89
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
L. Lamberson et al. (eds.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society
for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59947-8_16
Chapter 16
The Development of Split Hopkinson Tension-Torsion Bar
for the Understanding of Complex Stress States at High Rate
Yuan Xu, Lukasz Farbaniec, Clive Siviour, Daniel Eakins, and Antonio Pellegrino
Abstract A novel Tension-Torsion Split Hopkinson Bar apparatus conceived to achieve an arbitrary combination of tension
and shear at high strain rates in a single loading event is proposed. The apparatus allows for the population of the failure and
yield surfaces with experimental data and, in turn, for a better understanding of materials during deformation and failure. The
proposed loading system enables combined direct-shear stress pulses of duration exceeding one millisecond to be applied to
the sample under consideration; hence, allowing for the achievement of a wider range of strain rates than what currently
exists in the open literature. Force, torque, and velocity histories at the interface between bars and specimen are measured
via a data analysis procedure based on the method of characteristics and on D’Alambert’s solution of wave equations.
Additional velocity measurements are conducted by means of photon Doppler velocimetry. The capability of the apparatus
is validated by means of bar-attached tests and will be further demonstrated via experiments on commercially available pure
titanium. The rate-dependent behavior of the material subjected to complex stress states will be analyzed and reported. The
failure surface of specimens subjected to a prescribed combination of tension and torsion will be assessed by means of optical profilometry and scanning electron microscope.
Keywords Split Hopkinson bar · Tension-torsion · Photon Doppler velocimetry · Combined tensile-shear loading
Failure surface
16.1 Introduction
Aircrafts face the risk of foreign object collision during every single flight. In particular, bird strike and foreign object ingestion by jet engines are safety-critical. During these events, fan blades are subjected to complex stress states that can lead to
their failure. High plastic deformation of the blades can induce load unbalance and oscillations of the rotating components
that may, in turn, cause the failure of the whole engine. Hence, deformation and failure of fan blades upon bird strike/ingestion play a significant role in the damage tolerant design of aircraft engines.
At present, the deformation and failure mechanism of aerospace materials under pure tension, compression, and shear
loading at a wide range of strain rates have been broadly studied and gained a solid research foundation. During the short
period of foreign object ingestion, however, the blade would endure a rather complicated stress/strain state at high rate such
that an arbitrary combination of both normal and shear stress exists at failure. Therefore, the dynamic responses and failure
mechanism under combined tensile and shear loading are of great interest but far from being understood.
Characterizing the dynamic responses of engineering materials under combined loading is key for a better understanding
of advanced aerospace materials at failure so that material distribution can be optimized. For this reason, a Tension-Torsion
Hopkinson Bar (TTHB) system is proposed to achieve the combination of tension and torsion at high rates in a single load.
This novel experimental apparatus aims to deliver materials data corresponding to the loading regime experienced within
aero-engines and allow for the direct population of the failure and yield surfaces.
Y. Xu (*) · L. Farbaniec · C. Siviour · D. Eakins · A. Pellegrino
Department of Engineering Science, University of Oxford, Oxford, UK
e-mail: yuan.xu@eng.ox.ac.uk; lukasz.farbaniec@eng.ox.ac.uk; clive.siviour@eng.ox.ac.uk; daniel.eakins@eng.ox.ac.uk;
antonio.pellegrino@eng.ox.ac.uk
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
L. Lamberson et al. (eds.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society
for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59947-8_16
Chapter 16
The Development of Split Hopkinson Tension-Torsion Bar
for the Understanding of Complex Stress States at High Rate
Yuan Xu, Lukasz Farbaniec, Clive Siviour, Daniel Eakins, and Antonio Pellegrino
Abstract A novel Tension-Torsion Split Hopkinson Bar apparatus conceived to achieve an arbitrary combination of tension
and shear at high strain rates in a single loading event is proposed. The apparatus allows for the population of the failure and
yield surfaces with experimental data and, in turn, for a better understanding of materials during deformation and failure. The
proposed loading system enables combined direct-shear stress pulses of duration exceeding one millisecond to be applied to
the sample under consideration; hence, allowing for the achievement of a wider range of strain rates than what currently
exists in the open literature. Force, torque, and velocity histories at the interface between bars and specimen are measured
via a data analysis procedure based on the method of characteristics and on D’Alambert’s solution of wave equations.
Additional velocity measurements are conducted by means of photon Doppler velocimetry. The capability of the apparatus
is validated by means of bar-attached tests and will be further demonstrated via experiments on commercially available pure
titanium. The rate-dependent behavior of the material subjected to complex stress states will be analyzed and reported. The
failure surface of specimens subjected to a prescribed combination of tension and torsion will be assessed by means of optical profilometry and scanning electron microscope.
Keywords Split Hopkinson bar · Tension-torsion · Photon Doppler velocimetry · Combined tensile-shear loading
Failure surface
16.1 Introduction
Aircrafts face the risk of foreign object collision during every single flight. In particular, bird strike and foreign object ingestion by jet engines are safety-critical. During these events, fan blades are subjected to complex stress states that can lead to
their failure. High plastic deformation of the blades can induce load unbalance and oscillations of the rotating components
that may, in turn, cause the failure of the whole engine. Hence, deformation and failure of fan blades upon bird strike/ingestion play a significant role in the damage tolerant design of aircraft engines.
At present, the deformation and failure mechanism of aerospace materials under pure tension, compression, and shear
loading at a wide range of strain rates have been broadly studied and gained a solid research foundation. During the short
period of foreign object ingestion, however, the blade would endure a rather complicated stress/strain state at high rate such
that an arbitrary combination of both normal and shear stress exists at failure. Therefore, the dynamic responses and failure
mechanism under combined tensile and shear loading are of great interest but far from being understood.
Characterizing the dynamic responses of engineering materials under combined loading is key for a better understanding
of advanced aerospace materials at failure so that material distribution can be optimized. For this reason, a Tension-Torsion
Hopkinson Bar (TTHB) system is proposed to achieve the combination of tension and torsion at high rates in a single load.
This novel experimental apparatus aims to deliver materials data corresponding to the loading regime experienced within
aero-engines and allow for the direct population of the failure and yield surfaces.
Y. Xu (*) · L. Farbaniec · C. Siviour · D. Eakins · A. Pellegrino
Department of Engineering Science, University of Oxford, Oxford, UK
e-mail: yuan.xu@eng.ox.ac.uk; lukasz.farbaniec@eng.ox.ac.uk; clive.siviour@eng.ox.ac.uk; daniel.eakins@eng.ox.ac.uk;
antonio.pellegrino@eng.ox.ac.uk