Question 1

Elastic constants – Relationship among E (Young’s modulus), C (shear modulus), and K (bulk modulus) Select the correct formula that relates Young’s modulus E, shear modulus C, and bulk modulus K for an isotropic elastic material.

Accepted Answer

Introduction / Context: For isotropic, linear elastic materials, only two independent elastic constants are required. Various equivalent relationships link Young’s modulus E, shear modulus C (often denoted G), bulk modulus K, and Poisson’s ratio ν. Given Data / Assumptions: Material is homogeneous, isotropic, and linearly elastic. Standard elasticity relations apply. Concept / Approach: The well-known relations are E = 2C(1 + ν) and E = 3K(1 − 2ν). Eliminating ν between these gives a direct relation among E, C, and K: E = 9KC / (3K + C). Step-by-Step Derivation: Start with E = 2C(1 + ν) and E = 3K(1 − 2ν).Solve the pair for ν and eliminate it.Algebra yields E = 9KC / (3K + C). Verification / Alternative check: Another equivalent form is 1/E = 1/(9K) + 1/(3C), which rearranges to E = 9KC/(3K + C). Why Other Options Are Wrong: Option (a) lists two separate relations involving ν, not a direct three-constant relation; option (c) is correct but different in form (reciprocal); (d) is not valid for isotropic elasticity; (e) misses the factor 3 in the numerator. Common Pitfalls: Confusing symbols C and G; misapplying relations for anisotropic materials. Final Answer: E = 9 * K * C / (3 * K + C)

Question 2

Basic beam theory – Identify the correct statement Which of the following statements about loads and internal effects in beams is correct?

Accepted Answer

Introduction / Context: Beams in structural analysis are subject to various loads. Recognizing the correct definitions and relationships among load, shear force, and bending moment is essential for preliminary design and checks. Given Data / Assumptions: Standard sign conventions; statically determinate cases as typical examples. Uniformly distributed load (UDL) has constant intensity over its application length. Concept / Approach: Key facts: A continuous beam has more than two supports; the location of maximum bending moment generally occurs where the shear force changes sign (shear equals zero), not at maximum shear; for a simply supported beam with central point load, M_max = Wl/4. Step-by-Step Evaluation: Option (a): False—continuous beams have three or more supports.Option (b): True—UDL has constant load per unit length over the region where it is applied.Option (c): False—M is extremum where shear force V = 0.Option (d): False—correct value is Wl/4 at midspan.Option (e): False—support reactions create nonzero shear at the supports. Verification / Alternative check: Construct V–M diagrams for the central point load case to confirm M_max = Wl/4 and V = 0 at midspan. Why Other Options Are Wrong: They contradict standard beam theory definitions and results. Common Pitfalls: Confusing “distributed over whole length” with “over entire span” (UDL may act on part of the span); mixing locations of max shear and max moment. Final Answer: A uniformly distributed load acts uniformly over the entire span where it is applied.

Question 3

Compound member under preload – Rod inside a tube tightened by rigid washers A slender rod is enclosed centrally within a tube, and the assembly is pre-tightened by rigid washers (nutting). If a net external compressive load is applied to the assemb…

Accepted Answer

Introduction / Context: Preloaded compound members (a central rod within a surrounding tube) are classic examples of load sharing through differential stiffness and initial stresses. Understanding the initial state and its evolution under external loading is important for bolts, tie-rods, and composite columns. Given Data / Assumptions: Rod and tube are concentric and connected by rigid end washers/nuts. Preload is applied by tightening (nutting) without external load. Linear elastic behavior; compatible deformations at the ends. Concept / Approach: During tightening (no external force), the rod is placed in compression while the tube is stretched in tension; the rigid washers enforce equal end displacements. When an external compressive load is later applied to the assembly, it first reduces the tube’s tensile stress; only after a threshold might the tube go into compression, depending on load magnitude. Step-by-Step Reasoning: Initial state (preload only): rod in compression, tube in tension.Apply external compression: total tube stress = tension from preload minus portion of external load; rod compression increases.Unless the external load exceeds the preload in the tube, the tube remains in tension while the rod remains in compression. Verification / Alternative check: Force–displacement compatibility analysis for two springs in parallel with initial strains reproduces the same conclusion. Why Other Options Are Wrong: Options (a), (b), (c) ignore preload directions; both in compression occurs only beyond a specific external load; “stress-free” is impossible after tightening. Common Pitfalls: Neglecting the effect of preload; assuming immediate compression in both members under any external compressive load. Final Answer: Tube is under tension and rod is under compression

Question 4

Strength of materials – Resilience: Is the modulus of resilience defined as the proof resilience (i.e., maximum elastic strain energy) stored per unit volume of a material?

