Bumper Beam Design and Materials Selection

Advanced Machine DesignME517

Spring 2016

Jie Jing & Drew De RuiterGroup 3

Abstract

In the traditional automobile industry, the cost is the major concern in terms of vehicle

design. However, in modern industry, the reduction of automobiles weight is more critical to

create a fuel-saving and cost-efficient vehicle market. Bumpers, as one of the important

structures on vehicles, play an important, safe role in protecting vehicles and drivers from

accident damage. Additionally, in the bumper system, the major part is bumper beam which

assumes the highest strength when impact occurs, and in this way focusing on the study of

bumper beam is a better way to design a fuel and cost effective system. The shape design and

materials selection of the bumper beam has gained a lot of attention on the market and current

studies. The traditional material applied on bumper beam is metal, based on their higher strength

and lower cost. However, the limitation of metal materials is their larger mass. In this way, fiber

reinforced polymers draw a lot of attention due to their great yield strength performance and less

weight consumption. In our project, we completed the structure design and finite element

analysis (FEA) and study about effects of material selection, the cross-section shape and

roundness design on bumper beam performance. Results showed a better performance on I shape

beams, and for each material the sheet molding compound (SMC) is our optimized material

because it is both cost and mass effective. Additionally, a rounder shape of curvature structure

provides a better mechanical performance.

1. Introduction

In today’s market the automotive industry is under more and more pressure to

manufacture a better product, but offer it at a lower price. There are many criteria by which the

consumer considers one automobile to be better than another, but one of the areas highest up on

their checklist is safety. Just like being better overall there are many ways one vehicle can be

safer than another.

However, the federal government does not require auto manufacturers to go through

much testing in order to be deemed safe to be on road. The National Highway Traffic Safety

Administration (NHTSA) is the government office that regulates safety standards for

automobiles that will be driven in the United States. The standard set by the NHTSA is that a

vehicles exterior safety device has to “prevent the damage to the car body and safety related

equipment at barrier impact speeds of 2 mph across the full width and 1 mph on the corners.”

There are many parts that make up the exterior safety devices of vehicle, but the most

known is the bumper. A bumper system is usually made up of four main parts; a bumper fascia,

energy absorber, bumper beam and bumper stay, as shown in Figure 1. The bumper fascia is the

outside covering of the bumper that you would see. The energy absorber is usually made of a

foam material that is designed to absorb low impact energy. Next, the bumper beam is the core

of the bumper, and is needed to absorb the energy from all other impacts that are too large for the

energy absorb to contain. Finally, the bumper stay is what attaches everything to the frame of

the vehicle. Since, the bumper beam is the most important part of a bumper system it will be the

focus of this project.

Fig. 1 Diagram of bumper system

Designing and manufacturing a bumper beam is a complicated process because there are

multiple factors that have to be considered. First, and most important, is has to be able to absorb

kinetic energy while staying intact upon impact. Weight has to be considered because the lighter

the bumper is the better fuel efficiency the vehicle will get; as well as lower operation and

maintenance costs. Next, the bumper beam has to have a simple manufacturing process because

cheaper manufacturing leads to a cheaper product. Which is our last parameter, cost. In order to

achieve all these criteria major variables to consider are material and bumper beam shape.

There are many different materials that can be used to make a bumper beam.

Traditionally, steels, aluminum, and magnesium have been preferred, but lately there has been

great success when using polymeric-based composites. The use of a polymer composite material

can be very beneficial because they can be easier to manufacture, and the weight of the beam can

be reduced by 30% without sacrificing any bending strength. Some of the leading composite

materials being used are sheet molding compound (SMC), and carbon fiber reinforced plastic

(CFRP).

The shape of the bumper beam also has a major impact on its efficiency, and there are

almost an infinite number of designs that could be used. However, in order to simplify this area

this project will focus on three different cross-sections: square, I-beam, and saddle or C-beam.

All of these designs will have the same cross sectional area, and have a curved length.

2. Objectives

After searching about current studies and markets on bumper beam design we think there

are many parameters of bumper beam that we can study to obtain an optimized design. So what

is the optimized design? From the knowledge we learned, we determined that a cost and mass

effective design is what we currently need on market, and is also what we want to investigate in

our project.

Materials selection of bumper beam is on a wide range. Based on the data in 2012,

approximately 83% were made by steel, 16% were aluminum, and less than 1% were

composites. So this gives us the idea on material selection. In our project, steel, SMC and carbon

fiber epoxy are our targeted materials based on their individual strength including lower cost,

higher yield strength, and lower density. Referring to the Edupack we acquired materials

characteristics including yield strength, cost, density and young’s modulus to help us with

calculations and analysis.

We want to analyze different beam designs by FEA because it can help us understand the

stress-strain performance and, in this way even though we don’t have the instrument for testing,

we can have a general understanding of beam performance from the result. In this way, we need

to prepare our 3D model and set the loading condition. Based on the federal requirements on the

force withstanding of bumper beam, we set 95 kN as the loading force. Loads were added onto

the front surface of beam and beam had two ends fixed.

