W.J.T. Daniel

Three-component force balance for flows of millisecond duration

The authors investigate the feasibility of a new type of multiple-component force balance for measurements on models in hypervelocity hows of millisecond duration. The balance extends the concept of the single component stress-wave force balance to measurement of axial force, normal force, and pitching moment. Numerical modeling of the performance of the balance shows that coupled deconvolution techniques can be used to decouple the signals from axial strain measurements in the balance to determine the applied loads. Experiments performed in the T4 free-piston shock tunnel on a sharp cone at incidence indicate that the prototype balance performs satisfactorily, forces being measured to within 11% of their theoretical values, and the location of the line of force being measured to within 2.1% of the theoretical location as a fraction of model chord.

A study of the stability of subcycling algorithms in structural dynamics

Algorithms for explicit integration of structural dynamics problems with multiple time steps (subcycling) are investigated. Only one such algorithm, due to Smolinski and Sleith has proved to be stable in a classical sense. A simplified version of this algorithm that retains its stability is presented. However, as with the original version, it can be shown to sacrifice accuracy to achieve stability. Another algorithm in use is shown to be only statistically stable, in that a probability of stability can be assigned if appropriate time step limits are observed. This probability improves rapidly with the number of degrees of freedom in a finite element model. The stability problems are shown to be a property of the central difference method itself, which is modified to give the subcycling algorithm. A related problem is shown to arise when a constraint equation in time is introduced into a time-continuous space-time finite element model.

Design, modelling and analysis of a six component force balance for hypervelocity wind tunnel testing

A combination of modelling and analysis techniques was used to design a six component force balance. The balance was designed specifically for the measurement of impulsive aerodynamic forces and moments characteristic of hypervelocity shock tunnel testing using the stress wave force measurement technique. Aerodynamic modelling was used to estimate the magnitude and distribution of forces and finite element modelling to determine the mechanical response of proposed balance designs. Simulation of balance performance was based on aerodynamic loads and mechanical responses using convolution techniques. Deconvolution was then used to assess balance performance and to guide further design modifications leading to the final balance design

A partial velocity approach to subcycling structural dynamics

Subcycling, or the use of different timesteps at different nodes, can be an effective way of improving the computational efficiency of explicit transient dynamic structural solutions. The method that has been most widely adopted uses a nodal partition. extending the central difference method, in which small timestep updates are performed interpolating on the displacement at neighbouring large timestep nodes. This approach leads to narrow bands of unstable timesteps or statistical stability. It also can be in error due to lack of momentum conservation on the timestep interface. The author has previously proposed energy conserving algorithms that avoid the first problem of statistical stability. However, these sacrifice accuracy to achieve stability. An approach to conserve momentum on an element interface by adding partial velocities is considered here. Applied to extend the central difference method. this approach is simple. and has accuracy advantages. The method can be programmed by summing impulses of internal forces, evaluated using local element timesteps, in order to predict a velocity change at a node. However, it is still only statistically stable, so an adaptive timestep size is needed to monitor accuracy and to be adjusted if necessary. By replacing the central difference method with the explicit generalized alpha method. it is possible to gain stability by dissipating the high frequency response that leads to stability problems. However. coding the algorithm is less elegant, as the response depends on previous partial accelerations. Extension to implicit integration, is shown to be impractical due to the neglect of remote effects of internal forces acting across a timestep interface

Subcycling first- and second-order generalizations of the trapezoidal rule

Subcycling algorithms which employ multiple timesteps have been previously proposed for explicit direct integration of first- and second-order systems of equations arising in finite element analysis, as well as for integration using explicit/implicit partitions of a model. The author has recently extended this work to implicit/implicit multi-timestep partitions of both first- and second-order systems. In this paper, improved algorithms for multi-timestep implicit integration are introduced, that overcome some weaknesses of those proposed previously. In particular, in the second-order case, improved stability is obtained. Some of the energy conservation properties of the Newmark family of algorithms are shown to be preserved in the new multi-timestep extensions of the Newmark method. In the first-order case, the generalized trapezoidal rule is extended to multiple timesteps, in a simple way that permits an implicit/implicit partition. Explicit special cases of the present algorithms exist. These are compared to algorithms proposed previously

Simulation and Experimental Observations of Effect of Different Contact Interfaces on the Incremental Sheet Forming Process

Incremental sheet forming (ISF) is a promising forming process perfectly suitable for manufacturing customized products with large plastic deformation by using a simple moving tool. Up to now, however, the effects of contact conditions at the sheet interface are not well understood. The aim of this work is to study the effect of tool type and size on the formability and surface integrity during the forming process. Experimental tests were carried out on aluminum sheets of 7075-O to create a straight groove with four different tools (ϕ 30,ϕ25.4,ϕ20 andϕ10mm). One tool tip was fitted with a roller ball (ϕ 25.4mm) while the other three were sliding tips. The contact force, friction and failure depth were evaluated. A finite element (FE) model of the process was set up in an explicit code LS-DYNA and the strain behavior and thickness distribution with different tools were evaluated and compared with the experimental results. This study provides important insights into the relatively high formability observed in the ISF process. Microscopic observations of the surface topography revealed that a rolling tool tip produced better surface integrity as compared with a sliding tool tip, wherein, distinct scratch patterns in the tool traverse direction were evident.

