Capítulo 1. La Unión Europea. Un agente global
1.1. Un proceso integrador sui generis. Antecedentes, origen y desarrollo de la Unión Europea
7.1 Introduction
Motivation
It was demonstrated in the previous chapter that the vehicle fleet has increased in lateral stiffness continuously with vehicle design year. Figure 6.2 provided support to the third elementary research question of chapter three, that the newer vehicles are more resilient to intrusion. Although the degree of residual intrusion was shown to have decreased, it only explains half of the story. Chapter three also outlined that it was the dynamic inward motion97 of the encroaching door which causes the injury. Also discussed was the intruding velocity of the target vehicle’s door being governed by the longitudinal to lateral stiffness ratio of the bullet and target vehicles. The behaviour of the door during impact has long been a valid research topic. Other work has shown that the depth of dynamic door intrusion is greater than residual static intrusion [Prasad et al. (1991)], where the degree of strain hardening during the plastic deformation determines the final residual state. So residual crush does not accurately reflect the true injury potential [Warner et al. (2007)]. It was demonstrated that the severity of injuries was predominantly affected by a difference in velocity between the occupant and intruding door, and thus effective countermeasures should reduce relative velocity rather than just increasing structural strength [Strother et al. (1984)]. Also influencing door response is
97Or punching motion as described by Lau et al. (1991)
the presence of a nearside occupant [Monk et al. (1980)], and the seats ability to crush the middle tunnel98 [Bohlin (1980)].
Complementing the dummy response measurements obtained from the US NCAP barrier impacts (analysed in the previous chapter), one can obtain data describing the dynamic motion of the intruding door. For an array of tests99 accelerometers were fitted to the struck front (and rear) door. Time-history charts are accessible and can be differentiated (with regards to time) to obtain door intrusion velocity profiles. In doing so, one could additionally assess if the dynamic motion of the intruding door has changed with the vehicle fleet. The aim of the research reported in the chapter was to assess:
• if the dynamic motion of the intruding door has reduced in a similar manner as the residual intrusion.
7.2 Statistical Analysis
Introduction
A series of NCAP barrier tests (n=72) performed on four door sedans were analysed to average intruding door behaviour during the first 100ms of barrier impact [Watson et al.
(2009)]. Results are shown in Figure 7.1. In later work, the authors went on to discuss that barrier impact test exhibit three common characteristics; first peak, valley and second peak [Campbell and Cronin (2014)]. The first peak occurs immediately after the barrier contacts the door resulting in a rapid increase in velocity of the door [Crandall and Pilkey (1999)].
The velocity then decreases (valley) as the door contacts the occupant. The second peak is reached as the occupant rebounds off the door. In Figure 7.1, the first peak occurs at 20ms, the valley at 30ms, and the secondary peak shortly after [Watson (2010)]. The NCAP barrier strikes the driver’s side of the vehicle, yet the statistical evaluations in the previous chapters had not investigated the influence of the seating position. The seating position of the struck-side occupant was assumed not to influence the dynamic motion of the intruding door.
In the work that follows, we derive similar graphs for a pool of vehicles with different stiffness categories as well as pairs of similar vehicles.
98As the door is contacted, the rigid seat frame is able to displace sideways towards the center console and its structural members crush the middle tunnel located in the vehicle underbody
99Only available for specific vehicle of years 1997-2006
7.2 Statistical Analysis
Fig. 7.1 Average door intrusion velocity for four door sedans in NCAP barrier impacts. The blue section shows the maximum intruding velocity, the valley is shown in green, and finally the secondary peak is shown in orange. Source: [Campbell and Cronin (2014)]
Resources
The VCT database that was used in the previous chapter, was queried for all barrier tests where accelerometers were fitted to the nearside front door of the struck vehicles (n=197).
This corresponded to vehicles with a model year from 1997-2006. Table 7.1 shows the number of vehicles fitted with accelerometers per vehicle year. In some circumstances, a sole accelerometer was fitted within the door cavity whereas, in other tests multiple accelerometers were used100. In either case, pulses were first viewed to remove any obvious errors in the accelerometer response101 (eg, dislodging), and the maximum response taken for each vehicle. As each accelerometer recorded acceleration102in the lateral (y-) direction at a given frequency, one could obtain the dynamic response of the door. Assuming zero initial velocity, one could obtain the instantaneously lateral velocity at each time step by v= vo+ a·t.
Table 7.1 The number of vehicles fitted with accelerometers in the front struck door during US NCAP barrier impacts from all barrier impacts.
