6. Design of Joints, Fasteners and Connections

Introduction and engineering context

Machine elements rarely operate as isolated components. Shafts must be joined to hubs, covers must be attached to housings, pressure vessels must be closed, structural members must be assembled, and dissimilar materials must be connected without compromising strength, stiffness, serviceability, or maintainability. The elements that perform these functions are joints, fasteners, and connections.

A connection is successful only when it transfers the required forces and moments throughout its intended service life. This requires more than selecting a bolt diameter or a weld size. The designer must identify the load path, determine whether the loading is static, fluctuating, impact-like, thermal, or environmental, and account for local stress concentration, preload, friction, slip, deformation, corrosion, manufacturing tolerances, inspection, and disassembly requirements.

Connections are often the most vulnerable regions of a machine. Holes reduce net section, threads introduce notches, weld toes contain geometric discontinuities, adhesive layers may degrade with moisture and temperature, and interference fits generate residual stresses. A sound design therefore combines mechanics of materials, fatigue theory, materials engineering, manufacturing knowledge, and reliability-based decision-making.

This chapter develops methods for the design and selection of threaded fasteners, bolted joints, welded and bonded joints, riveted connections, keyed and splined joints, and interference fits. Emphasis is placed on load transfer, failure modes, design procedure, and the influence of fatigue, temperature, and environment.

Learning objectives

After studying this chapter, the reader should be able to:

1. Classify joints and select a suitable connection method for a specified application.

2. Determine bolt preload, external load sharing, tightening torque, and factors of safety for bolted joints.

3. Design threaded fasteners under tensile, shear, combined, and fatigue loading.

4. Evaluate welded, bonded, riveted, keyed, splined, and interference-fit connections.

5. Explain the effects of preload, stress concentration, residual stress, thermal expansion, corrosion, and fretting.

6. Identify likely failure modes and develop appropriate inspection and maintenance practices.

7. Apply a systematic connection-design workflow to an engineering case.

8. Recognize the role of standards, digital analysis, and responsible engineering judgment in connection design.

6.1 The load path: the central idea in connection design

The first question in connection design is not “What fastener should be used?” but “How does the force travel from one component to the next?” Consider a flange secured by bolts. An axial separating force acts on the flange faces, but the bolts do not necessarily receive the entire external force. If the joint is properly preloaded, part of the external load relieves compression in the clamped members while the remainder increases bolt tension. The stiffness of the bolt and the clamped members governs this division.

For a preloaded bolted joint subjected to an external tensile force P, the increase in bolt load may be approximated by

ΔFb = C P,

where C is the joint stiffness factor,

C = kb/(kb + km).

Here kb is the bolt stiffness and km is the stiffness of the clamped members. The bolt load is therefore Fb = Fi + CP, while the compressive load remaining in the members is approximately Fi - (1 - C)P. Fi is the initial preload. Separation begins when the remaining clamping force approaches zero. This explains why a properly preloaded joint can resist external loading with relatively small changes in bolt stress.

A connection must also be assessed for secondary effects. A bolt group may experience direct shear and moment-induced shear; a weld may carry eccentric loading; a key may transmit torque but create a severe shaft stress concentration; and an interference fit may transfer torque only while contact pressure remains adequate. The nominal load must therefore be transformed into the local forces, stresses, and contact pressures that govern failure.

6.2 Selection of a connection type

The choice between a bolted, welded, bonded, riveted, keyed, splined, or shrink-fit connection depends on several interacting requirements. Bolted joints are preferred when disassembly, inspection, or field replacement is important. Welds provide high structural efficiency and low part count but may introduce distortion and heat-affected zones. Adhesive joints distribute load over an area and can join dissimilar materials, although temperature and surface preparation are critical. Rivets remain useful in thin-sheet and aerospace structures where controlled, permanent assembly is required.

Keys and splines are used mainly to transmit torque between a shaft and hub. A key is simple and inexpensive, whereas splines provide multiple load-sharing teeth and may permit axial sliding. Interference fits are compact and free of holes or external fasteners, but assembly forces, dimensional control, and thermal effects must be carefully managed.

