Bolted and Welded Joints

Joint types and how they fail

Joints are where structures usually fail: a part of a machine seldom breaks in its plain section, but at a hole, a weld or a fastener. A joint has to carry the load from one part to another and, in a machine, often do so for millions of cycles. The common methods are compared below.

Joint	Strengths	Weaknesses	Typical failure
Bolted (preloaded)	Dismountable, strong, no heat distortion	Holes weaken the parts; loosening, fatigue	Bolt fatigue, loosening, thread stripping, joint slipping
Welded	Strong, rigid, continuous, no extra parts	Heat distortion, residual stress, low fatigue strength, not dismountable	Fatigue cracks at the weld toe or root, lack of fusion
Riveted	No heat, good for dissimilar and thin materials	Holes; strength depends on the installation	Shear of the rivet, bearing and tear-out of the plate
Adhesive (bonded)	Spreads the load, joins any material, seals	Needs surface preparation; low peel strength; temperature and aging	Peel and cleavage failure, a poor surface
Press, shrink and keyed	No fasteners; high torque (see Press, Shrink and Expansion Fits)	Precision, assembly effort	Slip, fretting

Bolts: sizes, grades and stress area

A bolt is described by its thread (M10 × 1.5: 10 mm nominal diameter, 1.5 mm pitch, coarse), its property class and its length. The threaded part carries the load over the tensile stress area A_t, a little smaller than the shank, which is the area to use in strength calculations: A_t = π ÷ 4 (d − 0.9382 p)². The class (4.6, 8.8, 10.9, 12.9) encodes the strength: the first number × 100 is the tensile strength in MPa, and the second ÷ 10 is the yield ratio.

Property class	Proof strength Sp (MPa)	Yield Sy (MPa)	Tensile Su (MPa)	Use
4.6	225	240	400	Light, general (low carbon steel)
5.8	380	420	520	General
8.8	600	660	830	The standard structural and machine bolt (medium carbon, quenched and tempered)
9.8	650	720	900	Small sizes (M16 and under)
10.9	830	940	1 040	High strength, machinery
12.9	970	1 100	1 220	Very high strength (socket cap screws); sensitive to hydrogen embrittlement and notch
A2-70, A4-80 (stainless)	450 to 600	450 to 600	700 to 800	Corrosion-resistant; galls when tightened dry (use anti-seize)

Thread (coarse)	At (mm²)	Preload Fi = 0.75 Sp At (kN, class 8.8)	Torque T = 0.2 d Fi (N·m, dry)
M6	20.1	9.0	11
M8	36.6	16.5	26
M10	58.0	26.1	52
M12	84.3	37.9	91
M16	157.0	70.7	226
M20	245.0	110.3	441
M24	353.0	158.9	762

Preload and tightening torque

A bolt in a good joint is not a simple pin: it is a spring that is stretched during tightening and clamps the parts together with the preload F_i. The preload is what holds the joint, keeps the nut from loosening, and above all makes the bolt see only a small part of any external load (see below), which protects it against fatigue. A rule for a reusable joint is F_i = 0.75 × S_p × A_t (75 percent of the proof load; 90 percent for a permanent joint). The tightening torque that produces it is T = K d F_i, where the nut factor K is about 0.2 for plain dry steel, 0.15 for lightly oiled and 0.18 for zinc plated; about 90 percent of the torque goes into overcoming friction under the head and in the threads and only 10 percent into stretching the bolt, so the preload scatters ±25 to 30 percent with torque control.

The joint diagram: stiffness and external load

The bolt stretches and the clamped members are compressed by the preload, so the bolt and the members are two springs with stiffnesses k_b and k_m. When an external tensile load P pulls the joint apart, it relieves some of the compression of the members and adds only a part to the bolt: F_b = F_i + C P and F_m = F_i − (1 − C) P, with the joint constant C = k_b ÷ (k_b + k_m). Because the members are normally much stiffer than the bolt, C is small (0.1 to 0.3): most of the external load is taken by relief of the clamping force, not by the bolt. The joint separates only when P exceeds P₀ = F_i ÷ (1 − C).

