Fracture Loci in Sheet Metal Forming: A Review

doi:10.1007/s40195-015-0341-6

Abstract

Abstract:

Fracture in sheet metal forming usually occurs as ductile fracture, rarely as brittle fracture, at the operating temperatures and rates of loading that are typical of real processes in two different modes: (1) tensile and (2) in-plane shear (respectively, the same as modes I and II of fracture mechanics). The circumstances under which each mode will occur are ide.jpgied in terms of plastic flow and ductile damage by means of an analytical approach to characterize fracture loci under plane stress conditions that takes anisotropy into consideration. Fracture loci was characterized by means of the fracture forming limit line and by the shear fracture forming limit line in the fracture forming limit diagram. Experiments were performed with single point incremental forming and double-notched test specimens loaded in tension, torsion and in-plane shear give support to the presentation and allow determining the fracture loci of AA1050-H111 aluminium sheets with 1 mm thickness. The relation between fracture toughness and the fracture forming limits was also investigated by comparing experimental values of the strains at fracture obtained from a truncated conical part produced by single point incremental forming and from double-notched test specimens loaded in tension.

Key words: Sheet forming, Fracture, Fracture forming limit diagram

M. Beatriz Silva, Kerim Isik, A. Erman Tekkaya, Paulo A. F. Martins. Fracture Loci in Sheet Metal Forming: A Review[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(12): 1415-1425.

Figures/Tables 10

Fig.1 Formability limits of sheet metal forming in the principal strain space: a Marciniaks vision [1]; b schematic representation of the forming limit curve (FLC) and of the fracture forming limit line (FFL)

Fig.2 Schematic representation of the fracture forming limit line (FFL) a and in-plane shear fracture forming limit line (SFFL) b in the principal strain space

Table 1 Summary of the mechanical properties of the AA1050-H111 aluminium sheets

Orientation	Modulus of elasticity (GPa)	Yield strength (MPa)	Ultimate tensile strength (MPa)	Elongation at break (%)	Anisotropy coefficient
0ºRD	72.7	115.4	119.0	7.1	0.71
45ºRD	67.9	120.4	121.2	5.2	0.88
90ºRD	71.8	123.0	120.8	5.6	0.87
Average	70.0	119.9	120.5	6.8	0.84

Fig.3 Method and procedure used for determining fracture toughness $R$: a schematic representation of a double-notched test specimen loaded in tension; b schematic evolution of the tensile force with displacement for test specimens with different lengths $c$ of the ligaments; c determining fracture toughness $R$ from extrapolation of the total energy per unit of area $w$

Fig.4 Formability limits by necking and fracture: a schematic procedure to determine the in-plane strains at the onset of necking; b schematic procedure to determine the gauge length strains at the onset of fracture; c the FLC of the AA1050-H111 aluminium sheets with 1 mm thickness

Table 2 Experimental test specimens utilized in the characterization of the formability limits by fracture


$w$	$c$	$d$	$w$	$c$	$d$	$r$	$r_{i}$	$c$	$d$	$r$	$r_{\text{tool}}$	$\psi_{0}$	$w$	$r_{\text{tool}}$	$\psi_{0}$
50	5C25	3	20	4	1	40	21	1.5C19	1	165	4C25	30	170	4	30

Fig.5 Fracture toughness $R$ in AA1050-H111 aluminium sheets with 1 mm thickness obtained from double-edge-notched test specimens loaded in tension: a experimental evolution of the tensile force with displacement for test specimens with different ligaments $c$ that were cut out from the supplied sheets at 0º with respect to the rolling direction; b average value of fracture toughness $R$ obtained from test specimens with different ligaments $c$ that were cut out from the supplied sheets at 0º and 90º with respect to the rolling direction

Fig.6 Determining fracture toughness directly from SPIF tests: a circumferential crack with notation and detail showing the hatched region corresponding to a thin boundary layer alongside the crack; b truncated conical part fabricated by SPIF with a detail of a circumferential crack

Fig.7 Experimental strains obtained from measurements in truncated conical SPIF parts and double-notched test specimens loaded in tension. The grey solid markers refer to the strain pairs at the onset of necking, the black solid markers refer to the strain pairs at the onset of fracture, and the elliptical dashed grey curves refer to the iso-effective strain contours

Fig.8 Experimental fracture strain pairs obtained from the tests listed in Table 2 that were utilized to determine the fracture loci of the AA1050-H111 aluminium sheets with 1 mm thickness

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