Issue 33

C. Bagni et alii, Frattura ed Integrità Strutturale, 33 (2015) 105-110; DOI: 10.3221/IGF-ESIS.33.14

The un-notched specimens as well as the notched samples were loaded in cyclic four-point bending at a frequency of 10 Hz. For the force-controlled fatigue tests the adopted failure criterion was the complete breakage of the specimens. The experimental results generated according to the above experimental procedure are shown in Fig. 1 together with the corresponding endurance limits. In this figure  0,MAX indicates the un-notched material endurance limits, whilst  MAX denotes the notch endurance limits referred to the nominal net area. The symbol R m is used to indicate the average value of the load ratio characterising any data sets. The above endurance limits were all estimated at 2  10 6 cycles to failure by post-processing the fatigue data according to the up-and-down method proposed by Dixon [14]. To conclude, it is worth observing that the determined endurance limits were estimated in terms of  0,MAX and  MAX because, under either cyclic axial loading or bending, the maximum stress in the cycle is seen to be a stress quantity capable of efficiently taking into account the mean stress effect in concrete fatigue [15].

Intermediate notches, r n

=12.5 mm

Un-notched Specimens

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

 max [MPa]

 max [MPa]

Failure Run Out

Failure Run Out

=5.1 MPa

 0,MAX

=4.6 MPa

 MAX

Batch B, R=0.33

Batch B, R=0.35

Batch A, R=0.11

Batch A, R=0.09

=3.3 MPa

=3.2 MPa

 0,MAX

 MAX

1

2

3

4

5

6

7

1

2

3

4

5

6

7

Specimen ID

Specimen ID

Blunt notches, r n

=25 mm

Sharp notches, r n

=1.4 mm

 max [MPa]

 max [MPa]

Failure Run Out

Failure Run Out

=4.7 MPa

=4.6 MPa

 MAX

 MAX

Batch B, R=0.40

Batch B, R=0.34

Batch A, R=0.07

Batch A, R=0.11

=3.3 MPa

 MAX

=3.1 MPa

 MAX

1

2

3

4

5

6

7

1

2

3

4

5

6

7

Specimen ID

Specimen ID

Figure 1 : Summary of the generated fatigue results and endurance limits estimated according to Dixon’s technique [14].

G RADIENT - ENRICHED TIP STRESSES TO DESIGN NOTCHED CONCRETE AGAINST HIGH - CYCLE FATIGUE

T

he accuracy of GE in estimating high-cycle fatigue strength of notched concrete was checked against the generated experimental data by using this approach according to two different strategies as described in what follows. Initially GE was applied along with the so-called Theory of Critical Distance (TCD) [16, 17]. The TCD postulates that the high-cycle fatigue strength of a notched component can be estimated via critical distance L which takes on the following value [16]: 2



    

K

1

 

L

(4)

th



0 

In the above definition  K th

is the range of the threshold value of the stress intensity factor, whereas  0

is the range of

the un-notched material endurance limit.

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