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Instrumented Impact Testing

testXpo 2018

03.08.2018

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Prüfen mit Verstand

Instrumented

Impact Testing

testXpo 2018

Helmut Fahrenholz


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Agenda

Why impact testing ?

Typical DWT standards

Impact methods

Energy, speed and mass in DWT

Results evaluation acc. standards


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This is a way of testing football helmets back in 1912

We can offer better

solutions

more reliable

less painful

Picture source: https://rarehistoricalphotos.com/testing-

football-helmets-1912/

https://rarehistoricalphotos.com/testing-football-helmets-1912/


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There are many good reasons: for example in Automotive …


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… in the aircraft industry, ….


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… in the building industry, ….

Pipe systems

PMMA roofings


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.. as well as in many situations of our daily life.

Eye protection googles

Safety shoes

Body protection

Safety

screens


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With increasing strain-rate:

Modulus increases

Max stress increases

Yield points disappear

Elongation decreases,

brittleness increases

With lower temperature:

Modulus increases

Max stress increases

Brittleness increases


Polymers are viscoelastic. Therefore their mechanical behavior

changes strongly with the strain-rate applied.


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Zwick covers all relevant strain-rate ranges, from creep to

quasistatic …

Creep (Messphysik) Static - Zwick


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… up to high speed impact test methods.

Pendulum Impact (Zwick, US) Drop Weight (Zwick Taicang) Hydraulic High Speed (Zwick UP)


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Agenda



Impact methods




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Pendulum Impact

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Zwick’s HIT pendulum impact series - a complete product

range for impact testing

Automation

5 Joule ISO

5.5 / 25 / 50 Joule universal, digital

Charpy Izod

tensile impact Dynstat

Instrumentation

Notch cutting machine

Manual notch cutter

../../../../../../../bm/branchen/1_plastics_(BM)/Fachmesse - K-Insel und DZ-TUN/Fachmesse 2010/FM IST Schulung/02_Vermarktung Kunststoffprüfgeräte/75_roboTestH_HIT25.wmv


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Pendulum Impact – Basic principle

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Instrumented pendulum impact is only standardized for the Charpy

method, but Izod and tensile-impact equipment is available.

Charpy

ISO 179, ASTM D 6110

IZOD

ISO 180, ASTM D 256

Tensile Impact

Here: ISO 8256 method A

Dynstat

DIN 53435


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Pendulum Impact – Basic principle

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Charpy is the recommended test method in the ISO standard.

Standards:

ISO 179 Part 1 and 2

ASTM D 6110

Notched or not notched

Evaluate type of break optically

ISO standard: always use the

biggest possible pendulum hammer

only use 10 to 80% of the pendulum

hammer‘s energy capability

impact strength normally is

measured in kJ/m²


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E – energy

m – mass of the pendulum hammer

h – drop height

g – gravity acceleration (9,81 m/s²)

Pendulum Impact – Conventional method

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In the conventional method, impact resilience is measured as

single-point data from the angle of raise of the pendulum.

h1

h2

𝐄𝟏 = 𝐦 ∗ 𝐠 ∗ 𝐡𝟏

𝐄𝟐 = 𝐦 ∗ 𝐠 ∗ 𝐡𝟐

𝐄 𝐒𝐩𝐞𝐜𝐢𝐦𝐞𝐧 = 𝐦 ∗ 𝐠 ∗ (𝐡𝟏 − 𝐡𝟐)


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Instrumented Pendulum Impact Testing

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Instrumented pendulum impact means force measurement

during impact. This offers supplementary characteristics.

used in R&D, TS and QA Charpy

Izod

tensile impact

Fracture mechanics

co

nve

ntio

na

l ha

mm

er

instr

um

ente

d h

am

mer


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Use of standard load cells

Test setup remains unchanged

compared to standardized

tensile-impact

Same pendulum hammers,

same yokes.

Mass of the grip leads to lower

natural frequencies compared to

Charpy and thus limits the range

of application of this method.

Instrumented Pendulum Impact Testing

For tensile-impact, the force is measured at the level of the

grip, which is part of the vice.


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The force-travel diagram provides supplementary materials

data obtained under high deformation rates.

E specimen

E specimen

𝐹

∆𝑠

E – energy

F – force

s – travel

EP = 𝐹 ∗ 𝑠 The conventional method may show

same results for completely different

stress-strain behavior.

Instrumented impact methods allow

to distinguish such situations, while

conventional impact can’t.

