What are the common failures in a boomilever structure, and how to test for them

The Science Olympiad competition is right around the corner! It’s time for teams to gather their designs and review the best performing structures from prior years. ことし, we wanted to take a deeper look into the reasons Boomilever’s can fail and what can be done to strengthen your structure.

Designing a Boomilever

Have you modeled a structure in SkyCiv yet? 最初にこの記事を読むことから始めることをお勧めします: SkyCivでBoomileverを設計する方法. It will show you how to model and design your first structure and simulate your Boomilever’s performance 前 you build it!

次の記事では、構造をテストして分析する方法について説明します, モデルの問題を事前にシミュレートして発見できるようにするため.

Our Design

The most important part of your design is the way you arrange your members. Are you going to use a Truss? トラスのどのようなスタイル? これは、負荷がかかった状態での構造のパフォーマンスと全体の重量の両方に最も大きな影響を与えます。. それはあなたがしなければならない唯一の最も重要な決定です.

記事上で, we are going to base our design on the following simplified design:

This is comprised of the following elements:

ソース: Boomilever Wiki

あ: Support/Base
B: テンションメンバー
C: Distal End
D: Compression Cross Members

Now let’s review to check all the different ways these members can fail, and how we can help strengthen our design.

Failure Cases

Compression Failure

Compression forces are those that compress メンバー, または crush inwards.

We want to pick a failure criteria we can use. From our article we know that the Compression Failure Stress of Balsa Wood is around 7 MPa. Because wood is an anisotropic material, the strength depends on the quality of the wood and it’s grain direction, but our assumption of failure stress will be a good rule of thumb to use in this design

Our goal is to ensure all compression members have a compression stress no greater than 7 MPa. It is unlikely that the member will fail due to pure compression, but this is important concept to understand and is important for the below checks.

Tension Failure

Tensile forces are those that induce テンション within the member, または pull outwards

Balsa wood is twice as strong in tension as it is in compression. It is highly unlikely the structure will fail due to pure tension forces. We can disregard this as a failure criterion.

Failure Checks

1. Bending Stress Failure

We’re going to start with bending stress as it is a common failure case. その名のとおり, this occurs as the member is loaded perpendicular to its Neutral Axis (NA) it will start to bend, causing distribution of stress along the cross-section of the member. An obvious sign of bending occurring, particularly in wood, if the deflection of the member along its span from its original shape.

私たちの場合には, all of the wood members are straight before loading, so any deflection tells us that the member is bending. A member experiencing bending stress will look something like this:

The top of the member is in compression (-) and the bottom (+) 緊張している. の “M” is the positive moment force that is inducing the stress distribution in this case.

 

How to identify Bending Failure

After running the analysis of the Boomilever in Structural 3D, we will operate from the post-processing, または Solve Window. You can use the 可視性の設定 on the right side of the screen to prompt some viewing/filtering options.

Our goal is to check and ensure compression stress due to bending does not exceed 7 MPa. Use the right Result Visibility option to show any stresses that are above 7 MPa:

This type of failure can be seen in the following video:

The video shows the struts that connect the tension and compression chords are going to fail with a combination of bending and buckling. This coincides with our model above that shows the key failure points are at these connections.

 

How to strengthen against Bending Failure

Here we can see there are four members that are prone to bending failure – as their negative values exceed our compression limit of 7 MPa. Now that I have identified these weaker members, I can strengthen them by increasing the section height:

The increase in height of the member will increase its Moment of Inertia, a section property that directly relates to the cross sections strength. この場合, the as the height of the member increases, bending stress decreases, およびその逆. Imagine trying to bend the same piece of wood, but with these two shapes, which would be harder to break?

By making this change I was able to reduce the amount of stress caused by bending forces to a maximum of about 6.7 MPa.

Another option, is to add a cross member to help distribute the force across another member. これは構造に重みを加える可能性があるため、オプションよりも常に好ましいとは限りません 1 (you’ll have to consider the difference in adding one member as opposed to increasing the section size and weight of multiple members. この場合, 構造を強化するためにブレース部材を追加しました:

円式の慣性モーメント, adding that one bracing member helped to distribute the forces more evenly along the three members. It even alleviated the excessive stresses of the members on the other side (reduced from -7.31 に -4.895 MPa). Note however, as shown this will disrupt any symmetry in your structure.

The pattern on these bracing members (or truss members) depends on your design. ここにあるいくつかの トラスの種類 and their strengths and weaknesses.

2. Buckling Failure

This is a very common failure for slender, thin members. 座屈 is the failure mode of a structural member experiencing high 圧縮性 突然の横方向のたわみを引き起こす応力. Imagine pushing down on a member like so, then it kicks out and collapses like so:

In the case of our boomilever, the relative ratio of the cross section dimensions to the length of the members makes our members more prone to buckling. We can test for Buckling by running a buckling analysis in the software under 解決する. This will check your model to see if any members are at risk of buckling:

As the alert suggests, a number less than 1 indicates buckling. So our Boomiliever is OK at the moment for buckling. If there were any buckling issues, they would show up as red members on the structure so that you can identify the critical members and modify your design.

注意: Buckling is especially important in Science Olympiad’s Towers competition as there are a lot of column members.

3. Connections/Supports

The base and significant connections (like your distal end) should also be designed ahead of time. This part of the design can make or break your structure’s success… literally! Let’s look at the support base first. This connects two members in tension to the the main board. Your structure should not be failing at the base. If you need some help with this, refer to Aia’s guide on designing a boomilever, it has a great guide on an effective base design that weighs ~1.5g and will support 18-19 kg.

 

TL; DR

We recommend checking the following to identify any failure areas of your Boomilever:

• Identify any stresses exceeding 7 MPa. Toggle through the ストレス results with a stress limit of 7 MPa to identify these. If members are failing, あなたが試すことができます:
  • Increase the area of the cross section
  • Add bracing members
  • Change structure formation or truss style
• Run a buckling analysis (especially for column or vertical members) and look for a value of greater than 1
  • Shorten the length of the member
  • Increase the area of the cross section
  • Add bracing members along the way
• Make sure you have a strong base, it should not be the cause of failure
  • If it is, check out Aia’s guide for a strong base design.