FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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The two components of the frp composite are very different in mechanical
behaviour. The glass fibres are brittle, exhibiting little strain prior to
tensile fracture, whilst the matrix is a plastic which exhibits visco-elastic
flow [1] rather than sudden failure. To some degree, the composite tends
towards the characteristics of the matrix when in compression and the fibres
when under tension. Under normal load regimes, a boom will experience tenoion
on its upper surface and compression on its lower surface. The tensile properties
are superior to those under compression thus, if the load is such as to
cause failure, the normal mode is compressional collapse.
Corapressional failure is, from an acoustic emission standpoint, very active. It
is initiated by failure of the matrix and results in the large scale expulsion
of fibres, as in Figure 1. All the processes involved are good generators of
acoustic emission and thus defects that would lead to this failure are readily
detectable.
In rare but no unknown circumstances [2], tensile failure can occur. An hypothesis
describing this behaviour has been presented [3] but has not been
confirmed. Briefly, the scenario is as follows. Below a certain threshold, the
fibres will maintain a tensile load Indefinitely. At loads above this threshold,
small scale fibre breakage will occur. The load previously supported by
the broken fibres will be instantaneously transferred to the surrounding
matrix. The matrix then undergoes visco-elastic flow and, in turn, transfers
its load to the remaining fibres, thus adding to their overload. Again, this
results in more fibre breakage and more load shedding through the matrix to the
remaining fibres. It is reasonable to suggest that the time taken for the
matrix to transfer its load due to fibre breakage, ie, through the flow process,
decreases as the load to be transferred increases. Thus, once the process
starts it accelerates to an avalanche type failure, assuming the load is
maintained. This can result in a very sudden failure in contrast to the
relatively slow compressional failure mode.
Figure 2 illustrates the relationship between fibre breakage and time and also
indicates the effect of changing load. Figure 3 illustrates how the time to
failure is dependent on load and also the effect of increasing the fibre count.
Both of these figures are generated based on the above hypothesis. However,
McElroy [2] 1 s demonstrated the relationship in the laboratory, his results
being illustrated in Figure 4. Figure 4, reproduced from reference 2, also
illustrates the effect of removing or changing the load during the process.
Initially, acoustic emission monitoring was applied to bucket trucks solely to
enhance the inspectability of the boom. It soon became apparent that the metal
components also generated acoustic emission under the proof test conditions. At
first this was considered a nuisance and "noise" sources were eliminated eg, a
poorly lubricated pivot pin, but advantage was taken of this and the test was
soon expanded to cover all major structural items in a single, comprehensive
test. In fact, current indications are that defects in metallic components are
discovered far more frequently than defects In the frp components [4],