Article
Articles, Issue 47 - Winter 2012

Determining the Crashworthiness of a Lightweight Vehicle

In Issue 38, proActive published an article about Lotus Engineering Inc.’s development of a lightweight vehicle based on a Toyota Venza.

Exterior styling of Lightweight vehicle

Exterior styling of Lightweight vehicle

The project was born out of the ever brightening spotlight on fuel economy and emissions and its relationship to vehicle mass.

Phase one of the program was a great success, yielding a 38% mass reduction in non-powertrain systems with a cost increase of just 3%.

However, reducing the mass of a vehicle is only one piece of the puzzle. Vehicular safety is of great concern as well, with several safety standards becoming more stringent and more automakers debuting advanced active collision avoidance systems.

So what happens when the Lotus lightweight structure actually crashes? It may be a surprise to the public, but it turns out that despite the reduced mass, the structure actually fares as well as, if not better than the Venza it’s based on.

Review

California Air Resources Board (CARB) commissioned Lotus Engineering to design a mass reduced crossover vehicle based on the 2009 Toyota Venza with the intent of highlighting reducing mass as a way to increase fuel economy and reduce emissions.

The results showed that it is possible to reduce the mass of a common crossover utility vehicle (CUV) by 38% with only a 3% cost increase by using a holistic approach to vehicle design.

This mass reduction study was continued in Phase two with crash test analyses and a further cost study.

Methodology

Crash analyses started with a topology analysis based on derived suspension loads (using 50% bending and 50% torsional loading) of an FEA model of the lightweight body in white (BIW) developed in the Phase one study.

From Lotus’ experience, this leads to lighter weight solutions due to material selection, than immediately applying crash loads. Three topology analyses were conducted on single material BIWs using aluminium, magnesium, and steel.  The results of these analyses, displaying relative material energy strain densities, are shown in the Topology Results figure. Red indicates areas in which the peak material strain levels were exceeded and green indicates areas well below the threshold.

These results were then used in conjunction with mass, cost, and recyclability concerns to further optimise the material selection of the Phase one model for crash modeling.

A variety of manufacturing processes were considered including casting, extruding, injection molding, hydroforming, electromagnetic forming and stamping, as well as variants, e.g. brake bent, welded extrusions. The processes were selected based on their contribution to reducing mass and cost.

Joining processes are also integral to building a BIW as they affect the weight, durability, strength, and cost of the BIW. Ultimately, an adhesively bonded and friction spot joined (FSJ) body was selected. For dissimilar metals where galvanic corrosion was of concern, rivets and mechanical fasteners were used.

Modelling results

In total, over 500 design refinements were made to the original Phase one model across 26 different versions as testing continued. This yielded what is predicted to be one of the stiffest vehicles on the market with a torsional stiffness of 32,900 Nm/degree and a BIW that is predicted to perform as well as, if not better than the baseline Toyota Venza in every crash category. Model V26 predicts the lightweight BIW meets or exceeds every load case defined by the customer CARB. Due to space limitations, not every crash test result is reviewed in this article, but the crash tests required by CARB were as follows:

Front Impact:

  • FMVSS 208 35 mph Flat Barrier 0°
  • FMVSS 208 25 mph Flat Barrier 30°
  • FMVSS 208 25 mph 40% Offset Deformable Barrier
  • IIHS 6 mph Centerline Bumper
  • IIHS 3 mph 15% Offset Bumper

Side Impact:

  • FMVSS 214 33.5 mph 27° Moving Deformable Barrier
  • FMVSS 214 20 mph 75° Pole Impact (seat at 5th percentile Female)
  • FMVSS 214 20 mph 75° Pole Impact (seat at 50th percentile Male)

Rear Impact:

  • FMVSS 301 50 mph 70% Offset Moving Deformable Barrier
  • IIHS 6 mph Centerline Bumper
  • IIHS 3 mph 15% Offset Bumper

Roof Crush:

  • FMVSS 216 Quasi Static Crush

Other:

  • FMVSS 210 Quasi Static Seat Belt Pull
  • FMVSS 213 Child Restraints Systems

FMVSS 208

FMVSS 208 represents a variety of frontal collisions and the particular FMVSS 208 test shown here is a straight-on test with no offset. A maximum of 10% less than the Toyota Venza, which recorded a peak deceleration of 50 g-force in this test, result was used as the baseline. This translates to a maximum deceleration of 45 g-force allowed. The results of this test are shown below along with the baseline Venza.

After the initial crash, Lotus and TRW, a tier 1 safety restraint system supplier, examined the data and determined the crash pulse was conventional in the automotive industry and would not require special tuning of the restraint systems. Lotus further refined the BIW after it performed a sensitivity analysis and determined that reducing the material thickness by 10% would yield a 30% reduction in acceleration levels during the first 30 ms of the impact.

