Question 1

Material behavior — term for permanent deformation A structural member that does not regain its original shape after the load causing deformation is removed is said to exhibit which behavior?

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Introduction / Context: Understanding elastic versus plastic behavior is critical in structural design, as permanent deformations affect serviceability and residual capacity after overloads or forming operations. Given Data / Assumptions: The load is removed after causing observable deformation. The member does not return to its original dimensions. Concept / Approach: Elastic behavior is characterized by full recovery of strain on unloading. Plastic behavior produces permanent (residual) strain. Rigid is an idealized concept implying no deformation at all under load. Step-by-Step Solution: Identify that non-recovery on unloading implies residual strain.Residual strain is the signature of plastic behavior.Therefore, the correct term is plastic. Verification / Alternative check: Typical stress–strain curves for metals show linear elastic region up to yield, followed by plastic region; unloading from plastic region leaves a permanent offset. Why Other Options Are Wrong: Elastic: contradicts the observed non-recovery. Rigid: implies zero deformation, inconsistent with the scenario. Viscous-elastic only: viscoelasticity involves time-dependent recovery but not necessarily permanent set for purely elastic–viscous models. Common Pitfalls: Confusing time-dependent (creep/viscoelastic) effects with plasticity; assuming small residuals are elastic. Final Answer: Plastic

Question 2

Point of contraflexure — where bending moment changes sign In which types of beams can a point of contraflexure (a location where bending moment changes sign and curvature reverses) occur under typical loading?

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Introduction / Context: The point of contraflexure is significant for detailing reinforcement and checking sign changes in bending stress. It is where bending moment shifts from positive (sagging) to negative (hogging) or vice versa. Given Data / Assumptions: Standard loading scenarios for common beam types. Prismatic members and small deflection theory. Concept / Approach: In a simply supported prismatic beam without overhangs, the bending moment diagram maintains a single sign between supports (no sign change), so no point of contraflexure exists. In overhanging beams and continuous beams, the bending moment diagram typically crosses zero between regions of positive and negative moment, creating one or more points of contraflexure. Step-by-Step Solution: Consider a simply supported beam: M is zero at supports and positive inside for typical downward loads — no sign change.Overhanging beam: negative moments can develop over supports and positive between, yielding a zero crossing.Continuous beams: hogging over supports and sagging in spans produce sign changes within spans. Verification / Alternative check: Plot basic M–x diagrams for the cases to confirm sign behavior; contraflexure points occur where M(x) = 0 between two regions of opposite sign. Why Other Options Are Wrong: Only cantilevers/only continuous/only overhanging: each omits valid cases. All beam types including simple simply supported: incorrect, as a plain simply supported beam does not have a sign change under usual loads. Common Pitfalls: Confusing zero bending moment at a support with a point of contraflexure inside the span. Final Answer: Overhanging and continuous beams (not in a simple prismatic cantilever under usual loads)

Question 3

Thin cylindrical boiler — ratio of hoop (circumferential) stress to longitudinal stress For a thin-walled cylindrical boiler under internal pressure, what is the ratio of the tensile (normal) stress in the circumferential (hoop) direction to the ten…

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Introduction / Context: In thin pressure vessels, two principal membrane stresses arise: the circumferential (hoop) stress and the longitudinal (axial) stress. Correctly identifying their magnitudes is crucial for safe shell thickness design. Given Data / Assumptions: Thin-wall assumption: wall thickness t is small compared to diameter D. Internal pressure p is uniform; end caps are integral. Concept / Approach: Classical thin-cylinder formulas: sigma_hoop = p * D / (2 * t)sigma_longitudinal = p * D / (4 * t)Taking the ratio gives 2. Step-by-Step Solution: Write equilibrium of half-cylinder in the hoop direction to get sigma_hoop.Write equilibrium of the cylindrical shell end to get sigma_longitudinal.Compute ratio: (pD / 2t) / (pD / 4t) = 2. Verification / Alternative check: Dimensionally consistent; widely cited in pressure vessel design texts. Why Other Options Are Wrong: 4 or 3: overestimates; not supported by thin-wall equilibrium. 1: would imply equal stresses, which is incorrect. 1/2: reversed ratio. Common Pitfalls: Using diameter versus radius inconsistently; mixing thick-wall formulas with thin-wall assumptions. Final Answer: 2

