Foundamentals of Automotive Crash Safety

Lecture 1 Fundamentals of Vehicle Crash Safety

• impact energy absorption The kinetic energy is converted to work by deforming all the related objects
• Three levels of impact
  • First impact: between vehicle front-end structure and external objects
  • Second impact: between occupant and restraint system/interior
  • Third impact: between occupant’s internal organs
• Crash safety
  • a matter of reducing the relative velocity between occupants and vehicle interior to help reduce risk of injury to occupant during a collision
• Vehicle Crashworthiness
  • Measure of the vehicle’s structural ability to plastically deform and yet maintain a sufficient survival space for its occupants in crashes involving loads**

Lecture 2 Vehicle Frontal Impact Response and

Occupant Ride-Down

• Crash pulse measurement

  • Mounted in non-crash zone of occupant compartment
  • Avoid low stiffness locations

• Injuries are produced by the occupant motion relative to vehicle interior

• Purpose of restraint system is to make the contacting speed lower

• The occupant kinetic energy is absorbed by three sources

  • The restraint system
  • The human body deformation
  • The vehicle structure through restraint coupling (The Ride-Down Energy)

• Achieve the lowest force and acceleration, good to occupant protection in a general sense

• The occupant response

  • The occupant response depends on both of the vehicle pulse and the occupant restraint system
  • The effect of the occupant restraint has a limit

• Crash duration

  Definition: from the time when the vehicle starts contacting the obstacle to the time when the acceleration diminishes to a negligible level (about 80~120 ms)

  • Vehicle type – the stiffer the structure, the shorter the crash duration
  • Crashing type – the softer the crash mode, the longer the crash duration

Lecture 3 Human Body Injuries in Vehicle Collisions

• Human body injury

  deformation of anatomical structures beyond their failure limits resulting in damage of tissue

• Mechanical causes of body injuries:

  • direct body contact
  • no external contact : Inertial effect leads to internal squeeze or stretch

• human body injury tolerance

  Mechanical parameter and value to gage severity of certain injury and define acceptance level $$ HIC = MAX \Bigg( \Big[\frac{1}{t_2 -t_1} \int_{t_1}^{t_2} a_g(t) dt \Big]^{2.5} (t_2 - t_1\Bigg) $$ Note: head injury is related both magnitude and duration of acceleration

Lecture 4 Vehicle Crash Safety Assessment

• Dummy: Anthropomorphic Test Device
• Hybrid III 50th Male has been adopted in world-wide automotive safety regulations, which is also the only daummy with broad basis in biofidelity.
• Other dummies were scaled from Hybrid III 50th Male.(Not only scale mass and size, but also injury thresholds)
  • H3-50M neck: Match nech angle-torque response data in forward and rearward directions for humans seated in automotive posture.(Note the slits)
  • H3-50M chest assembly: Represented by six high strength steel ribs with polymer based on damoing material.
    • Chest acceleration
    • Chest deflection
  • H3-50M calibration
    • Dummy mush be calibrated before test to guarantee its state
• Requirements of carsh test dummies
  • Biofidelity
    • Measure its human-likeness
    • Size, mass, kinetic and dynamic responses under specified loadings.
  • Sensitivity to injury parameters(sensitive and robust)
  • Repeatability
    • Same responses under the same impact test conditions
  • Durability
    • No damages under the conditions of the required impact test
• Two levels of biofidelity requirements
  • First level to control overall kinematics
    • Joint stiffness between articulated body regions
    • Mass properties of individual assemblies
  • Second level to control response of critical assemblies
    • Chest compression stiffness, head impact response, femur compressive response, etc
• Sled tests
  • Low cost
  • Quick
  • Controllable
  • Good repeatability and reliability
  • Drawback: 1-D, no pitch

Lecture 5 Seatbelt and Airbag

Intro

• Modern seatbelts: a three-point anchorage system
• Seatbelt components: Belt, Retractor, Slip ring, Buckle, Anchor
• Typical seatbelt consists of two webbing sections:
  • Lap belt
  • Shoulder belt
• The spool effect: Belt webbing can be stetched to some extent: for belt force on occupant gradually increasing and occupant’s stopping not too abrupt
• Retrctor lock: vehicle and webbing sensitive
• Aurbag: package, inflator, crash sensors(ECU)

Safety functions

• Seatbelt

  • Restrain occupant and reduce risk of occupant contact with vehicle interior
  • Spread force across stronger parts of occupant body(shoulder and pelvis)

• Airbag

  • Fill space between ocuupant and steering wheel or dashboard
  • Apply force on occupant during occupant-airbag interactraction
  • Reduce head rotation and protect head and neck

  ###Risks and hazards of airbag

  • Occupant should not strike airbag until it is fully inflated(And it should deplot in a fraction of a second to be effective)
  • Unbelted or out of position occupants can be seriously injured or killed by airbag

  ###risks of seatbelt

  • Submarining: occupant slides down underneath lap-belt in frontal collision
  • Concentrated loading to chest

Lecture 6 Seat as Occupant Impact Protection Device

Structures

• Rail system
  • Adjust seat position forward and backward
  • Secure seat position for safety

