Bolt Preload Effect on Fatigue Life

Bolt Preload Effect on Fatigue Life

Preloaded bolts and rods are present in a very wide range of engineering design solutions and geometries where the effects of high cyclic loading are critical. These include, but are not limited to, high-speed railway infrastructure components, renewable energy assets subject to wind or aerodynamic loading, rotating machinery, or seismicity, to mention a few.

With the increasing demand in loading (heavier and faster trains, increased wind speeds, etc.) and assets design life (increased number of load cycles), both for new and existing infrastructure, engineers need to be able to analyse the preloaded systems and connections in detail; failure to understand the occurring stress flows will lead to overdesign, or even worse, unannounced structural failure with disastrous consequences.

A classical loading sequence happening in a preloaded system or connection may be the following:

The existing bolts or rods are preloaded to a desired level, accounting for any short and long-term preload losses. The preload leads to an initial state where these (bolts, rods) are in tension, and any surrounding elements are manly in compression (steel plates, concrete plinths, etc.).
Some of the existing permanent loading may subsequently “consume” some or part of the preload, and potentially cause partial or total separation of the areas initially in contact.
The structure or system is then subject to high, variable loading such as wind, rotational/inertia forces or ground motion, for instance.

The aim of a good design is to make sure that bolts and rods will operate within a controlled stress range, maximising the asset’s life and integrity. The preload means bringing the starting condition of the bolts “close” to their maximum capacity (for instance 70 or 75% of their ULS capacity, although this depends on the application and design constraints). Due to the preload, any further effects (forces, stress) caused on the bolts by live load effects will become manageable or even negligible.

The force history of a preloaded bolt vs a non-preloaded bolt as a function of the applied separation load is graphically shown in Figure 1. The plastic region of the bolt is indicatively shown for completeness. In fatigue-controlled designs, the plastic region of the bolt will not be reached.

Figure 1: Indicative force history comparison between a preloaded and a non-preloaded bolt

ADINA provides all the required capabilities to accurately analyse bolt force/stresses, regardless of their geometry, leading to optimum designs, cost savings and more sustainable solutions for long-lasting assets:

Ability to create, import or automate the generation of any arbitrary geometries.
The automatic bolt preload feature allows the user to define a target preload to each required bolt in any desired sequence. This means that the short-term preload loses arising from elastic shortening are automatically dealt with by ADINA through “special bolt iterations”, so the engineer doesn’t need to do manual iterations and calculations. The different bolt modelling techniques available in ADINA are shown in Figure 2.
The powerful contact capabilities in ADINA allow to accurately model friction and lift-off between plates, bolts, plinths and any other components that may be present in the model and slip or lift relative to each other.
The nonlinear loading sequence allows to apply all the required loading in the desired sequence, “locking” stresses and carrying them forward from one loading stage to the next one.
Powerful solvers and multi-thread capability which allows numerous runs, design iterations and sensitivity analyses leading to design optimisation in the shortest amount of time.
Comprehensive stress analysis which enables an agile visualisation of all required stress components.

Figure 2: Different bolt modelling techniques in ADINA

The following figures show a baseplate model with bolt preload created in ADINA using all the functionalities above. The model uses the 3D bolt modelling technique. Friction contact has been defined between the faces of the plate, plinth and bolt heads and nuts. The bolts are de-bonded from the concrete at their sides. An initial target preload has been defined and a bending moment inducing load is subsequently applied to the column which causes an increase of tensile force in two of the bolts.

Figure 3: General view of baseplate model with bolt preload in ADINA

Figure 4: Live load is applied causing plate lift off and eventual bolt preload loss and plate slippage

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The bolt force function obtained in ADINA, presented in Figure 6, shows how the bolt force increases in a nonlinear manner as the applied loading acts and consumes the initial preload. As can be seen, the plastic region of the bolt has been reached. This resembles the preloaded (blue) graph shown in Figure 2. The curve in ADINA transitions smoothly, rather than showing a sharp change in slope when approximately 50% of the load has been applied; this is due to the flexibility of the connecting components, such as the plate in bending. A stiffer plate would result in a sharper transition leading to two differentiated zones in the curve.

Figure 6: Bolt stresses (N/m²) as a function of the applied load ratio