Question 4

Degrees of freedom at a member end in a 2D (plane) frame — bending moment components In a two-dimensional plane frame (bending occurs about the out-of-plane axis), how many independent bending moment components can act at a single member end (i.e., …

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Introduction / Context: Plane (2D) frame analysis considers in-plane forces and a single bending moment component about the out-of-plane axis. Understanding end actions and degrees of freedom clarifies what internal forces/moments may be transferred at joints and which releases change the behavior. Given Data / Assumptions: Structure is a 2D frame (no torsion about the member longitudinal axis). Forces considered: axial force N, shear force V, and bending moment M_z (about z, perpendicular to the plane). Concept / Approach: At any member end in a 2D frame, the internal resultant consists of N, V, M_z. Only one independent bending moment component exists (about the out-of-plane axis). Therefore, the maximum number of possible bending moment components at a single end is one. Step-by-Step Solution: Identify the coordinate system: frame lies in x–y plane; bending is about z.Recognize that torsion about the member axis is not part of 2D frame bending.Therefore, there is exactly one end moment component M_z at a member end. Verification / Alternative check: Finite element beam elements for 2D frames have three DOFs per node (u, v, theta), where theta corresponds to the single bending rotation, confirming one bending moment component conjugate to theta. Why Other Options Are Wrong: Two/three/four: would imply additional bending axes or torsion considered; that is 3D frame behavior. Zero: only true if the end is fully released (pin), but the question asks maximum possible. Common Pitfalls: Confusing 2D frame action with 3D beam behavior; counting axial or torsional effects as separate bending components. Final Answer: One

Question 5

Axial deformation of a steel rod — elongation under tensile load A steel rod of diameter 2 cm and length 5 m is subjected to an axial tensile load of 3000 kg (kgf). If Young's modulus E = 2.1 × 10^6 kg/cm^2 (consistent units), compute the elongation…

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Introduction / Context: Axial deformation calculations are fundamental for serviceability checks. Using consistent units with the material modulus avoids large numerical errors. Given Data / Assumptions: Diameter d = 2 cm ⇒ radius r = 1 cm. Length L = 5 m = 500 cm. Axial load P = 3000 kgf. E = 2.1 × 10^6 kg/cm^2. Linear-elastic behavior; uniform stress; small strains. Concept / Approach: Axial elongation for a prismatic bar: delta = (P * L) / (A * E)with area for a circle: A = π * r^2 Step-by-Step Solution: A = π * (1 cm)^2 = 3.1416 cm^2.Stress sigma = P / A = 3000 / 3.1416 ≈ 955 kg/cm^2.Strain epsilon = sigma / E = 955 / (2.1 × 10^6) ≈ 4.548 × 10^-4.Elongation delta = epsilon * L = 4.548 × 10^-4 * 500 cm ≈ 0.2274 cm = 2.274 mm ≈ 2.275 mm. Verification / Alternative check: Compute directly: delta = (P * L) / (A * E) = (3000 * 500) / (3.1416 * 2.1 × 10^6) cm ≈ 0.2275 cm. Why Other Options Are Wrong: 0.2275 mm and 0.02275 mm: too small by factors of 10 and 100. 2.02275 mm: arithmetic inconsistency. None of these: unnecessary since 2.275 mm is correct. Common Pitfalls: Mixing SI (N, MPa, mm) and gravitational (kgf, cm) units; forgetting to convert metres to centimetres with E in kg/cm^2. Final Answer: 2.275 mm

Question 6

Elastic constants — identify the relation that does NOT hold For an isotropic, linear-elastic material, E is Young's modulus, C is the shear modulus (G), K is the bulk modulus, and m is Poisson's ratio. Which of the following relations is NOT correc…

