Light-Duty Truck Weight Reduction Study with Crash Model, Feasibility and Cost Analysis (September 24, 2015)
- Executive Summary
Consumers have come to expect a high level of safety from new vehicles, and there are obvious benefits to society as well as the individual for continual improvement in road safety. Canadian & U.S Federal government regulatory safety requirements exist and are continually being revised (as are emissions and fuel efficiency standards), but it has been consumer crash testing programs in particular that have driven many aspects of vehicle safety innovation in recent years. The National Highway Transport Safety Administration’s (NHTSA) new car assessment program (NCAP), and the Insurance Institute for Highway Safety’s (IIHS) own consumer crash testing program gives consumers objective information in relation to active safety technology (used to help avoid a crash) & passive safety (the inherent crashworthiness of a vehicles structure and the performance of its restraint systems to protect the occupants). The automotive manufacturers have seen the need to perform well in these tests to satisfy the demands of the consumer. Therefore significant resources are spent engineering new vehicles to ensure good results are achieved.
When a vehicle is the target of a light-weighting study, the crashworthiness is one of the key performance functions that must be maintained. Add in the many other structural performance functions such as noise, vibration & harshness (NVH) management, durability, styling & packaging, manufacturing considerations and of course cost pressures, the challenge for the engineering team not to compromise on any of these is significant.
This project aimed to investigate mass reduction strategies for a light-duty truck (LDT) for the purposes of improved fuel economy and reduced emissions, at the same time showing that achieving good results in the most severe of the consumer crashworthiness tests is possible, whilst maintaining the other key structural performance functions of the vehicle.
The project successfully demonstrated that two different LDT designs were capable of achieving a ‘Good’ structural rating on the Insurance Institute for Highway Safety (IIHS) Small Overlap Crash Test whilst meeting the performance functions of the original baseline vehicle:
- A LDT design utilizing Advanced High Strength Steel countermeasures, with only a moderate mass penalty incurred.
- An Aluminum intensive LDT design incorporating mass reduction strategies such as advanced design, material and manufacturing processes.
Both of these designs were developed using Computer Aided Engineering methods, which allowed for the assessment of the models crash and NVH performances. This development was supported by data obtained through a full vehicle crash test of a 2013 Silverado 1500 4x4 Crew Cab, shown in Figure 1.
Figure 1 – Small Overlap Physical Crash Test – MY 2011 Silverado 1500 Crew Cab 4x4
Text version of Figure 1 – Small Overlap Physical Crash Test – MY 2011 Silverado 1500 Crew Cab 4x4
The image on the left shows a MY 2011 Silverado Crew cab 4x4 just before impacting the barrier. The image on the right shows the MY 2011 Silverado Crew cab 4x4 after it has fully impacted the barrier and come to a stop. In this image the airbags have deployed and significant amounts of deformation have occurred to the front driver’s side portion of the vehicle.
The following processes were undertaken to complete the above objectives.
A Baseline 4x2 CAE Model of the Silverado was first validated then converted to represent a 4x4 model and was successfully correlated to the results of a Small Overlap crash test conducted by Transport Canada. Based on an initial assessment of the Silverado structure, and some preliminary CAE analysis in the Small Overlap loadcase, a ‘POOR’ structural ratingFootnote1 according to the IIHS protocol was expected, and this was confirmed during the physical crash test.
The correlation process involved various updates and improvements to many parts of the model, but was mostly concentrated on areas that undergo extreme loading during the small overlap, namely the suspension, frame & cab. Material testing of critical components was done in support of this work. The 4x4 powertrain was also tested to determine the center of gravity and inertia properties to input into the model. Various geometry, connection and joint updates were completed, however it was the implementation of failure in the various suspension components and cab spotwelds that was essential to obtaining good correlation between the model and the test vehicle.