The design decision should consider strength, fatigue life, stiffness, sealing, corrosion, manufacturability, assembly method, repairability, inspection access, cost, and consequences of failure. A permanent connection is not automatically superior: if a bearing must be replaced periodically, a permanent weld may create an unacceptable maintenance problem.

6.3 Threaded fasteners and bolted joints

Thread geometry and effective strength

The nominal diameter of a bolt is not the diameter governing tensile failure. The threaded portion has a reduced tensile stress area, At, which should be obtained from the applicable standard rather than approximated by the nominal cross-sectional area. Thread pitch, root radius, flank angle, nut height, engagement length, and thread form influence load distribution and stripping resistance.

The first engaged threads carry the greatest load because the bolt and nut deform differently. Increasing nut height beyond a suitable value does not indefinitely increase strength. Proper thread engagement, material compatibility, and avoidance of damaged or incomplete threads are essential.

Common bolt failure modes include tensile fracture, thread stripping, shear failure, bending due to poor fit, fatigue fracture at the first engaged thread, preload loss, and corrosion-assisted cracking. The connected members may fail by bearing, tear-out, net-section fracture, or separation.

Preload and tightening

Preload is the tensile force introduced into a bolt during tightening. It produces compression in the members and is central to joint reliability. A common engineering target for a reusable steel bolt is a preload related to proof strength, often expressed as

Fi ≈ 0.7 to 0.9 Sp At,

where Sp is the proof stress and At is the tensile stress area. The exact value depends on joint function, scatter in tightening, material, lubrication, fatigue requirements, and the risk of embedment or yielding.

The tightening torque is frequently estimated from

T = K Fi d,

where T is torque, d is nominal bolt diameter, and K is a nut factor incorporating thread friction, bearing friction, lubrication, and geometry. Since friction dominates this relation, torque control can produce substantial preload variation. A torque wrench may therefore be inadequate for critical joints unless calibration and friction control are carefully managed. Alternatives include angle-controlled tightening, hydraulic tensioning, direct-tension indicators, or ultrasonic measurement.

Preload must be high enough to prevent separation, slip, and fatigue damage, but not so high that the bolt or clamped parts yield. Soft gaskets, coatings, surface roughness, and embedment can cause preload relaxation. Locking devices, prevailing-torque nuts, chemical thread lockers, safety wire, or mechanical locking features may be required, but none compensates for an incorrectly designed joint.

Static design of a bolt

For a bolt subjected to direct tensile load F, the nominal tensile stress is

σt = F/At.

For direct shear V, the nominal shear stress may be estimated as

τ = V/As,

where As is the effective shear area. If tensile and shear stresses act simultaneously, an equivalent-stress criterion such as von Mises may be used:

σe = √(σt² + 3τ²).

The design is acceptable when the equivalent stress is below the appropriate allowable value, with a factor of safety selected according to uncertainty, consequence of failure, and governing code.

A bolt group under eccentric load must be analyzed by superposing direct load and moment-induced load. For a group of n bolts carrying direct shear V, the direct component per bolt is V/n. The additional shear caused by a moment M is proportional to the bolt’s radial distance from the group centroid:

Fi,m = M ri / Σrj².

The vector sum of the direct and moment components should be evaluated for each bolt. In tension-loaded flange joints, prying action may substantially increase bolt force beyond the simple division of external load by the number of bolts. Flexible end plates and thin flanges are especially susceptible to prying.

Fatigue of bolted joints

A preloaded bolt joint can have excellent fatigue performance because the external load is shared by the bolt and clamped members. The alternating component of bolt stress is approximately C times the alternating external load per bolt, while the mean bolt stress includes preload. Fatigue assessment should use the bolt’s appropriate S–N data, notch sensitivity, surface condition, size effect, and mean-stress correction.

Failure commonly initiates at the thread runout or first engaged thread. Rolled threads generally provide better fatigue performance than cut threads because of favorable surface condition and residual compressive stress. The bolt should be selected so that the joint remains closed under cyclic loading. Separation greatly increases the alternating bolt load and often causes rapid fatigue failure.

