PSI - Issue 64

Henrik Becks et al. / Procedia Structural Integrity 64 (2024) 1279–1286 Henrik Becks / Structural Integrity Procedia 00 (2019) 000 – 000

1285

7

FOS failure

95% of max. load

mean strain development

strain profile at 95% of max. load

30

6000

15000

a)

b)

c)

1. crack 2 3

100

prior to macro-cracking

strain [ ] FOS,mean  µm/m

5000 10000 strain [ ] FOS  µm/m

20

4000

yield point

50

FOS data anomaly

0

10

2000

5

loading [kN]

4

time

time

0

0

0

34 cm

1 - 5

Figure 5. FOS results of a monotonic tensile test with reinforcement: a) load-time curve, b) mean FOS strain-time curve, and c) FOS strain profile at 95% of max. load.

4.3. Fatigue behavior To efficiently test the FOS system under subcritical cyclic conditions, an accelerated fatigue test was conducted, i.e., a fatigue test with stepwise increasing amplitude. The applied loads can be seen in Figure 6a. Until the first crack emerges, the strain increases gradually (Figure 6b). At load step 16, there is a pronounced strain minimum (due to specimen eccentricity, cf. Section 4.2), which announces crack initiation.

20

100 80

max

upper and lower load level

c)

d)

20

a)

first crack

max

12 16

strain [ ] FOS,mean  µm/m

8 12 16

0 20 40 60

step 16

8 loading [kN]

min

step 7

0 4

loading [kN]

0 4

step 1

min

time

0

50

100

150

time

prior to macro-cracking

strain

[ ] FOS,mean  µm/m

0 20 60 40 140 120 strain [ FOS  ] µm/m 80 100

strain at max. load

1500

20

b)

e)

f)

max min

step 16

strain [ ] FOS,mean  µm/m

12 16

1000

step 1 step 7

8 loading [kN]

500

0 4

0

time

0 after macro-cracking

500

1000 1500

31 cm

strain

[ ] FOS,mean  µm/m

Figure 6. FOS results of a stepwise increasing fatigue tensile test with reinforcement: a) load-time curve, b) FOS strain profile at various steps, c-f) load-strain curve as well as mean FOS strain-time curve prior to and after macro-cracking.

Figure 6c and d illustrate the mean strain behavior leading up to crack formation. The influence of fatigue can be particularly observed in Figure 6d, when looking at the minimal strain: Despite a constant lower load level of 1 kN, the strain increases gradually. Similar to the other two presented experiments (cf. Figure 3f and Figure 4a), the first crack initiates at approximately 100 μm/m. Subsequently, the strain increases significantly, marking the onset of steel/steel-concrete-bond fatigue (Figure 6e and f). Several hundred cycles (before and after crack formation) evidently have no impact on the quality of the measurement.