Good Energy Absorbing Properties Engineering Essay

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This study presents an investigation of anti-whiplash features that can be implemented in a car seat to reduce whiplash injuries in case of a rear impact. The main emphasis is to design a car seat damper with good energy absorbing properties.

2. Aim

To design cost effective variable coefficient car seat damper having damping profile matching to the profile as developed by Himmotoglu et al (2007).

3. Introduction

Road traffic accidents frequently result into human neck injuries. The term whiplash is used to refer these kind of injuries in which the abrupt differential movement between the head torso causes hyper-extension and hyper-flexion of the soft neck tissues, as the body experiences sudden and forceful jerk during the rear end car accidents; and often result in damage of neck or spine tissues depending on severity of impact.

The term "whiplash-associated disorder" is used to describe the clinical symptoms of whiplash injury. The Québec Task Force on Whiplash-Associated Disorders describes these entities thus:

"Whiplash is an acceleration-deceleration mechanism of energy transfer to the neck. It may result from rear end (sic) or side-impact motor vehicle collisions, but can also occur during diving or other mishaps. The impact may result in bony or soft-tissue injuries (whiplash injury), which may lead to a variety of clinical manifestations (Whiplash-Associated Disorders)." (Spitzer et al., 1995)

Approximately 30% of all the traffic accidents are whiplash type neck distortions, and frequency of such accidents has increased over past two decades. (Richter et al.,2000). An American Institute from the year 2000 indicates an incidence of approximately 900,000 such injuries annually, which is 3 per 100 inhabitants (Quinlan et al., 2004).Other authors report an annual incidence of 1-4 per 100 inhabitants (Barnsley et al.1995; Sterner et al.,2003).Severity of Whiplash phenomenon is usually measured in terms of maximum Neck Injury Criterion (NICmax) (Böstrom et al.,1996,97,2000).

4. Identification of need

Though improving car seat head restraint geometry is the first step in reducing injury risk in case of rear impact, research has shown that seats with good head restraint geometry do not always offer good protection dynamically (Himmetoglu et al., 2007). If the seat is not properly designed, the occupant can deflect the seat back and head restraint unfavourably.

Moreover, seatbacks with strong structural cross members do not allow the occupant to sink into the seat back; this hinders energy absorption and acts against reducing the backset between the head and head restraint. This kind of situation leads him to S-shape deformation in the neck. A strong rebound of the seatback can also exacerbate injury, especially in higher severity car impacts. These problems can be overcome by designing a car seat damper that provides good energy absorption and/or early head support and reduces head to head restraint contact time as recommended by International insurance whiplash protection group (IIWPG, 2003).

5. Literature Review

5.1 Seat design developments

The first vehicle equipped with a safety system specifically aimed at reducing whiplash associated injury was the SAAB. The aim of the Viano and Olsen (2001) study was to evaluate the field performance of the Saab Active Head Restraint (SAHR) in reducing whiplash in rear crashes. Comparisons were made of single-event rear-end crashes (refer figure 1) involving Saab 9-5/9-3 equipped with SAHR and Saab 9000/900 fitted with standard head restraints, over a period of 18 months.

The design of the SAHR aims to ensure a horizontal trajectory of the head restraint, to lower the loads in an occupant's neck during rear impact. The seat modifications introduced with the SAHR system also addressed lower back injury risk. The seat provides uniform support of the spine by removing the stiff cross-seat structures adjacent to the thoracic and lumbar spine. In the field, the SAHR reduced whiplash injury risks by 75% (±11%): An 18% (±5%) incidence in 85 occupants with standard head restraints to 4% (±3%) in 92 occupants with SAHRs (CASR report series, 2006). No SAHR-fitted seats required repair or replacement after the crashes.

The SAHR was found to be effective in reducing the incidence of medium to long term whiplash-associated disorders in a sample of rear crashes in Sweden.

Recent research has investigated the effectiveness of these new head restraint and seat designs in reducing neck injury in rear impacts. Insurance Institute for Highway Safety study was based on the claims data supplied by three of the major US insurance companies, Nationwide, Progressive, and State Farm, (Farmer et al., 2003). Three different seats and head restraint design approaches were studied:

Improved geometry - To allow the head restraint to be positioned closer to most occupants' heads. Ford adopted this principle in their Ford Taurus and Mercury Sable models between 2000 and 2002.

Active head restraint - To allow the occupant's torso to sink back into the seat during a rear-end crash, and engage a mechanism in the seat back, which pushes the head restraint up and toward the back of the head. This design was adopted by Saab in 1997 (Viano and Olsen 2001) and in some General Motors and Nissan models.

