T.J. Cloete

Contributions to the experimental validation of the discrete element method applied to tumbling mills

Accurate 3D experimental particle trajectory data, acquired from a laboratory tumbling mill using bi‐planar X‐ray filming, are used to validate the discrete element method (DEM). Novel numerical characterisation techniques are presented that provide a basis for comparing the experimental and simulated charge behaviour. These techniques are based on fundamental conservation principles, and provide robust, new interpretations of charge behaviour that are free of operator bias. Two‐ and three‐dimensional DEM simulations of the experimental tumbling mill are performed, and the relative merits of each discussed. The results indicate that in its current form DEM can simulate some of the salient features of the tumbling mill charge, however, comparison with the experiment indicate that the technique requires refinement to adequately simulate all aspects of the system.

How hard is hard enough? An investigation of the force associated with lateral blunt force trauma to the porcine cranium

Blunt force trauma forms a substantial portion of deaths worldwide. However, few studies have attempted to determine the force involved with blunt force trauma to the lateral part of the head. Nor have many studies been conducted at velocities exceeding 10 m/s. The acquisition of human tissue for experimental studies is becoming increasingly difficult. As such, the current study investigates the trauma and the force involved with cranial blunt force trauma in a porcine model. Thirty whole porcine heads were subjected to single impact tests on the fronto-parietal region at velocities ranging from 10 m/s to 25 m/s. Half the specimens were subjected to impact by a short projectile resembling a hammer head and the other half were subjected to impact with a Hopkinson pressure bar (HPB). Both implements had the same impact diameter and were machined from the same material. The HPB is an apparatus commonly used in material testing. Its use to determine fracture force in whole cranial specimens is novel. Fractures appeared similar in both the hammer tests and HPB tests. Lacerations and fractures resembled the shape of the striker surface with the most common fracture observed being a semi-circular depressed fracture. The mean peak fracture force was 7760 N (± 4150 N), with a mean displacement of 3.1mm (± 1.1mm). Peak fracture forces concur well with previous studies although no clear trend appears to exist between level of trauma and peak impact force.

Blast characterization using a ballistic pendulum with a centrally mounted Hopkinson bar

A series of laboratory-scale blast characterization experiments are presented to show the degree to which two alternative configurations of an instrumented ballistic pendulum can provide an ideal impulsive load. Both the total impulses and blast pressure histories were captured, the latter using a centrally mounted Hopkinson bar. Repeatable and consistent total impulse values were achieved, while the Hopkinson bar technique was sufficient to capture the essential shape of the blast loads, although the fine detail of the peak pressure could not be resolved due to higher mode dispersion. A short stand-off configuration produced short duration blast loads that approached an ideal impulsive load condition with near uniform impulse distributions, although the blast pressure distributions were non-uniform. Conversely, a blast tube configuration produced blast loads with near uniform pressure and impulse distributions but did not approach an ideal impulsive load condition, that is, the pressure history must be accounted for in subsequent analytical work.

The behavior of cancellous bone from quasi-static to dynamic strain rates with emphasis on the intermediate regime

Previous studies, conducted using quasi-static and dynamic compression tests, have shown that the mechanical strength of cancellous bone is strain rate dependent. However, these studies have not included the intermediate strain rate (ISR) regime (1/s to 100/s), which is important since it is representative of the loading rates at which non-fatal injuries typically occur. In this study, 127 bovine bone specimens were compressed in 3 regimes spanning 8 distinct strain rates, from 0.001/s to 600/s, using three different devices: a conventional quasi-static testing machine, a wedge-bar (WB) apparatus and a conventional split Hopkinson pressure bar (SHPB) implemented with a cone-in-tube (CiT) striker and a tandem momentum trap. Due to the large sample size, a new robust automated algorithm was developed with which the material properties, such as the apparent Young׳s modulus and the yield and ultimate values of stress and strain, were identified for each individual specimen. A statistical summary of the data is presented. Finally, this study demonstrates that results obtained at intermediate strain rates are essential for a fuller understanding of cancellous bone behavior by providing new data describing the transition between the quasi-static and dynamic regimes.

A Tandem Momentum Trap for Dynamic Specimen Recovery During Split Hopkinson Pressure Bar Testing of Cancellous Bone

A novel method for dynamic specimen recovery using tandem momentum traps on an otherwise standard split Hopkinson pressure bar, is presented. The method is based on a pair of concentric tubes that are impedance matched to and co-axially aligned with the input bar and arranged to operate sequentially. The tandem momentum traps provide a single specimen loading event, of a predefined intensity and duration, without the need to initially offset the momentum traps from the input bar by accurate preset gaps. The method is relatively simple to set up and operate, which allows for routine specimen recovery during dynamic testing. The operation of the tandem momentum trap is demonstrated by an investigation of the dynamic mechanical properties of soft cancellous bovine bone specimens.

Fracture characterization in cancellous bone specimens via surface difference evaluation of 3D registered pre- and post-compression micro-CT scans.

To cite this article: M. Prot, G. Dubois, T. J. Cloete, D. Saletti & S. Laporte (2015) Fracture characterization in cancellous bone specimens via surface difference evaluation of 3D registered preand post-compression micro-CT scans, Computer Methods in Biomechanics and Biomedical Engineering, 18:sup1, 2030-2031, DOI: 10.1080/10255842.2015.1069608 To link to this article: http://dx.doi.org/10.1080/10255842.2015.1069608

What is the recommended size of a Volume of Interest for cancellous bone? A skeleton-based study

The study of the bone fracture is an important issue for osteoporosis and car safety. The behavior of cancellous bone is strongly linked to the micro-architecture, the strain rate (Prot et al. 2015...