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Impact and Drop Testing
with
ICP® Force Sensors
Robert Metz
PCB Piezotronics, Inc.
Automotive Testing Expo, North America
Novi, MI, USA
October 26, 2006
Overview
� Reasons for Impact Testing
� Energy and Impact Force
� Relationship Between Force and Distance
� Relationship Between Force and Time
� Drop Test Example
� Selecting a Force Sensor
� ICP® Force Sensor Configurations
� Conclusions
Reasons for Impact Testing
• Determine energy absorbed or required to damage UUT
• Validate design & ensure that it meets product durability &
safety requirements
– Safety critical components: Automotive bumpers,
protective sports equipment, headform testing of
hardhats/helmets
– Various SAE, MIL, ANSI or ASTM test specifications
• Destructive impact testing performed to document strength or
durability of non-safety critical items for industrial use
Work-Energy Principle
• Ave. impact force x distance traveled = change in kinetic energy
• Reduce impact force by extending stop distance via ‘crumple zones.’
Energy & Impact Force
• Energy not directly measurable
– Calculate from Work Energy Principle
• Conservation of energy - potential energy before event must equal kinetic energy after event
PE = KE
• Drop test conservation of energy equation is
mgh = ½ mv2
• Impact velocity independent of mass, neglecting drag caused by air resistance, velocity is calculated from:
v = √2gh
Relationship Between Force & Distance
• Change in Energy, or Net work during impact = average force
of impact x distance traveled during impact
• Measuring distance traveled after impact, d, the average
impact force, F, is calculated as
F = Wnet
d
Wnet = ½ mvfinal2 - ½ mvinitial
2
• In drop test, Wnet = ½ mvfinal2 since the (vinitial) = zero
Relationship Between Force & Distance
To get Energy, Test Engineer must measure Force and Distance
• What sensor should be selected? How to estimate the
expected Force?
• Use the formula in reverse order
• Must however estimate distance traveled before 1st impact test
• This is a function of the UUT hardness and whether or not
there is a perfectly elastic collision (perfect rebound)
• Not easy to estimate, so must make sample drop test and
measure indentation
Relationship Between Force & Distance
Work Energy Method using Estimated Displacements
Material h (m) m (kg) v final
(m/s) KE (J) d (m) F (lbs) F (N)
Steel 1 4.5 4.427 44.1 0.0001 99,137 441,000
Plastic 1 4.5 4.427 44.1 0.1 99 441
Foam 1 4.5 4.427 44.1 5 2 9
h
d
Relationship Between Force & Time
• Another way to estimate impact force - Newton’s 2nd law, F=ma
• From conversation of energy equation v = √2gh, compute resulting impact acceleration
• Acceleration dependent on impact pulse width, calculated from velocity change during impact time
a = dv = dv
dt tpulse
• Assume perfect rebound for steel on steel impact
• Initial & final velocities equal & opposite, thus add thus peak acceleration is
a = vinitial - vfinal = 2 * √2ghtpulse tpulse
Relationship Between Force & Time
• Do not confuse acceleration due to free fall gravity (g) used in
impact velocity calculation with the impact acceleration
• Impact force is then calculated from Newton’s 2nd law
F = ma
• Pulse width, and acceleration, vary as penetration distance
varied.
• Softer impact surfaces have lower impact force
• Soft surface slows down the impact, spreading pulse width
Relationship Between Force & Time
Pulse Width
Relationship Between Force & Time
Newton's 2nd Law Method using Estimated Pulse Widths
Material h (m) m (kg) v final
(m/s) KE (J) t pulse
F (lbs) F (N)
Steel 1 4.5 4.427 44.1 0.0005 18,050 80,294
Plastic 1 4.5 4.427 44.1 0.002 4,513 20,076
Foam 1 4.5 4.427 44.1 0.100 90 400
INSTRON® Drop Test Example
• Automotive bumper assemblies
designed to absorb and dissipate
impact energy.