Yielding seat back - To reduce the forward acceleration of the torso in rear-end crashes. The Volvo WHIPS seat design includes a specially designed hinge below the seat back, which allows rearward movement to reduce forward acceleration, without collapse of the seat (Lundell et al. 1998). The Toyota and Lexus whiplash injury lessening (WIL) system allows an occupant to sink farther into the seat back during a rear impact (Sekizuka 1998).

Overall, neck injury claims were reduced, with the benefits greater for women than for men. A 49% reduction was seen in claims for the Volvo S70 compared with similar cars before the WHIPS design was introduced. There was also a 43% reduction in neck injury claim rates for the Saab, General Motors and Nissan models with the active head restraints and an 18% reduction in Ford models with improved geometry. The Toyota WIL system did not show any reduction in neck injuries.

Though wide range of investigation is being carried out on car seat features related to Whiplash phenomenon especially on improving the head restraint geometry and the overall seat back features, hardly any articles have been found showing the evidence of investigatory work about whiplash mitigation damper system integrated to car seat.

6. Project background:

This project is based on the work carried out by S.Himmetoglu in his Ph.D. thesis titled "Car seat design strategies to reduce whiplash injuries" completed in 2007. The damping and stiffness profiles for anti-whiplash devices (AWDs) obtained after testing and validation of actual whiplash conditions by Himmetoglu largely forms the basis of the project.

The thesis provides an insight of the subject including preliminary research, testing and validation of actual whiplash conditions using Japanese automobile research Institute (JARI)(refer figure 2) sled tests and biofidelic 50th % male multi-body human model, whiplash injury mechanism, affecting factors and the evaluation of injuries. The theoretical information has been supported with the experimental analysis carrying out a series of multi-body simulations to study the effects of car seat design to mitigate whiplash injuries.

1. Sled crash test using BIORID dummy used in SAAB 2. JARI sled test

Himmetoglu also developed a generic multi-body car seat model to implement various feedback and recliner properties, AWDs, and head restraints. Using the same driving posture and the rigid seat (Lundell et al,1998) in the JARI sled tests as the basic consultation, several anti-whiplash seats are designed to allow different types of motion for the seat back and seat pan as shown in figure 3.

3. Anti-whiplash car seat design concepts (HR: head restraint, SB: Seatback, SP: seat-pan, OF: outer seatback frame, P: translational AWD, R &R*: rotational AWD) developed by Himmetoglu, 2007.

Though not much difference among the performance of last four anti-whiplash seat design concepts has been observed regarding their responses to the IIWPG standard pulse, it is concluded by that seats with inner seatback frame have some advantages over the ones without the inner seatback frame. DRFWMS performs the best as it does not let the head rise over the head restraint and also lowers the position of head relative to the vehicle floor. This offers good perception for the tall and unbelted occupants during rear impact.

Therefore DRFWMS concept has been selected for further development especially for design of damper to reduce rearward displacement of seatback minimizing head to head restraint contact time. The damping and stiffness profile obtained by Himmetoglu while studying Whiplash phenomenon are in figure 4.

img013

4. Damping and stiffness profile for translational AWD in DRFWMS

Further development of DRFWMS concept especially represents development of translation AWD.It involves finding an appropriate material, which exhibits similar profiles for damping and stiffness as well as designing a variable coefficient damper using the same material. Thus work done by Himmetoglu et.al 2007 as discussed above forms the base for the project.

7. PRODIUCT DSEIGN SPECIFICATION:

7.1 Performance:

Damper System must allow controlled rearward displacement of whole passenger seat by absorbing crash energy.

Seat Displacement must follow the force-displacement profile developed by S.Himmetoglu as closely as possible, in his PhD thesis, Car seat design strategies to reduce whiplash injuries.

Maximum rearward displacement of seat at medium impacts (∆V=16KPh; as specified by S.Himmetoglu et al., 2007) must not exceed 6 cm.

The damping system must be activated when any of the following condition satisfies.

∆V>10.5KPh

Breakaway force(force exerted by torso of the occupant on the seatback) ≥ 4KN

The passenger seat retraction after impact must be as quick as possible.

Damper unit must sustain maximum velocity of impact of 35Km/h( Linder et al,2001a).

7.2 Size:

Damper system must fit within current average car seat dimensions.

7.3 Material:

The material selected for the damper application must be such that it should provide necessary damping profile with minimum weight and cost and without the need of any type of actuators or controllers.

7.4 Environmental:

The damper system must work within the temperature range of -200C to 1000C.

The damper system must not produce any kind of hazardous gases, fumes or aerosols as a part of operation.

