Question 1

In strength of materials: The shape of the bending-moment diagram along a beam that carries a uniformly increasing (linearly varying) load over its entire span is always:

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Introduction / Context: Bending-moment (BM) and shear-force (SF) diagrams reveal how internal actions vary along a beam. When the external load varies linearly (uniformly increasing or triangular loading), it is important to recall the integration relationships between load, shear, and moment. Given Data / Assumptions: Prismatic beam with constant flexural rigidity assumed for action–effect reasoning. Load w(x) varies linearly along the span. Static equilibrium; small deflection theory. Concept / Approach: The fundamental relationships are: dV/dx = w(x) and dM/dx = V(x). If w(x) is linear in x, integrating once gives V(x) as a quadratic function (parabola). Integrating the quadratic shear once more gives M(x) as a cubic function. Step-by-Step Solution: Assume w(x) = a + b x (linear).Integrate: V(x) = ∫ w(x) dx = a x + (b/2) x^2 + C1 → quadratic (parabolic).Integrate again: M(x) = ∫ V(x) dx = (a/2) x^2 + (b/6) x^3 + C1 x + C2 → cubic.Therefore, the bending-moment diagram is a cubic (often called “cubical” in exam options). Verification / Alternative check: Special cases confirm the rule: for uniform load (constant w), V is linear and M is parabolic; increasing the load order by one increases the order of V and M by one in turn. Why Other Options Are Wrong: Linear/Parabolic: correspond to constant or uniform loads, not linearly varying loads.Circular/Exponential: not produced by integrating a linear function twice. Common Pitfalls: Confusing the shape of shear with moment; under a linearly varying load, shear is parabolic but moment is cubic. Final Answer: Cubical.

Question 2

Structural analysis terminology: The diagram that shows how the axial (thrust) force varies along the span of a beam or frame member is called:

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Introduction / Context: Engineers represent internal actions along members using standard diagrams. Besides shear and moment, some members (beams with axial load, frame members, arches) also carry axial force, commonly called thrust. Given Data / Assumptions: A member may carry axial force in addition to shear and bending. Axial force is taken as positive in tension (or as defined by sign convention). Small deformation theory; statics-based evaluation. Concept / Approach: The axial force at a cross-section is the algebraic sum of projections of all external forces on the member’s axis to the left (or right) of that section. Plotting this value versus position produces the thrust (axial force) diagram. Step-by-Step Solution: Cut a section at distance x, write ΣF_along_axis = 0.Evaluate the axial force N(x) at that section.Repeat along the length to obtain N(x) distribution; plot it to get the thrust diagram. Verification / Alternative check: For a pure beam with no axial loads, the thrust diagram is identically zero; for frames or members with axial loads, the diagram shows nonzero values. Why Other Options Are Wrong: Bending-moment/Shear-force diagrams plot M(x)/V(x), not axial force.Stress diagram is not a standard internal action plot name in this context.Load diagram shows external w(x) or point loads, not internal thrust. Common Pitfalls: Confusing sign conventions or mixing axial force with shear force due to poor axis definition. Final Answer: Thrust diagram.

Question 3

Thin cylinder basics: The normal stress in the wall of a cylindrical shell measured circumferentially (due to forces acting along the hoop) is known as:

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Introduction / Context: Pressure vessels develop distinct stress components in their walls: hoop (circumferential), longitudinal (axial), and a relatively small radial stress (for thin shells). Naming and distinguishing these stresses is essential in design. Given Data / Assumptions: Cylindrical shell under internal pressure. Thin-wall assumption so that through-thickness radial stress is small. Membrane theory applies (uniform stress across thickness). Concept / Approach: The circumferential stress acts tangentially around the cylinder and resists the tendency of the vessel to split longitudinally. It is called “hoop stress” (also termed circumferential stress). The longitudinal stress acts along the axis and is typically half the hoop stress for closed ends under the thin-wall formula. Step-by-Step Solution: Identify the direction: stress normal to longitudinal axis but tangent to circumference.Name this component: hoop (circumferential) stress.Recall thin-wall formulas: σ_hoop = p * D / (2 * t); σ_long = p * D / (4 * t). Verification / Alternative check: Cut the cylinder longitudinally; the internal pressure tends to open the cylinder along its length — resisted by hoop stress in the wall. Why Other Options Are Wrong: Longitudinal stress acts along the axis.Yield/Ultimate are material strength limits, not directional stress names.Radial stress acts through the thickness; small in thin shells. Common Pitfalls: Confusing hoop and longitudinal directions; forgetting that hoop stress is the larger of the two in closed-end thin cylinders. Final Answer: Hoop stress.

