Bearing and Bypass Loading in Fastened Structures

Bearing Stress at the Fastener Hole

Bearing stress is the compressive stress at the interface between the fastener and the hole wall. When a fastener transfers load from one plate to another, the fastener presses against one side of the hole wall in each plate, and the plate material bears against the fastener. The nominal bearing stress is the fastener bearing force divided by the projected bearing area: σ_br = F_bearing / (d × t), where d is the fastener diameter and t is the plate thickness. The projected bearing area is the diameter times the thickness — the rectangular area of the hole wall as seen from the load direction. The nominal bearing stress is an average — the actual contact pressure distribution is not uniform. It is peaked near the load-entry side of the hole and falls off toward the back, and it depends on the fastener fit, the hole condition and the load level. The nominal value is used for screening and for comparison with bearing allowables, which are themselves based on nominal stress.

  Nominal bearing stress:

  σ_br = F_bearing / (d × t)

  where  F_bearing = fastener bearing force (load transferred)
         d         = fastener diameter
         t         = plate thickness

  →  Nominal value — actual contact pressure is non-uniform
  →  Peaked near load-entry side, falls off toward the back
  →  Used for comparison with bearing allowables

Bypass Stress

Bypass stress is the tensile (or compressive) stress in the plate that passes through the hole location without being transferred by the fastener. It is the stress in the net section — the reduced cross-section of the plate at the hole, which has the area (w − d) × t, where w is the plate width and d is the hole diameter. The bypass load is the load that remains in the plate after the fastener at that station has transferred its share. In a multi-fastener joint, the bypass load at each hole is the plate load minus the fastener bearing load at that hole. The end fasteners have the highest bypass load because the full plate load passes through the end holes on one side. The bypass stress is σ_by = F_bypass / ((w − d) × t). The bypass stress is a net-section stress — it acts across the reduced section through the hole — and it contributes to net-section failure (tensile fracture across the hole).

  Bypass stress (net section):

  σ_by = F_bypass / ((w − d) × t)

  where  F_bypass = load passing through the plate at the hole
         w         = plate width (or pitch for a row of holes)
         d         = fastener diameter
         t         = plate thickness

  →  Net-section stress — acts across the reduced section
  →  Contributes to net-section tensile failure
  →  Highest at end holes (full plate load on one side)

Bypass stress is the net-section stress from the load that passes through the plate without being transferred by the fastener. The end fasteners have the highest bypass stress because the full plate load passes through the end holes on one side.

Combined Bearing and Bypass Loading

At every hole in a multi-fastener joint, the plate simultaneously experiences bearing stress (from the fastener load) and bypass stress (from the remaining plate load). The two stresses act at the same location — the hole — and they interact. The bearing stress compresses the plate material on one side of the hole; the bypass stress tensions the plate across the net section. The combination produces a complex stress state at the hole that determines both the static failure mode and the fatigue initiation location. The interaction is typically presented as a bearing-bypass interaction diagram — a plot with bearing stress on one axis and bypass stress on the other, with failure envelopes for each mode (bearing failure, net-section failure, shear-out). The diagram shows which failure mode governs for a given combination of bearing and bypass stress. A hole with high bearing and low bypass may fail in bearing; a hole with low bearing and high bypass may fail in net section; a hole with moderate values of both may fail in shear-out.

Bearing and bypass stresses act simultaneously at every hole. The interaction determines the failure mode — bearing, net-section or shear-out. The bearing-bypass interaction diagram shows which mode governs for a given combination.

Load Transfer and the Fastener-Hole Interaction

The fastener-hole interaction is the mechanism by which load is transferred from the fastener to the plate. The fastener bears against the hole wall, the contact pressure compresses the plate material, and the load diffuses into the plate around the hole. The diffusion is not instantaneous — the bearing stress is concentrated near the hole and decays with distance from the hole. The rate of diffusion depends on the plate stiffness and the fastener stiffness. The load transfer also produces a shear-out stress — the tendency for the material ahead of the fastener (between the hole and the free edge) to shear out. The shear-out stress depends on the edge distance: a short edge distance gives a small shear-out area and high shear-out stress; a long edge distance gives a large area and low stress. The load transfer mechanism is the same for metallic and composite plates, but the response is fundamentally different — metals yield and redistribute, composites do not.

Net-Section Load and Joint Efficiency

The net-section load is the load in the plate at the reduced cross-section through the hole. The net section has a smaller area than the gross section — the hole removes material — so the net-section stress is higher than the gross stress. The ratio of the net-section area to the gross-section area is the hole efficiency: η = (w − d) / w. For a single hole in a plate of width w with a fastener of diameter d, the efficiency is (w − d) / w — the fraction of the gross area that remains. For a row of holes at pitch p, the efficiency is (p − d) / p. The joint efficiency is the product of the hole efficiency and the load-transfer efficiency — the fraction of the plate strength that the joint can develop. A joint with 80% hole efficiency and 90% load-transfer efficiency has a joint efficiency of 72%. The joint efficiency is the key metric for comparing joint designs: a riveted joint with 72% efficiency is weaker than the unjointed plate by 28%, and the joint must be sized to carry the design load at this reduced capacity.

