Strain Rate Sensitivity and Crash Modeling of High Strength Steels
 

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Project Description


Circular Tube Crush Tests, TMAC [+]
Dynamic axial compression tests have been conducted on circular tube specimens using the intermediate strain rate servo-hydraulic test machine. These tests are conducted on different AISI material grades: DQSK (1.6mm), HSLA340 (1.6 mm), DP590 (1.6 mm), DP780 (1.2 mm), BH300 (1.6 mm), TRIP600 (1.6 mm), TRIP780 (1.6 mm), 440WGA (1.2 mm) and DP780 HDGI Steel (1.2 mm). For each of these materials, nine tests were run, corresponding to three replicates at three different test velocities. In each case, minimum two out of the three replicates were instrumented with a maximum of sixteen strain gages in locations of crush fold formation. The inner diameter (100 mm) and the height (180 mm) of all the tubes were the same for all material grades to allow for comparison and common tube fixturing. [+]


Octagonal Tube Crush Tests, TMAC [+]
In support of developing standard test methods for characterizing the behavior of sheet metal at high strain rates, dynamic axial compression tests were conducted on octagonal tube specimens using the intermediate strain rate servo-hydraulic test machine. The tests were conducted on three different AISI material grades: DP590 (t=1.6mm, GI coating), DP780 (t=1.2mm, GA coating), HSLA340 (1.6mm, GA coating). For each of these materials, nine tests were run, corresponding to three replicates at three different test velocities (0.06 m/s, 0.6 m/s and 6 m/s). In each case, two out of the three replicates were instrumented with a maximum of nine strain gages. The geometry of all the tubes were the same for all material grades to allow for comparison and common tube fixturing. [+]


Spotweld Tests, Drop Tower [+]
Static and dynamic strength tests were performed for spot welded specimens made of dual-phase (DP780) and mild steels (DQSK). Steels were provided by A/SP members and welds were made by RoMan Engineering. Lap-shear (LP) and cross-tension (CT) as well as a newly designed mixed mode specimens were tested using MTS hydraulic universal testing machine for static and drop weight tower for dynamic tests. Three weld nugget sizes for each steel and specimen geometry (LP and CT) were made. In the mixed mode test, only DP780 of one weld size was tested. Load and displacement as functions of time and failure mode of the spot welds were recorded in the tests. [+]


Base Material Tests, High Speed Hydraulics [+]
ORNL High Strain Rate Spot Weld Testing Proposal

Correlation of Mechanical and Optical Measurements of High Strain Rate Properties of Advanced High Strength Steels

 

Srdjan Simunovic, Donald Erdman and J. Michael Starbuck

Oak Ridge National Laboratory

 

ORNL Testing Proposal to Auto/Steel Partnership

Strain Rate Characterization Project

 

April 5, 2007

 

1. Purpose and Background

 

In support of the Auto/Steel Partnership (A/S-P), Strain Rate Characterization Project, ORNL will conduct a high rate experimental tests and analysis of base material specimens in tension configuration.  The objective of the test program is to provide the necessary experimental data in support of the A/S-P efforts to determine high-strain-rate mechanical properties of Advanced High Strength Steels (AHSS). The experiments and data will also enable better predictive modeling of base materials for finite element crash modeling.  The test program will consist of testing tensile specimens under strain rates of quasi-static, 0.1/s 1/s 10/s 100/s and maximum strain rates achievable in full open loop configuration and the gage length. The tests will be conducted on the new hydraulic test equipment at the Oak Ridge National Laboratory. All the tests will be conducted using the same apparatus. The machine allows for testing at speeds from quasi-static to 700 in/sec (18.5 m/s) over a range of 4 inches (100 mm) at maximum loads of 9000 lbf (40 KN). If an effective gage length for the chosen specimen is known equipment can run non-linear velocities in the drive file to achieve global strain control (engineering or true strain rate as desired).

