Digital simulation proves no match for physical testing

We live in a digital age. Everything is online and software driven, especially when it comes to design. The CAD companies are quick to tell you that physical prototyping is dead, and that 3D modelling and simulation is the way forward. However, while the phrase 'virtual prototyping' was coined some time ago, most in industry do not yet have the confidence to fully commit to it as a design philosophy.

"As good as digital simulation is, it can never replace physical testing," says Colin Bradsell, company director at Materials Testing and Technical Services (MTTS). "One of the issues with simulations is that the results you get out are only as good as the data that goes in and the person running the simulation, and that takes real specialism and expertise. And software can't necessarily advise you how to engineer out cost or how to make something stronger in quite the same way as a physical test can."

This is exactly the niche that Kingston based MTTS hope to fill. When Bradsell along with fellow company directors Dr Vanessa Izzard and Val Morris set up the company several years ago it had a clear vision of what it wanted to deliver.

The company is taking a somewhat old-school approach to the way it takes on projects, the equipment it uses and the in-house skills it can employ. And while it can be broadly billed as a testing specialist, it is keen to point out it is not a test house.

"We don't want to be sent 1000 samples for test and validation, that are not who we are or what we do," says Bradsell. "We like to get involved with projects that might be a bit obtuse, so where something might not fit in to a standard test procedure or machine, or if people want more analysis around what's actually being tested and not just a print out of the result. Many test houses might not have machines can effectively accommodate and test non-standard shapes and materials. For us though, that is our forte."

MTTS has a broad range of skills to call on that allow them to revert back to first principles and establish key fundamentals when addressing problems. While technical analysis of test data, determining failure modes and analysing specimens is core to MTTS, getting there often requires them to show a more practical side. The team are all experienced machinists with its workshop housing several hand operated lathes. These, as readers will know, require significant skill but fortunately Bradsell can revert back to his time served as an apprentice instrument maker at the National Physical Laboratory.

At first glance the lack of complete standard test rigs may seem bizarre. However, MTTS?instead has a raft of components from frames, to hydraulic pumps, load cells, deflection-meters and strain gauges that it assembles for purpose like a highly evolved Meccano set. And if it doesn't have an appropriate load cell for an application, it will simply make one.

"This approach allows us to get the most from tests," says Bradsell. "We can gather more effective data that we can then feed back to customers. Because we can do that, we've done a lot of prototyping work. We enjoy that process of designers coming up with a concept or idea, and then coming to us to help make it work physically, and practically. We've done folding bikes, an unusual orthopaedic chair and are currently helping a well know bolt manufacturer develop a new line of product."

Design it, make it, then break it
As MTTS explain, the thing with many designers is that they always want the most complicated part made first. And with many small manufacturers wanting to do a minimum batch of 100, prototyping can be very expensive.

Dr Izzard says: "It's not usual for products to be 'blindly' made and then for someone else to 'blindly' test it. Designers can end up with a broken prototype and some data on a paper read out and really they are are none the wiser.

"We want to be a one stop shop because we have machining skills, we can make the prototypes in low quantities and even one-offs. We can then test them and from doing all that we've got this pretty good picture of the product, of what works and what doesn't. That allows us to feedback things about the design, its use, its manufacture and show test data to back up everything we are saying."

The process of taking a conceptual design, making it, and then testing it allows an insight that is difficult to quantify and is perhaps a forgotten advantage of its fundemental approach.

"As you make it you can start to see real world problems," says Dr Izzard. "How is someone going to build or assemble this for example on a production line? As wonderful as 3D CAD is, it not will give you that assembly insight in quite the same way, and it allows you to design things that are sometimes physically impossible to make."

It is this practical feedback that marks MTTS?out. While its customers may not always think about things under load, it does, and it has been able to forge some good relationships this way.

"Not a lot of people have these skills," says Bradsell, "and they can be seen as old fashioned. But they are proving very valuable for us doing prototyping and customised testing work."

Photo-E still effective
While not many people use it these days it is quite difficult to analyse the equipment. What is really good is what's called a Full Field View. This method allows you to see all the stress concentrations around a part, and it is also good for dynamic. So if you get a camera that is strobing you can actually see the stress patterns as it is moving.

If you do that in FEA you are doing an idealised test. So the component is brand new, made of new steel, with no faults of anything. And then you have to make assumptions about wear or environment. What we can do is the actual component as it is.

So you either make the piece of plastic to suit your component, as it is a particular type of material, or you can coat the actual object of the material to see the patterns. So the method is well developed and historically quite old. Now, everybody loves strain gauges, great in most environments but not all.

You can use it to calculate strain, and even stress, but the real trick is you identify stress concentration areas. It tells you where the worst case of the part is and then you can put a strain gauges on that part, rather than all over a part.

Tension Control Bolts (TCB)
MTTS has undertaken work with bolt manufacturer TCB, well known in the railway industry for rivet replacements of railway infrastructure as well as Victorian bridges. Due to the historic nature of some of the infrastructure it was important that any replacements mimic the appearance of the originals.

TCB manufacture the historic bolts as a close tolerance fit, so to offer clamping through the preloading of the shank during the installation process. This mimics the function of the traditional rivets.

However, little published information was available about the performance in tension (friction) and shear simultaneously so MTTS were called in to test the performance of the Tension Control Bolts under dual loading and in-service conditions.

A test rig was designed that contained the non-faying surfaces to represent on site conditions where age / corrosion may lead to a loss of a contact between components. Strain gauges were fitted to the surface of the bolt at the rivet head in order to monitor its behaviour during installation and testing. Three bolt diameters were used – 16mm, 20mm and 22mm were used and tested in tension and under shear at reasonably slow strain rates with and without pre load.

The result was a fracture surface of the bolts which were consistent throughout the testing showed that the rig failed the bolts correctly with clean shear failures. The tensile strain produced as a result of the preloading is lost at approximately 70% of the maximum achievable load.

The state of preload in the Tension Control Bolts appeared to have no effect on the shear strength and the experimental shear strength was found to be approximately twice the published value. It was validation of the bolts so it could be confident in industry.

Fellow director Val Morris, says: "At the moment we are designing and testing a new fixing for them to extend their range. They had an initial concept, we took that and designed various possibilities, they picked two, and then we tested them and fed back that data. Now we are looking at making the bolts as mechanically efficient as possible."

Author
Justin Cunningham

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