2. Experimental determination of the dynamic fracture-
initiation toughness of high-strength metals
Engineering Fracture Mechanics
Contents lists available at Science Direct
journal homepage: www.elsevier.com/locate/engfracmech
AUTHORS:
MARIA JESUS PEREZ-MARTINA, BORJA ERICEA, FRANCISCO GALVEZB
• DEPARTMENT OF ENGINEERING SCIENCE, UNIVERSITY OF OXFORD, PARKS ROAD, OX1 3PJ
OXFORD, UNITED KINGDOM
• DEPARTMENT OF MATERIALS SCIENCE, E.T.S.I. DE CAMINOS, CANALES Y PUERTOS,
UNIVERSIDAD POLITÉCNICA DE MADRID (UPM), PROFESOR ARANGUREN 3, 28040 MADRID,
SPAIN
3. • Title
• Authors
• Abstract
• Introduction
• Materials and Procedures
• Experimental Procedure and Set Up
• Results and Discussions
• Concluding Remarks
• Acknowledgements
• References
4. A B S T R A C T
In the case of materials that may be subjected to dynamic loads or extreme conditions, it is crucial to
be aware of the evolution of their fracture behavior with the strain rate. This study describes the
detailed design and careful development of a novel experimental technique that allowed measuring the
dynamic fracture-initiation toughness for a wide range of loading rates based on a strong theoretical
background. For such a purpose, two high-strength metallic alloys were studied: (i) a very mild strain
rate dependent alloy, the AA7017-T73, and (ii) an armor steel with well-established rate effects the
Mars 240. The former was selected in order to validate the newly established experimental technique.
The measured dynamic fracture-initiation toughness, as expected, gave almost identical results,
therefore validating the technique. Once the technique was established, the dynamic fracture-initiation
toughness of the Mars 240 steel at different loading rates was obtained. The results showed an increase
on the dynamic fracture-initiation toughness with increasing loading rates.
5. INTRODUCTION
• Fracture toughness property describes the ability of a material containing a crack to resist
fracture and it may be a function of loading rate and temperature. In structural design, it is
important to know how the dynamic fracture-initiation toughness behaves with increasing
loading rates.
• In order to simulate dynamic loading situations under well-defined conditions in the
laboratory a quite extensive variety of testing devices, such as fast-driven servo hydraulic
testing machines, pendulums, drop weight towers, Hopkinson bars, gas guns and
explosive- driven machines, have been designed and modified for different loading rates.
Over the years, these experimental techniques have been further developed in order to
obtain more detailed and reliable information.
Continue…..
6. INTRODUCTION
• The aim of this study is to develop an exclusively experimental procedure that allows the
calculation of fracture-initiation toughness at loading rates ranging from low 10−1 up to
high 106 MPa・m1/2・s−1. For such a purpose, two high-strength metallic materials
were selected to study.
• First, a brief theoretical overview, that includes some preliminaries, of the dynamic
fracture-initiation toughness parameter is exposed. Having established several key
concepts, the selection and description of the materials used in this study is described and
the experimental procedures developed for this investigation are detailed. Subsequently,
the experimental campaign and its results are presented. Finally, such results are analyzed
in detail and discussed to determine the validity of the experimental procedures.
9. MATERIALS AND PROCEDURES
Two already-known high-strength metals were selected in order to carry out this study. The
first material, a AA7017-T73 alloy, was selected because of its very mild rate effects. Such
found aspect was considered an adequate characteristic to validate the experimental
techniques described later in this document. Once the developed experimental techniques
were validated, they were brought into play in order to obtain the dynamic fracture-initiation
toughness of a rate dependent material, the Mars®240 armor steel, at different loading rates.
A more detailed description of both materials is detailed below.
Continue…..
27. CONCLUSION
• A purely experimental procedure to obtain the dynamic fracture-initiation toughness of
two high-strength metals at very wide range of loading rates was presented.
• Three-point bending tests of pre fatigued specimens at different loading rates from low
10−1 up to high 106 MPa・m1/2・s−1 were carried out employing three different set-
ups: a servo-hydraulic universal testing machine, a modified split Hopkinson pressure bar
and an explosive load testing device.
Continue…..
28. CONCLUSION
• A “virtually” rate-independent high-strength metallic material, AA7017-T73 alloy, was selected in
order to validate the different techniques. The three experimental devices gave similar values of
fracture-initiation toughness of the aluminium alloy, showing the reliability of the devices for such a
purpose. The dynamic fracture-initiation toughness of AA7017-T73 alloy remained constant
regardless of the velocity at which the load was applied. In light of the obtained results, the test
procedures and devices that were used to test the materials under dynamic loading conditions were
considered to be validated.
• A rate-dependent high-strength metallic material, Mars® 240 steel, was selected in order to
determine the evolution of its dynamic fracture-initiation toughness with the loading rate. For such a
purpose, the previously validated experimental techniques were employed being confident that the
results were going to be reasonably accurate. The results revealed that the dynamic fracture
initiation toughness of the Mars® 240 steel increased with the loading rate.
29. ACKNOWLEDGEMENTS
This work was supported by the project BIA2011-24445 of the State Secretariat for
Research, Development and Innovation of the Spanish Ministry of Economy and
Competitiveness, and through the FPI 2012 predoctoral research grant BES-2012-051973.
30. REFERENCES
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