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Introduction / Context: In isotropic elasticity, only two independent constants are needed; the rest are linked by standard identities. Spotting an incorrect relation is a quick test of fundamentals. Given Data / Assumptions: Material is homogeneous, isotropic, and linear-elastic. Symbols: E (Young's modulus), C or G (shear modulus), K (bulk modulus), m (Poisson's ratio). Concept / Approach: Correct identities include: E = 2 G (1 + m)E = 3 K (1 - 2 m)G = E / [2 (1 + m)]K = E / [3 (1 - 2 m)]Any expression that contradicts these, such as replacing (1 - 2 m) with (1 + 2 m), is incorrect. Step-by-Step Solution: Check option (a): matches E = 2 G (1 + m) ⇒ correct.Check option (b): matches E = 3 K (1 - 2 m) ⇒ correct.Option (c): rearrangement of (b) ⇒ correct.Option (d): rearrangement of (a) ⇒ correct.Option (e): uses (1 + 2 m) instead of (1 - 2 m) ⇒ not correct. Verification / Alternative check: For m = 0.25, compute both sides numerically to see that (e) fails while others match. Why Other Options Are Wrong: (a)–(d) are standard and internally consistent for isotropic materials. Common Pitfalls: Sign error in the bulk modulus relation; mixing symbols G and C for shear modulus. Final Answer: E = 3 K (1 + 2 m)

Question 7

Solid circular shaft in torsion — radial variation of shear stress In a solid circular shaft subjected to a torque (pure torsion), how does the shear stress vary from the centre to the outer surface?

Accepted Answer

Introduction / Context: Torsion of circular shafts is a cornerstone topic in machine and structural design. Correctly recalling the shear stress distribution is essential for sizing shafts and checking combined stress states. Given Data / Assumptions: Solid circular shaft of radius R under torque T. Material is homogeneous and linear-elastic; Saint-Venant torsion applies. Concept / Approach: For a solid circular shaft, the shear stress varies linearly with radius: tau(r) = (T * r) / Jwhere J is the polar moment of inertia. Therefore, tau = 0 at r = 0 and tau = tau_max at r = R. Step-by-Step Solution: Write torsion formula: tau = T r / J.At r = 0 (shaft axis), tau = 0.At r = R (outer fibre), tau = T R / J (maximum value). Verification / Alternative check: The angle of twist formula theta = T L / (G J) also relies on J; using the linear tau–r law integrates to the correct torque–stress relationship. Why Other Options Are Wrong: Maximum at centre: contradicts tau = T r / J. Uniform: only true for thin-walled tubes under special approximations, not solid shafts. Maximum at mid-radius: incorrect; linear variation peaks at the surface. Common Pitfalls: Confusing solid and hollow shafts; misremembering that stress is linear (not parabolic) in torsion for circular shafts. Final Answer: Zero at the centre and maximum at the circumference

Question 8

Beam mechanics – neutral axis concept In the flexural analysis of a simply supported prismatic beam (small deflection theory), which statement best describes the behavior of material fibres exactly along the neutral axis during bending? Provide the …

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Introduction / Context: The neutral axis is a core concept in strength of materials and structural analysis. When a beam bends under transverse loads, fibres above and below a certain line experience compression and tension respectively. Understanding what happens exactly on that line (the neutral axis) is essential for stress–strain distribution, section design, and locating reinforcements. Given Data / Assumptions: Prismatic, homogeneous, and isotropic beam material. Linear elastic behavior (Euler–Bernoulli beam theory) and small deflections. Plane sections remain plane and perpendicular to the neutral axis after bending. Concept / Approach: Under pure bending, normal strain varies linearly with distance from the neutral axis (y = 0 at the axis). The classic relationship is: epsilon(y) = -k * ywhere k is curvature. Thus, at y = 0 (the neutral axis), the normal strain is zero and the corresponding normal stress is also zero. Above the axis (positive y), compression occurs; below (negative y), tension occurs. Step-by-Step Solution: Identify the neutral axis (through the centroid for homogeneous, symmetric cross-sections under bending).Use linear strain distribution: epsilon = -y / R → at y = 0, epsilon = 0.Stress follows sigma = E * epsilon → at the neutral axis, sigma = 0 as epsilon = 0.Therefore, fibres along the neutral axis neither shorten nor elongate; they undergo zero normal strain. Verification / Alternative check: The bending stress formula sigma = M * y / I shows sigma = 0 at y = 0 (neutral axis). Experimental strain gauge readings confirm zero strain along this axis for elastic bending. Why Other Options Are Wrong: Minimum strain suggests a non-zero extremum, which is incorrect—strain is exactly zero at the neutral axis. Maximum strain occurs at the extreme fibres, not at the neutral axis. None of these is invalid because the correct statement is provided in option A. Common Pitfalls: Confusing neutral axis with centroidal axis for non-homogeneous or transformed sections; in composite sections, the neutral axis may not pass through the geometric centroid but still corresponds to zero strain and stress. Final Answer: fibres do not undergo strain

