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1.1 Errors within Organizations

Next, we will be sharing the mistakes made by CAE engineers working in di!erent 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 di!erence 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 di!erence 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 speci"c 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 modi"cations 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

modi"ed model showed a drastic di!erence when compared to the original. After careful checking and

I 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.

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comparing the number of elements, the analyst realized that a few rigid elements were missing in the

"rst 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 "le) 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 quali"ed (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 quali"ed, 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 !oor, testing department, or "eld to study the

manufacturing, functioning, and failures of the components: Just sitting in front of the computer

in an air-conditioned o#ce 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 di!erent 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 "eld 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 speci"c software and a resistance to learn and use new ones: Engineers using a speci"c

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 speci"c software is good but takes more time in comparison to another one

for some speci"c application, then it is better to use the better one. Sometimes a combination of two

di!erent 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

speci"c software.

9) Not the CAE engineers but the design engineers are the most important person in the design

chain: CAE Engineers are usually highly quali"ed, 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

49

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 modi"cations, no consideration for the manufacturing constraints

and the cost e#ectiveness: Sometimes the CAE engineer gives suggestions which are either not

manufacturable or cost e!ective. 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 modi"cations 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 re$ects the

successful marketing of any company. Sweet talking, impressive infrastructure, and other facilities can

create an excellent "rst 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 "eld, 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 "nd someone else” or

“I want results and not the problems, don’t come to my o#ce 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 di!erently skilled CAE users.

CAE System Administrators:

1) Laziness in "xing 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

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to create problems. It will not be logical to expect the best performance from the team without

providing them good computers and "xing 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 a!ord 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 "nished, 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 o#ce. Another

good practice is to avoid working in on a single "le through out the process. Instead, save the "le with

a di!erent name after every 3 or 4 hours of work is recommended.

HR:

Inconsistent salaries for the same post and same job pro"le: In many (probably all) organizations

the most infamous department is HR. In CAE groups it is common to "nd inconsistent salaries for

the same quali"cation and the same job pro"le. When there is an urgent requirement, HR people

generally o!er higher salaries. When engineers come to know that the newly recruited person has

been o!ered 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 re$ects 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 insu#ciently de"ned 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 di!erent

modeling pitfalls are provided in the remaining chapters of this book.

Geometry simpli"cation

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 "llets may

be replaced by sharp edges, as shown in the images below. Even though this simpli"cation was/

is requested, keep in mind that your FEM model now “deviates” from the initial geometry.

Meshing

What kind of elements are you using in your model? Why are using this element type? Did you

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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 di!erent 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 "ner mesh to check for convergence of the simulation results. As a rule of thumb, areas of

interest should be meshed "ner (smaller element size).

Of utmost importance is the element quality. Keep in mind that the elements not only “re$ect”

the CAD model, but eventually the analysis is based on the "nite 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 di#cult 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 "gure 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).

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

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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.

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 "nite 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.

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 "rst 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).