Question 4

Simply supported beam with two equal point loads: A beam of span L carries two equal concentrated loads W placed at distances L/3 from each support (i.e., at x = L/3 and x = 2L/3). What is the maximum bending moment in the beam?

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Introduction / Context: Finding the location and value of maximum bending moment for beams with symmetric point loads is a standard analysis task. The shear sign change and zero-shear region pinpoint the maximum moment zone. Given Data / Assumptions: Simply supported beam, span L. Two equal loads W at x = L/3 and x = 2L/3. Static, linear analysis; self-weight ignored for clarity. Concept / Approach: By symmetry, reactions are equal: R_A = R_B = W. Between the two loads, the shear becomes zero, implying a constant bending moment there and indicating the maximum value occurs anywhere in that region. Step-by-Step Solution: Reactions: R_A = R_B = (2W)/2 = W.For a section at x in [L/3, 2L/3]: V(x) = R_A − W = 0 → moment is constant.Take x = L/3 or x = L/2 for convenience.At x = L/2: M = R_A * (L/2) − W * (L/2 − L/3) = W * L / 2 − W * L / 6 = W * L / 3. Verification / Alternative check: Compute at x = L/3: M = R_A * (L/3) − W * 0 = W L / 3. Same value confirms a constant maximum between loads. Why Other Options Are Wrong: WL/6, WL/4, WL/8: correspond to different loading configurations.2WL/3: not consistent with equilibrium and internal actions here. Common Pitfalls: Forgetting that zero shear implies constant moment and hence the maximum lies in that zone. Final Answer: W L / 3.

Question 5

Cantilever under pure end moment: A cantilever of length L is subjected to a constant bending moment M applied at the free end (no shear). What is the vertical deflection at the free end?

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Introduction / Context: Different loadings on cantilevers produce characteristic slope and deflection formulas. A pure moment at the free end creates a uniform bending moment along the length, simplifying integration. Given Data / Assumptions: Cantilever length L; flexural rigidity E I constant. Applied end moment M at the free end; no transverse load. Small deflection; Euler–Bernoulli beam theory (plane sections remain plane). Concept / Approach: The differential equation is E I * y'(x) = M (constant). Integrating twice with fixed-end boundary conditions (zero slope and zero deflection at the built-in end) yields the slope and deflection distributions. Step-by-Step Solution: E I y' = M → y' = M / (E I).Integrate: y' → y' dx → y' = (M / (E I)) x + C1.Boundary at fixed end x = 0: slope y'(0) = 0 ⇒ C1 = 0.Integrate: y = (M / (E I)) x^2 / 2 + C2.Boundary at fixed end x = 0: y(0) = 0 ⇒ C2 = 0.Deflection at free end: y(L) = M L^2 / (2 E I). Verification / Alternative check: The slope at the free end is θ(L) = M L / (E I); differentiating the deflection confirms consistency. Why Other Options Are Wrong: M L / (E I): that is the slope, not the deflection.Other denominators (3 E I) arise for different load cases (e.g., point load).Zero: a pure end moment definitely causes rotation and deflection. Common Pitfalls: Applying the point-load formulas instead of the constant-moment case. Final Answer: M L^2 / (2 E I).

Question 6

Euler buckling (both ends hinged): For a column of length l with least moment of inertia I and modulus of elasticity E, the Euler critical buckling load is:

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Introduction / Context: Long, slender columns fail by elastic buckling at loads much lower than material crushing strength. Euler's formula predicts the critical load for ideal columns with various end conditions. Given Data / Assumptions: Both ends hinged (pinned). Prismatic column; elastic behavior up to buckling. Initial imperfections and eccentricities neglected (ideal case). Concept / Approach: For a column with effective length l_e, Euler's load is P_cr = π^2 * E * I / l_e^2. For both ends hinged, l_e = l. The least I governs buckling in the weakest axis. Step-by-Step Solution: Identify end condition: hinged–hinged ⇒ l_e = l.Apply Euler formula: P_cr = π^2 * E * I / l^2.Select the option matching this expression. Verification / Alternative check: Other end conditions: fixed–free (l_e = 2l), fixed–fixed (l_e = l/2), fixed–hinged (l_e = l/√2). Substituting these recovers standard results. Why Other Options Are Wrong: Dimensional inconsistency or wrong placement of I and l (e.g., π E l^2 / I is incorrect).Correct dependence is inversely proportional to l^2 and directly proportional to I and E. Common Pitfalls: Using the wrong effective length for the boundary condition. Final Answer: (π^2 * E * I) / l^2.

