Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The fracture of ductile materials is currently regarded as a complex and challenging
phenomenon to characterise and predict. Recently, a bond-based, non-local theory was
formulated called the peridynamic theory, which is able to directly solve solid mechanics
problems that include fracture. The failure criterion is governed by a critical stretch
relation between bonds. It was found in literature that the critical stretch relates to the
popular fracture mechanics parameter called the critical energy release rate for predicting
brittle linear-elastic failure. It was also proposed that the non-linear critical energy
release rate or J-integral can be used to model ductile failure using peridynamics.
The aim of this thesis was to investigate the validity of using the J-integral to determine
the critical stretch for predicting ductile failure. Standard ASTM fracture mechanics
tests on Compact Tension specimens of Polymethyl methacrylate, stainless steel 304L
and aluminium 1200H4 were performed to determine the critical energy release rates
and non-linear Resistance-curves. Furthermore, a novel peridynamic-based algorithm
was developed that implements a critical energy release rate based failure criterion and
Digital Image Correlation (DIC) measured full surface displacement fields of cracked
materials. The algorithm is capable of estimating and mapping both the peridynamic
damage caused by brittle cracking and damage caused by plastic deformation. This
approach was used to validate the use of an energy release rate based failure criterion
for predicting linear-elastic brittle failure using peridynamics. Also, it showed a good
correlation among the test results for detecting plastic damage in the alloys when incorporating
the respective J-integral derived critical stretch values. Additionally, Modified
Arcan tests were performed to obtain Mode I, Mode II and mixed Mode fracture load
results of brittle materials. Mode I peridynamic models compared closely to test results
when using the Mode I critical energy release rate, derived critical stretch and served
as validation for the approach. Moreover, it was argued that Mode I failure criteria
cannot in principle be used to model shear failure. Therefore, it was proposed to rather
use the appropriate Mode II and mixed Mode critical energy release rates to predict the
respective failures in peridynamics. Also, for predicting ductile failure loads it was found
that using a threshold energy release rate derived from the R-curve yielded considerably
more accurate failure load results compared to the usage of the critical energy release
rate, i.e. J-integral.
In this thesis it was shown that there exists great potential for detecting and characterising
cracking and failure by using a peridynamic-based approach through coupling DIC
full displacement field measurements and the critical energy release rate of a particular
structural material. / AFRIKAANSE OPSOMMING: Duktiele breeking van materiale word tans beskou as 'n kompleks- en uitdagende fenomeen
om te voorspel en te karakteriseer. 'n Binding-gebaseerde, nie-lokale teorie is onlangs
geformuleer, genaamd die peridinamika teorie. Die laasgenoemde stel ons in staat om
soliede meganiese probleme met krake direk op te los. Die falings kriterium word bemagtig
deur die kritiese strekfaktor tussen verbindings. Daar was bewys dat die kritiese
strekfaktor in verband staan met die popul^ere breek meganika parameter, genaamd die
kritiese vrylatings-energie-koers vir die voorspelling van bros line^ere-elastiese faling. 'n
Onlangse verklaring meen dat die kritiese strekfaktor vir duktiele falingsgedrag, bereken
kan word met die nie-line^ere kritiese vrylatings-energie-koers, beter bekend as die J-
integraal.
Die doel van hierdie tesis was om te meet hoe geldig die gebruik van die J-integraal
is om die kritiese strekfaktor te bereken, om sodoende duktiele breking te ondersoek.
Standaard ASTM breukmeganika toetse op Polimetilmetakrilat, vlekvrye staal 304L en
aluminium 1200H4 is uitgevoer om die kritiese vrylatings-energie-koers en Weerstandskurwes
te bepaal. Verder was 'n nuwe peridinamies-gebaseerde algoritme ontwikkel.
Die laasgenoemde implementeer die berekening van 'n kritiese strekfaktor, gebaseer
op die kritiese vrylatings-energie-koers, sowel as Digitale Beeld Korrelasie (BDK) vol
oppervlaks-verplasings veld metings van gebreekte materiale. Dit is in staat om die
peridinamiese skade te bereken, tesame met die beeld wat veroorsaak was van bros
krake en plastiese vervorming in duktiele materiale. Hierdie benadering is aangewend
om die gebruik van 'n vrylatings-energie-koers gebaseerde falings kriterium vir bros
line^ere-elastiese falings in peridinamika te bekragtig. 'n Goeie korrelasie tussen toets
resultate is ook gevind vir die opsporing van skade wat veroorsaak is deur plastiese
deformasie in die legerings waar die onderskeilike J-integrale gebruik was as falings kriteria.
Daarbenewens, was Verandere Arcan toetse uitgevoer om die Modes I, Modes II
en gemenge Modes falingsresultate te verkry. Die Modes I peridinamiese model het goed
vergelyk met die toetsresultate en het gedien as bekragtiging vir die falingsbenaderings.
Verder was dit aangevoer dat Modes I falings kriterium in beginsel nie gebruik kan
word om skuiffaling te modelleer nie. Dus was dit voorgestel om eerder die toepaslike
Modes II en gemengde Modes kritieke vrylatings-energie-koerse te gebruik om onderskeie
falings te voorspel in peridinamiese modelle. Dit was ook gevind dat vir die voorspelling
van duktiele falingslaste die drumpel vrylatings-energie-koers, wat verkrygbaar is vanaf
die Weerstands-kurwe, aansienlik meer akkurate resultate gee, in vergelyking met die
gebruik van die kritiese vrylatings-energie-koers, m.a.w. die J-integraal.
In hierdie tesis was dit gewys dat daar groot potensiaal bestaan vir die opsporing en
karakterisering van krake en falings met 'n peridinamies-gebaseerde benadering, deur dit
te skakel met BDK vol verplasings veld metings en die kritiese vrylatings-energie-koers
van 'n bepaalde strukturele materiaal.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96755 |
| Date | 03 1900 |
| Creators | Conradie, Johannes Hendrik |
| Contributors | Turner, Daniel Z., Becker, Thorsten H., Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 146 pages : illustrations |
| Rights | Stellenbosch University |
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