Bolts in shear and tension

A bolt can carry a transverse load in two ways. In a bearing (shear) connection the bolt shank bears on the hole walls and is loaded in shear: the capacity per shear plane is about 0.6 S_u A (A the area at the shear plane: the shank, or A_t if the threads cross it). In a friction-grip (slip-critical) connection the preload clamps the plates and friction carries the load: the slip load is μ n F_i (μ = 0.3 to 0.5 for clean steel, n friction surfaces). Both are checked together with the bearing stress on the plate and the tear-out of the plate edge (edge distance at least 1.5 to 2 d).

Thread engagement and tapped holes

The threads must be long enough that the bolt breaks before the threads strip. In a nut of matched strength about 0.8 d of engagement is enough (a standard nut is about 0.8 d high). In a tapped hole in a weaker material (aluminum, cast iron, plastic) the internal thread is the weak part and needs more length: roughly 1 d in steel, 1.5 d in cast iron and 2 d or more in aluminum.

• Blind holes need a drilling depth beyond the thread length; a bolt too long can bottom out and not clamp. See the Tap Drill Size and Thread Pitch Lookup for the drill sizes.
• Fine threads have a larger stress area and resist loosening a bit better but strip more easily in soft material.
• Clearance holes are about 0.5 to 1 mm larger than the bolt (ISO 273 medium series): M10 → 11 mm; allow the thermal movement and the tolerance (see Fits and Tolerances).

Fatigue and loosening

A bolt under a pulsating load fails by fatigue at the first engaged thread, under the head, or at the thread run-out. The preload makes the bolt’s stress amplitude small: σ_a = C (P_max − P_min) ÷ (2 A_t). For the joint above with the load pulsing from 0 to 10 kN: σ_a = 0.25 × 10 000 ÷ (2 × 58) = 21.6 MPa, against an endurance limit of about 129 MPa for a rolled-thread class 8.8 bolt (Shigley): a large margin, as long as the preload is maintained (see Fatigue and Stress Concentration).

Cause of loosening	Prevention
Embedment (settlement) of the surfaces, loss of preload	Hard washers, fewer interfaces, a longer grip, retighten after the first load
Transverse vibration (slip of the head and thread)	High preload, thread-locking adhesive, prevailing-torque nuts, wedge-locking washers (Nord-Lock), serrated flanges
Thermal cycling	Disc (Belleville) springs, long bolts, matching expansion coefficients
Gasket creep	Metal-to-metal contact or stops, retighten
Vibration alone with no slip	Rarely loosens a well-preloaded bolt; check the preload and the joint design

Spring and split lock washers are almost useless against vibration; use proper locking methods, safety wire or a cotter pin for critical joints.

Welded joints

A butt weld with full penetration is as strong as the parent plate (if it is sound). A fillet weld is triangular, with a leg length s; the strength is calculated on the throat a = 0.707 s, the shortest section, in shear: the allowable shear stress is about 0.3 × the electrode tensile strength (AWS E70 electrodes: S_u = 482 MPa, so τ_all ≈ 145 MPa). The load capacity per unit length is a × τ_all.

• Weld groups in bending and torsion: treat the weld as a line with a second moment of area I_w about its centroid; the stress is then (M ÷ I_w) × distance combined with the direct shear.
• Fatigue: welds have a much lower fatigue strength than the parent metal, because of the notch at the toe and root, residual stress and defects. The design codes (EN 1993-1-9, IIW, AWS D1.1) give detail categories; the endurance limit of a fillet weld is often only 20 to 40 percent of the plate’s.
• Design for good welds: access for the welder, avoid weld stacking and sharp corners, balance the welds to limit distortion, specify the process, filler and inspection, and consider heat treatment for thick or alloyed parts.
• Heat effects: the heat-affected zone of aluminum alloys loses much of its strength (6061-T6 drops to about 40 to 50 percent of its strength): design the weld in a low-stress region.
• Quality: inspection (visual, dye penetrant, magnetic particle, ultrasonic, radiographic) for critical welds; follow the welding standard (ISO 5817 quality levels).

