Innovative test method for seat belt D-ring

Int. J. Vehicle Safety, Vol. 2, Nos. 1/2, 2007 87 Innovative test method for seat belt D-ring D. Dubois* Autoliv France ETS, Avenue de l Europe, Gournay en Bray 76220, France E-mail: david.dubois@autoliv.com, E-mail: dd.dubois@wanadoo.fr *Corresponding author H. Cord Autoliv ETN, Otto-Hahn-Strasse 4, P.O. Box 109, Elmshorn D-25333, Germany E-mail: hansjorg.cord@autoliv.com E. Markiewicz Laboratory of Industrial and Human Automation, Mechanical Engineering and Computer Science, University of Valenciennes, LAMIH, UMR CNRS 8530, Le Mont Houy, Jonas2, Valenciennes 59313, Cedex 9, France E-mail: eric.markiewicz@univ-valenciennes.fr Abstract: Seat belt assemblies are tested prior to vehicle crash tests in accordance with the UNECE R16 regulation. Thanks to these tests, global verifications of the restraint system performance are performed. Nevertheless, this common method is inadequate to analyse the local behaviour of seat belt components such as D-ring stability, because small variations of the dummy or the belt positions can create high perturbations in the test results. Besides, the cost of classical crash tests prevents planning test campaigns from focusing on the effect of these tiny variations. In this paper, an innovative test method is proposed to analyse webbing/d-ring/retractor subsystem of a seat belt assembly in a fully controlled manner. A test method is studied and evaluated by finite element analysis. An experimental bench is described and validated during a drop test campaign. Several load cases are described and a case of seat belt bunching phenomenon is reproduced. Keywords: seat belt; D-ring; webbing; retractor; experimental bench; drop tests; bunching phenomenon. Reference to this paper should be made as follows: Dubois, D., Cord, H. and Markiewicz, E. (2007) Innovative test method for seat belt D-ring, Int. J. Vehicle Safety, Vol. 2, Nos. 1/2, pp.87 102. Copyright 2007 Inderscience Enterprises Ltd.

88 D. Dubois, H. Cord and E. Markiewicz 1 Introduction Automotive manufacturers and suppliers commonly conduct crash tests in order to assess the performance of safety systems on new vehicles: this covers, for example, in accordance with international regulations (UNECE, 1995), the dummy response during a frontal collision. Crash tests dummies are used to collect the generated loads and accelerations, and based on biomechanics criteria, specified in regulatory as injury performance numbers, for assessing potential occupant injuries. During the vehicle deceleration, the dummies are subjected to the collision forces and in case of a frontal impact, they move forward. They will impact against the steering column, instrument panel, windshield, etc. when unrestrained. To prevent their interaction with vehicle interior/component that cause potential occupant injuries, 3-point seatbelt and/or other countermeasures are used to restrain the occupant s motion. The airbag, for instance, is a supplemental restraint system, which needs to be used with seatbelt systems. The webbing of a width of 48 mm of the belt restrain system has to resist dynamic loads up to 14 kn. Depending on the type of vehicles, about 3.5 m of belts are manufactured using polyester threads. It is woven on a Jacquard weaving loom in accordance with a two by two twill equally sided weaving pattern. In the warp direction, 280 or 400 yarns are used (dtex1670 and dtex 1100, respectively). In the weft direction, other yarns are used (dtex 550 or dtex 1100 or dtex 830) sometimes in combination with monofil yarns. In light of the new improvements of dyeing, a rainbow pallet of webbing colours is proposed to the car manufacturer to correspond with the shade of colour of their car interiors (Bader and Greva, 1996). Safety belts are usually combined with a load limiter retractor and also pretensioner fixed at the buckle or at the lower part of the B pillar (the post on which the rear doors are fixed). A D-ring (also called webbing guide) is used on the upper part of the B pillar in order to redirect the seat belt, which reels out from the retractor to the passenger shoulder. It is designed to adapt its angular position according to the passenger s motion. Numerous different kinds of D-rings are used in the current vehicle: most of them are made of a metal insert on which polymer part in nylon or in acetyl is moulded (see Figure 1(a)). In some cases, the ring frame and the guidance surface are made of one piece of metal (see Figure 1(b)). In the latest vehicles, the complete D-ring assemblies are integrated into the trim panel, and only the diagonal belt portion appears (see Figure 1(c)). Figure 1 D-rings For some belt geometries (i.e. the 3D position of the anchorage points of the seat belt on vehicle), the use of certain D-rings has led to a non-systematic instability, which is very disadvantageous (Dubois, 2003). The webbing, which should scroll without hindrance through the webbing guide, laterally shifts, bunches and produces the overturning of the

Innovative test method for seat belt D-ring 89 ring (Pedrazzi and Schaub, 2000, 2001) as shown in Figure 2, where the D-ring has been masked on purpose, for confidentiality reasons. Figure 2 Seat belt bunching phenomena This bunching phenomenon, first observed during the design stages of vehicles, is an issue because it might prevent the restraint systems connected to the webbing from working in a normal way (pretensioner, load limiter). To assess the seat belt performance, standard regulated tests are commonly performed in accordance with the legal requirements. The seat belts requirements are described in UNECE 16 regulation (see Figure 3, which defines dynamic tests procedures for seat belt system). It specifies the use of a sled test with a hard metal seat and a crash test dummy to evaluate the global behaviour of the seat belt. The UNECE 14 regulation deals with the test of seat belt anchorage points. These regulations are commonly used for seat belt components validations. However they are not adapted to the study of the webbing/d-ring interactions. Actually, between two consecutive tests, local dispersions are observed and can have huge influences on the global results. Figure 3 UNECE Regulation 16 deceleration pulse sled dynamic test-side view As the seat belt bunching phenomenon is an instability problem, small variations of the sled pulse also interfere on the results. Numerous sled tests would be necessary to evaluate the trends but the high cost of these sled tests does not permit to undertake a design of experiment based on the results of a complete campaign (Dubois, 2000).

90 D. Dubois, H. Cord and E. Markiewicz Starting from this state of the art, a new method of testing is studied in this paper. An experimental bench is developed to test dynamically D-rings in a fully controlled manner. 2 Technical approach 2.1 Dummy motion during a crash In order to evaluate the loading cases, which are applied to the D-ring/seat belt webbing subsystem, an analysis of the crash test dummy motion during an USNCAP Full-width frontal impact crash test (the vehicle crashes head-on into a rigid concrete barrier at 56 km/h) has been undertaken (Figure 4) using Pam-Safe. Figure 4 Simulation of a frontal sled test using Pam-Safe For this study, a MBS model of a Hybrid III 95% ile dummy was used (a Multibody System is a group of rigid bodies connected to each other by kinematic joints). This study shows that the motion of the first contact point between the webbing and the shoulder (Figure 5) can be divided into two phases: a first phase during which the passenger has a forward motion a second phase during which the passenger is bowing down. Figure 5 2D motion of the first contacting point between the shoulder and the seat belt

Innovative test method for seat belt D-ring 91 As the load is longitudinally applied to the seat belt webbing (in the warp direction) and as the loading point moves in accordance with the curve Figure 5, the loading direction varies during the crash. 2.2 D-ring/seat belt webbing orientation Two webbing portions usually define the seat belt webbing. The vertical portion between the Emergency Locking Retractor and the D-ring is called Retractor Belt. The diagonal portion between the shoulder and the D-ring is called Shoulder Belt. In order to analyse the belt geometries existing in the current cars, parametric positioning of the Shoulder Belt is necessary. To enable that positioning, the warp direction of the Shoulder Belt is projected into two orthogonal planes π 1 and π 2. The π 1 plane is coplanar with the plane defined by the Retractor Belt. The warp direction of the retractor belt belongs to the perpendicular π 2 plane. The alpha angle defines, in the π 2 plane, the angle between Shoulder Belt and the Retractor Belt; the beta angle is the equivalent angle in the π 1 plane (Figure 6). Figure 6 Angular orientation of the shoulder belt 3 Development of an innovative test method 3.1 Schedule of conditions To design an innovative experimental bench, which enables to test D-ring in a fully controlled manner, a precise specifications sheet has been defined. To enable the precise test of all the belt geometries seen in the real vehicle, the testing device must be modular. It must ensure realistic loadings. Moreover, specific instrumentations usually used on the dynamic field must be useable. In those aims, several characteristics have been listed: the device must be able to test all the D-ring types the angular orientations (α and β) must be met (within the ranges 25 < α < 65 and 25 < β < 65 ) the length of the shoulder belt and the retractor belt must be coherent

92 D. Dubois, H. Cord and E. Markiewicz the webbing scrolling length and its speed (measured by video tracking) must be equivalent to the one observed during real crash tests the fixture must enable the use of a high-speed camera and webbing load cells. 3.2 Technical solution: webbing guide drop test In accordance with the specifications sheet, the use of a drop test ring has been proposed (tests easy to set, low cost and reproducible loadings). In order to simplify the experimental bench, the loading direction has been considered as constant during the entire test. Nevertheless, the experimental bench has been developed to enable the individual test of all the observed loading directions. The following skeleton diagram (Figure 7) shows the technical solution developed. Figure 7 Skeleton diagram (WGDT) The principle of this bench is: a half cylinder drop weight, fixed to the sled of a drop test, impacts a horizontal webbing portion between a set of rods. The horizontal webbing portion is then pulled and the shoulder belt reels out. The D-ring is screwed on a vertical column. Its column is oriented at 45 to facilitate the shoulder belt orientation. The total energy applied to the system is controlled by the mass and the speed of the impactor. This test method has been chosen because of its simplicity. It is called Webbing Guide Drop Test (WGDT). This method does not enable to vary the loading direction during the test but all the belt orientations observed during a crash can be tested. 4 Numerical validation of the WGDT To validate the use of this loading method (i.e. the scrolling speed of the webbing through the D-ring, the loading level and the loading case to use), a WGDT simplified numerical model has been realised using the finite element method with Pam-Safe.

Innovative test method for seat belt D-ring 93 With this aim, the rods, the falling mass, the webbing and a simplified D-ring have been modelled. The main characteristics of that numerical model are described in Figure 8. Figure 8 Finite element model of the WGDT bench All the rods used for that model are considered as rigid and are fixed in the global space. Numerical contact algorithm Type 46 (PAM, 2001) has been defined to enable the sliding of the webbing on the four rods and the falling mass. The friction coefficient between the rods and the webbing has been fixed to 0.3 (classical friction coefficient between steel and Polyamide Nylon 66). For the following loading case: mass weight 80 kg and a falling speed 4 m s 1, that is, 640 J, Figure 9 shows several steps of the numerical simulation. Figure 9 Numerical model drop test principle using Pam-Safe The following curve (Figure 10) shows the load variation through the shoulder belt as a function of time. During the first stage, this curve shows a progressive load due to the use of the half cylinder falling mass shape (its shape resembles that of a dummy thorax). During the second stage, the loading limiter plate behaviour can be observed and eventually the load progressively decreases once the energy is absorbed. The total duration of the loading is 110 ms and is equivalent to the loading duration observed with Regulation 16 sled tests. The comparison of the loading cases measured through the shoulder belt with a numerical sled test and the WGDT numerical model shows a satisfactory correlation (Figures 11 and 12). The loading slopes are equivalent and the load limiting behaviour appears to be at the same level.

94 D. Dubois, H. Cord and E. Markiewicz Figure 10 Load through the webbing Figure 11 Load through the webbing for two numerical models versus displacement Figure 12 Load through the webbing for two numerical models versus time

Innovative test method for seat belt D-ring 95 5 WGDT angular adjusting devices The aim of the rods is to position and to maintain the webbing to avoid the disequilibrium of the falling mass. The vertical column is an assembling support for the D-ring and for the retractor. The webbing angular adjusting is achieved by: a vertical translation of the D-ring for the α angle adjusting (Figure 13) a D-ring rotation, using an angle (or plate) block for the β angle adjusting (Figure 13). Figure 13 WGDT angular variation 6 WGDT development Based on the previous skeleton diagrams and numerical computation validations, a mechanical welded assembly (using UAP80 and 40 40 beams) has been built by LAMIH, The Laboratory of Valenciennes University (Figure 14). Figure 14 WGDT views

96 D. Dubois, H. Cord and E. Markiewicz Several adaptive systems have been developed to enable the bench modularity needed by the D-ring shape diversity and the test parameters adjusting (Figure 15). Thus, a set of blocks and a machined steel plate have been used for the webbing angular adjusting. A drilled plate and a set of mounting holes on the assembly enable the length of the retractor belt to be varied. Horizontal oblong holes adapt the device to the width of the D-ring. Once fixed on the bench, the webbing is tined using specific fabric jaws (Figure 16). Figure 15 Adaptive systems for the test parameters adjusting Figure 16 Jaws for seat belt webbing An energy absorbing system has been adapted to avoid a possible damage of the falling mass in the event of a belt failure. The restraint system is composed of a set on two energy absorbing tubes on either sides of the half cylinder mass. A finite element model has been used to simulate responses of these two tubes with the following dimensions: diameter = 40 mm and thickness = 1.5 mm. To ensure their support, two backing shells have been machined (Figure 17).

Innovative test method for seat belt D-ring 97 In order to vary the mass and the corresponding energy applied to the seat belt, a set of steel plates (13 kg each) has been fixed on the half cylinder (Figure 18). Figure 17 Backing shells energy absorbing tubes Figure 18 Plates and half cylinder on the sled 7 WGDT implementation The WGDT and the falling mass have been fixed inside the drop test device. To analyse the seat belt behaviour through the D-ring, a high-speed camera is used on the left side of the drop test device. To measure the load applied to the webbing, a load cell is fixed on the vertical belt between the D-ring and the retractor (Figure 19). To facilitate the analysis of the webbing behaviour, parallel white lines have been drawn on the webbing every 3 cm (Figure 20). 8 Data acquisition system In order to measure the physical data of these drop tests, a specific data acquisition system designed for the field of dynamic loadings has been implemented. It is composed of an Entran webbing load cell EL20-16kN, a Nicolet Vision acquisition system, a Vishay Signal Conditioning amplifier and an high speed Ekta Pro camera with a live screen (Figure 21).

98 D. Dubois, H. Cord and E. Markiewicz Figure 19 WGDT fixed inside the drop test device Figure 20 Close-up view of the parallel white lines on the webbing Figure 21 Data acquisition chain

Innovative test method for seat belt D-ring 99 9 Method validation by experimental tests In order to validate the WGDT method, pretests have been performed. These tests have been made to verify the expected webbing scrolling through the rods, to control the bench stiffness and to analyse the influence of the combined mass/speed on the load applied to the webbing. Three tests have been focused on the WGDT bench validation. Between each test, one variable has been varied to analyse its influence (Table 1). To evaluate the risk of belt failure during the validation stage, the total energy has been progressively increased. Table 1 Loading cases Test 1 Test 2 Test 3 Mass 60 kg 60 kg 73 kg Speed 3.5 m s 1 5 m s 1 5 m s 1 Energy 370 J 750 J 910 J 10 Results analysis The comparison of the loading curves enables to analyse the effect of the mass on the load going through the belt in one hand and the speed on the load going through the belt in the other hand (Table 2). Table 2 Load cases/scrolling length Falling mass D-ring Test 1 Test 2 Test 3 Mass 60 kg 60 kg 73 kg Speed 3.5 m s 1 5 m s 1 5 m s 1 Energy 370 J 750 J 910 J Speed 1 m s 1 2 m s 1 2.2 m s 1 Scrolling 6 cm 9 cm 11 cm The use of a load cell on the retractor belt enables to analyse the chronology and the level of the load applied by the falling mass to the belt (Figure 22). An increase of the impact speed boosts the loading slope and an increase of mass weight amplifies the load limiter plate duration. In parallel, the length of scrolled webbing raises with the growth of the total energy. 11 Seat belt bunching phenomenon generated with the WGDT Based on the global understanding of the instability problem, a first belt geometry (Figure 23) has been tested with the WGDT. Five cases of D-ring instability have been consecutively reproduced with the following load case; mass: 87.4 kg and speed 7.5 m s 1 and the following belt orientation: α = 45 /β = 45 (see Figure 24).

