15 - Crashworthiness Flashcards
What is crashworthiness?
Comes under heading ‘passive safety’
Built-in, designed for safety
Always present without monitoring or power needs
Ready when needed
Last means of protection when all possibilities of preventing an accident have failed
Purpose of crashworthiness
Ensuring best possible outcome in event of collision
Protecting passengers and drivers/staff
Retaining a safe space
Reducing secondary impacts with seats, handrails etc.
Decelerating in controlled way
Absorbing energy
Simple 1D train crash model
Assume two vehicles with mass and velocity before the crash of m1, v1 and m2, v2
After crash the vehicles move together with speed V and combined mass m1+m2
Momentum is conserved unless an external force acts (i.e. assume no braking applied, momentum is mv)
Energy is conserved
Energy of collision is used in deforming metal, noise, etc.
Define impulse
Change of momentum
For a steady force is equal to the product of force and time
Types of accident
Collision between trains on high speed lines with train protection is very unlikely
Derailments due to vandalism, environmental factors and mechanical failures can still occur
Impact of train with track side structures such as bridges following derailment is also possible
Level crossings with risk of impact with road vehicles are widespread
Unlikely sequence of event leading to collisions (e.g. road vehicles falling onto line)
Factors influencing accidents
Speed: average train speeds have increased in recent years; most fatal accidents above 80km/h
Location of passengers: traditional locomotives being replaced with multiple-unit trains (i.e. underfloor power); any protection offered by heavy loco at front of train has been removed
Materials: many modern vehicles built from extruded aluminium alloy; in the future composites are likely to be used; collapse behaviour is different to that of steel
Design issues with energy absorption
For passenger vehicles, target is to avoid collapse of passenger space
Absorbing all energy of crash at single location is too difficult, but linear nature of train makes it possible to distribute energy absorption along train - multiple small energy absorbing events
Energy absorbers can be built in - plastic deformation used to absorb energy
Each vehicle intended to handle proportion of overall energy to be dissipated
Energy absorption
Plastic deformation is most viable way
Steel/aluminium tubes or other structures built in
Must sustain normal operating loads
Must collapse in controlled way under crash loads
Major issues include how to build in energy absorbers without adding too much weight
Metals have been used extensively, composites have potential to absorb energy for less added weight
How must tube-shaped energy absorbers collapse under compressive load?
Bending with small amounts of plastic flow will absorb little energy
Collapse with lots of folding and extensive plastic deformation is needed
Testing energy absorbers
Full scale tests must be carried out
Accompanying model work (e.g. FE) conducted to optimise
Important issues - high strain rate material behaviour, modelling large plastic deformations
Prevention of overriding
For energy absorbers to work fully, colliding vehicles need to be kept in line both horizontally (jack-knifing) and vertically (overriding)
Recent passenger vehicles fitted with ‘anti-climbers’ - engage in a crash preventing one vehicle riding over or into next
Prevention of jack-knifing
Can be reduced through design of inter-vehicle connection
Design issues with overriding and jack-knifing
Avoiding penetration of vehicle body or passengers being ejected
Experience shows people survive best if they remain inside vehicle and nothing from outside enters vehicle
Strong body shell construction
Windows that don’t break - laminated glass
Plastic interlayer - prevents passengers being thrown out
No seatbelts as research shows risk of getting trapped or colliding with rigid internal fixtures the belts would need
Strong vehicle structures
Since 70s rail coaches have used monocoque construction - vehicle skin is structural and supports its own weight and internal and external loads
Steel and aluminium versions exist
Recent vehicles favoured closed cell, double skin aluminium extrusions
Aluminium panels for roof, floor, sides prepared separately
Panels joined
Vehicle fitted out, bogies and wheels added, electrical wiring completed etc.
This form is extremely strong - excellent resistance to impact loading
Strong ‘tube’ can protect passenger space in collision
Often so strong, behaves as rigid body, but this won’t absorb energy so impact absorbing crumple zones added
Ladbroke Grove Junction case study
5th October 1999
Two trains involved, series of fires started in wreckage
31 deaths, 523 injuries
One train was aluminium construction diesel multiple unit train built of extruded body panels which didn’t behave as expected
Unexpected failure was due to weld failures
Longitudinal welds joining extruded sections, forming vehicle body, failed by fast fracture along HAZ/weld metal (WM) interface
Weld unzipping - describes this type of failure in which welds progressively open