Then we want to focus on the beam structure design. Actually, there are many parameters

we could alternate, such as the strength, the shape of cross section, thickness and curvature

structure. In order to analyze by FEA and also better applied our coursework knowledge onto our

project. We decide to alternate the cross section shape and keep the area the same. From our

course knowledge, the cross section shape does affect a lot on mechanical performance, such as

bending, buckling and torsion. So it might be a good try if we apply this idea in our project to

learn about their effects on bumper beam performance. We found many shape designs and

selected square beam, I beam and C beam as our targeted shape designs.

Besides these, we want to learn more to optimize the design. Although many studies

mentioned about the concept of roundness on bumper beam design but they didn’t pay much

attention on how it affects the performance. So in our project we want to investigate how the

roundness of the curvature structure influences the beam performance. Rounder structure has a

better performance or worse one.

In one word, our goal is to get an optimized design by altering these parameters and all of

the stress-strain analysis can be done by FEA. Most ideas of our project are from recent studies

and the understanding of current market. During the process we kept optimizing our thoughts

and ideas to make it more applicable and meaningful to a real case study.

3. Design Criteria

There has been a lot of research that has gone into the design of and materials used in

manufacturing bumper beams. Most of the research has been focused on answering the same

question as this project. What combination of material and design yields the most optimal

bumper beam?

Medium carbon steel was used for analysis since it is the market standard material used

for bumper beams. Steel has a very high yield strength, and very low cost compared to almost

any other material. However, it is also very heavy which is a major drawback when considering

the weight criteria.

It was found that one of the most common composite materials used to make bumper

beams is SMC. The reason SMC was chosen is because it is much lighter than traditional steel

while still maintaining the high strength that is needed in a bumper beam. It is also very easy to

manufacture with since it doesn’t have to be heated up to extreme temperatures like steel does.

This property also makes it easier on manufacturing tools by decreasing their wear rates, and it

becomes much safer for the works since they are not dealing with molten metals.

CFRP also shares many of the same traits of SMC, but with a great advantage in yield

strength, and lower weight. However, this comes at a major downfall of being much more

expensive. This material was used for analysis as a “price is not a concern” option.

When considering what cross section designs to use the three most come general shapes

were chosen. This area had to be simplified due to lack of time and CAD experience, so

additional, complication features (ie. strengthening ribs, indents, and corner flanges) were

excluded. Fortunately, the curved, flat faced design is representative of the effectiveness for the

different cross sectional shapes. After the initial data was analyzed, and the optimal material and

cross sectional shape found, that combination was analyzed again using FEA. This time the

beam was bent at a shaper angle, and used to determine if a more drastic curve is more or less

efficient.

4. Methods

4.1 Project proposal

The project was determined based on our knowledge strength and familiarity. We

communicated our thoughts and process by email during the whole project. After the project set

we started related searching. Jie decided the project outline and came up with the perspectives

and analytical methods we want to work on bumper beam design. Drew did some background

searching on material selection. Then we combined them together.

4.2 Project concept presentation

Based on the general idea in project proposal we did the concept presentation to help the

class understand the motivation, background information and ideas on our project. The

preparation and presentation part were distributed evenly. Drew worked on concept teaching on

bumper beam function, structure, design requirement and selection parts. Jie worked on the

teaching parts of available materials, bumper beam design study overview and how these ideas

applied into our project. From the study overview, we got the idea to evaluate the beam

performance from materials young’s modulus, yield strength and beam shape. By getting

materials information from Edupack and analyzing by FEA, they might give us the idea on an

optimized design.

4.3 Project process presentation

After a more detailed and concentrated searching, we started working on the process. As

before, we distribute our work on preparation and presentation. Jie did the bumper beam

structure design part. From several studies, Jie got the dimension of bumper beam and simplified

the bumper beam to a curvature structure. A 3D model of beam was completed with three

different cross sections (same area) and two different roundness. Drew did the material and load

selection. Drew got the federal requirement of the force an average car should withstand and also

calculate the yield strength for candidate materials. From the Edupack, Drew got characteristics

of five candidate materials as our material choices.

4.4 Final report

The rest work of our project was distributed. Drew finished the FEA on the model Jie

designed. And we got our optimized design after analyzing the strain-stress result from FEA and

the calculations on mass and cost. The composition of final report was distributed evenly.

5. Results

Upon uploading each cross section into Abaqus, and defining the properties for the

different materials a load of 95 kN (impact force of the average 1700kg car) was applied to the

front surface of the beam. Both ends of the beams had defined boundary conditions that

restricted their movement. The following figures show the results of the FEA.

Fig. 2 Stress Distribution of Steel Material in Square, I-beam, and C-beam Cross Sections

Fig. 3 Stress Distribution of SMC Material in Square, I-beam, and C-beam Cross Sections

Fig. 4 Stress Distribution of CFRP Material in Square, I-beam, and C-beam Cross Sections

From the figures it can be seen that stress is distributed very similarly for each material

when comparing the same cross section. Table 1 shows the max stress recorded from the FEA.

Even though the yield strengths for the three materials are quite different the max stresses for

each cross section are not significantly different; with none of them reaching the point of failure.

