Bolted & Pinned Composite Joints

Technical provenance

Applicable standards / specifications

• ASTM D3039/D3039M — Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials
• ASTM D6641/D6641M — Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials Using a Combined Loading Compression Fixture
• ASTM D7136/D7136M (2025) — Standard Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event

References

• CMH-17 — Composite Materials Handbook — Widely used reference for composite material characterisation, design allowables, test methods and structural substantiation.
• ASTM D7136/D7136M — Drop-weight impact damage resistance of fibre-reinforced polymer matrix composites — Relevant to impact-damage characterisation of laminated composites.

What Is It?

Bolted and pinned joints in composite structures transfer load between components through mechanical fasteners that pass through holes in the composite. Unlike metallic joints, where the ductile material yields and redistributes stress around the hole, composites are brittle and do not yield. The load transfer occurs through bearing — the fastener bears against the hole edge — combined with bypass stress in the remaining section. The behaviour is fundamentally different from metallic bolted joints and requires composite-specific analysis methods.

Why It Matters

Mechanical fasteners are necessary in many composite structures — for assembly, for disassembly, for load introduction and for fail-safe load paths. However, drilling a hole through a composite interrupts the fibre path and creates a stress concentration that cannot be relieved by yielding. The joint can fail through several distinct mechanisms, and the laminate architecture around the hole strongly influences which mode governs. Understanding bolted composite joint behaviour is essential for designing joints that are both efficient and reliable.

The hole interrupts the fibre path — the laminate must redistribute the load. Unlike metals, composites cannot yield to relieve the stress concentration. The joint design must manage the load transfer through laminate architecture and fastener configuration.

Failure Modes in Bolted Composite Joints

Bearing and Bypass

In a multi-fastener joint, each hole experiences two load components. The bearing load is the portion transferred through that particular fastener. The bypass load is the portion that passes through the laminate and continues to the next fastener. The local stress at the hole is determined by the combination of bearing and bypass stress. The interaction between bearing and bypass is not a simple superposition — the combined effect on failure is non-linear and is typically assessed using bearing-bypass interaction diagrams or detailed FEA.

Bearing stress:

σ_br  =  F_bearing / ( d · t )

Bypass stress:

σ_bp  =  F_bypass / ( ( w − d ) · t )

where:
F_bearing  =  load transferred through the fastener
F_bypass   =  load passing by the hole
d          =  fastener diameter
t          =  laminate thickness
w          =  laminate width

Why Composites Around Holes Differ from Metals

In a metallic joint, the material around the hole yields under bearing load. This yielding redistributes the stress — the peak stress is limited by the yield strength, and the load spreads over a larger area. In a composite, there is no yielding. The stress concentration remains at its elastic value, and the material fails by brittle fracture when the local stress exceeds the strength. This means composites are more sensitive to hole quality, fastener fit, clamp-up and laminate architecture than metals. A composite joint cannot rely on plastic redistribution — it must be designed for the elastic stress state.

Local Ply Orientation

The laminate architecture around the hole strongly influences the joint behaviour. A laminate with too few ±45° plies may be weak in bearing and shear-out. A laminate with too few 0° plies may be weak in net tension. The optimal laminate for a bolted joint typically has a balanced distribution of 0°, ±45° and 90° plies — the ±45° plies improve bearing and shear-out, the 0° plies improve net tension, and the 90° plies provide transverse restraint. The specific percentages depend on the joint geometry and loading, but a laminate designed only for in-plane stiffness without considering the bolted joint may perform poorly at the fastener holes.

COMPOSITE CHECK: Has the laminate architecture been assessed for the bolted joint, not just the remote laminate? A laminate optimised for global stiffness can be suboptimal at the hole where bearing, shear-out and net tension govern.

Clamp-Up

Clamp-up — the compressive force applied by the fastener to the joint — significantly affects bearing strength. A clamped joint has higher bearing strength than an unclamped joint because the clamp-up creates friction between the adherends that carries part of the load, and because the clamp-up constrains the material around the hole, reducing damage growth. However, clamp-up can be lost over time due to vibration, thermal cycling or relaxation. The bearing strength used in analysis should be consistent with the clamp-up condition expected in service.

Countersunk Fasteners

Countersunk fasteners present particular challenges in composites. The countersink reduces the effective thickness of the laminate at the hole, creating a stress concentration at the countersink edge. The reduced section has lower bearing capability. The countersink angle and depth must be controlled carefully — over-countersinking further reduces the bearing area. Countersunk fasteners in composites generally have lower joint capability than protruding-head fasteners and require careful analysis. The countersunk region can be the governing location for bearing failure.

Joint Modelling Approaches

Bolted composite joints can be modelled at several levels of detail. A simple analytical approach uses bearing stress and bypass stress with bearing-bypass interaction curves. Detailed FEA models the fastener, the hole and the local laminate with fine mesh, including contact between the fastener and the hole edge. The most detailed models include progressive damage modelling to predict the failure sequence. The choice depends on the engineering question — initial sizing may use analytical methods, while detailed assessment requires FEA with contact and damage modelling.

• Analytical — bearing/bypass interaction; fast for initial sizing
• FEA with contact — fastener-to-hole contact; local stress distribution; moderate detail
• FEA with progressive damage — predicts damage growth and failure sequence; computationally expensive
• Submodelling — global model for load distribution; local model for hole detail

Key Takeaways

• Bolted composite joints fail through bearing, net tension, shear-out, cleavage or pull-through — distinct modes
• Composites do not yield — the elastic stress concentration at the hole cannot be relieved by plastic redistribution
• Bearing-bypass interaction determines the stress state at each hole in a multi-fastener joint
• Laminate architecture around the hole must be designed for the joint, not just the remote laminate
• Clamp-up, hole quality and fastener fit significantly affect bearing strength