Every engineer who has specified mechanical fasteners for a structural joint knows the hidden costs: the drill time, the tap time, the fastener cost, the torque verification, the thread insert for soft materials, the galvanic corrosion between the fastener and the substrate, and the fatigue stress concentration at every hole. These are accepted as necessary costs of structural joining until a high-strength adhesive makes the tradeoff worth reconsidering. Ultra-high bond epoxy does not eliminate mechanical fasteners in all applications — but in a well-defined range of structural assembly scenarios, it replaces them with a joint that is lighter, less expensive to produce, more resistant to fatigue, and free of the stress concentrations that holes introduce into structural members.

The Engineering Case Against Fasteners in Structural Metal Joints

Mechanical fasteners join parts by clamping force and bearing load. Both mechanisms concentrate stress in ways that adhesive bonding does not. A drilled hole in a structural member removes material and creates a stress concentration factor — typically 2.5 to 3.0 for a circular hole in a flat plate under tension — that reduces the effective structural capacity of the member at that location. In fatigue loading, which includes any structure subject to vibration, repeated loading, or dynamic forces, this stress concentration is where cracks initiate and propagate.

Fastener contact bearing is another source of concentrated stress. Load transfers from one member to another through the fastener shank in bearing, loading a small area of each member with a high local stress. In thin-sheet assemblies, bearing failure — deformation of the hole and shank contact zone — can occur at loads well below the fastener’s rated tensile strength.

Adhesive bonding, by contrast, distributes load across the entire overlap area. In a well-designed lap joint with ultra-high bond epoxy, there are no stress concentrations from holes, no bearing surfaces, and no locations where the joint geometry focuses load onto a small area. The load distribution advantage is most pronounced in fatigue applications, where the absence of stress concentration sites dramatically extends the cycle life of the bonded joint relative to a mechanically fastened equivalent.

Where the Strength Case Closes

The decision to replace fasteners with ultra-high bond epoxy requires that the adhesive joint carry the same or greater load as the fastener group it replaces, with adequate safety factor. For this calculation to close, the bond area must be large enough, and the adhesive strong enough, to achieve the required joint capacity.

Ultra-high bond epoxy with a lap shear strength of 4,000 psi to 5,000 psi provides substantial load capacity per unit of bond area. A 25 mm × 50 mm overlap (1,250 mm²) with a 4,000 psi adhesive has a theoretical capacity of approximately 8,000 N — equivalent to two 8 mm grade 8.8 bolts in shear. In structural practice, the design allowable uses a fraction of the rated strength, typically 25 to 33 percent for structural applications, but the comparison still closes favorably for assemblies where significant overlap area is available.

The case for adhesive fastener replacement is strongest in thin-sheet assemblies — sheet metal structures, panel bonding, skin-to-frame connections — where the sheet thickness does not support large-diameter fasteners and the bearing capacity of the sheet metal is a limiting factor. In these applications, the adhesive joint distributes load across the full overlap area and avoids the bearing failure mode entirely.

Weight and Fabrication Cost Advantages

Fasteners add weight — not dramatically for individual joints, but cumulatively across an assembly with hundreds or thousands of fastened connections, the fastener mass is measurable and eliminates the weight saved by specifying lightweight substrate materials. Ultra-high bond epoxy has a cured density of approximately 1.1 to 1.4 g/cm³ and is applied at very small film thicknesses; the adhesive mass in a structural joint is typically a fraction of a gram. Replacing a fastener group with an adhesive joint removes the fastener mass entirely.

Fabrication cost reduction comes from eliminating the hole-making and fastener installation steps. Drilling, countersinking, deburring, and torquing fasteners are labor-intensive operations that can represent a significant fraction of assembly time in panel and frame fabrication. Adhesive bonding replaces this sequence with surface preparation, adhesive application, fixturing for cure, and cure time — a different process, but one that can be faster and less labor-intensive per joint for high-volume production.

Elimination of hole-making also preserves surface treatments and coatings applied before assembly. Drilling through an anodized, painted, or otherwise coated structure breaks the protective surface at every hole location. Adhesive bonding leaves surface treatments intact across the joint area.

If you want to work through a fastener replacement analysis for a specific joint — load requirements, substrate thickness, overlap area, and safety factor — Email Us and Incure’s engineering team can assist with the comparison.

Design Rules for Adhesive Fastener Replacement

Successful replacement of mechanical fasteners with ultra-high bond epoxy requires attention to joint geometry that mechanical fastener design does not demand. Fasteners handle peel loading naturally because each fastener is a rigid through-connection. Adhesive lap joints are sensitive to peel loading at the overlap edges, particularly in thin, flexible substrate assemblies.

The first design rule is to load the adhesive joint in shear rather than peel wherever possible. Long, narrow overlaps loaded along their length are loaded primarily in shear. Short overlaps loaded perpendicular to their length, or joints where one member can rotate relative to the other under load, have significant peel components.

The second rule is to keep the overlap length within limits where load distribution is reasonably uniform. Very long overlaps — more than approximately 30 to 50 mm for typical structural epoxies — do not deliver proportional increases in joint strength because the end regions of the overlap carry disproportionately more load due to the elastic compliance of the substrates. Optimum overlap lengths depend on substrate stiffness, thickness, and adhesive modulus.

The third rule is to ensure the joint geometry transfers load through the adhesive without imposing eccentric loading that causes rotation or peel. Double-lap joints, symmetric configurations, and load paths aligned with the bond plane all improve joint efficiency.

Hybrid Designs: Adhesive and Fasteners Together

In many structural assemblies, the appropriate outcome is not a pure adhesive replacement of fasteners but a hybrid design that uses both. Fasteners provide peel resistance and fixturing during cure; the adhesive provides shear load distribution, fatigue resistance, and sealing. This approach is common in aerospace panel-to-frame bonding, rail vehicle body assembly, and heavy equipment cab construction.

In hybrid designs, the fastener load contribution is often conservatively ignored in structural calculations, and the adhesive is sized to carry the full design load. The fasteners provide redundancy, peel resistance, and manufacturing alignment rather than primary load capacity.

Contact Our Team to discuss fastener replacement design for your specific assembly, including joint geometry, substrate materials, load case analysis, and appropriate safety factors.

Visit www.incurelab.com for more information.