Thermomechanical Modeling and Analysis of Rail Vehicles Disc Brake with Nonaxisymmetric Finite Element Method
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Date
2024-06
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Addis Ababa University
Abstract
Braking could have unfavorable consequences, including disc thickness variations, thermal
judder, crack, fade, surface wear, and limited service life as a result of thermal fatigue. To
counteract such damaging consequences, accurate prediction or determination of temperature
is fundamental in the design stage and during operation and maintenance. Eventually, several
FE (finite element) models have been attempted to assess the temperature, stress, and fatigue
life prediction of disc brakes. Despite this, the spatial variations of heat input load and boundary
conditions are not adequately taken into account in these models. Hence, accurate detection of
failure in the design stage and during preventive maintenance is a key problem. In this
dissertation, an FE-based non-axisymmetric moving heat source (NAMHS) algorithm that
takes into account the temporal and spatial change of thermal load and boundary conditions is
developed, and implemented in disc brake geometry and material selection, as well as in
evaluating the effect of braking energy. All input geometric parameters and braking conditions
implemented in thermomechanical modeling are extracted from the trailer and motor bogie of
Addis Ababa Light Rail Transit (AALRT). ANSYS parametric design language (APDL) is
implemented in coding the variations in thermal loads and the corresponding boundary
conditions, both spatially and timely. The model constitutes three separate analyses: thermal,
mechanical (stress assessment) and fatigue life. To consider space and time variation in heat
input and boundary condition, NAMHS is executed by the APDL programming model, similar
to FORTRAN written commands. Once the model is seen successful in disc brake analysis, its
applicability in other research areas is tested in three ways: comparative analysis geometry
selection, examining the effect of braking energy, and disc material comparative analysis. The
consideration of radial distance in the NAMHS model algorithm showed surface temperature
variation as high as 10% and 60% compared to traditional FE models of moving heat source
and axisymmetric, respectively. Besides, the partition of friction surface area into the heat input
and convection in NAMHS resulted in maximum circumferential variations of temperature,
von Mise stress, and fatigue life prediction as high as 49°C, 46MPa, and 2000 life (braking
times), respectively. Moreover, the friction surface is exposed to radial stress variations from
tensile stress of 20MPa to compressive stress of -125MPa. Stress variation between the leading
and trailing edge of the pad trace due to deceleration is illustrated 5ᵒC and 3MPa on late-braking
times, respectively. The applicability test of the NAMHS model algorithm revealed encouraging outcomes. Although the maximum friction surface temperature seems similar, its
variation is highlighted higher in the original disc geometry, compared to the modified.
Unexpectedly, the original disc’s stress is found more than twice the stress found in modified
disc geometry. Besides, the braking energy variation prevailed in emergency braking of the
motor bogie revealed twice the strain range in the motor bogie, compared to the service brake
in the trailer bogie. And, its applications in material selection displayed cooling times as the
main factor. Finally, the NAMHS model algorithm is applied to experimentally and
analytically studied solid disc brakes, and successfully validated. Therefore, this finding has
drawn our attention to the significance of considering the spatial variation of heat source in a
modeling disc brake, which couldn’t have been supported in traditional models. The results
reported here suggest that the NAMHS model algorithm could provide convincing evidence
and a reliable estimate of where a maximum temperature and stress were observed, and where
a crack could be initiated.
Hence, this study provides a first step towards a realistic and comprehensive representation of
FE modeling, which could be implemented in failure prediction. Hence, this NAMHS finding
will help us to predict thermal fatigue life in a better way, compared to any traditional modeling.
And, the model should find a broad range of applications in conducting comparative analysis
of geometries and materials under any braking type. We hope that our finding could influence
disc brake manufacturers, researchers, and maintenance personnel in disc brake damage
investigation. Furthermore, the proposed model could be easily implemented and suitable in
the area of linear or tangential sliding frictions in addition to disc brakes. These might include,
but not limited to thermal and stress analysis of tread brakes, drum brakes, engine pistoncylinder,
and camshafts are just to cite a few of its application areas.
Description
Keywords
ANSYS APDL, Disc brake, finite element, spatial temperature variation, moving heat source, thermal stress, thermal fatigue, geometry and material comparative analysis