7.5 Cost:

The market retail price of the damper unit must not exceed £ 20.

7.6 Installation:

The damper unit must be build such that minimum or no modifications in the present design of car seat is necessary.

The damper unit must be easy to install and if possible modular to suit the requirements of different types of car seats.

7.7 Service Life:

The service life of the damper unit must be nothing less than 20 years.

7.8 Aesthetics:

The damper unit must be aesthetic in overall appearance and suit the seat styling.

7.9 Reliability:

The damper system must provide optimum performance at all impact severities.

7.10 Weight:

Weight of the damper unit must not exceed 5Kg.

7.11 Maintenance:

Damper unit must be able to work with minimum monthly maintenance.

8. Design:

8.1 Concept Selection:

8.1.1 Types of damper systems considered:

Magneto Rheological Damper

Pneumatic two way cylinder type damper

Piston cylinder damper with oil as working fluid producing damping force and spring providing necessary stiffness

Rubber Damper

Table (1) represents 4 different types of damper systems considered and evaluated against criteria as mentioned in Product Design Specification.

8.2 Material selection:

Elastomers are the first choice as a damper material since some of this family provide an exponential force-deflection curve profile (refer figure 4), exactly as required in this application (ASTM Handbook of elastomers, 2008). Secondly the overall cost using elastomers i.e. machining cost, tooling cost, maintenance cost, fabrication cost is negligible compared to that using pure metals or metal-composite matrix. Thirdly the weight of the damper system should not be a burden to car, resulting in increase in gross weight of the car by considerable amount; in this context elastomers are the perfect pick for this application.

A number of elastomers are considered including basic elastomers as well as commercially available and pretested one. Table (2), shows the type of elastomers selected for comparison based on the criteria of desirable properties and feasibility in the operation.

The material properties necessary in this application are enlisted as below.

8.2.1 Material properties required:

Excellent damping properties are required. Ideally force deflection characteristics of the material should follow the profile has developed by S.Himmetoglu et. al., 2007.

Material should have good compressive and shear strength.

Material should be able to withstand wide temperature range of -220C to 700C.

Material should have good wear and aberration existence.

Material should have good resistance to aging and weathering i.e. UV.

Damper Types/Concepts

Cost

Manufacturing cost

7

7

7

8

Maintenance cost

5

5

5

9

Weight

4

6

7

9

Performance

8

8

8

9

Ease in obtaining required damping profile

7

8

7

10

Feasibility for use in car

7

8

8

10

Tooling/machining

5

6

6

9

Modifications required in car seat to accommodate damper system

5

5

7

8

Space Requirement

3

5

7

8

Accessories required for operation e.g. prime mover, tubes, controller etc.

3

3

7

10

Ease of installation

4

5

7

8

Safety

6

7

7

8

Fixtures/mountings needed

3

5

7

9

Life in service

7

7

6

9

Reliability

8

8

7

9

Disposal

6

7

7

8

Shipping

4

6

7

8

Size

4

6

7

8

Product cost

4

6

8

10

Total

98

118

132

167

Table 1: Concept Evaluation- Damper Concepts evaluated against Product design specification

Table 2: Properties and characteristics of elastomeric materials considered for damper application

Elastomer Characteristics

Natural Rubber (Latex foam)

Silicone Rubber

Polyurethane rubber

Neoprene rubber

Natural + Synthetic rubber (30%+70%)

(Polypropylene Diethene)

Cost/ Kg

1.2 US $

16US $

13US $

15 US $

12 US $

Mechanical Properties

Hardness

10 points(Shore D)

35 points(shore D)

50 points(shore D)

15 points(Shore D)

45 points (Shore D)

Density

920 kg/m3

1290 kg/m3

1200Kg/m3

960 kg/m3

1500 Kg/m3

Tensile yield strength

21MPa

2.5MPa

44.1MPa

0.52MPa

18.2 MPa

Compressive strength

30MPa

10 MPa

57.9MPa

18 MPa

35 MPa

Shear modulus

0.0007 GPa

0.01 GPa

0.008 GPa

0.0008 GPa

0.01 GPa

Poisson's Ratio

0.5

0.48

0.48

0.48

0.5

Stiffness to weight ratio

0.00217 MN-m/kg

0.00504 MN-m/kg

0.00208 MN-m/kg

0.0021 MN-m/kg

0.0023 MN-m/kg

Strength to weight ratio

22.8 KN-m/Kg

1.94 KN-m/Kg

36.8 kN-m/kg

20.57 KN-m/Kg

23KN-m/Kg

Glass transition temperature

(-20°C)