Seat and head restraint for anti-whiplash

• Seat and head restraint are critical for reducing neck whiplash injury risk
• Whiplash injury influence factors
  • Gender
  • Height
  • Seating position
    • Drivers have higher risk rate than passengers
      • Drivers tend to sit away from seatback
      • Passengers are usually more relaxed and lean further back in seats
• Mechanism
  • Most researchers agree that neck injuries are related to relative motion between head and torso

The debate between stiff and yielding seats

• Need yielding seatback to prevent whiplash in more frequent, minor rear crashes
• Need stiff (rigid) seats for occupant retention in infrequent, severe rear crashes
• Reducing both j and k has resolved the debate between rigid and yielding seats by separately considering seat characteristics for strength and stiffness

Lecture 7 Protection of Child Passengers

Difficulty and easiness

• Difficulty
  • Lack of data in injury mechanics
  • Large variations in body and age
• Easiness
  • May adopt “over protection”

Charateristics

• Relatively large head
• Relatively weak neck
• Childs’ ribs are more flexible than those of adults

Attentions

• Child in rear seat(away from airbag)
• Child mush be properly restrained
• Rear-fracing

Lecture 8 Adaptive Occupant Restraint System

**To provide individualized occupant crash protection to meet current and future challenges **

• Current occupant restraint design aimed for regulation requirements
• New needs and challenges
  • Crash protection of diverse and vulnerable road users
  • Crash protection of diverse and vulnerable road users
  • Vehicle safety under automous driving
  • Simulate real accidents
• Adaptive restraint system for individualized protection
  • Pre-crash warning can provide ORS with more information about crash and more preparation time
  • Reversible ORS can be tuned to optimal configurations prior to imminent crash
• Design space of adaptive restraint system
  • Seatbelt pretensioning
    • Time to fire
    • Pull-in length
  • Seatbelt force limiter
    • First level limiting
    • Second level limiting
    • When to switch
  • Seatbelt D-ring position
  • Airbag inflating/deflating characteristics
    • Time to fire
    • Mass flow scaling
    • Exhausting hole size
  • Seat position(a new factor)
• Optimization results
  • For 56km/h
    • Head injury reduced for all dummies
    • Chest compression largely reduced for small dummies
    • Seat position tends to be close to knee bolster for better posture control
    • Airbag effect on dummy reduced, help reducing airbag risk
    • Seatbelt
      • Force limiting level lowered for small dummies for reducing chest injury
      • Pretension increased, belt force increased in early stage
  • For 40km/h
    • Sit farther away from knee bolster, further reducing effect of airbag
    • Seatbelt
      • Pretension pull-in proportional to dummy size
      • Pull-in & 1st level limiting force decrease as dummy gets smaller
  • Summary
    • Seat position
      • In 56 km/h crashes, seat position is proportional to dummy height, close to knee bolster, for posture control
      • In 40 km/h crashes, seat position is farther away from knee bolster, for staying away from airbag
    • Airbag
      • Contact force with dummy is largely reduced
      • In low speed crash, airbag hardly contacts with occupant
    • Seatbelt
      • Greater pretension effect
      • Load limiting level is proportional to dummy size
      • Seatbelt becomes main restraining device, airbag force is greatly suppressed
  • Optimization of restraint system considering Chinese statures
    • Airbag mass flow is a main factor to chest injury of small stature and head injury of large stature
  • Safety development of autonomous driving vehicles
    • Characteristics of future traffic:
      • Smart
      • efficient
      • convenience
      • safe
    • Connected and autonomous vehicles moving in platoon : high-speed crash of multiple-cars
    • Technologies
      • pre-crash sensing
      • adaptive restraint system
      • individualized protection

Lecture 10 Occupant Head Impact Protection

• Background

  • Head injury is leading cause of severe injuries and fatalities in passenger car accidents

  • upper interior head impact requirements of FMSVSS 201 :

    To reduce severe head injuries due to secondary impact with upper interior components

  • Countermeasures

    • Padded interior to absorb head impact energy and reduce head injury

  • Problem

    • Padding takes too much interior space
    • Should use the least interior space to meet the FMVSS 201 requirements

  • Innovative trim – Plastic rib filled with polymer foam

    • Goal: early peak and small rebound
    • Trim developed through collaboration between supplier and OEM
    • Effective protection with minimum padding

  • Total stopping distance (EA space)

    • Three sources:
      • Deformation of padding
      • Compression of headform skin
      • **Elastic deflection of sheet metal **
    • Why elastic sheet metal deflection can absorb (manage) head impact energy
      • Answer: the event is over before sheet metal rebounds
      • The rebound part has little effect to the HIC calculation
      • But stiffness of the elastic structure affects HIC

Lecture 11 Pedestrain Impact Protection

• Injury mechanisms
  • Head impacts are most life threatening form
  • Lower extremity impacts
    • Severe knee joint injuries often cause permanent disability
  • Account for severe injuries
• Injury sources
  • Bumper, hood, and windshield
  • Improvements of vehicle front-end structures can reduce pedestrian injuries
    • Styling & packaging affecting pedestrian protection
  • Lower extremity impact protection
    • Design of vehicle front-end structures (bumper height, shape and stiffness) greatly affects leg bone fractures
      • First contact location (above, at or below knee) affects injury patterns
    • The influence of vehicle type on injury severity