Vehicle Year 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
nwith 25 20 26 16 27 15 17 19 25 7
Ntotal 25 20 26 16 28 17 17 19 28 25
100These were mounted at front-center, mid-rear and upper door positions
101Comments were attached to each measurement in which the data was classified as: questionable, channel failed or as measured. Only as measured was used in the analysis
102Acceleration was recorded in G’s and was converted to a velocity (m s−1)
Comparing vehicles of different stiffness clusters
Clustering vehicles by residual intrusion would allow for the dynamic door response to be compared. Figure 7.2 illustrates a scatter plot of residual intrusion resultant from the barrier impact as a function of vehicle year. Clusters of stiffness groups were then defined according to 20mm intrusion intervals. The red numbers on the right hand side, indicate the sample sizes within each stiffness cluster.
200 300 400 500
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Vehicle Year
Lateral Intrusion (mm)
Vehicles fitted with accelerometer in outborad front door Lateral Intrusion obtained from US−NCAP Barrier Test
n= 2
Fig. 7.2 Maximum residual intrusion from vehicles equipped with accelerometers in door from barrier impacts.
Determining average time-velocity response for a given stiffness cluster
The following section reports on the average response for a group of vehicles categorised within an equivalent stiffness cluster. Those vehicles equipped without tSAB and experienced 340-360mm resultant intrusion (n=8). For illustrative purposes, those vehicles with tSAB deployment and a similar stiffness were initially ignored (n=6). It was desired to maintain a simple dataset in order to create a series of explanatory figures. The accelerometer responses for the eight vehicles103104are plotted in Figure 7.3a.
At∼20ms, the difference in individual door responses appears noisy, however all eight vehicles in the selected cohort demonstrated an absolute maximum before velocity decreases again. This concurs with the previous research from the University of Waterloo, Canada [Campbell and Cronin (2014); Watson et al. (2009)]. In a raw analysis one could determine
103All measurements were taken from front-center position
104The dataset reduces to 8 vehicles from the original 22 in Figure 7.2, as vehicles fitted with tSAB were removed (n=6) and questionable/channel failed data (n=8) were removed.
7.2 Statistical Analysis
0 5 10 15
0.000.01 0.020.03 0.040.05 0.060.07 0.080.09 0.100.11 0.120.13 0.140.15 0.160.170.18 0.190.20
Time (s) Cohort of 340−360mm Resid. Intr vehicles
Driver's door intusion velocity during US−NCAP barrier tests
(a) Dynamic door behaviour for vehicles equipped with tSAB whom underwent 340-360mm maximum intrusion.
0 5 10 15
0.000.01 0.020.03 0.040.05 0.060.07 0.080.090.10 0.110.12 0.130.14 0.150.16 0.170.18 0.190.20
Time (s)
Median Lateral Velocity (m/s)
Cohort of 340−360mm Resid. Intr vehicles
Median (Driver's) door intusion velocity during US−NCAP barrier tests
(b) Median velocity-time response for the range of vehicles used in Figure 7.3a. The shaded areas represent a corridor of be-tween the 25 and 75thpercentiles.
Fig. 7.3 Velocity-time response of door.
the median response for the given vehicles at each time-step. By accounting for the 25 and 75th percentiles of the median, one was able to calculate a response corridor over each time-step. This is shown in Figure 7.3b. The blue lines indicates the average response in which an absolute (average) maximum intrusion was obtained at 20ms, before the valley-like characteristic response was incurred thereafter, and finally a secondary peak velocity at 30ms.
The median curve yields a plateau-like response after 30ms for another 20ms. This corresponds to the full engagement of the vehicle structure. Elements such as the door beam, the design of the floor sill and additional work of the B-pillar are now deforming in synchronisation – absorbing impact energy. Velocity further decreases from 50ms where the maximum deformation has occurred and the vehicle begins to move laterally. The rebound phase of the barrier begins at roughly 70ms, where a slight increase in velocity was the result of loading bearing structure collapse [Payne et al. (1997)].
The width of the corridor was simply too wide to confer meaningful results. One cannot expect when carrying out a similar analysis with a different (vehicle) stiffness cluster that, average response (inclusive of corridor) would differ.
One of the reasons that the intervals remain wide was due to the time-shift jitter associated with each vehicle. This was evident from the width of response curves derived in the period where velocity first increases in Figure 7.3a. Whilst all accelerometers are initiated at a given time prior to impact, the relative width of the responses in the first 10ms was concerning. To investigate this further the maximum intrusion velocity obtained for each of the vehicles was analysed. For this, refer to Figure 7.4.