Slip-resistant and bearing-type joints

In a bearing-type joint, external shear is eventually transferred by contact between the bolt and hole. The designer checks bolt shear, plate bearing, net-section tension, and tear-out. In a slip-resistant joint, preload creates friction between faying surfaces, and slip is not permitted under the specified service load. The slip resistance depends on preload, friction coefficient, number of interfaces, and surface preparation. Slip-critical designs require strict control of coating, cleanliness, bolt installation, and inspection.

6.4 Welded connections

Welded joints transfer load through fusion or solid-state bonding between components. Fillet welds are widely used because they tolerate fit-up variation and are economical. Their effective throat is approximately 0.707 times the leg size for an equal-leg fillet weld. The effective throat area is the throat length multiplied by the effective weld length.

A fillet weld under direct shear may be checked using

τw = F/Aw.

For eccentric loading, direct and moment-induced stresses are combined over the weld-group geometry. Weld groups should be analyzed in a manner analogous to bolt groups, but the continuous or segmented nature of the weld and the effective throat must be considered.

Welded joints are influenced by the base material, filler metal, heat input, cooling rate, residual stress, distortion, and heat-affected-zone microstructure. Fatigue cracks often begin at weld toes or roots because of geometric discontinuity and tensile residual stress. In fatigue-critical structures, weld profile improvement, toe grinding, controlled dressing, suitable joint detail, and nondestructive examination may be more important than simply increasing weld size.

Design standards classify welded details according to fatigue strength. The nominal stress approach, structural hot-spot stress approach, and effective notch stress approach provide progressively more refined assessments. A large weld does not necessarily solve a fatigue problem if it increases restraint, distortion, or residual stress.

6.5 Bonded joints

Adhesive joints transfer load through an adhesive layer between adherends. Unlike a single fastener, an adhesive can distribute load over a broad area and reduce local stress concentration. Lap joints, scarf joints, butt joints, and stepped joints are common configurations.

An ideal bonded joint keeps the adhesive primarily in shear and avoids peel stresses. A simple average shear stress is τavg = F/(bL), where b is joint width and L is overlap length. Actual shear stress is nonuniform, with high concentrations near overlap ends. Peel stresses caused by eccentricity, bending, adherend flexibility, or abrupt termination can dominate failure.

Adhesive design requires attention to surface preparation, cleanliness, roughness, primer, bond-line thickness, cure cycle, aging, and compatibility. Environmental exposure may cause moisture uptake, plasticization, hydrolysis, thermal softening, or loss of adhesion. Structural adhesives should be characterized at the relevant temperature, humidity, strain rate, and aging condition rather than by room-temperature lap-shear strength alone.

Mechanical fasteners and adhesives are sometimes combined. The adhesive distributes load and seals the interface, while the fastener provides immediate clamping and fail-safe capability. However, stiffness mismatch can cause undesirable load sharing, and drilling holes after bonding may damage the joint.

6.6 Riveted joints

Riveted joints are permanent mechanical connections formed by inserting and upsetting a rivet. They are common in sheet structures and remain valuable where heat input from welding is undesirable or where controlled, inspectable assembly is required.

A riveted lap joint is checked for rivet shear, plate bearing, plate tearing across the weakest net section, and tear-out at the edge. For a single-rivet connection, the nominal shear capacity is related to the rivet shear area, while bearing capacity depends on rivet diameter, plate thickness, and allowable bearing stress. Joint efficiency is the ratio of the strength of the joint to that of the unperforated plate.

Rivet pitch, edge distance, hole quality, and load eccentricity strongly affect performance. Excessive pitch causes plate bending and leakage; insufficient edge distance promotes tear-out. Fatigue-sensitive riveted structures require attention to hole quality, residual clamping force, fretting, and corrosion around the faying surfaces.

6.7 Keys and splines for torque transmission

A keyed connection transmits torque from a shaft to a hub through a key fitted in matching keyways. The transmitted tangential force at shaft radius d/2 is

F = 2T/d,

where T is torque and d is shaft diameter.

