Laboratory experiment 3 – CMOD, crack growth rates and LEFM
Three experiments will illustrate some aspects of fatigue crack propagation.
1.
Fatigue crack growth rate and the determination of Paris law parameters
2.
Determination of the linear fracture toughness KIc
3.
Determination of the crack length a from measured CMOD using the compliance
method.
The laboratory work will be performed jointly between the two parallel laboratory groups in two
test machines. The reason for this is to allow the fatigue growth experiment to run for at least
some mm. Three different specimens will be used, one for each measurement.
The testing machines are potentially hazardous. Do not work in the active area of the piston
when the hydraulic is on without LOAD PROTECT. Ask for help at the slightest uncertainty!
Crack growth rates
A standard CT-specimen will be supplied by the test leader. Measure the specimen overall
dimensions required for determining KI according to page 271 in the Solid Mechanics Handbook.
For the crack length, make an estimate based on visual inspection of the specimen.
The control program requires a start load. Calculate a load that for your specimen corresponds to
K = 16 MPam^1/2 at R = 0.1.
The crack growth rate will be measured during some mm of extension. The program CT will
command the test machine and acquire necessary data. The compliance is measured in real time.
Next, the instant crack length and the force corresponding to a desired K are calculated. The
calculated force is then automatically sent as a command to the machine.
Start the test
Make sure LOAD PROTECT is activated at for instance 0.2 kN and that the machine is in
position control.
Apply hydraulic pressure, LOW and HIGH in sequence.
Mount the CT specimen in the fixture. Use the jogging buttons to align the specimen and fixture
holes for mounting the load pin.
Mount the clip gage that is used to measure CMOD to the edges at the crack mouth.
Apply some small tensile pre-load. Use for instance LOAD PROTECT at 0.2 kN and the jogging
buttons.
Start the computer program CT (press ctrl F9). In the program a lot of parameters can be set
(suggested values are given inside [ ]):
W
B
Bn
E
a0
da1
dKmin
aStop
kvot
R0
frekvens
Fstart
tEdge
ndat
loop
nlevel0
hLim
lLim
vand
[your data]
[your data]
[your data]
[208]
[a+0.2]
[0.25]
[10]
[a+4 mm]
[1.1]
[0.1]
[14]
[your data]
[0]
[1500]
[1]
[4]
[100]
[60]
[-1]
Distance from the load line to the specimen rear side.
Gross thickness.
Net thickness, measured at the bottom of possible side grooves.
Young’s modulus.
Starting point for load shedding.
Distance over which the growth rate is measured.
Lowest level for K. Not used.
Final crack length.
Load shedding ratio.
Load ratio.
Frequency.
Start load at fatigue
Thickness of the edges.
Number of measured points for each a calculation.
Save a loop-file.
Measurements on the same level during load shedding.
Upper limit for points used to calculate the compliance.
Lower limit for points used to calculate the compliance.
Used to switch from load shedding to increasing load levels.
Insert data for the experiment to be placed in the documentation file and continue to next page.
Press REMOTE (flashing light) on test machine.
Start program to take control of experiment by pressing any key (except <s>!) on the computer
twice.
Note the initial growth result on the computer screen. Press <A> for K control and start of
da/dN measurements. Some initial cycles are required before the stable results are presented.
Press <K> to give control K value: K = 20 MPam^1/2. Wait some cycles for stable values on
the screen and note the measured crack length, a. Press <P> to update the values a0 = a + 0.2 and
aStop = a + 4. (You need to do the update fairly rapidly, else the test will halt.) The computer
output file is used for the result analysis. The experiment continues automatically during the
laboratory exercise. The load will be increased some 2 – 4 times* and new growth rates will be
measured and documented on the output file.
At the end of the laboratory exercise you will return and finish the growth rate part of the
experiment.
Determination of crack length
You will now change test machine for the next two parts of the laboratory work. A second
specimen will be used, which crack length shall be determined using the compliance method.
Measure the specimen dimensions and estimate the crack length roughly by doing a visual
inspection. Compute the initial load level that corresponds to the growth load K = 16 MPam^1/2.