(-127°C)

(-31°C)

(-20°C)

(-40°C)

Maximum Working temperature

70 °C

200 °C

80°C

93.3°C

70 °C

Elongation at break

650%

180%

390%

125%

510%

Rupture work

150 MJ/m3

4.5 MJ/m3

172 MJ/m3

150 MJ/m3

150 MJ/m3

Resilience

5250 kJ/m3

192 kJ/m3

8820 kJ/m3

5300 kJ/m3

5250 kJ/m3

Shelf Life

up to 20 years

up to 25 years

up to 25 years

up to 20 years

up to 30 years

Source: Handbook of Speciality Elastomers by Robert C. Klingender, ASTM, 2008

Note 1: Material properties shown in above table follow ASTM D 2000 and SAE J200 standard norms for testing of elastomers.

Note 2: Highlighted columns show desirable properties in selection of rubber for damper application

8.3 Biomechanics and guidelines:

Though several injury mechanisms have been suggested by different researchers, the injury mechanism suggested by S.Himmetoglu, 2007 supports the purpose behind this project. In order to be able to know what engineering efforts to make, the accident experience and the results of all realistic injury mechanism research need to be condensed. An effort to do this presented in the following three guidelines. The guidelines summarize a holistic approach to the whiplash problem, aiming to address all existing theories and cover all possible situations. The four guidelines are:

Reduce head to head restraint contact time.

Reduce occupant acceleration.

Minimize relative movements between adjacent vertebrae and in the occipital joint, i.e. the curvature of the spine shall change as little as possible during the impact.

Minimize the forward rebound into the seat belt.

8.4 Mechanism for damper:

8.4.1 Basis for the mechanism:

When car experiences a sudden impact from rear side, the car seat is accelerated forwards, though torso of the occupant, due to its inertia, resists to change its original position and as a result exerts a force on the seatback much higher in magnitude i.e. approximately five times the sitting force for standard mass of 80Kg in normal conditions.

Thus when torso of the occupant exerts a force of approximately 4KN on the seatback, the backset starts moving rearwards, due to which the time of contact between the head and the head restraint of the car seat occupant increases ,resulting in hyper flexion and hyperextension of neck, which ultimately gives rise to whiplash injury.

8.4.2 Description of the mechanism:

As shown in the figure 3.,the damper mechanism consists of polypropylene diethene rubber damper block(1), fixture for the rubber block welded to the chassis of the car(2), rectangular slot of 6cm of depth with the inner guideway(3) ,a slider sliding in and out within the slot to compress the rubber block to provide necessary damping force(4) ,a rectangular mate to side in the slot provided in the back of the seatback(5) and the fasteners(6) to locate rubber block on the fixture firmly.

(1)

(2)

(4)

(3)

(5)

(6)

(6)

5. Mechanism for rubber damper

8.4.3 Working of the mechanism:

When the seatback starts moving rearwards, the rectangular mate in integral with the seatback also starts moving rearwards and in turn compresses the rubber block which provides necessary damping force. The rubber block absorbs the crash energy experienced by seatback, helping seatback to move forward and minimises the time of contact between head and the head restraint of the occupant.

The rectangular slot of 6cm is provided to ensure that slider within the slot will have translation distance equal to 6 cm as per the requirement mentioned in product design specification. The slider is having recesses along the potion that seats in the slot and the slot is provided with the mating wedge profile along its length to facilitate ease of translator motion of slider along the slot as used in case of drawers. The working drawing of the above mechanism is attached as an appendix.

9. Scheme Calculations:

Using Newton's Second Law of Motion, F=mÃ-a, in this application where, F=Impact force in N; m= gross weight of vehicle in Kg; a=Maximum Acceleration/deceleration during impact in m/s2.

9.1 Assumptions:

It is assumed that average gross weight of most of the compact cars is 1200kg.

Though in today's cars, rear bumpers are provided to absorb most of the shock, it is assumed that the rear bumper absorbs only half the total impact force. This is for the reason that designing rubber damper for the impact force more than expected to be experienced, will cover the dynamic factors as well as aberrant changes due to acceleration of car during impact and will ensure passenger safety.

The maximum acceleration is assumed to be 16g to consider bumping and bouncing in worst case, confirming the Product design specification requirement.

The basic calculations considering above assumptions are divided in two parts as shown below.

9.1.1 Part 1:

Total Rear Impact Force = Mass of the car Ã- Maximum acceleration

= 1200Ã-16Ã-9.81=188.35 KN

As per assumption rear bumpers are capable of absorbing only half the total impact force, remaining half force is transmitted to chassis to seat which is then absorbed by the rubber damper.