Accepted Answer

Introduction / Context: Modulus of resilience is a key property in strength of materials that quantifies the amount of elastic strain energy a material can absorb per unit volume right up to its elastic limit (also called proof stress for design). This concept is crucial when selecting materials for components that must endure impact or cyclic loading without permanent deformation, such as springs, couplings, and energy-absorbing devices. Given Data / Assumptions: The discussion is limited to linear elastic behavior up to the elastic limit. Stress–strain relationship is taken as linear with slope E (Young's modulus). Proof resilience is the total elastic strain energy stored at the elastic limit. Concept / Approach: Strain energy density (energy per unit volume) in uniaxial linear elasticity is the area under the stress–strain curve up to a specified stress. For a straight line up to the elastic limit, this area is a right triangle. Therefore, the modulus of resilience equals the triangular area up to proof stress. Step-by-Step Solution: Let sigma_p = proof stress (elastic limit), and epsilon_p = sigma_p / E.Strain energy density U = area under stress–strain curve up to sigma_p.For linear elasticity: U = 0.5 * sigma_p * epsilon_p.Substitute epsilon_p: U = 0.5 * sigma_p * (sigma_p / E) = sigma_p^2 / (2 * E). Verification / Alternative check: This same result can be written using resilience terminology: modulus of resilience = proof resilience per unit volume = sigma_p^2 / (2 * E). Units check: (stress^2)/(modulus) gives energy per volume, as required. Why Other Options Are Wrong: Incorrect: contradicts the standard definition used in design handbooks. Not applicable: the definition is broadly applicable to linear elastic materials. Depends on loading type: while multi-axial states need corresponding forms, the uniaxial definition remains per unit volume up to elastic limit. Common Pitfalls: Confusing modulus of resilience with toughness (the latter is the entire area under the stress–strain curve to fracture). Another mistake is using yield strength beyond proportional limit without clarifying the definition of proof stress. Final Answer: Correct

Question 5

Torsion basics – definition: The product of the tangential (circumferential) force acting on a shaft and its radial distance from the shaft axis is called what?

Accepted Answer

Introduction / Context: Rotating machine elements such as shafts transmit power through torsion. The fundamental measure that relates the force causing twist to its lever arm is the twisting moment, commonly called torque. Correct terminology distinguishes torque from bending moment, which acts in a different plane and produces curvature rather than twist. Given Data / Assumptions: A circular shaft carries a tangential force at some radius r from its center. The force acts perpendicular to the radius, creating a couple about the shaft axis. Static equilibrium and small-deformation torsion theory apply. Concept / Approach: By definition, a moment (or couple) equals force multiplied by the perpendicular distance to the axis. When the force is tangential around a shaft and the distance is the shaft radius, the resulting moment twists the shaft. This is the twisting moment or torque, denoted T. Step-by-Step Solution: Given tangential force F at radius r.Compute moment about the shaft axis: T = F * r.This torque causes an angle of twist theta related by T * L / J = G * theta, where L is length, J is polar moment, and G is shear modulus (for uniform shafts).Hence, the correct name for F * r is twisting moment (torque). Verification / Alternative check: Bending moment would involve a transverse force producing curvature about a neutral axis, not a circumferential force about the shaft axis. Flexural and torsional rigidities are stiffness measures (E * I and G * J respectively), not moments. Why Other Options Are Wrong: Bending moment: corresponds to bending, not torsion. Torsional rigidity: equals G * J, a stiffness parameter with units of force * length^2, not a moment from F * r directly. Flexural rigidity: equals E * I, relevant to bending stiffness. Common Pitfalls: Mixing torque with power. Remember, power P = T * omega, where omega is angular speed; torque alone is independent of speed. Final Answer: Twisting moment (torque)

Question 6

Section properties – polar moment: For a solid circular shaft of diameter D, what is the polar moment of inertia J about its own axis?

