Forensic Engineers & Investigators
Accident Reconstruction
Accident reconstruction is a general term to indicate a collection of processes used to try and understand how an accident unfolded. Using various means and techniques we are able to reconstruct the events that occurred in the time leading up to the accident and during the accident itself. This allows us to analyse various parts of the accident and determine it’s contributing factors.
Capabilities and services:
Construction of wreckage plots. We use this in air accident investigation to plot the locations where wreckage is found on land or on the seabed floor. If the accident involved an inflight break up, the locations of as found wreckage pieces can indicate the order that they left the aircraft and therefore help to determine the cause of the accident.
Reconstruction of road accident scenes. We can do this graphically, numerically or if appropriate, physically in a driven car, utilising instrumentation to record relevant parameters for later analysis.
Reconstruction of fire and risk inducing conditions. In a safe and controlled manner it is possible for us to reconstruct conditions that were observed or recorded to be in place at the time of the incident or accident in order to analyse their significance.
Reconstruction of industrial accident scenes. We are able to undertake this either graphically from photographs or physically from eye witness reports for the purposes of causative analysis. Depending on the context it may also be possible to use these for training purposes in order to prevent a recurrence.
Case Study 1: Vehicle Collision – Truck/ Sign Gantry
A semi trailer hit a vertical leg of an overhead gantry. We were able to determine the approximate speed of the truck before it impacted the gantry by comparing as found damage with that predicted by the engineering computer simulations.
Evidence Presented
Evidence presented included photos of the gantry after the impact. Following the incident, it was observed that the gantry leg was severely bent but still connected to its foundations. It was also known that the vehicle was brought to rest upon hitting the gantry.
Accident Reconstruction
Estimating Vehicle Speed
The geometry and material properties of the gantry were known, so we were able to create an accurate FEA model of the gantry itself, including fracture of the gantry material once it reached a specified strain. We used a semi trailer truck model from a library of FEA vehicle models and modified it to match the length and mass of the vehicle in question. The truck was then set up to impact the gantry structure at various speeds (30km/h, 45km/h and 60km/h) to examine the damage imposed on the gantry structure.
The animations below show the results of the three FEA runs (30km/h top, 45km/h middle, 60km/h bottom):
It is clear that in the 30km/h simulation, the gantry stops the truck but the gantry leg only has a slight deformation as a result of the impact. This small deformation is less than that observed in the actual impact, which suggests the truck was travelling faster than 30km/h.
The 60km/h impact speed simulation actually shears the bottom of the gantry leg off and allows the truck to continue travelling past the gantry. In the actual impact, the gantry remained intact and brought the truck to rest, suggesting the truck was travelling at less than 60km/h.
In the 45km/h impact simulation, the gantry leg is bent and the truck is brought to rest, which is generally consistent with the observations made from the actual event, indicating that the impact speed was likely to be around 45km/h.
Velocity and Energy Outputs
In addition to the speed estimate, other useful information such as the rate of deceleration and the energy involved in the collision can be determined as shown in the plots below:
The velocity plot shows how the vehicle decelerates during the impact event for the three different initial speeds, and it can be seen the 30km/h and 45km/h runs come to rest (zero velocity) while the 60km/h run slows down initially but then continues to travel at around 30km/h once the gantry leg is broken off.
The energy balance plot shows that initially, the majority of energy in the system is kinetic energy from the mass of the moving vehicle. Once the impact occurs and the vehicle starts slowing, the kinetic energy reduces and is converted into internal energy, which ramps up as the material in the gantry and truck structures is strained and deformed. Total energy in the system remains relatively constant, although a small amount of energy is lost through contacts and other numerical effects.
Visualization Materials
The FEA results can also be used to create photo realistic imagery and animations of the scenario in question. Such visualization helps to convey accurately and scientifically what occurred in the event. Another major benefit that can be highlighted is that these visualizations are based on actual engineering data and analysis rather than subjective analysis.
Note in the animations above, the shockwave travelling across the top of the gantry due to the impact, again highlighting that full dynamic effects are taken into account during the engineering simulations.
Summary
Whilst a vehicle crash has been used in this example, the same approach can be used to reconstruct almost any event or scenario. Advanced engineering FEA analysis such as this can be utilised with great effect to confirm or disprove a forensic hypothesis on the basis of engineering data and analysis, and can create accurate visualizations based on the analysis results.
With respect to computer modelling for simulating crash and failure events, we utilize the services of our technology partner Bremar automotion, who have contributed to this simulation.
Case Study 2: In Flight Breakup of Robinson R44 Helicopter
The inflight break up of a Robinson R44 resulted in wreckage that was scattered on the sea bed floor. Utilising a side scan sonar facility it was possible to reconstruct certain aspects of the in-flight break up and determine the order of break up. A side scan sonar facility was able to plot the GPS coordinates of items of interest which were later found by divers. Buoys were anchored at the GPS coordinates and divers were able to find these items by tying rope to the buoy anchors and using the rope to effect a spiral search of the sea bed floor around the buoy anchor. This was successful in finding a small fragment of main rotor blade, which after locating on the wreckage plot map, showed that it was the first fragment to leave the helicopter. This was instrumental in establishing the likely cause of the accident.
Following recovery from sea bed, the helicopter fragments were reassembled and supported by temporary structures. This type of reconstruction was undertaken to identify the way in which the helicopter broke up in flight so that the cause of the break up could be determined.
Reconstruction of helicopter tail boom and tail rotor assemblies