For a rectangular sunk key of width b, length L, and thickness h, the approximate shear stress is

τk = 2T/(d b L),

and the bearing stress, depending on the contact geometry, is commonly estimated using the effective bearing area. For a key with half its thickness engaged in bearing, a useful approximation is

σb = 4T/(d h L).

The key length is selected so that both shear and bearing requirements are satisfied. The shaft must also be checked because the keyway reduces its torsional and bending strength and creates a stress concentration. A long key is not always beneficial if it increases distortion or produces poor load distribution.

Splines use multiple integral teeth and therefore provide greater torque capacity, improved centering, and the possibility of axial movement. Important spline parameters include tooth form, pressure angle, major and minor diameters, tooth engagement length, fit class, and load distribution. Failure may occur by tooth shear, tooth bending, surface pitting, wear, fretting, or shaft fracture at the spline root. Lubrication, alignment, hardness, and contamination control are particularly important in sliding splines.

6.8 Interference-fit and shrink-fit connections

An interference fit is produced when the assembled shaft diameter exceeds the hub bore diameter by a controlled amount. Elastic deformation generates contact pressure p at the interface. That pressure creates frictional resistance to torque and axial movement.

For a simplified interface, the torque capacity may be estimated by

T = μ p π d L (d/2),

where μ is the friction coefficient, d is interface diameter, and L is engagement length. The axial holding capacity is approximately F = μ p πdL. More accurate pressure calculations require thick-cylinder elasticity and account for shaft and hub diameters, material moduli, Poisson’s ratios, and geometry.

Assembly may be achieved by pressing, heating the hub, cooling the shaft, or combining both methods. The design must consider assembly temperature, thermal expansion, residual stress, yielding, and the possibility of reducing contact pressure during operation. If a steel hub is heated excessively, material properties, seals, and surface treatments may be damaged.

Interference fits are vulnerable to fretting when cyclic micro-slip occurs. Fretting produces wear debris, surface damage, and sometimes fatigue cracks. Adequate interference, surface finish, alignment, and protection against vibration are necessary. The designer must also ensure that the hub does not split or that the shaft does not yield during assembly.

6.9 Thermal, environmental, and fatigue effects

A connection exposed to temperature change experiences differential expansion whenever the joined materials have different coefficients of thermal expansion or different temperature distributions. In a constrained bolted joint, the bolt load may increase or decrease according to the relative thermal expansion of bolt and clamped members. A simplified elastic estimate is based on the difference in free thermal expansion multiplied by the effective joint stiffness. Temperature-dependent elastic modulus, yield strength, creep, and gasket relaxation should be included for high-temperature service.

At low temperatures, toughness may decrease and brittle fracture becomes a concern. At elevated temperatures, preload relaxation, stress relaxation, creep, and loss of adhesive strength may dominate. Thermal cycling can loosen joints, damage seals, and generate fatigue through repeated differential movement.

Environmental degradation includes general corrosion, galvanic corrosion, pitting, crevice corrosion, hydrogen embrittlement, stress-corrosion cracking, and oxidation. High-strength fasteners are particularly sensitive to hydrogen embrittlement under certain plating and cleaning conditions. Material selection, coatings, drainage, isolation of dissimilar metals, sealing, and inspection must be integrated into the design rather than added after failure.

Fatigue design should distinguish nominal stress, local stress, and contact stress. Stress concentrations at holes, threads, keyways, weld toes, and fit transitions require suitable fatigue data or notch-sensitive analysis. Mean stress, residual stress, surface finish, size, corrosion, and assembly preload all affect fatigue life.

6.10 A systematic design workflow

A defensible connection design can follow this sequence:

1. Define the service function, required life, maintenance strategy, allowable movement, and failure consequences.

2. Identify all load cases, including static, transient, cyclic, thermal, pressure, vibration, assembly, and accidental loads.

