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frontal impact: comparison between direct loading in
isolated tests and sleds
Philippe Beillas, Anurag Soni
To cite this version:
Philippe Beillas, Anurag Soni. Assessment of abdominal loading by shield CRS in frontal impact: comparison between direct loading in isolated tests and sleds. 12th International Conference on Protection of children in cars (VD-TUV), Dec 2014, Munich, Germany. �hal-01720995�
Assessment of abdominal loading by shield CRS in frontal impact:
comparison between direct loading in isolated tests and sleds
Philippe Beillas*, Anurag Soni
Université de Lyon, Ifsttar-Université Claude Bernard Lyon 1, UMR_T9406, Bron, France
Abstract:
In this study, complementary approaches (direct loading on a material testing machine, sled testing with a Q3 dummy, geometrical analyses and simulations with 3 YO human model) were developed to study the interactions of shield Child Restraint Systems with the thoracic and abdominal regions. Overall, the results suggest that while shields can effectively distribute the loads over different body regions in direct loading, this distribution is very dependent on the position of the shield and does not necessarily apply to sled configurations. High abdominal pressures observed in sleds performed in this and other studies may be the consequence of a relative lack of pelvic restraint. In direct loading, abdominal pressures obtained using Abdominal Pressure Twin Sensors were found to be useful to help understanding the load distribution between body regions. They could also be helpful in sled testing as direct observations of the interactions of shield CRS with the dummy are especially difficult.
___________________________ Introduction:
In Europe, the safety of integral Child Restraint Systems (CRS) is assessed in Regulations ECE R44 and R129. In frontal impact, restraint loads applied to the lower body are typically expected to be transmitted to the pelvic region rather than the abdomen. For example, the text of the regulation R129 supplement 2 states that “All restraint devices utilizing a lap strap must positively guide the lap strap to ensure that the loads transmitted by the lap strap are transmitted through the pelvis. The assembly shall not subject weak parts of the child’s body (abdomen, crotch, etc.) to excessive stresses” (Article 6.2.1.5, R129 Text, 2014). However, the definition of excessive stress is not provided and the assessment could be difficult due to the lack of instrumentation of the Q dummies in that region.
While transferring loads to the pelvis may not be an issue for current 5-point harnesses, shield CRS may also transfer significant restraining loads to the abdomen and thorax. In Hybrid III 3 years old and Q3 sled testing and simulations, Tanaka et al (2009) typically found higher thoracic deflections with shields than harnesses. The abdominal loads were not directly assessed but the lack of pelvis involvement in some cases suggests that significant abdominal loads could have been present. In Q3 dummy tests, Beillas et al. (2012a) and Johannsen et al. (2012) also reported higher thoracic deflections and higher abdominal pressures (assessed using Abdominal Pressure Twin Sensors, APTS) with shields than harnesses.
However, the relationship between this loading pattern and real world injuries seems difficult to assess with certainty. Comparing shields and harness CRS, Langwieder et al (1999) found a trend of lower injury severity with shields. However, the group 1 frontal sample included no serious injuries in the sense of the AIS scale (19 shield cases: 10 AIS1; 30 harness cases: 24 AIS1 and 4 AIS2). Also, 4 point harnesses, which are not in use anymore, were included in the harness sample. Based on the CHILD and CASPER Project accident database, Johannsen et al. (2012) reported that thoracic and abdominal injuries (including spine) represented 42% of the MAIS2+ with shields vs. 17% with harnesses. However, the results were only based on 31 cases for shields. These cases were
frontal and side impact. In the USA, Edgerton et al. (2006) found higher risks of abdomen/pelvic and chest MAIS2+ with shield booster seat than 5 point harness (12 injuries for 16 cases vs 1 injury for 30 cases, respectively). The risk differences were statistically significant, with large odd ratios and wide confidence intervals due to the low number of samples. For the abdomen, there was no injury for the harness sample and 25% of AIS2+ (4 out of 16) for the shield sample. However, besides the small sample numbers, the study grouped both frontal and side impacts together. Overall, due to relatively low case numbers, methodological differences and other limitations, these published results seem too limited to support definitive conclusions on real world abdominal injury risk associated with the use of shield CRS in frontal impact.