Break types can automatically be

detected

Information about fracture

mechanical characteristics can be

obtained.


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Pendulum Impact - Tests and Curves

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FI – First impact maximum

No contact between

pendulum hammer and

specimen

FM – maximum force

sM – deflection at

maximum force

Several points in a travel-deflection diagram are characteristic

for instrumented Charpy tests

The specimens natural

frequency has a square-

root function with the

materials tensile modulus


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This part of the curve shows the natural

frequency of the measurement system

The curve directly after break shows the resonance frequency of the

measurement system. No measurement beyond this frequency is possible.

Fo

rce

in N

Resonance ≥ 3 x natural frequency

of the specimen


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Zwick developed an automatic recognition of test curve types

according to ISO 179 part 2 in collaboration with Borealis.

tough break brittle break spall break

no break partial break

Complete break typesHinge break

Type of break can be

identified by instrumentation

Automatic classification of

the statistics by the type of

break

Safe and reliable test results

are obtained even with many

operators and in night shifts

Problems in test setup and

specimen handling become

visible and thus also

traceable.


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Fracture mechanic data can be obtained by using

instrumented impact and a stop-bloc.

Pre-cracked specimen

are submitted to impact

in Charpy configuration

The impact is stopped at

a given deflection

Under condition of stable

crack growth, the crack

can be stopped and its

length measured.

The result are R-curves.

Elastic materials behavior without and including crack propagation

energy

Fma

x

fma

x

f

[mm]

F [N

]

Ael

fgy

Fgy

Apl

RA

Fma

x

fma

x

f

[mm]

F [N

]

Ael

fgy

Fgy

Apl

f [mm]

f1

F [N

]

f2 fi

Apl

AelAel


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The resilience (energy) can still be obtained by the conventional

method – or by integration of the force deflection curve.

𝐹

∆𝑠

E specimen = 𝐹 𝑠 𝑑𝑠E specimen = 𝐹 ∗ 𝑠

E – energy

W – work (energy)

F – force

s – specimen deformation

Note:

The travel is obtained by double

integration of the acceleration (a)

which is obtained from the force

signal and the mass of the

pendulum hammer.

E specimen


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Instrumented drop-weight and HTM

Impact characteristics can be obtained by drop-weight testers

or by hydraulic high-speed testing machines (HTM)

Drop weight tester Zwick HIT 230F High-speed Testing Machines (HTM 5020 and HTM 2512)


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Agenda



Impact methods




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Instrumented Drop-Weight Testers

The products range from single purpose instruments to universal drop

weight testers, covering materials characterization and parts testing.

HIT230F Multiaxial

H= 2600mm

Mass Min= 23.5kg

V1= 4.40m/s

E= 230J

HIT1100F

H= 3270mm

Mass Min= 9.27kg

Mass Max= 29.42kg

V1= 4.4m/s

V2= 14.1m/s @ 9.27kg

E= 1126J

HIT2000F

H= 3870mm

Mass Min= 9.27kg

Mass Max= 29.42kg

V1= 5.42m/s

V2= 19.4m/s @ 9.27kg

E= 2044J

Remark: H=Machine Height, V1= Max Velocity without acc, V2= Max Velocity with acc, E= Max Energy

HIT230F CAI

H= 2600mm

Mass Min= 2.02kg

Mass Max= 10.08kg

V1= 4.40m/s

E= 120J

HIT600F CAI

H= 3150mm

Mass Min= 2.04 kg

Mass Max= 10.03kg

V1= 5m/s

E= 125J

HIT600F Multiaxial

H= 3150mm

Mass Min= 4.43kg

Mass Max = 40.43kg

V1= 5m/s

V2= 8.15m/s @ 4.43kg

E= 647J


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The basic principle is that of a free

falling mass, accelerated only by gravity.

The available energy is determined by

the mass and the drop height only.

Epot = Ekin

m g H = ½ m v²

The impact speed only depends on the

drop height.

v = √ (2gH)


The available potential energy is generated by the mass and

the drop height. The height determines the impact speed.

Epot = m g H

H

Ekin = ½m v²


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calculates, that the available

potential energy must be at least

2,8 times the energy needed to

impact the test piece.

This means that at a drop height

of 1 m @ 23kg giving 230 J of

potential Energy, it is possible to

test specimen where the

consumed energy is up to 83 J.

Typical values for 2 mm plastics

are below 50 J.


