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  3. Training Module #12: Joints

Training Module #12: Joints

Note

Joints were introduced in an earlier training module. This module provides a closer look at the types of Joints available in HyperX and how they can be used to represent and analyze various types of real-world joints.

HyperX Joints are the HyperX interpretation of a joining mechanism between two Zones – which can be Analyzed or Sized in the same manner as Zones.

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Structural joints are defined on the FEM with HyperX. Different FEA loads processing techniques are provided to analyze and design fastened, riveted, and bonded joints – concurrently with panels. Read more on the Feature Page on our main website, found here.

HyperX recognizes three different types of Joint entities based on the elements defining the membership:

  1. Edge Joints - Do not contain any connector elements and physically exist on the boundary between Zones.

  2. Point Joints - Contain at least one connector element (spring element, spring + RBE combination, etc.) and are used to represent a fastened or riveted connection.

  3. Non-FEA Joints - Do not originate from finite elements or Zone boundaries and, therefore, have no association with the FEM. They are manually created in HyperX and used with Non-FEA Loads.

Edge Joints

joint_edgejoint-example.png

Edge Joints are used to represent the connections between panels abstracted along panel boundaries, rather than being modeled explicitly. They are common in earlier design stages of fastened connections, shear-dominated fastened joints, and bonded joints.

  • Do not contain any discretely modeled connector or adhesive elements and physically exist on the boundary between Zones.

  • Flexible design options, supporting configurational trades.

  • Convert shell element forces to Joint-level loads.

  • Customizable Sizing routines to control MS resolution. See "Sizing and/or Analyzing" Joints.

Applicable Designs

Any Joint Design can be assigned to an Edge Joint. See Joint Design Properties (in Type > Joint section) for the full list of concepts within each Property.

jointDesigns_Ribbon.png

Loads

Edge Joints require their own logic for loads extraction from the FEM and processing into Design-to Loads. You can find more information on that here.

Average, statistical, element-based, neighbor average, and peak Load Properties are applicable to these Joints. For more information on these methods, see FEA Loads Processing.

Training Examples

Point Joints

joint_pointjoint-example.png

Point Joints contain at least one connector element (spring element, spring + RBE combination, etc.) and represent a fastened or riveted connection.

Applicable Designs

  • Fastened Joints

    DesignProp_joints-fastened.png

  • Riveted Joints

    DesignProp_joints-rivet.png

Loads

Point Joints require their own logic for loads extraction from the FEM and processing into Design-to Loads. You can find more information on that here.

Average, statistical, element-based, and peak Load Properties are applicable to these Joints. For more information on these methods, see FEA Loads Processing.

Training Examples

Non-FEA Joints

nonFEAJoint.png

Non-FEA Joints are Joint entities that do not originate from finite elements or Zone boundaries. They are used when working with Non-FEA Design Loads.

Applicable Designs

Any Joint Design can be assigned to a Non-FEA joint. See Joint Design Properties (in Type > Joint section) for the full list of concepts within each Property.

jointDesigns_Ribbon.png

Loads

Non-FEA Joints do not use Load Properties. A Non-FEA Load is assigned directly to a joint, thereby applying the corresponding loads to the selected joint(s).

Bonded Joints

HyperX provides a flexible and practical Analysis/Sizing tool to rapidly predict the strength of bonded joints. It handles a variety of joint configurations and allows easy configurational trades, assessment of margins after new external loads are dropped, etc. 

executiveSummary1.png

The HyperX Bonded Joint tool is uniquely situated in the tool space because of its speed and relative accuracy, making it great for configurational trades. HyperX Bonded Joint analysis is:

  • Far faster (and simpler) than FEA.

  • More accurate than traditional methods / legacy tools.

Overall, HyperX Bonded Joint Analysis is designed for the everyday stress engineer. It greatly simplifies the process of determining valid joint geometry without needing to repeatedly build complex FEMs.

typicalApplications.png

Workflow

reminderOfHyperXWorkflow.png

Applicable Design Properties

bondedJointDesignConfigurations.png

Loads Processing

For typical “structural joint” cases (e.g. single lap, double strap, stepped lap, etc.), HyperX will automatically extract the loads along an edge and apply to the Joint.

From there, HyperX features various customizable Joint load processing techniques which can:

  • Handle thousands of load cases.

  • Feature different processing approaches to envelope or evaluate element-by-element.

  • Manage coordinate system transformations.

  • Etc.

automatedLoadExtraction.png

Note

For Bonded Joints: Average, Element based, Neighbor Average, Peak Load, and Statistical Load Properties are all applicable. For more information on these methods, see FEA Loads Processing.

