1 Introduction
This document will describe the experimental apparatus and procedure for conducting material
creep testing. The apparatus will use a weight and lever to apply constant tensile load to the sample
during testing. The necessary sample loads and forces applied to the test apparatus are estimated
based on approximated sample sizes and material properties from similar materials. The first
section will discuss the estimation of forces and loads. Then this document will discuss the test
apparatus and instrumentation, test specimen specifications and fabrication, and the design of
experiments and test procedure.
2 Forces and Loads
The test specimens will be thin walled composite tubes with fiber winding angle as close to 90° as
possible. The samples will be loaded axially to measure the transverse material properties. The
thin samples are estimated to have a wall thickness of 2-3 mm and are constructed from glass fiber
reinforced polymer (GFRP). Material properties for a similar GFRP published in literature (Ha et
al. 2006) is used to estimate the load that must be applied to the sample, forces on the test apparatus,
and possible deformation. Table 1 provides approximate sample sizes and material properties.
Estimations will be made with the samples being tested at 40% and 60% of their tensile strength.
Table 1: Values for various parameters necessary for stress and strain analysis.
Parameter
Inner radius
Outer radius
Length
Elastic modulus
Transverse tensile strength
Value
25.4 mm
27.4 – 28.4 mm
101.6 mm
8.27 GPa
31 MPa
Beginning with the stress applied to the sample is 40% or 60% of the tensile strength,
𝜎𝑠𝑎𝑚𝑝 = 12.4E6 MPa, or 18.6E6 MPa
(1)
Then the force associated with this stress is given by
𝐹𝑠𝑎𝑚𝑝 = 𝜎𝑠𝑎𝑚𝑝 𝐴𝑠𝑎𝑚𝑝
(2)
where the sample area is between Alow = 0.331x10-3 m2 and Ahigh = 0.507x10-3 m2. Then the sample
load is given in Table 2.
Table 2: Creep stress for 40% and 60% load creep testing. Values are given for the lower and
upper bound of the cross-sectional area.
FSamp(%)
40%
Alow
4,113.7 N
Ahigh
6,287.5 N
60%
6,170 N
9,431.2 N
The load is applied to the sample with the lever arm, connected to part 8 in the assembly, appendix
A. Redrawing the lever as a free body diagram yields Figure 1.
Figure 1: Free body diagram of the lever arm with applied load, Fapp, load from the sample,
Fsamp, and the support load at the fulcrum, Fsup, sitting at an arbitrary angle θ.
In this figure the fulcrum is a distance rA and rB from the sample, FSamp, and the applied load, FApp
respectively. The fulcrum is a pin type support and therefore restricts movement in the horizontal
and vertical directions, and rotations about the vertical, y, or horizontal, x, axis. It can rotate about
the out of plane, z, axis. The applied load is caused by gravity and acts vertically, and the sample
load is also vertical. Of these only the rotation about the z axis and forces in the vertical direction
are non-trivial. Then to produce the desired sample load, the necessary applied load and required
support at the fulcrum can be found as a function of the angle of the lever θ.
∑ 𝑀𝑆 = 0 = 𝐹𝐴𝑝𝑝 cos(𝜃) − 𝐹𝑆𝑎𝑚𝑝 cos (𝜃)
𝐹𝑆𝑎𝑚𝑝 𝑟𝐴
𝐹𝐴𝑝𝑝 =
𝑟𝐵
(3)
(4)
(5)
∑ 𝐹𝑦 = 0 = 𝐹𝑆𝑢𝑝 − 𝐹𝑆𝑎𝑚𝑝 − 𝐹𝐴𝑝𝑝
𝑟𝐴
𝐹𝑆𝑢𝑝 = 𝐹𝑆𝑎𝑚𝑝 (1 + )
(6)
𝑟𝐵
Using equation (2) and the sample load in Table 2, the required load can be applied to the end of
the lever. Using the current solid model, available in appendix A, and assuming the total length of
the lever is 1 m then
𝑟𝐴
𝑟𝐵
= 0.118. The applied load at the end of the lever, FApp, is given in Table
3 in N and kg.
Table 3: Load required at the end of the lever arm to apply the appropriate sample stress. The
applied load is given in N and kg for both cross sectional areas
FApp(%)
40%
60%
Alow
485.4 N 49.5 kg
728.1 N 74.2 kg
Ahigh
741.9 N
75.6 kg
1,112.9 N 113.4 kg
Based on this analysis the test apparatus must be able to support a load of approximately
𝐹𝑆𝑢𝑝 = 10,544.1 𝑁
(7)
At the lever arm fulcrum and 9,431.2 N at the base of sample, part 6. Deformation is estimated
using the values in equation (1). The initial deformation is calculated using Hooke’s law and the
results are provided in the table below.
