Failure Prediction of AISI 420 Martensitic Stainless Steel Using the Theory of Critical Distances

Siqueira, Marcelo de Oliveira; Carvalho, Eduardo Atem de

doi:10.1590/1980-5373-MR-2020-0494

Marcelo de Oliveira Siqueira ^**e-mail: marcelo.osiqueira@pq.uenf.br

Universidade Estadual do Norte Fluminense Darcy Ribeiro, Rio de Janeiro, RJ, Brasil

https://orcid.org/0000-0002-3280-1529

Eduardo Atem de Carvalho

Universidade Estadual do Norte Fluminense Darcy Ribeiro, Rio de Janeiro, RJ, Brasil

https://orcid.org/0000-0001-8141-7518

*e-mail: marcelo.osiqueira@pq.uenf.br

Thumbnail

Figure 1
Coordinate system considered by Creager and Paris¹⁸18 Creager M, Paris PC. Elastic field equations for blunt cracks with reference to stress corrosion cracking. Int J Fract Mech. 1967;3:247-52..

Thumbnail

Figure 2
Graphical representation of the working principles of (a) PM and (b) LM.

Thumbnail

Figure 3
Linear–elastic stress versus distance curve for notches with different features.

Thumbnail

Figure 4
Geometry of specimens tested specimens (dimensions in millimeters).

Thumbnail

Figure 5
Microstructure of AISI 420 etched with 10wt% oxalic acid.

Thumbnail

Figure 6
Microstructure of AISI 420 etched with Vilella’s reagent.

Thumbnail

Figure 7
Engineering stress–strain curves of the tested specimens.

Thumbnail

Figure 8
Microcracks in the region around the notch tip of the specimen with the thickness of 7 mm and notch tip radius of 1 mm: (a) notch tip apex, 216×; (b) notch tip sideline, 430×; (c) sketch indicating the observed regions.

Thumbnail

Figure 9
Fracture surface: (a) thickness of 14 mm and radius of 1 mm, (b) thickness of 14 mm and radius of 0.17 mm, (c) thickness of 7 mm and radius of 1 mm, and (d) thickness of 7 mm and radius of 0.17 mm.

Thumbnail

Figure 10
Fracture surface observed using a scanning electron microscope with a low magnification (50×).

Thumbnail

Figure 11
Scanning electron microscopy images of the fracture surface near (a) and far (b) from the notch tip in two different specimens (magnification of 3000×).

Thumbnail

Figure 12
Stress field analysis according to PM: (a) thickness of 7 mm and (b) thickness of 14 mm.

Thumbnail

Figure 13
Stress field analysis according to LM: (a) thickness of 7 mm and (b) thickness of 14 mm.

Thumbnail

Table 1
Chemical composition of the purchased AISI 420 (in wt%).

Thumbnail

Table 2
General dimensions of the tested specimens.

Thumbnail

Table 3
Parameters for stress intensity factor—V-notched.

Thumbnail

Table 4
Mechanical properties of the tested AISI 420.

Thumbnail

Table 5
Average load-bearing capacity of the tested specimens.

Thumbnail

Table 6
Theoretical predictions of K_c^theor. (MPa√m) for the tested specimens under bending loading.

Thumbnail

Table 7
Experimental values of K_c^exp (MPa√m) for U-notched specimens.

Thumbnail

Table 8
Percentage error for U-notched stress intensity factor prediction.

Thumbnail

Table 9
Assessing V-notched specimen failure.

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Fe	C	Cr	Mn	P	S	Si	Ni	Mo
85.7	0.367	12.790	0.459	0.013	<0.001	0.349	0.126	0.013
V	Nb	Cu	Al	Co	Pb	Sn	W
0.067	0.027	0.020	0.019	0.014	<0.03	<0.01	<0.01

U-NOTCHED PLATE
Thickness t (mm)	Notch Tip Radius ρ (mm)	Notch Depth d (mm)	Number of Tested Specimens
7	0.17	5	5
7	1		5
14	0.17		4
14	1		5
V-NOTCHED PLATE (60°)
Thickness t (mm)	Notch Tip Radius ρ (mm)	Notch Depth d (mm)	Number of Tested Specimens
7	0.17	5	5
14	0.17	5	5
V-NOTCHED ROUNDED BAR
Angle (2α)	Notch Tip Radius ρ (mm)	Ligament b (mm)	Number of Tested Specimens
60°	0.39	3.08	4
90°	0.59	3.68	3