Finite element modelling of a three-component force balance for hypersonic flows

Measurement of aerodynamic forces on a model craft exposed to hypervelocity flow in a shock tunnel must be performed during the period of steady flow which may be less than a millisecond. Measurement of drag has previously been achieved in this time frame at The University of Queensland, by deconvolution of strain signals measured in a sting attached to the model craft. This procedure has been extended to include measurement of lift and pitching moment. Finite element modelling has played a major role in the design of the device used to achieve this. Limitations of finite element predictions of strain signals are discussed, as is the applicability of two- and three-dimensional models in the design process. Finite element modelling has enabled questions to be answered that cannot easily be investigated experimentally: in particular, establishing what strain signals can be successfully processed to recover the input loading and what physical configurations produce acceptable strain signals. As well, the sensitivity of the procedure to the time history of loading, the distribution of loading and the flexibility of the model is studied. The chosen configuration for lift measurement involves mounting the craft to the sting by means of symmetrical triangulated bars, in which the axial strains are measured. Experimental tests on this support arrangement are compared to the finite element simulations.

Three-dimensional orthotropic viscoelastic finite element model of a human ligament

Ligaments undergo finite strain displaying hyperelastic behaviour as the initially tangled fibrils present straighten out, combined with viscoelastic behaviour (strain rate sensitivity). In the present study the anterior cruciate ligament of the human knee joint is modelled in three dimensions to gain an understanding of the stress distribution over the ligament due to motion imposed on the ends, determined from experimental studies. A three dimensional, finite strain material model of ligaments has recently been proposed by Pioletti in Ref. [2]. It is attractive as it separates out elastic stress from that due to the present strain rate and that due to the past history of deformation. However, it treats the ligament as isotropic and incompressible. While the second assumption is reasonable, the first is clearly untrue. In the present study an alternative model of the elastic behaviour due to Bonet and Burton (Ref. [4]) is generalized. Bonet and Burton consider finite strain with constant modulii for the fibres and for the matrix of a transversely isotropic composite. In the present work, the fibre modulus is first made to increase exponentially from zero with an invariant that provides a measure of the stretch in the fibre direction. At 12% strain in the fibre direction, a new reference state is then adopted, after which the material modulus is made constant, as in Bonet and Burtons model. The strain rate dependence can be added, either using Piolettis isotropic approximation, or by making the effect depend on the strain rate in the fibre direction only. A solid model of a ligament is constructed, based on experimentally measured sections, and the deformation predicted using explicit integration in time. This approach simplifies the coding of the material model, but has a limitation due to the detrimental effect on stability of integration of the substantial damping implied by the nonlinear dependence of stress on strain rate. At present, an artificially high density is being used to provide stability, while the dynamics are being removed from the solution using artificial viscosity. The result is a quasi-static solution incorporating the effect of strain rate. Alternate approaches to material modelling and integration are discussed, that may result in a better model.

Investigation of the impact of locomotive creep control on wear under changing contact conditions

This paper presents the locomotive traction controller performance with respect to the track wear under different operation conditions. In particular, an investigation into the dynamic response of a locomotive under changing wheel–rail friction conditions is performed with an aim to determine the effect of controller setting on track wear. Simulation using a full-scale longitudinal–vertical locomotive dynamic model shows that the appropriately designed creep threshold, controller, settings can effectively maintain a high tractive effort while avoiding excessive rail damage due to wear, especially during acceleration under low speed.

Influence of deceleration profiles on occupant velocity differential and injury potential

Abstract. This paper compares two hypothetical and identical vehicle deceleration profiles mirrored in time, one linearly descending with time and the other linearly ascending with time. The differences of such profiles on occupant velocity differential and by implication, injury levels at the point of occupant impact are presented. An indifference point is established to assist in comparing which occupant body part will benefit from the altered crash pulse. It is shown that for occupant proximity distances below the indifference point, an ascending profile results in lower injury risk. Above the indifference point, the result is reversed.