan tegasan dikenakan pd arag merentas lintang drp arah gentian, kirakan modulus kenyal. Bandingkan nilai yang diperolehi dengan nilai di bhg.(i)

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**Indicate whether the statements are TRUE of FALSE**

1) Usually the matrix has a lower Young’s Modulus than the reinforcement 2) The main objective in reinforcing a metal is to lower the Young’s Modulus 3)The properties of a composite are essentially isotropic when the reinforcement is randomly oriented, equiaxed particles

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**Mark the correct answers**

The matrix Is always fibrous Transfers the load to the reinforcement Separates and protects the surface of the reinforcement Is usually stronger than the reinforcement Is never a ceramic

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The specific modulus Is given by 1/E where E is Young’s modulus Is given by Eρ where ρ is density Is given by E/ ρ Is generally low for polymer matrix composites Is generally low for metallic materials

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Hybrids Are composites with two matrix materials Are composites with mixed fibers Always have a metallic constituents Are also known as bidirectional woven composites Are usually multilayered composites

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**Compared with a ceramic, a polymer normally has a**

Greater strength Lower stiffness Lower density Better high temperature performance Lower hardness

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