A laminate's longitudinal modulus is \( 139 \) GPa but a quasi-isotropic layup of the same material gives about \( 52 \) GPa. The fraction retained is nearest to
Answer: A
Dividing, \( 52/139 = 0.374 \). Making the laminate direction-independent throws away nearly two thirds of the unidirectional stiffness, which is why tailoring the layup to the load path is where the real benefit of composites lies.
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Assertion (A): Almost all practical laminates are made symmetric about their mid-plane.
Reason (R): Symmetry removes the coupling between in-plane loading and bending.
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Answer: B
Both are true and the second explains the first. Without it a panel warps as it cools from cure and bends when loaded in its own plane, neither of which is acceptable in a structure that must hold an aerodynamic shape.
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Assertion (A): Transverse properties of a lamina are far poorer than longitudinal ones.
Reason (R): Across the fibres the constituents carry the same stress, so their compliances add.
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Answer: C
Both statements are true and the second is the reason for the first. The result sits close to the matrix modulus, some sixteen times below the longitudinal value, which is why laminates are built from plies at several angles.
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Assertion (A): Composite compression panels are often sized by compression after impact.
Reason (R): Barely visible impact damage can cut compressive strength by forty per cent.
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Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the sizing logic. Damage that leaves almost no mark on the surface can still delaminate the interior and cut compressive strength by around forty per cent, and since it cannot be relied on to be found in service, the structure must be assumed to carry it. Compression after impact therefore becomes the governing case for most composite panels.
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Assertion (A): Bolted joints in composites cannot rely on local yielding to share load between fasteners.
Reason (R): Composite laminates behave essentially elastically up to failure.
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Answer: A
Both statements are true and the second is the reason for the first. A metal redistributes plastically around a hole and evens out the fastener loads; a composite does not, so the elastic peak is what the joint actually experiences.
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Assertion (A): A conductive mesh is incorporated into the outer surface of a composite aircraft.
Reason (R): Carbon composite conducts far less well than an aluminium skin.
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Answer: B
Both are true and the second explains the first. Without a conduction path a lightning strike deposits its energy locally and can burn through the laminate, so the mesh or foil provides the route an aluminium skin would have offered naturally.
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Assertion (A): Carbon composite must be isolated from aluminium in a joint.
Reason (R): Carbon is conductive and noble relative to aluminium, so the aluminium corrodes galvanically.
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Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the mechanism. Carbon fibre conducts and sits well towards the noble end of the galvanic series, so with aluminium and an electrolyte present the aluminium becomes the anode and corrodes rapidly. The standard remedy is a glass fibre interleaf or a sealant layer that breaks the electrical path.
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Assertion (A): Composite design allowables are quoted for a hot and wet condition.
Reason (R): Absorbed moisture and elevated temperature together soften the matrix.
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Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the physics. Absorbed moisture plasticises the polymer matrix and lowers its glass transition temperature, and elevated temperature softens it further, so the two act together. Since matrix-dominated properties - compression, shear, bearing - suffer most, allowables are quoted at the worst credible combination of the two.
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Assertion (A): Composite certification relies more heavily on testing than metallic certification.
Reason (R): Composite failure is less reliably predicted from first principles than metal yielding.
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Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason explains why the building block approach exists. A metal yields in a way that is well understood and predictable from a handful of properties; a laminate can fail by fibre breakage, matrix cracking, delamination or a combination, and the mode depends on layup, process and environment. So certification proceeds through thousands of coupon, element and sub-component tests up to full-scale articles.
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Assertion (A): A quasi-isotropic layup discards much of the benefit of composite material.
Reason (R): It retains only about a third of the unidirectional longitudinal stiffness.
Select the correct answer from the options below.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason quantifies the first. Making the laminate quasi-isotropic means spending three quarters of the plies on directions other than the principal load path, so the longitudinal stiffness falls to roughly a third of the unidirectional value. It buys direction-independence and easier analysis, and a designer who can identify the load paths will do better with a tailored layup.
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