Performance of pin-loaded carbon fibre reinforced polymer straps at elevated temperatures
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Date
23/06/2022Author
Stankovic, Danijela
Metadata
Abstract
Fibre reinforced polymer (FRP) composites are ideal for demanding structural engineering
applications which require high-strength, stiffness, and durability. This is particularly
the case for carbon fibre-reinforced polymers (CFRPs). In structural engineering,
the use of CFRPs is reasonably widespread for providing additional (externally bonded)
reinforcement to existing structural members, but CFRPs may also be used in standalone
structural engineering applications. One such application is their use as tensile
elements in the form of looped CFRP members (straps). Until now, all research that
has considered CFRP straps (either in static or fatigue loading) has been performed at
ambient temperature conditions. The absence of experimental data at elevated temperatures
damages confidence when utilising CFRP straps in applications where elevated
temperatures may occur. This thesis investigates the tensile performance of model,
laminated, unidirectional (UD), titanium pin-loaded CFRP straps as temperatures increase
up to 600 °C. In addition, it also investigates the fretting fatigue performance of
pin-loaded straps at sustained service temperatures of 60 °C.
Two main experimental series are presented, including tests both at ambient and at
elevated temperatures (in both steady state and transient thermal conditions). The UD
CFRP straps were manufactured either as non-pretensioned – using an existing mould
– or pretensioned during manufacture using a new pretensioning mould developed as
part of this thesis.
First, the performance of pretensioned, laminated, UD, CFRP straps is assessed at
elevated temperatures. Two types of tests are presented: (1) steady state thermal, and
(2) transient state thermal. Nine steady-state target temperatures in the range of 24
to 600 °C were selected based on results from dynamic mechanical thermal analysis
(DMTA) and thermogravimetric analysis (TGA). The transiently-heated pretensioned
straps sustained three tensile load levels, namely 10, 15, and 20 kN, corresponding
to 25%, 37%, and 50% of their ambient temperature ultimate tensile strength (UTS).
The straps were able to retain about 50% of their ambient temperature UTS at 357
°C. Similarly, the strength degradation curve of non-prestressed straps at eight target
temperatures between 24 and 410 °C was obtained.
To better understand the stress concentration regions within the straps leading to failure,
an 1/8 finite element (FE) model is presented. A series of parametric studies is
used to show that the most influential parameter is the pin geometry; an elliptical pin
could decrease the stresses of the straps around the vertex area (at the contact region
between the pin and the strap) by almost 19%. In addition, the strains of the outermost
plies of the 1/8th scale FE model were compared to the strains obtained with optical
fibre measurements made experimentally, and are found to be in reasonable agreement.
Resultant stresses from the FE modelling are implemented in a failure criterion by
Schürmann to predict the load at which first fibre failure is likely to occur; this is found
to be in reasonable agreement (less than 3% difference) compared to the experimental
load at which fibre breakage occurs.
The tensile performance of larger pretensioned CFRP straps is investigated following
the same testing conditions as noted above (same thickness and width; longer straight
shaft). Initially, two configurations of large straps are transiently heated for which their
winding start/end point and a ±45o ply is located: (1) at the centre of the straight shaft
length (inside the chamber) and (2) close to the vertex area (outside the chamber).
The two configurations were chosen in order to determine whether the large straps’
performance with increasing temperature would be affected. The transiently heated
large straps sustained the same three tensile load levels as the standard straps: 25%,
37%, and 50% of their ambient temperature UTS. It is found that the large straps with
the start/end point of the winding and a ±45o ply nearer to the vertex area perform
better in the transient tests. Subsequently, steady state tests of large pretensioned
straps at eight target temperatures between 24 and 600 °C are presented, in which the
straps’ winding start/end point with a ±45o ply was outside the chamber. Overall, the
large pretensioned straps are able to sustain 50% of their ambient temperature UTS at
550°C.
With respect to the second experimental series, the fretting fatigue performance of
laminated, titanium pin-loaded standard CFRP straps at ambient and elevated service
temperature is examined. A frequency of 10 Hz and a load ratio of R=0.1 up to three
and/or 11 million loading cycles are used in all fatigue tests in order to establish the
maximum stress over the number of cycles to failure (S-N) curves. Prestressed straps
are fatigue tested at ambient (laboratory) conditions at six upper stress levels (USLs)
between 800 and 1300 MPa, while non-prestressed straps are fatigue tested at a slightly
elevated service temperature (60 °C) at nine USLs between 650 and 1400 MPa. The
fatigue limit observed from the S-N curve at 24 °C shows a slight increase of 50 MPa,
when compared to the fatigue life determined from non-prestressed straps at 24 °C.
Additional tests at 11 million loading cycles could provide a complete picture of the
results obtained in this thesis regarding the prestressed straps at 24 °C. Regarding the
fatigue limit of non-prestressed straps at 60 °C (proposed maximum service temperature),
it is concluded that the fatigue of the non-prestressed straps is not obviously
affected at higher USLs, and only exhibits a slight decrease of 50 MPa when compared
to the fatigue limit derived from the S-N curve of non-prestressed straps at 24 °C.
The findings of this thesis provide evidence for designers as to the tensile and fatigue
performance of CFRP straps; this evidence can potentially be used to enable the application
of such straps for bridge deck suspender cables or shear reinforcements for
reinforced concrete slabs and beams.