Question 9

Maximum bending moment for a simply supported beam under UDL A simply supported beam of span L carries a uniformly distributed load of intensity w (force per unit length) over its entire length. What is the maximum bending moment (in consistent unit…

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Introduction / Context: Uniformly distributed loads (UDL) are common in slabs, bridge decks, and purlins. Determining the peak bending moment quickly and correctly is foundational to sizing members and reinforcement and to checking deflections and crack control. Given Data / Assumptions: Beam is simply supported with clear span L. Load is uniformly distributed along the full length with intensity w (force/length). Linear elastic, small-deflection theory; self-weight may be included in w. Concept / Approach: For a simply supported beam with UDL over the entire span, reactions are equal and the bending moment diagram is a parabola. The maximum bending moment occurs at midspan and equals w L^2 / 8. Using W = w L (total load), the same result can be written as (W * L) / 8. Step-by-Step Solution: Support reactions: R_A = R_B = w L / 2.Moment at a section x from the left: M(x) = R_A * x − w x^2 / 2.Differentiate: dM/dx = R_A − w x = 0 → x = L / 2 (midspan).Maximum moment: M_max = (w L / 2) * (L / 2) − w (L / 2)^2 / 2 = w L^2 / 8. Verification / Alternative check: Area method: the shear diagram is a straight line from +wL/2 to −wL/2; the area under shear up to midspan equals w L^2 / 8, which is the peak moment. Why Other Options Are Wrong: w L / 8 has wrong dimensions (moment must be forcelength). W L / 8 is correct only if W = w L is clearly defined; option C is a note, not a fundamental expression. w L^2 / 12 and w L^2 / 4 are standard values for other boundary conditions or checks but not for this case. Common Pitfalls: Confusing w (intensity) with W (total load). Dimensional analysis is a quick sanity check: bending moment must carry units of forcelength. Final Answer: w L^2 / 8

Question 10

Shear deflection of a cantilever (Timoshenko effect ignored) For a cantilever of length L, cross-sectional area A, and shear modulus G carrying a concentrated load W at its free end, what is the tip deflection contribution due to shear alone (ignori…

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Introduction / Context: Total deflection of a beam comprises bending and shear components. For short, deep members or materials with low shear modulus, the shear component can be significant. Recognizing and estimating the shear deflection helps avoid serviceability issues and informs when Timoshenko (shear-deformable) theory must be used instead of Euler–Bernoulli (bending-dominant) theory. Given Data / Assumptions: Cantilever of length L, uniform area A, shear modulus G. End load W applied at the free tip. Shear deflection only is requested; bending deflection is not part of this specific answer. Uniform shear strain distribution is assumed (no shear correction factor k applied). Concept / Approach: From elementary shear deformation theory, the average shear strain gamma is shear stress divided by shear modulus: gamma = tau / G. For a prismatic member with end shear V = W (constant along the length), the average shear stress is tau_avg = V / A. The shear rotation per unit length is then gamma = V / (A G), leading to a tip shear deflection equal to this rotation integrated over length L. Step-by-Step Solution: Shear force along the cantilever: V = W (constant).Average shear strain: gamma = V / (A G) = W / (A G).Assuming uniform gamma → shear rotation is constant = W / (A G).Tip deflection due to shear: delta_s = (W / (A G)) * L = W L / (A G). Verification / Alternative check: In Timoshenko theory, a shear correction factor k (≈ 1.2 for rectangles) modifies the formula to delta_s = W L / (k A G). With k = 1, we retrieve the elementary expression asked in the problem. Why Other Options Are Wrong: W L^3 / (3 E I) and W L^2 / (2 E I) are bending deflection forms, not shear deflection. W / (A G) omits length. 2 W L / (A G) overestimates by a factor of 2 without theoretical basis. Common Pitfalls: Mixing bending and shear deflections; forgetting the role of shear modulus G; or applying a shear correction factor unintentionally when the problem asks for the basic expression. Final Answer: W L / (A G)

Question 11

Locations of zero bending moment in a three-hinged arch For a three-hinged arch with hinges at both springings and at the crown, at which hinge locations is the internal bending moment identically zero under any loading?