The new ‘Task 3 Baseline 4x4 CAE Model’ (T3-BL Model) showed good agreement with; the intrusions according to the IIHS protocols, other deformation measurements in the floor pan and door frame, failure timing of chassis and powertrain parts, body panel weld failures, accelerometer data, global vehicle kinematics, and general deformation of the vehicle. Objective rating criteria (Sprague Geers and ANOVA metrics) were also used to verify that the acceleration, or ‘pulse’, results were within the expected variation of real life crash tests.
Figure 2 – IIHS Small Overlap Structural Rating – Test vs CAE Models
Text version of Figure 2 – IIHS Small Overlap Structural Rating – Test vs CAE Models
The image shows the amount of intrusion in centimeters measured or modeled at 16 different points of the occupant compartment on the IIHS structural rating chart. The image shows that the actual results for the crash test and the “Task 3 – Baseline model” fall in the “poor” rating zone overall. The image shows that the “Task 4 – Good/Acceptable Model” and “Task 5 – Light Weighted Model” achieve a “good” rating for all 16 intrusion points.
The correlated model was then assessed for a variety of other loadcases to show that previous correlation had not been adversely affected by the updates in the model. A variety of Frontal Impact, Side Impact, Rear Impact and Roof Crush loadcases all showed reasonable correlation compared to the available test data, bearing in mind the difference in powertrain configurations since most tests had been conducted on 4x2 vehicles.
The Baseline 4x4 CAE Model was also shown to still provide good correlation to the NVH static torsion and bending loadcases, as had the Baseline 4x2 CAE Model.
IIHS Small Overlap Performance Improvement
Countermeasures were developed for the vehicle with the aim of achieving a good or acceptable IIHS structural rating. The strategy employed consisted of early engagement of the barrier with energy absorbing structures to limit the maximum forces applied to the safety cell, and a stronger safety cell to allow the Cab to maintain its structural integrity and limit intrusions into the occupant compartment. A ‘Good’ structural rating was achieved with an associated mass penalty of 45.5kg in the ‘Task 4 Good/Acceptable 4x4 CAE Model’, (T4-GA Model).
40.9kg of countermeasures were added to the Cab, with 4.6kg added to the frame. A combination of new reinforcements (in the frame, cab’s fenders, bodysides & underbody), material substitutions (particularly the use of Advanced High Strength Steels), modified gages & improved connections (critical areas with revised spotweld configurations in conjunction with structural adhesive bonding) were used to achieve the improvement in performance without excessive mass penalties.
It is important to note that this T4-GA solution was not optimized for mass, and also had countermeasures such as an improved B-Pillar which was a sensible addition to the upgraded safety cage, but was not strictly necessary for the Small Overlap Test performance. The T4-GA configuration was assessed in the other crash and NVH loadcases and was found to maintain or in some cases significantly improve the performance compared to the T3-BL Model.
Figure 3 – Cab Deformation – Test vs CAE Models w/ plastic strains
Text version of Figure 3 – Cab Deformation – Test vs CAE Models w/ plastic strains
The photo on the top left shows a view of the cab portion of a 2011 Silverado Crew cab 4x4 post crash. Significant amounts of intrusion can be seen in both the upper and lower portions of the occupant compartment. The image on the top right is of “Task 3 - Baseline model”, from the same view point, after the model crash. Similar amounts of deformation can be seen when compared to the actual crash vehicle with less front rough bend seen in the “Task 3 Baseline Model”. The bottom left image is of the “Task 4 – Good/Acceptable Model” and the bottom right image is of the “Task 5 – Light Weighted Model” from the same view point as the top two images, (view of the cab), post crash. Both of these images show much less observable intrusion than either of the top two images.
Light Weight Vehicle Development with IIHS Small Overlap Performance Maintained
Finally, the ‘Task 5 Light-Weighted 4x4 CAE Model’ (T5-LW Model) was developed by combining the light weight vehicle systems from the EPA Project with an updated Aluminum intensive cab, achieving the objective of also attaining a ‘Good’ structural rating in the IIHS Small Overlap Test. The T5-LW Model achieved equivalent or better performance in the other crash and NVH loadcases compared to the T3-BL Model as required, and in most cases was actually comparable to the T4-GA Model.
