FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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Metal components with classical acoustic emission sources (eg, propagating
crack) exhibit the Kaiser effect. This effect describes the phenomenon whereby
acoustic emission will only be generated when stress exceeds the previous highest
value. A new component will tend to yield copious acoustic emission when
first loaded. If the load were removed and reapplied, no acoustic emission
would result until the load was further increased above its previous value.
This can be beneficial in that if a structure is loaded to the onset of acoustic
emission, an indication of load history is gained. If a periodic proof load is
imposed, such as 150% working load, acoustic emission will only be generated if
active degradation has occurred since the last proof load.
The Kaiser effect is less well exhibited by frp composites. This is because the
resin material flows and thus relaxation occurs on load removal. This was
demonstrated in the laboratory, Figure 5. Here an frp specimen was subjected to
a three point bend test such that acoustic emission occurred. The load was then
removed. When it was reapplied with little delay, almost no acoustic emission
was generated, in accordance with the Kaiser effect. However, if a delay of
24 h ensued prior to reloading, much of the acoustic emission was recovered.
This phenomenon has implications where a test is performed following an accident
to a boom, such as a known overload.
Some reference has been made in the literature to the ultimate strength of frp
components with respect to acoustic emission monitoring. The ultimate tensile
strength is truly the load that just fails to initiate the avalanche failure
mechanism. Whilst loads much higher than this can be sustained for finite
periods of time, they do impart damage and do reduce the load level required to
initiate avalanche failure. Also, as failure is a time dependent phenomenon,
above the threshold load acoustic emission will continue to be generated up to
ultimate failure. Initially this may be very sparse but becoming more prolific
as fracture approaches.
For loads below the avalanche threshold, the Kaiser effect (short-term) tends to
hold. For loads above the threshold, acoustic emission will always occur. If a
boom is loaded, unloaded and then reloaded and acoustic emissions recur, then
the threshold load has been reached. In this case, damage will have been done
and acoustic emission will be re-established at a slighly lower load dependent
on the degree of degradation. The ratio of these two loads is known as the
"Felicity Ratio" and is used as a measure of damage.
PERFORMANCE OF THE ACOUSTIC EMISSION TEST
With the boom in the lowered state, but with outriggers operating, sensors are
attached. It is important that the sensors are well coupled to the component.
This is as simple as having a clean surface, using a small amount of viscous
fluid such as vacuum grease between sensor and the surface and holding the
sensor firmly in position. Sensor hold-downs are extremely varied and may range
;rora a magnetic clamp for steel components to a simple band of adhesive tape
for the boom Figure 6. Some care must be taken to ensure that the hold-down is
not going to move, and hence produce its own acoustic emissions, during the
test.