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Full-Scale Accelerated Fatigue Test on a Rail Car Designed and Analyzed with nCode GlyphWorks 

The Transportation Technology Center, Inc. (TTCI), headquartered in Pueblo, Colorado, is a wholly owned subsidiary of the Association of American Railroads (AAR). TTCI is a world-class transportation research and testing organization, providing emerging technology solutions for the railway industry throughout North America and the world. We recently had a discussion with Juan C. Valdes-Salazar, Principal Investigator II for TTCI who describes their new challenge:

"We are simulating 4.5 million miles of regular service for structural reliability using a shaker test and are presenting the creation of this custom accelerated fatigue program —as well as the resulting data cleaning and analysis — primarily using nCode GlyphWorks."

Challenge and project scope


Accelerated fatigue programs are used to simulate decades of revenue service in a period of a few months (or weeks in some cases). They provide a powerful tool for both rail car manufacturers and fleet owners that can be used for both technical and financial decisions in the short term.

The project's main goal was to simulate 4.5 million miles of service in a short period of time based on recorded data obtained during an Over the Road Test (OTR test).  A typical OTR test involves putting an instrumented rail car into regular service operation for several months to capture the different excitations experienced by the rail car, and use those excitations to create equivalent inputs to a hydraulic full size shaker to impose the same damage on the structure as seen during the OTR test but in considerably less time.

TTCI uses a hydraulic shaker system known as the Simuloader Unit (SMU) to run the accelerated fatigue test

Identifying parameters for fatigue analysis and accelerated fatigue test


TTCI performed the shaker test on a computer controlled, electro-hydraulic structural test device used for applying dynamic inputs directly to full-scale railcar bodies, highway vehicles and other heavy structures. The Simuloader uses up to 11 actuators with capacities up to 750,000 lbs and a 6 inch stroke, a longitudinal actuator operating in FORCE control mode, vertical and lateral actuators operating in displacement control mode.

In order to correctly design the accelerated test, the following input signals forming the Environmental Data for both a Fatigue Analysis and an Accelerated Fatigue Test were needed:

  • a longitudinal force signal
  • strain signals on the carbody structure for the fatigue analysis
  • vertical and lateral displacement signals derived from the Over The Road (OTR) test
Illustrations of the sensors placed before the launch of the accelerated test

OTR test data fatigue analysis using nCode GlyphWorks


TTCI were able to analyze the data gathered during the accelerated test using solely nCode GlyphWorks. "GlyphWorks enables us to import the test data. GlyphWorks fully supports filtering, spikes removal, data editing and offset. All the data treatment is done using standard embedded processes within GlyphWorks that require no customization. It was easy to get our signal data ready to be analyzed for fatigue damage," explains Juan C. Valdes-Salazar, Principal Investigator II at Transportation Technology Center.

The fatigue damage is then calculated from all the strain channels and the resulting damage is correlated with the environmental variables of vertical and lateral accelerations as well as the longitudinal coupler force. Accelerations at the carbody to suspension interface (truck bolster) are analyzed at the corresponding critical damage events to identify the associated carbody motions.

Illustration of relationships among variables in GlyphWorks

SMU driver files creation for the accelerated fatigue test


After the fatigue analyses have been performed, the procedure to generate the driver files to be used as inputs for the SMU is the following:

  • Identification of non-damaging sections of data
  • Removal of the non-damaging sections of data
  • GlyphWorks editing capabilities allows for an efficient identification and removal of the non-damaging sections by means of the assessment and slicing process found in the Damage Editor Glyph
Illustration of the Damage Editor
Vertical and lateral accelerations are converted to displacements by a double integration process.  These calculated displacements are compared against displacements directly measured on the test car during an on-site test where the car passes through a special precision track test with known perturbations. Both accelerations and displacements are recorded.
Measured and calculated displacements comparison
The strain-coupler force and strain-accelerations relationships are determined. These relationships allow to monitor the proper development of the test, making sure that the correct damage is being created during each block of data.
Strain-coupler force and strain-accelerations relationships
The last step is to generate equivalent sinusoidal signals to be used as the inputs for the SMU. These signals are designed to create the same damage level seen during the OTR test and to reduce even more the duration of the accelerated fatigue test.

Single tool for performing fatigue analyses and accelerated fatigue test


The procedure described in this article allows TTCI to successfully conduct both Fatigue Analyses and Accelerated Fatigue Test using nCode GlyphWorks. "We were able to generate the SMU driver files for the accelerated test by using nCode GlyphWorks. No customization was necessary during the process and we were able to accurately monitor the accelerated test and ensure proper damage was being applied. The results of the accelerated test confirmed the pre-test damage fatigue calculations," concludes Mr. Valdes-Salazar.

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