Question 7

Design of solid circular shaft under torsion: A solid shaft transmits torque T. If the maximum shear stress must not exceed f_z, what diameter d is required?

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Introduction / Context: Power shafts are sized by limiting shear stress under torque. For solid circular shafts, the elastic torsion formula relates torque to maximum shear stress via the polar section modulus. Given Data / Assumptions: Solid circular shaft of diameter d. Applied torque T (steady). Allowable maximum shear stress f_z (τ_allow). Elastic torsion; no stress concentration considered. Concept / Approach: For a solid circular shaft, τ_max = 16 T / (π d^3). This comes from T / J = τ / r with J = π d^4 / 32 and r = d/2, giving τ_max = 16 T / (π d^3). Solving for d yields the required diameter. Step-by-Step Solution: Start with τ_max = 16 T / (π d^3).Set τ_max = f_z (allowable): f_z = 16 T / (π d^3).Rearrange: d^3 = 16 T / (π f_z).Therefore, d = (16 T / (π f_z))^(1/3). Verification / Alternative check: Dimensional check: T has units of force*length; dividing by stress (force/area) gives length^3, consistent with d^3. Why Other Options Are Wrong: Square-root forms are for thin tubes or unrelated cases.Algebraic rearrangements with π misplaced do not satisfy τ_max relation. Common Pitfalls: Using hollow-shaft formulas or forgetting the 16/π factor for the solid section. Final Answer: d = (16 T / (π f_z))^(1/3).

Question 8

Shear in a prismatic square beam under longitudinal loading: Where is the shear stress maximum and along which plane does it act most critically?

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Introduction / Context: In beams with rectangular or square cross-sections, transverse shear stress varies parabolically over the depth. Understanding where it peaks is important for web sizing and checking shear capacity. Given Data / Assumptions: Square (rectangular) prismatic beam. Transverse shear due to bending action. Elastic behavior; Saint-Venant shear distribution applies. Concept / Approach: For a rectangular section, τ(y) = (3/2) * V / (b * h) * (1 − (2y/h)^2), which is a parabola with maximum at the neutral axis (y = 0) and zero at the top and bottom fibres. Step-by-Step Solution: Recognize the parabolic distribution across depth.At the neutral axis (middle fibre), τ_max = 1.5 * V / (b * h).At extreme fibres, τ = 0.Therefore, the critical shear occurs along the horizontal plane through the neutral axis. Verification / Alternative check: Shear flow q = V * Q / I b is maximum where first moment Q is maximum, i.e., at the neutral axis. Why Other Options Are Wrong: Lower/top fibres: shear is zero there.Equal at every fibre: false; distribution is parabolic.Corners/diagonals: not where τ is maximum in rectangular sections. Common Pitfalls: Confusing shear with bending stress (which is maximum at extreme fibres). Final Answer: On the middle fibre along the horizontal (neutral axis) plane.

Question 9

Fatigue behavior of metals: A member subjected to repeatedly reversible tensile and compressive stresses may fail at a stress lower than the ultimate strength. This failure phenomenon is called:

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Introduction / Context: Many components operate under fluctuating or cyclic loads. Even if the peak stress is below the ultimate strength, repeated cycling can initiate cracks and cause failure—this is the realm of fatigue design. Given Data / Assumptions: Stress varies from tension to compression repeatedly (fully reversed or fluctuating). Number of cycles can be high (high-cycle fatigue) or relatively low (low-cycle). Material behaves elastically within each cycle until crack initiation. Concept / Approach: Fatigue is the progressive, localized structural damage that occurs when a material is subjected to cyclic loading. Design uses S–N curves (stress–life), endurance limits (if applicable), and mean-stress corrections (e.g., Goodman, Soderberg) to ensure adequate life. Step-by-Step Solution: Identify loading as reversible (alternating) tension/compression.Recall that such loading can cause failure at stress levels far below ultimate strength.This behavior is termed fatigue. Verification / Alternative check: Classic rotating-beam tests produce S–N data demonstrating stress levels for a given life, often well below static strength. Why Other Options Are Wrong: Elasticity: ability to recover shape; not a failure mode.Plasticity: permanent deformation capacity; not repeated-load failure.Workability: ease of shaping/manufacturing.Creep: time-dependent deformation under constant load, usually at elevated temperature. Common Pitfalls: Equating fatigue with overload failure; ignoring notch effects and surface finish which strongly influence fatigue strength. Final Answer: Fatigue of the metal.