V26 Front of dash intrusion displacements FMVSS 207 at 35 mph against a flat barrier at zero degrees

V26 Front of dash intrusion displacements FMVSS 207 at 35 mph against a flat barrier at zero degrees

Also cause for concern are intrusions into the passenger compartment. In this regard, the lightweight BIW also fares well with a maximum dash intrusion of just 20 mm in the upper dash. Maximum dash intrusion in the passenger compartment footwell was less than 10 mm. A diagram of the dash intrusion levels can be found below.

FMVSS 208 (35 mph flat barrier 0 degree)

FMVSS 208 (35 mph flat barrier 0 degree) [Click image to view animation]

FMVSS 214

This U.S. federal crash test involves a side impact with a deformable barrier and a pole at two specified locations – one where the pole impacts the seating location of a 5th percentile female and the second impact location is where a 95th percentile male would be seated. The two pole locations ensure that the vast majority of drivers will be safe in the event of a side impact. The case discussed here involves a 75-degree impact with the pole located at the 5th percentile female location. The diagram below shows the pole location relative to the vehicle.

V26 5th percentile female side impact FMVSS 214 at 20 mph and 75 degree pole

V26 5th percentile female side impact FMVSS 214 at 20 mph and 75 degree pole

The ultimate measurement criteria for this side impact test are intrusion levels into the passenger compartment. Intrusion levels are shown in the figure below, with a maximum intrusion level of 160 mm at the door beltline and 142 mm at the driver’s pelvis. For reference, the seats are 300 mm inboard of the zero reference of the pole, indicating the seats were still 140 mm away from the maximum intrusion.

FMVSS 214 (20 mph 75 degree pole)

FMVSS 214 (20 mph 75 degree pole) [Click image to view animation]

FMVSS 216

FMVSS 216 is a roof crush test designed to ensure occupant safety in the event of a rollover accident, whereby the BIW is required to protect the occupant headform envelope at three times the vehicle curb weight. The Insurance Institute for Highway Safety (IIHS), an independent safety organisation in the U.S., requires vehicles to withstand four times the vehicle curb weight, which was the target for the lightweight body. The figure below shows the setup of the plate for the roof-crush test.

V26 FMVSS 216 quasi static roof crush

V26 FMVSS 216 quasi static roof crush [Click image to view animation]

The lightweight body exceeded all requirements – both FMVSS and IIHS, by withstanding six times the vehicle curb weight . One of the key factors in obtaining such performance is the high strength steel B-pillar, which is the main load-bearing structure under the plate.

FMVSS 301

In order to provide a basic, 360-degree view of the safety of the lightweight vehicle, the last standard reviewed here is a rear impact to test the integrity of the fuel storage and delivery system. However, designing all the ancillary components of a full fuel delivery system were beyond the scope of this vehicle, so only the fuel tank and filler neck were tested.

V26 FMVSS 301 model setup

V26 FMVSS 301 model setup [Click image to view animation]

With a maximum plastic strain of 10 percent in the fuel storage system, the results indicate there should be no rupturing of the tank or filler neck. The figure below details the plastic strain found and the locations.

V26 FMVSS 301 fuel tank strain

V26 FMVSS 301 fuel tank strain

Changes

Derivations of the body were made as testing analysis progressed, ultimately yielding numerous material and material gauge changes to optimise the crash load paths. The table below details the changes made from Phase one to Phase two:

Summary of Changes from Phase one HD to Phase two HD

Body Subsystem Venza (kg) Phase one HD (kg) Phase two HD (kg) Phase two Material Shift Reason for Change
Underbody/Floor 113.7 83.8 92.7 Mix to mostly aluminium Manufacturing
Front structure and radiator crossmember 25.2 18.6 17.1 Magnesium to aluminium Frontal impact, FMVSS 208
Body-side A-pillar 18.2 12.8 9.1 Magnesium to aluminium Roof crush, frontal impact
Body-side B-pillar 37.19 17.13 17.48 Magnesium to aluminium and HSS Roof crush, side impact
C-pillar 12.8 10.2 3.5 Magnesium to steel and aluminium Roof crush, side impact
Roof 27.8 16.8 16.9 Magnesium to aluminium Roof crush

Magnesium was used extensively in the front structure, roof, and A-pillar design of the Phase one HD design, but the metal proved too brittle to meet crash standards. The validated Phase two HD model uses primarily aluminium for all these structures. The lower A-pillar inner however, is integrated into the magnesium dash casting and the move to an aluminium A-pillar allowed for an increase in cross-sectional area to stiffen the body and increase torsional stiffness. Changes were made to the C-pillar design as well, moving from a magnesium structure to an aluminium and steel structure for the same reasons as the A-pillar.

