Design Optimisation Of Automobile Crash Box

Students' Corner February 2019 Design Optimisation Automobile Crash Box
Design Optimisation Of Automobile Crash Box

Muskan Taneja, a third-year Mechanical Engineering student at the Birla Institute of Technology and Science (BITS) Pilani University, KK Birla Goa Campus, has diligently focussed on developing a solution to minimise impact on a vehicle and the passenger in case of a car crash. This eye-catching innovation has been widely appreciated within the automotive industry, and helped her win the Indian Automotive Technology and Innovation Awards (IATIA) 2018 ‘Student Innovator of the Year’ award.

INTRODUCTION

Given the contemporary fast-paced life and its dependency on transportation, it is imperative to focus on accidents and their impact. According to various available statistics, around 1.3 mn people globally die in road crashes every year, which amount to 3,287 deaths on an average a day. Further, an additional 20-50 mn either sustain injures or are disabled. India, unfortunately, is the largest contributor to worldwide accidents and fatalities – 1.5 lakh people die in road accidents in over five lakh road accidents every year.

On the vehicle side, despite various research and innovations carried out for crash boxes (a major component of the frontal impact zone in a car), none have been implemented over a decade due to the cost-effectiveness of the existing model.

To tackle this problem, Muskan along with various professors from both Pilani and Goa, worked on developing a more efficient design with minimum cost and volume requirements. The prototype proposed helps increase the efficiency of the crash box by 29 % along with reduction in space required for the component by as much as 40 %, while keeping the weight constant.

The innovation started with a base thought of increasing the packing fraction of the crash box to reduce the space occupied, and to further utilise this space to better design and performance characteristics of a car. The students worked on introducing circular tubes inside the hollow space of the crash box to facilitate distribution of force in all directions and avoid straight wave propagation in case of modern day crush cans. These students further used the super fold element theory to introduce corrugations along with filling in cylinders in the crush can.

The design was minimised for cost by optimising it according to manufacturing conditions and selecting material that can be easily obtained from recyclable products. For example, the cylinders used were made of recyclable aluminium that can be obtained for cheap, using the recycled beverage cans. The design was further optimised using design experiments for various lengths and thicknesses of the parts used and tested using finite element analysis to report a 100 kJ decrease in impact on the passenger inside, when tested in a crash at 80 km/h. The fluctuation frequency and amplitude were also decreased to minimise passenger discomfort and prevent spinal injuries that occur due to high force fluctuations.

THE PROBLEM STATEMENT

Crashworthiness is defined as the degree to which a vehicle will protect its occupants from the effects of an accident. Muskan’s aim was to maximise crashworthiness as much as possible to exceed the governing norms such as Bharat New Vehicle Safety Assessment Programme (BNVSAP) or Federal Motor Vehicle Safety Standards (FMVSS) of the US with the least weight and material to fit in the competitive world.

The problem statement specifically was to propose a design that improves the performance characteristics of a crash box with minimum cost addition. It is pertinent to point out that despite multiple research papers on the use of different geometries and materials none have been implemented in India owing to budget constraints. Hence, most accidents have high damage problems. Given the fast-paced life and increasing focus on mobility and transport, it is important to look at the safety aspects of an automobile vehicle.

Muskan and her team identified force dissipation in one direction as one of the probable reasons for inefficient force distribution

THE SOLUTION

Muskan proposed an alternate geometry. The team identified that the first problem of high impact force was the straight wave propagation of the force. They identified the force dissipation in one direction to be one of the probable reasons for inefficient force distribution. “Working for the college Baja team, I observed how the frame was made of circular tubes to allow better transmission of impact. Our idea was to inculcate this in the frontal impact zone,” Muskan said.

And to achieve this goal, she proposed an alternate design bearing in mind the super-fold element theory. “The design was a corrugated thin walled steel casing stacked with aluminium cylinders with axis aligned perpendicular to line of impact,” she said. By filling the hollow space using cylinders, the force is distributed throughout the body as instead of a straight wave propagation, the hexagonal stacking of cylinders helps the dissipation of force and hence energy in multiple directions. This distribution allows for a lower jerk along with maintaining the amount of energy absorbed. The corrugations, on the other hand, allow the progressive symmetric deformation mode increase the amount of energy absorbed as compared to a straight wall crush can.

The proposed design achieves an increase in efficiency along with reduction in jerk experienced by the passenger by 100 kN with minor alterations in the current industry model, thereby adding minimal cost and making it implementable in the industry. The frequency and amplitude of oscillations of force and the space required for a crush can were also reduced. The model was found to have improved design parameters, namely higher crush force efficiency, reduced frequency and amplitude of force fluctuations, lower impact force and reduced volume requirements – the key achievement being a rise of 29 % in crush force efficiency of the crash box. Thus, this model was found to deliver better results than the design widely used throughout the industry. The model has also been optimised for simplifying manufacturing considerations, and recyclable, easily procurable materials were used in order to minimise added costs.

Fast forward to now, the prototype is in its final stages of development. Designed and developed indigenously, this crash box has garnered significant interest from various automotive OEMs, who have extended their technical and product support to cater to the customised requirements of the prototype.

CONCLUSION

This design not only offers designing and technical advantages such as reduction in space & weight, improving stacking density and protecting other expensive car parts, but also promotes higher passenger safety in case of an accident. The reduction in space occupied and weight reduced from the crumple zone could be well utilised to improve the other design aspects of an automobile – be it aesthetics or performance characteristics.

Most importantly, this design ensures improved crash force efficiency, which is the ratio of the mean dynamic force experienced by the crash box to the maximum initial force. It also provides passenger comfort in case of a crash by minimising the frequency and amplitude of the force experienced. The sudden jerk i.e. the buckling force experienced at the time of a crash is reduced by 36 %. Potentially, it can minimise deaths that occur due to spine breakage, in case of high sudden impact during road accidents.

The proposed design is easily implementable, is cost effective and can be the next big innovation in the crash industry, claims Muskan. Further work on the actual testing of the crash box is currently in process and the product will be industry-ready by the end of 2019, either individually or in partnership with Indian OEMs, based on how the development conditions unfold.

(Muskan’s colleague at BITS PILANI, Shashank Singh Tomar worked as her partner in this project)