Standards require that the speed drop during impact remains

less than 20% of the initial impact speed

Impacto

r

Specimen

4,4

m/s

> 0

.8 * 4

,4 m

/s

trave

l

speed


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The drop weight tester needs to be

specified by

the impact speed = drop height

the available kinetic energy

The complete working range

including all available masses and

speeds, can be shown in a diagram

ENERGY / Impact SPEED


For a given specimen we need a defined minimum kinetic

energy at a fixed speed.

Speed, m/s

Energ

y in J

Both axes are logarithmic !

Min required

Energy

2,2

m/s

4,4

3 m

/s

10

m/s


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Purpose: Multiaxial impact tests

Standards: ISO 6603-2, ASTM D 3763

Puncture test at a speed up to 4,4 m/s

Spherical impactor, diam. 20 mm or 10 mm

Design of clamps as fixed by the standards

Fast and easy operation for pre-cooled

specimen

Instrumented Drop-Weight Testers, HIT230F

The Amsler HIT230F are single purpose drop-weight testers.

Impactor and clamps Brittle and ductile specimen Amsler HIT230F in multiaxial impact configuration


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Multiaxial impact tests to ISO and ASTM standards

at 1 m drop height (4.43 m/s)

Multiaxial impact to automotive specifications at low speed

of 2.2 m/s

Multiaxial impact to automotive specifications at higher

speed of 6.6 m/s

Multiaxial impact for research purposes at up to 8 m/s

Multiaxial impact testing with pre-cooled specimen

Multiaxial impact tests for plastic film, ISO 7765-2

Instrumented Charpy and Izod tests

CAI tests at variable and low impact work including second

impact prevention


The Amsler HIT600F is a universal drop-weight tester for

materials testing, covering many standards and methods.


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HIT600F in multiaxial impact configuration

The working range covers the whole speed range

from 2.2 m/s to 8.1 m/s with a maximum available

potential energy of more than 600 J.


The HIT600F provides a large working range covering different

test methods and automotive specifications.

Working range

Working range, CAI

HIT600F in CAI configuration

The work range covers the needed low energy

range from less than [email protected] m/s up to over

125J@5 m/s

Multiaxial impact,

Charpy, Izod

CAI configuration


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Test methods:

Instrumented multiaxial impact tests

Instrument Charpy and Izod tests

Finished parts testing

Important features:

Very rigid guides to allowing for side

loads occurring during the tests

Impact speed of up to 19 m/s

Possible integration of a temperature

chamber

Instrumented Drop-Weight Testers

The Amsler HIT1100F and HIT2000F cover both, materials

testing and testing of components.

Amsler HIT1100F Amsler HIT2000F


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Agenda



DWT versus other impact methods




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Polymers designed for impact applications are typically quite

ductile. Examples are PC, PMMA, PP

Fo

rce in k

N

Indentation travel in mm

Ductile PC specimen of 2 mm thickness

tested in multiaxial impact


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Travel is normally measured by

double integration of the load signal.

F = m * a

a = F/m

s = ∫∫ a dt dt

testXpert III integrates the

force/travel diagram to determine

consumed energy to any relevant

point:

Energy up to FM

Energy up to 50% FM


The surface below the stress strain curve represents the

energy being consumed by the specimen.

Fo

rce

Travel

Energy /

Work


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lM – deflection at maximum force

EM – Energy at maximum force

½ FM – detection point for overall

deflection

lP – puncture deflection

EP – Energy at puncture deflection


Several characteristic points in the force-travel diagram are

defined as the result of this test.

Fo

rce

, F

Deflection, l

FM

½ FM

lM

lP

Sta

ble

cra

ckin

g

EM EP


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These oscillations mainly result from the

natural frequency of the measurement

system.


is by half the amplitude too high

lM – deflection at maximum force

lP – puncture deflection

may strongly scatter

The influence is only little for

EM – Energy at maximum force

EP – Energy at puncture deflection


Oscillations of the signals may have some influence on the

test results, but in many cases they are not possible to avoid.

Fo

rce

, F

Deflection, l

FM

½ FM

lM

lP

EM EP

½ Amplitude


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Brittle materials show low deformation and thus the test time

is often less than 1 millisecond. Sensor oscillations may

become visible and relevant.

Fo

rce

in

kN

Indentation in mm

Brittle PBT GF specimen tested in

multiaxial impact


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Thank you for your

attention !

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