Higher crash safety of compos­ites through simu­la­tion with LS-DYNA

Dear reader,

crash­wor­thi­ness is one of the most impor­tant factors that every car manu­fac­turer must take into account during the devel­op­ment phase of a vehicle. The safety of occu­pants and pedes­trians depends to a large extent on the crash­wor­thi­ness of the vehicle. An optimal design of the vehicle body in terms of crash perfor­mance ensures that occu­pants and pedes­trians are maxi­mally protected in traffic acci­dents. At the same time, regu­la­tions for vehicle crash­wor­thi­ness are becoming increas­ingly strin­gent. In order to reli­ably and effi­ciently predict and demon­strate the crash­wor­thi­ness of light­weight composite compo­nents, special exper­tise is required in the simu­la­tion of highly dynamic processes as well as an in-depth under­standing of the composite mate­rials used.

In this blog article, we will there­fore give you an insight into the dynamic simu­la­tion of fibre composite compo­nents in the event of a crash, and how we at ar engi­neers contribute to the devel­op­ment of lighter and safer vehicle compo­nents as a result. Learn how our exper­tise in the crash eval­u­a­tion of compos­ites can help you reach your goal faster and lighter.

Have fun reading!

Designing for Crash­wor­thi­ness in Auto­mo­tive

In recent decades, isotropic struc­tural mate­rials such as aluminium and steel have been used for the vast majority of struc­tural compo­nents in vehicle construc­tion. Nowa­days, however, espe­cially with elec­tric vehi­cles, the weight of the car is such a driving factor in devel­op­ment that car manu­fac­turers are replacing many metallic struc­tural compo­nents with high-quality compos­ites such as carbon fibre-rein­forced plas­tics (CFRP) in order to maximise their light­weight poten­tial. These compos­ites must also be opti­mally designed not only for static load cases, but also with regard to their crash behav­iour. In order to be able to esti­mate the crash­wor­thi­ness of fibre composite compo­nents, it is very impor­tant to deter­mine the absorbed energy and the complex defor­ma­tion and frac­ture behav­iour of compos­ites in the event of an impact.

During a crash of a conven­tional passenger car, a high energy transfer takes place due to the impact speed as well as the mass of the vehicle - the conver­sion of kinetic energy into defor­ma­tion energy. The struc­tural compo­nents are plas­ti­cally deformed, which leads to compo­nent failure. Aluminium or steel alloys are univer­sally used in many appli­ca­tions and are often suffi­ciently char­ac­terised with compu­ta­tional para­me­ters that are rele­vant for under­standing and simu­lating a crash. Fibre-rein­forced compos­ites have many advan­tages, but exhibit a signif­i­cantly more complex mechan­ical behav­iour, which also mani­fests itself in crash calcu­la­tions in partic­ular. Here we use our many years of exper­tise to be able to provide our customers with valid results within the scope of the calcu­la­tion.

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

Numer­ical simu­la­tion enables manu­fac­turers to meet the crash­wor­thi­ness perfor­mance require­ments of their compo­nents more quickly and cost-effec­tively, bringing more effi­cient, safer vehi­cles to market. The tools and model­ling tech­niques used to analyse crash behav­iour should provide the most accu­rate results possible with a high degree of control­la­bility by the user to success­fully perform complex crash simu­la­tions with composite mate­rials.

LS-DYNA is one of the most advanced and renowned FEM soft­ware systems used by over 80% of auto­mo­tive OEMs to simu­late compo­nent, occu­pant and pedes­trian safety. LS-DYNA is also a stan­dard tool for us and offers complex non-linear mate­rial models for metals, plas­tics and compos­ites as well as a wide variety of model­ling options and control options. Coupled with our exten­sive expe­ri­ence and exper­tise in the design and calcu­la­tion of composite compo­nents, we thus ensure for you that your compo­nent meets the highest safety and light­weight construc­tion require­ments.

