Hydrocarbon Processing - July 2022 - 79

Maintenance and Reliability
adhesive strength of a material when bonded between two ridged
metallic substrates. Two samples with dimensions of 100 mm ×
25.4 mm × 2 mm were overlapped lengthwise by approximately
12.7 mm and bonded to a minimal bond-line thickness with the
proprietary adhesivea (FIG. 2). The specimen was allowed to cure
before being attached to a 25-kN tensometer using suitable grips.
The tensometer then applied a load at a fixed rate of 1.3 mm/
min, exerting a cleavage force on the specimen until bond failure.
Fatigue resistance: ISO 9664:1993. Fatigue resistance is
the highest stress that a material can withstand for a given number
of cycles without breaking. A standard static tensile shear
adhesion test was conducted to determine the mean breaking
stress-24.17 MPa was used as the mean stress in fatigue testing.
The mean breaking stress value was 35%; therefore, the
35% mean shear stress was 24.17 MPa × 35% = 8.461 MPa. At
four different alternating stresses, fatigue testing was conducted
at 30 Hz until failure. Results were as follows:
1. 80% = 6.8 MPa (8.461 MPa × 80%) stress amplitude
cycles between
2. 60% = 5.1 MPa (8.461 MPa × 60%) stress amplitude
cycles between
3. 57.5% = 4.9 MPa (8.461 MPa × 57.5%) stress amplitude
cycles between
4. 55% = 4.7 MPa (8.461 MPa × 55%) stress amplitude
cycles between.
The ISO 9664:1993 fatigue stress cycle is shown in FIG. 3.
Impact resistance: ASTM D256-10 (2018). Impact tests
can be used to assess the toughness of a material. A material's
toughness is a factor of its ability to absorb energy during plastic
deformation. Brittle adhesives have low toughness due to
the small amount of plastic deformation that they can endure.
Conversely, tougher materials can absorb greater energy during
fracture; therefore, they have improved impact resistance.
The Izod impact test enables samples to be tested in two forms:
notched or unnotched. In this case, the testing was notched, with
a V-shaped notch of approximately 2.5 mm in depth, and a total
defect angle of 45° in the center of the specimen sample, with
dimensions of 12.7 mm × 12.7 mm × 65 mm. The notch concentrates
stress and enables a crack propagation measurement.
Three-point load test. This non-standard test was used
to assess the relative flexibility of adhesives when applied to a
metallic substrate. In this test, mild steel panels of dissimilar dimensions
(Panel 1's thickness was 550 mm × 50 mm × 10 mm,
and Panel 2's thickness was 225 mm × 50 mm × 10 mm) were
stressed to the point that the adhesive failed. The panel was held
in position at two points-one at either end of the sample-and
was gradually stressed at a single point in the center of the specimen
via a hydraulic press (FIG. 4). The greater the displacement
(i.e., the further the press travels until failure), the more flexible
the adhesive. The thickness of the adhesive will influence the
degree of flexibility, so the analysis should be duplicatable for
repeatability purposes.
In the case of this testing (the manufacturing stage), the
specimens were compressed by hand pressure only to replicate
in-field applications of achieving below the maximum bond-line
thickness of 2 mm.
FIG. 3. ISO 9664:1993 fatigue stress cycle.
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FIG. 4. Three-point load testing.
Hydrocarbon Processing | JULY 2022 79
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Hydrocarbon Processing - July 2022

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