Failure Criteria

The analysis formulation is based on cylindrical bending assumptions (plate theory, traction-separation, etc.) combined with CLT and direct application equilibrium to obtain governing ODEs leading to a full 3D stress and strain state throughout the joint.

Two fundamental approaches for assessing failure: Stress-based and VCCT. These are compared in the table below. Many users choose VCCT because of its advantages and stability.

Stress Criteria

VCCT Criteria
failureAnalysisStressCriteria.png
failureAnalysisVCCTCriteria.png

Output - Full in-plane and out-of-plane stresses and strains

Output - Strain energy release rate components at crack tip

Failure Criteria - Comparison of stresses to allowables using interaction equations

Failure Criteria - Assess crack growth using strain energy release rates and material toughness

Representation - Typically assume perfect joint.

Representation - Assume embedded crack(s) due to manufacturing or operation

Sensitivity - Strong mesh dependence; stress concentration sensitivity

Sensitivity - Less sensitive to peaking elements and mesh density

Results Interrogation

When analyzing a Bonded Joint in HyperX, the software is assessing stresses / strains at many hundreds or thousands of points through the Joint, but the data provided in the interface only corresponds to the critical point for each criterion. Users can more deeply interrogate the simulated stresses and strains throughout the entire Joint through a dedicated series of output files.

Detailed Outputs

For performance reasons, detailed file outputs are not produced at all times. To produce them:

  1. Run a single Joint with a single candidate (no Sizing).

  2. Run with a User FEA or User General Load with a single load case.

  3. Look for the files in the project TEMP directory (C:/HyperX/Projects/Temp/[Project Name]).

outputFilesExplorer.png

There you will find 5 files:

  1. BondedJointResult.BDF - A “surrogate FEM” to visualize the simulation results. This is not a “real” FEM and cannot be run in Nastran. It is only used to provide a way to view and import the tool’s outputs.

  2. BondedJointResult.f06 - An empty file, not needed.

  3. BondedJointResult.csv - Discrete Field data that can be imported into HyperX and visualized on the surrogate FEM.

  4. BondedJointResult.OUT - Adherend midplane forces and displacements. Adhesive stresses and strains. Strain energy release rates (if running VCCT).

  5. BondedJointResult_LOC.OUT - Full ply-by-ply stresses and strains.

Postprocessing

Understanding Coordinate Systems

The Joint coordinate system used in the output files is slightly different than the one referred to in HyperX when applying loads and boundary conditions.

postprocessingCoordSystem.png

The X-axis runs along the overlap, starting at the left side, with Z-axis pointing upward, (\(\sigma_{zz}\) is the peel stress, \(\tau_{xz}\) is transverse shear stress, etc.). There are other minor differences in the output geometry (e.g. some joints may appear “upside down” compared to HX figures).

Surrogate FEM and Discrete Fields

  1. Create a new Project in HyperX.

  2. Import the surrogate FEM (BondedJointResult.bdf).

  3. Select the entire Joint in the Viewport and create a Structure out of it.

  4. Edit the Discrete Field CSV header to say “Grid ID” so HyperX can import it.

    outputFilesCSV.png

  5. Import the Discrete Field data (“Model” tab in HyperX 2023.3.1+, otherwise “Result” tab of the Ribbon).

  6. Plot via the Legend.

    outputFilesLegend.png

OUT File

The OUT file contains a series of 1D datasets, organized into distinct groups:

  • One group per adherend, with midplane plate displacements, curvatures, forces, and moments.

  • One group per adhesive with midline stresses and strains.

The file also contains computed strain energy release rates at the crack tip for VCCT runs (at the top of the file). This file can be loaded into Excel to generate plots for further postprocessing the analysis results.

outputFilesExcel.png

Note

This file uses “a” in the adhesive to indicate the through-thickness (z) direction.

E.g. SIGa == \(\sigma_{zz}\) ; TAUax = \(\tau_{xz}\) ; etc.

_LOC.OUT File

This file typically is best post-processed with Scripts. It contains 2D blocks of data showing the full ply-by-ply stresses and strains throughout the Joint. The basic format is:

outputFilesLocOutFormat.png

Bolted Joints

In HyperX, Joints are entities that allow for the Analysis and Sizing of the joining mechanism between Zones. Bolted Joints contain Fasteners or Rivets which can be modeled discretely in the FEM (making them Point Joints) or simulated along Zone boundaries (in the case of Edge Joints).