Rivets, adhesives and other joints

• Rivets (solid, blind/pop, self-piercing) are loaded in shear and bearing, like bolts, and are quick for sheet; the strength comes from the rivet’s shear area and the plate’s bearing, and they have little preload so are poor against tension and vibration.
• Adhesive joints carry shear well over a large overlap (a lap shear strength of 15 to 30 MPa for structural epoxies) but peel and cleavage poorly: design so that the bond is loaded in shear, with a large overlap, a clean prepared surface and the right cure. Combine with fasteners for safety.
• Threaded inserts, rivet nuts and press-in nuts add strong threads in thin or soft parts.
• Snap fits and clips in plastics are loaded by the elastic deflection of a beam (see Beam Bending and Deflection).
• Clamps, hose clips and V-bands use friction and the capstan effect like belts.

Troubleshooting

Symptom	Likely cause	Check
Bolts loosen in service	Low preload, embedment, vibration slip, no locking	Torque and method; higher preload; a locking element; check the joint for slip
Bolt breaks below the head or at the first thread	Fatigue from low preload, too high a preload, a bending load or a misaligned face	Preload (K factor), flatness of the seat, grip length, a loose joint
Threads strip	Short engagement in soft material, over-tightening, a damaged thread	Engagement length, thread inserts, the torque used
Joint opens or leaks	Preload lost or the load above the separation load	Preload, gasket, stiffness; more bolts or a stiffer flange
Cracks at a weld toe	Fatigue, a notch, residual stress	Weld detail; smooth the toe, post-weld treatment, a larger section
Stainless bolt seizes (galling)	Dry tightening, high speed, fine threads	Anti-seize, lower speed, different materials
Adhesive joint peels	Peel loading, a poor surface, aging	Redesign for shear; surface preparation

Related

• Structures and Strength of Materials: the overview of this guide.
• Stress, Strain and Safety Factors, Torsion, Shear and Combined Loads and Fatigue and Stress Concentration: shear, combined stress and fatigue.
• Bolt Torque and Thread Engagement Tool and the Tap Drill Size and Thread Pitch Lookup.
• Worked Design Checks: a worked bolted-joint check.
• Fits and Tolerances: clearance holes and fits.

FAQ

What is the right torque for a bolt?

It depends on the size, the class and the friction: T = K d F_i with F_i about 75 percent of the proof load and K 0.2 for dry steel, 0.15 oiled. Use the manufacturer’s or the engineering specification value for a critical joint, and the tool above for a first estimate.

Why does preload matter so much?

Because the clamped parts are stiffer than the bolt, a preloaded bolt takes only a small part of the external load, so its stress hardly changes and it does not fatigue. A bolt that is not preloaded sees the whole load and soon fails.

Can I reuse a bolt?

A bolt tightened within its elastic range (to 75 percent of the proof load) can be reused if the threads are undamaged. Torque-to-yield and high-strength structural bolts should be replaced.

Bolt or weld?

Bolts when the joint must be taken apart, or loaded in fatigue and the welding is hard; welds for continuous, rigid, leak-tight joints and where weight matters. Many structures use both.

References

• Shigley, J. E. and Mischke, C. R., Mechanical Engineering Design, McGraw-Hill: screws, fasteners, welded and bonded joints.
• ISO 898-1 (mechanical properties of fasteners), ISO 262 and ISO 724 (metric threads), VDI 2230 (systematic calculation of bolted joints).
• AWS D1.1 (structural welding code), EN 1993-1-8 (design of joints), Bickford, J. H., An Introduction to the Design and Behavior of Bolted Joints.

Last reviewed: October 2026. Strengths and factors are typical; follow the applicable design code and test critical joints.