Thus, Rubber damper experiences the impact force equal to (1/2Ã-188.35KN) = 94.17KN ≈ 95KN-(1)

The material chosen for the damper application is a mixture of natural and synthetic rubber, for which compressive stress= 35MPa.Thus to determine dimensions of rubber, considering mechanics of rubber, compressive stress= (Force)/(Area of c/s of rubber damper block).

Thus, 35= (95Ã-103)/AC-S gives Ac-s=2714.28 mm2--- (2)

The shape of rubber damper block which gives damping profile as determined by S.Himmetoglu, is found to be annular cylinder with outer diameter of cylinder =(1/2) inner diameter of cylinder (Handbook of speciality elastomers,2008).

h

t

d

D

6. Shape profile for rubber block-annular cylinder

The c/s area of rubber damper block is annular, i.e. Ac-s= (Ï€/4) (D2-d2) --- (3), where, D= outer diameter of annular rubber damper (mm), d=inner diameter of annular rubber (mm).

From (2) and (3), 2714.28=(π/4)(D2-(D/2)2)= (π/4)((3/4)D2)= (π/16)D2 which gives, D=117.57 mm =4.629"≈4.8"----(4)

Thus d= 58.78mm=2.314" ≈2.4"----- (5)

Thickness of rubber damper block= (D-d)/2=13.75mm= 1.2"----- (6)

Height of the rubber damper block is assumed to be 10 cm. Thus, h≈ 4 ----- (7)

The exact height is to be determined from further experiments and testing.

9.1.2 Part 2:

This part gives the general idea about total weight of the damper system.

Mass of the rubber damper block is calculated as below.

Mass= Volume of the rubber blockÃ- density of rubber block= AC-SÃ- thickness of rubber blockÃ- height of rubber blockÃ- density of rubber block material

Thus, mass of rubber block (Kg) = (2714.28mm2Ã-100mm)Ã- (10-3)3m3Ã-1500(Kg/m3) ≈0.5 Kg----(8)

Therefore assuming the mass of fixture for rubber damper block, stainless steel hinges for operation of mechanism, fasteners; the total mass of the system would be approximately 5 Kg.

10. Future work:

10.1 Manufacturing of rubber block:

Rubber block should be incurred from speciality rubber manufacturing company .A speciality Rubber Manufacturing Company named; 'Quingdao Jier Engineering Rubber Co.Ltd.' has been contacted with the size and specification of rubber and requested for quotation. The above named company mainly manufactures rubber dock fenders available in variety of shapes and sizes. The shape of the rubber block proposed in damper application is exactly similar to one of the products of the company i.e. cylindrical rubber dock fender, which would be ideal for absorbing crash energy. Also this rubber is pretested based on industrial standards, and therefore may save time and cost for testing the rubber block in the lab.

10.2 Mechanism:

The fixture, slider and rectangular mate should be dimensioned using the strength and bending calculations. These parts then can be manufactured in mechanical engineering department workshop .The mechanism after assembly should be tested in actual situation by installing it to sled used in JARI sled test which represents the model to study actual whiplash phenomenon. This is further discussed in section 10.4.

10.3 Modification in car seat:

Modification in the seatback of car seat is very much necessary to incorporate the rubber damper mechanism. In general, compact cars e.g. Kia Picanto, ,Toyota Yaris etc. are considered in the project for the design of damper, though for different cars different types of seta may require less or moderate modifications in the original design.

10.4 Testing and Validation:

The proposed rubber damper unit should be incorporated in the car seat (in this case to the sled) and tested dynamically under actual whiplash conditions with acceleration of 16g and maximum speed of 35Km/h. The result obtained in terms of tensile force, shear force acting on neck as well as NICmax(Neck injury criterion value in m2/s2 ) which decides severity if whiplash phenomenon will then be compared and validated using results obtained by Himmetoglu et.al,2007 for DRWFMS car seat concept as proposed in Ph.D. thesis, Car Seat Design Strategies to reduce whiplash injuries.

10.5 Time plan for remaining project activities:

Figure 7 represents the time plan for the remaining project activities considering the examinations and available time as well as deadline for submission of thesis.

Damper Project.gif

7. Time plan for remaining project activities

10.6 Scope:

As stated in the Product design specification, the attempt will be made to modularize the damper unit to fit to any car seat, so that it can be easily installed in car seats presently available in the market without any need of internal modification. The basic damper design, its testing and validation using IIWPG pulse as per industry standards would be followed by designing and manufacturing the modular damper unit if time permits.

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