3. Establish the load path and idealize the joint geometry.

4. Select candidate connection types using strength, stiffness, manufacturability, inspection, and disassembly criteria.

5. Determine preliminary dimensions and material grades from applicable standards.

6. Check primary failure modes: tensile fracture, shear, bearing, tear-out, slip, separation, stripping, weld fracture, adhesive peel, fretting, and fatigue.

7. Evaluate local stresses, stress concentrations, prying, eccentricity, residual stress, and thermal effects.

8. Specify installation controls, tolerances, surface preparation, lubrication, tightening method, and inspection requirements.

9. Verify the design using hand calculations, finite-element analysis, testing, or a combination of methods.

10. Document assumptions, safety factors, acceptance criteria, and maintenance requirements.

Finite-element analysis is especially useful for bolt-group load distribution, flange separation, weld hot spots, adhesive peel, interference pressure, and local contact. It should not replace basic free-body diagrams or validated material data. Models must represent contact, preload, friction, nonlinear material behavior, and geometric details sufficiently to answer the design question.

Illustrative case study: redesign of a vibrating pump-flange joint

Background

An illustrative process pump uses a bolted steel flange to connect a discharge pipe. The joint operates at moderate temperature and is exposed to vibration and occasional pressure cycling. After several months, one bolt fractured near the first engaged thread and the flange developed a small leak.

Situation and diagnosis

The original design used eight bolts tightened by approximate hand torque. The flange was relatively flexible, the gasket was soft, and the external pressure load was cyclic. Inspection found uneven gasket compression, fretting marks around several holes, and corrosion beneath the washer faces. The failed bolt showed a fatigue origin at the thread runout rather than a single overload fracture.

The initial design calculation had divided the maximum pressure force equally among the eight bolts and checked only nominal tensile stress. It ignored preload scatter, joint stiffness, flange flexibility, gasket relaxation, and the possibility of partial separation. Consequently, several bolts carried much larger alternating loads than predicted.

Analysis and redesign

The redesign began with a revised free-body diagram and determination of pressure, piping reaction, and vibration loads. A bolt-joint stiffness model was used to estimate the load fraction C. Flange flexibility and prying were then assessed using a plate model and finite-element verification. The required preload was selected to maintain gasket compression without exceeding bolt proof strength or crushing the gasket.

The bolts were upgraded to a suitable fatigue-resistant grade with rolled threads, and the grip length was increased so that the threaded portion did not lie in the highest bending region. Hardened washers improved load distribution. A controlled hydraulic tightening procedure, cross-pattern sequence, and calibrated verification method replaced approximate hand torque. The flange facing and gasket system were revised, and a corrosion-resistant coating compatible with the temperature and gasket was specified.

Outcome

The redesigned joint maintained compression during the pressure cycle, reduced bolt stress amplitude, and eliminated visible fretting during qualification testing. A maintenance specification required periodic inspection of preload indicators, corrosion, flange alignment, and gasket condition. The principal improvement did not come from merely increasing bolt size; it resulted from controlling the load path and installation variability.

Learning points

The case demonstrates that bolted-joint failures are frequently system failures rather than isolated bolt-strength failures. Preload, stiffness, gasket behavior, flange flexibility, thread geometry, corrosion, and installation practice must be considered together. It also illustrates the value of combining analytical models with inspection and controlled assembly.

Common design errors, professional responsibilities, and best practice

Typical mistakes include using nominal bolt area instead of tensile stress area, assuming torque directly determines preload, ignoring prying action, placing welds at fatigue-critical locations without detail assessment, designing adhesive joints from room-temperature strength alone, neglecting the shaft weakening caused by a keyway, and specifying interference without considering assembly temperature.

Another common error is applying a large safety factor to an inaccurate model. A safety factor cannot compensate for an omitted failure mode, uncontrolled friction, poor surface preparation, or an incorrect load path. Critical designs should use standards such as those governing structural bolting, pressure equipment, welding qualification, fastener property classes, adhesive testing, and nondestructive examination. The governing standard depends on industry and jurisdiction.

