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Development of a High-Speed Data Acquisition System to 
Assess Spectrum Fatigue Test Performance, 18-9206

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Principal Investigator
Peter C. McKeighan
Frederick E. Fess II
Forrest S. Campbell
Michael Petit

Inclusive Dates: 07/13/00 - 12/20/00

Background - One of the most frequently performed tests in the Institute's Solid and Fracture Mechanics Laboratory is the variable load amplitude, fatigue crack growth test. In this test, cyclic loads of varying amplitude are applied to a fracture mechanics specimen with a growing crack to simulate service loading conditions and correlate analytical life prediction models. However, the newest generation of digital, servohydraulic controllers used to control the test have to be completely relied on to ensure that the proper loads are applied. The focus of this program was developing a data acquisition tool that could be used to measure the actual loads applied to the specimen.

Approach - The objective of this program is to develop a data acquisition and analysis system that can be used to quantify controller performance under spectrum loading conditions by continuously recording end level errors. The approach intended to achieve this objective includes the following:

  • Development of the data acquisition and analysis system using A/D board hardware and the LabVIEW programming language
  • Application of the system to different analog and digital control platforms available in the laboratory to quantify overall performance using both simple and complex continuously varying load magnitudes.
  • Development of an engineering methodology to interpret the subsequent fatigue life error observed for different control modes.

Accomplishment - This program yielded a number of useful findings including:

  • In using the developed system, the team has shown that significant errors can be unknowingly incurred when running these tests. SwRI's current systems are not sufficiently well-developed to indicate that less than optimum control is occurring.
  • Validation that some of SwRI's existing servohydraulic control systems work properly and identification of deficiencies in other (digital) systems available in the laboratory. These deficiencies are currently being addressed by the system manufacturers.
  • Development of an engineering approach that can be implemented in a control system to provide a continuous measure of loading error. One commercial system developer has stated their intent to implement our technical approach in their system.
  • Development of a highly specialized tool that SwRI can use in marketing to demonstrate a higher level of added value when performing spectrum testing at the Institute. This ability better positions SwRI to continue as an industry leader in this area.
  • SwRI is currently involved in a dialog with two controller manufacturers regarding this data acquisition system developed and its use. SwRI has offered to work with them in improving their control algorithms, provided, of course, they fund our validation efforts. Discussions are currently underway with a major user of spectrum test methodologies, to sell them an SwRI-developed data acquisition system. However, the system may have to be refined further before SwRI sells the company a system.

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