Abstract:
Off-road mobility defines the ability of land-based vehicles to accelerate, pull
loads and climb gradients. Historically, much of the work completed to relate
mobility to vehicle and terrain characteristics has been empirical in nature, and
has therefore been susceptible to errors caused by the inherent variability of
naturally occurring soils. This thesis addresses the off-road mobility of wheeled
vehicles, and in particular the use of a finite element (FE) based approach to
predict performance metrics such as rolling resistance, drawbar pull and tractive
efficiency. Tools to predict these measures of performance have a wide range
of applications, including military, agricultural and leisure vehicles, aircraft
operating from temporary airfields and planetary landing craft.
The current study extends the virtual proving ground (VPG) concept, where a
single vehicle model can be subjected to a series of virtual test conditions, to
include the assessment of off-road mobility. Throughout, modelling has been
carried out using LS-DYNA, a commercially available non-linear dynamic
analysis code. Unlike previous studies using FE techniques to investigate
wheel / soil interaction, an Eulerian representation of the soil has been
employed, which permits a consistent approach to be applied to both purely
cohesive and purely frictional soils, as well as those displaying a combination of
cohesive and frictional behaviour. At each stage of the research, the validity of
the FE-based modelling approach has been assessed using data from
controlled experimental testing at Cranfield University’s off-road dynamics
facility.
A key finding of the work completed is that rate sensitivity in moisture containing
soils can significantly affect the accuracy of (empirical, analytical and numerical)
model predictions, even at low translational speeds (less than 0.1m/s). This
finding highights the adverse impact of simplifications in many previous
experimental and modelling studies, which have assumed that rate effects may
be ignored providing the translational speed of the vehicle is low.