Engineers have a professional responsibility to document traceable assumptions, use qualified materials and procedures, provide inspection criteria, and communicate residual risks. A connection should be designed for its real maintenance environment, not an ideal laboratory assembly. If failure can endanger people, containment, or the environment, independent review and proof testing may be justified.

Digital tools increasingly support connection design. Parametric CAD systems can control tolerances; finite-element models can evaluate preload and contact; digital torque tools can record installation data; and statistical analytics can identify preload drift or recurring failures from maintenance records. Machine-learning methods may assist anomaly detection, but they do not replace mechanics-based verification, qualified data, or engineering accountability.

Emerging practice emphasizes lightweight hybrid joints, friction-stir and laser welding, structural adhesives with improved environmental resistance, additive-manufactured integrated joint features, smart fasteners with preload sensing, and digital-twin monitoring of critical connections. These developments expand design options but also introduce new qualification, repair, data-governance, and certification requirements.

Chapter Summary

Joints and connections are load-transfer systems whose reliability depends on geometry, material, installation, environment, and service loading. Bolted joints require attention to preload, joint stiffness, external load sharing, separation, torque scatter, fatigue, and flange flexibility. Welded joints must be assessed through weld geometry, heat-affected zones, residual stress, and fatigue-sensitive details. Bonded joints distribute load effectively but are sensitive to peel, surface preparation, temperature, and moisture. Riveted joints are checked for rivet shear, bearing, net-section fracture, and tear-out. Keys and splines transmit torque but introduce local shaft stresses, wear, and fretting. Interference fits rely on contact pressure and friction and require control of dimensional tolerances and assembly temperature.

A robust design begins with the load path and proceeds through failure-mode identification, analytical checks, installation controls, inspection, and validation. Static strength alone is insufficient where cyclic loading, thermal variation, corrosion, creep, or maintenance variability exists. Standards, digital analysis, and monitoring enhance reliability, but engineering judgment remains essential.

Key Terms

Preload: Initial tensile force introduced into a fastener during tightening.

Tensile stress area: Effective threaded area used to calculate tensile capacity of a bolt.

Joint stiffness factor: Fraction of an external tensile load that increases bolt load.

Prying action: Additional bolt force caused by deformation of a flexible connected member.

Slip-resistant joint: Joint designed to prevent relative slip through friction generated by preload.

Bearing-type joint: Joint in which shear is transferred through contact between fastener and hole.

Fillet weld: Triangular weld joining two surfaces, commonly characterized by its effective throat.

Heat-affected zone: Region adjacent to a weld whose microstructure and properties are altered by heat.

Adhesive lap joint: Bonded joint in which overlapping adherends transfer load through an adhesive layer.

Rivet efficiency: Ratio of joint strength to the strength of an equivalent unperforated plate.

Key: Removable machine element transmitting torque between a shaft and hub.

Spline: Multiple-tooth shaft connection used for torque transmission and, in some designs, axial sliding.

Interference fit: Assembly in which the shaft is larger than the hub bore before assembly.

Fretting: Surface damage caused by small-amplitude repeated relative motion under contact pressure.

Thread stripping: Failure caused by shearing of engaged bolt or nut threads.

Preload relaxation: Reduction in joint clamping force due to embedment, creep, or stress relaxation.

Net-section failure: Fracture through the reduced cross-section remaining after holes or keyways are introduced.

Stress concentration: Local increase in stress caused by geometric discontinuity.

Hydrogen embrittlement: Loss of ductility and cracking susceptibility caused by absorbed hydrogen in susceptible materials.

Proof stress: Specified stress level used to verify that a fastener can sustain load without unacceptable permanent deformation.

Review Questions

Short Answer Questions

1. Why is the tensile stress area used instead of nominal bolt area in tensile design?

2. Define the joint stiffness factor and explain its significance.

3. State two principal failure modes of a bonded lap joint.

4. Why does a keyway reduce the fatigue strength of a shaft?

5. What is fretting, and why is it important in interference-fit connections?

Long Answer Questions

1. Explain the mechanics of a preloaded bolted joint subjected to external tensile loading, including separation and stiffness effects.