Table 4: Instantaneous deformation of test samples when the creep load is first applied.
FSamp(%)
Instantaneous
deformation
40%
152.3x10-3 mm
60%
228.5x10-3 mm
Based on these results it is reasonable to expect creep deformation to be on the order of 10-5 m.
The above analysis will be used to inform the design of the test apparatus and the selection of
sensors to ensure subsequent equipment will meet the test requirements.
3 Material Creep Testing
3.1 Design of Experiments
Table 5: Treatment combinations table showing all the combinations of creep load and testing
temperature.
Applied Load
40% YS
60% YS
Test Temperature
25
40
65
80
105
25
40
65
80
105
o Blocking
3.2 Test apparatus
Technical drawings for the testing apparatus can be found in appendix A. The primary components
of the test apparatus are the frame, mounting fixtures, and instrumentation.
The frame is constructed from standard sized T-slotted aluminum bars 1.5 inches square and either
1 foot or eight inches long. These are held together with L-brackets. These components are readily
available from commercial suppliers such as McMaster-Carr (USA). Instrumentation will be
discussed in greater detail in the next section.
The lower mounting fixture is a solid steel bar held in place with pipe brackets. The middle of the
bar has a coupling which slides inside the test sample tab and is held in place with a 12 mm steel
pin. This provides a ridged support structure to hold the sample in place and can be adjusted to
line up with the upper fixture ensuring the sample is loaded purely in the axial direction. The cross
bar is constructed from solid steel to ensure it is significantly more ridged than the sample to
minimize deflection of this part. The upper fixture is constructed from a short steel bar that acts as
a coupling between the end of the lever arm and the test sample. Again, these components are
constructed from steel to minimize deformation and are held in place with 12 mm steel pins
A tensile load sensor added to the test apparatus to replace part 7 to continuously record the creep
load on the test specimen. The strain gauge will be surface mounted to the specimen using an
epoxy adhesive. A silicone heater will then be wrapped around the sample and set to the
appropriate temperature. Electrical leads from these sensors then run to controllers then computer
to be recorded. These sensors are discussed in greater detail in the following sections.
3.3 Instrumentation
3.3.1 Heater
The heating elements necessary for elevated temperature testing will be required to maintain
temperatures from 25°C to 120°C for between 1 and 12 hours depending on the specific
experimental requirements. The heating elements will also have to wrap around the cylindrical
shaped samples and provide uniform heating of the sample.
The silicone rubber strip heater, part number STR001, is an example of an appropriate heater is
produced by Wattco Inc. (Lachine, Quebec, Canada). This type of heater has a maximum rated
operating temperature of 200°C, rated input voltage of 120V, and a built in adjustable thermostat.
The heater is available in a strip 5” wide and 12” long, appropriately sized for the test samples,
with Velcro attachments on the short edge to facilitate application to the samples. When contacted
the company made clear that this heater will be able to maintain the set temperature within 1°C as
required. As a premanufactured component this heater would easily integrate into the test
apparatus.
Cost: Quantity 1 = ~400CAD, 10 = 186CAD/unit
Commented [MS1]: Will have to change for a less
expensive load sensor. New sensor Omega LCM111-1k
3.3.2 Strain gauge
3.3.3 Load sensor
Omega LCM111-1k:
•
•
•
•
S type load sensor will be placed under the sample, replacing part 6. Fixed between two
plate. Sample applies tensile load to top plate, sensor records load.
3” x 2” x 1”
Compatible Meters: DP41-S, DP25B-S
Sensor: ~525CAD, Meter: DP25-S = ~425CAD
3.3.4 Data Acquisition
3.4 Sample Fabrication
3.4.1 Shape and dimensions
3.4.2 Filament winding
3.4.3 Cutting/post processing/documentation
3.5 Test procedure
To begin an experiment
1.
2.
3.
4.
5.
6.
7.
8.
Install sample
Connect strain gauges and load sensors
Calibrate sensors
Set temperature and begin heating
Check sensors are functioning properly, reached correct temperature
Begin recording
Apply load
Begin timer
After completion
1. Remove load and turn off heat
2. Stop data recording
3. Remove sample from apparatus