Case StudyRear Impact Guard Study

INTRO

Rear impact guards are an important safety device on large commercial vehicles and trailers. Indeed they are a device in that they are designed to absorb the energy of a rear impact with a smaller vehicle while deforming permanently in the process - much like the crumple zone of a vehicle - and in doing so help reduce risk of injury. Our client required a rear impact guard for their custom semi-trailer design in order to receive transport certification.

Rear impact guard requirements are laid out in the Motor Vehicle Safety Regulations, specifically Standard 223 (Rear Impact Guards). This standard allows for either physical testing or software simulations and provides multiple specific goal based criteria that must be met for a passing design.

Business Problem

Standard impact guard designs are expensive to buy and usually include a licensing fee. By working with Innovex our client was able to own and produce their own design on as many products as they would like.

Technical Problem

The design needed to be either physically tested or computer simulated using more advanced models that allow for permanent damage. When we say damage in this context we technically mean plastic deformation. Plastic deformation requires a non-linear finite element analysis (FEA) approach which is not as common as typical linear elastic analysis FEA. If possible to simulate the testing procedure required by the regulation, then our client could test many designs for far less cost than building and running actual physical testing.

FEA model showing stress distribution upon impact

Some of the specific configuration and testing requirements of the regulation are:

Configuration Requirements

The configuration requirements are given in subsections 5-8 and are included below:

(5) The horizontal member of a rear impact guard shall have a cross-sectional vertical height of at least 100 mm at any point across the guard width when installed on a trailer.
(6) The outermost surfaces of the horizontal member shall extend outboard to within 100 mm of the longitudinal vertical planes that are tangent to the side extremities, but shall not extend outboard of those planes.
(7) When the trailer is resting on level ground, unloaded, with its full capacity of fuel, its tires inflated and its air suspension, if so equipped, pressurized in accordance with the manufacturer’s recommendations, the ground clearance shall not exceed 560 mm at any point across the full width of the horizontal member. However, rounded corners may curve upward within 255 mm of the longitudinal vertical planes that are tangent to the side extremities.
(8) At any height above the ground clearance, the rearmost surface of the horizontal member shall be located as close as practicable to a transverse vertical plane tangent to the rear extremity of the trailer, and no more than 305 mm forward of that plane, as shown in Figure 1. However, the horizontal member may extend rearward of the plane.

Test Requirements

The test requirements are given in subsections 9 – 12 and are summarized below:

Test 1: Deflect less than 125mm when subjected to a 50 000N force on P1 and P2 as shown in the following figure, and
Test 2: Either

a) Deflect less than 125mm when subjected to a 350 000N, and absorb at least 20 000J of plastic deformation energy, or
b) Deflect less than 125mm when subjected to a 700 000N uniform load (no plastic deformation requirement)
c) Have a ground clearance not exceeding 560mm after the uniform load test has been completed.

Solution

Innovex conducted a review of other designs and collaborated with the client to develop a design specific to outfit their trailer. Designs and 3D guard sub-assembly models were developed and pared down with client review using fabrication cost as a key determinant. We had a current CAD model of the whole trailer, having created it earlier for another project, and so the fitment of the guard sub-assembly could easily and virtually be checked.

The models were tested using a non-linear FEA software package. The testing procedure was entirely virtually simulated. The guard deflection can be seen and measured since the software produces a deformed shape after the load is applied and allows for plastic deformation and other non-linear effects.

After the FEA model produced satisfactory results, a separate production CAD model was built. Fabrication drawings were easily created using this model. Should any adjustments be required for future trailer designs, the drawings link directly to the CAD models and so an update to the design gets reflected to the drawing file automatically reducing time and cost for our client.

The proposed rear impact guard passed all the configuration requirements of Transport Canada’s Motor Vehicle Safety Regulations Section 223. The proposed guard also passed the load tests (Test 1 and Test 2) which was confirmed by FEA results.