- Email: [email protected]
Development of Adaptable CFRP Energy Absorbers for Car Crashes
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 26784–26791
www.materialstoday.com/proceedings
DAS_2017
Development of Adaptable CFRP Energy Absorbers for Car Crashes Nikola Schmidováa*, Tereza Zavřelováa, Michal Vašíčekb, Filip Zavadilb, Milan Růžičkaa, Martin Rundc a
Department of Mechanics, Biomechanics and Mechatronics, Faculty of Mechanical Engineering, Czech Technical University in Prague. Technická 4, Praha 6, 160 00, Czech Republic. b Centre of Vehicles for Sustainable Mobility, Faculty of Mechanical Engineering, Czech Technical University in Prague. Technická 4, Praha 6, 160 00, Czech Republic. c Comtes FHT a.s., Průmyslová 995, Dobřany, 334 41, Czech Republic.
Abstract This paper focuses on the possibilities of adapting the force and energy response of the composite beam when used as a car crash absorber. Several variants of a composite beam with the initial part for controlling the absorbed energy by cracking of the composite material were described and tested. The results obtained are compared with those from a finite element analysis. It was shown that the impact force and energy absorption of the tested composite beam can be effectively influenced. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Committee Members of 34th DANUBIA ADRIA SYMPOSIUM on Advances in Experimental Mechanics (DAS 2017). Keywords: energy absorber; composite, beam; car crash; adaptation; passive safety
* Nikola Schmidová. Tel.: +240 22 435 5605 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Committee Members of 34th DANUBIA ADRIA SYMPOSIUM on Advances in Experimental Mechanics (DAS 2017).
Schmidová/ et al. Materials Today: Proceedings 5 (2018) 26784–26791
26785
1. Introduction The passive safety of a vehicle is one of the basic parameters defining its overall safety. It is determined primarily by the ability to protect passengers in case of unavoidable accidents. It is given by the ability of the structure (body) to absorb the kinetic energy of the impact by conversion to potential energy during its deformation and/or damage. In present designs this ability is given by the design of the structure (shape and choice of material). It is obvious, that such a structure meets the safety requirements in different collision scenarios but in none of them optimally. This could mean that cars are safe only in configured collisions considered during the design. There is an unlimited amounts of real crash scenarios which indicate the need to adapt the design with respect to real crash conditions, mainly velocity [1], involved masses and geometrical conditions. Optimal – in terms of a minimum Overall Severity Index - deceleration curves for different crash velocities are displayed in Fig. 1 and illustrate different requirements on the body structure which are not achievable using conventional materials.
Fig. 1. Optimal deceleration curves for different crash velocities [1]
Due to all these facts our intention is to develop composite beams that can adapt its force response to that required by the actual situation. Fiber composite materials compared with conventional structural materials excel in their ability to absorb the damage energy and subsequent fragmentation. The price of these materials is still quite high, and this has so far prevented their wider application. An increased demand for electro-mobility and related requirement for a better mass absorbed energy ratio will nevertheless require their use. For further development it is necessary to find a reliable mechanism of damage initiation and to fully understand the mechanism of fragmentation. Several possibilities of influencing energy absorption with a composite beam during impact were already described in literature. We focused on methods which have a potential to affect the material response very quickly. The properties of a composite material can be modified by temperature. Thermoset and thermoplastic matrices could be used. Both can be affected by heat as described in [2]. Results published in [3] confirmed that initiation of fragmentation can be achieved by localized weakening before impact. Another possibility is that the structure of the absorber device is placed at the end of the composite beam and facilitates the initiation of fragmentation of the composite beam in case of a crash. The device should have the form of a head with holes permitting parts of the ruptured material going through them. This option is used in the aerospace industry [4]. Based on published results it was assumed that controlled circumferential damage of the beam before the front impact could adequately influence the force response of the beam during a traffic accident. Controlled circumferential damage could cause both the rupture of fibres and delamination of the composite material, both of which can influence the stiffness of the absorber during impact.
