The understanding of the dynamic behaviour of materials subjected to impulsive loading is of paramount importance in the fields of transportation, military, automotive and aerospace industries. The consequences of impact loading due to collision or explosion are generally severe, often threatening the safety of people involved. Hence the desire to understand the dynamic response of engineering materials under loading conditions representative of real case scenarios to design impact resilient structures and materials.

These conditions are generally not uniaxial but deform the materials in multiple directions. Researchers in experimental mechanics have made extensive efforts to reproduce such loading conditions. The Split Hopkinson Bar (SHB) is a well-established dynamic loading method used for the characterisation of the dynamic behaviour of engineering and natural occurring materials.

Main research Objective – Development of a novel Tension-Torsion Hopkinson Bar apparatus

The main research goal of this work package is to develop a novel experimental facility enabling the measurement of the high-rate response of engineering materials in controlled and synchronised combined direct-shear stress conditions. This new Tension-Torsion Hopkinson Bar facility system employs innovative impulsive loading generation techniques to provide controllable, well-defined stress waves.

Particular attention is devoted to aspects of critical importance within the Impact Engineering research community, such as the ability to control key impulsive loading parameters (rise time, magnitude, duration, generation of tension-torsion combined impulsive loads of arbitrary duration and amplitude) and establishing their correlation to the overall response and damage mechanisms of advanced engineering materials.

Experimental technique and analysis

The first experiments conducted on this new facility (Fig 2.1) demonstrate the capability of the apparatus to generate simultaneous tensile and torsional stress waves in a single experiment. The employment of both longitudinal and torsional actuators at the far-end of the incident bar and the release of the stored elastic energy by the rapid disengagement of a clamp allow longitudinal and shear stress waves to be synchronised upon loading of the specimen.

The analysis of the experimental data shows that dynamic equilibrium conditions and steady strain rates were achieved during the experiments. The assessment of the loading paths shows that nearly proportional strain loading was attained during testing. The evolution of the stress trajectories during tension-torsion dynamic loading describes complex trajectories characterised by an approximately steady proportion between normal and shear stresses in the plastic regime.

The experimental results generated using the TTHB apparatus allow us to delineate, for the first time, the failure stress locus of engineering materials over a wide range of stress states including pure torsion, shear-dominated combined tension-shear, tension-dominated combined tension-shear and plain tension.

Existing theoretical constitutive laws are employed to approximate the failure envelopes, assess the rate sensitivity of engineering materials and visualise/predict any asymmetry of the failure stress loci both at quasi-static and dynamic strain rates. Additionally, the failure surface of samples tested in different loading conditions can be examined and analysed to assess the influence of loading mode and strain rate on deformation and failure mechanisms.

The quasi-static and dynamic failure envelopes, depicted in the normal stress vs shear stress space or in the principal stresses space, motivate the development of accurate and effective constitutive models.

Fig 2.1 – components of the TTHB bar apparatus


The system uses high-frequency acquisition systems, miniaturised strain gauges and high-speed cameras to capture the evolutions of strains and stresses during deformation and failure of materials under dynamic tension-torsion loading.

Additionally, the system employs the Photon Doppler Velocimetry (Fig 2.2) technique to capture simultaneously propagating elastic waves of longitudinal and shear nature in a Tension-Torsion Hopkinson Bar apparatus.

A prototype of the Photon Doppler Velocimetry (PDV) diagnostic (1 optical channel) was constructed to demonstrate its capability for capturing the propagation of incident and reflected waves. More recently the PDV system was built and its capability of recording up to 8 optical channels simultaneously was successfully demonstrated.

Fig 2.2. Schematic of the PDV channel and the data analysis process.

Specimen design

A four-ligament specimen geometry (Fig 2.3) comprising of four flat dog-bones circumferentially arranged around the axis of the sample is proposed for combined tension-torsion loading experiments.

Experimental results were obtained on ligament specimens made of various titanium alloys subjected to simultaneous tension and torsion in the quasi-static and dynamic loading regimes. The tensile and torsional behaviours measured from the ligament specimens were compared with those from cylindrical dog-bone specimens and thin-walled tubular specimens. Results showed a good agreement of the stress-strain curves measured from the ligament specimens and the other conventional specimen geometries.

Fig 2.3 – four ligament and tubular specimen designs

Advanced pulse generation

An innovative double-clamp TTHB system (Fig 2.4) allowing the flexible synchronisation, duration and timing of tensile and shear stress waves was developed and is currently being tested on a range of lightweight materials.

The system comprises two independent loading units and automated clamps to store tensile and torsional elastic energy and is provided with an ultra-fast trigger mode to control the timing and duration of tensile and shear stress waves.

The above pulse generation design allows for the complete flexibility of the dynamic loading sequence. In other words, it is possible to generate combined tensile-shear stress pulses loading the sample as it follows:

  • simultaneously
  • with shear stress waves reaching the sample before the longitudinal stress wave
  • with shear stress waves reaching the sample after the longitudinal stress wave

The duration of both torsional and axial loading waves can be controlled by varying the positioning of the two independent clamps.

Fig 2.4 – schematic of the double clamp tension-torsion Hopkinson Bar architecture.

Antonio Pellegrino
Antonio PellegrinoDepartmental Lecturer
Antonio Pellegrino is Departmental Lecturer in Impact Engineering at University of Oxford. His expertise focuses on the development of novel experimental apparatuses for the measurement of mechanical response of engineering materials at high strain rate. His research concentrates on strain rate and environmental effects on the response of lightweight reinforced polymers, titanium alloys and natural occurring materials. He is also interested in applications of artificial intelligence in solid mechanics. Within CORNERSTONE, his attention is focused on developing a novel combined loading tension-torsion Hopkinson Bar capable of a controlling the loading history of direct and shear stressed.

COVID was as unwelcome in the CORNERSTONE project as it was everywhere else. It slowed progress in a number of ways but has not prevented us from achieving the main objectives and neither has it undermined the value of the work. Clearly, the virus had a major negative impact on the aerospace industry in the short term and no aerospace company or airline went unscathed in this.

The main negative effect arose from the need to work from home for many months and avoid visits and in-person meetings. Laboratory work suffered due to the direct effect of being unable to have people present and due to the indirect effect on suppliers. However, thanks to the effective planning on both the academic and Rolls Royce sides, WP2 was up to date, able to meet without any significant delays the milestones planned for the project.

The consortium is extremely grateful to EPSRC for the accommodation of an extension in the end date of the award to April 2023.

COVID Statement