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 mvinitial2 • 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 * √2gh tpulse 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

Crosshead with integral force sensors

Bumper

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 0.010 sec tpulse

[85.7 g peak]

Eqn. 3 793.8 lb * 33,060 in/sec2 = 68,000 lb F = ma = W *a = 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 Force & Energy vs. Time for Bumper

KE = 3,196 J Force = 36,035 lb (160.3 kN) Pulse Width = 15.17 msec Average impact force

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 / µ inch Or kN / µm

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 output

Photo 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

•

Rise Time = 52 micro sec 120000

Force (Lbs.)

100000

80000

60000

40000

20000

0 300

400

500

600

700

Time (micro sec)

800

900

1000

1100

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

4500 60 mph 90 mph

4000 3500

Force (lb)

3000 2500 2000 1500 1000 500 0 -500 0.00

0.25

0.50

0.75

1.00

1.25

Time (ms)

4 ea. 208C05 general purpose

1.50

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

e os Cl o up so en fs g in nt ou rm

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