Like the original EPA Project cab, the T5-LW cab exploited the low density and manufacturing methods specific to Aluminum, however geometrically was based on the superior loadpaths of the T4-GA cab. Extrusions and castings were used to meet and exceed the static bending and torsion requirements with mass efficient solutions. Structural adhesive bonding in conjunction with self-piercing rivets (SPRs), a proven technology for high volume production, was used for the assembly of the cab. Failure criteria were implemented for both of these connection types, which showed that the design provided sufficient crash performance for the demands of the Small Overlap crash test. The connection scheme of the adhesive and SPRs also provide good stiffness and durability performance. The light weight frame developed in the EPA Project was also modified with some additional countermeasures.
The mass reductions of the T5-LW cab (79kg) and frame (21kg) compared to the T3-BL Model together totaled 100kg. The total vehicle curb weight reduction of the T5-LW Model compared to the T3-BL Model was 455kg, or 19% of the 2454kg curb weight.
The performance functions of the original baseline vehicle have been maintained for the T5-LW model. The Crash & NVH performance has been demonstrated. Ergonomics & Aesthetics are considered to be maintained, since both the internal packaging (passenger compartment) and external styling surfaces were effectively unchanged. Fuel efficiency is expected to be improved due to the reduction in vehicle mass (with further fuel efficiencies achievable assuming a downsizing of the powertrain to match the reduced vehicle mass). The remaining performance factors are considered to be maintained by the use of the EPA Project’s Optimized vehicle components in the T5-LW model, which had already investigated these factors.
A number of limitations to the CAE analysis exist that should be recognized;
- The baseline model having been built predominantly through reverse engineering techniques.
- T4-GA & T5-LW configurations using material properties from public domain data.
- Some factors that would normally be a part of the regular OEMs design & analysis routine have not been considered, such as a full design for manufacture assessment & accounting for forming effects on material properties.
- The lightweight vehicle components used in the T5-LW model, taken from the EPA Project have not been engineered.
- No ‘base’ material failure has been implemented in any of the three models.
- The T3-BL suspension failure mechanisms, while successfully correlated to the physical test, are limited in their capacity to be predictive. The suspension behaved similarly in the T4-GA model, but required some time based failure to enforce the appropriate failure timing in the T5-LW model.
- In all three model configurations, the failure criteria for the cab connections were developed using limited data found in open literature. The T3-BL spotweld failure was able to be correlated to the full vehicle test result, however the T4-GA spotweld model, the T5-LW SPR model and the structural adhesive model have not been validated. This results in an inherent level of uncertainty regarding the prediction of joint failure in the cab.
It should also be noted that the light weight EPA front suspension, front brakes, steering rack and wheels were not adopted for the T5-LW Model. While it is expected that lightweight alloy wheels could be implemented in a large pickup truck capable of achieving a Good or Acceptable result, as indeed many vehicles in other classes have done, predicting the likely changes in the structural response due to the complex failure modes associated with these components was beyond the scope of this investigation.
The original T3-BL Model configuration for these parts was instead maintained, which provided consistent suspension failure modes & wheel kinematics to allow back to back comparisons of the cab and frame performance. These components represent an additional 89kg of mass compared to the EPA Lightweight configuration, and a further potential mass saving opportunity.
Since the objective of the investigation was to achieve a ‘Good’ or ‘Acceptable’ result in the IIHS structural rating, it is considered that the ‘Good’ ratings achieved provide an adequate margin to account for the uncertainty associated with the above limitations, and the expected test-to-test variability.
Predictive cost calculations were performed for the changes itemized in the Task 5 TC Light-Weighted vehicle design. These calculations were performed using the same costing methodology applied to the original EPA Silverado Project. This study included the cost impact of the EDAG revised components, the cost impact of the non-revised components as well as the projected cost impact for the total vehicle.
The projected cost impact for the total vehicle was calculated to be $2115, which equates to $4.65/kg across the 455kg total mass saving achieved.
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