PSI - Issue 28

M.R. Tyutin et al. / Procedia Structural Integrity 28 (2020) 2148–2156 TyutinM.R./ Structural Integrity Procedia 00 (2020) 000–000

2151

4

At the strain hardening stage, the activity of AE signals practically does not change. The next peak of AE activity is associated with the development of multiple fracture before reaching the ultimate strength. At the last stage of fracture (IV), a low AE activity is observed up to the final fracture of the specimen. A plateau appears on the dependence of the cumulated number of AE signals (Fig.2a). When testing specimens made of medium carbon steel (Fig.2b), the intense acoustic activity is observed at the plastic flow and strain hardening stages up to the value of relative deformation ε* = 0.6. At deformation of ε* = 0.6, a local peak in the AE activity and a subsequent drop in the b AE -value are observed, which, as will be shown below, is associated with the development of multiple fracture before reaching the ultimate strength. With further deformation, the intensity of acoustic emission significantly decreases until the fracture of the specimen. Features of the development of acoustic emission parameters during tensile deformation of stainless steel specimens consist in a more uniform activity and the absence of the AE peak at the yield point (Fig.2c). The maximum AE activity corresponds to the strain hardening stage at ε* ~ 0.2. The local drop in the b AE -value corresponds to a deformation of ε* = 0.2–0.3. When the ultimate strength is reached, the b AE -values decrease (Fig.2c). The plots in Fig.2d show that changes in AE characteristics during tensile tests of bainitic steel specimens appear at an early stage at a value of ε* ~ 0.2-0.4 [29]. At ε* ≈ 0.4, the intensity of acoustic emission (Fig.2d, curve 3) decreases, and a plateau appears on the deformation dependence of the total number of AE signals (Fig.2d, curve 2), which corresponds to the beginning of the localization of plastic deformation. a b -1 Ṅ AE , s -1

Ṅ AE , s

 N AE 1200

6

700

500

25

6000

I

II

III

IV

I

II

III

IV

2,0 b AE

15

600

1

 N AE 4000

400

20

1000

b AE 4

1

500



800

10

400

4

300

15

1,5

2

600

300  , MPa

4

200  , MPa

2

10

400

5

200

2000

1,0

200

100

3

100

5

3

0

0

0

0

0,0

0,2

0,4

0,6

0,8

1,0



0

0

0,5

0



0,0

0,2

0,4

0,6

0,8

1,0

-1

c

d

Ṅ AE , s

I

II

III

IV

3x10 4

200

1200

I

III IV

16

II

8000

600

3 b AE

150 Ṅ , s -1

1

14



2,0

ΣN AE 6000

1000

2x10 4 Σ N AE

2

500

12

2

1,5 b AE

800

10

400

2

3

100

600

 MPa

8

4000

300

4

 , MPa

1x10 4

400

4

6

1,0

50

200

1

4

200

2000

3

100

2

0

0,5

0

0

0,0

0,2

0,4

0,6

0,8

1,0



0

0

0

0

0,0

0,2

0,4

0,6

0,8

1,0



Fig.2. Stress-relative strain diagrams (curves 1) of specimens from low-carbon (а), medium-carbon (b), austenitic stainless (c) and bainitic (d) steels, combined with deformation dependencies of cumulated number of AE events (2), AE activity (3) and b AE -value (4).

The stages of changes in the parameters of acoustic emission are highlighted in the graphs (Fig.2). They are associated with the end of elastic deformation when the yield point is reached (stage I), with a kink on the dependencies