Simulating Complex Fasteners and Connections – HiLock Fastener

Within the aerospace industry, a need arose to create a new fastener to replace the HiShear fastener, more commonly known as a rivet.  The prefixed collar of the HiShear fastener limited its versatility.  Prompted by this need, the HiLock fastener was designed.  The collar of the HiLock is threaded and both the pin and the collar itself were designed to fulfill a wide range of applications and installations.  The HiLock fastener also is easier to install and remove, eliminating the need for highly specialized tooling.  Along with its expanded versatility comes the challenge to accurately model and simulate the HiLock fastener.  Altair HyperWorks addresses this challenge by using the connector technology in HyperMesh to automate the modeling of this complex fastener.

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Numerous model assembly techniques are available for welding and bonding metallic parts.  Many of them must be idealized for CAE analysis and, as a whole; the computer-aided engineering (CAE) community has standardized many of these connection types. With composites becoming more prevalent within both the aerospace and automotive industry sectors, the requirement to join composite structures has grown significantly. Hence, HiLock. The joining of these structures is complex and is achieved primarily via a fastener-type connection. Often engineers face the need to join not only composite to composite, but, also metal to composite parts. As with any connection, the dimension, material properties and stiffness of the fastener connection must be considered to ensure correct load distribution.

Challenges:

Rivet Idealization:

  • Highly repetitive – for any connection type, the repeatability and consistency needs to be achievable when creating one or more connections.
  • Manual creation – today, the HiLock realization would either be created manually or by some form of automation script type of solution often requiring inputs for the connecting points/nodes. This activity is a major undertaking when creating several hundred HiLock connections.
  • Element configuration – selecting the correct configuration of elements to represent the connection type.

Rivet Stiffness Calculations:

  • Calculating the rivet stiffness is very complex.
  • Typically, the calculations would be computed manually or with Excel spreadsheets and possibly macro-type solutions.
  • The HiLock realization has a predefined stiffness calculation which requires the following information:-
    • Connecting parts, thickness.
    • Rivet information such as diameter and material properties.
    • Connecting part information, such as composite to composite or metal to composite.

Requirements:-

  • High levels of automation so that the model assembly time can be reduced.
  • Accuracy – reduction in potential errors by simplifying processes and removing manual operations.
  • Decrease the dependency on engineering time and judgment – the engineer doesn’t have the time to generate hand calculations for hundreds of fasteners.
  • Ensure the solution is completely managed by software, versus further customized solutions.
  • Standardization so that repeatability can be achieved easily.

Overview of HiLock:-

A connector is an entity within HyperMesh which defines the CAE idealized representation of a spot (fastener), bolt, seamweld (MIG, TIG) or area (adhesive) type of connection.

The connector technology within HyperMesh enables semi-automated full model assembly. The connectors can be realized by importing weld or connection data from CAD or PLM and reading them into HyperMesh.  A realized connector means that the idealized CAE definition has been successfully created and the connecting parts can be considered as connected.

The HiLock fastener was released as a new connector realization type within HyperMesh 10.0SA1-120.

The specifics of a realized HiLock fastener are complex.  The HiLock realization creates 1D elements which consist of a mixture of rigid bars (RBAR), beams (CBAR) and springs (CBUSH) elements (refer to Figure 1).

Figure 1:- Breakdown of HiLock fastener realization within Altair HyperMesh.

The outer extensions of the HiLock, which protrude through the connecting parts, are calculated by the thicknesses of the outer shell elements. The inner nodes of the RBAR element are connected to the shell elements, whereas the inner nodes of the CBAR elements are coincident to the shell nodes only. Between the appropriate connected and coincident nodes, CBUSH’s are created. Each outer node connects one CBAR and one RBAR. Each HiLock fastener connection generates its own coordinate system with the z-axis collinear to the HiLock direction. All affected nodes are assigned to this newly created coordinate system. The coordinate system is used for the degrees-of-freedom (DOF) definition of the CBAR elements, for the stiffness calculation of the CBUSH elements and for the DOF of the nodal constraint.

The HiLock fastener realization uses the shell properties and materials (PSHELL or PCOMP) and a predefined HiLock material to calculate the exact position of the outer nodes and the stiffnesses of the PBUSH elements.

Realizing the HiLock fastener requires a property script. A connector realization property script is used to precisely produce the idealized connection representation with specific attributes that are related to the connection in question-in this case, HiLock. The HiLock property script organizes all the newly created elements into component collectors and creates and organizes PBAR and PBUSH properties that define the diameter, translational and rotational stiffnesses. The property script also creates the boundary constraint, local co-ordinate system and materials that are specific to HiLock. The HiLock realization can be configured by updating the configuration file and the property script.

Once a HiLock fastener connector is realized within HyperMesh, it will be displayed within the connector browser (see Figure 2). The connector browser contains all of the capabilities to configure and assemble any model. The connector browser provides full find-and-display control to interrogate part connectivity. All connection types are displayed within a folder.  In this instance it’s called “HiLock”. The benefit of grouping connections per realization type is that it enables fast and easy find, display, review and editing if they are required.

Figure 2:- HiLock fastener realization within Altair HyperMesh Connector Browser.

Connector technology within Altair HyperMesh now makes possible a fully automated HiLock model assembly.  All calculations are fully automated within the connector realization itself, yielding huge gains in repeatability, efficiency and usability.

Stuart Sampson

Vice President- Modeling & Visualization Program Management at Altair
Stuart Sampson

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