Forensic Crash & Failure Analysis
Our Finite Element Analysis (FEA) software can be used to simulate vehicle crashes and structural failures in order to determine the initial conditions that preceded the crash or failure. Typical questions are “what speed was the vehicle travelling at when it hit the structure?” or “could some operating cases be sufficient to cause failure of the structure?”
Prosolve are a New Zealand based forensic engineering firm who carry out investigations into air, sea, road accidents and industrial failures. Bremar has collaborated with Prosolve on various projects, supplying FEA analysis which allows the comparison of simulated loading scenarios to be compared with actual “as found” deformations. If the deformations of the simulated cases match the as found damage, then we can learn more about the causal factors which contributed to the failure or accident.
Generally, the FEA simulations involved in such a comparison are highly non-linear, requiring advanced simulation techniques involving dynamic impact, contact, non-linear material properties and material fracture. The FEA analysis applies engineering calculations to determine stresses, strains and deformations of the structure in question. Results of interest can then be plotted on a graph, while images and animations can also be output to visualize the event and help understand how the failure or accident occurred. Due to the complexity of the calculations required, the computer runs typically take several hours to calculate less than a second of the actual event occurring.
The case study shown here is similar to one assignment undertaken recently with Prosolve Ltd. In this case, 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 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.
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 our 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:
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.
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, which is useful for an audience with limited technical knowledge. 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.
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. It’s a powerful tool and one that Prosolve will continue to leverage in the future.
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