Kazim-PhD Thesis.pdf (4.07 MB)
Investigation of the effect of relative humidity on additive manufactured polymers by depth-sensing indentation
Version 2 2019-12-13, 15:13
Version 1 2011-07-05, 08:41
thesis
posted on 2019-12-13, 15:13 authored by Kazim AltafAdditive manufacturing methods have been developed from rapid prototyping
techniques and are now being considered as alternatives to conventional techniques
of manufacturing. Stereolithography is one of the main additive methods and is
considered highly accurate and consistent. Polymers are used as stereolithography
materials and exhibit features such as high strength-to-weight ratio, corrosion
resistance, ease of manufacturing and good thermal and electrical resistance
properties. However, they are sensitive to environmental factors such as temperature,
moisture and UV light, with moisture being identified as one of the most important
factors that affect their properties. Moisture generally has an adverse effect on the
mechanical properties of polymers. Investigation of the effects of moisture on
polymers can be carried out using a number of experimental techniques; however,
the benefits of the depth sensing indentation method over bulk tests include its ability
to characterise various mechanical properties in a single test from only a small
volume of material and the investigation of spatial variation in mechanical properties
near the surface.
The aim of this research was to investigate the effects of varying relative humidity on
the indentation behaviour of stereolithography polymers and to develop a modelling
methodology that can predict this behaviour under various humidities. It was
achieved by a combination of experimental and numerical methods. Depth sensing
indentation experiments were carried out at 33.5 %, 53.8 %, 75.3 % and 84.5 % RH
(relative humidity) and 22.5 °C temperature to investigate the effects of varying
humidity on the micron scale properties of the stereolithography resin, Accura 60. In
order to minimise the effects of creep on the calculated properties, appropriate
loading and unloading rates with suitable dwell period were selected and indentation
data was analysed using the Oliver and Pharr method (1992). A humidity control unit
fitted to the machine was used to condition the samples and regulate humidity during
testing. Samples were also preconditioned at 33.5 %, 53.8 %, 75.3 % and 84.5 % RH
using saturated salt solutions and were tested at 33.5 % RH using humidity control
unit. It was seen that properties such as indentation depth increased and contact
iv
hardness and contact modulus decreased with increasing RH. The samples
conditioned and tested using the humidity control unit at high RH showed a greater
effect of moisture than the preconditioned samples tested at 33.5 % RH. This was
because the samples preconditioned at high RH exhibited surface desorption of
moisture when tested at ambient RH, resulting in some recovery of the mechanical
properties. In order to investigate these further, tests were performed periodically on
saturated samples after drying. Ten days drying of samples conditioned for five days
at 84.5 % RH provided significant, though not complete, recovery in the mechanical
properties. These tests confirmed that Accura 60 is highly hygroscopic and its
mechanical properties are a function of RH and removal of moisture leads to a
significant recovery of the original mechanical properties.
History
School
- Mechanical, Electrical and Manufacturing Engineering
Publisher
Loughborough UniversityRights holder
© Kazim AltafPublication date
2011Notes
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.EThOS Persistent ID
uk.bl.ethos.546325Language
- en
Supervisor(s)
I.A. Ashcroft ; R. HagueQualification name
- PhD
Qualification level
- Doctoral