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frequency domain within “nCode 11.1 glyphworks” using the FFT function. 5 to 200 Hz was chosen for the BEV RESS test profiles within this study and is illustrated in Figure 25.

A peak frequency of 200 Hz was chosen to ensure that it could be replicated on hydraulic shakers (which typically display performance run off above 250 to 300 Hz [45, 117]), whilst a 5 Hz starting point was chosen so that the profile could be conducted on electromagnetic shaker facilities which typically cannot replicate frequencies below 5 Hz (due to the lower displacement associated with these

facilities). Reviewing PSD’s of the test data highlighted that the majority of vibration energy occurred between 0 to 120 Hz.

Figure 25: Illustration of Selected Frequency Bandwidth for Synthesised Test Profile

3.4.5 Optimise Test Duration

Within this study, the test was optimised with respect to time for two different test durations of 50 hours and 150 hour per axis. 50 hours was chosen so that 1 hour was representative to 2000 miles of durability loading in the desired test axis. 150 hours per axis was chosen as a conservative test duration that would result in lower shock loading and subsequently a higher degree of correlation with the in-service environment. Figure 26 shows a SRS of each of the two test durations compared to the SRS analysis of the pre sequenced data in gn (as shown by the red dashed line

in Figure 26).

Figure 26: Effect of Test Duration of Derived PSD’s SRS in Relationship to the SRS of Un- sequenced Surface Data

Majority of vibration energy occurs from 0 to 120 HZ Test spectra start frequency of 5Hz chosen to allow for limitations of electromagnetic shakers

Test spectra end frequency of 200 Hz chosen to allow for limitations of hydraulic shakers.

It can been seen that the 150 hour test duration is well within the SRS of the baseline SRS, whilst the 50 hour test has a greater shock loading than the pre sequenced data from 5 to 20 Hz and 55 to 70 Hz which indicates that this test is over accelerated with respect to its time compression within these frequencies. Subsequently the 50 hour test duration is not considered to be representative of the in service condition.

3.4.6 Derived Test Profile

To define a generic test profile that would be suitable for a wide range of passenger BEV’s the generated PSD for each axis from each vehicle were overlaid. The peak values were selected. These peak values were then used to derive a simplified PSD that enveloped the greatest vibration witnessed by the RESS within A and C segment BEV’s within this study. This peak enveloping process is illustrated in Figure 27.

Figure 27: Example Z-Axis Test Profile Derived by Peak Enveloping of Derived PSD for Each Test Vehicle

The derived profiles were defined via no more than 15 break points of variable frequency spacing. This was to ensure that they could be uploaded into a wide range of shaker system controllers whilst maintaining suitable PSD resolution to define the desired vibration loading.

The subsequent synthesised vibration test profiles, representative of 100,000 miles of durability for the X, Y and Z axis of a RESS are illustrated in Figure 28, whilst the break points of the synthesised profile are defined in Table 10.

Figure 28: Synthesised Test PSD’s for 150 Hours Test Duration per Axis

Table 10: Break Points for Derived BEV Random Vibration Profile Representative of 100,000 Miles of Customer Usage in UK for 150 Hours Test Duration

Frequency (Hz) X Axis PSD for Battery (150 Hours Test Duration) gn 2 /Hz Grms = 0.361 Frequency (Hz) Y Axis PSD for Battery (150 Hours Test Duration) gn 2 /Hz Grms = 0.269 Frequency (Hz) Z Axis PSD for Battery (150 Hours Test Duration) gn 2 /Hz Grms = 0.524 5 0.0016585 5 0.0019155 5 0.003062 9 0.0084295 10 0.0011905 9 0.0078005 14 0.0024565 14 0.002253 11 0.010018 19 0.0038555 18 0.0015989 13 0.010018 23 0.0038555 28 0.00068665 19 0.002771 32 0.001272 39 0.001444 25 0.010439 39 0.0013865 67 0.00026895 26 0.010439 60 0.0002894 85 0.0002043 30 0.003168 100 0.0001643 120 0.00005535 32 0.0032675 200 0.000019045 200 0.000016065 45 0.0021935 68 0.00134415 100 0.00027795 120 0.0001233 150 0.0001044 200 0.000088275

Comparison of Test Standards to Measured Data

3.5

Within this Chapter the FDS and SRS of the synthesised profiles (presented in Figure 28 and Table 10) are compared to the FDS and SRS of the RESS tests standards presented in Chapter 2. The purpose of this is to understand the suitability of contemporary standards for vibration durability assessments via comparisons to test profiles that have been derived from real world BEV RESS measurements to replicate 100,000 miles of UK customer usage.