26786
Schmidová/ et al. / Materials Today: Proceedings 5 (2018) 26784–26791
Damage in the form of circumferential holes on two levels located near the impacted end of the beam was chosen to be tested first. In this study the results are discussed of the quasi-static and impact tests and numerical simulations. 2. Experimental procedure 2.1. Specimen description and experimental procedure Undamaged tubes and tubes with two damage sizes were prepared with an aim to investigate the influence of the damage size on the energy absorption capability of the composite absorbers. The size of the tube specimens was chosen in agreement with the space in real cars. The length of the specimens is 170 mm, the inner diameter is 70 mm. The thickness of the wall is 3 mm. The specimens comprised 8 layers of the fabric prepreg (carbon fibres and epoxy resin) SIGRAPREG C W305 - TW2/2 - E323/42%. All layers were oriented so that the 0° direction was identical with the longitudinal axis of the specimen. The damage of the specimens was prepared in the form of holes drilled on two height levels. All specimens are shown in figure Fig. 3. Specimens without damage are marked X. Specimens with damage in the form of circular holes are marked A and B. For both variants 50 % of the bearing cross-section was removed. Compared with type A specimens type B specimens have twice as many holes. The distance between the two height levels of the holes was determined in such a way, that the connecting line between the centres of holes on different levels and the connecting between the centres of holes on the same level form an angle of 45°.
Fig. 3. Specimen overview
Fig. 2: Photo of the test arrangement
To decrease the primary peak after impact, different types of initiators were tested. Initiators in the form of halfholes was used (specimens XI10, XI20, AI20, BI10). An initiator in the form of a chamfer was also tested (specimen XCH). Combinations of both mentioned initiators were also tested (specimens XI10CH, AI20CH, BI10CH). Each specimen was tested in a vertical configuration compressed between two strait plane as shown in Fig. 2.
Schmidová/ et al. Materials Today: Proceedings 5 (2018) 26784–26791
26787
2.1. Quasi-static tests In order to verify the finite-element models of tube specimens and to obtain a basic overview of the crushing behaviour of the examined tubes, quasi-static tests were performed first. The course of the force relative to the displacement can be seen in Fig. 5 and Fig. 4. Quasi-static tests were performed using a Heckert EU 100 testing machine. Loading speed was 25 mm/min. Further modifications of initiators based on the results of the quasi-static tests of specimens X, XI10, XI20, B, BI10, A and AI20 were proposed for the dynamic tests. The results of the quasi-static tests are in Fig.6.
Fig. 5: Comparison of the dependence the force on the displacement for the group of specimens X, quasi-static tests
Fig. 4: Comparison of the dependence the force on the displacement for the group of specimens A and B, quasi-static test
2.2. Impact tests The dynamic tests were performed using a drop weight Imatek IM10 impact tester. Parameters of the tests can be found also in Fig.6. The measured data for the impact test are depicted in figures 7 – 12. Errors in data logging occurred during measurements on specimens.
26788
Schmidová/ et al. / Materials Today: Proceedings 5 (2018) 26784–26791
Fig. 6: Overview of performed tests
From the results for damaged and undamaged specimens it can be seen, that the initiator in the form of a chamfer evidently decreases the peak force. Initiators in form of half-holes did not work well although during static tests they showed good results. When the measured data for specimens with and without holes were compared, an overall decrease of the acting force could be observed. For type A specimens with holes with a 20 mm diameter, a drop of the force caused by the holes could be observed. The sinusoidal behavior of the force could be observed. In type B specimens with holes with a 10 mm diameter this effect is much less pronounced. In order to evaluate the effect caused by damage in the case of circular holes, an average force was calculated from data measured in the displacement range from 10 mm to 40 mm for type X specimens with an initiator (specimens XCH and XI10CH) and type B specimens with an initiator (BI10CH_1, BI10CH_2, BI10CH_3). The average force for specimens without damage was 61 kN (Fig. ). The average force for damaged type B specimens decreased by 26 % to 45 kN (Fig. 12). The decrease of average force for type A specimens was not evaluated due to the evident sinusoidal behavior of the force (Fig. ), which was evaluated as inappropriate for the intended use of the energy absorber.
Schmidová/ et al. Materials Today: Proceedings 5 (2018) 26784–26791
26789
Fig. 7: Comparison of the dependence the force on the displacement for the group of specimens X
Fig. 8: Comparison of the dependence the force on the displacement for the group of specimens X with chamfer and their average value
Fig. 9: Comparison of the dependence the force on the displacement for the group of specimens A
Fig. 10: Comparison of the dependence the force on the displacement for the same shape of specimens from group A with chamfer
Fig. 11: Comparison of the dependence the force on the displacement for the group of specimens B
Fig. 12: Comparison of the dependence the force on the displacement for specimens from the group B with chamfer and their average value
3. Numerical simulations Numerical simulations of the drop weight tests were performed using a PamCrash explicit FEM solver with an aim to predict the response of the samples and to study the effects of initiators and designs. The tube samples were modeled using a single layer shell approach, where one shell layer corresponds to one lamina. An inter-laminar interface was modeled using the 1D node-shell approach. MATYP 131 material model – a Multilayered Orthotropic Bi-Phase incorporating Fabric Composite Global Ply model (ITYP7) was used for the laminate shells and MATYP 303 – Tied 2 incorporating Pickett Delamination Model [5] for the 1D inter-laminar interface. A setup of these material models was made on the basis of the results of a coupon test (tension, compression, shear, cyclic tension, cyclic shear, DCB, ENF) performed in the past on the same type of laminate and manufacturing technology.
