Sled-based tribometer and process for measuring friction coefficients

Abstract

Embodiments of a device and methodology for measuring friction coefficients of different irregularly shaped objects against different materials are described. One example device includes a sub-frame slidably coupled to a frame. The device further includes an object mount coupled to the sub-frame, a sliding surface under the object mount, and an instrumented sled on the sliding surface. The instrumented sled includes a support platform to support a material sample under the object mount. The device further includes a tension loadcell coupled to the instrumented sled and a tension load application coupled to the tension loadcell. The instrumented sled is configured to measure a normal force applied to the material sample by an irregularly shaped object coupled to the object mount during operation of the device. The tension load application is operable to apply a tangential force to the instrumented sled. The tension loadcell is operable to measure the tangential force.

Description

Attorney Docket: 222204-2950 SLED-BASED TRIBOMETER AND PROCESS FOR MEASURING FRICTION COEFFICIENTS CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Application Serial No. 63 / 515,214, filed July 24, 2023, titled “SLED-BASED TRIBOMETER AND PROCESS FOR MEASURING FRICTION COEFFICIENTS,” the entire contents of which are hereby incorporated herein by reference. BACKGROUND

[0002] The ability of helmets to reduce head injury risk is evaluated through impact testing using dummy headforms. Various standards and independent test methods use different headforms having different outer material compositions. The head and helmet frictional interface can affect the head’s impact response. Additionally, as headforms have an irregular, non-flat shape, measuring their coefficients of friction (COF) with devices currently on the market is not possible. SUMMARY

[0003] The present disclosure is directed to embodiments of a tribometer device and methodology for operating the same that can be implemented to measure COFs of various irregularly shaped, non-flat objects. The tribometer device and methodology can be implemented to measure friction coefficients of different irregularly shaped, non-flat objects against different materials. The tribometer device is embodied as a sled-based tribometer device in various examples.

[0004] The sled-based tribometer device includes an instrumented sled that is positioned on a relatively low friction sliding surface under an irregularly shaped, non-flat object mounted on a vertically slidable sub-frame. The instrumented sled is operable to move horizontally on the low friction sliding surface and is configured to measure a normal force applied in a vertical direction to a top surface of the instrumented sled in some examples. The instrumented sled is also configured to measure a normal force applied to a material sample positioned on the top surface of the instrumented sled in other examples. The instrumented sled can measure the normal force applied by the irregularly shaped, non-flat object when a curved surface of the object contacts the material sample or the top surface of the instrumented sled. The sled-based tribometer device further includes a tension loadcell coupled to the instrumented sled and a tension load application coupled to the tension loadcell. The tension load application is operable to apply a tangential forceAttorney Docket: 222204-2950 in a horizontal direction to the instrumented sled while the normal force is being applied to the material sample or the top surface of the instrumented sled. The tension loadcell is operable to measure the tangential force. The sled-based tribometer device also includes a computing device coupled to the instrumented sled and the tension loadcell. The computing device is configured to compute a coefficient of friction corresponding to at least one of the irregularly shaped, non-flat object or the curved surface thereof based at least in part on the normal force and the tangential force.

[0005] Aspects and advantages of embodiments of the present disclosure will be set forth in part in the following description or can be learned from the description or through practice of the embodiments. Other aspects and advantages of embodiments of the present disclosure will become better understood with reference to the appended claims and the accompanying drawings, all of which are incorporated in and constitute a part of this specification. The drawings illustrate example embodiments of the present disclosure and, together with the description, serve to explain the related concepts of the present disclosure.

[0006] According to one example embodiment, a device for measuring coefficients of friction includes a frame and a sub-frame slidably coupled to the frame. The device further includes an object mount coupled to the sub-frame, a sliding surface under the object mount, and an instrumented sled on the sliding surface. The instrumented sled includes a support platform to support a material sample under the object mount. The device further includes a tension loadcell coupled to the instrumented sled and a tension load application coupled to the tension loadcell. The instrumented sled is configured to measure a normal force applied to the material sample by an irregularly shaped object coupled to the object mount during operation of the device. The tension load application is operable to apply a tangential force to the instrumented sled. The tension loadcell is operable to measure the tangential force.

[0007] According to another example embodiment, a method for measuring coefficients of friction includes mounting an irregularly shaped object on an object mount. The method further includes positioning a material sample on a support platform of an instrumented sled that is positioned under the irregularly shaped object. The method further includes applying a normal force to the object mount to cause a curved surface of the irregularly shaped object to contact a top surface of the material sample. The normal force is applied until a target normal force is detected by the instrumented sled. The method further includes applying a tangential force to the instrumented sled based at least in part on detecting the target normal force to cause the instrumented sled and the material sample to move in a horizontal direction away from the irregularly shaped object. The tangential force is applied until movement of the instrumented sledAttorney Docket: 222204-2950 is detected. The method further includes determining a coefficient of friction of the irregularly shaped object based at least in part on the target normal force and a detected tangential force that causes the movement. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Many aspects of the present disclosure can be better understood with reference to the following figures. The components in the figures are not necessarily to scale, with emphasis instead being placed upon clearly illustrating the concepts of the disclosure. Moreover, repeated use of reference characters or numerals in the figures is intended to represent the same or analogous features, elements, or operations across different figures. Repeated description of such repeated reference characters or numerals is omitted for brevity.

[0009] FIG. 1A illustrates a front view of an example tribometer device and an example implementation thereof according to various aspects and embodiments of the present disclosure.

[0010] FIG. 1B illustrates a front view of an example instrumented sled of the example tribometer device of FIG. 1A according to various aspects and embodiments of the present disclosure.

[0011] FIG.2 illustrates another front view of the example tribometer device of FIG.1A and another example implementation thereof according to various aspects and embodiments of the present disclosure.

[0012] FIG.3 illustrates a flow diagram of an example method that can be implemented using the example tribometer device of FIG. 1A according to various aspects and embodiments of the present disclosure.

[0013] FIG. 4 illustrates a plot of an example calculated COF over time for a single trial performed using the example tribometer device of FIG. 1A in accordance with at least one embodiment of the present disclosure.