V-NOTCHED PLATE (60°)
Angle (2α)	Tip Radius ρ (mm)	q	r₀ (mm)	λ ₁	μ ₁	χ _b1	χ _c1	χ _d1	ω ₁	η_θθ(0)
60°	0.17	1,667	0.068	0.512	−0.406	1.312	3.283	0.096	0.970	0.970
V-NOTCHED ROUNDED BAR
Angle (2α)	Tip Radius ρ (mm)	q	r₀ (mm)	λ ₁	μ ₁	χ _b1	χ _c1	χ _d1	ω ₁	η_θθ(0)
60°	0.39	1.667	0.156	0.512	−0.406	1.312	3.283	0.096	0.970	0.969
90°	0.59	1.500	0.196	0.545	−0.345	1.841	2.506	0.105	0.810	0.810

σ _y	Yield strength	1159	MPa
E	Young’s modulus	197228	MPa
σ _u,t	Ultimate tensile strength, true value	1796	MPa
ε _u,t	True strain at rupture	0.0386	mm/mm
AR%	Area reduction	5.60%
n	Strain hardening exponent	0.160
K	Strain hardening coefficient	3156.6	MPa
K _Ic	Plane strain fracture toughness	35.4	MPa√m
	Rockwell C Hardness	51	HRC

U-NOTCHED PLATE
Thickness (mm)	ρ = 1 mm	COV	ρ = 0.17 mm	COV
7	23221 N	2.0%	14819 N	7.6%
14	46921 N	5.0%	29604 N	2.7%
V-NOTCHED PLATE (60°)
Thickness (mm)	ρ = 0.17 mm			COV
7	15547 N			12.6%
14	32642 N			7.6%
V-NOTCHED ROUNDED BAR
Angle (2α)	ρ			COV
60°	0.39 mm	20559 N		8.6%
90°	0.59 mm	26998 N		5.6%

ACCORDING TO THE EQUATION 2
	METHOD
Tip radius	PM	LM
1 mm	56.5	61.5
0.17 mm	34.5	41.0
ACCORDING TO THE GRAPHICAL ANALYSIS
	METHOD
Thickness \| Tip radius	PM	LM
t = 7 mm \| ρ = 1 mm	185.2	184.8
t = 7 mm \| ρ = 0.17 mm	112.0	112.5
t = 14 mm \| ρ = 1 mm	187.4	186.8
t = 14 mm \| ρ = 0.17 mm	112.1	112.8

	Specimen Thickness
Tip radius	7 mm	14 mm
ρ = 1 mm	186.2	188.1
ρ = 0.17 mm	112.7	112.7

ACCORDING TO THE EQUATION 2
	ERROR
Thickness \| Tip radius	PM	LM
t = 7 mm \| ρ = 1 mm	229.8%	202.7%
t = 7 mm \| ρ = 0.17 mm	226.4%	174.8%
t = 14 mm \| ρ = 1 mm	233.2%	205.8%
t = 14 mm \| ρ = 0.17 mm	226.5%	174.9%
ACCORDING TO THE GRAPHICAL ANALYSIS
	ERROR
Thickness \| Tip radius	PM	LM
t = 7 mm \| ρ = 1 mm	0.54%	0.77%
t = 7 mm \| ρ = 0.17 mm	0.63%	0.17%
t = 14 mm \| ρ = 1 mm	0.34%	0.72%
t = 14 mm \| ρ = 0.17 mm	0.58%	-0.04%

V-NOTCHED PLATE (2α = 60°; ρ = 0.17 mm)
Thickness (mm)		Method	K_c^theor. MPa.m^(1-λ1)	K_c^exp MPa.m^(1-λ1)	Error %
7		LM	122.3	122.6	0.21
7		PM	118.5	122.6	3.44
14		LM	122.4	129.8	6.07
14		PM	118.6	129.8	9.51
V-NOTCHED ROUNDED BAR
Angle (2α)	Tip radius ρ mm	Method	K _c ^theor.	K _c ^exp	Error
Angle (2α)	Tip radius ρ mm	Method	MPa.m^(1-λ1)	MPa.m^(1-λ1)	%
60°	0.39	LM	144.2	116.8	-19.02
60°	0.39	PM	138.0	116.8	-15.37
90°	0.59	LM	215.4	157.7	-26.77
90°	0.59	PM	207.1	157.7	-23.83

[1] *e-mail: marcelo.osiqueira@pq.uenf.br