Examination of functionality of car deformation zones recovered after the car accident.
Doctoral Dissertation, Technological Science, Transport Engineering 03T Prepared at Kaunas University of Technology in 2005-2009 and supported by Lithuanian State Science and Studies Foundation. Defended on 28 December 2009 at Kaunas University of Technology. Scientific Supervisor: Prof. Dr Habil. Jonas Sapragonas
The objectives of the dissertation are:
* to determine the influence of car deformation zones recovered after a car accident on car safety;
* to develop a methodology of evaluating the effectiveness of a deformation zone.
The tasks of the dissertation are:
* to analyze the existing methodologies for evaluating frontal deformation zones;
* to analyze the behaviour of car safety zone elements and their influence on total passive car safety in case of frontal impact;
* to identify damaged deformation zones that occur during car accidents and subsequent repairs;
* to determine the effect of damages on the functionality of structural elements in deformation zones;
* to develop dynamic models of the car for predicting the effectiveness of the entire deformation zone and its individual elements.
Scientific novelty. The methodology has been developed for evaluating the hazardousness of damages potentially suffered during car operation on the basis of the amount of energy absorbed.
Using the finite element method, the numerical model has been developed enabling to predict the behaviour of deformation zones during car impact loads that cause major plastic deformations in the case of damaged deformation zone elements.
The dynamic model has been developed allowing a simulating course of the dynamic process occurring during a car accident and the interaction between individual elements of the deformation zone.
Practical value. The most dangerous changes in the characteristics of the frontal deformation zone during a car accident and subsequent repair were defined.
The effect of deformation zone damages made during a car accident and its repair on deformation zone characteristics and the effect of these changes on the course of a subsequent car accident were determined.
The developed methodology may be used for determining the objective criteria that define the reliability of deformation zones and other passive safety elements used in vehicles as well as their suitability for operation after car accident loads.
The author's contribution to the subject of research. The numerical methodology has been developed for predicting the amount of energy absorbed by thin-walled profiles potentially damaged during operation and commonly used in car safety zones and deformation course during impact loads that cause the loss of stability. For this purpose:
* the numerical models of damages were developed;
* using quasi-static and dynamic tests, the fidelity of models to the behaviour of real profiles was verified;
* the effect of potential damages, i.e. geometric deviations and changes in the mechanical characteristics of materials and technological connections, on the deformation of such elements after stability loss was determined.
Statements presented to defence:
* the numerical model for predicting the course of the deformation of damage elements and the amount of energy absorbed;
* the effect of interaction among deformation zone elements on the course of the deformation of individual elements;
* the effect of geometric deviations and changes in the mechanical characteristics of materials and technological connections to the deformation course of thin-walled elements and the amount of energy absorbed;
* the effect of the deformation zone with changed characteristics on the passive safety of the car.
1. The course of the repeated deformation of damaged deformation zone elements was examined. The findings show that the amount of energy absorbed may be used as the main criterion for safety zone effectiveness; however, the process of impact must be simulated in parallel.
2. The dynamic model was developed enabling to assess the effect of the individual deformation zone section on the entire deformation process. The deformation course and amount of energy absorbed were found to be affected by adjacent sections; for this reason, damages made to a single zone may affect the entire deformation process. To evaluate the effectiveness of the deformation zone is necessary to analyze different ranges of impact speeds, namely below 50 km/h and above 50 km/h.
3. Quasi-static and dynamic natural experiments with a part of the deformation zone revealed that damages lead to changes in the characteristics of elements. The quasi-static axial crushing of specimen with the recovered geometry resulted in a 20% reduction in critical buckling force and the amount of energy absorbed was reduced by 5% reaching the same level of specimen deformation. Axial crushing experiments showed that the amount of energy absorbed by dynamically crushed specimen was 90% greater comparing to quasi-static crushing.
4. The effect of geometric imperfections in the course of deformation was found to be ambiguous. The effect of unitary geometric imperfections of 0.5 mm in size on specimen deformation was found to be insignificant and unitary imperfection of 1.0 mm in size was found to change the position of forming the first plastic deformation wave, however, the amount of energy absorbed per length unit was found to have no significant effect.
5. The results of simulations obtained using numerical finite element models match the findings of natural experiments. Making adjustments in describing materials and technological connections is sufficient for a description of damages to deformation zone elements.
6. Simulations enabled to observe that the altered characteristics of spot-welds, depending on their positions, may significantly affect the nature of specimen deformation and the amount of energy absorbed. Weakening spot-welds by 50% in the central zone of the specimen used and deforming specimen by 0.5 of its length, the amount of energy absorbed is reduced by 8%. When the amount of energy absorbed by this element is 1.77 kJ, the axial shortening of crushed specimen increases up to 9% when compared to the specimens of all original spot-welds. A 50% axially-directed weakening of spot-welds situated in the zone of the first plastic deformation waves is equal to their complete absence.
7. Non-standard parts of the deformation zone used for repair have a significant effect on car safety. Using nonstandard parts and the presence of a repaired deformation zone almost doubles HIC (Head Injury Criterion) parameter.
8. The following effects of changes in the characteristics of individual deformation zone sections on their functionality were determined:
* stiffening elements in deformation zones negatively affect car safety in the entire range of tested speeds; though this negative effect decreases with the increasing speed of impact, nevertheless it remains significant;
* element stiffness reduction in deformation zones positively affects car safety below the impact speed of 67 km/h; above this speed, the weakening of deformation zone elements has a negative effect on car safety, and this effect grows with increasing impact speed.
Citation of the Summary of Doctoral Dissertation in Other Research Papers: Juodvalkis, D. 2009. Examination of Functionality of Car Deformation Zones Recovered after the Car Accident. Summary of Doctoral Dissertation. Kaunas: Technologija. 30 p.
Rubric prepared by Assoc. Prof. Dr Olegas Prentkovskis, Managing Editor of Journal TRANSPORT
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|Title Annotation:||DEFENDED DOCTORAL DISSERTATION|
|Date:||Mar 1, 2010|
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