As commented in previous posts, composite material joints require an in-depth analysis due to the problems that can appear if are not performed correctly. That’s why in this post it is going to show the advantages and disadvantages of using bolted joints, typical failure modes of the composite material and the preferred laminate stacking sequence for bolted joints.
Advantages and disadvantages of bolted joints
One of the major advantages of bolted joints are that it is possible to disassemble.
However, it has some disadvantages too. The stress concentrations caused by the fastener hole. The disruption of the fibre continuity during drilling, which severely reduces the load carrying capacity, undesirable damages induced by drilling the holes which drastically reduce strength against fatigue, thus degrading the long-term performance of composite laminates.
Bolt preload
To optimize the performance of a preloaded joint in composite materials, it must satisfy three fundamental criteria:
1. Sufficient Bolt Strength: Ensuring that the bolt possesses ample strength to withstand the applied loads is paramount. This ensures structural integrity and prevents failure under load.
2. Demonstration of Separation Factor of Safety: The joint should exhibit a separation factor of safety at limit load. Essentially, this implies that the joint must remain intact even when subjected to the maximum anticipated load.
3. Adequate Fracture and Fatigue Life of the Bolt: The bolt should possess adequate fracture and fatigue life to withstand cyclic loading and prevent premature failure over the expected lifespan of the joint.
Selecting the appropriate size and strength class of fasteners necessitates consideration of the coefficient of utilization (γ). This coefficient represents the maximum pretension stress divided by the yield stress of the fastener material, calculated as σV/σy. Typically ranging from 0.5 to 0.8, with a recommended starting value of 0.65, γ can be adjusted to fine-tune joint performance.
To calculate the nominal preload in the fastener (FV), the following formula is utilized:
A standard torque distribution among these components typically consists of:
– 10% stretching bolt
– 30% absorbed by friction in threads
– 50% absorbed by friction under the head of the bolt or nut
– 10% prevailing torque of a self-locking thread
Understanding and applying these principles facilitate the design and optimization of threaded fastener joints, ensuring their reliability and performance in various applications within composite materials.
Bolted Joints Failure Modes
1. Bearing Failure: This occurs when the load-bearing surface of the composite material deforms or fails due to excessive pressure from the bolt head or nut. For example, bearing failure may occur in a bolted joint connecting a composite panel to a structural frame under high tensile loads.
2. Shear-Out Failure: Shear-out failure happens when the composite material around the bolt hole is unable to withstand the shear forces applied during loading, leading to material failure around the hole. This can occur in bolted joints connecting composite components subjected to shear loads, such as in aerospace wing structures.
3. Delamination: Delamination failure involves the separation of layers within the composite material, often caused by tensile or shear forces applied perpendicular to the layers. For instance, delamination may occur in bolted joints joining composite laminates under cyclic loading conditions, leading to reduced structural integrity.
4. Matrix Cracking: Matrix cracking occurs when the resin matrix in the composite material experiences tensile or compressive stresses beyond its strength limit, resulting in cracks. This can happen in bolted joints under high loading conditions, leading to localized damage and reduced load-carrying capacity.
5. Interlaminar Fracture: Interlaminar fracture involves the separation of layers within the composite material along the interface between adjacent laminae. This failure mode can occur in bolted joints subjected to bending or torsional loads, leading to structural instability and reduced stiffness.
Parameters that affects bolted joints strenght
The joint strength depends on different parameters, such as:
- joint configuration
- bolt torque
- laminate and bolt materials
- laminate lay-up
- temperature.
Stress concentrations
The laminae orientation combinations influence both the magnitude and the shape of the stress variation near the hole. The high stresses at the edge of the hole may initiate fracture. If the laminate fails as a brittle material, fracture will be initiated when the maximum tensile stress at the edge of the hole equals the strength of the unnotched material.