• Steel supports typically used, but
lighter materials save fuel
• INSTRON® developed test machine
used to qualify alternative bumper
materials
INSTRON® Drop Test Example
• Model 8150 Dynatup® drop tower
• Capable of generating 27.8 kJ of energy from a
drop height of 96 in (2.4 m) and mass of 1,000
lb (454 kg)
Test parameters:
• Required energy of 3.2 kJ
• Drop mass 793.8 lb (360 kg)
• Drop height 35.4 in (0.9 m)
• Estimated crumple zone pulse width 10 msec
INSTRON® Drop Test Example
Bumper
Crosshead with
integral force sensors
INSTRON® Drop Test Example
Eqn. 1
V = √2gh = √2*385.92 in/sec2*35.4 in = √27,323.1 in2/sec2 =165.3 in/sec
Energy
KE = ½mV2 = ½*793.8 lb * (165.3 in/sec) 2 = 28,101.5 lb-in
385.92
= 3175.2 N-m = 3175.2 J
Eqn. 2
a = 2 * √2gh = 2*165.3 in/sec = 33,060 in/sec2 [85.7 g peak]
tpulse 0.010 sec
Eqn. 3
F = ma = W *a = 793.8 lb * 33,060 in/sec2 = 68,000 lb
g 385.92 in/sec2
INSTRON® Drop Test Example
Close up of Model 8150 crosshead
shows ICP® force sensor cable exiting
the striker
• Crosshead supported by
4 ea. PCB model 203B
ICP force rings
• Each having a 20 klb (90
kN) compression rating
• Total impact range 80 klb
(355.9 kN)
INSTRON® Drop Test Example
Average impact force
Force & Energy vs. Time for Bumper
KE = 3,196 J
Force = 36,035 lb (160.3 kN)
Pulse Width = 15.17 msec
INSTRON® Drop Test Example
Approx 1.5 inch
Cross check the math with displacement
• Use work-energy principle derived earlier
• Displacement of bumper after impact was 1.5 in (0.038 m)
INSTRON® Drop Test Example
Estimate average force from curve 19,108 lbs (85 kN)
Energy is:
Wnet = F * d = 19,108 lb * 1.5 in = 28,662 in-lb = 3,238 N-m
= 3,238 J
Selecting a Force Sensor
• Select a force sensor several times stiffer than UUT
• If not, sensor will absorb some impact, resulting in measurement inaccuracy
• Strain gage technology commonly taught & widely used
• Not very stiff
• Stiffness = amount of force required to displace one inch
lbs. force / µµµµ inch
Or
kN / µµµµm
Selecting a Force Sensor
• Strain gage load cell
requires deflection of
0.001 to 0.003 in to
reach full-scale output
• Equates to stiffness
of 0.03 to 6.7 lbs/µin for
100 lb and 10 klb
respectively
• Bending required to
create outputPhoto shows flexure deflection
Selecting a Force Sensor
• Quartz Piezoelectric force sensors react to stress, not large displacements 1E-6 in (0.2 µm)
• Few orders of magnitude stiffer than strain gage load cell of equivalent measuring range
• Depending on physical shape, stiffness 6 to 100 lbs/µin
Selecting a Force Sensor
• Measure to several tens of kHz
• Well beyond ringing frequency of strain gage load cells
• Additional benefits of high stiffness
• small size
• low mass
• overload protection
500 lb ICP® sensor on right,
250 lb load cell on left
Selecting a Force Sensor
• Rise time of force sensor must be faster than expected
pulse width to measure properly
• Rise time defined as the time it takes a sensor to rise
from 10% to 90% of final value when subject to step input
0
20000
40000
60000
80000
100000
120000
300 400 500 600 700 800 900 1000 1100
Time (micro sec)
Force (Lbs.)
Rise Time = 52 micro sec
Selecting a Force Sensor
• Rise time for force sensor affected by frequency– The more mass, the lower the natural frequency
– The lower the natural frequency, the slower the rise time
• ICP® force sensor rise time estimated as 1/2 of natural period
Tp = 1/2*(1/fn)
Where, fn = natural frequency and Tp = time to peak
• Example, PCB ICP® impact force sensor model 203B – Natural frequency 60 kHz
– Rise time would be 8.3 µsec
ICP® Force Sensor Configurations
• ICP® force sensor configurations commonly available
– General purpose
– Ring
– Impact
– Penetration
– 3-axis
208C05 205C 200C50 208A22 260A11
ICP® Force Sensor Configurations
• ICP® impact force sensors supplied with specially designed impact
caps
• Convex surface transmits impact loads evenly
– Better measurement results
– Preventing sensor damage
• Caps also compensate for misalignment of UUT or drop mass
• Provides replaceable wear surface if damaged
-500
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0.00 0.25 0.50 0.75 1.00 1.25 1.50
Time (ms)
Force (lb)
60 mph
90 mph
4 ea. 208C05 general purpose
ICP® Force Sensor Configurations
• In some cases, much higher force range is required
• Multiple force ring style ICP® sensors may be used in series
between an impact plate and base plate
• Each sensor within the structure absorbs 25% of force
• Voltage signals may be monitored individually or summed
Upper Impact Plate
ICP Force Rings
Base Plate
ICP® Force Sensor Configurations
Upper Impact Plate
ICP® Force Rings
Base Plate
ICP® Force Sensor Configurations
Impact test on
automotive interior
vinyl trim material
Curved impact cap
keeps sensor prom
penetrating material
Selecting a Force Sensor
• Impact force simultaneously in 3 orthogonal directions
• PCB ICP® 260 series, 3-component force ring
• Each x-y-z axis provides independent output signal
• Summing 4 in series provides 6 DOF
– Fx,y,z and Mx,y,z
Impact testing on Space Shuttle External Fuel Tank Foam
Selecting a Force Sensor
Close up of sensor mounting
Conclusions
• Impact force measurement is a proven way to document
that proper energy obtained during impact test
• Selection of force sensor measuring range possible by
– Using conservation of energy and estimate pulse width for the
planned test
– Use of Newton’s 2nd law
• Attributed to high stiffness, quartz piezoelectric ICP® force
sensors
– Measure high impact forces with fast rise times
– Have durability required to perform in harsh test conditions
• Various sensor configurations for impact applications
– Allows the test engineer to perform testing with great ease