Designing a Safety-Critical Device Using FEA with SimScale
February 7th, 2018
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BlogFEADesigning a Safety-Critical Device Using FEA with SimScale
This post is the third in a series of three about discovering the benefits of cloud-based simulation and the SimScale finite element analysis software in a business environment.
In the first and second article of this series, we discovered the potential of using SimScale for finite element simulation and revealed the results of a professional consulting project. It was shown that SimScale allowed the tackling of a relatively demanding simulation case with ease, reliability, and agility. Subsequently, a more complex and critical structural analysis project will be detailed.
Let’s take a case from one of my company’s consulting projects—a workplace safety equipment provider. For this article, my customer agreed to share some details of this simulation.
It is required by law that every safety device installed for fall protection must be backed up by engineering calculations. A stress engineer’s responsibility is to ensure that the design of the device meets the safety requirements.
Testing a Safety-Critical Device
The safety-critical device to be analyzed is a harness anchor point, made of carbon steel and designed to be welded to the bottom of a steel girder. The design is illustrated in Figure 1.
The safety code for fall protection requires that the anchor device is able to withstand a static load of 23 kN for each harnessed worker. The load could be applied in any direction, and the device must retain its integrity after the load is applied.
The preliminary hand calculations for the stress check show that the device should be able to withstand the load, but complex effects such as stress concentration at the corners and plasticity require a more detailed analysis. For that, it is necessary to use the finite element analysis with the SimScale platform.
Modeling and FEA of an Anchor Point
A three-dimensional solid model was built to recreate the design of the safety-critical device. Second order tetrahedral elements were chosen for maximum fidelity. The finite element mesh is shown in Figure 2, which has the following statistics:
Number of nodes: 148 561
Number of elements: 95 344
For the finite element analysis, a nonlinear plastic simulation was chosen. This allows for checking of the design integrity even after the yield limit is surpassed. Stress redistribution, stress concentration, and the probable rupture points can be identified using this method. The material model is a standard piecewise-linear stress-strain curve, with elastic, plastic, and strain-hardening regions. The data is standard for A36 structural steel.
The considered load case is for a downward vertical load of 23 kN, applied uniformly to the inner lower face, and constraints at the top plate vertical faces, to simulate the restrictions imposed by an all-around welding bead.
Results: Deformation and Stress Concentration Points
The nonlinear simulation of the safety-critical device took 91 minutes to finish (a preliminary linear statics simulation was carried out to validate the model). Figure 3 shows the resultant deformation plot.
The maximum deformation is approximately 0.05 mm, thus no global plastic deformation occurs for the considered load. This is corroborated by looking at the displacement plot for the point with maximum deformation, shown in Figure 4.
The von Mises stress plot is then considered to examine the stress state:
It can be seen that the maximum stress (307 MPa) is over the yield point (250 MPa) and that it occurs at the stress concentration points. To yield a better picture of the stress state and to contrast with the common von Mises stress plot, it is important to look at the signed von Mises stress plot, which shows the regions that are in tension and in compression. It is interesting to note the load path in this plot, and have some idea of material regions that could be removed to save weight if this were to be the goal.
In Figure 6, it can be appreciated that the compressive bearing stress region under the load application face, the flexural stresses on the top plate (note the flexion in both plane directions and the complex moment distributions, consider for a moment how this could be quantified by analytical methods), and the stress concentration points on the inner corner and in the weld bead.
To assess the safety of the part, it must be noted that even though yielding occurs, it is localized and it doesn’t appear to have global effects. Also, the ultimate stress (400 MPa) levels are not achieved. Therefore, for the considered load case, it is expected that the part retains its structural integrity.
A structural safety assessment for a safety-critical device was carried out for a professional consultancy project. SimScale’s high computing power and nonlinear analysis capabilities were leveraged to take into account all the necessary aspects for the Finite Elements Analysis to be considered complete and valid. It was found that the part is safe for the considered conditions. To arrive at this assessment, various post-processing techniques were applied, as well as engineering judgment.
This concludes our three-article series about discovering the power of Finite Element Analysis for professional industry projects and its use through the SimScale cloud-based platform. If you want to explore the platform’s capabilities in FEA, feel encouraged to give it a try, and share your findings with the community.
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