Seat Belt Assembly Tensile Test Procedure Development

Wietholter, Kedryn; Wetli, Alaine · 2021 · ROSA P / United States. National Highway Traffic Safety Administration, Vehicle Research and Test Center

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Summary

This report details the development of an updated tensile test procedure for seat belt assemblies to better represent in-vehicle restraint angles, addressing limitations in Federal Motor Vehicle Safety Standard (FMVSS) No. 209. The existing standard utilizes a two-dimensional test setup with discrete angles, whereas actual in-vehicle geometry is three-dimensional. The National Highway Traffic Safety Administration (NHTSA) conducted this research to evaluate whether incorporating representative in-vehicle angles into tensile testing provides a more accurate assessment of seat belt performance. The methodology involved collecting in-vehicle measurements using a coordinate measuring machine on eight vehicle models equipped with various seat belt technologies, including load limiters and inflatable belts. Hybrid III anthropomorphic test devices representing small (5th percentile female or 6-year-old) and large (95th percentile male) occupants were used to determine representative angles for retractor, guide loop, and hardware attachments. These measurements were translated into two-dimensional fabrication drawings to create custom fixtures that held original equipment manufacturer hardware at the measured angles. Ten seat belt assemblies were tested under these conditions, applying tensile force at 51 mm/min until reaching force criteria of 22,241 N for pelvis loops and 13,345 N for upper torso loops, with an elongation limit of 508 mm. Results indicated that four of the ten tested assemblies failed to meet FMVSS No. 209 performance criteria under the updated procedure. Failures included hardware fractures, such as a broken weld in the 2014 Volkswagen Tiguan buckle and a torn retractor bracket in the 2011 Ford Explorer inflatable belt, as well as exceeding elongation limits. The study also evaluated testing procedures for assemblies with load limiters using stroke-limited machines. Testing with no webbing on the spool or minimal webbing allowed assemblies to reach the force criterion, though load limiters were not fully engaged in the minimal webbing condition. Actuated retractor pretensioner testing failed to achieve the required load. The study concludes that the updated test procedure is feasible and successfully converts three-dimensional in-vehicle geometry into a representative two-dimensional tensile test. The developed protocol includes detailed methods for collecting in-vehicle angles and fabricating unique fixtures that incorporate all vehicle attachment hardware. This approach provides a more realistic evaluation of seat belt assembly performance compared to the standard discrete-angle testing, highlighting potential failure modes in specific hardware configurations that may not be captured by current regulatory procedures.

Key finding

Four out of ten seat belt assemblies failed FMVSS No. 209 performance criteria when tested using the updated in-vehicle angle procedure, primarily due to hardware fracture or exceeding elongation limits.

Methodology

lab_experiment

Sample size: 10

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• seat belt use

Information type

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• Applied Guidance: standards test procedures