Beyond their limitations, these studies may have included different CRS evolutions and designs, distributing differently the restraining loads on the pelvis, abdomen and thorax. Without a direct assessment of abdominal and thoracic loading and in the absence of sufficient epidemiological data, it seems difficult to distinguish between different CRS designs in regard to abdominal and thoracic injuries. This limits the ability to compare the performance between different shield designs, between shields and harnesses, or between past and current strategies. It also does not set limits on the future evolutions of CRS design. For the thorax, since Q dummies have a deformable ribcage, deflection – rather than chest acceleration – may be useful to assess thoracic loading in regulation and consumer testing. This is however not the focus of this study. For the abdomen, there is currently no standard equipment in Q dummies. Pressure measured using APTS has shown promises to assess abdominal loading by a belt in Q dummies, and a risk curve based on accident reconstructions has been proposed (Beillas et al., 2012b). However, the experience is more limited with shields. For example, it is not completely clear if pressure amplitudes measured in shield CRS testing can be interpreted in the same way as belt loading results.
The objectives of this study were therefore to develop a better understanding of the interactions between shield CRS, abdomen and APTS as a preliminary step towards a possible evaluation procedure.
Methodology
Three approaches were developed concurrently: (1) direct loading on a material testing machine (2) sled testing (3) geometrical measurements and simulations with a human FE model.
1) Direct trunk loading on a material testing machine
All tests were performed using a Q3 dummy equipped with APTS on a material testing machine. An overview of the test setup is provided in Figure 1. After removing head, neck, arms, tibia and feet, the Q3 dummy was mounted face down inside a rectangular steel frame bolted to the fixed side of the testing machine. The dummy was secured using screws mounted in the lumbar spine mount. An aluminum spacer was used in the back of the thoracic spine to provide a flat contact surface with the frame while preventing lumbar flexion due to the contact of the back of the pelvis. The hips were extended to prevent load transfer between the shields and the thighs.
The shield components of shield CRS were used to load directly the dummy trunk. Three shields models, which will be referred to as SH1, SH2 and SH3, were used for the study. These are currently sold on the European market. The shield was positioned against the trunk of the dummy and held in place using a standard vehicle belt (46mm wide) passed inside the shield guide. The belt was attached to a square steel beam mounted on the piston. The beam was perpendicular to the piston and to the dummy. The distance between belt attachments was adjustable. It was set such that the belt was in a vertical plane. Finally the belt length was adjusted such that the starting position of the piston was always the same in the initial position before pulling (beam just above the frame).
Figure 1: overview of the test setup for the direct loading on a material testing machine (with SH1 and SH2) Tests were then performed by moving the piston at 1m/s. The following configurations were tested:
- Shield SH1 (baseline): 3 positions were tested by moving the shield up and down the abdomen. In the lower position, the shield could directly engage the pelvis and abdomen but not the thorax. In the upper position, the shield could engage the thorax and abdomen but not the pelvis. In the middle position, the belt was aligned with the mid abdomen.
- Shield SH2 and SH3: only the middle position was tested
- Belt only: the shield was removed and the standard belt was used to load directly the abdomen (middle position) or to engage the upper pelvic skin. Additionally, the mid test was repeated using a 32mm wide belt. The standard vehicle belt will be referred to as the wide belt and the other one as the narrow belt.
To document the position of the shield and the engagement of anatomical regions, surface scans of the shield component were positioned relatively to a surface scan of the Q3 trunk. Illustrations of the positions are provided in the Figure 2.
Measurements included applied force (computed as the sum of the forces measured on the two sides of the belt), actuator displacement, thoracic deflection (string potentiometer), and abdominal pressures (APTS). Since the setup is symmetric, left and right pressures were averaged to have only one representative pressure trace.
Figure 2: Shield configuration used in direct loading tests. Top: external view. Bottom: partial section showing the limits of the thorax, abdomen and pelvis in relation to the shield structure.