Accepted Answer

Introduction / Context: The polar moment of inertia J quantifies a shaft’s resistance to torsional deformation. For circular shafts, J is directly used in torsion equations to compute shear stress and angle of twist, which are critical in power transmission and drive-train design. Given Data / Assumptions: Circular cross-section, solid, diameter D. Homogeneous, isotropic material. Axis of interest is the shaft’s centroidal axis. Concept / Approach: For circular sections, J equals the sum of the second moments of area about two orthogonal centroidal axes: J = I_x + I_y. For a solid circle, I_x = I_y = (pi / 64) * D^4. Therefore, J = 2 * (pi / 64) * D^4 = (pi / 32) * D^4. Step-by-Step Solution: Recall: I_x(solid circle) = (pi / 64) * D^4.By symmetry: I_y = I_x.Compute J: J = I_x + I_y = 2 * (pi / 64) * D^4 = (pi / 32) * D^4.Use in torsion: tau_max = T * R / J, theta = T * L / (G * J). Verification / Alternative check: Dimensionally, J has units of length^4, matching D^4. Values appear in design tables for quick reference and align with standard formulae. Why Other Options Are Wrong: (pi / 64) * D^4: this is I_x or I_y for a solid circle, not J. (pi / 16) * D^4 and (pi / 8) * D^4: overestimate J by factors of 2 and 4, respectively. Common Pitfalls: Confusing polar moment J with area moment I. Remember: torsion uses J, bending uses I. Final Answer: J = (pi / 32) * D^4

Question 7

Pure bending – stress distribution: In a beam subjected to pure bending, the intensity of normal stress in any fibre is __________ the distance of that fibre from the neutral axis.

Accepted Answer

Introduction / Context: Understanding how bending stress varies across a beam’s depth is central to safe and economical design. Under pure bending (constant bending moment, zero shear), the normal stress varies linearly with the distance from the neutral axis, reaching zero at the neutral axis and maximum at the extreme fibres. Given Data / Assumptions: Euler–Bernoulli beam theory applies. Plane sections remain plane and perpendicular to the neutral axis after bending. Material behavior is linear elastic and homogeneous. Concept / Approach: The flexure formula provides the relationship between bending stress and the fibre distance y from the neutral axis: sigma = M * y / I, where M is the bending moment and I is the second moment of area about the neutral axis. This shows a direct proportionality to y. Step-by-Step Solution: Start from curvature–strain relation: epsilon = y / rho.Hooke’s law: sigma = E * epsilon = E * y / rho.From moment–curvature: M / I = E / rho → E / rho = M / I.Combine: sigma = (M / I) * y → sigma ∝ y. Verification / Alternative check: The linear stress distribution is consistent with zero stress at the neutral axis and equal magnitudes of tension and compression at symmetric extreme fibres for symmetric sections under pure bending. Why Other Options Are Wrong: Equal to / less than / more than: these are not general relationships; the correct dependence is proportional, not a fixed inequality. Common Pitfalls: Forgetting sign convention: fibres on one side are in tension (positive), and on the other in compression (negative). Magnitudes still scale linearly with |y|. Final Answer: Directly proportional to

Question 8

Elastic constants – E and K: Select the correct relationship between Young's modulus (E) and bulk modulus (K) for an isotropic material in terms of Poisson's ratio v.

Accepted Answer

Introduction / Context: In isotropic linear elasticity, the elastic constants E (Young’s modulus), G (shear modulus), K (bulk modulus), and v (Poisson’s ratio) are interrelated. Knowing one pair allows computation of the others, which is especially useful when test data report different constants than those needed for analysis. Given Data / Assumptions: Material is homogeneous and isotropic. Small strains and linear elastic behavior. Standard definitions of elastic constants apply. Concept / Approach: The classical relationships among elastic constants are: E = 2 * G * (1 + v) and K = E / (3 * (1 - 2v)). Rearranging the second yields E = 3 * K * (1 - 2v). These formulas ensure consistency between volumetric and deviatoric responses in isotropic media. Step-by-Step Solution: Start from bulk modulus definition in isotropy: K = E / (3 * (1 - 2v)).Rearrange to solve for E: E = 3 * K * (1 - 2v).Optionally cross-check with E, G, v relation: E = 2 * G * (1 + v) and K = (2 * G * (1 + v)) / (3 * (1 - 2v)).Thus, the correct relation between E and K is E = 3 * K * (1 - 2v). Verification / Alternative check: Dimensional consistency is satisfied. For typical metals with v ≈ 0.3, the factor (1 - 2v) ≈ 0.4, giving E ≈ 1.2K, which aligns with typical ranges. Why Other Options Are Wrong: E = 2 * K * (1 + v): confuses K with G; the coefficient 2 belongs with G. E = K / (1 - 2v): missing factor 3; overestimates E. E = 9 * K * v / (1 + v): not a standard relation between E and K. Common Pitfalls: Using these isotropic relations for anisotropic materials (e.g., composites) where they do not hold. Final Answer: E = 3 * K * (1 - 2v)