 

The dynamic testing procedures from recent studies sponsored by International Iron and Steel Institute [1,2] and published literature [3,4] will be followed. High-speed video recording will be used for will provide detailed record of the test and will be used for strain measurement correlation to mechanical displacement data.

 

2. Scope of Work

 

2.1 Specimen Design

 

The dog bone-shape tensile specimen configuration will be used. Configuration variation of laboratories labeled B and C from Reference 2 will be used. Specimens will be made by the ORNL. Specimen configuration must accommodate for strain gage attachment in the load measurement section. Figure 1 shows schematics from Reference 3.

 

Figure 1. Tensile specimen configuration

 

The necessity of installing backing plates will be determined by the ORNL by conducting preliminary tests of specimens with and without backing plates and evaluating the results. The preliminary results will be evaluated jointly by A/SP and ORNL to determine specimen modifications. If it is determined that the addition of backing plates is necessary, the plates will be added by the ORNL.

 

2.2 Test Speeds

 

We propose to conduct test under 5 speeds in order to achieve rates of : quasi-static, 0.1/s 1/s 10/s 100/s and maximum strain rates of the order of 500+/s. Specimen gage lengths may need to be modified to provide sufficient time for specimen equilibration under higher loading speeds [1,2].

 

2.3 Test Instrumentation

 

Measurement of forces, displacements, stresses and strains will be based on the instrumentation listed below. Tests at rates higher than 1/s will be recorded by a high-speed camera. Several lower rate tests will be recorded during the test development to make sure that the measurement method is consistent across different speeds.

Forces will be measured by:

  1. Equipment load cell
  2. Equipment load washer
  3. Calibrated strain gage on the specimen tab [2, 3].

Strains will be measured by:

  1. Actuator displacement,
  2. Optical measurements from high-speed movies
  3. Strain gage in the test gage region [2, 3].

 

The multiple measurement methods of both forces and displacements will allow for correlation of the results and verification of the method. All the measurements will be synchronized using the central trigger.

2.4 Test Materials

 

We propose to test seven different materials per A/SP request. Materials are:

  1. DP980T/550Y
  2. DP980T/650Y
  3. DP780T/420Y
  4. DP780T/550Y
  5. DP590T/340Y
  6. DP500T/300Y
  7. 590R

 

Material thickness is to be defined by the A/SP. Thicknesses up to 2 mm are possible. Thickness between 1.0 mm and 1.6 mm is preferred. Tests will be conducted with 3 replicates for each rate and each material.

 

3. Deliverables

 

Deliverables will be text files, high-speed movies, analyzed data and web-based presentation of the results similar to the current ORNL A/SP project web site for tube crush and material data. See:

http://thyme.ornl.gov/ASP_Main/crashtests/crashtests.cgi

We will also provide a written Final Report documenting the experimental procedure and test results.  The raw data from each test will be provided in an ASCII file format.

 

4. Schedule

 

The tests will be completed by September 30, 2007 provided that the funds and materials are available during  May 2007. The high-speed testing is not standard, and the proposed project involves significant research component in instrumentation, test configuration and test interpretation.

 

5. References

 

  1. Recommendations for Dynamic Tensile Testing of Sheet Steels, International Iron and Steel Institute, 2005. http://www.worldautosteel.org/pdf_hsrt/DynTestingRecomPract.pdf
  2. C. Wong, IISI-AutoCo Round-Robin Dynamic Tensile Testing Project, International Iron and Steel Institute, 2005. http://www.worldautosteel.org/pdf_hsrt/RptRndRobResults.pdf
  3. D. Matlock, J. Speer, Constitutive Behavior of High Strength Multiphase Sheet Steels Under High Strain Rate Deformation, AISI/DOE Technology Roadmap Program, Report TRP 9904, 2005.
  4. D.M. Bruce, “Dynamic Tensile Testing of Sheet Steels and Influence of Strain Rate on Strengthening Mechanisms in Sheet Steels”, Ph. D. Thesis # MT-SRC-003- 018, Colorado School of Mines, 2003.

 

 

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