Robert Metz

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


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


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


• 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


Pulse Width


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


• 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


Bumper

Crosshead with

integral force sensors


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


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)


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


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)


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


• 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


• 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


• 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


• 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


• 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® 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


• 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


Upper Impact Plate

ICP® Force Rings

Base Plate


Impact test on

automotive interior

vinyl trim material

Curved impact cap

keeps sensor prom

penetrating material


• 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


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

Robert Metz

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