Abstract :
Cracks in thin structures often are subjected
to combined in-plane and out-of-plane loading conditions
leading to complex mixed mode conditions in
the crack tip region. When applied to ductile materials,
large out-of-plane displacements make both experimentation
and modeling difficult. In this work, the
mixed-mode behavior of thin, ductile materials containing
cracks undergoing combined in-plane tension
(mode I) and out-of-plane shear (mode III) deformation
is investigated experimentally. Mixed-mode fracture
experiments are performed and full, threedimensional
(3D) surface deformations of thin-sheet
specimens from aluminum alloy and steel are acquired
using 3D digital image correlation. General characteristics
of the fracture process are described and quantitative
results are presented, including (a) the fracture
surface, (b) crack path, (c) load-displacement response,
(d) 3D full-field surface displacement and strain fieldsprior to crack growth, (e) radial and angular distributions
of the crack-tip strain fields prior to crack growth
and (f) singularity analysis of the crack-tip strains prior
to crack growth. Results indicate that the introduction
of a mode III component to the loading process (a)
alters the crack tip fields relative to those measured
during nominally mode I loading and (b) significantly
increases the initial and stable critical crack-openingdisplacement.
The data on strain fields in bothAL6061-
T6 aluminum and GM6208 steel consistently showthat
for a given strain component, the normalized angular
and radial strains at all load levels can be reasonably
represented by a single functional form over the range
of loading considered, confirming that the strain fields
in highly ductile, thin-sheet material undergoing combined
in-plane tension and out-of-plane shear loading
can be expressed in terms of separable angular and radial
functions. For bothmaterials, the displacement and
strain fields are (a) similar for both mixed-mode loading
angles = 30◦ and = 60◦ and (b) different from
the fields measured for Mode I loading angle = 0◦.
Relative to the radial distribution, results indicate that
the in-plane strain components do not uniformly exhibit
the singularity trends implicit in the HRR theory.
