Chp 14- Design Problem 12

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© 2010 Elsevier Inc. All rights reserved.

doi:10.1016/B978-1-85617-694-1.00014-7

2009

CHAPTER

Design Problem 12

Structural Validation of Trailer Chassis(Design Problem Courtesy of Wright Resolutions Ltd)

KEY FEATURES INTRODUCED IN THIS DESIGN PROBLEM

Key features

1 Automatic Bonded Contacts-Welded Fabricated Structure Analysis

2 Multiple-Loads

3 Planar X,Y,Z Stress Plots

4 Interpretation of results with Stress Singularities present

INTRODUCTION Wright Resolutions Ltd is a design consultancy specializing in agricultural cultivation and

crop establishment machinery. Current clients include a number of well known UK and

European agricultural machinery manufacturers.

As part of a project to design a new concept for a trailer chassis, it was necessary to determine

the loadings on and strength/deflections of a conventionally manufactured trailer chassis, as

can be seen in the following picture. Such chassis are generally manufactured from hollowsection steel, together with flame-cut steel plates and flat bar parts.

Initially, the trailer was modeled in Dynamic Simulation to determine the loads at all critical

areas including the drawbar, axle spring mountings, tipping cylinder, and rear hinges to the

body. Maximum load situations during tipping were then taken and applied via FEA to deter-

mine the parameters listed on following page. One such situation, simplified, is used for this

design problem.

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CHAPTER 14 Design Problem 12

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The requirement of this design problem is to determine

1. Maximum compressive and tensile stresses in the chassis.

2. Maximum deflection of the chassis under load.

3. Factor of safety.

4. Key stress zones for potential reinforcement when designing an alternative chassis.

In addition to the above requirements, the design criteria to be used for this design problem

are the following.

■ Material to be used is EN 50D/S355J2G3 steel.

■ Factor of safety required is 1.5.

WORKFLOW OF DESIGN PROBLEM 12

OPTIMIZATION1– Change thickness and/or add stiffening plates

RUN SIMULATION AND ANALYZE

1– Analyze and Interpret results

BOUNDARY CONDITIONS

1– Apply multiple loads and constraints

IDEALIZATION

1– Include welds as part of geometry (such as fillets)


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CHAPTER 14Design Problem 1

PART 1 – CHASSIS DESIGN WITH WELDS ANDRHS CHANNEL RADII

Idealization

To simplify the analysis of the fabricated chassis, the welds have been modeled as fillets

within the components and will greatly help to reduce the number of contacts produced.

As the strength and characteristics of RHS are dependent on the corner radii, it is importantto include these for more meaningful results. If welds are modeled separately to the RHS,

the joints created in FEA are often complex and can be based on very thin slivers (highly dis-

torted mesh elements) at the limits of the corner radii. Stress singularities produced can be

very high. In practice, provided welds are correct and homogenous to the sections to which

they are applied, such slivers are not present. Extruding the weld as part of the original sec-

tion can represent nearer to a realistic situation. It is important to simulate welds in a manner

that represents reality, as closely as possible, for the results to be meaningful . The use of filler mate-

rials to bridge over the joints, or partial V butt welds, for example, would alter the strength

and integrity of the structure in practice and lead to different results from those simulated.

1. Open Chassis .iam

2. Select Environments Tab Stress Analysis

3. Select Create Simulation Specify Chassis-Analysis for Simulation Name Click OK


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Boundary Conditions

4. Select Automatic Contacts to detect adjacent faces between components and welds

A total of 111 contacts will be created within the Weldment Assembly.

Many more contacts would have been created if welds were modeled separately as a

weldment assembly.

The chassis is attached to the tractor via a drawbar arm which is secured to the chassis vialocking pins.

Therefore, we will apply pin constraints to secure the chassis.


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5. Select Pin Constraints Select the faces of both holes as shown Click OK

6. Select Pin Constraints again Select the back faces of the two middle slots as

shown Click OK

With the aid of Dynamic Simulation, the trailer is used to simulate tipping to determine the

maximum reaction forces on the chassis.


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10. Select Bearing Load Select the two internal circular faces of the bushings Specify

top face of plate to specify direction of force as shown Specify 2.5e4 * 2 for

Magnitude

11. Specify 0.7 to reduce size of force display Specify Reaction-load-1 for

Name Click OK



Magnitude


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Name Click OK



Magnitude


Name Click OK

16. Select Bearing Load Select the four internal circular faces of the bushings Specify

top face of chassis to specify direction of force as shown Specify 2.1e4 * 4 for

Magnitude


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Name Click OK

18. Select Mesh Setting Specify Create Curved Mesh Elements Deselect Use part

based measure for Assembly mesh Click OK

19. Select Mesh View

41,489 Elements are generated with the default mesh size. With Use part based measure for

Assembly mesh selected would have resulted in excess of 100,000 elements.

Number of elements created may differ.

Run Simulation and Analyze

20. Select Simulate Run Analysis

21. Deselect Mesh View Select Undeformed for Displacement Scale Select Show

Max value in Display

Maximum stress value may differ.

The maximum stress value is around the weld areas and is largely due to stress singularities

as a result of discontinuity in the geometrical shape. Refining the mesh around these areas

will not necessarily reduce stresses and in most cases will further increase the stresses.


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As the result stands, the safety factor relating to maximum stress indicates failure at a value

around 0.8.

If the stress singularities are a very low percentage of the total joint area, local yielding

can occur initially until the load is transmitted at lower stress by the full joint. As a rule ofthumb, if such singularities result from static loading and are concentrated in small localized

areas, they can be ignored for purposes of calculating the overall safety factor, for example.