2) Sled testing
The restraint kinematic was studied in sled testing with a Q3 dummy on a R129 bench and a R44 pulse. Four configurations were tested (Figure 3a), with one test in each configuration:
- Baseline test (S1): the CRS was held by the belt only (no ISOFIX).
- Test without backrest (S2): in order to facilitate the observation of the interactions between dummy and CRS, the test was repeated after removing the back of the CRS and the back foam of the bench. Thin aluminum tubes (painted back) holding light white spheres were mounted on the lumbar and neck attachments (Figure 3b). The initial dummy position was adjusted to be the same as in S1 by measuring points on the dummy. An adjustable rod near the neck and a horizontal polystyrene plate behind the head were used to maintain the dummy initial position (Figure 3b). Paper tape was used to maintain the head in place during the sled acceleration. As soon as the deceleration started and the dummy moved forward with respect to the bench, the tape broke and the contact with the support components was lost.
- Reduced bench angle (S3): the main difference with S1 was that the bench cushion angle was reduced to 5 degree (instead of 15 degrees in the baseline standard bench).
- Reduced bench angle and strapped CRS (S4): the main difference with S3 is that straps were added to attach firmly the CRS to the bench.
The shield component used in S3 and S4 was the same model as SH1. The shield component used in S1 and S2 was very similar but not identical to SH1 (there were some differences visible on the plastic belt guide). No differences were visible on the bases and backs used in the various tests.
Relevant instrumentation included abdominal sensors (APTS), chest deflection (IR-TRACC), belt forces at the shoulder belt, lap belt and buckle. Four cameras were used to document the tests. All tests were performed by Dorel SA.
a) Overview of the four tests (S1, S2, S3, S4). A normal bench cushion was used for S1 and S2 and a reduced angle cushion (5 degrees) was used for S3 and S4. In S4, the CRS was strapped to the bench at the base (red), lower back (blue) and upper back (not visible).
b) Test S2: Back targets (white spheres) and systems used to maintain the initial position (thin black rod point down below the upper targets and polystyrene plate around the head) Figure 3: Overview of the sled test setup
3) Geometrical analysis and human simulation
Simulations were performed using a 3 years old (YO) human model. The model is based on the nonlinear scaling (Dual Kriging) of a 6 YO child model to match the anthropometric dimensions predicted by GEBOD for a 3 YO child. The 6 YO model validation included a variety of thorax and abdomen loading cases (PMHS studies from the literature) as well as sled testing (volunteer or PMHS data) (Beillas et al., 2013, 2014).
In order to prepare the simulations with the human model, a geometrical analysis was first performed with a dummy. A Q3 was seated on a shield CRS placed on a Peugeot 806 rear seat. All components were scanned successively using a laser surface scanner. The car seat was scanned alone first, then with the CRS base, back and the Q3 dummy, and finally with the shield added. For this analysis, a CRS similar to SH1 (same CRS without lower base) was used. Then the human 3 YO model was positioned using the dummy as a reference.
The test configuration S4 was then simulated using a R129 bench model provided by TU Berlin (described in Beillas et al., 2014). The CRS was simulated using different rigid components based on the surface scans for the base, back, and shield. The components were attached to the bench using springs to simulate the straps. The simulation was mainly analyzed in terms of kinematics. A preliminary analysis of the load distribution by body region was performed by separating the contacts between the human model and the environment as described in Figure 4.
Figure 4: Contacts defined on the 3 YO human model to separate the loads applied to different body regions. From left to right: head/neck, thorax, abdomen, pelvis and lower extremities (grouped on the picture here) Results
1) Direct trunk loading on a material testing machine
For reference, the results obtained using the belts are summarized on Figure 5. The stiffest response (in terms of force vs. applied displacement) was obtained for the wide belt in the lowest position (partially engaging the pelvis), followed by the wide belt on the mid abdomen and the narrow belt on the mid abdomen. Force vs. abdominal pressure relationships were in the reverse order: for a given force, the highest pressure was obtained for the narrow belt, and the lowest pressure was obtained for the wide belt when the upper pelvis was engaged. However the pressures vs. belt displacements relationships were similar for all three cases: they were almost identical for the two wide belt cases, and slightly lower for the narrow belt.