[0014] FIG. 5 illustrates a chart of example static friction coefficients obtained for different headforms against multiple materials in accordance with at least one embodiment of the present disclosure.

[0015] FIG. 6 illustrates a diagram of example static friction coefficients obtained for different headforms of different condition against multiple materials in accordance with at least one embodiment of the present disclosure.

[0016] FIG. 7 illustrates a chart of example different headforms static COFs against high- density foam obtained in accordance with at least one embodiment of the present disclosure.Attorney Docket: 222204-2950 DETAILED DESCRIPTION

[0017] Various helmet standards and independent test methods use different headforms, including the European Standard EN960, Hybrid III, and National Operating Committee on Standards for Athletic Equipment (NOCSAE) headforms. These headforms have known limitations to their biofidelity, including their friction characteristics. Each headform has a different outer material composition resulting in different frictional interfaces between the headform and helmet. The EN960 headform does not include a skin layer and is entirely comprised of magnesium. Unlike the EN960, the Hybrid III and NOCSAE headforms have outer layers to mimic skin. The Hybrid III headform has a vinyl plastisol skin, and the NOCSAE headform has a polyurethane skin.

[0018] Multiple studies have evaluated the frictional characteristics of the Hybrid III and EN960 headforms. The Hybrid III headform reported coefficient of friction (COF) values to range from 0.75 to 1.07. Due to concerns that these values are too high, some test methods cover the Hybrid III headform with a stocking or skull cap to reduce friction. One study reported a COF of 0.26 when the Hybrid III was equipped with a skull cap and a COF of 0.17 when it wore a wig. Magnesium headforms have a lower friction coefficient with reported COF values ranging from 0.16-0.23. Although the NOCSAE headform is widely employed for helmet testing, no documented friction coefficients exist. Even though friction coefficients for the EN960 and Hybrid III headforms have been previously investigated, varying experimental methods with different materials make comparisons between headforms difficult.

[0019] One study measured the COFs of magnesium and Hybrid III headforms against a helmet lining material and directly compared them to the human head. The study concluded that both headforms were significantly different from the human head. However, human head COFs were measured on cadaveric samples and did not account for the power-law relationship between normal force and friction. COF comparisons between dummy and human heads must be made at matched normal forces because COFs will decrease with increasing normal force. Another concern that previous work has yet to examine is how the frictional characteristics of headforms change over time. Polyurethane and vinyl plastisol oxidize and degrade over time, and many headforms undergo repetitive impacts resulting in wear markings.

[0020] Standardizing headform frictional properties has the potential to enhance the biofidelity of impact responses during helmet testing and can provide information to optimize helmet designs. Toward that end, embodiments of the present disclosure include a tribometer device and methodology for measuring friction coefficients of different irregularly shaped, non- flat objects against different materials. Some embodiments describe a sled-based tribometer deviceAttorney Docket: 222204-2950 and methodology for implementing the same to, for instance, (1) quantify the EN960, Hybrid III, and NOCSAE headforms’ COFs, (2) compare COF across headforms and against multiple materials, and (3) quantify the effect of adding friction-reducing skull caps to the headforms. Example applications of the embodiments include optimizing headform biofidelity, refining helmet testing protocols, and defining frictional boundary conditions for computational models, among others.

[0021] The sled-based tribometer was used in various examples to quantify friction coefficients for the EN960, Hybrid III, and NOCSAE headforms across multiple materials. The sled-based tribometer was used in some examples to compare friction coefficients between new and used samples of each headform and further used to measure the effect of friction-reducing skull caps in other examples. In one implementation, the static friction coefficients for each headform against high-density foam were 0.29 ± 0.01 for the EN960, 0.97 ± 0.17 for the Hybrid III, and 1.16 ± 0.06 for the NOCSAE and the static friction coefficients against a helmet lining material were 0.48 ± 0.04 for the EN960, 0.83 ± 0.03 for the Hybrid III, and 0.99 ± 0.10 for the NOCSAE [mean ± standard deviation (SD)].

[0022] Turning to the figures, FIG.1A illustrates a front view of an example tribometer device 100 and an example implementation thereof according to various aspects and embodiments of the present disclosure. FIG. 1B illustrates a front view of an example instrumented sled 130 of the tribometer device 100 according to various aspects and embodiments of the present disclosure. The tribometer device 100 is embodied as a sled-based tribometer device that can be used to measure the COFs of different irregularly shaped, non-flat objects including different headforms against multiple materials according to at least one embodiment of the present disclosure. Although various examples of the present disclosure describe the implementation of the tribometer device 100 to measure the COFs of different headforms against different materials, the present disclosure is not so limited. The tribometer device 100 can be implemented to measure the COFs of various irregularly shaped, non-flat objects such as objects having one or more curved surfaces, for instance.

[0023] Referring amongst FIGS. 1A and 1B, the tribometer device 100 includes a frame 105 and a sub-frame 115 that is slidably coupled to the frame 105. The frame 105 is embodied as a fixed frame having vertical frame members 110a, 110b, 110c, 110d (or “members 110”) and horizontal frame members 111a, 111b, 111c, 111d (or “members 111”). Only the vertical frame members 110a, 110b and the horizontal frame member 111a are illustrated and denoted in FIG. 1A for clarity. The sub-frame 115 is embodied as a slidable sub-frame that is slidably coupled to the vertical frame members 110a, 110b, 110c, 110d of the frame 105. The sub-frame 115 is slidablyAttorney Docket: 222204-2950 coupled to the vertical frame members 110a, 110b, 110c, 110d in a manner that allows the sub- frame 115 to move up and down in a vertical direction along the members 110.

[0024] The tribometer device 100 further includes an object mount 120 coupled to the sub- frame 115. The object mount 120 is operable to receive and secure in place an irregularly shaped, non-flat object 190 (or “object 190”). The object 190 in various examples includes a curved surface. The object 190 can be embodied as at least one of a round object, a circular shaped object, an elliptical shaped object, a headform, a helmet coupon, or another irregularly shaped, non-flat object having a curved surface. The object mount 120 can receive and secure in place various types of irregularly shaped, non-flat objects such as different headforms in some examples. The object mount 120 can also receive and secure in place other types of irregularly shaped, non-flat objects having at least one curved surface in other examples.