Mount the specimen in the machine following the same procedure as above. Apply a low load,
for instance with aid of the load-protect function and thereafter remove load-protect. Switch to
LOAD CONTROL (press LOAD - GOTO - 0.2 kN – ENTER).
When the machine is in LOAD CONTROL at the pre-load level, you shall program a double
ramp to the new load level at a speed corresponding to a cycle time of say 20 s. For load control,
select WAVEFORM, RAMPS and DRAMP. Here you enter the ramp load and rate. Select
MORE and enter return load level and rate. Remember that you start from a pre-load level.
Start the computer program DragJQ1, give an identification name to the test and chose
appropriate scales:
xLo
xHi
yLo
yHigh
Jk
xVar
tid
ASCII
koeff0
koeff1
koeff2
[your data]
[your data]
[your data]
[your data]
[1.000]
[2.000]
[0.200]
[1.000]
[0.000000]
[1.000000]
[0.000000]
Displacement (min)
Displacement (max)
Load (min)
Load (max)
Scale factor at J-calculations
0 for position, 3 for clip-gauge, 2 for both
Sampling interval > 0.1 s
1 for ASCII, 0 for Pascal
Constant term in the CG-equation
Coeff. in front of the linear term
Coeff. in front of the 2nd degree term
Continue to the point where the program records data.
Press START to perform the ramp.
When the ramp is done (HELD will lit). End the ramp by pressing FINISH.
Repeat the load ramp by pressing START again. The repetition should continue until stable
compliance curves are achieved on the computer screen.
When HELD is lit for the last ramp, stop the computer program (<s>) and press FINISH.
Change to position control POSITION-IMMED and activate LOAD PROTECT.
Determine the fracture toughness KIc
Change specimen in the test machine to the third one. Measure the specimen dimensions and
estimate the crack length.
Start the program DragJQ1, give an identification name to the test and chose appropriate scales.
Program a single 20 mm ramp with a displacement rate of 0.6 mm/min using the WAVEFORM
buttons. This displacement rate falls within the stress intensity factor rate according to standard
(0.55 ≤ 𝑲̇≤ 2.75).
Perform the ramp by pressing START.
After the specimen has broken, stop the test by pressing FINISH and immediately activate
LOAD PROTECT. Stop the computer program by pressing <s> on the key-board.
Copy the result files with your data to your own USB for result analysis.
Disassemble the specimen from the test machine. Measure the crack length at five different
points to use for your evaluation.
End crack growth simulation
Return to the first test machine and end the crack growth experiment by pressing stop <s>. The
machine will automatically change to POSITION CONTROL and activate LOAD PROTECT.
Disassemble the specimen from the test machine.
Copy your file with crack growth data to your USB.
Data analysis and report
The laboratory report shall contain:
• Diagram with crack growth result da/dN vs. KI (both measured results and fitted curve)
• Paris law parameters from the diagram both in C, n format and in the “unit free” Cm and Kcm
format.
• Estimate of crack length by compliance method1. The report shall include a diagram with the
P-CMOD cycle that was used to determine a.
• Estimate KIc.
Data columns in the output file
DragJQ1:
•
•
•
•
CT:
•
•
•
•
•
Time in seconds
Piston position in mm
Load in kN
Clip gage in mm
0 is when program starts
0 is when program starts
Actual data
CMOD is set to 0 when program starts.
Crack length in mm
K in MPam^1/2
da/dN in nm/cycle
Cycle number
da/dN in nm/cycle
Alternatively computed value.
*Comment
The growth simulation for Paris Law parameter values are usually determined using the load
shedding procedure, i.e. a decreasing KI load range. Either the R-ratio is constant at a low value
or the maximum load value Kmax is constant throughout the experiment1. The experiment is
typically terminated when da/dN < 1 nm/cycle. Here increasing and rather high loads were used
to speed the experiment.
1E647–00
Standard Test Methods for Measurement of Fatigue Crack Growth Rates, ASTM, West Conshohocken, 2005, p 310.