Multi-Axial Fatigue Research

1. DEACON
E N G I N E E R S
Mul!-axial Fa!gue Design Methodology – R&D Project
Many structural and mechanical pieces of equipment are subjected to repeated loadings. These
repeated
loadings
may
lead
to
unpredicted failure since the resul!ng stresses may
be below the material's strength point. Such failure
is described as fa!gue failure.
Several interna!onal standards and codes
provide guidelines on stress life predic!ons due to
fa!gue loading. These have been well accepted by
major industries. These standards and codes however have been based primarily on uniaxial test
data which is predominant in most structures undergoing fa!gue type loading.
While these prac!ces may be adequate for structures,
mechanical
equipment
is
o"en
subjected to biaxial or mul!-axial stress
spectrums and this calls into ques!on the
applicability of these prac!ces.
Many
mechanical equipment designs fail prematurely due
to fa!gue even though they conform to relevant
design standards.
Various approaches for dealing with biaxial and
mul!-axial stresses have been studied and
laboratory tested in recent !mes. The cri!cal plane
approach is one method which accounts for
mul!-axial stresses. It seeks to resolve stresses
onto a series of planes near about a poten!al crack
plane in order to find that crack ini!a!on plane or
cri!cal plane. Deacon Engineers believe that if
these techniques are applied to the design phase of
mechanical equipment, then design reliability will
increase.
The objec!ve of Deacon Engineers R&D project is
to develop a code of prac!ce / process that will
enable an engineer to safely and accurately design
equipment subjected to mul!-axial loadings. The
code of prac!ce / process will extend to cover
equipment incorpora!ng welded joints.
By
colla!ng various laboratory studies and their
results, we believe that through several
amendments we can adapt these codes and
standards to address these loadings. The process
developed will then be replicated under laboratory
condi!ons to test the performance against some
real life scenarios.
DEACON
E N G I N E R S
In addi!on to normal stresses in mechanical structures the presence of shear stresses area are also
o"en present. Some!mes the magnitude of these
shear stresses is significant and thus should not be
ignored. Uniaxial methods of signal coun!ng
ignore the effect of these stresses. A pathdependent cycle coun!ng technique has been proposed and accounts for the resul!ng shear stresses
on a crack ini!a!on plane.
Deacon Engineers have developed a test facility to
generate mul!-axial fa!gue test data in order to
validate new approaches to mul!-axial fa!gue
analysis.
The photograph below shows the test facility with
test sample that will be loaded and run to destruc!on.
deaconengineers.com.au