Abstract
Quantification of the dynamic fracture toughness of
structural materials is
essential to a wide
range of problems from nuclear power plant integrity
assessments to structural
engineering
applications. The difficulties associated with accurate
determination of the
fracture resistance
under dynamic loading are considerable. Although a lot of
research work has
been directed
to the problem, there are no standardized procedures for
the precracked Charpy
type
specimen. In this study a new single specimen test method
and testing facility
for evaluating
dynamic fracture toughness has been developed. The method
is based on the
application of a
new pendulum type instrumented impact tester equipped
with an optical crack
mouth opening
displacement (COD) extensometer. The fracture toughness
measurement technique
uses the
Double Displacement Ratio (DDR) method, which is based on
the assumption that
the
specimen is deformed as two rigid arms that rotate around
an apparent centre of
rotation.
This apparent centre moves as the crack grows, and the
ratio of COD versus
specimen
displacement changes. As a consequence the onset of
ductile crack initiation
can be detected
on the load-displacement curve. Thus, an energy-based
fracture toughness can be
calcu-
lated. In addition, the testing apparatus can use
specimens with the double
ligament
size as compared with the standard Charpy specimen which
makes the impact
testing
more appropriate from the fracture mechanics point of
view.
The novel features of the testing facility and the
feasibility of the new DDR
method
have been verified by performing an extensive
experimental and analytical
study. An
experimental comparison of the inertia effects and
specimen oscillations
revealed that
from the measurement point of view the registered
load-time curve of the
conventional
testing geometry has more oscillation effects than the
inverted testing
geometry. The
experimental results indicated quite clearly that the
oscillation amplitude
from the first
to the third oscillation was slightly less in the new
impact tester. The new
DDR
method gave comparable values for the ductile fracture
initiation toughness as
could be
obtained by applying different methods based on the
multi-specimen technique.
Fracture resistance curve information, stretch zone width
measurements and
specimen
instrumentation were used for determining the ductile
fracture initiation
toughness.
Good agreement was observed for three different materials
and two specimen
sizes.
The DDR method was successfully applied in studying the
effects of side
grooves,
specimen size and irradiation embrittlement on the
ductile fracture initiation
toughness.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
|
Supervisors/Advisors |
|
Award date | 14 May 1993 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 951-38-4258-4 |
Publication status | Published - 1993 |
MoE publication type | G4 Doctoral dissertation (monograph) |
Keywords
- destructive tests
- impact tests
- dynamic tests
- fracture tests
- fracture strength
- toughness
- impact strength
- construction materials
- test methods
- tests
- test equipment
- optical measuring instruments
- crack propagation
- theses