BoltedJointsIntro.png

Bolted Joints, like all other Joints, can be organized in Structures and Sets in the Structures Tree. As mentioned above, Bolted Joints can either be Edge or Point Joints depending on whether the connector elements are discretely modeled in the FEM. They can be classified further based on the members of the joint:

  • 1 Member Joints - Edge Joints with discontinuous skin over the joint.

  • 2 Member Joints - Edge Joints with continuous skin over the joint OR Single Shear Point Joints.

  • 3 and 4 Member Joints - Double and Triple Shear Point Joints, respectively.

Modeling

The following sub-sections describe how to represent Bolted Joints as Edge and Point Joints (including connector modeling techniques) to help you get the most use from HyperX's joint analysis capabilities.

Edge Joints

Edge Joints are defined on the boundary between two Zones. To create an Edge Joint: select a boundary, right-click, and select "Create Joint".

CreateCenter2.png

From here you can select whether you want this to be a 1 or 2 Member Joint. Make your selection based on whether the skin over the joint will be discontinuous or continuous, respectively.

While this form is open, the Viewport will also show arrows pointing from the Joints based on the current selections in that form; these represent the pull-off direction of the joint. Additionally, the Zones on either side of the Joint have been color-coded to indicate which Zone will be which member.

CreateCenter4.png

Once the Joints are created, you can see the model updates to show them. These Joints are now Structures which can be selected in the Viewport or the Structures Tree.

Note

To learn how Edge Joints are used when extracting and processing loads, see Edge Joint Loads.

Point Joints

For a Bolted Joint to be a Point Joint, its connectors must be discretely modeled in the FEM so the process for creating them is a bit different than for Edge Joints. First, you must import the FEM ensuring that "1D Connectors" are imported.

pointJoint_FEMImport.png

Next, we need to create Structures for the Point Joints. Navigate to the FEM Tree, select the connector elements, right-click, and select "Create Structure".

pointJoint_createStructures.png

Note

Organizing structures in the imported FEM with INCLUDE files can be useful for quickly selecting Joint Structures from the "Inc" view of the FEM Tree.

On the "Structure Creation" form, you will see FEM properties referenced by the elements selected, PBUSH in this case. Select "Create point joints per connector element" and "Create structures per include files" (if using INCLUDE files) and press "Create".

CreateJoints1.png

When done you can find the new Joints in the Structures Tree and on the model in the Viewport.

CreateJoints2.png

Note

To learn how Point Joints are used when extracting and processing loads, see Point Joint Loads.

Connector Modeling

To ensure the best results, it is important to understand the relationship between connector modeling techniques and HyperX Joint Load Extraction/Processing methods. Load extraction and processing differs based on whether the Joint is analyzed as an Edge Joint or Point Joint.

Note

Bolted Joints can be represented as Edge or Point Joints. However, when the connector elements are discretely modeled, they must be represented as Point Joints.

multiLapJointDiagram.png

To make the most of HyperX's Bolted Joint capabilities, reference the image above and make the following considerations when modeling connector elements:

  • CBUSH elements must be attached either directly or via RBE to different Zones on both ends. If they are not connected to different Zones, HyperX will create 0 or 1 member Joints which have limited functionality.

  • RBE may be RBE2 or RBE3. RBE3 is recommended.

  • The outer-most 'Grid A' represents the connection to 'Sheet 1'. For countersunk fasteners, this will be the countersunk skin.

  • It is important to align 'Grid A' and 'Grid B' directions for each spring in series. However, the local shear (y-z) planes do not have to be perfectly aligned between springs - HyperX will make the proper transformations, into-sheet coordinates defined by material vectors, and shear force vector sums to compute the sheet bearing for Sheet 2.

  • HyperX can handle zero-length CBUSH elements (where 'Grid A' and 'Grid B' are coincident) for single and multi-lap joints with the following considerations:

    • They require a reference coordinate system, and the X-axis direction of that reference CS becomes the axial direction in HyperX.

    • Zero-length CBUSH elements in multi-lapped joints must all reference the same or identical coordinate systems, or HyperX won’t recognize them as being part of the same joint.

    • You can create a multi-lap joint that is a mix of zero-length and non-zero length CBUSHs, as long as the zero-length CBUSHs have a CS X-axis that aligns with the Grid A to B direction of the non-zero length element.

Workflow

reminderOfHyperXWorkflow.png

Note

While specific Design Properties and Failure Criteria differ between Fastened and Riveted Joints, they function almost identically from a UI perspective. As such, the following tutorials can be used for either style of Joint as long as appropriate connector elements and Properties are used.

Initial Setup

You must assign a Design, Load, and Analysis Property to a Bolted Joint to Size and/or Analyze it. To do that:

  1. Select the Joint.

  2. Drag the desired Property from the Property Tree onto the selected Joint. The assignment will be confirmed by the Joint momentarily changing color in the Viewport.

