4.2.1 Stage I
4.2.1.1 Defining Joint & Loads
The precept on which the guideline is based is that the joint being analysed can be isolated from the surrounding structure. The joint can then be represented by one of five main joint categories, which depend on its configuration and applied forces. Therefore, the first task is to define the physical extent of the joint within the structure, and identify its category (see Section 5). It is assumed that an analysis of the structural system in which joint exists has been carried out in sufficient detail to specify the loads transmitted via the joint.
4.2.1.2 Combined Loading
The case of combined axial and bending loading is treated as an eccentric axially loaded joint, and dealt with in Section 8. A method is given in Section 9 to deal with combined axial and shear loading which is assumed to be the most frequently encountered.
4.2.1.3 Dynamic Loading
In all but exceptional cases the working load on the joint is dynamic. This is dealt with in more detail in each section, as appropriate. However, it is recommended that the engineer, at an early stage in the design process, is aware of the influence of varying external joint loads on the loads transmitted by the fastener and at the clamped interface. Reference is therefore made to Section 7.4, which discusses the force ratio.
4.2.1.4 Thermal Loading
Analysis of preloaded joints operating in a varying temperature environment is needed to ensure that they meet functional requirements across the full temperature range. Section 6.3.5 covers the effects of temperature change on the preload. Furthermore, certain joints have requirements for thermal conductivity to be either above or below certain limits. Section 10 covers the analysis of thermal conductivity.
4.2.2 Stage II
4.2.2.1 Joint Analysis & Design Procedures
Depending on the category of joint, the user is directed to the appropriate section, which details the equations needed to calculate Margins of Safety (MoS) for all relevant failure modes. Worked examples are provided to illustrate the application of the proposed design procedures.
4.2.2.2 Embedding & Relaxation
For many joint designs the effects of embedding and relaxation are important. These effects are covered in Section 6.4.2.
4.2.2.3 Fatigue & Fracture Mechanics Aspects
For a more detailed analysis of either fatigue or fracture mechanics, the user is referred to Section 11, which treats these subjects in more detail.
If, within the general design constraints, the joint meets neither the basic nor the detailed fatigue criteria, it can be necessary to reconsider the initial joint design concept.
4.2.3 Stage III
4.2.3.1 Corrosion
Environmental effects are identified as an important design parameter. Section 13 discusses a number of corrosion mechanisms relevant to joint components and provides a checklist against which the proposed design can be assessed and possible problem areas identified. Where a severe or intractable corrosion problem is predicted it can be necessary to review the initial structural concept, which can lead to a change in the joint’s materials or detailed design.
4.2.3.2 Methods of Preloading & Lubrication
It is expected that at some stage the fastener assembly method is specified, together with the use or not of a lubricant. The method of preloading can be critical as this determines the accuracy with which the fastener preload can be achieved.
Section 12 covers a number of different methods of installing preloaded fasteners. Design data and estimates of achievable accuracy for the preload are given where appropriate.
Section 14 contains advice on lubrication aspects and contains details of the characteristics of a selected range of fastener lubricants.
4.2.3.3 Preloading Method
The calculation procedures need that a particular preloading method is specified at the start of the design process. It is necessary to check later that the specified preloading method is capable of ensuring the preload accuracy to be achieved, and also that it is compatible with joint location and access constraints.
4.2.4 Stage IV
4.2.4.1 Manufacturing Quality Control Procedures
Section 15 provides the engineer with a guide to the factors that need to be controlled during production to guarantee adequate joint performance.
4.2.4.2 Joint Validation by Testing
Consideration needs to be given to demonstrating that the joint design is adequate. This can be achieved either by testing the complete structure or by isolating and testing individual joints. These aspects are covered in Section 16.