18-05-2012, 11:59 AM
Investigation of strain transfer to a sensor protection system embedded in concrete using finite element analysis
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INTRODUCTION
When a structure is subjected to an external load it
experiences strains which can be measured experimentally
using strain gauges. It is assumed that the strain parallel
to the free surface of the concrete structure is faithfully
recorded by the strain gauges. Fibre optic sensors
systems offer the possibility of measuring strains inside
the concrete structure. A number of previous publications
have successfully demonstrated the deployment of
embedded optical fibre sensors (OFS) for structural
integrity monitoring of civil structures [1-4]. In general,
two sensor designs dominate this field, namely, fibre
Bragg gratings (FBG) and fibre Fabry-Perot (FFP) sensors
[5, 6].
Sensor protection system and test specimens
A schematic illustration of the three sensor protection
systems, investigated in this study is presented in
Fig. 2 along with the relevant dimensions. The SPS was
constructed from stainless steel. The relative dimensions
and the location of the SPS within the concrete test
specimens are shown in Fig. 1. Points A and B in Fig. i
correspond to locations along the SPS surface where the
FE analysis focused on shear stress and direct strain. A
summary of the relevant material properties is given in
Table 1. The concrete cylinder was subjected to a compressive
load of 100 kN.
Finite element modelling and test specimens
The following assumptions were made in developing
the FE model:
i. the material is homogenous and isotropic in nature;
ii. the test specimen is only subjected to elastic loading;
iii. preliminary analysis were performed based on the
following type of contact between the concrete-SPS
interface:
a. no separation (where the target and contact surfaces
are tied but they are allowed to slide once the ultimate
shear stress is achieved),
b. rough (perfectly rough frictional contact corresponding
to infinite friction and hence ignores MU), and
c. bonded (contact integration points that are either initially
inside the pinball region or that once involve contact
always attach to the target surface along the normal
and tangent directions to the contact surface so that sliding
is permitted at the ultimate shear stress);
Finite element analysis of SPS without flanges
An initial FE analysis for a SPS without flanges but
assuming perfect bond between concrete and SPS, was
performed by applying a compressive load of 100 kN
(12.73 MPa) to the concrete cylinder with an embedded
stainless tube having a diameter of 2.04 ram. The results
are shown in Fig. 6. This was found to generate an average
shear stress of 2.22 kN/mm 2 at the tube-concrete
interface. Since this value was higher than the interfacial
bond or anchorage strength