Crack growth data A and m

(1) A and m are obtained from crack growth measurements on standard notched specimens orientated in the LT, TL or ST direction (e.g. see Figure B.3.1) using standardised test methods (e.g. see Reference B.8.3). The specimen design must be one for which an accurate stress intensity factor (K) solution (i.e. the relationship between applied load and crack size 'af) is available.

Two holes diameter 0,25 w +

Increment

Increment

Fatigue precrack x

*

S

A-i >

w± 0,005w

1,5w ± 0,010w

Recommended thickness: w/20 <b<w/4

Mouth opening displacement

(Symmetric)

Recommended thickness: w/20 <b<w/4

Fig.B.3.1. Typical crack growth specimen (example from ref.B.8.3)

(2) The test entails computer controlled cyclic loading of the specimen at constant applied stress intensity ratio (Kmin/KmaxX for R° - testing conditions or at constant Kmax for Kmaxc - testing conditions (see ref. B.8.7) and accurate measurement of the growth of the crack from the notch.

(3) If discrete values of crack length 'a' are obtained, a smooth curve is fitted to the data using the method specified in the test Standard. The crack growth rate, da/dN at a given crack length is then calculated as the gradient of the curve at that fa' value.

(4) The corresponding value of the stress intensity factor range, AK, is obtained using the appropriate K solution for the test specimen, in conjunction with the applied load range. The results da/dN versus DK are plotted using logarithmic scales.

(5) For general use, crack growth curves may be required for different R values. Figure B.3.2 shows a typical set of da/dN vs AK curves for the aluminium extrusion alloy AA6005A-T6 (AlMgSi0.7). In Fig.B.3.2(a) the testing condition was constant ratio of stress intensity K^n/K^, Rc, and in Fig.3.2(b) the result of a K^0 - test at a constant Kmax of 10MPa(m)1/2 is combined with the conservative branches of the curves from Fig.B.3.2(a). This combination of the results of the Rc and the Kmax0 data is a conservative engineering approximation and can be used for the fatigue life prediction in case of high residual tensile stresses or short fatigue crack evaluations. The values of m and A for Fig. B.3.2. are given in tables B.3.2(a) and (b).

(6) The assumption made in ref. B.8.1, equation A4-11, that the fatigue crack propagation rates of metals are proportional to the cube of the ratio of the Young's moduli with respect to steel is used as a scale to compare the FCGR of different aluminium alloys. In Fig.B.3.3(a) the Rc-FCGR of wrought aluminium alloys of R=0.1 are plotted and in Fig.B.3.3(b) the corresponding data for R=0.8 are added. Figure B.3.4 shows the set of Rc-FCGR curves of three gravity die cast alloys at R=0.1 and R=0.8. Figure B.3.5 represents the combined data of Rc and K^0 - tests of wrought aluminium alloys for R=0.1 and R=0.8. The values of m and A of the upperbound FCGR envelopes for Figs.B.3.3 to B.3.5 are given in tables B.3.3 to B.3.5 respectively.

(7) Corrosive environments can effect A and m. Test data obtained under conditions of ambient humidity will be adequate to cover most normal atmospheric conditions.

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

1E-06

1E-07

1E-08

1E-09

1E-10

1E-05

10 20 30 40 50

a) R = constant (kmin/kmax) various R-ratios

10 20 30 40 50

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

10 20 30 40 50

b) kmax = constant (10MPaVm) various R-ratios

10 20 30 40 50

Figure B.3.2. Typical fatigue crack growth curves for aluminium alloy 6005A T6LT

Table B3.2(a) Fatigue crack growth rate data for EN AN 6005-T6 LT,

R-Kmin/Kni,1=constant

AK MPam0'5

m

A

AK MPam0'5

m

A

0,100

3,30 4,50 8,00 32,4 41,61 60,00

15,00 7,51 2,96 11,97 11,97 11,97

0,1657E-18 0,1293E-13 0,1673E-09 0,4100E-23 0,4100E-23 0,4100E-23

0,500

2,00 2,72 4,20 6,50 21,00 29,16 42,50

16,29

3.85

4.86 2,80 12,23 12,23 12,23

0,1243E-15 0,3174E-10 0,7414E-11 0,3406E-09 0,1211E-21 0,1211E-21 0,1211E-21

0,200

2,90 3,80 7,50 29,60 37,98 55,00

18,53 5,86 2,92 12,43 12,43 12,43

0,2679E-19 0,5949E-12 0,2227E-09 0,2253E-23 0,2253E-23 0,2253E-23

0,650

1,50 1,95 2,20 3,55 6,00 15,00 22,17

16,93 4,42 2,38 4,76 3,05 12,00 12,00

0,1042E-13 0,4418E-10 0,2206E -09 0,1068E-10 0,2326E-09 0,6084E-20 0,6084E-20

0,300

2,60 3,40 7,35 26,00 34,49 50,00

18,67 5,23 2,82 12,40 12,40 12,40

0,1774E-18 0,2470E-11 0,3060E-09 0,8411E-23 0,8411E-23 0,8411E-23

0,800

1,00 1,28 1,55 3,50 4,60 9,20 13,48

13,03

4,99

2,50

6,03

3,11

15,93

15,93

0,9999E-11 0,7289E-10 0,2168E-09 0,2611E-11 0,2225E-09 0,9830E-22 0,9830E-22

Table B3.2.(b) Fatigue crack growth rate data for EN AA-6005A-T6 LT, K,n«-100MPa(m)0,5 = constant