Question 10

Cantilever with combined loading — UDL over full length and an upward point load at the free end A prismatic cantilever beam of length L carries a uniformly distributed load of total magnitude W acting over its entire length. In addition, an upward …

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Introduction / Context: This problem tests superposition for beam deflections. A cantilever with both a uniformly distributed load (UDL) and a point load at the tip can be treated as two separate load cases. The algebraic sum of the individual tip deflections gives the net deflection and its direction at the free end. Given Data / Assumptions: Cantilever beam of length L, constant E and I. Uniformly distributed load with total magnitude W over the full length (so intensity w = W/L). Upward point load P = W acting at the free end. Small deflection Euler–Bernoulli beam theory and linear superposition are valid. Concept / Approach: Use standard tip-deflection formulae and add effects with signs. For a cantilever with a tip point load P (acting upward), δ_tip = + PL^3 / (3EI). For a cantilever with a UDL of intensity w (downward), δ_udl = − wL^4 / (8EI). Here w = W/L, so δ_udl = − WL^3 / (8EI). The net tip deflection is their algebraic sum. Step-by-Step Solution: Given P = W and w = W/L.δ_point = + WL^3 /(3EI).δ_udl = − (W/L)L^4 /(8EI) = − WL^3 /(8EI).δ_net = δ_point + δ_udl = WL^3 /(EI) * (1/3 − 1/8) = WL^3 /(EI) * (5/24).Since 5/24 > 0, deflection is upward. Verification / Alternative check: If instead W meant UDL intensity (force/length), the two magnitudes would not be directly comparable. The wording explicitly says “total magnitude W over its entire length”, so the above interpretation is consistent and yields a clear direction (upward). Why Other Options Are Wrong: Downward: would require the UDL effect to exceed the tip-load effect; numerically it does not for equal W as defined. Zero: cancellation would require W values chosen to satisfy WL^3 /(3EI) = WL^3 /(8EI), which is not true. None of these: not applicable because a unique direction (upward) is obtained. Common Pitfalls: Confusing W as total load with w as load intensity; always check units. Forgetting the sign convention when superposing deflections. Final Answer: upward.

Question 11

Elasticity and materials — identify the incorrect statement Which one of the following statements is incorrect regarding material behaviour and terminology (kern distance, viscoelasticity, isotropy/orthotropy)?

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Introduction / Context: Understanding standardized material terminology is fundamental in strength of materials. This question examines the definitions of kern distance, viscoelasticity, isotropy, and orthotropy and asks you to spot the incorrect statement among them. Given Data / Assumptions: Axially loaded members may carry eccentric loads; kern defines the no-tension zone. Material classes include elastic, plastic, and viscoelastic; time dependence matters. Directional property variation is captured by terms “isotropic” and “orthotropic”. Concept / Approach: Compare each statement with standard definitions: the kern distance is the limiting eccentricity from the centroid that still maintains compressive stress over the whole section. Viscoelastic materials exhibit time-dependent stress–strain (creep/relaxation). Isotropy means properties are identical in all directions. Orthotropy means three mutually perpendicular preferred directions with distinct properties (e.g., wood: longitudinal, radial, tangential). Step-by-Step Evaluation: (a) Kern distance definition: correct.(b) Viscoelasticity implies time dependence: correct.(c) Isotropy means same properties in all directions, so the statement claiming “different” is incorrect.(d) Orthotropy definition: correct. Verification / Alternative check: Engineering handbooks and textbooks consistently define isotropy as directional uniformity (E, ν, G invariant with direction), while anisotropy/orthotropy cover directional variation. Hence (c) is unequivocally wrong. Why Other Options Are Wrong (as answers): (a), (b), (d) are correct statements; choosing them as “incorrect” would be a misinterpretation. “All the above” cannot be true since not all statements are incorrect. Common Pitfalls: Confusing isotropic with anisotropic; “iso” means same. Overlooking that orthotropy is a specific type of anisotropy with three orthogonal symmetry axes. Final Answer: An isotropic material has different mechanical properties in different directions.

