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Common Mistakes and Errors
This chapter includes material from the book “Practical Finite Element Analysis”. It also has been reviewed and has additional material added by Matthias Goelke and Gareth Lee.
1.1 Errors within Organizations Next, we will be sharing the mistakes made by CAE engineers working in different organizations.
Common CAE mistakes
CAE Engineers
Marketing Engineers
Managers
System Administrators
HR
CAE Engineers : 1) Submission of a job without the proper checking (should be cross checked by ideally 2 CAE engineers): Checking the work of someone else is a rather boring and not willingly accepted job. But it is very important and a job of high responsibility. Checking and rechecking all the details, ensures good quality and minimal mistakes. For example, a CAE service providing company submitted a meshing job to their regular client. Everything was perfect except for the material properties. The analyst at the customer end was used to error free models being submitted by this CAE team over the years. He blindly started the analysis without checking the material properties. At a later stage in the design process, a big difference was noticed in the results of the current analysis and the previous one carried out for a similar model. After checking both the models carefully, the analyst realized the difference in the material properties. Please be careful before submitting your work and check it several times and then ask your colleague to check it as well. Also, always request that your client check the model in every aspect before starting the analysis. 2) Import / Export errors : Some of the preprocessors do not export all the elements and boundary conditions that exist, unless special options are turned on or special translators are used. One CAE group exported a big mesh model with the template set for a specific external solver. Some of the special elements (RBE3 elements) were not exported due to a translator problem. These were extra rigid connections and were not resulting in rigid body modes during the free-free check. The analysis was carried out As-It-Is by an analyst. Based on the CAE results, the CAD engineers released the drawings and a prototype was prepared. The test results were not satisfactory and further modifications were suggested. Updated CAD data was provided again to the same meshing group. The changes were local and it was to be carried out on the earlier submitted model. By this time, the CAE group had an upgraded version of the pre-processing software and the export operation was 100% successful (all the elements including the ones that were missed earlier were exported properly). The results for the modified model showed a drastic difference when compared to the original. After careful checking and 47
comparing the number of elements, the analyst realized that a few rigid elements were missing in the first model. Hundreds of engineers had worked on the job in the mean time (CAD, prototype, testing, planning etc.). Who is responsible for this delay and cost? Is it the analyst, the service provider, or the pre-processing software? It’s strongly recommended to import the mesh model before submission to the client (in a new file) and apply all of the quality checks as well as compare the number of elements of each type (like number of tria, quad, rigid, spring, mass etc.). 3) Experienced engineers are the best guides and teachers for newcomers and less experienced colleagues: CAE engineers are usually highly qualified (education wise) and having years of experience means that a lot of know-how and knowledge is available within the team. The best teachers for newcomers in any organization are undoubtedly the senior engineers working in the same group. Software trainers or consultants do not know exactly what is required by the customer. Every company should encourage and pay special incentives for experienced engineers to share their knowledge with the newcomers. 4) Meshing is considered as low level work, post graduates and PhDs are reluctant to spend time on meshing: Sometimes a dangerous trend is observed among post graduates and PhDs. They feel meshing is a low level job and being highly qualified, they should not waste time in such low level work. A building cannot be built on a weak foundation. Meshing is the foundation of CAE. At least in the initial years, analysts should be encouraged to mesh the components. 5) CAE engineers are reluctant to visit the shop floor, testing department, or field to study the manufacturing, functioning, and failures of the components: Just sitting in front of the computer in an air-conditioned office and submitting nice analysis reports is not going to make the analysis successful. What is absolutely necessary is to regularly visiting the test department, observing the components on the structure, and comparing the real life performance with the computer model. These days, many times a CAE team is located in a different country than the manufacturing and testing facility. The quality of the CAE work would be much better if there is an opportunity to know the product, manufacturing process, testing and on field behavior. 6) Providing basic training related to data acquisition and testing: At least a basic training on the data acquisition and testing methods is strongly recommended for CAE engineers. 7) Unnecessary emphasis on modeling the minute details without giving due consideration to available time, hardware, and software capabilities: Finite Element Analysis is an approximate approach. Modeling the things to the minute details without giving due consideration to the capabilities of the available software and hardware could unnecessarily complicate the problem. For example, when analyzing a structure and the failure is expected at the body, a bolt should not be simulated by modeling the minute details like threads. Instead a beam element and connections using rigids in the washer area could adequately model the bolt ( in the linear static domain). 