Question 9

Bending sign convention: Across the neutral axis of a beam under bending, the fibres on one side are in tension and on the other side are in compression. Do you agree?

Accepted Answer

Introduction / Context: When beams bend under transverse loads, internal normal stresses develop that vary from compression on one side to tension on the other, with a zero-stress surface known as the neutral axis in between. Understanding this distribution is fundamental to sizing members and checking serviceability. Given Data / Assumptions: Euler–Bernoulli beam assumptions (plane sections remain plane). Linear elastic, homogeneous material. Conventional sagging moment (concave up) places top in compression and bottom in tension for prismatic, horizontally oriented beams. Concept / Approach: From the flexure formula sigma = M * y / I, stress is proportional to the distance y from the neutral axis. The sign depends on the curvature and the side of the axis. Hence, one side experiences positive (tension) and the opposite negative (compression) stress, separated by the neutral axis where sigma = 0. Step-by-Step Solution: Identify the bending moment sign (sagging or hogging).Apply sigma = M * y / I with sign convention for y above or below the neutral axis.Conclude that stresses have opposite signs across the neutral axis.Note that the neutral axis passes through the centroid for homogeneous sections under pure bending. Verification / Alternative check: Deflection shape visualization: fibres on the concave side shorten (compression), while those on the convex side lengthen (tension), confirming the opposite nature of stress. Why Other Options Are Wrong: Disagree / conditional options: The statement holds broadly for beams in bending regardless of support type, provided material is homogeneous and within elastic range. Common Pitfalls: Confusing sign conventions or misidentifying the neutral axis location in non-homogeneous or composite sections. Final Answer: Agree

Question 10

Torsion capacity comparison: Two shafts of the same material are compared. Shaft A is solid with diameter 100 mm. Shaft B is hollow with outer diameter 100 mm and inner diameter 50 mm. The torque that shaft B can transmit (at the same allowable shea…

Accepted Answer

Introduction / Context: Designers often prefer hollow shafts to save weight while maintaining torsional strength. Torque capacity at a given allowable shear stress is proportional to the polar moment of inertia J of the section. Comparing J for solid and hollow circular sections quantifies the efficiency of material distribution. Given Data / Assumptions: Same material and allowable shear stress for both shafts. Solid shaft A: diameter D = 100 mm. Hollow shaft B: outer diameter D_o = 100 mm, inner diameter D_i = 50 mm. Length and boundary conditions are comparable so that torque capacity scales with J. Concept / Approach: For a circular shaft, maximum shear stress tau_max = T * R / J. At a fixed tau_allow, torque capacity T_allow ∝ J / R. For shafts with the same outer radius R, the ratio of allowable torques equals the ratio of J values. Here, both have the same outer radius, so T ratio simplifies to J ratio. Step-by-Step Solution: Solid shaft: J_s = (pi / 32) * D^4.Hollow shaft: J_h = (pi / 32) * (D_o^4 − D_i^4).With D_o = 100 and D_i = 50: J_h / J_s = (100^4 − 50^4) / 100^4.Compute: 50^4 = (1/16) * 100^4 → J_h / J_s = 1 − 1/16 = 15/16. Verification / Alternative check: Numeric check confirms 0.9375 fraction. The modest reduction in capacity versus large weight saving highlights the advantage of hollow shafts. Why Other Options Are Wrong: 1/6, 1/8, 1/4: greatly underestimate hollow shaft capacity; they do not reflect the fourth-power diameter dependence. Common Pitfalls: Comparing torque per unit weight without accounting for equal outer diameter; or forgetting to use the fourth power in J calculations. Final Answer: 15/16

Question 11

Neutral axis behavior: At the neutral axis of a beam under bending, which statement best describes the state of the fibres?