Question 12

Beam with equal overhangs under full-length UDL — midspan bending moment condition A simply supported beam with equal overhangs a on both sides (total length l + 2a) carries a uniformly distributed load over its entire length. The bending moment at …

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Introduction / Context: Beams with overhangs subjected to a uniformly distributed load (UDL) develop hogging moments over the supports and sagging moments within the span. For certain overhang-to-span ratios, the sagging at midspan can be exactly cancelled by the overhang effects, making the midspan bending moment zero. This is a classic contraflexure balance problem. Given Data / Assumptions: Equal overhangs each of length a; clear span between supports is l. UDL of constant intensity along entire (l + 2a). Linear elastic behaviour; small deflections. Concept / Approach: By symmetry, reactions are equal. The midspan moment is influenced by span loading (sagging) and by negative moments transmitted from the loaded overhangs (hogging at supports). Setting the resultant midspan moment to zero yields a relationship between l and a. Step-by-Step Solution (outline): Write support reactions from global equilibrium.Develop the bending moment function in the span considering contributions from span load and overhang-induced effects.Impose M at x = l/2 equal to zero.Solving the resulting expression gives l = 2a as the condition for zero midspan moment. Verification / Alternative check: Standard beam tables confirm that for equal overhangs under continuous UDL, the midspan moment changes sign through zero at l = 2a. For l < 2a, the overhang effect dominates (midspan hogging), and for l > 2a, sagging persists at midspan. Why Other Options Are Wrong: l = 4a, l > a, l > 3a: do not satisfy the precise balance condition. l < 2a: implies reversal but not zero at midspan; the equality is required for zero specifically at the middle. Common Pitfalls: Mistaking the condition for zero support moment rather than zero midspan moment. Final Answer: l = 2a.

Question 13

Octahedral shear stress from principal stresses At a point in a solid, the three principal stresses are σ1 = 100 kgf/cm², σ2 = 100 kgf/cm², and σ3 = −200 kgf/cm². Compute the octahedral shear stress τ_oct (in kgf/cm²).

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Introduction / Context: Failure criteria that depend on distortional energy (e.g., von Mises) and octahedral stresses are widely used in design. The octahedral shear stress τ_oct is a scalar measure derived from the three principal stresses and represents the shear acting on planes equally inclined to the principal axes (the octahedral planes). Given Data / Assumptions: Principal stresses: σ1 = 100 kgf/cm², σ2 = 100 kgf/cm², σ3 = −200 kgf/cm². Small-strain continuum; standard octahedral definitions. Concept / Approach: The octahedral shear stress is computed from the principal stresses via the invariant-based expression: τ_oct = sqrt( ( (σ1 − σ2)² + (σ2 − σ3)² + (σ3 − σ1)² ) ) / (3sqrt(2)). This comes from resolving the stress state on the octahedral planes (planes making equal angles with all three principal directions). Step-by-Step Solution: Compute differences: σ1 − σ2 = 0; σ2 − σ3 = 100 − (−200) = 300; σ3 − σ1 = −200 − 100 = −300.Square and sum: 0² + 300² + (−300)² = 0 + 90000 + 90000 = 180000.Take square root: sqrt(180000) = 424.264.Divide by (3sqrt(2)) = 31.4142 = 4.2426.τ_oct = 424.264 / 4.2426 ≈ 100 kgf/cm². Verification / Alternative check: Using the alternative form τ_oct = (1/3) * sqrt( ( (σ1 − σ2)² + (σ2 − σ3)² + (σ3 − σ1)² ) / 2 ) yields the same numerical result since 1/(3sqrt(2)) = (1/3) * (1/sqrt(2)). Why Other Options Are Wrong: 141.4 corresponds to dividing by 3 only (missing sqrt(2)). 173.2, 200, 244.9 are inconsistent with the invariant formula. Common Pitfalls: Using maximum shear ( (σ_max − σ_min)/2 = 150 ) instead of octahedral shear; they are not the same. Dropping the sqrt(2) factor in the denominator. Final Answer: 100 kgf/cm².