Accepted Answer

Introduction / Context: Three-hinged arches are widely used in bridges and long-span roofs. Hinges (pins) release moment at their locations, which has direct consequences for internal force distributions and influence lines. Given Data / Assumptions: Hinges exist at both springings (supports) and one hinge at the crown. Arbitrary static loading within the plane of the arch. Rigid arch rib between hinges. Concept / Approach: By definition, an ideal hinge cannot transmit bending moment. Therefore, at each hinge of a three-hinged arch, the internal bending moment is zero regardless of loading. The crown hinge also divides the arch into two determinate segments, each with zero moment at the hinge location. Step-by-Step Solution: Identify hinge locations: left springing, crown, right springing.Apply the hinge property: M_hinge = 0 at each hinge.Conclude that bending moment vanishes at all three hinges under any loading. Verification / Alternative check: Influence lines for moment in a three-hinged arch always pass through zero at each hinge; structural analysis textbooks and codes rely on this fact for determinate solution procedures. Why Other Options Are Wrong: Options A, B, and C restrict zero moment to fewer hinges, contradicting the fundamental behavior of an ideal hinge. Common Pitfalls: Confusing hinges with fixed joints; a fixed joint can transfer moment, but a hinge cannot. Final Answer: at all the three hinges

Question 12

Torsion – comparative strength of solid vs. hollow circular shafts Let a solid circular shaft of diameter D be compared with a hollow circular shaft of external diameter D and internal diameter d. What is the ratio of their torsional moments of resi…

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Introduction / Context: Designers often select hollow shafts for weight savings while maintaining torsional capacity. Comparing the torsional “moment of resistance” (polar section modulus) clarifies why hollow shafts can be efficient. Given Data / Assumptions: Solid shaft diameter = D. Hollow shaft external diameter = D, internal diameter = d. Elastic torsion (Saint-Venant) with circular sections. Concept / Approach: The torsional moment T that produces a given extreme shear stress tau_max is: For solid: T = (pi / 16) * tau_max * D^3For hollow: T = (pi / 16) * tau_max * (D^4 − d^4) / DThe torsional “moment of resistance” Z_p is T / tau_max, so: Z_p(solid) = (pi / 16) * D^3Z_p(hollow) = (pi / 16) * (D^4 − d^4) / DTherefore the ratio (solid : hollow) is: [(pi/16) D^3] : [(pi/16) (D^4 − d^4) / D] = D^4 : (D^4 − d^4) Step-by-Step Solution: Write Z_p(solid) and Z_p(hollow) as above.Cancel common factor (pi/16).Multiply both sides by D to clear denominator → ratio = D^4 : (D^4 − d^4). Verification / Alternative check: Limiting cases: if d → 0, hollow → solid and ratio → D^4 : D^4 = 1, consistent. If d → D, hollow capacity → 0, ratio → infinite, also consistent. Why Other Options Are Wrong: Options with powers 1–3 confuse torsion with bending section modulus patterns; torsion depends on polar properties leading to fourth-power terms for circular shafts. Common Pitfalls: Using J (polar moment of inertia) directly without converting to Z_p = J / R; neglecting the outer radius R = D/2 in the hollow expression. Final Answer: D^4 : (D^4 − d^4)

Question 13

Bending moment diagram shape – cantilever with tip load For a cantilever beam carrying a single concentrated load at its free end, what is the shape of the bending moment diagram along the span?