  Hole efficiency (single hole):

  η = (w − d) / w

  Hole efficiency (row of holes at pitch p):

  η = (p − d) / p

  Joint efficiency:

  η_joint = η_hole × η_load_transfer

  →  Fraction of the unjointed plate strength that the joint can develop
  →  Key metric for comparing joint designs

Metallic Structures: Bearing Deformation and Plasticity

In metallic structures, the bearing-bypass interaction is governed by the plastic behaviour of the plate material. When the bearing stress exceeds the bearing yield strength, the plate material at the hole wall yields and deforms plastically — the hole elongates. The plastic deformation redistributes the bearing stress: the yielded material has a lower stiffness, and the load spreads to a larger area of the hole wall. This plastic redistribution is beneficial — it increases the bearing capacity beyond the initial yield and delays final bearing failure. The net-section failure in metals is also governed by plasticity: the net section yields across the reduced width, and the full plastic capacity of the net section can be mobilised before fracture. The bearing-bypass interaction in metals is therefore relatively forgiving — the plastic redistribution provides reserve strength beyond the first yield. The bearing allowables for metallic materials (in MMPDS / MIL-HDBK-5) include both the bearing yield and the bearing ultimate strengths, and the interaction diagram is typically constructed using these allowables with appropriate factors.

In metallic structures, plastic bearing deformation redistributes the stress and provides reserve strength beyond first yield. The hole elongates plastically, the load spreads to a larger area, and the bearing capacity increases beyond the initial bearing yield. The interaction is relatively forgiving.

Composite Structures: No Plastic Redistribution

In composite structures, the bearing-bypass interaction is fundamentally different because composites do not yield plastically. The bearing stress at the hole wall produces local damage — matrix crushing, fibre microbuckling, delamination — rather than plastic deformation. The damage does not redistribute the stress in the way that metallic plasticity does; instead, the damage accumulates and the bearing stiffness degrades. The net-section failure in composites is a brittle fracture — the fibres break across the net section without prior yielding. The shear-out failure is also brittle — the material ahead of the fastener shears out without plastic redistribution. The absence of plasticity makes composite joints more sensitive to the bearing-bypass combination: there is no reserve strength beyond first damage, and the failure mode can transition abruptly from bearing to net-section to shear-out as the combination changes. The bearing-bypass interaction diagram for composites is therefore more critical — the failure envelopes are tighter, and the allowables are lower relative to the unnotched laminate strength.

Composite joints have no plastic redistribution. Bearing damage is brittle (matrix crushing, delamination), and failure can transition abruptly between modes. The bearing-bypass interaction is more critical — the failure envelopes are tighter and the allowables are lower relative to the unnotched strength.

Edge Distance, Pitch and Failure Interactions

The edge distance (e) and the pitch (p) are the geometric parameters that most strongly influence the bearing-bypass failure mode. The edge distance is the distance from the centre of the hole to the free edge of the plate, measured in the direction of the load. The pitch is the distance between adjacent holes in the same row. The edge distance governs the shear-out strength: a short edge distance gives a small shear-out area and a low shear-out strength — the joint fails by shear-out before bearing or net-section failure. The pitch governs the net-section strength: a small pitch gives a small net-section area and a high bypass stress. The rule of thumb for metallic joints is e/d ≥ 1.5 and p/d ≥ 3 for full bearing strength — below these values, the shear-out or net-section strength falls below the bearing strength and governs the design. For composites, the requirements are typically more stringent (e/d ≥ 2.5–3, p/d ≥ 4–5) because of the absence of plastic redistribution and the lower shear-out strength. The engineer should consult the applicable design allowables for the specific material and configuration.

Edge distance and pitch govern the failure mode. Short edge distance causes shear-out failure; small pitch causes net-section failure. For full bearing strength, metallic joints typically need e/d ≥ 1.5 and p/d ≥ 3; composites need more — e/d ≥ 2.5–3, p/d ≥ 4–5. Consult the specific material allowables.

Key takeaways

• Every fastener hole experiences bearing stress (from the fastener load) and bypass stress (from the load remaining in the plate). The two must be analysed together — the interaction governs joint strength and fatigue life.
• The end fasteners in a multi-fastener joint have the highest bearing stress and the highest bypass stress — they are the most critical holes in the joint.
• In metallic structures, bearing-bypass interaction is governed by plastic bearing deformation and net-section yielding. In composites, it is governed by bearing damage, net-tension fracture and shear-out — with no plastic redistribution.
• Edge distance and pitch directly affect the bearing and shear-out capacity. Insufficient edge distance reduces the shear-out strength; insufficient pitch increases the bypass stress at adjacent holes.