KLK349: Thermal Stresses in Pyrotechnic Initiators Used in Automotive Supplemental Restraint Systems

Principal Investigator:

Karl Rink

Project Objectives:

To analytically predict and quantify micro-cracks in airbags and adaptive seatbelt initiators caused by manufacturing or in-service use; to model micro-crack potential and locations using closed form and FEA models; to verify closed form and FEA models with SEM and optical photographs; to experimentally quantify leakage using radioisotope leak detection facility; to provide design suggestions to eliminate or reduce initiator micro-cracking.

Project Background:

The use of pyrotechnic initiators continues to expand as world wide production of driver, passenger, side impact, and curtain inflation systems exceeds 100 million units yearly. In this potentially life-saving application, it is critical that these electro-explosive devices operate reliably without failure or diminished performance for the intended service life of the vehicle – typically specified as 15 years. Recent research at the University of Idaho indicates that initiators are often not able to meet the stringent quality control requirements. In simple terms, there is evidence that the initiators have leaks cause by either manufacturing or from in-service use. These leaks have the potential to allow water into the initiator cavity; the water in turn degrades the connections and the result is that there is potential for the restraint system to either function poorly or not at all. The proposed project is follow-on to previous work currently funded by NIATT.

The geometry of the initiators is a stainless steel outer ring, filled with glass with the conducting rod placed either symmetrically or non-symmetrically within the glass. The difference in thermal properties between the steel and glass cause residual stresses during manufacturing. To further complicate the process, a combustible filament is welded between the outer ring and the conducting rod. This welding process introduces further residual stresses. These residual stresses are much like the manufacturing stresses found in composite materials. Therefore, similar modeling techniques that have been used by the author will be used to predict potential material stresses caused by the thermal differences. [1,2,3,4].

With current funding we are now completing our radioisotope leak detection facility. This facility will allow us to use radioactive isotopes in order to detect leaks in the initiators to verify analytical and optical results.

Task Descriptions:

• Task 1  Complete simple closed formed solutions to the problem for symmetric configurations

• Task 2  Obtain the constitutive law for the glass insulation including properties during the cooling of the glass during manufacture

• Task 3  Complete an FEA model of non-symmetric initiator configurations including realistic material properties and thermal conditions

• Task 4  Complete the photographing of the initiators using optical or SEM magnification

• Task 5  Perform leak testing of initiators using the radioisotope leak detection facility

• Task 6  Write at least one journal article comparing the results and offering manufacturing changes to alleviate the problems

• Task 7  Luke Thompson completes MS thesis

Milestones:

• Project start date: August 24, 2004

• FEA model completed: by October, 2004

• SEM and optical photographs: by November, 2004

• Presentation of conference paper: by May, 2005

• Submission of a Journal Article: by April, 2005

• Project end date: August 31, 2004

Budget Information:

The total UTC funding for this project is $20,533

Student Involvement:

This project will involve two graduate students.

Relationship to NIATT Strategic Plan and Other Research Projects:

This proposal fits well with the NIATT Center for Clean Vehicle Technology (CCVT). The CCVT mission includes a commitment to safety. The proposal supports two of the three identified federal and state priorities: technologies that support the development of new vehicle technologies, and training for engineers who work in vehicle technology industries.

Education is well supported by the involvement of a graduate student from ME. Research Selection is supported by the fit this work has with the NIATT Strategic Goals, as indicated above. Research Performance is supported by the presentation of a professional paper at a conference and publication of a journal article. Technology Transfer benefits are the direct application to the restraint and automobile industry.

Technology Transfer Activities:

The technology will be transferred by the presentation of a paper at a conference yet to be determined. The project will also be disseminated by the publication of a paper in an established journal. We will also make a presentation at a national conference. We will also be presenting our results to Autoliv of Salt Lake City.

Potential Benefits of the Project:

As the public outcry for improved safety increases, it is important to develop and provide systems that are robust and yet minimally increase vehicle weight and maintain or increase occupant space within the vehicle. A final goal of this project is to reduce or eliminate the need to replace pyrotechnic devices on existing and future vehicles by being able to determine if these systems still adequately function. We expect the results of this work to dramatically change the way in which initiators are manufactured and maintained. This project will put the UI at the national forefront of pyrotechnic restraint systems.

Project status:

Complete

Final Report:

N06-05 (pdf)