Accepted Answer

Introduction / Context: Recognizing standard shear force and bending moment diagram (SFD/BMD) shapes speeds up preliminary design and checking of members in frames, bridges, and machine elements. Given Data / Assumptions: Cantilever with fixed end at the wall and free end at the tip. Single point load W applied at the free end. Linear elastic, small deflection theory. Concept / Approach: The bending moment at a section located a distance x from the fixed end equals the end shear times the lever arm from the section to the free end: M(x) = −W * (L − x). This relation is linear in x, varying from −W L at the fixed end to 0 at the free end, which graphically is a straight line—hence a triangular diagram when plotted against the beam axis. Step-by-Step Solution: Take section at distance x from fixed end; free end distance = L − x.Bending moment: M(x) = −W (L − x) → linear in x.At x = 0 (fixed end): M = −W L (maximum hogging).At x = L (free end): M = 0.A straight-line variation from −W L to 0 plots as a triangle. Verification / Alternative check: The shear force is constant V(x) = −W along the entire span, whose integral gives a linear (triangular) bending moment diagram. Why Other Options Are Wrong: Rectangle corresponds to constant bending moment, which does not occur here. Parabola or cubic parabola are associated with distributed loads or varying loads, not a single point load on a cantilever. Common Pitfalls: Reversing sign conventions or plotting from the free end; always check boundary values: BMD must be zero at the free end and maximum at the fixed end for this case. Final Answer: triangle

Question 14

Definition of yielding point in a stress–strain curve In a standard tension test on ductile materials, the stress level at which extension increases much more rapidly compared to the accompanying increase in load is called the:

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Introduction / Context: The stress–strain curve of ductile materials (such as mild steel) has characteristic regions: elastic, yield, strain hardening, and necking leading to fracture. Correctly naming these regions is vital for design, as service stresses are typically kept below the yield point with appropriate safety factors. Given Data / Assumptions: Standard tensile test under quasi-static loading. Ductile material (e.g., low-carbon steel) showing a distinct yield phenomenon. Engineering stress–strain interpretation. Concept / Approach: At the onset of yielding, a small increase (or even a slight decrease) in load can produce a disproportionately large elongation. This marks the transition from purely elastic behavior (recoverable strains) to plastic behavior (permanent deformation). The corresponding stress level is the yielding point. Step-by-Step Solution: Load up to proportional limit → linear relation sigma = E * epsilon.Approach elastic limit → slight nonlinearity precedes yield.At yield → significant strain increase with little load change, defining the yield point.Beyond yield → plastic flow and strain hardening until ultimate stress, then necking to fracture. Verification / Alternative check: In mild steel, upper and lower yield points may appear; the lower yield point is commonly taken for design checks. For materials without a clear yield, a 0.2% proof stress is defined to represent the yield condition. Why Other Options Are Wrong: Elastic point refers to the purely elastic regime below yield. Plastic point is not a standard term for the onset of large inelastic strains. Breaking point is the fracture stress, far beyond yield. Common Pitfalls: Confusing yield with ultimate strength; ultimate tensile strength corresponds to the peak engineering stress and occurs after considerable plastic deformation. Final Answer: yielding point

Question 15

Composite bar under uniform heating – identify internal force sense A composite member (two different materials 'a' and 'b') was made at 25°C and then heated uniformly to 45°C. Material 'a' has a higher coefficient of thermal expansion than material…

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Introduction / Context: Composite bars made of different materials experience internal self-stresses when the temperature changes because each material wants to change length by a different free thermal strain. Compatibility (same final extension) forces the materials to “negotiate” through internal forces, which are equal and opposite, producing compression in one and tension in the other. Given Data / Assumptions: Two materials a and b, perfectly bonded, same original length at 25°C. Temperature is increased uniformly from 25°C to 45°C (ΔT = 20°C). Coefficient of thermal expansion: alpha_a > alpha_b. Ends are free of external load but compatibility (no slip) is enforced. Concept / Approach: If the two bars were free, their free thermal strains would be: epsilon_free,a = alpha_a * ΔTepsilon_free,b = alpha_b * ΔT Because alpha_a > alpha_b, material a wants to expand more. Bonding imposes a common final strain epsilon that lies between the two free strains. Hence, the higher-expanding material a must be compressed back from its free expansion, while b must be pulled ahead to catch up. Step-by-Step Solution: Let epsilon be the common strain. Then for a: epsilon = alpha_a * ΔT + (σ_a / E_a)For b: epsilon = alpha_b * ΔT + (σ_b / E_b)Force equilibrium for the composite (no external axial load): A_a * σ_a + A_b * σ_b = 0.Solving reveals σ_a is compressive and σ_b is tensile because alpha_a > alpha_b. Verification / Alternative check: Think physically: material a tries to expand more and is restrained by b, so a is squeezed (compression); b is dragged along (tension). Why Other Options Are Wrong: (a) reverses the correct sense of stress. (b) and (c) cannot both be true due to action–reaction balance in an unloaded composite. (e) ignores compatibility; even with free ends, bonding creates internal forces. Common Pitfalls: Confusing free thermal expansion (no stress) with constrained thermal expansion (self-stress). The key is the bond imposing equal net strain. Final Answer: Material a will be in compression and material b in tension