100 D. Dubois, H. Cord and E. Markiewicz Figure 22 Load through the belt measurement Figure 23 View and sketch of the tested set up Figure 24 Bunching phenomenon generated with the WGDT

Innovative test method for seat belt D-ring 101 12 Conclusion The WGDT is a consistent method to test D-rings in dynamic loading conditions. It generates loading levels equivalent to those observed during R16 sled tests and the test results are completely repeatable. The loading directions are realistic as well as the length belt portions. Nevertheless, the experimental bench does not take into account the inertial forces generated by the pulse of vehicle crash on all the components on the safety belt. The varying loading direction generated by the forward motion of the passenger is not represented but the entire observed loading directions can be tested. This WGDT device does not aim to replace the regulation 16 sled tests, but it aims to complete the understanding of the webbing behaviour through the ring during a dynamic loading (Dubois, 2004). 13 Perspectives The perspective for this study is to perform a complete design of experiment focused on the seat belt behaviour and the instability issue. The load case, the D-ring type, the webbing type and also the belt orientation will be the factors to vary during that experimental study. The results of that analysis will enable to organise into a hierarchy the parameters that influence seat belt bunching and to control its occurrence. Acknowledgements The authors would like to thank Nicolas Evrat and Denis Lesueur for their valuable contributions during the development of the WGDT bench. References Bader and Greva (1996) Analyse dynamique des sangles de ceinture de sécurité, PhD Thesis, LPMT Laboratoire de Physique et Mécanique Textile. Dubois, D. (2000) Internal Report, D.E.A. Study, Seat Belt Behaviour Simulation Project, Autoliv UK, Valenciennes University, LAMIH Laboratory of Industrial and Human Automation, Mechanics and Computer Science, Le Mont Houy, Valenciennes, France, 11 September. Dubois, D. (2003) F.E. Analysis of seat belt behaviour under dynamic loading, ICD2003 Second International Crashworthiness and Design Congress, Lille, France, 4 December. Dubois, D. (2004) Prevention and prediction of the seat belt bunching phenomena, PhD Thesis, Valenciennes University LAMIH Laboratory of Industrial and Human Automation, Mechanical Engineering and Computer Science, France, 4 June. PAM-SAFE (2000) Solver Notes Manual, V2000, Contacts Algorithms, ESI Software, 99 rue des Solets SILIC 112, 94513 Rungis cedex, France. Pedrazzi, E. and Schaub, T.R.W. (2000) Occupant Restraint Systems Gmbh & Co, KG, Alfdorf, Simulation of Belt Movement in the D-ring During a Crash CAD-FEM User s Meeting Internationale FEM-Technologietage Graf-Zeppelin-Haus, Friedrichshafen, 20 22 September.

102 D. Dubois, H. Cord and E. Markiewicz Pedrazzi, E. and Schaub, T.R.W. (2001) Occupant restraint systems Gmbh & Co, KG, Alfdorf, Aspects of Seat belt Material Simulation Third European LS-DYNA Conference, Dynalis, Paris, June. UNECE agreement (1995) Addendum 15 Regulation 16, Safety Belts and Restraint Systems for Occupants of Power-Driven Vehicles.