Figure 5: Results of the direct belt loading tests. The belt position is shown in the upper left pictures. Stiffness responses (upper right) and pressure vs force responses (lower left) were more variable than the pressure vs. applied displacement responses (lower right). Legends: Belt 50 corresponds to a standard vehicle belt (wide)
and Belt 32 to a narrower belt (narrow). The position is mid abdomen by default.
Responses for the baseline shield SH1 in the three positions are summarized in Figure 6. The stiffness responses (applied force vs. applied displacement) of SH1 were between the lower and mid wide belt loading configurations, and the effect of the shield location on the response was somewhat limited. The variations were much larger when considering the thoracic deflection and the pressure vs. force responses. These variations seemed related to geometrical involvement of abdomen or thorax. For example, when the shield was low, the thoracic deflection was the smallest (as a function of force or displacement) and similar to the wide belt low configuration. On the opposite, chest deflections were largest for the mid and upper shield locations. Pressures for belt mid abdomen and high shield (both cases without pelvic involvement) were also similar up to 1000 N loading while the belt low and other shield configurations were similar up to 2000 N (cases with some pelvic involvement). The regional responses (deflection and pressure) seemed therefore more affected by the shield position than the overall stiffness. The applied displacements vs. pressures characteristics were similar for all SH1 configurations and belts.
The responses with the shields SH2 and SH3 are provided in Figure 7. The response with SH3 was similar to SH1. However, SH2 had a lower stiffness response and a higher pressure for a given force than SH1 or the wide belt in the mid abdomen. Pressures vs. applied displacements characteristics were again similar with these shields.
Figure 6: Results for the baseline shield SH1. Top left: positions used in the test. Top middle and right: thoracic deflections as a function of applied displacement or applied force. Bottom: force vs. applied displacement (stiffness) and vs. pressure responses. Legends: Belt 50 corresponds to a standard vehicle belt (wide) and Belt
32 to a narrower belt (narrow). The position is mid abdomen by default. Shield 1 is SH1.
Figure 7: Results for the shields SH2 and SH3. Top left: positions used in the test. Top middle and right: thoracic deflections as a function of applied displacement or applied force. Bottom: force vs. applied displacement (stiffness) and vs. pressure responses. Legends: Belt 50 corresponds to a standard vehicle belt (wide) and Belt
32 to a narrower belt (narrow). The position is mid abdomen by default. Shield 2 is SH2 and Shield 3 SH3. Mid Low High
2) Sled testing
All sleds where the CRS was restrained by the belt (S1, S2, and S3) resulted in a kinematics with relatively large vertical rotations of the CRS towards the end of the test (left to right from the dummy viewpoint). The CRS did not move forward in the case of S4 due to the strapping. The shield interactions with the trunk were difficult to observe in the tests S1 and S3 due to the back hiding the interaction. The view was more open in the case of S2 even though a quantitative video analysis was difficult: due to the rotations, the lower spheres were partially masked from one side early in the test and contacted the base after 60ms. An illustration of the positions in the test S2 at 60ms is provided in Figure 9. A rotation of the lower spheres mount is visible on the left camera, associated a forward motion of the knee target visible on the right camera.
Corresponding measurements on the thorax (deflection) and abdomen (pressures) are summarized in Figure 10. Response curves were similar for all tests, with more dissymmetry in the pressure responses for the flat bench cases (S3, S4) and a higher pressure on the right side. All peaks were reached around 90ms. Peak values were much higher than in the direct loading test. Peak pressures were similar for S1 and S2 (36 and 37mm, 1.3 and 1.3 bar, respectively). They were higher however for the tests with the flat bench: 43mm and 2.1 bar in S3 and 43mm and 1.8 bar in S4.
Figure 9: Test S2 (backless) illustrations of the kinematics. Initial position (left) and 60ms (right) from the left (top) and right (bottom) cameras. Notice the evolutions of the lower spheres frame and the forward motion of
the knee target.
