Download - Chp 14- Design Problem 12
-
8/9/2019 Chp 14- Design Problem 12
1/19
© 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.
14
-
8/9/2019 Chp 14- Design Problem 12
2/19
CHAPTER 14 Design Problem 12
18
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)
-
8/9/2019 Chp 14- Design Problem 12
3/19
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
-
8/9/2019 Chp 14- Design Problem 12
4/19
CHAPTER 14 Design Problem 12
20
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.
-
8/9/2019 Chp 14- Design Problem 12
5/19
CHAPTER 14Design Problem 1
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.
-
8/9/2019 Chp 14- Design Problem 12
6/19
-
8/9/2019 Chp 14- Design Problem 12
7/19
CHAPTER 14Design Problem 1
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
12. 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 4.1e4 * 2 for
Magnitude
-
8/9/2019 Chp 14- Design Problem 12
8/19
CHAPTER 14 Design Problem 12
24
13. Specify 0.7 to reduce size of force display Specify Reaction-load-2 for
Name Click OK
14. 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 2.9e4 * 2 for
Magnitude
15. Specify 0.7 to reduce size of force display Specify Reaction-load-3 for
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
-
8/9/2019 Chp 14- Design Problem 12
9/19
CHAPTER 14Design Problem 1
17. Specify 0.7 to reduce size of force display Specify Reaction-load-4 for
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.
-
8/9/2019 Chp 14- Design Problem 12
10/19
CHAPTER 14 Design Problem 12
26
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.
-
8/9/2019 Chp 14- Design Problem 12
11/19
CHAPTER 14Design Problem 1
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.
-
8/9/2019 Chp 14- Design Problem 12
12/19
CHAPTER 14 Design Problem 12
28
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)
-
8/9/2019 Chp 14- Design Problem 12
13/19
CHAPTER 14Design Problem 1
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
-
8/9/2019 Chp 14- Design Problem 12
14/19
CHAPTER 14 Design Problem 12
30
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
-
8/9/2019 Chp 14- Design Problem 12
15/19
-
8/9/2019 Chp 14- Design Problem 12
16/19
CHAPTER 14 Design Problem 12
32
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
-
8/9/2019 Chp 14- Design Problem 12
17/19
-
8/9/2019 Chp 14- Design Problem 12
18/19
CHAPTER 14 Design Problem 12
34
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.
-
8/9/2019 Chp 14- Design Problem 12
19/19
CHAPTER 14Design Problem 1
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