Repeatability and Reproducibility

The repeatability values obtained are the result of several sources of error in our system. Inher- ently, the accelerometer precision can contribute to error of up to 5% for the force and impact energy measurements of specimen 1622, and 2% for the measurements of the other specimens, however this will not affect the repeatability of the impact duration. The QDAC gain accuracy was determined to be negligible, accounting for only 0.1% error. The remaining sources of error are due to experimental setup, procedure and post processing.

The difficulty in maintaining identical skull positioning hit to hit probably accounted for the majority of error observed. The specimen was susceptible to shifting thus altering the initial impact condition. This shifting was the result of a number of reasons including the pot sliding in the bracket or the skull shifting on the carriage bolt when the rubber grommits got squished or displaced. In an attempt to alleviate this, a laser pointer was used to mark the position of the specimen before each hit, however this introduced a new inconsistency. Specifically, the operator realigning the laser marker may have different standards of precision. Specimen 1643 sites 4 and 5 as well as specimen 1652 site 2 was tested twice with and without diligence in positioning the head with respect to the laser allowing us to comment on the effect

Chapter2. Design andDevelopment of aHeadImpactorSystem 71

of laser use. Figure 2.14 indicates that the deviation of sites with significant laser diligence were smaller than those obtained by the initial test for nearly all repeatability levels and impact characteristics suggesting that care in intermitting positioning can have a drastic effect. This however is not supported by the RMSE values, as the two conditions result in an inconclusive effect (see Figure 2.15).

Post processing can be attributed to the larger standard deviations in the impact duration deviations. The automated peak width code identified peaks by marking the distance between two threshold values which were defined as the point where the curve reached 10% of the overall peak height. To locate these points, the code first identified all points lying above this threshold and then found the two particular values that corresponded with the first and last elements of a peak (see Appendix C. Errors in the execution of this code occurred when noise or drift jumped above the 10% threshold misidentifying the true event start or finish. In some cases, this could cause deviations of up to 20% in the impact duration repeatability, however extreme cases were double checked manually and rectified. Despite our best efforts, larger deviations (such as the maximal deviations of the impact duration in Figure 2.9) may be due to similar errors that were not double checked.

The material and structural properties of the specimen were expected to affect the repeata- bility of the testing, however this was not fully confirmed with testing. For example, specimen 1625 and 1652 were observed to have very porous bone while sanding locations for gauge application, and therefore it was expected that micro breaks in the trabeculae would affect the overall repeatability of these specimens. However, the deviations in peak force and impact en- ergy as well as the RMSE values of these specimens were comparable to those of the remaining specimens (see Figures 2.9, 2.10, 2.11, and Table 2.2). Nevertheless, there was a distinct in- crease in deviation of the impact duration (see Figures 2.9c, 2.10c, 2.11c) for these specimens which suggests that weaker material properties inconsistently alter the impact duration of each strike.

Chapter2. Design andDevelopment of aHeadImpactorSystem 72

(a) First level repeatability comparison of laser use. Deviation values are the mean deviation values of all specimens and sites of a particular impact characteristic.

(b) Second level repeatability

(c) Third level repeatability

Figure 2.14: Laser use comparison of deviation values. Deviation values are the mean deviation values of all specimens and sites with two trials completed (with and without diligent use of laser; Sp. 1643 S.4 and S.5 and Sp. 1652 S.2) of a particular impact characteristic.

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Figure 2.15: The average RMSE values of both heights for each condition (Sp. 1643 site 4 and site 5 and Sp. 1652 site 2) with and without laser use

Chapter2. Design andDevelopment of aHeadImpactorSystem 74

to facial and cranial sites were comparable, however the facial sites were apparently more sus- ceptible to faulty impacts such as the double peaked strikes, most likely due to the number of protrusions around a limited spatial area that might impinge on different areas of the im- pactor surface (for example, the nasal bone impinging on the projectile before the strike at the intended site). Specifically, four of the six events with bad trials (see Figure 2.9) were facial sites, and a fifth is site 3 which can include protrusions such as the brow bone validating this argument. Furthermore, although it is not supported by the deviations nor the RMSE values, testing observation seemed to suggest that shifting of the skull on the carriage bolts was found to occur more often on the facial sites because of the tilted orientation of skull. This may be supported weakly by the third level repeatability results. For example, 70% of all deviations between the first three trials and the last three trials over 15% were found among facial sites and all were found among sites 1, 2 and 3 (Figure 2.11). This suggests that at some point during testing a large cumulative deviation occurred, most likely accounted for by subtle differences in initial impact conditions throughout the repeated impacts due to shifting.

Finally some inconsistencies in repeatability may be accounted for by inconsistent friction losses in the tubed track, however this is most likely minimal for freefall condition considering the lack of effect height seems to have on repeatability (see Figure 2.12). Most likely inconsis- tent friction losses occurred with differences in the initial release of the projectile. For example, inconsistent pin removal may cause the projectile may be nudged upon release causing the ini- tial begin falling with a wobbling motion. This wobbling motion would both inconsistently increase the friction of the edges of the projectile on the walls of the tube, and would also alter the initial impact condition accounting for an undetermined component of the overall deviation.