S1 D=36mm@94ms PR=1.2 bar (90ms) PL: 1.3 bar (82ms) S2 D=37mm (93ms) PR=1.3 bar (93ms) PL=1.2 bar (90ms) S3 D=43mm (93ms) PR=2.1bar (91ms) PL=1.6 bar (90ms) S4 D=43mm (91ms) PR=1.8 bar (90ms) PL=1.5 bar (89ms) Chest de fl. (m m) Ab d o Pre ss ure (b ar)
Figure 10: Chest deflection and abdominal pressure time histories and peak values (D means deflection, PR pressure right and PL pressure left).
3) Geometrical analysis and human simulation
Geometrical results are summarized on Figure 11. After scanning the position of the Q3 in the shield CRS and overlaying the human model, it appeared that the pelvic region seemed below the lower edge of the shield near the dummy (Figure 11a, right). For the human, this was confirmed by removing the flesh and other components (Figure 11b). The shape of the shield was similar to the human shape (Figure 11b). This similarity was even more striking when comparing with the dummy (Figure 11c): the internal shield shapes of SH1 and SH3 were almost exactly matching the Q3 abdomen. There were more differences between SH2 and Q3 abdomen, and between SH1/SH3 and Q1.5 (which has a different diameter)
a) Seating positions for the Q3 and the child model. Left: Q3 scan on shield CRS (SH1 shield) on top of a Peugeot 806 back seat. Center: Overlay of the human model and Q3 scan. Right: zoom in the pelvic region
b) Shield vs. human model shapes. Left: the pelvis is located below the lower edge of the shield. Center and right: the shapes of the shield and human model are somewhat conforming whether in a sagittal or axial plane
SH1 and Q3 SH2 and Q3 SH3 and Q3
SH1 and Q1.5 c) Dummy abdomen shapes vs. shields: the shape is very conforming for the Q3 and the shield SH1 and SH3 but not as much for the shield SH2 and the Q3, or the shield SH1 and the Q1.5
Results from the simulation of S4 with the human model positioned as in Figure 11 are summarized in Figure 12. In terms of overall kinematics, trends shown in Figure 12a suggest some differences between dummy and human: the spine seemed more curved for the human, especially in the thoracic region. As a consequence, the upper body of the human model appeared to be more forward and above the shield than the dummy. When looking at the internal kinematics (Figure 12b), the pelvis did not seem to be engaged by the shield. The pelvis passed in the space between shield and base and this was associated with lumbar flexion (submarining like behavior?). Resulting forces by body regions (Figure 12c) were highest for the thorax followed by the abdomen (both between 3 and 4kN). Forces to other body regions were small until a hard contact was established between head and shield (both modelled as rigid) which should not be considered in the analysis.
a) Test S4 vs simulation at 96ms. The human spine seemed more curved than the dummy
b) Detail at 76 ms: the pelvis is clearly below the shield in that case and a thoracic compression is visible
c) Force distribution by region. The peaks at 90 ms correspond to a head contact with the shield. The forces are very high as both were modeled as rigid components.
Figure 12: Summary of simulation results Discussion
Three approaches were developed concurrently to study the interactions of shield CRS with the abdominal and thoracic regions.
Direct trunk loading tests suggested that, in some cases, shields can effectively distribute loads compared to belts (e.g. higher force for a given abdominal pressure) but that this distribution is very sensitive to the position of the shield. In general, abdominal pressure and thoracic deflection seemed to reflect the shield interactions with the pelvis, thorax and abdomen. When the pelvis was not engaged at all (SH1 high), the responses were similar to direct abdominal loading in the mid abdomen by a standard lap belt. One reason may be that a standard width belt covers most of the
shield was found to conform to the Q3 abdomen (similar to a belt). When the pelvis was engaged (upper pelvic region), pressures and deflections were lower. The responses of shields SH1 and SH3 in mid position (engaging the pelvis) were similar to the standard belt engaging the pelvis. In contrast, the abdominal pressure was higher for a given force for the shield SH2 (until 1500N approx.). To the contrary of SH1 and SH3, SH2 shape does not match the Q3 abdomen at the beginning of the loading (Figure 11), which may explain this initial additional pressure. In all cases, it seemed that the abdominal pressure represented the displacement applied to the belt more than the force. This could be explained by the fact that, for the configurations tested, the belt displacement always resulted in abdominal compression while the forces were sometimes transferred to other structures (thorax and pelvis).