0 5 10 15
0.00 0.01 0.02 0.03 0.04 0.05
Time (s)
Lateral Velocity (m/s)
Veh.m Veh.n Veh.o Veh.p Veh.q Veh.r Veh.s Veh.t Maximum (Driver's) door intusion velocity
Fig. 7.4 Time at which maximum intrusion velocity for vehicles used in Figure 7.3a was obtained.
Maximum peak intrusion velocity was obtained between 15-20ms for the given vehicles.
As the response corridor developed in Figure 7.3b accounted for the standard deviation at each time-step, the width of the corridor allowed for plenty of room to wiggle. In other words, where a velocity may have reached its peak at 20ms, another vehicle’s response may be reducing in velocity - thus the wide interval. This highlights a limitation associated with the raw analysis to derive average response corridors. That the given door intrusion characteristics which were described by the University of Waterloo research pieces are likely suppressed by slight differences in time response associated with each vehicle.
One manner to overcome such issues would be to normalise the time at which maximum intrusion velocity was obtained by manner of cross correlation. Yet the primary aim if the chapter was to assess if the dynamic motion of the intruding door has reduced in a similar manner as that seen with the residual intrusion. Therefore, a much simpler analysis can be conducted. It is known that:
• Maximum intrusion velocity is obtained∼20ms
• Post this initial peak, door velocity decreases (into a valley)
• A secondary peak intrusion velocity is incurred at∼30ms
Therefore, one can simply establish the (absolute) maximum velocity obtained over the entire impact duration, and a secondary (localised) peak velocity obtained after 27ms. Consequently, these peak values (absolute and localised) can easily be compared between the vehicles of different stiffnesses.
7.2 Statistical Analysis
Results
Comparing vehicles of different stiffness clusters, absolute maximum velocity
The maximum intrusion velocity is plotted in Figure 7.5 for an array of vehicle stiffness clusters. The clusters are defined based on 20mm intrusion intervals and are summarised in the legend105. Reading from right to left, the vehicles increase in stiffness - as they underwent less intrusion from the barrier impact. The inter-quartile range of the boxplots are generally overlapping to some extent, so no (statistically) significant result can be inferred, however a distinct trend becomes apparent. The median values for maximum intrusion velocity steadily decrease from right to left. As the energy imparted into the collision from the barrier remains constant, the negative-sloped trend suggests that the stiffer vehicles, not only reduce the maximum residual intrusion but also the peak intrusion velocity of the door. As there are few outliner points in Figure 7.5, one was able to interpret the whiskers106as containing all the values of absolute maximum intrusion velocities. The two extreme stiffness groups (most and least) contain overlapping whiskers and this lends support to only a trend being inferred from the data.
0 5 10 15 20
Vehicle Stiffness Clusters
Lateral Velocity (m/s)
s.280−300 (n=15) s.300−320 (n=15) s.320−340 (n=16) s.340−360 (n=21) s.360−380 (n=6) s.380−400 (n=15) s.400−420 (n=18) Maximum driver's door intusion velocity
Fig. 7.5 Peak maximum intrusion velocity for the clusters of vehicles stiffnesses with barrier impact at 61.90± 0.80kmh−1.
105s.280-300 are the cluster of vehicles whom underwent 280-300mm of static residual intrusion
106Tukeystyle boxplots as described in Chapter 5
Comparing vehicles of different stiffness clusters, secondary maximum velocity
Figure 7.6 analyses the secondary peak of intrusion velocity, that incurred after 27ms. The trends in Figure 7.6 and 7.5 are similar and indicate that a reduction in a secondary peak velocity was associated with newer vehicles. This secondary peak velocity correlates to the dummy rebounding off the door. The median velocity for the stiffest vehicles (s.280-300) was 8.56m s−1, whereas for the least stiff (s.400-420) vehicles, the velocity was 11.12m s−1. The inter-quartile range for these two groups were not overlapping, thus inferring with a greater level of certainty, that the door intrusion velocity and dummy rebound, has been reduced. The median vehicle year for the s280–300 and s400–420 groups were 1998 and 2002, respectively, where the distributions between the two groups significantly differed (Wilcoxon’s rank sum , W=253.5, p-value <0.001).
0 5 10 15 20
Vehicle Stiffness Clusters
Lateral Velocity (m/s)
s.280−300 (n=15) s.300−320 (n=15) s.320−340 (n=16) s.340−360 (n=21) s.360−380 (n=6) s.380−400 (n=15) s.400−420 (n=18) Time > 27ms
Secondary maximum driver's door intusion velocity
Fig. 7.6 Secondary peak maximum intrusion velocity for the clusters of vehicles stiffnesses with barrier impact at 61.90± 0.80kmh−1, where time >27ms.