R-ratio

Stress

m

A

Ratio

Stress

m

A

Intensity

Intensity

AK

AK

MPam0'5

MPam0'5

0,100

0,85

11,09

0,6069E-10

0,500

0,85

11,09

0,6069E-10

1,16

3,74

0,1807E-09

1,16

3,74

0,1807E-09

1,60

2,68

0,2969E-09

1,60

2,69

0,2960E-09

8,00

2,96

0,1673E-09

5,55

4,76

0,1081E-11

32,40

11,97

0,4103 E-23

6,50

3,05

0,2326E-09

41,61

11,97

0,4103 E-23

21,00

12,04

0,6081E-21

29,16

12.04

0,6081E-21

0,300

0,85

11,09

0,6069E-10

0,650

0,85

11,09

0,6069E-10

1,16

3,74

0,1807E-09

1,16

3,74

0,1807E-09

1,60

2,71

0,2935E-09

1,60

2,69

0,2960E-09

6,70

5,51

0,1413E-11

4,95

4,76

0,1081E-10

7,35

2,82

0,3060E-09

6,00

3,05

0,2326E-09

26,00

12,40

0,8421 E-23

15,00

12,04

0,6081E-20

34,49

12,40

0,8421 E-23

22,17

12,04

0,6081E-20

0,800

0,85

11,09

0,6069E-10

1,16

3,74

0,1807E-09

1,60

2,71

0,2927E-09

4,15

6,01

0,2689E-11

4,60

3,11

0,2225E-09

9,20

15,93

0,9819E-22

13,48

15,93

0,9819E-22

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

10 20 30 40 50

10 20 30 40 50

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

10 20 30 40 50

10 20 30 40 50

Figure B.3.3 Typical crack growth rate curves for various wrought alloys

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

10 20 30 40 50

10 20 30 40 50

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

10 20 30 40 50

AK (MPaVm)

10 20 30 40 50

AK (MPaVm)

Figure B.3.4. Typical fatigue crack growth curves for various cast alloys da/dN (m/cycle)

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

AK (MPaVm)

1E-06

1E-07

1E-08

1E-09

1E-10

1E-05

AK (MPaVm)

1E-04

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

1E-05

1E-06

1E-07

1E-08

1E-09

1E-10

1E-11

10 20 30 40 50

AK (MPaVm)

10 20 30 40 50

AK (MPaVm)

Figure B.3.5 Typical fatigue crack growth curves for various wrought alloys

Table B3.3. Fatigue crack growth rate data for wrought alloys, R = Kmu/Kou^onstant

R-ratio

Stress Intensity AK

m

A

MPam0'5

a) 0,100

1,68

3,3

0,254IE-18

1,89

3,4

0,4065E-10

2,96

4,1

0,4886E-09

4,75

6,6

0,2951E-12

6,70

2,8

0,4838E-09

19,51

5,9

0,4080E-13

28,71

9,8

0,3072E-17

b) 0,800

0,87

10,43

0,4276E-10

1,24

3,33

0,1959E-09

2,27

2,98

0,2603E-09

3,40

6,36

0,4155E-11

5,44

8,34

0,1454E-12

11,45

8,34

0,1454E-12

Note: These values are upperbound envelopes derived from the curves shown in Fig.B.3.3(a) and (b)

Table B3.4. Fatigue crack growth rate cast alloys R=Kl„ill/KIIMX = constant

R-ratio

Stress Intensity AK

m

A

MPam0,5

a) 0,100

3,28

35,46

0,5102E-29

3,45

11,01

0,7184E-16

4,60

6,50

0,705 IE-13

8,85

3,85

0,2260E-10

23,07

19,12

0,3475E-31

27,30

19,12

0,3475E-31

b) 0,800

1,42

21,24

0,6086E-14

1,76

5,47

0,4520E-10

5,82

12,34

0,2537E-15

8,70

12,34

0,2537E-15

Table B.3.5 Fatigue crack growth rate data for wrought alloys, Km„

=10MPa(m)0,5 constant

R-ratio

Stress Intensity AK

m

A

MPam"2

0,100

0,76

9,13

0,1211E-09

1,26

2,77

0,5266E-09

19,50

5,95

0,4190E-13

28,71

8,79

0,3072E-17

34,48

8,79

0,3072E-17

0,800

0,76

9,30

0,1268E-09

1,22

2,84

0,4560E-09

4,37

5,28

0,1243E-10

6,76

11,02

0,2128E-15

11,45

11,02

0,2128E-15

0 0

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