[0025] The tribometer device 100 also includes a compressive normal load application coupled to at least one of the frame 105, the sub-frame 115, or the object mount 120. The compressive normal load application is operable to apply a compressive normal force FN to a top portion of at least one of the object mount 120 or the object 190 during operation of the tribometer device 100 (e.g., as shown in FIG. 1A). The compressive normal load application can generate a variable or constant compressive normal force FN. The compressive normal load application can be embodied as or provided by a mechanical driver (e.g., a spring, piston, lever, weight, human force), an electrical driver (e.g., a motor driven piston), a hydraulic driver (e.g., a hydraulically driven piston), a pneumatic driver (e.g., a pneumatically driven piston), or another type of driver that can generate and apply a compressive normal force such as the normal force FNto a top portion of at least one of the object mount 120 or the object 190 during operation of the tribometer device 100.

[0026] The tribometer device 100 further includes a base plate 125 positioned under the object mount 120. The base plate 125 is coupled to at least one of the frame 105 or to a surface upon which the frame 105 is positioned or coupled to. The base plate 125 includes a base plate surface 126 (or “surface 126”) located on a top portion of the base plate 125. The base plate surface 126 is embodied as a relatively low friction sliding surface. At least one of the base plate 125 or the base plate surface 126 can be embodied as and / or include at least one of Teflon or a Teflon cover, sheet, or layer in some examples.

[0027] The tribometer device 100 also includes an instrumented sled 130 (or “sled 130”) positioned on the base plate 125. An upper portion of the instrumented sled 130 includes a support platform 135 (or “platform 135”). The support platform 135 includes a support platform surface 136 (or “platform surface 136”) located on a top portion (e.g., a top surface) of the support platformAttorney Docket: 222204-2950 135. The instrumented sled 130, the support platform 135, and the support platform surface 136 are collectively operable to allow the support platform 135 to receive and support a material sample 195 (or “sample 195”) under the object mount 120 during operation of the tribometer device 100 in some examples. The instrumented sled 130, the support platform 135, and the support platform surface 136 are also collectively operable to allow the support platform 135 (e.g., the support platform surface 136) to directly contact, interface with, and move (e.g., horizontally) relative to a curved surface of the object 190 during operation of the tribometer device 100 in other examples. The material sample 195 can be embodied as and / or include at least one of a foam material, a helmet liner, polyester, Teflon, aluminum, sandpaper, or another material. A lower portion of the instrumented sled 130 includes a base 145. The base 145 includes a base surface 146 located on a bottom portion (e.g., a bottom surface) of the base 145. The instrumented sled 130, the base 145, and the base surface 146 are collectively operable to allow the base 145 (e.g., the base surface 146) to directly contact, interface with, and move (e.g., horizontally) along the base plate surface 126 of the base plate 125 during operation of the tribometer device 100. At least one of the base 145 or the base surface 146 can be embodied as and / or include at least one of Teflon or a Teflon cover, sheet, or layer.

[0028] During operation of the tribometer device 100 the sub-frame 115 is operable to move the object mount 120 and the object 190 up and down in a vertical direction to cause a curved surface of the object 190 to contact a top surface of the material sample 195. The compressive normal load application is operable to then apply the compressive normal force FNto a top portion of at least one of the object mount 120 or the object 190 (e.g., as shown in FIG. 1A). The compressive normal force FNis then transferred through the object 190 to the top surface of the material sample 195. The instrumented sled 130 is configured to measure a compressive normal force such as the compressive normal force FNapplied to the top surface of the material sample 195 by the object 190 during operation of the tribometer device 100. The instrumented sled 130 can be embodied as and / or include a pancake load cell that is configured to measure a compressive normal force such as the compressive normal force FN, for example. The pancake load cell can be embodied as and / or include a LCF 450 Low Profile Universal Pancake, Resolution 0.4 N in one example, although another pancake load cell may be relied upon in other examples.

[0029] The tribometer device 100 further includes a tension loadcell 150 coupled to the instrumented sled 130 and a tension load application 160 coupled to the tension loadcell 150. The tribometer device 100 also includes lines 165, 170 and a pulley 175 in the example shown. Each of the lines 165, 170 can be embodied as at least one of a cable, a wire, a rope, a belt, a chain, or another type of line or in-line connector or coupling component. The line 165 couples the tensionAttorney Docket: 222204-2950 loadcell 150 to the instrumented sled 130. The line 170 couples the tension loadcell 150 to the tension load application 160. The pulley 175 is coupled to at least one of the base 145 or a surface upon which the base 145 is positioned or coupled to. The pulley 175 supports the tension loadcell 150, the tension load application 160, and the lines 165, 170. During operation of the tribometer device 100 the pulley 175 guides the line 165 and the instrumented sled 130 along a horizontal path on the base plate surface 126 and further guides the line 170, the tension loadcell 150, and the tension load application 160 along a vertical path.

[0030] The tension load application 160 is operable to apply a tensile tangential force FTANto a side portion of the instrumented sled 130 by way of the tension loadcell 150, the lines 165, 170, and the pulley 175 during operation of the tribometer device 100. The tension load application 160 can generate a variable or constant tensile force FT in a vertically downward direction (e.g., as shown in FIG. 1A). The tensile force FT transfers through the line 170, the tension loadcell 150, the line 165, and the pulley 175 during operation of the tribometer device 100 to apply the tensile tangential force FTAN to a side portion of the instrumented sled 130. The tension load application 160 can be embodied as or provided by a mechanical driver (e.g., a spring, piston, lever, weight, human force, bucket with fluid), an electrical driver (e.g., a motor driven piston), a hydraulic driver (e.g., a hydraulically driven piston), a pneumatic driver (e.g., a pneumatically driven piston), or another type of driver that can generate the tensile force FT to cause the tensile tangential force FTANto be applied to a side portion of the instrumented sled 130 during operation of the tribometer device 100. The tension loadcell 150 is operable to measure a tensile force such as the tensile force FTand the tensile tangential force FTANapplied to the instrumented sled 130 by the tension load application 160 during operation of the tribometer device 100. The tension loadcell 150 can be embodied as and / or include an in-line tension load cell that is configured to measure a tensile force such as the tensile force FTand the tensile tangential force FTAN, for example. The in-line tension load cell can be embodied as and / or include a LCM300 Tension and Compression Loadcell Resolution 0.4 N in one example, although another in-line tension load cell may be relied upon in other examples.

[0031] The tribometer device 100 further includes a computing device 180 coupled to the instrumented sled 130 and the tension loadcell 150. The computing device 180 can be embodied as and / or include at least one of a computer, a laptop, a tablet, a smartphone, or another computing device. As described in further detail herein the computing device 180 is configured to compute a COF based at least in part on the compressive normal force FN and the tensile tangential force FTAN respectively applied to the instrumented sled 130 by the object 190 and the tension load application 160 during operation of the tribometer device 100. The computing device 180 is configured toAttorney Docket: 222204-2950 compute the COF based at least in part on a system friction between the base plate surface 126 of the base plate 125 and the base surface 146 of the base 145 on the instrumented sled 130. The computing device 180 is further configured to compute the COF based at least in part on a material- object friction between a top surface of the material sample 195 and a curved surface of the object 190.

[0032] FIG.2 illustrates another front view of the example tribometer device 100 and another example implementation thereof according to various aspects and embodiments of the present disclosure. FIG.2 illustrates an example implementation of the tribometer device 100 to determine a COF for a curved helmet coupon 290 (or “coupon 290”) applied directly against the support platform surface 136 of the support platform 135 on the instrumented sled 130 according to various aspects and embodiments of the present disclosure. In this example the curved helmet coupon 290 is embodied as a helmet shell coupon (e.g., sample) that is mounted to the object mount 120 in place of the object 190. The material sample 195 is omitted in the example shown to allow a curved surface of the curved helmet coupon 290 to directly contact and interface with the support platform surface 136 of the support platform 135 on the instrumented sled 130 during operation of the tribometer device 100. FIG. 2 demonstrates the versatility of the tribometer device 100 to determine COFs for various types of irregularly shaped, non-flat objects against different materials of interest, including a curved helmet sample surface (e.g., outer surface of a helmet shell sample) against a clean top surface (e.g., the support platform surface 136) of the instrumented sled 130.

[0033] FIG.3 illustrates a flow diagram of an example method 300 that can be implemented using the tribometer device 100 according to various aspects and embodiments of the present disclosure. The method 300 can be implemented using the tribometer device 100 to determine friction coefficients of different irregularly shaped, non-flat objects against different materials. The method 300 includes operations performed in a particular order for purposes of illustration and discussion. Those of ordinary skill in the art using the disclosures provided herein will understand that various operations or steps of method 300 or any of the other methods disclosed herein can be adapted, modified, rearranged, performed simultaneously, removed, include operations not illustrated, and / or modified in various ways without deviating from the scope of the present disclosure.

[0034] At 302, the method 300 includes mounting an irregularly shaped object on an object mount. For instance, the object 190 or the curved helmet coupon 290 can be mounted on the object mount 120 as described herein and illustrated in FIGS.1A and 2, respectively.

[0035] At 304, the method 300 further includes positioning a material sample on a support platform of an instrumented sled that is positioned under the irregularly shaped object. ForAttorney Docket: 222204-2950 instance, the material sample 195 can be positioned (e.g., placed, adhered) on the support platform 135 of the instrumented sled 130 which can be positioned (e.g., located) under the object 190 as described herein and illustrated in FIG. 1A. Alternatively the material sample 195 can be positioned on the support platform 135 of the instrumented sled 130 which can be positioned under the curved helmet coupon 290 mounted to the object mount 120 in another example.

[0036] At 306, the method 300 further includes applying a normal force to the object mount to cause a curved surface of the irregularly shaped object to contact a top surface of the material sample. For instance, the compressive normal force FNcan be applied by the above-described normal load application to a top portion of at least one of the object mount 120 or the object 190 to cause a curved surface of the object 190 to contact a top surface of the material sample 195 as described herein and illustrated in FIG.1A. Alternatively the compressive normal force FN can be applied by the normal load application to a top portion of at least one of the object mount 120 or the curved helmet coupon 290 in some examples to cause a curved surface of the curved helmet coupon 290 to contact a top surface of the material sample 195 or to contact the support platform surface 136 directly in other examples where the material sample 195 is omitted. The normal load application can apply the compressive normal force FN in various examples until a target normal force is detected by at least one of the instrumented sled 130 or the computing device 180.

[0037] At 308, the method 300 further includes applying a tangential force to the instrumented sled based at least in part on detecting a target normal force to cause the instrumented sled and the material sample to move in a horizontal direction away from the irregularly shaped object while the target normal force is maintained. For instance, upon one or both of the instrumented sled 130 or the computing device 180 detecting the aforementioned target normal force applied by the normal load application, the tension load application 160 can generate the tensile force FTto apply the tensile tangential force FTANto a side portion of the instrumented sled 130 as described herein and illustrated in FIG. 1A. The applied tensile tangential force FTANcan then cause the instrumented sled 130 and the material sample 195 to move away from the object 190 in a horizontal direction along the base plate surface 126 while a target compressive normal force FN is maintained. The tension load application 160 can generate the tensile force FT to apply the tensile tangential force FTAN in various examples until movement of the instrumented sled 130 is detected by at least one of the instrumented sled 130 or the computing device 180.

[0038] At 310, the method 300 further includes determining a coefficient of friction of the irregularly shaped object based at least in part on a detected target normal force and a detected tangential force that causes movement of the instrumented sled. For instance, the computing device 180 can use Equations 1 and 2 as defined and described herein to compute a COF for the objectAttorney Docket: 222204-2950 190 based at least in part on the aforementioned target normal force (e.g., a target value for the compressive normal force FN) and some magnitude of the tensile tangential force FTAN that is found to cause the instrumented sled 130 to move away from the object 190 in a horizontal direction along the base plate surface 126 while the target normal force is maintained.

[0039] Referring again amongst FIGS. 1A and 1B, the tribometer device 100 includes the object mount 120 such as a headform mount (e.g., embodied as or coupled to the fixed frame 105), the instrumented sled 130, and the tension load application 160. An inverted headform can be secured to the vertically slidable sub-frame 115 of the fixed frame 105 using a custom headform mount in some examples. This configuration allows for the compressive normal force FNon the instrumented sled 130 to be controlled by adding weights to the top of the headform mount.

[0040] The tribometer device 100 can be used to measure the static friction coefficient of irregularly shaped, non-flat objects including different dummy headforms. FIG. 1A depicts an example embodiment of the total tribometer device 100 including the fixed frame 105, the slidable sub-frame 115, the object mount 120, the base plate 125, the instrumented sled 130, the tension load application 160, and the tension load cell 150, among other components. FIG.1A also depicts the fixed frame 105 and the inverted mounted headform (e.g., the object 190), showing the normal and tangential forces (e.g., the compressive normal force FN and the tensile tangential force FTAN). FIG.1A further depicts the instrumented sled 130 containing the material sample 195 being tested, a pancake load cell of the instrumented sled 130 to measure the normal force application, and a low friction sliding surface (e.g., the base plate surface 126).

[0041] The instrumented sled 130 in the example shown includes a pancake load cell (e.g., LCF 450 Low Profile Universal Pancake, Resolution 0.4 N) placed between the material of interest (e.g., the material sample 195) and the low friction sliding surface (e.g., the base plate surface 126). The pancake load cell measures the compressive normal force FNapplied by the mounted object 190 to the material sample 195 being tested. After reaching a target normal force, the tension load application 160 embodied as a bucket in one example can be attached to the instrumented sled 130 with a steel cable (e.g., the line 165) laid over a pulley (e.g., the pulley 175). The tensile force FT and tensile tangential force FTAN can be incrementally increased by adding 0.20 grams (g) to the bucket until movement is generated. This applied tangential force can be measured and recorded in one example using the tension loadcell 150 embodied as an in-line tension load cell (e.g., LCM300 Tension and Compression Loadcell Resolution 0.4 N).

[0042] During operation of the tribometer device 100 the instrumented sled 130 experiences two frictional forces. The first frictional force is between the material of interest (e.g., the material sample 195) and the mounted headform (e.g., the object 190). The second frictional force is theAttorney Docket: 222204-2950 friction of the system which is between the base surface 146 of the base 145 on the instrumented sled 130 and the base plate surface 126 of the base plate 125. In one example the system frictional force is the friction between the base surface 146 embodied as a Teflon-covering over the base 145 of the instrumented sled 130 and the base plate surface 126 embodied as a Teflon-covering over the base plate 125. Examples herein assume the pulley 175 is frictionless. Before, during, and after a test configuration, the COF of the system can be measured at a matched normal force (e.g., a matched compressive normal force FN) to the headform (e.g., the object 190) test three times in one implementation. The friction of the system can then be used to compute the COF of the headform on different materials of interest using Equation 1 (Eq.1) defined below.Eq.1 where T is the tangential force (e.g., the tensile tangential force FTAN) in Newtons (N) from the tension in the cable (e.g., the lines 165, 170), ^system is the average static COF of the base surface 146 of the instrumented sled 130 and the base plate surface 126 over three trials, msled is the mass in kilograms (kg) of the instrumented sled 130, FH is the normal force (e.g., the compressive normal force FN) in Newtons (N) measured by the pancake loadcell of the instrumented sled 130 applied by the headform (e.g., the object 190), and mbucketis the ending mass (kg) of the bucket (e.g., the tension load application 160).

[0043] The static COF occurs just before movement of the instrumented sled 130 occurs and therefore the acceleration is zero, reducing the COF equation to Equation 2 (Eq.2) defined below.

[0044] The duration of impact events is typically between 5-15 milliseconds, and the static COF plays a dominant role in determining the response due to the rapid nature of the event. Furthermore, prior research has indicated that, when it comes to helmeted applications, the difference in human head static and dynamic COFs can be disregarded. As a result, example methods described herein involve examining the static COF of headforms and headform alterations.

[0045] The sled-based tribometer device 100 was validated in one implementation by comparing five static friction coefficients of different materials collected using a classic angle-Attorney Docket: 222204-2950 based inclined plane friction measurement and the tribometer device 100. Materials tested in this implementation included polyester, Teflon, helmet liner used in a KASK Protone Icon, aluminum, polylactic acid, and wood, all against aluminum. A Bland-Altman analysis was also performed to determine the mean bias between systems. A paired t-test and 95% limits of agreement were used to evaluate the difference in the two measurement techniques, with a significance level of 0.05.

[0046] In one implementation, the sled-based tribometer device 100 was used to quantify headform static friction coefficients against multiple materials defined in Table 1. These materials included EPP high-density foam (50 kg / m3), a helmet liner material (e.g., from a KASK Protone Icon), Teflon, aluminum, and polyester fabric. To evaluate the effects of degradation and oxidation, new and laboratory-used NOCSAE, Hybrid III, and EN960 headforms were tested in this implementation. The used NOCSAE headform was 5 years old, while the Hybrid III headform was 3 years old, and EN960 headform was 13 years old in this implementation. Each used headform has been used regularly for conducting impact tests and has noticeable wear. Based on previous research, it was anticipated that the Hybrid III and NOCSAE headforms would produce COFs much higher than what has been measured from humans. For this reason, the effects of adding a skull cap to each of the Hybrid III and NOCSAE headforms were examined. Five trials for each headform and material condition were conducted in this implementation. Additionally, three system COF measurements were conducted in this implementation, during the five trials after the first, third, and fifth trials. These trials consisted of loading the instrumented sled 130 to a matched normal force using free weights as the normal load application, then performing a collection of data. This allowed for quantifying the COF of the low-friction base (e.g., the base plate surface 126). Table 1: Factor levels for static COF testing Headform Material Condition Modification NOCSAE Foam New Skull Cap Helmet Hybrid III Liner Used None EN960 Polyester Teflon Aluminum

[0047] In this example, each headform was securely mounted, leveled, and weighted to apply an 80 N normal force to the tested material. An 80 N normal force was used as it is within theAttorney Docket: 222204-2950 range of normal forces previously used to capture human head COF and for direct comparison to defined human head static COF at 80 N. The normal force was measured using the sled-mounted pancake load cell (e.g., the instrumented sled 130) of FIGS.1A and 1B for each test. The tangential force was then incrementally increased through tension in the cable (e.g., the lines 165, 170) by adding 0.20 g in this example to a bucket (e.g., the tension load application 160) until movement was generated. Data were collected at 400 Hertz (Hz) from both the pancake load cell (e.g., the instrumented sled 130) and the in-line tension load cell (e.g., the tension loadcell 150). Force signals were filtered using a 4-pole phaseless lowpass Butterworth filter with a 17 Hz cut-off frequency. Force signals were then smoothed using a 50 millisecond (ms) moving average window. The COF overtime was calculated (e.g., via the computing device 180) for each trial using Eq.1 and the static COF was quantified. The static COF was defined as the peak COF right before movement occurred, after which COF either leveled off or decreased. The dynamic COF was defined as the average of the plateau during movement.

[0048] FIG. 4 illustrates a plot of an example calculated COF over time for a single trial performed using the sled-based tribometer device 100 in accordance with at least one embodiment of the present disclosure. The shaded region up to approximately 69 seconds (s) is the start of applied force before movement occurs, and the shaded region beyond that point is the start of instrumented sled 130 movement after overcoming the static COF. The static COF was defined as the peak COF before movement occurred and the dynamic COF was defined as the average of the plateau during movement.

[0049] There were no differences in measured static COF between the sled-based tribometer device 100 of FIGS. 1A and 1B and the angle-based devices across all materials as seen in Table 2. When comparing the sled-based tribometer device 100 with the angle-based device, a mean bias of 0.00 was found with a 95% limit of agreement of -0.01 to 0.02 in this example. The upper and lower limits of agreement represent the range within which 95% of differences between the two measurement methods are expected to lie.

[0050] Table 2 includes the static COF [mean ± SD] data measured by an angle-based inclined plane friction device and the sled-based tribometer device 100 of FIGS.1A and 1B in this example. The paired t-test showed no difference in measurement technique. The mean bias and 95% limits of agreement were calculated based on a Bland-Altman analysis.Attorney Docket: 222204-2950 Table 2: Static COF [mean ± SD] Material Against Angle-Based Static Our Tribometer Static p-value Aluminum COF COF Polylactic Acid 0.23 ± 0.02 0.23 ± 0.06 0.92 Wood 0.22 ± 0.01 0.21 ± 0.08 0.80 Aluminum 0.21 ± 0.00 0.22 ± 0.04 0.72 Teflon 0.09 ± 0.01 0.09 ± 0.05 0.99 Polyester 0.23 ± 0.01 0.22 ± 0.11 0.88 Helmet Liner 0.23 ± 0.01 0.22 ± 0.02 0.37 Mean bias 0.00 and 95% limit of agreement -0.01 to 0.02

[0051] For the example new headforms static COF of the EN960 was measured as 0.29 ± 0.01, the Hybrid III as 0.97 ± 0.11, and the NOCSAE as 1.16 ± 0.06 against high-density foam in one test. Against the helmet lining material, the static COFs were 0.48 ± 0.04 for the EN960, 0.83 ± 0.03 for the Hybrid III, and 1.00 ± 0.10 for the NOCSAE in one test. The material also affected COF. Moreover, the COF of each headform varied against each material in the following order: Teflon < aluminum < polyester < helmet liner < foam as demonstrated by FIG.5 and Table 3.

[0052] FIG.5 illustrates a chart of example static friction coefficients (mean ± SD) obtained for the EN960, Hybrid III, and NOCSAE headforms against multiple materials (i.e., Teflon, Polyester, Helmet Liner, High-Density Foam, Aluminum) in accordance with at least one embodiment of the present disclosure.

[0053] Table 3 includes static COF [mean ± SD] data obtained for each headform and headform modifications tested according to one example. Table 3: Static COF [mean ± SD] NOCSAE Hybrid III Material NOCSAE Hybrid III EN960 + Skull Cap + Skull Cap Foam 1.16 ± 0.06 0.97 ± 0.11 0.29 ± 0.01 0.29 ±0.02 0.29 ± 0.02 Helmet 0.99 ± 0.10 0.83 ± 0.03 0.48 ± 0.04 0.38 ±0.02 0.37 ± 0.02 Liner Teflon 0.65 ± 0.02 0.45 ± 0.04 0.12 ± 0.01 - - Polyester 0.99 ± 0.05 0.99 ± 0.04 0.21 ± 0.01 - - Aluminum 0.79 ± 0.06 0.96 ± 0.10 0.22 ± 0.02 - -Attorney Docket: 222204-2950

[0054] Headform condition influenced COF in one test. The used headforms COFs against foam were 0.92 ± 0.04 for the NOCSAE, 1.20 ± 0.02 for the Hybrid III, and 0.31 ± 0.03 for the EN960 in one test. Against a helmet liner, the static COF was 0.83 ± 0.05 for the NOCSAE, 0.91 ± 0.04 for the Hybrid III, and 0.34 ± 0.02 for the EN960 in one test as denoted in FIG.6.

[0055] FIG. 6 illustrates a diagram of example static friction coefficients obtained for different headforms of different condition against multiple materials in accordance with at least one embodiment of the present disclosure. The example static coefficients of friction (COF) [mean ± SD] denoted in FIG. 6 provide a comparison of the condition of EN960, Hybrid III, and NOCSAE headforms against high-density foam and helmet liner material according to one test performed. In this test, the used condition decreased NOCSAE COF and increased Hybrid III COF.

[0056] FIG.7 illustrates a chart of example different headforms static coefficients of friction (COF) mean ± SD Cis against high-density foam obtained in accordance with at least one embodiment of the present disclosure. Adding a skull cap reduced the COF of the NOCSAE and Hybrid III headform.

[0057] In one test performed, the addition of a skull cap significantly reduced the static COF of both the NOCSAE (ǻ = -0.51, 95% CI = [-0.56 to -0.46]) and Hybrid III (, ǻ = -0.58, 95% CI = [-0.63 to -0.53]) headforms against foam and the helmet liner material, as demonstrated by FIG. 7 and Table 3.

[0058] In one test, the dynamic COF did not differ from the static COF, as demonstrated by Table 4.

[0059] Table 4 includes dynamic COF [mean ± SD] data obtained for each headform and headform modifications tested according to one example. Table 4: Dynamic COF [mean ± SD] Material NOCSAE Hybrid III EN960 Foam 1.15 ± 0.03 0.97 ± 0.10 0.29 ± 0.01 Helmet 1.00 ± 0.03 0.83 ± 0.01 0.48 ± 0.04 Liner Teflon 0.64 ± 0.02 0.47 ± 0.02 0.13 ± 0.01 Polyester 0.99 ± 0.04 1.00 ± 0.02 0.21 ± 0.00 Aluminum 0.75 ± 0.08 1.04± 0.09 0.23 ± 0.03

[0060] The frictional interface between a headform and helmet has been suggested to affect rotational impact response. The sled-based tribometer device 100 and the methods describedAttorney Docket: 222204-2950 herein were implemented to investigate the frictional properties of commonly used dummy headforms in helmet testing. Due to the unique shape of the headforms, the tribometer device 100 was designed to measure COFs of irregularly shaped, non-flat objects. Validation of the tribometer device 100 and COF calculation method found a mean bias of 0.00 compared to an angle-based friction measurement and suggested that 95% of all measurements will be between -0.01 and 0.02 of the incline plane’s measurement. The methods of the present disclosure allowed for quantifying COFs for the different interface conditions investigated reliably. The tribometer device 100 also exhibits comparable sensitivity to commercially available tribometers, which typically offer a sensitivity of 0.002 - 0.04.

[0061] By implementing the sled-based tribometer device 100 according to examples described herein, it has been identified that the static COF varied between headform models, condition of the headforms, modification, and material tested against. Though the NOCSAE headform is used across standards and lab protocols, previous studies have yet to quantify the NOCSAE headform’s COF. In some examples described herein, the NOCSAE COF was quantified and found to be similar to the Hybrid III. Previous studies reported a friction coefficient of 0.75 ± 0.06 and 1.07 for the Hybrid III headform against a polyester material. According to embodiments described herein, a friction coefficient within the range of previous work (0.99 ± 0.05) was determined. The EN960 headform’s COF has previously been reported as 0.16 ± 0.03 and 0.20 against polyester, and via implementation of embodiments herein, a similar COF of 0.20 ± 0.02 for the EN960 against polyester was determined. Another study found a COF of 0.23 for the EN960 against padding inside an HJC helmet. Although tests against similar padding were not performed, when compared against high-density foam, another compressive material, a higher COF for the EN960 0.30 ± 0.01 was identified. The differences between the example values obtained herein according to various examples and those previously published values could be due to differences in the tested polyester materials or applied normal force.

[0062] Due to the higher friction of the Hybrid III skin, some existing test methods cover the Hybrid III headform with a stocking or skull cap to reduce friction. With skull caps, the static COF of the new NOCSAE headform was determined in examples herein to decrease by 62% against the helmet liner material and 75% against foam, and the new Hybrid III was reduced by 56% against the helmet liner material and 70% against foam. A similar COF to a certain existing study was identified in an example herein when a skull cap was applied to the Hybrid III, 0.26 compared to 0.29 ± 0.02 obtained using the sled-based tribometer device 100. However, to determine the meaningfulness of reducing the friction coefficient with a skull cap, it is imperative to compare headform friction coefficients to human head friction coefficients.Attorney Docket: 222204-2950

[0063] By implementing the sled-based tribometer device 100 according to examples described herein, it was determined that the material used in helmets can affect the COF of headforms. This is an important factor to consider, especially since many helmets use a poly-blend comfort liner that can have various properties affecting the frictional interface. It is worth noting that the COF results mentioned in the present disclosure may only be relevant to the specific liner or polyester material tested. While embodiments described herein observe slight differences between the COF of the helmet liner and the polyester tested, it is still unknown how headform COF impacts oblique helmet testing. Therefore, it is unclear whether these minor changes in friction have any significant impact on head impact kinematics. By implementing the sled-based tribometer device 100 according to examples described herein, it was also determined that the dynamic COF for each headform was within the SD of static COF. Therefore, for helmeted applications, like the human head, the difference in static and dynamic headform COF can also be disregarded.

[0064] Moreover, the results obtained using the sled-based tribometer device 100 according to the examples described herein can be used to evaluate the influence of COF on an impact event because the headform COF was taken with the same technique and force across all headforms and alterations.

[0065] The results obtained from implementing the sled-based tribometer device 100 according to examples described herein also showed that the degradation and oxidation of the Hybrid III vinyl plastisol and NOCSAE polyurethane affect their static COFs. This was not observed with the EN960. The COF of the Hybrid III increased, which could have resulted from vinyl plastisol becoming tacky with wear. An opposite effect occurred for the COF of the NOCSAE headform in one test. The COF of the NOCSAE headform decreased for the laboratory- used headforms in this test. This may be due to the oxidation and hardening of the NOCSAE polyurethane skin. However, implementing the sled-based tribometer device 100 according to examples described herein did not involve quantifying the degradation, oxidation, or amount of use of the used headforms tested. Thus, the results obtained from such implementations could not draw direct conclusions as to the effects of each of these properties, and this should be considered in the future. Based on implementing the sled-based tribometer device 100 according to examples described herein, it was determined that modifying the NOCSAE and Hybrid III headform with a skull cap reduces their COFs to similar levels and using a skull cap for these headforms would reduce concern in headform-to-headform variance over time.

[0066] Another limitation identified in implementing various embodiments of the present disclosure is that the embodiments described herein do not account for the frictional interfaceAttorney Docket: 222204-2950 between the skull and scalp. One exiting study determined that the scalp affected both linear and rotational acceleration. Still, the implementation of the various embodiments of the present disclosure did not determine if this result was due to the reduction in friction or the addition of another slip plane. In the embodiments described herein, it was also assumed that the pulley was frictionless, but its effect is likely negligible. Another limitation identified in implementing various embodiments of the present disclosure is that the embodiments described herein do not identify what effect the reported differences in COF have on head kinematics during helmet testing.

[0067] By implementing the sled-based tribometer device 100 according to examples described herein, the COF of headforms can be measured. The present disclosure presents a comprehensive example analysis of friction coefficients for the EN960, Hybrid III, and NOCSAE headforms across multiple materials, including the condition of the headforms, as well as with and without skull caps. Adding a skull cap to the NOCSAE and Hybrid III headform may be needed to reduce the effect of COF variance between headforms and changes in condition over time. Standardizing headform frictional properties may be necessary to ensure consistency between test methods and accurate representation of the human head. For now, the effect of headform COF on kinematic response during helmet testing needs further investigation. Ultimately, these results obtained herein according to various embodiments may be used to develop improved headforms, more realistic helmet testing standards, and improved boundary conditions in computation models.

[0068] Disjunctive language, such as the phrase “at least one of X, Y, or Z,” unless specifically stated otherwise, is to be understood with the context as used in general to present that an item, term, or the like, can be either X, Y, or Z, or any combination thereof (e.g., X, Y, and / or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain embodiments require at least one of X, at least one of Y, or at least one of Z to be each present.

[0069] As referred to herein, the terms “include,” “includes,” and “including” are intended to be inclusive in a manner similar to the term “comprising.” As referenced herein, the terms “or” and “and / or” are generally intended to be inclusive, that is (i.e.), “A or B” or “A and / or B” are each intended to mean “A or B or both.” As referred to herein, the terms “first,” “second,” “third,” and so on, can be used interchangeably to distinguish one component or entity from another and are not intended to signify location, functionality, or importance of the individual components or entities. As referenced herein, the terms “couple,” “couples,” “coupled,” and / or “coupling” refer to chemical coupling (e.g., chemical bonding), communicative coupling, electrical and / or electromagnetic coupling (e.g., capacitive coupling, inductive coupling, direct and / or connected coupling), mechanical coupling, operative coupling, optical coupling, and / or physical coupling.Attorney Docket: 222204-2950

[0070] It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications can be made to the above- described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Claims

Attorney Docket: 222204-2950 CLAIMS Therefore, at least the following is claimed:

1. A device, comprising: a frame; a sub-frame slidably coupled to the frame; an object mount coupled to the sub-frame; a sliding surface under the object mount; an instrumented sled on the sliding surface, the instrumented sled comprising a support platform to support a material sample under the object mount; a tension loadcell coupled to the instrumented sled; and a tension load application coupled to the tension loadcell, wherein: the instrumented sled is configured to measure a normal force applied to the material sample by an irregularly shaped object coupled to the object mount during operation of the device, the tension load application is operable to apply a tangential force to the instrumented sled, and the tension loadcell is operable to measure the tangential force.

2. The device of claim 1, wherein the sub-frame is operable to move the object mount and the irregularly shaped object up and down in a vertical direction to cause a curved surface of the irregularly shaped object to contact a top surface of the material sample during operation of the device.

3. The device of claim 1, further comprising: a normal load application coupled to at least one of the frame, the sub-frame, or the object mount, the normal load application being operable to apply the normal force to a top portion of at least one of the object mount or the irregularly shaped object during operation of the device.

4. The device of claim 1, wherein the instrumented sled comprises a pancake load cell configured to measure the normal force.

5. The device of claim 4, wherein the pancake load cell comprises a LCF450 Low Profile Universal Pancake, Resolution 0.4 N.Attorney Docket: 222204-2950 6. The device of claim 1, wherein the tension loadcell comprises an in-line tension load cell configured to measure the tangential force.Tension and Compression Loadcell Resolution 0.4 N.

8. The device of claim 1, further comprising: a computing device coupled to the instrumented sled and the tension loadcell, the computing device being configured to compute a coefficient of friction based at least in part on the normal force and the tangential force.

9. The device of claim 8, wherein the computing device is configured to compute the coefficient of friction based at least in part on a system friction between the sliding surface and a bottom surface of the instrumented sled.

10. The device of claim 8, wherein the computing device is configured to compute the coefficient of friction based at least in part on a material-object friction between a top surface of the material sample and a curved surface of the irregularly shaped object.

11. The device of claim 1, wherein the irregularly shaped object comprises a curved surface that contacts a top surface of the material sample during operation of the device.

12. The device of claim 1, wherein the irregularly shaped object comprises at least one of a round object, a circular shaped object, an elliptical shaped object, a headform, or a helmet coupon.

13. The device of claim 1, wherein the material sample comprises a foam material, a helmet liner, polyester, Teflon, aluminum, or sandpaper.

14. A method, comprising: mounting an irregularly shaped object on an object mount; positioning a material sample on a support platform of an instrumented sled that is positioned under the irregularly shaped object;Attorney Docket: 222204-2950 applying a normal force to the object mount to cause a curved surface of the irregularly shaped object to contact a top surface of the material sample, the normal force being applied until a target normal force is detected by the instrumented sled; applying a tangential force to the instrumented sled based at least in part on detecting the target normal force to cause the instrumented sled and the material sample to move in a horizontal direction away from the irregularly shaped object, the tangential force being applied until movement of the instrumented sled is detected; and determining a coefficient of friction of the irregularly shaped object based at least in part on the target normal force and a detected tangential force that causes the movement.

15. The method of claim 14, further comprising: determining a system friction between a bottom surface of the instrumented sled and a sliding surface on which the instrumented sled is positioned.

16. The method of claim 14, further comprising: determining a material-object friction between the top surface of the material sample and the curved surface of the irregularly shaped object.

17. The method of claim 14, wherein determining the coefficient of friction of the irregularly shaped object comprises: determining the coefficient of friction of the irregularly shaped object based at least in part on a system friction between a bottom surface of the instrumented sled and a sliding surface on which the instrumented sled is positioned.

18. The method of claim 14, wherein determining the coefficient of friction of the irregularly shaped object comprises: determining the coefficient of friction of the irregularly shaped object based at least in part on a material-object friction between the top surface of the material sample and the curved surface of the irregularly shaped object.

19. The method of claim 14, wherein the irregularly shaped object comprises at least one of a round object, a circular shaped object, an elliptical shaped object, a headform, or a helmet coupon.Attorney Docket: 222204-2950 20. The method of claim 14, further comprising: covering the curved surface of the irregularly shaped object with a skull cap prior to applying the normal force and the tangential force.

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