Standardization of high strain rate tensile test techniques for automotive plastics

Scope

Background:

Recent regulations by the National Transportation and Highway Safety Administration (NHTSA) regarding automobile safety have served as motivation for the development of analytical models of an automobile crash event. These models have mandated the need for realistic constitutive data at high strain rates. This data is required as inputs for computational design models. If accurate numerical results can be obtained from the computational models and high strain rate test data, the need to conduct expensive crash tests may be dramatically reduced. This not only translates into significant cost savings, but also will allow the automobile manufacturer to make minor adjustments in their designs in a more timely and efficient manner. Material properties of interest include yield and failure stress, yield strain, modulus, and strain to failure.

Unfortunately, however, generating high rate tensile data is much more complicated than generating static tensile data. The assumptions generally true in static uniaxial tension tests do not always apply during high strain rate testing. At high strain rates, the gage section of a specimen may not be under a homogeneous stress or strain state. Stress waves propagate along the specimen and are reflected and transmitted at each interface along the line of travel. To minimize this effect, small test specimens are used so that an average stress state is simulated. Because only an approximate homogeneous stress state is achieved, high strain rate testing may not provide "true" material properties. Additionally, a great deal of engineering judgement and manipulation is required when reducing high strain rate data. Because of this, the tensile data obtained from the high strain rate test specimens should only be compared with data obtained from tests having similar experimental setups and data reduction schemes.

Plastic manufacturers and users would like to directly compare their results with other published data on the same or similar material. Because high strain rate data is very sensitive to specimen geometry and test and analysis techniques, directly comparing test data without having a full understanding of how that data was generated is not valid. Currently no standard exists for conducting high strain rate tests or for reducing the data generated from these tests. Significant variations in high strain rate test and analysis techniques exist both between and within companies. It is for these reasons that a standard that specifically addresses issues related to high strain rate testing and data analysis is needed. A standard such as this would enable data comparison among companies, facilitate the development of analytical models and eliminate the need for repetitive testing within companies.

Objective:

The objective of this project is to identify the high strain rate tensile testing variables and analysis techniques that may have a significant effect on the resulting data and to draft a standard that specifically addresses the issues related to high strain rate tensile testing of plastics.

Methodology:

Task 1: Conduct Literature Search

A literature search will be conducted initially and throughout the program to help develop the surveys, ad names of contacts to whom the survey(s) should be sent, and to augment the results obtained from the survey. The current state-of-the-art in high strain rate testing will be assessed. Effort will be made to ensure that the proposed project does not duplicate a current or previous effort.

Task 2: Survey Companies

Surveys will be sent out to those companies who indicated an interest in participating in this project. These surveys will request information regarding high strain rate test procedures and data analysis techniques as well as data generated from various plastics. Participants will be asked to provide a definition of modulus, strain rate, yield stress, and yield strain and strain-to-failure. Where applicable, information regarding modeling techniques, data input requirements, test equipment, and capabilities will be determined. Results from this survey will be sorted and used to help identify testing, data analysis, and material variables that may have a significant effect on high strain rate data. Surveys sent out to various test facilities, universities, and research institutes will help identify potential test sites for future round robin testing of the resultant test standard. Five separate surveys will be created and sent out to the various types of organizations. Surveys will be sent out electronically and via regular mail. The surveys will also be available on the SAE website:

2.1 Automotive Companies
2.2 Plastics Suppliers
2.3 Modelers
2.4 Test Equipment Manufacturers
2.5 Universities and Test Facilities
Task 3: Ruggedness Testing

A series of high strain rate tensile tests will be conducted at a single location, using a skeleton draft standard. This testing will be conducted to assess the ruggedness of the test technique and to identify possible variables which may have a significant affect on the resultant data. Information obtained from the literature review and surveys will be used to identify the test matrix for this task. The proposed test matrix will be presented to the industrial participants at a meeting to be held at SAE. The test matrix will be discussed and modified accordingly. The modified test matrix will be voted on by SAE, UDRI, and the industrial participants.

Task 4. Draft, Edit and Review Standard

A draft standard will be written using the ASTM D638 standard as a guide and following SAE format. Information obtained from the survey(s), literature search, and ruggedness testing will be incorporated into the draft document. The draft standard will be sent out to a working group for review. The document will be edited using the comments generated from this review. This will be done for both the first and second drafts. Where applicable, the drafts of the standard will be mailed electronically to the participants for review. Once complete, the proposed standard will be submitted to SAE and presented at a final meeting.

Possible Follow-On Effort

Laboratories capable of conducting high strain rate tests in accordance with the draft standard will be identified. Recommendations regarding round robin testing as well as a summary of the standardization activity will be included in a final report.