Accepted Answer

Introduction / Context: The neutral axis (NA) is a central concept in beam theory. It is the locus of points within the cross-section that experience zero normal stress during bending. Correctly identifying conditions at the NA helps in understanding strain distribution and in locating the centroidal axis for homogeneous sections. Given Data / Assumptions: Linear elastic bending assumptions (plane sections remain plane). Homogeneous material, no residual stresses. Pure bending or general bending with NA defined by zero normal stress. Concept / Approach: Normal strain varies linearly through the depth: epsilon = y / rho. At y = 0 (the NA), epsilon = 0. By Hooke’s law, sigma = E * epsilon, the normal stress is also zero at the NA. Thus, fibres along the NA neither elongate nor shorten; they undergo no normal strain. Step-by-Step Solution: Take curvature 1 / rho from bending.Relate strain to distance from NA: epsilon = y / rho.At NA: y = 0 → epsilon = 0 → sigma = 0.Therefore, the layers at the NA neither tensile nor compressive strain—no normal strain develops there. Verification / Alternative check: Experimental strain gauge readings across a beam depth show zero reading at the NA, confirming theory. Why Other Options Are Wrong: Maximum/minimum bending stress occurs at extreme fibres, not at the NA. Compression at NA is incorrect; stress changes sign across the NA and is zero at it. Common Pitfalls: Assuming the NA always passes through the geometric centroid; for composite or non-homogeneous sections, the NA may shift relative to the centroid. Final Answer: The layers do not undergo any strain

Question 12

Spring terminology: The load (force) required to produce a unit deflection of a spring is termed as what?

Accepted Answer

Introduction / Context: Springs are characterized by how much they deflect under load. The direct proportionality constant between load and deflection in the elastic range is the spring stiffness, a critical parameter for designing suspensions, valves, and vibration isolators. Given Data / Assumptions: Small deflections in the elastic range (Hooke’s law holds). Deflection measured along the spring’s working direction. Material and geometry fixed. Concept / Approach: For linear springs, F = k * x, where F is load, x is deflection, and k is spring stiffness (spring rate). Therefore, the load required for unit deflection is simply k. Other listed terms are stiffness-related but apply to beams or torsional members, not directly to overall spring response in translation. Step-by-Step Solution: Define spring law: F = k * x.Set x = 1 (unit deflection): F_unit = k.Hence, the required term is spring stiffness.For helical compression springs, k = G * d^4 / (8 * D^3 * N) (for reference), where d = wire diameter, D = mean coil diameter, N = active coils, and G = shear modulus. Verification / Alternative check: Dimensionally, k has units of force per unit displacement (e.g., N/mm), matching “load per unit deflection.” Why Other Options Are Wrong: Flexural rigidity (E * I) applies to beam bending stiffness, not directly to a spring’s axial rate. Torsional rigidity (G * J) is a torsional stiffness measure of a shaft or rod. Young’s modulus is a material constant, not the overall element stiffness. Common Pitfalls: Confusing material modulus with system stiffness; stiffness depends on geometry and boundary conditions in addition to material properties. Final Answer: Spring stiffness

Question 13

Beam diagrams – UDL case: For a simply supported beam carrying a uniformly distributed load of intensity w per unit length over the whole span, the bending moment diagram has which shape?

