Composite Crash Analyses (LS-DYNA)

Dear Reader,

crashworthiness is one of the most important factors that every car manufacturer must consider in the design phase of a vehicle. The safety of occupants and pedestrians is highly dependent on the crashworthiness of the vehicle. Optimal design of the vehicle body in terms of crash performance ensures that occupants and pedestrians are maximally protected in traffic accidents. For this reason, vehicle crashworthiness regulations are becoming increasingly stringent. In this article, we give you an insight into the dynamic simulation of fibre composite components in a crash and how this contributes to the development of lighter and safer vehicle components.

Have fun reading!

Composite Crash Components

Over the past few decades, automotive designers have used standard isotropic materials such as Aluminium and Steel for many structural components. During a crash incident, a high amount of energy transfer takes place due to the impact velocity of the vehicle. The structural components undergo plastic deformation leading to failure. Aluminium or Steel alloys are universally used in many applications and are often sufficiently characterized with all mechanical engineering properties relevant for understanding and simulating a crash.

Nowadays, the efficiency of an automobile is one of the driving factors, which is why automotive manufacturers are replacing many structural components with lightweight, high-end composite materials such as carbon fibre-reinforced plastics. These composite materials should also be optimally designed not only for static load cases but also in terms of crash behaviour. To estimate the crashworthiness of the composite parts, it is very important to determine the absorbed energy and the failure behaviour of composites during impact.

LS-DYNA - How we leverage the world’s leading Crash Simulation Software

Numerical simulation allows manufacturers to meet the performance requirements for crashworthiness more efficiently, which allows them to design the structure optimally with better materials to distribute the crash energy and thus achieve the required safety rating iteratively. The tools and modelling techniques used for crashworthiness analysis should be of high fidelity and provide accurate results. These tools should also provide high user controllability to perform complex crash simulations using composite materials.

LS-DYNA is one of the most advanced and renowned explicit dynamics simulation systems in the industry used for performing complex simulations for impact and penetration, crash, passenger safety, etc. where complex nonlinear material models for composites and adhesives can be modelled along with multiple modelling possibilities with a large contact formulation database and simulation controls to accurately perform explicit dynamics simulations.

Real-world Example:

To encapsulate all the challenges and features of performing a crash simulation of composite parts using LS-DYNA, a real-time test setup from the automotive industry is explained in this blog. The impact attenuator test (also, crash box test) is a standard test performed in automotive and motorsport where crash boxes are placed generally in the front structure of the vehicle to absorb the impact energy at low velocities, thereby avoiding large structural damages to the surrounding structure in the impact region. Factors such as energy absorption, maximum deformation, acceleration, and reaction forces determine the safety score of the vehicle during the crash box test.

For the physical test, a part of the front bulkhead along with the Anti-Intrusion Plate (AIP) is considered. An aluminium honeycomb structure is adhesively bonded to the AIP using a structural adhesive. During the test, the entire crash component is placed on a flat surface and a load of 300 kg at an initial velocity of 7 m/s is dropped onto the test setup.

These are a few of the deciding factors for a crash box test defined by Formula Student Germany:

-????????? A minimum energy of 7350 J should be absorbed by the crash box.

-????????? AIP should not deform by 25 mm.

-????????? Peak acceleration should not exceed more than 40 g.

-????????? Mean acceleration should not exceed more than 20 g.

In this example, the crash box test is simulated in LS-DYNA. The entire simulation process will be presented throughout the next couple of articles for this blog series. In this part, a 1:10 scaled model of the crash box test without the bulkhead is initially simulated to determine the basic simulation parameters required along with the required simulation settings.

The rigid wall is assigned with the material card (MAT_020) from the LS-DYNA material library (Fig. 05). The honeycomb structure is assigned with a piecewise linear plasticity material model (MAT_024) where the plasticity region of Aluminium 5052 is represented based on the plastic strain and yield stress (Fig. 04). The AIP is defined using glass fibre unidirectional and twill-fabric composite plies, using the material card MAT_054 (Enhanced composite damage).

A symmetric composite stack-up with a total of 26 layers is defined for the AIP [twill 0°-90° / twill 0°-90° / UD -45° / UD +45° / UD 0° / UD 90° / twill +-45° / twill +-45° / UD 0° / UD 90° / UD -45° / UD +45° / twill 0°-90°] s. The total thickness of the AIP is 3.8 mm. The contact between the honeycomb structure and AIP is represented by a tied contact, and the impact interaction between all the components is defined using an automatic contact with a coefficient of friction of 0.2. The outer edges of the AIP are constrained in all degrees of freedom. An impact simulation with a rigid wall of 30 kg mass, with an initial velocity of 7 m/s is performed in LS-DYNA.

Results and Summary:

As mentioned above, the main importance of crash box tests in the motorsport industry is to make sure that the crash box absorbs the energy on impact and that the underlying structural components are not severely damaged.

From the deformation plots, it’s clear that most of the energy during impact is absorbed by the honeycomb structure.

The total mass of the aluminium honeycomb structure is 44.8g. This results in a?total specific energy of 167.41 (kN-mm/kg) for an energy absorption of 7.5?kN-mm during the crash box test.

Using this 1:10 scaled model of the crash box test, the required engineering properties for 3D-Continuum Honeycomb material (MAT_026) are calculated. Also, the global simulation settings are verified. This data is further used to simulate the global crash box test. Stay tuned for more information regarding the global crash box test and further interesting real-world explicit dynamics simulation examples in LS-DYNA!

Soon, we will give you exciting insights into the 1:1 crash test of the complete AIP in LS-DYNA and show you how we can efficiently and realistically reproduce the complex behaviour of such a component by means of numerical simulation - so that you can save considerable testing costs and time.

Be curious!

Dear Reader

We hope that we have been able to bring you closer to our company and increase your curiosity about engineering in lightweight construction. If you would like to know more about other interesting topics presented, please contact us. We will show you how you can best benefit from our lightweight engineering expertise - so that you can achieve your goals more easily.

Many thanks and best regards,

yours

Axel Reinsch

ar engineers GmbH

Ingenieurbüro

Kühneh?fe 20

22761 Hamburg

+49 (0) 40 228 680 980

[email protected]