Offshore steel jacket with wave load from the left,
leading to buckling of an inclined brace. The graph
shows the development of the bending moment
as function of the top displacement.
Radial foil bearing (1) built by compliant bump foil
structure (2) and compliant top foil sensed and
controlled by piezoelectric patches.
(a) X-ray tomography scan of a crack showing
localization of deformation. (b) The cross-section
shows extensive crack tunnelling. (c) Indentation
will be used to investigate material hardening in
the fracture process zone.WW
Cyclic plasticity - material model and implementation of offshore structures
Offshore oil and gas exploitation in the North
Sea is challenged by a changing wind and wave
climate and settlement of the sea floor at
older fields. This calls for improved models that
can predict structural behavior up to eventual
failure. The present project on cyclic plasticity
models for materials and structures is a
collaboration between MEK DTU, Mærsk Oil, and
Rambøll Oil and Gas. It consists of three phases:
i) development of a general yet simple and
robust model for cyclic plasticity at the material
level, ii) a generalization of this model to the
level of a structural element for analysis of
steel jacket structures, and iii) implementation
of the structural model in the offshore analysis
program RONJA and demonstration of its applicability
on some actual offshore structures. The
project is now in its final phase.
A cyclic plasticity model on the stress-strain
level has been developed based on energy
in terms of internal and external variables,
combined with a yield surface and a plastic flow
potential. The model includes the possibility of
parameter evolution such as changing stiffness
and yield limit by cyclic straining. It provides excellent
representation of available experimental
results of cyclic plasticity even including the
phenomenon of ratcheting, where a constant
load component can increase the deformation
caused by another cycling component - e.g.
cyclic torsion of a bar exposed to a constant
tension load.
The material model has been generalized to
plastic yield hinges in beam elements, where a
general format in terms of section forces and
moments has been developed for the yield
surface and flow potential. In the current final
phase of the project the model is implemented
in the RONJA program, as illustrated in the figure.
The graph refers to a similar but somewhat
simplified structure and includes computed as
well as experimental results.
Smart controllable fluid film bearings for Industry 4.0
Industry 4.0 can be defined as the overlay of
several technological developments involving
products as well as processes. It is related to
the so-called Cyber-physical systems which
describe the merger of digital with physical
workflows. Cyber-physical systems combine
mechanics, electronics, computation, and capacity
of data storage and use the Internet as a
communication medium. Industry 4.0 embraces
a set of technologies enabling smart products.
Smart products are characterized by the
capability of performing computations, storing
data, communicating and interacting with their
environment. Piezoelectric Materials (PMs)
and Shape Memory Alloys (SMAs) are typical
representatives of smart materials (SMs), which
have the capability of changing form, similar
to human muscles (actuator), dependent on
the electric voltage and induced temperature
changes (signal processing and control inputs).
SMs though, are not typically used in connection
to bearing technology and the goal of this
project in the framework of Industry 4.0 is to
investigate the feasibility of integrating PMs
and SMAs into air lubricated bearings with the
aim of overcoming their drawbacks aided by
sensing, control, and software. The use of smart
controllable fluid film bearings – with higher
load capacity, lower energy losses, and capable
of running faster (increase in production) without
vibration instabilities – should lead to more
efficient machines when compared to their
passive conventional counterparts. Moreover,
the storage of big measured data obtained from
sensing systems composed of PMs installed
into the smart bearings can be analyzed with
the help of dedicated computational tools to
assist predictive maintenance of machines and
increase their reliability.
Advanced damage models with intrinsic size effects
Improved performance and expanded utilization
of ductile metals, without engaging in costly
development and test phases, are the goals of
this research proposal. Development of new
materials, or the application of existing materials
in new technological areas, is often time
consuming, but the process can be reduced by
improved modelling techniques that may qualify
new materials or validate existing materials
fit for new applications. This research project
aims to redefine and extend the modelling basis
for ductile fracture in metals, from the basic
concepts to practical applications. The focus is
on the fact that today’s well known material
size effects at the micron scale are neglected
from the state-of-the-art modelling techniques
of ductile failure, even though the underlying
mechanisms unfold in the micrometer range,
thus hindering quantitative modelling capabilities.
In essence, the intrinsic length scales set
by the void size and spacing are averaged out
in today’s models by adopting the void volume
fraction to describe damage evolution.
To bring the length scales back in to play this
research project will:
1) Develop high-fidelity micro-mechanics based
models for porous materials where void size
and spacing are new input variables that enable
proper modelling of intrinsic size effects.
2) Perform experimental validation and
benchmarking of new material models through
experiments that also serve to identify material
parameters relevant for structural level applications.
3) Employ the new micro-mechanics based
models to develop models for ductile fracture
and quantitative cohesive zone modelling that
may be utilized in future large-scale structural
models.
The aim is to bring quantitative accuracy into
typical modelling techniques for porous metals,
but without costly and cumbersome changes to
the existing numerical tools utilized in industry.
Contact:
Steen Krenk, e-mail: sk@mek.dtu.dk,
Lasse Tidemann, e-mail: lastid@mek.dtu.dk
Contact:
Ilmar Santos, e-mail:ifs@mek.dtu.dk
Contact:
Christian Niordson, e-mail: cn@mek.dtu.dk
Kim Lau Nielsen, e-mail: kin@mek.dtu.dk
40 Solid Mechanics