Underground Construction - December 2017 - 67

chamber, one on the chamber wall and the other on the surface of the test
specimens.
The uniaxial tension tests were performed when the temperature reading from the two thermocouples reached the testing temperature. An
extensometer with a gage length of 50.8 mm and travel length of 25.4
mm was used to measure the axial strain, along with electrical resistance
strain gage with a gage length of 10 mm. The strain gage was used to measure the strains below 1%. The tests were performed up to 50% strain.
Strain rates were varied between 0.003/min and 0.300/min and the three
test temperatures were 22°C, 60°C and 90°C, based on potential exposure
condition for the coatings. The strain rates were determined from the
extensometer reading.
The three material parameters, initial elastic modulus (Ei), yield
strength (y) and yield strain (y) were determined from the engineering
stress-strain relationships. The initial modulus was determined as the initial slope of the stress-strain relationship. The yield strength was the maximum engineering stress carried by the specimen. The secant modulus at
yield was obtained by dividing the yield strength by the yield strain.

Material Characterization
The materials were characterized using the bulk density, FT-IR response
and SEM microstructural analyses, based on testing 36 samples.
* The average density of PP was 0.80 g/cm3 and the maximum coefficient
of variation (COV) was less than 0.5%. This average density of PP was
comparable to the density reported in the literature.
* The average density of GM was 0.79 g/cm3 and the maximum coefficient of variation (COV) was less than 1%. It was very close to the
density of the pure polypropylene due to the type of glass microsphere
fillers used in the composite. The glass microsphere content was 30%
by volume of the composite and had an average diameter of 30 m.
* The average density of GF was 1.95 g/cm3 and the maximum coefficient of variation (COV) was less than 1.5%. It is 2.4 times higher than
the density of PP and was used to provide stability to the subsea pipeline, as well as for its thermal properties. A similar polypropylene composite was used as a subsea pipeline coating with a reported density of
2.30 g/cm3.
The SEM micrograph of GM in PP is depicted in Figure 4; part (a)
shows the overall distribution of glass microspheres in the polypropylene
matrix. At higher resolution, Figure 4(b), the glass microsphere was 30
m in diameter.

Figure 5: SEM micrograph of glass filled polypropylene (GF); (a) magnification of
12 KX, and (b) magnification of 123 KX.

Test Results
The engineering tensile stress-strain relationships of pure polypropylene, and polypropylene with glass microsphere filler and 65% glass filler,
under different strain rates and temperatures are shown in Figure 6(a),
Figure 6(b) and Figure 6(c), respectively.
At least three specimens were tested under each condition, and the
average properties are summarized in Table 2. The maximum coefficient
of variation (COV) for the yield strength, initial modulus and secant modulus for all the materials tested were 2%, 4% and 3%, respectively. No
clear necking was observed in any of the specimens during the tests. The
ratio of secant modulus at yield (Esy) to initial elastic modulus (Ei), Esy/Ei,
represents the nonlinear behavior of the material.

Table 2: Avreage properties of PP, GM and GF.

Figure 4: SEM micrograph of polypropylene with glass microsphere filler (GM); (a)
magnification of 0.1KX, and (b) magnification of 4.6 KX.

The SEM results of GF are shown in Figure 5. In part (a), the polypropylene matrix has been clearly identified separately from the glass particles. The concentration of the glass particles were about 65% (volume
fraction) calculated from the bulk density. The glass particles' average size
was less than 1 m, as observed at higher resolution in Figure 5(b).
December 2017 Underground Construction

The engineering tensile stress-strain relationship of PP was nonlinear in the elastic domain, as shown in Figure 6(a). The yield strength was
taken to be the stress at the point of maximum change engineering stress,
which corresponded in most cases to the maximum stress in the stressstrain curve. After yielding, the stress-strain relationship was nearly
perfectly plastic behavior at all strain rates and temperatures. Quantifying the temperature effect at the same strain rate, the overall stress level
decreased with increasing temperature. At a strain rate of 0.300 min-1
the yield strength was 25.6 MPa at 220C and 10.0 MPa at 900C, a 61%
decrease in yield strength for about a 300% increase in temperature. On
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Underground Construction - December 2017

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