Development of a High-Speed Data
Acquisition System to
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Assess Spectrum Fatigue Test Performance, 18-9206
Peter C. McKeighan
Frederick E. Fess II
Forrest S. Campbell
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
- Development of an engineering methodology to
interpret the subsequent fatigue life error observed for different control
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.