Accepted Answer

Introduction / Context: Shear force and bending moment diagrams are essential tools for beam design. Recognizing the characteristic shapes for common loading scenarios allows quick checks and aids in sizing members without re-deriving equations each time. Given Data / Assumptions: Simply supported beam. Uniformly distributed load (UDL) of intensity w acting over the full span. Static, linear elastic behavior. Concept / Approach: Relationships between load w(x), shear force V(x), and bending moment M(x) are: dV/dx = −w(x) and dM/dx = V(x). For constant w, shear varies linearly with x, and bending moment varies quadratically, resulting in a parabolic bending moment diagram. Step-by-Step Solution: Load: w(x) = w (constant).Integrate once: V(x) = R_A − w * x → linear variation from support A to B.Integrate again: M(x) = R_A * x − (w * x^2) / 2 → quadratic in x.Hence, the bending moment diagram is a parabola with maximum at midspan M_max = w * l^2 / 8. Verification / Alternative check: Symmetry dictates V is zero at midspan for symmetric UDL; substituting x = l/2 into M(x) yields M_max = w * l^2 / 8, consistent with standard formulas and confirming the parabolic shape. Why Other Options Are Wrong: Horizontal/vertical line: would imply zero or undefined change; inconsistent with quadratic bending moment under constant load. Inclined straight line: corresponds to constant shear (e.g., point load case), not UDL. Common Pitfalls: Confusing the shapes: for a point load at midspan, the bending moment diagram is triangular (piecewise linear), while for UDL it is parabolic. Final Answer: A parabolic curve

Question 14

Fundamentals of stress and strain: Which statement below is incorrect? Select the single wrong definition among linear strain, Poisson’s ratio, volumetric strain, and bulk modulus.

Accepted Answer

Introduction / Context: Engineering mechanics relies on precise terminology. Confusing basic definitions of strain measures and elastic moduli leads to wrong designs. This question checks conceptual clarity on linear strain, Poisson’s ratio, volumetric strain, and bulk modulus. Given Data / Assumptions: Small deformations (engineering strain) are considered. Isotropic, homogeneous materials in the elastic range. Standard sign conventions (tension positive). Concept / Approach: Recall the standard definitions:Linear (longitudinal) strain ε = ΔL / L.Poisson’s ratio ν = − (lateral strain / longitudinal strain); the magnitude equals lateral/longitudinal.Volumetric strain ε_v = ΔV / V.Bulk modulus K = hydrostatic (volumetric) stress / volumetric strain.Young’s modulus E, not K, is the ratio of linear stress to linear strain in uniaxial loading. Step-by-Step Solution: Check option (a): matches ε = ΔL / L → correct.Check option (b): definition aligns with ν when longitudinal strain is used → correct in magnitude.Check option (c): ε_v definition is standard → correct.Check option (d): this is the definition of E, not K → wrong. Verification / Alternative check: Elasticity relations: σ = E * ε (uniaxial); p = K * ε_v (hydrostatic). These confirm the mismatch in option (d). Why Other Options Are Wrong: They are not wrong; (a), (b), and (c) are standard textbook definitions.“None of these is wrong” is invalid because (d) is incorrect. Common Pitfalls: Mixing up E and K; treating Poisson’s ratio without noting that lateral strain is of opposite sign; confusing volumetric and linear strains. Final Answer: Bulk modulus is the ratio of linear stress to linear strain.

Question 15

Composite bar of copper and steel heated to 40°C: What is the nature of stress induced in the copper bar (assuming the two are rigidly bonded and free expansion is restrained)?

Accepted Answer

Introduction / Context: Temperature changes in composite members cause self-equilibrating thermal stresses when free expansion is restrained. Understanding the sign of thermal stress in each material is essential for safe composite design. Given Data / Assumptions: Copper bar is rigidly bonded to a steel bar (composite system). Uniform heating to 40°C above the reference temperature. Free expansion is prevented by compatibility (bonding) and overall restraint. Typical properties: α_cu > α_steel, and E_cu, E_steel finite and positive. Concept / Approach: Free thermal strain for each material is epsilon_free = α * ΔT. Because α_cu > α_steel, copper wants to expand more than steel. Bonding enforces equal total strain, so copper is forced to expand less than it would freely → copper experiences compressive mechanical strain; steel is forced to expand more than freely → tensile mechanical strain. Step-by-Step Solution: Compute free strains: ε_cu_free = α_cu * ΔT, ε_st_free = α_st * ΔT.Compatibility: ε_cu_total = ε_st_total.Mechanical strains: ε_mech = ε_total − ε_free.Since ε_cu_free > ε_total, copper’s ε_mech is negative → compression; steel’s ε_mech is positive → tension. Verification / Alternative check: Force equilibrium requires internal compressive force in copper to balance tensile force in steel. Analytical solution yields sigma_cu = − (E_cu * E_st / (E_cu A_cu + E_st A_st)) * (α_cu − α_st) * ΔT * A_st in compression (sign negative). Why Other Options Are Wrong: Tensile/Zero/Shear: contradict thermal compatibility and equilibrium for α_cu > α_steel.“Depends only on cross-section” is false; coefficients of expansion are decisive. Common Pitfalls: Assuming no stress appears under uniform heating; ignoring restraint due to bonding; mixing sign convention for tension/compression. Final Answer: Compressive

Question 16

Short column criterion: When is a column classified as “short”? Choose the standard slenderness-based definition.