Within this study, the FDS and SRS were calculated using an assumed damping of 5 % (ܳ = 10). This value of damping was chosen based on the work discussed within [71]. All FDS within this study are calculated for a comparative dimensionless analysis; they use a default system stiffness (ܭ) of 1 N/m3 as the proportional constant between displacement and stress for a single degree-of-freedom, and a Basquin coefficient ܥ of 1. In the calculations the value of the Basquin exponent material parameterܾ, was set to 4. 4 was chosen as the value for the material parameter of ܾbased on the study discussed in [71] and the analysis of empirical data discussed in [113, 118]. A summary of the parameters utilised within this study are shown in Appendix E to allow for the replication of the results from this investigation in future studies.

For the analysis of ISO12405, the 21 hour test duration was chosen within this comparison as this is the longest test duration provided and therefore assumed to be the most damaging. For USABC Procedure 10, the long and short test durations for the random profile procedures are both presented. Within the swept sine tests for USABC procedure 10, the additional 6000 sine cycles for the X, Y and Z axis were performed at 13 Hz (for the Z axis) and 19 Hz (for the X and Y Axis). These frequencies were selected by reviewing where natural frequencies were witnessed within the measured vehicle data discussed in [26] and within the PSD profiles shown in Figure 28 and Table 10.

As discussed in Section 3.3.6, FDS is presented as a comparative measure within this study of fatigue potential of different test profiles and is not presented as an absolute measure of fatigue damage. The FDS and SRS functions with nCode V11.1 were utilised to develop the data presented within this Chapter. Whilst nCode V11.1 contains specific IP which produces refined SRS and FDS data it utilises the theory defined within Appendix A and B within the calculation of these spectra. 3.5.1 Comparison of FDS – Random Test Procedures

Figure 29 shows a comparison of the FDS of the derived BEV profiles to current test standards. Reviewing the potential fatigue damage for the Z-axis (as shown in Figure 29a), it is noticeable the derived profiles have a comparable fatigue damage from 30 to 70 Hz as the USABC Procedure 10 standard. However, the synthesized profiles typically have a greater fatigue loading from 5 to 15 Hz. Both the ISO12405 and BS62660 Z-axis profiles have a greater fatigue loading than the profiles

generated from BEV battery measurements, indicating they are too aggressive for durability assessments.

a)

b)

Figure 29: FDS of Standards Utilising Random Profiles vs FDS from Derived Test Profiles a) Z -axis, b) X and Y-axis

Assessing the X and Y-axis FDS (as shown in Figure 29b), it is noticeable that the potential fatigue damage of the USABC Procedure 10 standard is lower than that generated by the synthesised test data from 15 to 30 Hz for the X-axis. This

demonstrates the importance of having two separate test profiles for the X and Y- axis of the vehicle and not relying on a simplified profile for the horizontal and longitudinal axis. However, it must also be noted that the USABC Procedure 10 also has been developed from North American market data, wherein the profiles developed in this study have been generated from European surfaces weighted to a UK customer usage. Also two “A segment vehicles” (Smart ED and Mitsubishi iMiEV) were measured within this study, both of which are likely to have greater roll and pitch moments due to their compact dimensions that the vehicles used within the development of the USABC manual such as the General Motors (GM) EV1 [119].

Like the Z-axis both BS62660 and ISO12405 show a significantly higher fatigue damage potential than profiles derived from 100,000 miles of durability suggesting that they are too aggressive to determine the degradation of battery assemblies, with normal customer use and are only suitable for robustness studies. It is also notable that the fatigue potential of all the random profiles currently available is significantly higher in all three axis at frequencies above 70 Hz than the profiles derived from BEV measurements.