FEA_Tutorial_2007.pdf

Introduction to Solid Modeling Using SolidWorks 2007

COSMOSWorks Tutorial

Page 1

In this tutorial, we will use the COSMOSWorks finite element analysis (FEA) program to analyze the

response of a component to an applied load. Finite element analysis is a powerful tool that allows

engineers to quickly analyze and refine a design. It can be applied to problems involving vibrations, heat

transfer, fluid flow, and many other areas. The most common use of FEA is in structural analysis, and

this introductory tutorial will be limited to that use.

There has been much discussion during the past decade over who should be using FEA software. As the

software has become easier to use, the potential for misuse has risen. An inexperienced user can quickly

obtain results, but the interpretation of the results requires knowledge of the applicable engineering

theories. In this tutorial, we will point out where choices and assumptions are made that could affect the

accuracy of the results.

The part to be analyzed is the bracket from the tutorial of Chapter 3.

Open the part file. From the main menu, select Tools: Add-Ins and

check the COSMOSWorks 2007 box.

If you check the box to the right

of the add-in name, then that

add-in will be activated

whenever SolidWorks is started.

Most users will prefer to activate

COSMOSWorks only when it is

needed for an analysis.

When COSMOSWorks is activated, a new menu item is created,

and a tab for the analysis manager is added above the

FeatureManager.

Click the analysis tab. From the main menu, select COSMOSWorks:

Study. A study defines a specific analysis and its results. A single part file

can have multiple studies associated with it.

Name the study “50 lb

Load”. Click the check

mark to accept the default

element type (solid mesh)

and analysis type (static).

Introduction to Solid Modeling Using SolidWorks 2007

COSMOSWorks Tutorial

Page 2

Element Type: There are many element types, such as plates, shells, truss members, beam elements,

and solid elements. COSMOSWorks allows for solid elements to be created from solids, or shell

elements to be created from either surfaces or solid mid-surfaces. Although solid elements are

typically chosen when a solid model is available, solid elements are not always the best choice for

many applications. Often, a few beam or shell elements will provide more accurate results than

hundreds of solid elements.

Analysis Type: In a static analysis, we assume that that loads are applied slowly. If loads are applied

almost instantaneously, then dynamic effects need to be considered. A linear static analysis assumes

that the response of the structure is linear – for example, a 20-lb load produces stresses and deflections

that are exactly twice that of a 10-lb load. However, if the deflections are relatively large, then the

stiffness of the part changes as the part deflects. In this case, a large-deflection analysis, in which the

load is applied incrementally and the stiffness re-calculated at every step, may be required.

From the main menu, select COSMOSWorks: Material: Apply Material to All. Check “From

library files” as the source of the material data, as select ABS from the list of plastics. (ABS stands

for Acrylonitrile butadiene styrene,

a common thermoplastic used in a

variety of applications). Change

the units to English and click OK.

Material: One of the most important inputs to the model is the elastic modulus E of the material. The

elastic modulus defines the stiffness (resistance to deflection) of the material. Its value is determined

from material tests. A material with a high value of E will deflect less than one with a lower value of

E. For comparison, steel has an E value of about 30,000,000 psi (pounds per square inch). Aluminum

has an E value of 10,000,000 psi. The ABS plastic that we have chosen has a value for E of 290,000

psi, so it is about 100 times less stiff than steel.

An assumption of our model is that the material’s behavior is perfectly linear, so that the deflection is

exactly proportional to the load. This model is an idealization for many plastic materials, which

exhibit some amount of non-linear behavior.

Most materials reach a point before they break at which additional loading produces much larger

deflections. We say that the material has yielded at this point, and our model is not valid beyond the

yield point of the material.

Introduction to Solid Modeling Using SolidWorks 2007

COSMOSWorks Tutorial

Page 3

From the main menu, select COSMOSWorks: Load/Restraint: Restraints. Click on the back face

of the bracket. By default, a fixed restraint will be created. Click the check mark to apply the

restraint.

Boundary Conditions: When a component is isolated for analysis, the way in which that component is

attached to another must be simulated with boundary conditions. In this case, we have chosen a fixed

restraint, which means that every point on the back face of the bracket is prevented from moving in any

direction. While this seems to be a reasonable assumption, it may not be entirely accurate. If screws

are used to attach the bracket to a wall, then the top screws may stretch enough to allow the top of the

bracket to separate from the wall. Also, the wall itself may deflect slightly. The choice of proper

boundary conditions to simulate actual constraints is often one of the most important decisions to be

made for an analysis.

From the main menu, select COSMOSWorks: Load/Restraint: Pressure. Click on the face around

the ½-inch hole as shown here.

Set the pressure as 84.9 psi (be sure to set the units to English IPS).

The pressure is calculated from the 50-lb load applied to the surface, which is

one inch in diameter with a ½-inch hole in the center:

4 1 0.5 84.9

Note that often a load or constraint is to be applied to only a portion of an

existing face or edge. In these cases, the use of a split line can be helpful. A

split line simply divides a face into multiple faces that can be selected

separately. See the SolidWorks help files for information about creating split

lines.

Introduction to Solid Modeling Using SolidWorks 2007

COSMOSWorks Tutorial

Page 4

From the main menu, select COSMOSWorks: Mesh: Create. Move the slider bar toward the right

(fine) and click the check mark.

When complete, the mesh will be displayed.

Mesh Size: A finer mesh, with more elements, will generally produce more accurate results at the

expense of longer processing time. For simple parts and a relatively fast computer, the longer

processing time is not significant. However, for complex analyses (such as non-linear and time-

dependent analyses), mesh size can significantly impact processing time. How many elements are

needed for accuracy? Sometimes it is necessary to experiment with different meshes until the results

converge to a solution. In other cases, the mesh can be refined to create more elements in a local area

where stresses are greatest.

From the main menu, select COSMOSWorks: Run.

While the analysis is being performed, a status box will appear on

the screen. For this analysis, about 8600 elements were created

(your number may be more or less, depending on how far to the

right you moved the mesh size slider bar). There are about 16,000

nodes, or points where the elements meet. Each node has three

degrees of freedom, or possible displacement, except for those on

the back face that have been constrained. Each degree of freedom

has an associated equation for its displacement. While the solver

is running, these equations are being formulated and solved.

This analysis should take only a few seconds on a

reasonably fast computer. (A remarkable feat,

considering there were about 42,000

simultaneous equations to be solved!)

After the analysis is complete, results can be

viewed in several ways.

Introduction to Solid Modeling Using SolidWorks 2007

COSMOSWorks Tutorial

Page 5

In the analysis manager, under Results, right-click on the

item called Stress1 and choose Show.

A color contour plot of the stresses in the bracket is displayed.

We will now change some of the default settings of the plot.

Right-click on Stress1 and choose Edit Definition. Leave

the type of stress shown as von-Mises, and set the units to

psi. Click the check mark.

Right-click on Stress1 again and choose Chart Options. Check the box labeled “Show max

annotation”. Also, change the numeric display to floating, with no decimal places shown. Click the

check mark.

The resulting plot is shown here. Note the

value of the maximum stress, which occurs in

the center rib.

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