The main limitation of the direct loading study is that the displacement metric used to represent the compression is the applied displacement (piston). This does not account for variations of belt extensions or shield deformations in various tests. While it is believed that this effect may have been minimized by the use of a similar start position and that the deformations are relatively small considering the applied forces, this was not demonstrated and the lack of video data in most tests prevented a more in depth analysis. Another limitation is that the forces applied and resulting pressures were much lower than those observe in sled testing. This may have prevented the observation of nonlinear phenomena later in the response.
In the four sled tests, abdominal pressures that were measured (1.3-2.1 bar) were similar or larger than IARV proposed for belt loading in Beillas et al., (2012). Thoracic deflections (36 to 43 mm) were also relatively high. The kinematic analyses were made difficult by the CRS back and the rotation of the CRS. However, results obtained in the backless test seemed to suggest that the pelvis was not engaged by the shield (submarining like behavior?), hereby resulting in abdominal loading and relatively high pressures. The geometrical analyses on the dummy scan and human model (Figure 11), as well as the human simulation results (Figure 12) seem to concur with this interpretation. Similar lack of pelvis engagement was also observed for some cases in Tanaka et al (2009), and hypothesized in previous studies such as Johannsen et al (2012). The tests with reduced cushion angle (5 degrees) led to further increase in abdominal pressure compared to the standard bench (15 degrees). This may suggest that the 15 degrees angle contributed to the pelvic (and CRS) restraint and reduced the load seen by the abdomen. The relative pressure reduction (2.1 in S3 vs. 1.8 bar in S4) when the CRS was strapped could suggest that part of the CRS mass (around 9.3kg in total) may have also contributed to the body loading in S3. These hypotheses could be tested either with additional instrumentation or by performing dummy simulations. Overall, while obtained with a single shield CRS model, these results suggest that APTS could be sensitive to restraint conditions. Other results in the literature (e.g. Johannsen et al., 2012) seem to suggest that they could also be sensitive to the CRS model used (in a given restraint condition).
The main limitation of the sled study was that no repetitions were performed due to the lack of available test time and experimental difficulties encountered for the backless test. The shield CRS model also differed slightly from the baseline SH1 in the last two tests.
Results from the human simulations have to be considered as preliminary. Only one simulation (S4) was shown in the current study. Simulations of the other tests were attempted but the X and Z rotations observed in the tests were not captured and would require more investigation. Besides the limited 3 YO specific validation (partially due to the scarcity of 3 YO data in comparison to 6 YO), many simplifying assumptions (rigid CRS, arm position not raised, no pretension in the belt, etc.) also have to be considered as limitations. Work is ongoing on model improvement and positioning methods within the FP7 EC Funded project PIPER (www.piper-project.eu). For now, it seems however difficult to formulate hypotheses which, considering the pelvis location shown in Figure 11, would lead to significant pelvic engagement during the deceleration.
Conclusions
Overall, the results suggest that while shields can distribute effectively the loads over different body regions in direct loading, this distribution is very dependent on geometrical parameters and does not necessarily apply to sled configurations. High abdominal pressures observed in sleds in this and other studies (e.g. Johannsen et al., 2012) may be the consequence of a relative lack of pelvic restraint. In direct loading, abdominal pressures were found to be useful to help understanding the load distribution between body regions. They could also be helpful in sled testing as direct observations of the interactions of shield CRS with the dummy are especially difficult.
Acknowledgements
The work performed in the current study was supported by the French Ministry of Industry and the Pays de la Loire Regional Council though the ProEtech Competitive Cluster project. The authors would like to acknowledge Dorel SA and its testing team for performing the sleds tests. The authors would also like to thank A. Ratzek (ADAC) for fruitful discussions along the study and for providing some of the CRS used in testing. Finally, the authors thank the TU Berlin for making the bench model available.
References
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