8) Loyal to specific software and a resistance to learn and use new ones: Engineers using a specific software for years, are not willing switch to other one. No commercial software is perfect and every software has its own plus and minus points. In the service industry, what matters most is the time and quality of the work. If a specific software is good but takes more time in comparison to another one for some specific application, then it is better to use the better one. Sometimes a combination of two different software works faster. For example, meshing in one software and then performing quality improvement or remeshing in other. A CAE engineer should be loyal to his/her duty rather then a specific software. 9) Not the CAE engineers but the design engineers are the most important person in the design chain: CAE Engineers are usually highly qualified, paid higher salaries and sometimes it leads to a superiority complex (that they are the most important people in the design cycle process). But it 48
should always be remembered that the Design Engineer is the most important person and the role of CAE engineer is to provide analysis services to him/her (other service providers are test, purchase, manufacturing, etc.). 10) While suggesting the modifications, no consideration for the manufacturing constraints and the cost effectiveness: Sometimes the CAE engineer gives suggestions which are either not manufacturable or cost effective. For example, it is very easy to increase the thickness of the parts showing a higher stress, or to suggest that high strength (costly) material should be used, or to suggest geometry modifications without considering the manufacturing constraints. Sometimes CAE engineers are adamant about their proposals and are not willing to carry out further iterations as per suggestions from the design or manufacturing engineer.
CAE Marketing Engineers : 1) Accepting jobs beyond their capabilities: Sometimes marketing engineers accept jobs just because it is from a reputed company or because the volume of work is very large, without giving due consideration to the capabilities and limitations of their technical team and available software / hardware. 2) Promising unrealistic time schedule: Sometimes marketing engineers promises to deliver results in a time span that is not possible with the current strength of the team and the number of software seats available. Maintaining a strict time schedule with good quality work is necessary and reflects the successful marketing of any company. Sweet talking, impressive infrastructure, and other facilities can create an excellent first impression with the client during initial visits, but it will vanish in no time if the delivery schedule isn’t maintained and the quality of the work is poor.
CAE Managers and Group Leaders: 1) Committing the job without consulting the CAE engineer: In particular, during a visit to the clients, managers whose domain is not CAE are involved in the meetings and they sometimes commit to the job without consulting the responsible person. 2) CAE manager / group leader should be someone who has spent several years in the field, not the one who is an expert in another area or a non-technical manager: A CAE experienced manager understands the problems faced by CAE engineers and is capable of helping them personally when required. A CAE job is supposed to be a white collar job, but it really requires day and night hard work and involves lot of mental stress due to tight delivery schedules. The following replies from managers or group leaders could be very frustrating and demoralize any CAE engineer “don’t ask me, that’s your job” or “you should know these simple things” or “if you cannot do it we will find someone else” or “I want results and not the problems, don’t come to my office without the results understood!”. ……. CAE Process Management: CAE Process Management is helping organization to capture knowledge and ensure best practices for CAE. It can be used for CAE load case automation, process guidance, and process integration. It lets organizations implement standardized CAE processes that automate the load case setup, interface with CAD, PDM systems, databases, and other IT systems and applications by capturing the “best practices” as templates. It also helps them retain knowledge, even though people may quit and leave organizations besides improving productivity and reliability of doing CAE between differently skilled CAE users.
CAE System Administrators: 1) Laziness in fixing computer hardware / software related problems: In a group of 15 CAE engineers, a minimum of 15 workstations are required. Computers are, after all, machines and bound 49
to create problems. It will not be logical to expect the best performance from the team without providing them good computers and fixing the hardware, software problems immediately. A knowledgeable and prompt system administer is a very valuable asset for any CAE group. 2) Improper data backup process: One cannot afford a data loss at the midpoint or at the project completion phase. Imagine what the impression of the organization would be if they say to the client that although the job was almost finished, the system crashed and we did not have proper data backup. It is the responsibility of a system administrator and CAE manager to ensure the simple arrangement for the daily backup and forcing all the users to backup before leaving the office. Another good practice is to avoid working in on a single file through out the process. Instead, save the file with a different name after every 3 or 4 hours of work is recommended.
HR: Inconsistent salaries for the same post and same job profile: In many (probably all) organizations the most infamous department is HR. In CAE groups it is common to find inconsistent salaries for the same qualification and the same job profile. When there is an urgent requirement, HR people generally offer higher salaries. When engineers come to know that the newly recruited person has been offered much more than what they are getting, they feel frustrated. Sometimes this results in HR recruiting one new engineer and as a result, two existing engineers leave.