26790
Schmidová/ et al. / Materials Today: Proceedings 5 (2018) 26784–26791
Schmidová/ et al. Materials Today: Proceedings 5 (2018) 26784–26791
26791
observed for type BI10CH specimens. Research shows that the variation of the force response during the crushing of the composite absorber can be efficiently controlled. Further investigation will continue into adaptive energy absorbers applying initial damage. Acknowledgements This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS16/214/OHK2/3T/12 - Development of the adaptive composite materials and structures for crash applications.
References [1] Witteman, W. J., Numerical Optimization of Crash Pulses. [Online] http://www.mate.tue.nl/mate/pdfs/1083.pdf, Euro-Pam 1999. [2] Kwang-Hee Im, et al., Effects of temperature on impact damages in CFRP composite laminates. Composites. Part B, 2001, vol. 32, pages 669-682. [3] Troiani, Enrico, et al., Influence of Plying Strategies and Trigger Type on Crashworthiness Properties of Carbon Fiber Laminates Cured through Autoclave Processing., Journal of Mechanical Engineering. 2014, vol. 60, pages 375-381. [Online] [Citation: 30.11.2015]: http://ojs.sv-jme.eu/index.php/sv-jme/article/view/sv-jme.2013.1506/pdf_20. [4] Heimbs, S., et al., Composite crash absorber for aircraft fuselage. [Online] [Citation: 30.11.2015] http://www.heimbsonline.de/Heimbs_2010_CrashAbsorber.pdf. [5] Greve L, Pickett AK. Delamination testing and modelling for composite crash simulation. Composites Science and Technology. 66. 816-826. 10.1016/j.compscitech.2004.12.042.
ScienceDirect Materials Today: Proceedings 5 (2018) 26784–26791
www.materialstoday.com/proceedings
DAS_2017
Development of Adaptable CFRP Energy Absorbers for Car Crashes Nikola Schmidováa*, Tereza Zavřelováa, Michal Vašíčekb, Filip Zavadilb, Milan Růžičkaa, Martin Rundc a
Department of Mechanics, Biomechanics and Mechatronics, Faculty of Mechanical Engineering, Czech Technical University in Prague. Technická 4, Praha 6, 160 00, Czech Republic. b Centre of Vehicles for Sustainable Mobility, Faculty of Mechanical Engineering, Czech Technical University in Prague. Technická 4, Praha 6, 160 00, Czech Republic. c Comtes FHT a.s., Průmyslová 995, Dobřany, 334 41, Czech Republic.
Abstract This paper focuses on the possibilities of adapting the force and energy response of the composite beam when used as a car crash absorber. Several variants of a composite beam with the initial part for controlling the absorbed energy by cracking of the composite material were described and tested. The results obtained are compared with those from a finite element analysis. It was shown that the impact force and energy absorption of the tested composite beam can be effectively influenced. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Committee Members of 34th DANUBIA ADRIA SYMPOSIUM on Advances in Experimental Mechanics (DAS 2017). Keywords: energy absorber; composite, beam; car crash; adaptation; passive safety
* Nikola Schmidová. Tel.: +240 22 435 5605 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Committee Members of 34th DANUBIA ADRIA SYMPOSIUM on Advances in Experimental Mechanics (DAS 2017).
Schmidová/ et al. Materials Today: Proceedings 5 (2018) 26784–26791
26785
1. Introduction The passive safety of a vehicle is one of the basic parameters defining its overall safety. It is determined primarily by the ability to protect passengers in case of unavoidable accidents. It is given by the ability of the structure (body) to absorb the kinetic energy of the impact by conversion to potential energy during its deformation and/or damage. In present designs this ability is given by the design of the structure (shape and choice of material). It is obvious, that such a structure meets the safety requirements in different collision scenarios but in none of them optimally. This could mean that cars are safe only in configured collisions considered during the design. There is an unlimited amounts of real crash scenarios which indicate the need to adapt the design with respect to real crash conditions, mainly velocity [1], involved masses and geometrical conditions. Optimal – in terms of a minimum Overall Severity Index - deceleration curves for different crash velocities are displayed in Fig. 1 and illustrate different requirements on the body structure which are not achievable using conventional materials.
