FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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ACOUSTIC EMISSION TESTING OF MAN-LIFT DEVICES
J.A. Baton
Ontario Hydto Reie.an.ah
Toronto, Ontario
INTRODUCTION
Safety is a very controversial issue. Whereas we may find it an acceptable risk
to cross a busy street or to ski down a steep slope, we may have some reservation
about flying in a commerical airliner or to be raised 20 m above the ground
in a bucket of an aerial lift device unless we have some positive perception
regarding the structural Integrity of the equipment. Much of this assurance is
developed through nondestructive test procedures but, until recently there has
been no method which could provide reliable information as to the mechanical
worthiness of the fibre reinforced plastic (frp) boom of man-lift devices
("bucket trucks" or "cherry pickers").
Dielectric testing of booms is mandatory but this test, usually performed semiannually,
is a poor indicator of mechanical integrity. Under dry conditions a
boom on the point of mechanical failure could pass this electrical test.
Conversely, seepage of moisture Into surface layers would result in dielectric
failure but may not have implications in terms of mechanical fitness.
Tradition NDT methods do not work well on composite materials such as frp.
Radiography will show fibre direction and gross voids but little else. Ultrasound
suffers high attenuation plus the fibres give rise to a myriad of reflections.
Liquid pénétrants will show surface imperfections but may also cause
chemical degradation of the plastic resin. However, acoustic emission monitoring
can potentially detect and locate "active" defects in both frp and metallic
components and can be used in a straightforward, all encompassing test.
FAILURE MODES
As a generality, nondestructive testing is performed to detect potential future
failure of a component or structure. If such a test is to be worthwhile, it
must be sensitive to the precusors of possible failure modes. Further, the test
must be performed at a frequency such that the progression from the earliest
reliable detection of incipient failure to actual failure cannot occur. This
point will not be dealt with here but does raise some questions as to the
efficacy of periodic testing of frp booms.