2. Describe the design and fatigue considerations for a welded connection under eccentric loading.

3. Compare bolted, welded, bonded, and riveted connections with respect to strength, maintenance, manufacturing, and environmental resistance.

4. Develop a design procedure for a keyed shaft-hub connection transmitting a specified torque.

5. Explain how temperature, corrosion, and preload relaxation influence the long-term reliability of fastened joints.

Analytical / Application Questions

1. A bolt group carries direct shear and an overturning moment. Describe how the load should be distributed among the bolts and identify the checks required before accepting the design.

2. A flange joint leaks after pressure cycling even though the nominal bolt stress is below yield. Using the concepts in this chapter, diagnose at least four possible causes and propose corrective actions.

3. Select and justify a connection type for a rotating shaft that must transmit high torque, permit occasional axial movement, operate in a contaminated environment, and be disassembled during scheduled maintenance.

22. Multiple Choice Questions with Answers

Question 1

For a preloaded bolted joint subjected to an external tensile load P, which expression approximates the increase in bolt load?

A. ΔFb = (1 − C)P

B. ΔFb = CP

C. ΔFb = P/C

D. ΔFb = Fi − CP

Correct Answer: B. ΔFb = CP

Question 2

Which condition most directly causes separation of a properly preloaded bolted joint?

A. The remaining clamping force is reduced to approximately zero

B. The bolt torque is below the nominal thread torque

C. The bolt has a diameter larger than the hole

D. The clamped members remain in compression

Correct Answer: A. The remaining clamping force is reduced to approximately zero

Question 3

Why can torque-controlled tightening produce significant preload scatter?

A. Torque is independent of friction

B. Most torque is consumed in overcoming friction at threads and bearing surfaces

C. Bolt proof stress changes with washer diameter only

D. Torque directly measures tensile stress area

Correct Answer: B. Most torque is consumed in overcoming friction at threads and bearing surfaces

Question 4

In a slip-resistant bolted joint, service shear is intended to be transferred primarily by

A. Bending of the bolt

B. Bearing at the edge of the hole

C. Friction between preloaded faying surfaces

D. Tensile yielding of the connected plate

Correct Answer: C. Friction between preloaded faying surfaces

Question 5

A fatigue crack in a welded joint most commonly initiates at a weld toe or root because these locations

A. Have no geometric discontinuity

B. Experience reduced temperature during service only

C. Contain stress concentration and may retain tensile residual stress

D. Always have compressive residual stress

Correct Answer: C. Contain stress concentration and may retain tensile residual stress

Question 6

For a keyed shaft-hub connection, the tangential force associated with torque T on a shaft of diameter d is approximately

A. T/d

B. d/(2T)

C. 2T/d

D. Td/2

Correct Answer: C. 2T/d

Question 7

Which adhesive-joint design practice generally improves performance?

A. Maximizing peel stress at the overlap ends

B. Loading the adhesive mainly in shear and controlling surface preparation

C. Using room-temperature strength for all service conditions

D. Ignoring bond-line thickness

Correct Answer: B. Loading the adhesive mainly in shear and controlling surface preparation

Question 8

An interference fit transmits torque primarily through

A. Friction generated by contact pressure at the interface

B. Tensile stress in an external bolt

C. Shear stress in a rivet head

D. Adhesive peel resistance only

Correct Answer: A. Friction generated by contact pressure at the interface

Question 9

Which factor is especially important when selecting a high-strength fastener for corrosive service?

A. Hydrogen embrittlement susceptibility

B. Elimination of all preload

C. Use of an unthreaded grip of zero length

D. Ignoring coating compatibility

Correct Answer: A. Hydrogen embrittlement susceptibility

Question 10

In the pump-flange case, the most effective redesign was based primarily on

A. Increasing the bolt diameter without changing installation

B. Dividing pressure force equally among bolts and ignoring the gasket

C. Controlling preload, joint stiffness, flange flexibility, fatigue, and installation variability

D. Replacing bolts with a single oversized rivet

Correct Answer: C. Controlling preload, joint stiffness, flange flexibility, fatigue, and installation variability