Experience of the effect of such high stress points is needed to ensure that the correct

interpretation is made of FEA results . For example, dynamically loaded situations can have

stress reversals; where these occur at welded joints, there is a high chance of fatigue failure

occurring. In these situations, we can make use of the color bar to better understand the

results as suggested in the following steps.

22. Reselect Von Mises Stress

23. Select Color bar Unselect Maximum Specify 355 MPa Click OK

355 MPa is the yield limit of the material used for the chassis.

As the concentration of red color display is extremely low, and in this case stress reversals are

unlikely, we can assume the safety factor of the design is above 1 for this case.


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But to determine what the safety factor actually is, and to illustrate zones of high stress

requiring design change, we can further manipulate the color bar. In the first instance, we

will change the maximum value of the color bar again to 260 MPa and then to 245 MPa.

Use Contour Shading rather than Smooth Shading to help isolate Stress Singularity Stresses.

Change minimum value of the color bar in addition to the maximum value to help identify

the areas of high stress.

Change the number of legends to help identify the maximum value to be used for calculat-

ing the safety factor, despite having stress singularities.

24. Select Contour Shading

25. Reselect Color bar Unselect Maximum Specify 260 Unselect

Minimum Specify 220 Click OK

By changing the color bar range between 260–220 MPa, we can see localized areas of red

color display indicating where the maximum stress occurs.


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26. Reselect Color bar Specify 245 for Maximum Value Click OK

As soon as we change the color bar max value to 245, we can see that the stresses occurring

above 240 MPa are now also at the outside of the main members, and a picture of areas

requiring redesign to optimize the chassis is becoming clear.

By altering the color bar max value, we can pinpoint the value of max stress (248) in the area

of interest as shown below.

IMPORTANT—you may need to alter color bar max value so that the red display just starts to appear on

the outside channels. (The legend value underneath max is the value we are manipulating, by altering

max value, and the one to be used to calculate safety factor)

Now by using the value of 248 MPa, we can manipulate the color bar max value and number

of legends to achieve the Von Mises plot below. Use your own value to calculate safety factor,

as mentioned above, as it may slightly differ (eg 245)


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The above Von Mises plot confirms that the stress value around 248 (247.5) starts to appear

on the outer sides of the channel.

So for the purposes of calculating safety factor, we will use the value of 248 MPa or use your

calculated value

Factor of Safety 355

2481 43.

As the value is close to the design limit of 1.5, we will look at the planar stresses primarily

occurring on the long channels, due to bending.

27. Double Click Stress YY

28. Select Color bar Specify 300 for Maximum Value Specify −300 for Minimum

Value Increase number of colors to 12 Click OK

29. Select Back View

This displays tensile stresses along the long RHS member of the chassis. As indicated earlier,

the highest stresses are along the length of the chassis.

The stress display above 250 MPa appears to be significantly relative to the width of the cross

member, and also occurs near a weld.

30. Reselect Color bar Specify 320 for Maximum Value Specify −320 for Minimum

Value Click OK


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31. Select Front view to display compressive stress

The compressive and tensile stress display shows values above 266.7 MPa, which are rela-

tively small in comparison to the width of the cross member. As loadings in this case are

gradually applied and relatively infrequent, we will use this value to calculate safety factor.

Factor of Safety 355

266 71 33

..

This value is below the design limit.

32. Double Click Displacement

The maximum displacement of the chassis is 122 mm and indicates that this value is rela-

tively high to the overall length of the chassis (approximately 7000 mm). This confirms the

stress and factor of safety values determined above.

Basically, this analysis suggests that this design needs to be further stiffened to meet the

design goal. In the next example, we will further idealize the chassis by removing the radii

from the RHS channel. This will simplify the model further but at the same time will furtherstrengthen the chassis and hence will not represent reality; however, it will give us an indica-

tion of whether the chassis is more rigid.

33. Close the file


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38. Double Click RL00024:1 Component Suppress Fillet1 Select Return

39. Double Click RL00034:1 Component Move End of Part below Shell1

feature Suppress Fillet1 Select Return

40. Select Environments tab Stress Analysis

41. Right Click Contacts Select Update Automatic Contacts

42. Right Click Mesh Select Update Select Mesh View


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46. Rotate component around (You may need to alter maximum and minimum values

using color bar to get a better understanding of the results)

Apart from the stress singularities around the mounts, the maximum stress occurs on the

main member away from the cross member, unlike before.

There are no stress singularities between the cross member and the main member as before.

IMPORTANT—to calculate Factor of Safety use the color legend value below max (or minimum for com-

pressive stresses)

Tensile Factor of Safety 355

235 81 5

..

The safety factor increases although it does not reflect the actual chassis, and it does give a

further indication where the chassis should be stiffened.

47. Double Click Displacement, the displacement has reduced from 122 mm to 110 mm

The value has reduced due to increased stiffness of the model by eliminating the radii of the

RHS members as expected.

Now, we will optimize the design in the next section to meet the design goals.

Optimization

Based on the above analyzes of the chassis, the results indicate that the design does not meet

the design criteria. To meet the design criteria we have two options.

1. Increase the thickness of the RHS members; however, this approach is not cost

effective when compared to option 2 or if already produced.


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2. To place a plate (suggest thickness 10mm) between the mounts and the RHS

members for a distance that can be determined from the FEA results above (see

picture below.)

If the chassis design is not built, a combination of options 1 and 2 can be used to manufac-

ture a more rigid chassis.

The following also illustrates another possible design for the new generation of trailer

chassis.

In, practice, additional loading scenarios are analyzed, for example, when the chassis has a torsional

load applied down its length. In this case, stress reversals are often present that need taking into

account when determining the final design of reinforcements to be made.

48. Close the file

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Chp 14- Design Problem 12