Real-world example: Impact Atten­u­ator (Crash Box) Test

In order to illus­trate all the chal­lenges and pecu­liar­i­ties of performing a crash simu­la­tion of fibre composite compo­nents with LS-DYNA, this blog explains a prac­tical test setup from motor­sport. Crash boxes are gener­ally placed in the front struc­ture of the vehicle in motor sports but also in passenger cars in order to absorb the impact energy at low speeds and thus avoid major struc­tural damage to the surrounding struc­ture and the passenger cell as well as to protect occu­pants from exces­sive accel­er­a­tion. Factors such as energy absorp­tion, maximum defor­ma­tion, accel­er­a­tion and reac­tion forces deter­mine the vehicle’s safety rating. The shock absorber test (also known as the crash box test) is a stan­dard test that is carried out in motor­sport, among other areas, for the prac­tical vali­da­tion of crash safety - including at HAWKS Racing e.V., the Formula Student Team at HAW Hamburg.

For the phys­ical test, a part of the front parti­tion of the vehicle is consid­ered together with the so-called Anti Intru­sion Plate (AIP). The AIP is usually a fibre composite compo­nent - mostly made of CFRP - and protects against intru­sive objects in the front area. An aluminium honey­comb struc­ture is bonded to the AIP with a struc­tural adhe­sive. The aim is to convert the impact energy into defor­ma­tion energy through the honey­comb struc­ture without damaging or exces­sively deforming the composite AIP. During the test, the entire compo­nent is placed on a flat surface and a load of 300 kg is dropped onto the test struc­ture with an initial velocity of 7 m/​s. The load is applied to the honey­comb struc­ture.

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.

The correct cali­bra­tion of calcu­la­tion models and vali­da­tion of assump­tions about mate­rial prop­er­ties and boundary condi­tions are essen­tial to obtain precise simu­la­tion results that are useful for real appli­ca­tions. There­fore, we always pay special atten­tion to exactly these points and first check the model assump­tions in down­scaled test simu­la­tions. In this example, the crash box test is simu­lated in LS-DYNA. Above all, the exact model­ling of the mate­rial behav­iour of the composite AIP is crucial here. Fibre compos­ites are anisotropic and there­fore exhibit direc­tion-depen­dent behav­iour under load - this applies both in statics and in the crash.

In this blog post, we first simu­late a 1:10 scale model of the crash box test without the parti­tion to deter­mine the basic simu­la­tion para­me­ters and the required simu­la­tion settings. The entire simu­la­tion process on the global model (1:1) of the AIP will be presented in the next arti­cles of this blog series.

The rigid wall is assigned the mate­rial card (MAT_​020) from the LS-DYNA mate­rial library (Fig. 05). The honey­comb struc­ture is assigned a multi­linear plas­ticity model (MAT_​024) in which the plas­ticity range of aluminium 5052 is repre­sented based on plastic strain and yield stress (Fig. 04). The AIP is defined with unidi­rec­tional glass fibre and twill fabric layers, using the MAT_​054 (Enhanced composite damage) mate­rial card.

A symmetric composite stack-up with a total of 26 layers is defined for the AIP as follows:

[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 connec­tion between the honey­comb struc­ture and the AIP is repre­sented by a rigid contact and the impact inter­ac­tion between all compo­nents is defined by an auto­matic contact with a fric­tion coef­fi­cient of 0.2. The outer edges of the AIP are locked in all degrees of freedom. An impact simu­la­tion with a rigid wall of 30 kg mass and an initial velocity of 7 m/​s is performed in LS-DYNA.

As already mentioned, crash box tests in motor­sport are mainly concerned with ensuring that the crash box absorbs the impact energy and that the under­lying 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.

Outlook

This 1:10 scale model of the crash box test is used to calcu­late the required mechan­ical prop­er­ties for the homogenised aluminium honey­comb (MAT_​026), which is used in the 1:1 scale global model. In addi­tion, the global simu­la­tion settings are checked to ensure a stable simu­la­tion with accu­rate and plau­sible results. This data is then used to simu­late the global crash box test.

In the next article of this series, 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!