Understanding the distinct advantages and disadvantages associated with a fixation method is crucial. By utilizing efficient joint design strategies, it becomes possible to maximize the benefits while minimizing the drawbacks, thus guaranteeing the durability of the joint. Adhesive bonding follows the same principle. By adhering to fundamental design principles, it is possible to attain resilient and long-lasting joints.
The Initial Design
A good design plays a key role in optimizing the many advantages of adhesives compared to other fixation methods. To ensure optimal performance, it is crucial to design joints that minimize stress concentrations, since adhesives excel in larger surface areas rather than single points or lines. As an illustration, let's consider the transition from fillet welding to structural bonding for a typical T-joint. This shift enables the distribution of stress over a broader area, as depicted by the orange region in the design.
Consider the following factors when designing a bonded joint:
- Mechanical properties of the adhesive
- Substrate characteristics (type, size,mechanics, surface conditon, etc.)
- Geometrical features
- Loading case
- Reduction factors
Basic Design Principles and Best Practices
In bonded joints, adhesives are typically exposed to various types of stress, either individually or in combination (see Figure 2). Torsion and bending can result in stress patterns that resemble combinations of these stress types.
Shear forces are effectively resisted by adhesives, as stress can be evenly distributed across the entire bonded area. Thus, incorporating shear design elements is a wise choice. However, it's important to note that shear stress peaks occur at the ends of the overlap, where most of the load is carried. The intensity of these stress peaks depends on the relative stiffness of the adhesive and the adherends. When a flexible adhesive is subjected to shear, stress is more uniformly distributed, leading to lower peak stresses at the end of the overlap.
Compression and tension loads, exerted by rigid substrates, create a uniform stress distribution within the adhesive layer. This can be advantageous in terms of design. Compression loads are preferable, as adhesives generally exhibit higher strength under compression. Conversely, tensile loads can result in peeling or cleavage if the adherends deflect or if the applied load is offset to some extent. Peeling and cleavage concentrate high stresses along a single boundary line of the joint, potentially causing premature failure, especially when stiff or rigid adhesives are used.
Joints subjected to peel or cleavage stresses, as well as bending, should be redesigned to optimize their suitability for bonding applications.
Best Practices for Bonded Joint Design
The fundamental guidelines for designing bonded joints can be succinctly summarized by adhering to the principles illustrated below:
- Minimize peel and cleavage stresses and prioritize compression and shear stresses
- Maximize the bond area to the greatest extent possible
- Optimize the joint design to promote even distribution of stresses
Dimensioning with Reductions Factors
After establishing a suitable design, the dimensions of the joint can be calculated by considering the adhesive's ability to sustain load throughout its operational lifespan. It is essential to account for external environmental conditions, as well as fatigue and creep forces, which gradually degrade the mechanical properties of the adhesive over time. Failure to address these factors can result in premature joint failure. Additionally, the adhesive thickness and service temperature must be taken into consideration during the design process, as they also impact the final material strength.
Hence, when dimensioning the joint, it is crucial to rely on material data that represents critical scenarios that may arise during the joint's service life. A simplified approach involves utilizing reduction factors (γ) for parameters such as temperature, thickness, environmental aging, lifetime loads, etc., applied to the reference or characteristic strength (τc).
Furthermore, a design safety factor (Sd) is employed to calculate the maximum allowable strength (τa) using the following formula:
τa = τc *γtemp *γthick *γageing *γload......./Sd
For specific Sika products, reference sets of reduction factors are available in our technical documentation to facilitate the dimensioning process.