7.3 Discussion
In chapter six it was demonstrated that vehicles with a late-model vehicle year generally yielded less residual intrusion than earlier model vehicles in the event of a lateral barrier impact. The results from the current chapter indicated that those vehicles which underwent less residual intrusion, also demonstrated reductions in maximum and secondary peak door intrusion velocity. Such a result indicates a correlation between reduced residual intrusion and velocity of the intruding door when maximum peak velocity was obtained and when
7.3 Discussion
the dummy rebounds off the intruding door. These two trends have contributed to improved fleet-wide crashworthiness ratings for lateral collisions. Thereby, the concerns of Strother et al. (1984) in which it was stressed that the relative velocity of the intruding door needs to be reduced has been addressed by vehicle structural and safety engineers. The ability of the tSAB to reduce the second peak velocity was not investigated solely due to the small sample sizes available.
The findings of this chapter, complement those from chapter five, in that one must meticulously account for changes in fleet stiffnesses if they are able to accurately estimate tSAB efficacy. As the units have been continuously integrated into a fleet of vehicles which also offer stronger intrusion resistance (both in terms of static and dynamic behaviour), much of the previous literature identified in the Side Airbag Accidentology section of chapter two failed to account for such stiffness changes. Nonetheless, significant improvements have been achieved with optimisation of the vehicle structure and this has consequently improved occupant safety in the event of a lateral impact.
It is important to recall the Discussion section from chapter 3.2. Volvo engineers con-ducted lateral crashes using two 1992 and 2008 model vehicles, to replicate a traditional barrier impact style collision. It was demonstrated that although residual intrusion was reduced, the door velocity at the time of occupant loading was similar [Sunnevang et al.
(2010)]. Yet, from the Controlled Collision Environment, it was just shown the velocity of the door at time of occupant rebound has been reduced. This trend, however, was only associated with one loading case, in which a fixed barrier collided against the vehicles at a given speed, injecting a given amount of energy into the impact. While the Volvo tests aimed to replicate the barrier-style impact, the bullet object was a real vehicle. One knows that the vehicle’s longitudinal stiffness has also increased with vehicle year (for example, as shown by Esfahani et al. (2011); Swanson et al. (2003)). In chapter two, a fundamental concept of vehicle crash compatibility indicated that the velocity of the intruding door is governed by longitudinal to lateral stiffness ratio of the two colliding vehicles [Careme (1991)]. So while the barrier stiffness has remained constant, the longitudinal stiffness of the vehicle fleet has increased. Therefore, one cannot infer that such a result from the Controlled Collision Environmentcan be projected to the real-world.
Proposed methodology to further evaluate the research question
The statistical analysis was restricted by the sample size and lack of current model vehicles.
The use of computer simulations could again be implemented to further analysis of the con-tributory research question outlined in Figure 1.1. It would provide the means of quantifying the improved vehicle rigidity associated with the current vehicle fleet. The use of such tools
could easily be traced within a popular vehicle model, such as the Volkswagen Golf. This could be achieved by integrating the material property card from the a previous vehicle model to the successive model. Likewise, the structural optimisations (such as those indicated in Figure 3.3) could be iteratively included into a simulation of the same vehicle model.
7.4 Chapter summary
The purpose of the chapter was to:
• account for the previously mentioned limitation of crash analysis and assess if the dynamic motion of the intruding door has reduced in a similar manner as residual intrusion
The analysis of a given set of vehicles indicated that in a fixed-energy barrier impact:
• A correlation was found between residual lateral intrusion and dynamic motion of the intruding door
– The magnitude of intruding door-velocity for newer and generally stiffer vehicles (in terms of residual intrusion) was less than that incurred by older, less stiff vehicles. This conclusion bridges one of the gaps of current knowledge by investigating the dynamic properties of the impacted door.
• The dummy-based thoracic benefit which was attributed to the tSAB deployed vehicles in chapter 7 was likely to be partly attributed to the motion of the door.
The chapter highlights one of the reasons why evaluation of the tSAB is a complex issue, especially when considering thoracic injury. As
• Newer, stiffer vehicles not only undergo less residual intrusion, but also the velocity of the intruding door has been reduced
– If contact was to be made with the occupants thoracic region, it would be
’punched’ with lower magnitude
• The greater stiffness of the fleet parallel to the increased availability of the tSAB may account for the series of literature which suggested a protective effect of the tSAB in field data.