Question 14

Uniaxial tension — shear stress on an oblique plane A prismatic member with a normal cross-sectional area A is subjected to a tensile force P (uniaxial tension). What is the shear stress τ on a plane inclined at an angle θ to the normal (transverse)…

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Introduction / Context: Stress transformation under uniaxial loading is a core topic in strength of materials. On planes inclined to the axis of loading, both normal and shear components exist. This question asks for the shear stress on such an oblique plane under a simple uniaxial tensile state. Given Data / Assumptions: Uniaxial stress σ = P/A acts along the member axis. Plane of interest is inclined at angle θ to the normal cross-section (i.e., to the transverse plane). Continuum, small deformation, standard Cauchy stress transformation applies. Concept / Approach: For uniaxial stress σ, the stress components on a plane at angle θ to the normal plane are: normal stress σ_n = σcos²θ and shear stress τ = σ(sin 2θ)/2. These arise from resolving σ along the plane’s normal and tangential directions or via Mohr’s circle (uniaxial case has centre at σ/2 and radius σ/2). Step-by-Step Solution: Let σ = P/A.Shear on the inclined plane: τ = σ*(sin 2θ)/2.Therefore, τ = (P/A) * (sin 2θ) / 2. Verification / Alternative check: From Mohr’s circle for uniaxial stress, the point corresponding to plane angle θ from the transverse plane is located at an angular position 2θ on the circle; the ordinate is τ = (σ/2) sin 2θ, confirming the result. Why Other Options Are Wrong: (b) and (c) miss the correct trigonometric dependence. (d) gives a normal component form, not shear. (e) is dimensionally incorrect across the full range of θ. Common Pitfalls: Mixing up the “plane angle” with the “Mohr’s circle angle”; remember the factor of 2. Final Answer: τ = (P / A) * (sin 2θ) / 2.

Question 15

Simply supported beam of span L with triangular load (zero at one end, maximum at the other) A simply supported prismatic beam of span L is subjected to a linearly varying (triangular) load that is zero at the left support and increases to a maximum…

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Introduction / Context: Linearly varying (triangular) loads are common idealizations of wind, soil, or hydrostatic pressures. For a simply supported beam, the position of maximum bending moment is not necessarily midspan; it is located where the shear force becomes zero. Expressing the answer in terms of total load W avoids ambiguity between W (total) and w (intensity). Given Data / Assumptions: Span L, triangular load intensity w(x) increasing from 0 at x = 0 to w0 at x = L. Total load W = (1/2) w0 L. Small deflection linear theory; supports are simple (pin and roller). Concept / Approach: Let w(x) = k x where k = w0/L. Determine reactions from equilibrium, write the shear V(x) by integrating the load, find x where V(x) = 0, then evaluate M(x) there to obtain M_max. Finally express the result in terms of W and L. Step-by-Step Solution: Total load W = ∫₀ᴸ k x dx = k L² / 2 ⇒ k = 2W / L².Moments about A: R_B L = ∫₀ᴸ k x · x dx = k L³ / 3 ⇒ R_B = k L² / 3.Hence R_A = W − R_B = k L² / 2 − k L² / 3 = k L² / 6.Shear: V(x) = R_A − ∫₀ˣ k ξ dξ = k L² / 6 − k x² / 2.Set V = 0 ⇒ x² = L² / 3 ⇒ x = L / √3.Moment: M(x) = R_A x − ∫₀ˣ k ξ (x − ξ) dξ = (k/6)(L² x − x³).At x = L/√3: M_max = (k L³) / (9√3) = (2W/L² · L³) / (9√3) = (2 W L) / (9√3). Verification / Alternative check: Numerically, (2/(9√3)) ≈ 0.1283, a well-known coefficient for this loading case. The location x = 0.577L matches the zero-shear condition from standard beam tables. Why Other Options Are Wrong: W L / 8 and W L / 6 are for other load patterns (e.g., UDL or central point load, respectively). W L / 9 and (W L)/(3√3) do not satisfy equilibrium/compatibility for this load shape. Common Pitfalls: Confusing W (total) with w (intensity). Always convert consistently. Assuming maximum moment at midspan; here it occurs at x = L/√3 from the zero-load end. Final Answer: M_max = (2 W L) / (9√3).

Question 16

Strain energy equivalences For a linearly elastic member under load, the strain energy stored can be equated to which of the following expressions (most generally correct)?