Question 16

Concurrent tensile forces at a joint – three forces equally inclined A pin-joint in a frame is acted upon by three concurrent tensile forces P, Q, and R that are equally inclined to one another (120° apart). If P = 10 tonnes, what are Q and R for th…

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Introduction / Context: At a pin-joint, concurrent forces must sum vectorially to zero for equilibrium. When three forces are equally separated in direction by 120°, symmetry allows a quick conclusion about their magnitudes. Given Data / Assumptions: Three tensile forces (pulling away from the joint) P, Q, R. Azimuthal separation between each pair is 120°. P is given as 10 tonnes (metric tonnes). Concept / Approach: For three coplanar forces 120° apart to be in equilibrium, their magnitudes must be equal. This follows from polygon of forces: a closed equilateral triangle is formed if and only if all three sides (magnitudes) are equal. Step-by-Step Solution: Write vector equilibrium: P⃗ + Q⃗ + R⃗ = 0⃗.Geometric condition: directions at 0°, 120°, 240° imply symmetry.Therefore |P| = |Q| = |R|.Given P = 10 tonnes ⇒ Q = 10 tonnes, R = 10 tonnes. Verification / Alternative check: Resolve along any two perpendicular axes; equal magnitudes 120° apart yield zero net components. Why Other Options Are Wrong: (a), (b) give zero for one force, breaking symmetry and equilibrium. (c) and (d) provide scalar relations that do not guarantee vector balance. Common Pitfalls: Adding magnitudes algebraically instead of as vectors; always use vector geometry for non-collinear forces. Final Answer: Q = 10 tonnes and R = 10 tonnes

Question 17

Slope–deflection method – stiffness factor for a prismatic member For a prismatic beam member of length L and constant second moment of area I, what is the joint rotational stiffness factor (end moment per unit rotation at the near end) when the far…

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Introduction / Context: In the slope–deflection and moment distribution methods, the rotational stiffness of a member at a joint is required to relate end moments to joint rotations. For prismatic members the stiffness factor has well-known values depending on the boundary condition at the far end. Given Data / Assumptions: Prismatic (constant E and I) beam member, length L. Small deflection, linear elasticity. Near-end rotation is applied; far end is fixed against rotation and translation. Concept / Approach: The stiffness factor k is defined as the end moment required at the near end to produce unit rotation at that end with the far end fixed and with no joint translation (sidesway). Classical beam theory yields k = 4EI / L for this case. If the far end were simply supported, k would be 3EI / L. Step-by-Step Solution: Definition: k = M_required / θ, with θ = 1 rad at near end.Slope–deflection derivation gives: M_near = (4EI/L) * θ_near + (2EI/L) * θ_far.With far end fixed, θ_far = 0 ⇒ M_near = 4EI/L. Verification / Alternative check: Compare to standard tables used in moment distribution: joint stiffness = 4EI/L (far end fixed) and 3EI/L (far end hinged). Why Other Options Are Wrong: 2EI/L is too small; corresponds to carry-over relation, not stiffness here. 3EI/L applies when the far end is hinged. 6EI/L and EI/L^2 are not the standard rotational stiffness factors for this boundary condition. Common Pitfalls: Confusing stiffness with carry-over factor; remember: carry-over moment = 1/2 of near-end moment for prismatic members. Final Answer: 4EI / L

Question 18

Impact loading on an elastic member – stress under sudden application A load of magnitude W is applied suddenly (without impact velocity) to an axially loaded elastic member. The maximum stress intensity produced is x times the stress that would res…

