Preliminary tests
5.2 Implementation of the DCPD method: initial configura- configura-tionconfigura-tion
In total six M(T) specimens and two C(T) specimens are tested with the initial configuration of the DCPD method. All relevant specimen data such as loading condition, name, main dimensions etc. are listed in table B.1 and table B.4 in appendices B.1 and B.2 for the M(T) and C(T) specimen, respectively.
A set of M(T) and a set of C(T) specimens are cut out of one steel plate which are referred to asVJ and VH, respectively. In addition there are M(T) specimens tested from another steel plate of the same material which are referred to asJS.
The experiments are performed at room temperature and start with tests of M(T) specimens followed by C(T) specimens. All used specimens are tested with constant amplitude loading at a nonzero mean stress, see section 2.2.2. The used M(T) specimens are tested at the two load ratiosR= 0.1 andR= 0.5 and all C(T) specimens at the load ratios R = 0.2 andR= 0.4 where the second load ratio of each specimen is used to create beach marks, see section 2.3.2.
5.2. Implementation of the DCPD method: initial configuration 43
yU= 6 mm
yI= 75 mm Voltage 1 Voltage 2
1
3 2
Figure 5.1 Wire locations of the M(T) specimen using the DCPD initial configuration. Thin red wires are potential measurement lead wires and thicker black and red wires are current input wires. Voltage 1 is measured between point 1 and 2, Voltage 2 is measured between point 1 and 3.
The higher first load ratio for the C(T) specimens (compared to the M(T) specimens) is chosen to ensure that the resonant testing machine is working correctly because of difficulties with generating very low loads.
If crack detection gauges are used they have to be glued at a position where a sufficient precrack exists, thus the notch has no influence on the fatigue crack growth and the crack tip is sharp. According to ASTM [ASTM 2015] the minimum required fatigue precrack length a0−an is 0.1B, hor 1 mm whichever is greater and precrack and notch should lie within the required envelope shown in figure 3.4 a). In Walters [Walters and Voormeeren 2013] it is found for linear elastic conditions that precracks larger than 0.5h cause less than 1 % error of the stress intensity factor. In this study it is also found that if the enclosing angle of the required envelope is smaller than 42◦ an error of less than 1 % occurs. Hence, a precrack length of 0.5 mm and 2 mm is sufficient for the M(T) and C(T) specimen, respectively, and an opening angle at the notch tip ofθ= 30◦ for both specimen geometries. Therefore if crack gauges are used for the M(T) or C(T) specimen they are positioned 2 mm in front of the specimen notch.
5.2.1 M(T) specimen
All M(T) specimens are loaded with a maximum load of Pmax = 50 kN which lead to a minimum load of Pmin = 5 kN forR= 0.1 and Pmin = 25 kN for R= 0.5.
The first M(T) specimenJS17 is equipped with two crack detection gauges and the load ratios are applied for 75000 and 15000 cycles where the latter corresponds to the load ratio R= 0.5. The measurements are averaged and stored every 20 seconds which is changed for all following specimens to 2 seconds because the data of the crack detection gauges is averaged within a too long period which results in impractical data. Also the potential drop readings
yU= 6 mm yI= 36 mm
1
3 2
Figure 5.2Wire locations of the C(T) specimen using the DCPD initial configuration. Thin red wires are potential measurement lead wires and thicker black and red wires are current input wires. Voltage 1 is measured between point 1 and 2, Voltage 2 is measured between point 1 and 3.
are not correct. Thus no data of specimenJS17 is used for further analysis.
For specimenVJ4 no crack detection gauges are used. To reduce environmental influences a cold chamber, which is already positioned around the resonant testing machine for planned fatigue tests at low temperatures, is put around the specimen. The temperature inside the chamber is set to a constant temperature of 23◦C. In figure 5.3 the applied maximum and minimum loads of specimenVJ4 are shown where the inhomogeneities at the maximum and minimum load is due to measuring errors of the system. The minimum load switches in accordance with the predefined load ratios. At the switching point to the second load ratio the dynamic force of the resonant testing machine is decreasing quickly and then increasing again which can be seen at the blue vertical lines of the maximum load. In total seven beach marks are created on the fracture surface which are depicted in figure 5.4.
0 10 20 30 40 50 60 70 80
0 100 200 300 400 500 600 700
Load P [kN]
Number of cycles N ×103 [-]
Max. load, specimen VJ4 Min. load, specimen VJ4
Figure 5.3 Applied load, specimen VJ4.
The voltage drop, calculated as explained in section 5, of specimen VJ4 is depicted in
5.2. Implementation of the DCPD method: initial configuration 45
Figure 5.4 Beach marks of specimen VJ4. The light bands correlate with the load ratio R= 0.1, the dark bands correlate with the load ratio R= 0.5.
figure 5.5 with unit mV. The global trend is an increasing voltage drop with an increasing number of cycles which is as expected because the crack length increases with an increasing number of cycles, thus the electrical resistance raises, see section 2.3. With the present electrical equipment the noise of the potential measurement was already reduced within the preliminary tests as much as possible.
During this test the temperature inside the chamber increases by about 1.8◦C which has an negligible effect on the voltage drop.
0.0022 0.0024 0.0026 0.0028 0.003 0.0032 0.0034 0.0036 0.0038 0.004
0 100 200 300 400 500 600 700
Voltage drop U [mV]
Number of cycles N ×103 [-]
Specimen VJ4
Figure 5.5 Voltage drop, specimenVJ4.
For specimenVJ1 the number of cycles of the load ratioR= 0.5 is increased to 20000 in order to obtain thicker beach marks. Here the cold chamber is also put around the specimen but without heating it because no temperature influence was recognised at the measurements of the previous specimen.
This specimen is equipped with two crack detection gauges for which better results are obtained compared to specimen JS17, see figure 5.6. These measurements have a stepped shape because at each disconnected strand the strain increases immediately.
The applied load is shown in figure B.2 a) in appendix B.1 and it can be seen that
se--500 0 500 1000 1500 2000 2500 3000
400 450 500 550 600 650 700
Strain [Ûm/m]
Number of cycles N ×103 [-]
Crack detection gauge 1, specimen VJ1 Crack detection gauge 2, specimen VJ1
Figure 5.6 Crack detection gauges, specimenVJ1.
ven beach marks are created. The voltage drop is shown in figure B.2 b) and globally increases.
SpecimenJS21 lasts much longer than the other M(T) specimen and 18 beach marks are created. The reason for this is potentially the different steel plate. For the test with this specimen the cold chamber is not closed because of a damaged door but this circumstance shows no effect on the obtained results. Nevertheless the results of specimenJS21 are not used for further analysis because it cracked asymmetrically, see figure 5.7.
Figure 5.7 Asymmetrically cracked specimenJS21.
5.2.2 C(T) specimen
The C(T) specimen VH1 is tested with a maximum load of Pmax = 16 kN and the second specimen VH2 with a maximum load of Pmax = 12 kN which leads to a minimum load of