Accepted Answer

Introduction / Context: Columns are classified as short or long based on their tendency to fail by crushing or by buckling. Slenderness ratio governs the dominant failure mode and the analysis method (material strength vs. stability). Given Data / Assumptions: Slenderness ratio = L / k_min, where L is effective length and k_min is least radius of gyration. Typical steel design heuristics: short if L/k_min ≲ 80 (some codes use different thresholds). Elastic behaviour up to failure criterion. Concept / Approach: Low slenderness implies stocky columns with minimal buckling tendency; failure is governed by material crushing (compressive stress limit). High slenderness increases susceptibility to elastic instability (Euler buckling), requiring stability checks. Step-by-Step Solution: Compute slenderness: λ = L / k_min.If λ is small (≈ 80 or below for steel), classify as short.Use compressive strength-based design (including end conditions, imperfections as per code). Verification / Alternative check: Compare with Euler critical load formula P_cr = π^2 E I / L_e^2; small L/k increases P_cr sharply, making buckling less critical, consistent with “short column.” Why Other Options Are Wrong: Absolute length or area alone (a, b) do not decide buckling behaviour.“More than 80” (d) corresponds to a long/slender column.Load magnitude (e) is not a classification criterion. Common Pitfalls: Ignoring effective length (end restraints); using area instead of radius of gyration; applying the same slenderness limit to all materials without checking the relevant code. Final Answer: When the slenderness ratio (L / k_min) is less than about 80

Question 17

Thick-walled cylinder under internal pressure p: The circumferential (tangential) stress is tensile while the radial stress is compressive across the thickness. Is this statement correct?

Accepted Answer

Introduction / Context: Pressurized cylinders (gun barrels, pressure vessels, pipes) exhibit non-uniform stress fields. Distinguishing tangential (hoop) and radial stresses is crucial in thick-cylinder design using Lame’s equations. Given Data / Assumptions: Internal pressure p acts; external pressure may be zero unless stated. Thick cylinder theory (Lame) applies; elastic, isotropic material. Ends closed; plane strain in axial direction if long. Concept / Approach: In a thick tube:sigma_theta (hoop) is tensile and highest near the inner radius.sigma_r (radial) is compressive, equal to −p at the inner surface and reduces in magnitude toward the outer surface (often to zero if external pressure is zero). Step-by-Step Solution: Lame’s forms: sigma_r = A − B/r^2, sigma_theta = A + B/r^2.At r = r_i: sigma_r = −p (compressive), sigma_theta is tensile and maximum.At r = r_o: if p_ext = 0, sigma_r = 0, while sigma_theta remains tensile but smaller than at r_i. Verification / Alternative check: Plotting sigma_r across thickness shows negative values increasing toward zero; sigma_theta always positive for internal pressure only, confirming the statement. Why Other Options Are Wrong: “Incorrect” contradicts thick-cylinder solutions.Dependence on external pressure alone is false; sign pattern holds for p_ext = 0.Truth limited to a surface is incorrect; the sign trend holds throughout thickness. Common Pitfalls: Using thin-cylinder formulas; confusing sign conventions; assuming radial stress is tensile at the outer surface without external pressure. Final Answer: Correct

Question 18

Welded joints: Shear strength of a double parallel fillet weld (two sides) of length l and effective throat s, using allowable shear stress τ, is equal to?