Accepted Answer

Introduction / Context: Strain energy is the elastic potential energy stored in a body due to deformation. For linearly elastic loading from zero to a final value, the load–displacement curve is a straight line; the strain energy equals the triangular area under this curve. Recognizing equivalent forms helps in solving a broad class of problems quickly. Given Data / Assumptions: Material follows Hooke’s law within the loading range. Loading is monotonic from zero to final value (no unloading). “Average resistance” means average load over the displacement path. Concept / Approach: For a generalized single-degree description, strain energy U = ∫₀^Δ P dδ. With linear behaviour, P varies from 0 to P_final, so U = (1/2) P_final * Δ, which is “average resistance × displacement”. In distributed form, U = ∫_V (1/2) σ ε dV; for uniform stress/strain this reduces to (1/2) σ ε × volume. Any correct global statement must capture the factor 1/2 from linearity and the appropriate extent (volume), not just area. Step-by-Step Solution: Global: U = ∫₀^Δ P dδ = (1/2) P_final Δ.Distributed: U = ∫_V (1/2) σ ε dV ⇒ for uniform state, U = (1/2) σ ε V.Therefore, a correct compact wording is “average resistance × displacement”. Verification / Alternative check: For an axially loaded bar of stiffness k: P = kΔ ⇒ U = (1/2) kΔ² = (1/2) PΔ, aligning with the average resistance interpretation. Why Other Options Are Wrong: “Stress × strain × area” lacks the length factor and the essential 1/2 factor. “Stress × strain × volume” omits the factor 1/2; correct form is (1/2) σ ε V. “(Stress)^2 × volume + E” is dimensionally inconsistent and incorrect. Common Pitfalls: Forgetting the factor 1/2 for linear loading. Confusing force–displacement area (global) with distributed energy density without integrating over the volume. Final Answer: Average resistance × total displacement.

Question 17

Changing beam proportions — effect on central deflection under a midspan point load A simply supported beam with a central point load has a rectangular cross-section of width b and depth d (depth is vertical). If the width and depth are interchanged…

Accepted Answer

Introduction / Context: The central deflection of a simply supported beam with a central load depends inversely on the flexural rigidity E I. For rectangular sections, the second moment of area I depends strongly on the depth (about the bending axis) as I ∝ b d^3. Interchanging width and depth changes I markedly, thus changing deflection significantly. Given Data / Assumptions: Simply supported span, single central point load P. Initial section: width b (horizontal), depth d (vertical). After swap: width = d, depth = b. Same material (E), same load P, same span L. Concept / Approach: For a central point load, δ = P L^3 / (48 E I). With a rectangular section bending about the strong axis, I = b d^3 / 12. After swapping, I′ = d b^3 / 12. Since deflection is inversely proportional to I, the ratio δ′/δ equals I/I′. Step-by-Step Solution: I = b d^3 / 12.I′ = d b^3 / 12.δ ∝ 1/I and δ′ ∝ 1/I′ ⇒ δ′/δ = I / I′ = (b d^3) / (d b^3) = (d^2)/(b^2) = (d/b)^2. Verification / Alternative check: If d > b (a common case), (d/b)^2 > 1, so swapping makes the beam much more flexible (deflection increases)—consistent with intuition because the “depth” has been reduced from d to b. Why Other Options Are Wrong: b/d or d/b underestimate the cubic sensitivity of I to depth. (b/d)^2 inverses the correct ratio. (b/d)^3 has no basis for deflection ratio in this swap scenario. Common Pitfalls: Forgetting that I for a rectangle varies with the cube of the depth about the bending axis. Final Answer: (d/b)^2.

Question 18

Principal planes and shear — property of the plane of maximum principal stress Along a principal plane that carries the maximum principal normal stress at a point in a stressed body, which statement is true?