Table 1. Maximum Stress Recorded by FEA for Each Material/Cross Section Combination (kN/m2)

Square I-beam C-beamSteel 33.83 30.78 36.86SMC 34.22 30.11 37.05CFRP 33.96 30.58 36.92

This data also shows that the I-beam cross section was the most efficient because it

recorded the lowest maximum stress for every material. For this cross section it was also found

that SMC was the best material when solely looking at strength. However, there are more factors

to consider when deciding which material is the optimal choice. From the FEA it is only

determined that the I-beam cross section is the most optimal out of the three that were analyzed.

The other major factors that need to be considered are price and weight. Table 2 shows

these material properties for each of the materials used. Steel is by far the cheapest material with

a price between $0.263-$0.268 per pound; with CFRP being the most expensive with a price

between $17.00-$18.90 per pound. The exact opposite is true when comparing weights with

CFRP’s density being 93.6-99.9 lb/ft3, and steel’s being 487-493 lb/ft3. SMC is in the between

the other materials for both of these properties.

Table 2. Price and Density for Each MaterialPrice (USD/lb) Density (lb/ft3)

Steel 0.263-0.268 487-493SMC 1.49-1.75 112-125CFRP 17-18.9 93.6-99.9

When considering all of the main design factors deciding what material and cross section

combination is option is not straight forward. The results show that the I-beam cross section is

the most efficient, but choosing the material to make it out of cannot be seen. Each material has

their strong points and their weak points. All three of the materials can withstand the force

required by the NHTSA. Medium carbon steel is the cheapest material that was tested, but it was

also very heavy compared to the others. Carbon fiber reinforced plastic is the lightest, but the

most expensive by over $15/lb. Sheet molding compound was in the middle for both of these

factors because it was cheaper than CFRP and lighter than steel.

With these results there is no single material that is optimal for every situation. Each

manufacturer would have to decide what the value more; cheapest price, lowest weight, or

something in the middle on both.

6. Discussion

Some problems came out when we were dealing with the project.

The first problem for us is the design methods. What are the ideas to make our design

more applicable and practical? After a study overview, it gave us a lot of ideas and allows us to

focus on the design of materials selection, cross section and curvature roundness.

The second problem occurred when we continued the analysis on FEA. We didn’t know

how to apply composite properties on FEA. After that we obtained the knowledge from the

lectures. When we went back on our project we realized that the composites we used were not

lamina and also all of them are isotropic so finally we set them the same way as steel but the

differences are their values on young’s modulus, cost and density.

The third problem is the loading solution. Dr. P. Shrotriya gave us the advice on

considering the solution about a curved beam rather than straight beam. After both of us efforts

on searching we can’t find a matched solution on our situation. So the solution not changed at

this point.

Some development we can make is to make the model more complicated and closer to

the real case such as the addition of barrier in front of bumper beam to support the load; another

improvement we can make is to change the applied tensile strength into impact because this is

the load case the real beams withstand. Or maybe we can even do more analysis on materials

energy absorption because this is the major requirement for a bumper beam.

7. References

[1] Cheon, S. S., Choi, J. H., & Lee, D. G. (1995). Development of the composite bumper beam

for passenger cars. Composite Structures, 32, 491-499.

[2] Davoodi, M. M., Sapuan, S. M., & Yunus, R. (2008). Conceptual design of a polymer

composite automotive bumper energy absorber. Materials and Design, 29, 1447-1452.

[3] Davoodi, M. M., Sapuan, S. M., Ahmad, D., Ali, A., Khalina, A., & Jonoobi, M. (2010).

Mechanical properties of hybrid kenaf/glass reinforced epoxy composite for passenger car

bumper beam. Materials and Design, 31, 4927-4932.

[4] Davoodi, M. M., Sapuan, S. M., Ahmad, D., Aidy, A., Khalina, A., & Jonoobi, M. (2011).

Concept selection of car bumper beam with developed hybrid bio-composite material. Materials

and Design, 32, 4857-4865.

[5] Yedukondalu, G., Srinath, D. A., & Kumar, D. S. (2015). Crash Analysis of Car Cross

Member Bumper Beam. K L University.

[6] A. Hambali, S. M. Sapuan1, N. Ismail and Y. Nukman2, Scientific Research and Essay Vol.

4 (4) pp. 198-211

[7] J. Marzbanrad, M. Alijanpour, M. S. Kiasat, Thin-WalledStructures47(2009)902–911

[8] Maheshkumar V. Dange. Design and Analysis of an Automotive Front Bumper Beam for

Low-Speed Impact. www.iosrjournals.org

[9] Michael F. Ashby. Materials Selection in Mechanical design. Third edition 2005.

[10] D. Kim, H. Kim, H. Kim. Design optimization and manufacture of hybrid glass/carbon fiber

reinforced composite bumper beam for automobile vehicle. Composite Structures.

131(2015)742-752.

Acknowledgements

Prof. P. Shrotriya for his help teaching the concepts needed to complete this project

Iowa State University for the use of the software programs Abaqus for FEA and CES

Edupack for material properties.

Appendices

Dimensions and structure design were based on reference 7 and 8:

Loading force solutions were based on our textbook: