TECHNOLOGY AREA(S): Human Systems, Materials/Processes ACQUISITION PROGRAM: PMA202 Aircrew Systems

OBJECTIVE: Develop a non-destructive testing capability to detect when the strength of webbing is no longer capable of withstanding the load for which it is designed.

 

DESCRIPTION: A capability is needed to detect when the load-bearing strength of a webbing has decreased into an unsafe zone. The strength of webbing is degraded with each use, and the degradation depends upon the webbing material, and the exposure to environmental factors. Because degradation is typically silent and invisible, unexpected failure during an emergency is a hazard that the Navy wishes to avoid. A non-destructive way to detect degradation has long been sought by the narrow fabrics industry. The demand is there, but the technology is lacking. Incorporating multiple indicator technologies rather than relying upon a single technology is an approach that may be more achievable than expecting one technology to detect every kind of failure mode. Incorporating load indicators in the load-axis of the structure without affecting the load-bearing capability is yet another challenge.

Embedding sacrificial visual wear indicators, like those used in road tires, in the webbing�s binder or marker yarns could indicate excessive abrasion. A dye that fades predictably and measurably under ultraviolet (UV) exposure or ozone may be another approach. Solid state mechanochromic luminescent dyes could potentially indicate when a load threshold has been breached. The goal of this STTR effort is to incrementally develop an indicator integral to a common webbing type or develop a portable test/inspection method that can be used on a common webbing type end-item in situ (e.g., on a restraint seat harness installed in the aircraft cockpit).

 

Current failure detection methods are limited to visual inspection by the naked eye using somewhat vague and incomplete criteria. Reference 1 directs the parachutist to check the webbing �for damage� and the harness for signs of �completeness, cuts, broken stitching, acid and signs of chafing and wear.� The Parachute Industry Association [Ref 2] states, �Any cuts, nicks or heavy abrasion to webbing should be shown to a certified rigger before the next jump.� Direction to check for loss of pliability or color change or loss is missing, as is direction to check specific areas of webbing exposed to flex fatigue and hardware surfaces, such as friction adjusters in buckles. Cascading failure begins on the molecular level with a weakened chemical structure [Ref 3]. Degradation of the fiber follows. Mechanical stress can cause failures of the yarn comprising these weakened fibers and Hearle, Lomas, and Cooke in Chapter 38 [Ref 4] show scanning electron microscope images of the filament fractures of ejection seat webbing yarns attributed to shear loading, flex fatigue, and friction from hardware components. The next step is failure of the end-item itself. Accelerated aging harness testing that excluded mechanical degradation showed that aged webbing tended to break before the stitched seam more frequently with a corresponding loss in tensile strength [Ref 5]. As the sewing thread is subject to the same degradants as the webbing, stitches were observed to fail before the webbing did, but less frequently.

 

There is currently no standard method to conduct surveillance testing of webbing. Of the very few surveillance testing studies that have been published, the criteria for accelerated aging can vary greatly by the purpose of the end item. For example, nylon is known to be degraded by exposure to UV radiation and a combination of high heat and humidity, and therefore those conditions are usually included in accelerated aging testing of ejection seat webbing. On the other hand, even though ejection nylon webbing is vulnerable to abrasion from blowing sand/dirt, ejection seat testing typically excludes that degradant because the closed cockpit shields the webbing from exposure.

Military helicopter seat harnesses and aircraft tie-downs, however, always include blowing sand and dust, and ship exhaust on carriers as degradants due to their exposed conditions.

 

Reference 5 identifies static tensile strength as the main variable, and interpreted strength loss by age trends to determine the probability of maintaining 1.5/1 margin of safety factor over three years. Small-scale dynamic testing and static testing of elongation by age was used by the European Aviation Safety Agency [Ref 6]. The Code of Federal Regulations provides pass/fail criteria in terms of allowed percentage loss for automotive seat belt assemblies, using abrasion, UV exposure, micro-organisms as independent variables, and elongation and breaking strength as dependent variables.

 

Proposals for Phase I should include a background section with explanatory figures describing the basic principles of the proposed technology concept, and publications or other references that outline the application being

considered.

 

It is recommended, but not required, that partnering with original equipment manufacturers be considered.

 

PHASE I: Demonstrate feasibility with an analysis that supports the proposed technology concept. Provide experimental work that demonstrates that the indicating technology is capable of detecting a 25 percent decrease in strength and elongation. Include a 3-tiered work breakdown structure with Gantt chart of Phase I design activities, and include make/break criteria and events. Provide Technical Performance Measures for Government review and approval that will be tracked throughout Phases I-III. The Phase I effort will include prototype plans to be developed under Phase II.

 

PHASE II: Incrementally develop the strength loss indicator technology. Demonstrate the technology using PIA-W- 4088 Type VII Class I webbing [Ref 8], which is commonly used in both commercial and military applications, or another webbing with similar width, thickness and strength properties. Include a 3-tiered work breakdown structure with Gantt chart of Phase II design activities, and include make/break criteria and events. Track performance against agreed-upon Technical Performance Measures quarterly. Develop quality assurance measures. Demonstrate the capability of the prototype on, or incorporated in, five sets of ten webbing test articles plus one control set conditioned per MIL-STD-810 procedures [Ref 9]: Set 1 should be exposed to UV radiation; Set 2 to combined high heat and humidity; Set 3 to impact cycling; Set 4 to fluid contaminants, salt fog, blowing sand/dust, and stack gas exposure; and Set 5 to all four (in sequence, as laboratory concomitant exposure is not yet possible). Produce a final Phase II report that includes raw data, photography and/or video recording, data recording sheets, documentation of test devices (manufacturer, model, serial, accuracy, calibration status) and test reports written in accordance with any specified standards. Develop a performance specification to document the Phase II prototype technology.

 

PHASE III DUAL USE APPLICATIONS: Finalize the developed strength loss indicator technology for webbing in performance specification and engineering drawings in accordance with military standards. Develop and perform required operational testing, document the quality assurance test program in accordance with industry best practices, and transition into military and commercial webbing markets. This technology may benefit the private sector in such markets as industrial fall arrest harnesses and tethers, commercial aircraft and automotive seat harnesses, and recreational airborne sports such as skydiving, hang-gliding, and parasailing.

 

REFERENCES:

1.   Parachute Rigger Handbook. Federal Aviation Administration, Flight Standards Service, Washington, DC, 2015 https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/prh_change1.pdf

 

2.   �Technical Standard 135: Performance standards for personnel parachutes and components, Rev 1.4 (PIA TS 135).� Parachute Industry Association, 2010. https://www.pia.com/piapubs/TSDocuments/TS-135v1.4.pdf

 

3.   Segars, R.A. �The degradation of parachutes: age and mechanical wear (NATICK/TR-92-035).� http://www.dtic.mil/dtic/tr/fulltext/u2/a252243.pdf

 

4.   Hearle, J.W.S, Lomas, B., and Cooke, W.D. Atlas of Fibre Fracture and Damage to Textiles (2nd ed). Woodhead Publishing: Cambridge, UK, 1998. https://www.sciencedirect.com/science/book/9781855733190

 

5.   Maire, R. and Wells, R.D. �Engineering evaluation of age life extension, T-10 harness, risers, and T-10 troop chest reserve parachute canopies (TR-72-59-CE).� United States Army Natick Laboratories, March 1972. http://www.dtic.mil/dtic/tr/fulltext/u2/742668.pdf

 

6.   Robinson, L., Atkin, C.J., Payne, T., Harper, C., and Frost, G. �Seat Belt Degradation, Phases I and II (EASA.2010.C21/EASA.E2.2011.C11 SEBED).� https://www.easa.europa.eu/sites/default/files/dfu/SEBED%20Report_Final_5-2010.pdf

 

7.  Seat belt assemblies, 49 CFR �571.209 (2004). https://www.law.cornell.edu/cfr/text/49/571.209#b