Accepted Answer

Introduction / Context: Designing fillet welds requires computing load capacity from effective throat area and allowable shear stress. A double parallel fillet weld places fillets on two opposite sides of a joint, doubling the effective area. Given Data / Assumptions: Two parallel fillet welds of length l each. Effective throat size = s (for a right-angled fillet, s ≈ 0.707 * leg size). Allowable shear stress in weld metal = τ. Concept / Approach: Shear strength = allowable shear stress * effective shear area. For parallel fillets on two sides, total effective throat area equals 2 * s * l (neglecting end returns). Therefore capacity equals P_allow = 2 * s * l * τ. Step-by-Step Solution: Compute area for one fillet: A_1 = s * l.Two fillets in parallel: A_total = 2 * s * l.Strength: P_allow = A_total * τ = 2 * s * l * τ. Verification / Alternative check: Compare with code formulas where effective throat = 0.707 * leg; substituting s = 0.707 a gives the same scaling factor of 2 for two weld lines. Why Other Options Are Wrong: 0.5 s l τ and s l τ underestimate by ignoring the second fillet or using an incorrect factor.4 s l τ overestimates; there are only two parallel fillets, not four.Option “2.s.l.τ” is the same as the correct expression but duplicated; we select the standard formatting in option (c). Common Pitfalls: Confusing leg size with throat size; forgetting that two-sided fillets double the area; neglecting reduction factors for intermittent welds or eccentric loading (not applicable here). Final Answer: 2 s l τ

Question 19

Mohr’s circle basics: The radius of Mohr’s circle equals which combination of the two principal stresses?

Accepted Answer

Introduction / Context: Mohr’s circle graphically represents plane stress transformation. Knowing the center and radius allows immediate computation of principal stresses and maximum shear stress. Given Data / Assumptions: Principal stresses σ1 and σ2 are real values (plane stress state). Standard plotting: σ-axis horizontal, τ-axis vertical. Positive shear convention consistent with Mohr’s construction. Concept / Approach: The circle’s center is at C = (σ1 + σ2)/2 on the σ-axis, and the radius is R = (σ1 − σ2)/2. The maximum in-plane shear equals this radius. Step-by-Step Solution: Plot points A(σ1, 0) and B(σ2, 0) on the σ-axis.Center C is the midpoint of AB → (σ1 + σ2)/2.Radius R is half the length of AB → (σ1 − σ2)/2. Verification / Alternative check: General stress components (σx, σy, τxy) reduce to principal form by rotation; analytical solutions give σ1,2 = (σx + σy)/2 ± √[ ((σx − σy)/2)^2 + τxy^2 ], where the radical equals the radius R. Why Other Options Are Wrong: Sum or half-sum describe the center, not the radius.Full difference is twice the radius.Product has no direct geometric meaning in Mohr’s circle radius. Common Pitfalls: Mixing center and radius formulas; sign mistakes in τ that reflect points across the σ-axis. Final Answer: Half the difference of the two principal stresses

Question 20

Thick cylinder under internal pressure only: How does radial stress vary across the wall thickness?

Accepted Answer

Introduction / Context: Design of thick-walled pressure components requires correct understanding of radial and hoop stress distributions. Errors in the sign or boundary conditions can seriously compromise safety. Given Data / Assumptions: Internal pressure p acts at the bore; external pressure is zero unless stated. Elastic, isotropic material; Lame’s solution applies. Static loading; no thermal gradients. Concept / Approach: From Lame’s equations: sigma_r = A − B/r^2. Boundary conditions for internal-only pressure are sigma_r(r_i) = −p (compressive) and sigma_r(r_o) = 0. Thus the largest magnitude of radial stress occurs at the inner radius and decreases to zero at the outer radius. Step-by-Step Solution: Apply inner boundary: sigma_r(r_i) = −p (maximum compressive).Apply outer boundary: sigma_r(r_o) = 0 (free surface).Therefore, radial stress varies from −p at r_i to 0 at r_o; it is not uniform. Verification / Alternative check: Plotting sigma_r versus r yields a monotonic curve increasing from −p to 0. Hoop stress sigma_theta remains tensile and larger than |sigma_r| near the inner radius. Why Other Options Are Wrong: Options (a) and (c) reverse the boundary conditions.Option (b) is incomplete; it omits that sigma_r at the outer surface equals zero when external pressure is absent.Uniform distribution is a thin-wall assumption for hoop stress, not for radial stress in thick walls. Common Pitfalls: Assuming thin-wall results; forgetting that radial stress at a free outer surface is zero when no external pressure acts. Final Answer: Maximum at the inner surface and zero at the outer surface (when no external pressure)

Strength of Materials Questions