JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year COMPOSITE MATERIALS Previous Question Paper (July/August - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
Explain the characteristics of particulate, fibrous and laminated composites.
Answer
Composite materials are classified based on the geometry of their reinforcement phase, which dictates their mechanical behavior and anisotropy. The three primary categories are particulate, fibrous, and laminated composites.
Particulate Composites consist of particles of one material suspended in a continuous matrix. These are typically isotropic, meaning their properties are uniform in all directions. The particles can be metallic, ceramic, or polymeric, and their primary role is to improve the matrix's stiffness, hardness, or wear resistance rather than providing high-tensile strength. Examples include concrete (aggregate in cement) and cermets.
Fibrous Composites utilize fibers as the reinforcement phase, which provide high strength and stiffness along the fiber axis. The performance of these composites is highly dependent on fiber orientation, length, and the fiber-matrix interface. They are classified as continuous (long) or discontinuous (short) fiber composites. Continuous fibers offer superior load-bearing capacity in the direction of alignment, making them highly anisotropic. Examples include glass-fiber reinforced polymers (GFRP) and carbon-fiber reinforced polymers (CFRP).
Laminated Composites are composed of multiple layers (laminae) of different materials or the same material with different fiber orientations, bonded together. This structure allows engineers to tailor the mechanical properties of the material to specific loading conditions by stacking layers in various sequences. Laminated composites are designed to be strong and stiff in multiple directions, effectively managing complex stress states. Examples include plywood and advanced aerospace laminates used in aircraft wings and fuselage structures.
What fiber factors contribute to the mechanical properties of a composite. Explain each.
Answer
The mechanical properties of a composite are heavily influenced by the following fiber factors:
- Fiber Orientation: Fibers provide maximum strength and stiffness along their longitudinal axis. Unidirectional alignment yields high anisotropic strength, while random or multi-axial orientation provides more isotropic properties.
- Fiber Volume Fraction: This represents the ratio of the volume of fibers to the total volume of the composite. Increasing the volume fraction generally improves the tensile strength and modulus, up to a critical point where insufficient matrix remains to transfer the load.
- Fiber Aspect Ratio: Defined as the ratio of fiber length to diameter. A high aspect ratio (long fibers) is essential for effective stress transfer from the matrix to the fiber through shear at the interface. Short fibers may lead to pull-out failures rather than fiber fracture.
- Fiber Surface Treatment: Chemical treatments or sizing applied to fibers improve the interfacial bond strength between the fiber and the matrix, ensuring efficient load distribution and preventing premature debonding.
Give the advantages of composite materials.
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Describe the fabrication process of glass fibers with a neat sketch.
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Highlight the desirable properties of carbon and aramid fibers.
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Explain the interfacial interactions occur in composites.
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Discuss properties and applications of carbon fibers.
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With neat sketches explain the following processes:
filament winding
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pultrusion.
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Explain the liquid infiltration technique used for manufacturing of CMCs.
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Discuss on toughness of CMCs.
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Explain how the MMCs are produced using the In Situ process.
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Highlight important applications MMCs.
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Derive rule of mixtures for predicting Young’s Modulus of a composite in terms of volume fractions and elastic modulus of fiber and matrix.
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Calculate the longitudinal modulus and tensile strength of a unidirectional composite containing 55 percent by volume of Sisal fibers in epoxy matrix. The modulus and strength of fiber is 30 GPa and 600 MPa respectively and the same for matrix is 3.5GPa and 100MPa respectively. Calculate:
- elastic Modulus of composite
- the fraction of load taken by fibers in the composite.
Answer
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