Fig. 1. Optimal deceleration curves for different crash velocities [1]
Due to all these facts our intention is to develop composite beams that can adapt its force response to that required by the actual situation. Fiber composite materials compared with conventional structural materials excel in their ability to absorb the damage energy and subsequent fragmentation. The price of these materials is still quite high, and this has so far prevented their wider application. An increased demand for electro-mobility and related requirement for a better mass absorbed energy ratio will nevertheless require their use. For further development it is necessary to find a reliable mechanism of damage initiation and to fully understand the mechanism of fragmentation. Several possibilities of influencing energy absorption with a composite beam during impact were already described in literature. We focused on methods which have a potential to affect the material response very quickly. The properties of a composite material can be modified by temperature. Thermoset and thermoplastic matrices could be used. Both can be affected by heat as described in [2]. Results published in [3] confirmed that initiation of fragmentation can be achieved by localized weakening before impact. Another possibility is that the structure of the absorber device is placed at the end of the composite beam and facilitates the initiation of fragmentation of the composite beam in case of a crash. The device should have the form of a head with holes permitting parts of the ruptured material going through them. This option is used in the aerospace industry [4]. Based on published results it was assumed that controlled circumferential damage of the beam before the front impact could adequately influence the force response of the beam during a traffic accident. Controlled circumferential damage could cause both the rupture of fibres and delamination of the composite material, both of which can influence the stiffness of the absorber during impact.
26786
Schmidová/ et al. / Materials Today: Proceedings 5 (2018) 26784–26791
Damage in the form of circumferential holes on two levels located near the impacted end of the beam was chosen to be tested first. In this study the results are discussed of the quasi-static and impact tests and numerical simulations. 2. Experimental procedure 2.1. Specimen description and experimental procedure Undamaged tubes and tubes with two damage sizes were prepared with an aim to investigate the influence of the damage size on the energy absorption capability of the composite absorbers. The size of the tube specimens was chosen in agreement with the space in real cars. The length of the specimens is 170 mm, the inner diameter is 70 mm. The thickness of the wall is 3 mm. The specimens comprised 8 layers of the fabric prepreg (carbon fibres and epoxy resin) SIGRAPREG C W305 - TW2/2 - E323/42%. All layers were oriented so that the 0° direction was identical with the longitudinal axis of the specimen. The damage of the specimens was prepared in the form of holes drilled on two height levels. All specimens are shown in figure Fig. 3. Specimens without damage are marked X. Specimens with damage in the form of circular holes are marked A and B. For both variants 50 % of the bearing cross-section was removed. Compared with type A specimens type B specimens have twice as many holes. The distance between the two height levels of the holes was determined in such a way, that the connecting line between the centres of holes on different levels and the connecting between the centres of holes on the same level form an angle of 45°.
Fig. 3. Specimen overview
Fig. 2: Photo of the test arrangement
To decrease the primary peak after impact, different types of initiators were tested. Initiators in the form of halfholes was used (specimens XI10, XI20, AI20, BI10). An initiator in the form of a chamfer was also tested (specimen XCH). Combinations of both mentioned initiators were also tested (specimens XI10CH, AI20CH, BI10CH). Each specimen was tested in a vertical configuration compressed between two strait plane as shown in Fig. 2.
Schmidová/ et al. Materials Today: Proceedings 5 (2018) 26784–26791
26787
2.1. Quasi-static tests In order to verify the finite-element models of tube specimens and to obtain a basic overview of the crushing behaviour of the examined tubes, quasi-static tests were performed first. The course of the force relative to the displacement can be seen in Fig. 5 and Fig. 4. Quasi-static tests were performed using a Heckert EU 100 testing machine. Loading speed was 25 mm/min. Further modifications of initiators based on the results of the quasi-static tests of specimens X, XI10, XI20, B, BI10, A and AI20 were proposed for the dynamic tests. The results of the quasi-static tests are in Fig.6.
Fig. 5: Comparison of the dependence the force on the displacement for the group of specimens X, quasi-static tests
Fig. 4: Comparison of the dependence the force on the displacement for the group of specimens A and B, quasi-static test
2.2. Impact tests The dynamic tests were performed using a drop weight Imatek IM10 impact tester. Parameters of the tests can be found also in Fig.6. The measured data for the impact test are depicted in figures 7 – 12. Errors in data logging occurred during measurements on specimens.
