Manually Enter Bearing Forces
The Bearing Force, P can be manually extracted directly from the FEM using grid point forces. The bearing force and load angle can be entered directly into the Bolted Joint Analysis form.
There are several options for extracting or manually entering bearing force on a laminate hole. Select from one of the three following approaches for defining bearing force on the laminate.
Manually Enter Bearing Forces
The Bearing Force, P can be manually extracted directly from the FEM using grid point forces. The bearing force and load angle can be entered directly into the Bolted Joint Analysis form.
Compute Bearing Force, Shear Flow
A shear flow analysis is most accurate for structures that have orthogonal connections, like wing skins to spar joints. By studying the internal load path, it is easy to see the shear load transfers between adjoining structures.
Assuming that the entire shear load is transferred through the fasteners, the bearing force is calculated from the user-defined fastener pitch and the number of fastener rows,
|
P = |
q S |
|
n |
where S is the fastener pitch, and n is the number of rows.
There are two methods for load extraction when performing a shear flow analysis, manual and automatic.
In the first method, the Joint Shear Load, q is manually extracted from the model and is entered directly into the form. Enter user-defined shear load (lb/in). The fastener pitch and number of rows are defined to calculate the bearing force.
Note that since the shear load is manually entered into the form, this value is not load set dependent.
In the second method, the Joint Shear Load, q is extracted from the Object Loads tab on the Sizing form. Note the object membrane loads are determined with an average, 0-sigma approach.
Select the components of load to consider as bearing load. In this case, the joint shear load q, is calculated by superimposing the Nxy and Ny loads.
Automatically Extract Bearing Forces from FEM
Unlike metals, composite materials are not ductile and do not yield. As a result, the bearing load distribution in a bolted composite joint is nonuniform so the outer rows of fasteners pick up more bearing force than the inner rows of fasteners.
To determine the bearing force, the fasteners are physically modeled in the FEM. With this approach the Bearing Force, P is automatically extracted from the finite element model. To automate the load extraction, HyperSizer imports FEA-computed CBUSH element forces for all mechanical and thermal load sets.
The use of Nastran CBUSH elements is a common fastener modeling technique. The CBUSH elements are defined with in-plane stiffness which provides a load path between two grids. For components with multiple fasteners, the bolted joint analysis is performed for every CBUSH element attached to the component. You may view the CBUSH elements on the FEM in the FEM viewer.
The controlling CBUSH force is displayed on the Bolted Joint Analysis form.
Several steps are automated through this process such as the handling of multiple load cases, computing the load angle, and calculating the bypass loads around the fastener. See Bypass Loads.
