Testing of real area of contact in tool-workpiece
interface under normal pressure q and longitudinal
elongation el.
Polymer samples (right) produced by injection
moulding using inserts (left) with gratings
1000 nm. They showed different colour when
the cameral was at different angle or lightning
condition.
Numerical simulation illustrating exothermic heat
generation in the laminate during pultrusion of
CF-Epoxy spare caps for wind turbine blades.
Modelling of real contact area in relation to friction in metal forming
One of the main results of a recent research
project, COMESURF, is the advancement in
modelling of the real area of contact in the
tool-workpiece interface in metal forming.
Friction modelling in metal forming has long
been based on the real area of contact, and the
state-of-the art modelling was developed in the
1970s at DTU MEK and known as the Wanheim-
Bay model. Deficiencies of the model were,
however, that strain hardening could only be
taken into account in an average sense and that
deformation of the underlying material was not
taken into account. These two deficiencies have
now been solved by numerical simulation and
corresponding, verifying experiments. The figure
shows one of the experimental setups for
flattening of model asperities by a normal pressure,
while being subjected to longitudinal, bulk
elongation under tension. Other experiments
were made with elongation under longitudinal
compression.
Experiments and computer simulations show
that the real area of contact under a given
normal load increases rapidly with longitudinal
strain due to the reduction of the necessary
yield pressure. The increased real contact area
leads to a corresponding increase of friction as
proportionality between friction and the real
area of contact has previously been proven.
While the pressures in bulk metal forming are
typically so high that full or close to full contact
is reached rather quickly, sheet metal forming
has a large range of growing contact area due
to in-plane elongation of the sheet materials.
Hence, the new modelling is especially important
for sheet metal forming, where implementation
of the new model into existing finite
element codes will potentially lead to improved
simulation of industrial sheet metal forming
processes. Advances of sheet metal forming are
possible with better prediction of process limits
and optimization of tool geometries for complex
components.
A new PhD project in collaboration with Chinese University of Hong Kong
A new PhD project titled “Integrating micro and
nano structures on steel surfaces – Process
chain implementation and validation” started in
September 2017 within the framework of MADE
Digital. The PhD project aims at developing and
implementing a complete process chain for the
establishment of micro-nano structures on the
surface of steel moulds. Associate Prof. Guido
Tosello is the main supervior; Senior researcher
Yang Zhang and researcher Matteo Calaon
participate as co-supervisors in the project. Part
of this PhD project is to investigate the replication
quality of micro and nano structures from
the metal insert to polymer replica by injection
moulding. Initial tests showed that the injection
moulded polymer samples reflecting different
colours when viewed at a certain angle (Figure).
The feasibility to transfer these submicronscale
periodic structures from the master mold
to plastics will be investigated and studied. The
proposed research attempts to develop a structural
colouration technique for plastic products
without using any chemical or additives. As a
potential option for the metal structuring Prof.
Ping Guo from the Chinese University of Hong
Kong will be the contact person for testing the
manufacturing of the micro gratings on inserts.
DTU tasks comprise:
• Development of tooling process chains for
the establishment of micro/nano structured
surfaces on mould steel for enhanced tool
performance (lifetime and replication capability)
• Development of the precision moulding
technology for the manufacture of precision
plastic components with micro/nano structured
surfaces for enhanced part performance (optical
and functional properties)
• Development of traceable methods allowing
for metrology and tolerance verification across
several length scales on both tools and moulded
components.
Modelling the multi-physics in industrial resin injection pultrusion (RIP)
The RIP process is a continuous manufacturing
process resulting in high productivity and
very little scrap material. Another advantage of
the RIP process is the high strength to weight
ratio generally associated with fibre-reinforced
polymers (FRP). The high strength to weight
ratio of FRPs are the main reason for using
pultruded profiles as spar caps in wind turbine
blades (cf. figure).
The project involves industrial collaboration
with Fiberline Composites A/S as well as
international academic collaboration with the
University of Twente and University of Warwick.
The objective of the project is to make a
digital twin of pultrusion capable of capturing
the multi-physics involved in the process, i.e.
fluid dynamics, thermodynamics, heat transfer
as well as stress analysis. One of the project
goals is to increase productivity of thick section
profiles while still obtaining a high degree of
cure of the thermoset resin and avoiding an
increasing amount of defects, hence, meeting
tolerances and ensuring good mechanical
properties.
This objective of using numerical simulations
for process design and process optimization is
highly in-line with the thoughts of industry 4.0.
Contact:
Chris Valentin Nielsen, e-mail: cvni@mek.dtu.dk
Contact:
Yang Zhang, e-mail: yazh@mek.dtu.dk
Contact:
Filip S. Rasmussen, e-mail: fsras@mek.dtu.dk,
Jesper Hattel, e-mail: jhat@mek.dtu.dk
34 Manufacturing Engineering