26788
Schmidová/ et al. / Materials Today: Proceedings 5 (2018) 26784–26791
Fig. 6: Overview of performed tests
From the results for damaged and undamaged specimens it can be seen, that the initiator in the form of a chamfer evidently decreases the peak force. Initiators in form of half-holes did not work well although during static tests they showed good results. When the measured data for specimens with and without holes were compared, an overall decrease of the acting force could be observed. For type A specimens with holes with a 20 mm diameter, a drop of the force caused by the holes could be observed. The sinusoidal behavior of the force could be observed. In type B specimens with holes with a 10 mm diameter this effect is much less pronounced. In order to evaluate the effect caused by damage in the case of circular holes, an average force was calculated from data measured in the displacement range from 10 mm to 40 mm for type X specimens with an initiator (specimens XCH and XI10CH) and type B specimens with an initiator (BI10CH_1, BI10CH_2, BI10CH_3). The average force for specimens without damage was 61 kN (Fig. ). The average force for damaged type B specimens decreased by 26 % to 45 kN (Fig. 12). The decrease of average force for type A specimens was not evaluated due to the evident sinusoidal behavior of the force (Fig. ), which was evaluated as inappropriate for the intended use of the energy absorber.
Schmidová/ et al. Materials Today: Proceedings 5 (2018) 26784–26791
26789
Fig. 7: Comparison of the dependence the force on the displacement for the group of specimens X
Fig. 8: Comparison of the dependence the force on the displacement for the group of specimens X with chamfer and their average value
Fig. 9: Comparison of the dependence the force on the displacement for the group of specimens A
Fig. 10: Comparison of the dependence the force on the displacement for the same shape of specimens from group A with chamfer
Fig. 11: Comparison of the dependence the force on the displacement for the group of specimens B
Fig. 12: Comparison of the dependence the force on the displacement for specimens from the group B with chamfer and their average value
3. Numerical simulations Numerical simulations of the drop weight tests were performed using a PamCrash explicit FEM solver with an aim to predict the response of the samples and to study the effects of initiators and designs. The tube samples were modeled using a single layer shell approach, where one shell layer corresponds to one lamina. An inter-laminar interface was modeled using the 1D node-shell approach. MATYP 131 material model – a Multilayered Orthotropic Bi-Phase incorporating Fabric Composite Global Ply model (ITYP7) was used for the laminate shells and MATYP 303 – Tied 2 incorporating Pickett Delamination Model [5] for the 1D inter-laminar interface. A setup of these material models was made on the basis of the results of a coupon test (tension, compression, shear, cyclic tension, cyclic shear, DCB, ENF) performed in the past on the same type of laminate and manufacturing technology.
26790
Schmidová/ et al. / Materials Today: Proceedings 5 (2018) 26784–26791
Schmidová/ et al. Materials Today: Proceedings 5 (2018) 26784–26791
26791
observed for type BI10CH specimens. Research shows that the variation of the force response during the crushing of the composite absorber can be efficiently controlled. Further investigation will continue into adaptive energy absorbers applying initial damage. Acknowledgements This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS16/214/OHK2/3T/12 - Development of the adaptive composite materials and structures for crash applications.
References [1] Witteman, W. J., Numerical Optimization of Crash Pulses. [Online] http://www.mate.tue.nl/mate/pdfs/1083.pdf, Euro-Pam 1999. [2] Kwang-Hee Im, et al., Effects of temperature on impact damages in CFRP composite laminates. Composites. Part B, 2001, vol. 32, pages 669-682. [3] Troiani, Enrico, et al., Influence of Plying Strategies and Trigger Type on Crashworthiness Properties of Carbon Fiber Laminates Cured through Autoclave Processing., Journal of Mechanical Engineering. 2014, vol. 60, pages 375-381. [Online] [Citation: 30.11.2015]: http://ojs.sv-jme.eu/index.php/sv-jme/article/view/sv-jme.2013.1506/pdf_20. [4] Heimbs, S., et al., Composite crash absorber for aircraft fuselage. [Online] [Citation: 30.11.2015] http://www.heimbsonline.de/Heimbs_2010_CrashAbsorber.pdf. [5] Greve L, Pickett AK. Delamination testing and modelling for composite crash simulation. Composites Science and Technology. 66. 816-826. 10.1016/j.compscitech.2004.12.042.