Accepted Answer

Introduction / Context: Stress at a point can be represented by normal and shear components on various planes passing through that point. Principal planes are special orientations where the shear stress component is zero and the normal stress reaches an extreme (maximum or minimum). Recognizing this property is vital for failure analysis and Mohr’s circle interpretation. Given Data / Assumptions: General 2D or 3D stress state at a point. Principal directions exist (symmetric Cauchy stress tensor). Standard sign conventions for stresses. Concept / Approach: On a principal plane, the traction vector is collinear with the plane’s normal; hence the shear component vanishes. The principal stresses σ1, σ2, σ3 are purely normal components acting on their respective principal planes. The maximum principal stress is the largest of these normal components, and on that plane there is no shear by definition. Step-by-Step Solution: Define principal planes: orientations where the shear stress τ = 0.Define principal stresses: normal stresses acting on principal planes (σ1 ≥ σ2 ≥ σ3).Therefore, on the plane carrying σ1 (maximum principal), τ = 0. Verification / Alternative check: Mohr’s circle: principal planes correspond to the intersection points of the circle with the horizontal axis (τ = 0), confirming no shear on those planes, including the one at σ_max. Why Other Options Are Wrong: “Maximum/minimum shear acts” contradicts principal-plane definition. “None of these” is incorrect since a definitive property applies. Common Pitfalls: Confusing planes of maximum shear (located 45° from principal planes in 2D) with principal planes themselves. Final Answer: No shear stress acts on that plane.

Question 19

Rankine–Gordon (Rankine) column formula — applicability across slenderness ranges The Rankine–Gordon formula accounts for both direct compressive (crushing) stress and elastic buckling. For which range of column lengths is this semi-empirical formul…

Accepted Answer

Introduction / Context: The Rankine–Gordon (often called Rankine) column formula blends crushing strength (short-column behavior) with Euler buckling (long-column behavior). It is widely used in civil and mechanical engineering to estimate the safe load for columns with a broad range of slenderness ratios. Given Data / Assumptions: Columns may be short, intermediate, or long based on slenderness ratio. Material is homogeneous and isotropic; end conditions are known. Rankine formula combines two failure modes: crushing and buckling. Concept / Approach: The Rankine formula is typically written in the form: P_rankine = P_crushing / (1 + P_crushing / P_euler)or equivalently using stresses: sigma_allow = sigma_c / (1 + (sigma_c / sigma_euler))It transitions smoothly: for very short columns, P_euler is very large so the denominator tends to 1 and crushing dominates; for very slender columns, P_euler becomes small and governs. Step-by-Step Solution: Identify limit cases: very short (crushing) and very long (Euler) behavior.Recognize Rankine is a combined criterion valid for all slenderness ratios.Thus, applicability is not restricted to one category; it spans short, intermediate, and long columns. Verification / Alternative check: Design codes and textbooks show Rankine matching test data reasonably across ranges, especially useful where Euler alone is unconservative for intermediate columns. Why Other Options Are Wrong: Only very long/long/short/intermediate: each restricts the applicability incorrectly. Common Pitfalls: Using Euler for intermediate columns without checking crushing; ignoring end conditions when computing Euler load. Final Answer: All of the above (works across short to long ranges)

Question 20

Shear in a transversely loaded rectangular beam When a rectangular beam is subjected to transverse loading (for example, vertical loads), on which planes does shear stress act within the beam depth?

Accepted Answer

Introduction / Context: In strength of materials, transverse loading on beams induces both bending and shear. For rectangular sections, the distribution of shear stress through the depth is a classic result that every engineer should know when checking web shear and designing reinforcements or web thicknesses. Given Data / Assumptions: Beam has a rectangular cross-section. Loading is transverse (producing shear force V and bending moment M). Material is linear-elastic; small deformation theory applies. Concept / Approach: The shear stress distribution in a rectangular section under transverse shear is parabolic: tau(y) = (3/2) * (V / A) * (1 - (2y/h)^2)where y is measured from the neutral axis and h is the depth. The stress is zero at the extreme fibres (top and bottom) and maximum at the neutral axis. Therefore, shear acts on every horizontal plane, not just at a single fibre. Step-by-Step Solution: Recognize transverse shear V exists due to external loads and support reactions.Use the known parabolic distribution for a rectangle to identify where tau is nonzero.Conclude: shear is present on every horizontal plane, peaking at the neutral axis and vanishing at the faces. Verification / Alternative check: Average shear V/A equals two-thirds of the maximum shear for a rectangle, consistent with the parabolic law (tau_max = 1.5 * V/A). Why Other Options Are Wrong: Top or bottom fibre only: shear is zero there. Only the middle fibre: although maximum at the neutral axis, it is nonzero at adjacent planes too. Only on vertical planes: vertical shear stress accompanies complementary horizontal shear; the statement is incomplete. Common Pitfalls: Assuming uniform shear stress; forgetting that design for web shear must consider maximum at the neutral axis. Final Answer: Every horizontal plane across the depth (maximum at neutral axis)

Strength of Materials Questions