1.2 Modeling and Visualization: While the above summary reflects errors and mistakes from an organizational point of view, the following high level summary is about modeling and visualization errors. During the analysis, the FEM solver will report Warnings and Errors. While warnings can be considered as hints e.g. element quality is bad, errors cause the analysis to stop. Errors may be related to extremely distorted elements, missing material properties, rigid body modes due to insufficiently defined constraints etc. The below listed modeling pitfalls can be considered as “appetizers” with the intention to make you think (and worry) more about the model set-up. More in depth details regarding the different modeling pitfalls are provided in the remaining chapters of this book. t Geometry simplification In many cases it is appropriate or even required to simplify the imported geometry in order to achieve a better mesh quality. For instance, the required minimum element size must be not smaller than x millimeters. In order to solve this (project) related requirement, small fillets may be replaced by sharp edges, as shown in the images below. Even though this simplification was/ is requested, keep in mind that your FEM model now “deviates” from the initial geometry.
t Meshing What kind of elements are you using in your model? Why are using this element type? Did you 50
use this element type before? You may mesh a thin walled 3D structure with 3D elements such as hexahedral or tetrahedral elements, or you may mesh the same structure with respect to its midsurface using 2D elements (trias or quads).
Model meshed with 3D elements
Model meshed with 2D elements Aside from the “decision” of whether to use 2D or 3D elements, there are other “uncertainties” (or even errors) related to the different numerical characteristics of quad versus trias and hexahedral versus tetrahedral elements (see the Chapters on 2D and 3D meshing). Another modeling error may be related to element size. The ultimate objective or aim is that the modeling results are independent of mesh size. Typically you need to re-run the analysis based on a finer mesh to check for convergence of the simulation results. As a rule of thumb, areas of interest should be meshed finer (smaller element size). Of utmost importance is the element quality. Keep in mind that the elements not only “reflect” the CAD model, but eventually the analysis is based on the finite elements. Hence, any deviation from the ideal element shape (e.g. perfect quadrilateral shape in case of a quad element) introduces numerical errors. The magnitude of which is generally difficult to assess. In the model shown below some elements are not coupled to each other (i.e. duplicated nodes exist), hence the mesh is locally incompatible. The area along the edge where the elements are not coupled is marked in red.
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Still, the FEM program does not prompt any warning or error messages as this may be an intended model behavior. If the mesh is not intentionally detached (and the model is not checked for free edges) then this model error may remain unknown until the results are fully checked and understood. As shown in the contour plot below, the displacements are not continuous across some parts of the mesh.
Also, keep in mind the orientation of the element normals. In the image below, a simple plate subjected to bending is shown. The stress contour plot (at the base of the elements Z1) reveals a sudden change of its sign from bending (positive) to compression (negative).
The following figure helps to understand this situation. In the green area Z1 is located at the top of the plate (tension) while in the blue area Z1 is located at the lower side of the element (compression).
t Material Inconsistencies in your unit system represent another likely source of error, i.e. mixing millimeters with meters, kilograms with tons, etc. Be especially cautious if you need to convert properties 52
from one system to another (e.g. pound-force lbf to Newton). There will be no warning message associated with any typos, except the “typo” will cause the entire model to “collapse” during analysis. t Boundary conditions and loads Errors are extremely prone with applying boundary conditions and loads as discussed in the Chapter on Boundary Conditions and Loads. To be mentioned exemplarily, a modeling error may be introduced into the model by applying the constraints (or forces) to what is named temporary working nodes (in HyperMesh displayed as yellow nodes).
As the temporary nodes (yellow nodes in the image above) are not the same as finite element nodes, it may happen that the structure is not constrained or loaded as intended. “Ideally”, this may lead to rigid body modes (error message) or to questionable results due to an improperly constrained or loaded model. t Visualization The blitheness that the analysis went through after struggling around with the model, may lower your attention regarding details while looking at the results. Quite often, especially while you are new to FEM, one becomes blinded by contour plots. Hence, always check the magnitude of displacements and stresses in the first step. Despite a reasonable looking displacements (or stress) contour plot, you may see displacement values in the order of 104 mm (small displacements assumed) or stresses far beyond 1000 MPa (linear elastic material).
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