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Experimental and numerical study on eect of forming
rate on AA5086 sheet formability
Cunsheng Zhang, Lionel Leotoing, Dominique Guines, Eric Ragneau
To cite this version:
Cunsheng Zhang, Lionel Leotoing, Dominique Guines, Eric Ragneau. Experimental and numerical
study on eect of forming rate on AA5086 sheet formability. Materials Science and Engineering: A,
Elsevier, 2010, 527, pp.967-972. �10.1016/j.msea.2009.09.013�. �hal-00979414�
Page 1 of 26
Accepted Manuscript
Experimental and numerical study on effect of forming
rate on AA5086 sheet formability
Cunsheng ZHANG
1∗
, Lionel LEOTOING
2
, Dominique GUINES
2
, Eric RAGNEAU
2
1
Shandong University, Key Laboratory of Liquid Structure and Heredity of
Materials (Ministry of Education)
73 Jingshi Road, Jinan, Shandong Province, P.R.CHINA, 250061
Email: zhangcs@sdu.edu.cn
Tel : +86(0)53181696577 Fax : +86(0)53188395811
2
Université Européenne de Bretagne, France, INSA-LGCGM - EA 3913
20, avenue des Buttes de Coësmes 35043 RENNES Cédex
Tel : +33(0)2 23 23 86 64 Fax : +33(0)2 23 23 87 26
Abstract
With increasing a pplication of aluminum alloys in automotive or aeronau-
tic industries, it is necessary to characterize their deformation behaviors at
large strains, high strain rates a nd elevated temp erat ures, which is r elatively
lacking today. The aim of this paper is to experimentally and numerically
investigate the influence of fo r ming rate and temperature on formability of
an AA508 6 sheet. F ir stly, tensile tests are carried out at different temper-
atures (20, 230, 290 and 350°C) and at different forming rates (10, 750 a nd
1000 mm/s). A technique of digital image correlation (DIC) associated with
a high-speed camera is applied to evaluate the surface strains and a com-
plete procedure is built to detect the onset of localized necking during the
experiments. The influences of initial t esting temperature and forming rate
on the sheet formability are analyzed. Then in order to numerically deter-
mine the formability of this sheet, a form of Voce’s constitutive law taking
Preprint submitted to Materials Science and Engineering: A September 4, 2009
Manuscript
Page 2 of 26
Accepted Manuscript
into account the temperature and strain rate is proposed. An inverse anal-
ysis is carried out to identify the material parameters of the law for the
tested aluminum alloy. F inally, with the above identified law, tensile tests
are simulated. The experimental and numerical results show that the testing
temperature and forming rate have a great influence on sheet formability.
At high forming rates, the sheet formability of AA5086 is lowered up to a
certain temperature, above this temperature, the for mability is greatly en-
hanced. Furthermore, the agreement between experimental and numerical
results indicates that the proposed constitutive law and the identified mate-
rial parameters can be appropriate to model the sensitivity of AA5086 sheet
towards strain rate and temperature.
Key words: Forming Limit Diagrams (FLDs); Dynamic tensile test;
Digital Image Correlation (DIC); Inverse analysis
1. Introduction
Sheet metal forming is a very commonly used method for producing var-
ious components in automotive or aeronautic industries. Especially with the
innovative techniques, such as hydroforming and incremental forming, the
manufacture of complex parts with low cost can be realized. However, in
sheet metal forming operations, the sheet can be deformed only to a certain
limit. This limit is usually imposed by the onset of localized necking, which
may lead to early failure. The ability of sheet metal to deform into desired
shape without local necking or fracture is defined as its formability. Fo r mabil-
ity depends on many factors like material properties (e.g. strain hardening
coefficient, strain rate sensitivity, anisotropy ratio ) or process parameters
2
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Accepted Manuscript
(e.g. strain rate, temperature) [19]. Thus, understanding and characteriz-
ing the formability of metal sheets are crucial for controlling final product
quality and then the success of the sheet forming operation, especially with
the increasing use of aluminum alloys that exhibit low formability compared
with typical mild steels [13, 7].
One important technique to evaluate t he formability of sheet metals is the
use of forming limit diagrams (FLDs) developed by Keeler and Backofen in
the 1960 s [9]. For sheet metal forming, FLDs are realistic and efficient diag-
nostic tools for evaluating formability. In addition, due to the improvement
of production ra t es, strain rates in sheet forming processes can be located
in the range o f intermediate strain rates (10
−2
to 500s
−1
). Nevertheless, the
mechanical behavior of materials at intermediate and high strain rates can be
considerably different from that observed with quasi-static loading because
of the strain-rate sensitivity of the material and propagation of stress waves
[21]. Further, strain-rate sensitivity has been identified as an important fac-
tor determining formability of sheet metal and can alter substantially t he
level and the shape of FL Cs. Of particular interest here is the influence of
strain rate on the forming capacity of metals during the forming process. A
review of currently available literature shows that relatively little attention
has been paid t o models for FLDs taking strain rate into account.
Different analytical models have been developed that fo cus either on dif-
fuse or localized necking. These models not only help to understand the
necking phenomenon but they represent also useful tools to successfully and
rapidly predict the formability of sheet metals in industrial practice. The
strain rate sensitivity has been studied mainly through the classical M-K
3
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Accepted Manuscript
model. One o f the first studies taking into account this effect was proposed
by [8]. Considering a von Mises’ yield function, the effect of strain rate
sensitivity have been evaluated. Afterwards, anisotropy of sheet metals was
introduced in this classical model by using different yield function, like the
Hill’s criterion [1] or the Logan’s and Hosford’s criterion [6]. In these analyt-
ical models, the strain hardening behavior and the strain rate sensitivity of
materials are generally represented by simplistic power laws. For some ma-
terials, especially for a luminium alloys, this formulation is not well adapted
to model the strain rat e effects and then the onset of local necking [3]. More-
over, it has been shown that these simplistic laws only permit the study of
the influence of the strain rate sensitivity index but not the influence of the
value of the strain rate and temperature [23].
Experimentally, the Nakazima [14] and Marciniak tests [12] are widely
used to study sheet formability at quasi-static loa ding. Under dynamic con-
ditions, it is relatively difficult to adapt these conventional tests. Percy [17]
analyzed the influence of strain rate on FLDs by explosive forming and con-
cluded that FLDs level was dependent on the strain state and forming rat es.
Broomhead et al. [2] performed bulge forming over a range of strain rates
from 10
−3
to 70s
−1
and concluded that the position o f FL Ds for biaxial tensile
conditions was lowered with increasing strain rate. The effects of tempera-
ture and forming speed on FLCs of an Al-Mg alloy (5 083-O) sheet have been
investigated by Naka and Yoshida [15] for different speeds (0.2-200 mm/min)
and at temperatures of 20- 180 °C. Experimental results showed that the level
of FLC increases with increasing temperature and decreasing forming speed.
By deep drawing o f rectangular parts at a certain strain rate (1 s
−1
) con-
4