Cost analysis

As part of this study, a much more detailed cost analysis was conducted – including developing a manufacturing plant and costs associated with this assembly process. The cost analysis was broken down into the following categories: piece cost, assembly, tooling, paint, and NVH materials.

With the aid of EBZ Engineering (a manufacturing plant engineering firm who has worked with companies such as Porsche and Audi) and Intellicosting (a manufacturing and piece cost estimation company whose clients include many of the world’s large automotive manufacturers), Lotus developed accurate full vehicle cost estimations. The assembly plant, with the exception of the building, was designed from scratch for this assembly process and the assembly costs based on this; tooling costs were based on low-volume tools; paint and NVH materials were derived from the Toyota Venza benchmark and held constant across both vehicles; and piece costs were derived based on CAD models of the BIW. In addition to determining the total cost of producing the vehicle, a variety of amortisation schedules were looked at and are broken out in the table below. The ‘Recommended Schedule’ includes amortising all investment over five years with the exception of the coordinate measuring machine (CMM), which is amortised over seven years. The three and five year schedules are straight amortisation over those time periods.

Fully Amortised BIW Costs

Category Recommended 3 Year  5 Year 
Piece cost USD 1,930 USD 1,930 USD 1,930
Capital costs USD 184 USD 301 USD 186
Labor USD 108 USD 108 USD 108
Tooling cost USD 156 USD 156 USD 94
Utilities USD 49 USD 49 USD 49
Interest USD 42 USD 42 USD 42
Freight USD 25 USD 25 USD 25
SG&A USD 24 USD 24 USD 24
Assembly Labor USD 432 USD 432 USD 432
Paint USD 540 USD 540 USD 540
NVH USD 39 USD 39 USD 39
BIW Total USD 3,529  USD 3,646  USD 3,469 

The estimated cost breakdown to produce a Toyota Venza is USD 15,878 broken down as follows:

Baseline Vehicle Cost Breakdown

Area/System Mass (kg) Cost (%) Cost (USD )
Body 382.5 18 2858
Closures/Fenders 143.02 10 1588
Bumpers 17.95 2 318
Thermal 9.25 1 159
Electrical 23.6 7 1111
Interior 250.6 22 3493
Lighting 9.9 1 159
Suspension/Chassis 378.9 13 2064
Glazing 43.71 3 476
Miscellaneous 30.1 0 0
Powertrain 410.16 23 3652
Total (excl. powertrain) 1290 12,226
Total (incl. powertrain) 1700 15,878

As shown by the table, the BIW itself is more expensive than the Toyota Venza, but due to the reduction in parts counts and cost savings elsewhere, the whole vehicle is only USD 130 more carrying costs from the Phase 1 study over as well. The Phase 1 cost breakdown is found below.

 Phase one Cost Breakdown

Area/System Mass (kg) Mass rem. (kg) Cost ratio (%) Cost (USD ) Cost Difference (USD )
Body 221.1 161.4 135 3858 1000
Closures/ Fenders 84 59 76 1207 -381
Bumpers 16 2 103 327 9
Thermal 9.3 0 100 159 0
Electrical 15 8.6 96 1067 -44
Interior 152.8 97.8 96 3354 -139
Lighting 9.9 0 100 159 0
Suspension/ Chassis 217 161.9 95 1961 -103
Glazing 43.7 0 100 476 0
Misc. 22.9 7.2 99 0 0
Powertrain 356.2 54
Total (excl. powertrain) 792 498 103 12,568 342
Total (incl. powertrain) 1148

Substituting USD 3,646, the cost of the three year amortised Phase two body, into the Phase one table results in an estimated total vehicle plus cost of USD 130 vs. the estimated baseline cost. Substituting the fully amortised Phase 2 BIW cost of USD 3,345 into Table 5 results in an estimated total vehicle savings of USD 171 vs. the estimated baseline cost. These figures do not include the powertrain cost.

Conclusion

In conclusion, mass reduction is one of the best ways to begin to reduce emissions and increase fuel economy as these two items come under increasing scrutiny. But by using a holistic approach to vehicle engineering, reducing mass does not mean giving anything up in vehicular safety.

Writer: Andrew Peterson

Related Post

Lotus Engineering adds Lightness to a Crossover Utility Vehicle (lotusproactive.wordpress.com)

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About lotusproactive

Lotus proActive is an e-magazine published quarterly by Lotus Engineering, covering engineering articles, industry news and articles from within Group Lotus (Cars, Engineering, Originals and Racing).

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