Application of the Theory of Critical Distances to As-Built Selective Laser Melted Ti-6Al-4V
Citation:
Gillham, Bobby, Application of the Theory of Critical Distances to As-Built Selective Laser Melted Ti-6Al-4V, Trinity College Dublin, School of Engineering, Mechanical & Manuf. Eng, 2024Download Item:
Bobby - Final Corrected PhD Thesis - February 2024.pdf (PDF) 28.10Mb
Abstract:
Metal additive manufacturing (AM) techniques such as selective laser melting (SLM) have become
an increasingly present feature of new and novel manufacturing methods in recent years. This is
owed to further technological advancements in a number of areas which have provided for
increased understanding and finer control of the process as a whole. Such progressions have
enabled the process’ capability to engineer components with complex shapes. Consequently, SLM
has acquired a variety of applications in various sectors, ranging from aerospace to biomedical
industries. Herein, metal alloys such as that of Ti-6Al-4V are employed, due to their
characteristically high strength-to-weight ratios along with other exceptional material properties.
Despite its growing acceptance however, many defects can arise throughout the SLM
manufacturing process. Coupled with the potential to impose greater design complexity within
AM parts, the extensive and sporadic nature of such sources of geometrical discontinuities can
lead to a greater degradation of mechanical properties in comparison to those of conventionally
manufactured materials. Furthermore, given the adoption of AM Ti-6Al-4V material in industries
whereby a lengthy and reliable service life is imperative, a strong knowledge of fatigue behaviour
is essential. The response of such material compositions to fatigue therefore needs to be
quantified in a way that is as timely and efficient as possible.
This work explores the potential to further verify and subsequently develop improved modified
fractures mechanics based techniques as a way to predict the fatigue performance of AM Ti64
components. The hypothesis states that the availability of improved forms of theoretical
assessment surrounding the dynamic capabilities of AM Ti64 are facilitated.
An investigation in to the available literature revealed an experimentally heavy approach to the
fatigue validation process of AM Ti64 material to date. This gave motivation towards evaluating
the possibility of encompassing a more theoretical approach to fatigue appraisal. However, a
review of traditional material failure theory identified certain gaps within the domain of a
universally applicable fatigue prediction approach being available. Developments in the area of
modified fracture mechanics (MFM) techniques served to breach these shortcomings somewhat.
The Theory of Critical Distances (TCD) was earmarked as being the most promising out of all MFM
methods to fulfilling these inadequacies. However, it was postulated that some key aspects still
required further attention. This work therefore represents attempts to attend to such needs.
The first practical task of this research was to verify the validity of applying TCD methods to
forecasting the fatigue strength of notched SLM Ti64 specimens. To achieve this, an experimental
study was formulated which involved evaluating various different sets of notched specimen
designs which contained gradually decreasing feature sizes. This included the scenario of a plain
specimen design with a rough SLM surface finish being considered as a micro-notch. In doing so,
the accuracy of TCD methods with regards to predicting the effect of process-inherent defects (i.e.
surface roughness) on resulting fatigue performance was evaluated. A review of results obtained
revealed that TCD methods could satisfactorily estimate the dynamic response of AM Ti64
material in the presence of a range of differing notch (Kt) feature sizes.
In an effort to develop TCD approaches beyond their current limitations, the ability of such
methods to perform in increasingly challenging environments was assessed. This involved the
analysis of specimen designs which incorporated progressively complex arrangements of stress-
rising elements within their composition. Novel ways of executing TCD methodology were
evaluated in an attempt to establish a superior universally predictive tool. Optimal ideologies were
identified and earmarked as being potential successors to original TCD methods going forward.
On the basis of the work that was accomplished, the hypothesis that the availability of improved
forms of theoretical assessment surrounding the dynamic capabilities of AM Ti64 could be
facilitated was accepted. Additional concluding remarks were made and suggestions for further
work which could potentially improve the limitations of current advancements were given.
Sponsor
Grant Number
SFI
Johnson & Johnson Services Inc.
European Regional Development Fund
Author's Homepage:
https://tcdlocalportal.tcd.ie/pls/EnterApex/f?p=800:71:0::::P71_USERNAME:GILLHAMBDescription:
APPROVED
Author: Gillham, Bobby
Advisor:
Lupoi, RoccoPublisher:
Trinity College Dublin. School of Engineering. Discipline of Mechanical & Manuf. EngType of material:
ThesisCollections:
Availability:
Full text availableKeywords:
fatigue, prediction, critical distance, TCD, Ti-6Al-4V, SLM, additive manufacturing, notches, defects, metal alloys, HCFLicences: