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Challenge Problem

Tube Crush - Challenge Problem

The Computational Structural Mechanics Working Group would like to present NAFEMS members with a nonlinear deformation challenge problem. The problem is outlined below. We’d love to learn how you would tackle the problem and look forward to comparing approaches.

Problem Description

The design department of your company are engaged in a project to design an impact limiter for a part to be moved by a crane lift. The design they want to go with is for two effectively rigid thick steel plates; the crane lifting points will be attached to the lower plate. The part that is required to be lifted will be attached to the upper plate, and the two plates must be separated by at most 350mm to provide the necessary clearance throughout the lift. The designers have determined that, for the energy content of the maximum credible drop height, a set of tubes that can each provide 100kJ of energy absorption would be sufficient, and they have found some stainless steel tubes in stores that they think will work. The tubes can be cut to the required 350mm length, and there are a large number of tubes immediately available which makes them very attractive to use. Simple hand calculations have shown that these tubes are capable of supporting the static force during the lift, including all credible dynamic loads from the crane.

The tubes have been inspected by the designer using a Vernier gauge and appear to all have a 4.0mm wall thickness, are visually round with no obvious ovalisation or other imperfections, and have an outside diameter of 174mm. The material certification cannot be found, although the company metallurgist has suggested that a mean plastic flow strength of 317MPa has been used in previous work which gave reasonable correlation to crushing problems such as this for this sort of material. The metallurgist is very busy (and also quite grumpy!) and won’t provide any more assistance in characterisation of the material. The designer is also very sorry, but there isn’t any money in the project for testing of the tubular material, either.

The designer wants to know if the tubes that have been found in stores are suitable for use and will absorb the required energy. What is your advice? What caveats, if any, do you think are necessary to be attached to your advice, noting that the project is under considerable time and cost constraints.

Tubes and plates

Figure 1: The tube and plates.

Responses are encouraged that cover a wide range of simulation approaches to the problem, augmented by existing published solutions where applicable. Innovations in simulation approach as well as efficiency in answering the question are particularly welcome (the ‘design department’ doesn’t have the budget to pay for very large amounts of time in analysis). The responses will be collated by NAFEMS and a summary report of the findings will be made available free of charge to all participants. Particularly noteworthy contributions can be highlighted and the names and affiliations of selected contributors can be included in the summary report, subject to the approval of all parties. There will be an opportunity to question the ‘designer’ to a limited extent (the department is busy after all and just wants answers from you!), and all queries (and their responses) will be made available on the NAFEMS website.


You are invited to send your results to the Computational Structural Mechanics Working Group at

The group don't need to see your detailed calculations they want to learn about the assumptions you have made, how you have generated confidence in your answer and your advice "can the tubes be used?". No more that 5 pages of A4 please.


Presentation Recording

The problem was described in detail by CSMWG Vice-Chair Adam Towse (our very own grumpy engineer) at the CSMWG community event on the 1st of July 2020. The presentation recording can be viewed below.


Presentation Slides

A detailed description of challenge problem is provided in this presentation.

Questions and Answers

How are the tubes be attached to the frame?

That detail has not been agreed yet – the designer wants the tubes to be attached in a way that doesn't add considerable cost. He would prefer not to weld the tubes as this is expensive and he may have to consider inspection of the weld.

How fast will the equipment be travelling?

The maximum speed will be achieved by a drop from 1 metre. This is approximately 4 – 4.5 m/s

Does the 100kg per tube account for all of the energy content, i.e. spreader beam, slings, upper frame?

The designer has not yet designed all of his equipment but he believes that the 100kg per tube limit accounts for all the equipment.

Is there space to use more tubes and can they be shorter than 350mm?

Definitely, there isn’t any limitation on space with this as long as the tubes are not longer than 350mm.

Can we alter the tubes such as bevel ends?

Yes this can be done but it would require additional cost. If this is additional manufacturing process is required to make the design work then it can be done.

Does the impact limited have to be reusable or does it only have to survive one drop?

The impact limiter is expected to be destroyed as it absorbs the energy from the drop. There is no expectation to reuse the limiter.

Is there any information on the mass distribution of the equipment?

No – the designer is working under the assumption that the upper and lower frames will remain parallel.

What are the dimensions of the frame and the dimensions of the different elements?

The frames can be considered as rigid.

Should we only consider collapse or could wall buckling as a failure mode?

The designer quizzically raises an eyebrow indicating that this is really a questions that the you should be answering.

If you have any additional questions please send them to