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Introduction / Context: Elastic members respond differently to loads depending on how the load is applied. A suddenly applied load creates transient elastic oscillations before settling, and the initial maximum stress can exceed the static value. This is crucial for safety factors in lifting, hoisting, and structural detailing. Given Data / Assumptions: Axially loaded prismatic member, linear elasticity. Load W applied suddenly from zero to full value (no initial kinetic energy). No damping during the first response peak. Concept / Approach: For a linear spring–mass analogy, a suddenly applied step load produces a displacement twice the static displacement at the first peak. Stress is proportional to strain in elasticity, so the maximum stress is twice the static (gradually applied) stress. This factor is often referred to as the “sudden load factor”. Step-by-Step Solution: Static case: σ_static = W / A (for axial load).Sudden step load: peak displacement = 2 * δ_static.Therefore peak stress: σ_peak = 2 * σ_static.Hence x = 2. Verification / Alternative check: Energy method: Work done by the load equals increase in strain energy. For sudden load, W * δ_peak = (1/2) * k * δ_peak^2, with δ_peak = 2δ_static. Why Other Options Are Wrong: 1 corresponds to gradual application. 3 and 4 require impact with velocity (additional kinetic energy) or different dynamics. 1.5 has no basis for the step-load case. Common Pitfalls: Confusing sudden application (step) with impact (drop). A falling load can cause factors higher than 2. Final Answer: 2

Question 19

Method of tension coefficients – definition of the coefficient In the method of tension coefficients for analyzing pin-jointed frames, the “tension coefficient” of a member is defined as:

Accepted Answer

Introduction / Context: The method of tension coefficients is a convenient approach for solving pin-jointed frames by resolving joint equilibrium in terms of member “tension coefficients,” avoiding repeated trigonometric calculations for direction cosines. Given Data / Assumptions: Pin-jointed truss with two-force members. Linear elasticity is not required; only static equilibrium and geometry are used. Members are massless for analysis purposes. Concept / Approach: The tension coefficient t_ij of member ij is defined as the member tension T_ij divided by its length L_ij: t_ij = T_ij / L_ij Joint equilibrium can then be written as the sum of t_ij times the respective coordinate differences, simplifying hand calculations. Step-by-Step Solution: Definition: T_ij acts along the member.Unit vector from i to j has components proportional to (Δx_ij / L_ij, Δy_ij / L_ij).Joint equilibrium at node i becomes Σ(t_ij * Δx_ij) = −Σ external_x and Σ(t_ij * Δy_ij) = −Σ external_y.Thus, the fundamental quantity is T_ij / L_ij, i.e., the tension coefficient. Verification / Alternative check: Compare to direction cosines method: replacing T_ij * (Δx_ij / L_ij) with t_ij * Δx_ij reduces arithmetic complexity. Why Other Options Are Wrong: (a) is stress, not a tension coefficient. (c) is a component of force, not normalized by length. (d) is just the force magnitude. (e) is strain (dimensionless), unrelated to this definition. Common Pitfalls: Mixing up stress (force/area) with tension coefficient (force/length). Keep track of geometry clearly. Final Answer: Tension in the member divided by its length

Question 20

Torsion of circular shafts – identify the false proportionality for shear stress In a solid circular shaft under torque T, the shear stress τ at a radius r obeys which of the following proportionalities? Choose the statement that is NOT correct.

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Introduction / Context: For circular shafts in pure torsion, the shear stress distribution is linear with radius. Recognizing the correct dependencies helps in sizing shafts and checking maximum shear at the outer surface. Given Data / Assumptions: Circular shaft, linear elastic, Saint-Venant torsion. No stress concentrations or warping considered. Polar moment of inertia J is constant for a given cross-section. Concept / Approach: The torsion formula for circular shafts is τ(r) = T * r / J where T is applied torque, r is radial position, and J is the polar moment of inertia. From this, τ ∝ r, τ ∝ T, and τ ∝ 1/J. It is not inversely proportional to r. Step-by-Step Solution: Write torsion relation: τ = T r / J.Inspect proportionalities: increases linearly with r; doubles if T doubles; halves if J doubles.Therefore the statement “τ inversely proportional to r” is false. Verification / Alternative check: At the center (r = 0), τ = 0; at the surface (r = R), τ = T R / J (maximum). This confirms τ grows with r. Why Other Options Are Wrong: (a) and (d) are consistent with τ = T r / J. (c) is also correct since larger J reduces stress for a given T. (e) is wrong because one incorrect statement exists, namely (b). Common Pitfalls: Confusing stress variation in torsion (linear) with bending (also linear for normal stress) but with different governing equations. Final Answer: τ is inversely proportional to the distance r from the axis

Strength of Materials Questions