TESTING AND SIMULATION OF ULTRALOW CYCLE FATIGUE AND FRACTURE IN STEEL BRACES

1
TESTING AND SIMULATION OF ULTRA-LOW CYCLE FATIGUE
AND FRACTURE IN STEEL BRACES

• Investigators
• Amit M. Kanvinde University of California at
  Davis (Principal Investigator)
• Gregory G. Deierlein Stanford University
  (Co-Principal Investigator)
• Benjamin V. Fell University of California at
  Davis (Graduate Student)
• Xiangyang Fu University of California at Davis
  (Graduate Student)
• Andrew T. Myers Stanford University (Graduate
  Student)

NEES Facilities Used University of California at
Berkeley (UCB)
Modeling Global and Local Buckling An important
issue that merits attention is the amplification
of the plastic strains due to buckling. The
global longitudinal strain is amplified due to
the bending and buckling of the entire brace.
This bending strain is further amplified by the
local buckling of the cross section.
Sophisticated continuum analyses are required to
capture the second level of amplification due to
local buckling (commercially available ABAQUS is
used). The figure below compares deformed
shapes from finite element analyses to those
observed during the experiment (Test 1 is shown
here). By comparing the analysis results to the
experimental findings, the figure illustrates the
ability of the FEM to simulate local buckling
during a compressive excursion, and shows that
the critical location of fracture can be
accurately determined through the stress and
strain gradients.
One important advantage of these models is that
they can be calibrated with relatively
inexpensive tension coupon type tests. This
calibration was done at Stanford University with
monotonic and cyclic testing on coupons from the
brace specimens.
HSS4x4x1/4
SCBF
Background The aim of this project is to
investigate Ultra-Low Cycle Fatigue (ULCF) in
large-scale steel bracing members. The tested
members represent braces in Special
Concentrically Braced Frame (SCBF) systems that
undergo severe cyclic inelastic deformations
accompanied by global and local buckling
eventually leading to ULCF-induced fracture. The
experimental findings are complemented by
detailed continuum-based FEM and
micromechanics-based models that capture the
fundamental processes of void growth, collapse,
and damage responsible for ULCF. Special
Concentrically Braced Frames (SCBFs) are one of
the more popular lateral load resisting
structural systems for steel buildings in
seismically active regions. Widespread fracture
observed in moment resisting frames (MRFs) during
Northridge, in addition to their flexibility and
stability concerns have further promoted SCBF
systems. However, SCBF systems are also
vulnerable to fracture, most notably at
connections and at brace plastic hinge locations
(above Figure). Despite these issues, research
regarding SCBFs is relatively less exhaustive
when compared to moment frame systems. Because
SCBFs dissipate energy through cyclic inelastic
buckling of bracing elements, their resistance to
fracture may ultimately govern system ductility.
In fact, recent experimental studies (Mahin et
al, 2004, Roeder, 2005) suggest that bracing
systems designed as per current code (AISC, 2005)
may fracture prematurely. Models Traditional
fracture and fatigue mechanics approaches such as
the J-integral, CTOD or the ?J cannot capture
fracture under the conditions that SCBF systems
present 1.) Large Scale Yielding 2.)
Ultra Low Cycle Fatigue 3.) Flaw Free
Geometric Details Consequently, continuum-based
models that capture the fundamental physics of
the fracture/ULCF phenomena are required to
capture the complex stress-strain interactions
leading to fracture. These models simulate the
micromechanical processes leading to fracture and
fatigue to predict fracture from a fundamental
physics-based perspective. They are fairly
general, and can be applied to a wide variety of
situations as they work at the continuum level,
and are relatively free from assumptions
regarding geometry and other factors.
Small-scale calibration at Stanford University
using monotonic and cyclic notched bar tests, FEM
and Scanning Electron Micrographs
Brace Tests The test specimens represented
several types of SCBF braces and were subjected
to reversed-cyclic loading histories to
characterize their performance. The cross
sections investigated in this study included two
square HSS (HSS4x4x1/4 and HSS4x4x3/8), two
PipeSTD (Pipe3STD and Pipe5STD), and one
wide-flange (W12x16). All experiments with
cyclic loading qualitatively follow a similar
sequence of events leading up to failure of the
brace. The initial elastic cycles do not induce
any visually observable deformation in the brace.
After the brace buckles, paint begins to flake
due to large strains at the middle plastic hinge
as well as at the gusset plates. As the amplitude
of loading increases, large scale yielding is
observed to occur at the gusset plate as well as
the middle hinge (shown below). Subsequently, a
local buckle begins to form at the middle hinge,
and soon thereafter, metal rupture is observed at
this location. Design Implications Fracture was
found to be governed by a combination of
slenderness and compactness. For example, the
wide-flange section with an undesirable
width-thickness ratio (three times the AISC
limit) exhibited excellent ductility, likely
because of its high slenderness. However, large
slenderness can reduce energy dissipation in the
brace, and place excessive tensile demands on the
connections (due to overstrength). Since brace
slenderness is a system level design variable, it
might not be feasible to provide large
slenderness with a sole view to prevent fracture.
On the other hand, the beneficial effects of
large slenderness may be leveraged to adjust
limits on compactness for highly slender braces,
recognizing that fracture is in fact governed by
a combination of the two factors.
Fracture Prediction The stress and strain data
from the critical node at the locally buckled
cross-section from the ABAQUS analysis is used to
predict the location of ductile crack initiation
loading (shown as a circle on the hysteretic
response). Comparing this point to the
experimental fracture instant (shown as a star on
the hysteretic figure) demonstrates the accuracy
of the ULCF models.
Models capture void growth and coalescence
leading up to fracture
Stress Modified Critical Strain (SMCS)
Ductile Fracture Hancock and Mackenzie (1976)
Acknowledgements National Science Foundation,
NEES _at_ Berkeley, Structural Steel Educational
Council, John A. Blume Earthquake Engineering
Center, Helmut Krawinkler (Stanford), Charles
Roeder (Washington), Steve Mahin (UC Berkeley),
Walterio Lopez and Mark Saunders (Rutherford and
Chekene).
Ultra-Low Cycle Fatigue (ULCF) Kanvinde and
Deierlein (2004)
Comparison of the simulated and experimental
buckled HSS4x4x1/4 brace