    Important

    The types of Design and Analysis Properties you can assign differs based on whether you are Sizing/Analyzing Fastened or Riveted Joints.

  3. Repeat for each desired Property.

assignSparSkinProps

Load and Analysis Property settings function similarly to those for Zones albeit using Joint-specific loads and criteria (see Fastened and Riveted criteria). The biggest differences between Sizing Zones and Joints comes from the Design Property settings.

Design Properties

Analysis of Bolted Joints involves assessing all objects in the joint - including the bolt, nut, fastened sheet(s), and so on. Joint Design Properties define the design space for the Joint by defining these elements. This information includes Material data for connected sheets, Fasteners or Rivets, and other variables such as sheet thicknesses, number of connector rows, etc.

BoltedJointsIntro.png

When viewing a Joint Design Property, the first thing to note is the Sizing/Analysis mode.

boltedJoints_DesignPropertyMode.png

There are 3 modes for a Joint Design Property:

  • Sizing - Size the Joint in the defined design space to find the minimum weight solution.

  • Advanced Sizing - Size the Joint in the defined design space with a user-defined order of operations.

  • Analysis - Evaluate MS for the Joint based on the currently defined design.

Note

These modes are discussed in more detail in the Sizing and Results Interrogation sections.

One key difference between Fastened and Riveted Joint Design Properties is whether they feature Fasteners or Rivets - otherwise the forms look and function nearly identically. The other key difference is in the kinds of Joints that can be represented by these Design Properties:

  • Fastened Joints

    Fastened Joints have one or more rows of Fasteners. HyperX supports Single, Double, and Triple Shear Fastened Joints.

    DesignProp_joints-fastened.png

  • Riveted Joints

    Riveted Joints have one or more rows of Rivets. HyperX supports Single Lap Riveted Joints.

    DesignProp_joints-rivet.png

Loads Processing

The same Load Properties can be applied to both Fastened and Riveted Joints. Any Bolted Joint can be assigned statistical, average, element-based and peak Load Properties. If modeled as an Edge Joint, neighbor-average Load Properties are also applicable.

For more information on how each Joint modeling method affects load processing, see Edge Joint and Point Joint loads pages. For general loads processing information, see FEA Loads Processing.

Failure Criteria

Available failure criteria for Bolted Joints depends on the type of connection modeled:

  • Fastened Joints

    The allowables used in Fastened Joint strength checks are taken from bolt/sheet material definition, as is applicable. For more information, see Fastened Joint Criteria.

  • Riveted Joints

    Rivet strength margins are written by comparing the Joint shear load the Joint shear allowables listed in the ‘Rivet Definition’ form. For more information, see Riveted Joint Criteria.

Sizing

When "Sizing" mode is selected, Joints can be Sized by:

  1. Selecting the desired Joints.

  2. Clicking "Size".

    Note

    When a Joint Design Property is set to Analysis mode, Analysis will be run instead of Sizing on that Joint when you click "Size".

image11.png

Advanced Sizing

The standard Sizing method for Joints will find the minimum weight design with all positive margins. However, that solution may not be easily manufactured.

A Joint in Advanced Sizing mode allows a user to specify multiple options for each variable, as well as the order of operations HyperX should use when resolving margins. This allows you to tailor the Sizing process to find the solution that works best for your situation.

AdvSizingOverview.png

A few things to notice here:

  1. By clicking the "Sizing" and "Advanced" buttons, we set the Design Property to Advanced Sizing mode.

  2. Using the "Type" and "Selection(s)" columns, we can specify one or more options for each available input. For instance, we can specify multiple Fasteners to potentially choose during Sizing.

  3. The "Seq..." column is how we specify the order of operations HyperX will follow when trying to resolve margins with different variables. After each change, it will re-Analyze the Joint. If the margins are still negative, it will try the next variable in the list. If margins are still negative after all operations, HyperX starts over at the next value for the first variable in the sequence.

  4. The "Order" column specifies the start and procedure for stepping through a range of options for a variable (i.e. start at the max or min and step down or up, respectively). In the image above, as HyperX tries different fastener spacings based on these settings, it would start at 2 inches, then try 1.75, 1.5, 1.25, and 1 inch.

Given the example above, HyperX will:

  1. Decrease the spacing between fasteners.

  2. Add a row of fasteners.

  3. Try the next fastener.

  4. Repeat from Step 1, if necessary (i.e. there are still negative margins).

Tip

For more information and practical examples, see the Edge and Point Joint Training Examples.

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