Evaluation of the strength of fasteners in a full-scale experiment and in the cloud CAE Prove.Design
Imagine that you needed to print some fasteners on a 3D printer. How do you know if the fastener will withstand the loads that will affect it during operation? What will be the margin of safety for such fasteners? Is it possible to change the fastener design to save on material and production time?
This question can be answered in at least two ways. Either we carry out field tests, or we calculate the fastener strength using a CAE program.
Using a CAE (numerical simulation program) looks like a very reasonable solution, but it can be fraught with a number of difficulties.
Such programs are quite demanding on the computing power of the computer.
CAE packages like Ansys are obscenely expensive to license. Those who decide to save money on piracy run the risk of bringing the virus to themselves.
It can take a long time to understand the tools and features of the program. If you need strength calculations infrequently, or even once, the time to master it can be wasted.
Natural tests:
Long and expensive.
Requires carefully designed experiment.
They still show inaccurate results.
In this article, we will demonstrate both in a simple experiment.
Task number 1 – hook
Given a hook 38 mm high and 5 mm in diameter, made of PLA plastic. This product is supposed to be used: to fix it by the upper part, and to hang the load from below.
Q: What is the load capacity of this product?
Before you know the answer, try to give your guess.
Task number 2 – record
In this problem, as a test item, we calculate a 10 cm long plate, also made of PLA plastic. The sectional size of the narrowest point of the plate is 5 mm.
The plate is supposed to be loaded in two ways:
Numerical hook calculation
To conduct numerical strength tests, we used our development – a cloud CAE service Prove Design. It has a number of advantages: it is free, convenient, not demanding on the preparation of the user and on his hardware, and most importantly – works directly from the browser and is freely available to anyone interested.
Here it will be easier to organize the process than in full-scale tests. We upload the model to the service, set the boundary conditions that are relevant for our task, namely: in the place where the hook is attached, we set a ban on movement; we apply a point force at the bend of the hook, which will simulate a suspended load.
We test the hypothesis. A product with a maximum dimension of 4 centimeters must support the weight 2 kg, or approximately 20 newtons. The product will be made of PLA plastic. We carry out the calculation, we get the following picture:
You can twist the result of the calculation yourself link
We immediately see the point of concentration of maximum stress. In this place we will expect destruction. The calculation tells us that at this point the maximum stress will exceed that allowed for the material at 4 kg loads.
Field tests of the hook
For a full-scale experiment, a plywood stand was assembled. After fixing on the stand, a load in the form of a plastic bottle of 5 liters was suspended from the product.
For the experiment, two identical hooks were used, pre-printed on a 3D printer.
The mass of an empty bottle with a rope was previously measured, which was 40 grams.
Further, by means of a measuring glass, water was slowly poured into a plastic bottle. At the beginning, 60 grams of water was added, then 100 grams were added to the bottle with an interval of several seconds between topping up.
Both hooks broke under load in 2.3 kg.
Numerical calculation of the plate (A)
Let’s impose a ban on movement on the left end of the plate, apply a vertical force to the surface of the hole on the right (simulate a hanging load). We calculate and get the following picture:
The calculation results show that the maximum stress is located on the edge closest to the place of fastening on the side of the jumper and destruction should occur already at a force of 1.2 kg.
Numerical calculation of the plate (B)
Similar to the first case, but now we apply a force along the plate. Mechanical intuition and strength of materials say that in this case the strength properties of the model will improve significantly – let’s check if this is so.
According to our calculations, in this state the plate holds 60 kg.
Well, let’s check!
Field tests of the plate (A)
The sample showed strength close to the calculated one, withstanding a load of 1.4 kg. Most likely, this is due to the diagonally oriented plastic fibers.
Field tests of the plate (B)
As we remember, the calculated strength of the plate under this type of load was 60 kg. Unfortunately, our setup does not allow us to check such a load with sufficient accuracy. However, we can reliably state that when loaded along the white plate, it can withstand a weight exceeding the weight of the experimenter (55 kg).
Conclusion
Our little experiment, however, gives some food for thought.
Thanks to the calculation engine underlying the Prove.Design engineering analysis service Fidesys – the best of the domestic programs in the field of solid-state engineering analysis, certified Nafems and having passed many practical tests, we can not be afraid for the correctness of the calculation of the mathematical model.
However, the very formulation of the problem creates assumptions. In the model used in Prove.Design, PLA plastic is considered an isotropic material, when in reality our product was printed in layers of 0.2 mm thick. This gave a significant error in the calculation of the hook, but already for the plate it turned out to be very accurate – 1.4 real loads against 1.2 theoretical ones. The CAE-Fidesys package, whose computational kernels we use, can simulate both anisotropic materials and materials with a complex – for example, composite – structure. In the next versions, we will definitely add this feature to the Prove.Design cloud service as well.
However, we still recommend that 3D printing enthusiasts who model fasteners with us or in any other similar software give their product a two to three times safety margin.
In both the hook and plate cases, we were able to accurately predict the break points.
In those cases where it turned out to be impossible to carry out tests in full scale, the program is still able to give an estimate that is quite accurate in order. This is very valuable when it comes to something more complex, strong and expensive than a plastic record.
Importantly, full-scale tests took many times more time and effort than calculation through the program.
cloud service Prove Design is the only cloud-based strength calculation service being developed in Russia. We combine Ansys-level accuracy with ease of use and accessibility for any user who cares about engineering and strength calculations.
Prove.Design allows you to perform static, modal, thermoelastic analysis. Our team is constantly developing ProveDesign, improving its reliability, usability, adding new features for engineering analysis.
You can see examples of other calculations on our website.
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