Instrumented Impact Testing

Transcription

1 Prüfen mit Verstand Instrumented Impact Testing Helmut Fahrenholz

2 Agenda Why impact testing? Typical DWT standards Impact methods Energy, speed and mass in DWT Results evaluation acc. standards 2

3 Why impact testing? This is a way of testing football helmets back in 1912 We can offer better solutions more reliable less painful Picture source: 3

4 Why impact testing? There are many good reasons: for example in Automotive 4

5 Why impact testing? in the aircraft industry,. 5

6 Why impact testing? in the building industry,. PMMA roofings Pipe systems 6

7 Why impact testing?.. as well as in many situations of our daily life. Eye protection googles Safety screens Safety shoes Body protection 7

8 Why impact testing? Polymers are viscoelastic. Therefore their mechanical behavior changes strongly with the strain-rate applied. 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 8

9 Why impact testing? Zwick covers all relevant strain-rate ranges, from creep to quasistatic Creep (Messphysik) Static - Zwick 9

10 Why impact testing? up to high speed impact test methods. Pendulum Impact (Zwick, US) Drop Weight (Zwick Taicang) Hydraulic High Speed (Zwick UP) 10

11 Agenda Why impact testing? Typical DWT standards Impact methods Energy, speed and mass in DWT Results evaluation acc. standards 17

12 Pendulum Impact Zwick s HIT pendulum impact series - a complete product range for impact testing Charpy Izod 5 Joule ISO Notch cutting machine tensile impact Dynstat 5.5 / 25 / 50 Joule universal, digital Manual notch cutter Instrumentation Automation 18

13 Pendulum Impact Basic principle 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

14 Pendulum Impact Basic principle 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 20

15 Pendulum Impact Conventional method In the conventional method, impact resilience is measured as single-point data from the angle of raise of the pendulum. E m h g energy mass of the pendulum hammer drop height gravity acceleration (9,81 m/s²) E Specimen = m g (h1 h2) E 1 = m g h 1 h 1 h 2 E 2 = m g h 2 21

16 conventional hammer instrumented hammer Instrumented Pendulum Impact Testing 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 23

17 Instrumented Pendulum Impact Testing For tensile-impact, the force is measured at the level of the grip, which is part of the vice. 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. 24

18 The force-travel diagram provides supplementary materials data obtained under high deformation rates. F E P = F s E specimen E energy F force s travel E specimen 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. s 25

19 Pendulum Impact - Tests and Curves Several points in a travel-deflection diagram are characteristic for instrumented Charpy tests F M maximum force The specimens natural frequency has a squareroot function with the materials tensile modulus s M deflection at maximum force F I First impact maximum No contact between pendulum hammer and specimen 26

20 Force in N Pendulum Impact - Tests and Curves The curve directly after break shows the resonance frequency of the measurement system. No measurement beyond this frequency is possible. Resonance 3 x natural frequency of the specimen This part of the curve shows the natural frequency of the measurement system 27

21 Pendulum Impact - Tests and Curves Zwick developed an automatic recognition of test curve types according to ISO 179 part 2 in collaboration with Borealis. no break partial break tough break brittle break spall 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. Hinge break Complete break types 28

22 F [N] F [N] F [N] Pendulum Impact - Tests and Curves 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. f 1 f 2 A pl f i F ma x F gy A pl f gy A el f ma x f [mm] F ma x F gy Elastic materials behavior without and including crack propagation energy A pl f gy A el A R f ma x f [mm] A el f [mm] A el 29

23 Pendulum Impact - Tests and Curves The resilience (energy) can still be obtained by the conventional method or by integration of the force deflection curve. F E specimen = F s E specimen = F s ds E energy W work (energy) F force s specimen deformation E specimen 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. s 30

24 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) 31

25 Agenda Why impact testing? Typical DWT standards Impact methods Energy, speed and mass in DWT Results evaluation acc. standards 34

26 Instrumented Drop-Weight Testers The products range from single purpose instruments to universal drop weight testers, covering materials characterization and parts testing. HIT230F Multiaxial HIT230F CAI HIT600F CAI HIT600F Multiaxial HIT1100F HIT2000F H= 2600mm Mass Min= 23.5kg V1= 4.40m/s E= 230J H= 2600mm Mass Min= 2.02kg Mass Max= 10.08kg V1= 4.40m/s E= 120J H= 3150mm Mass Min= 2.04 kg Mass Max= 10.03kg V1= 5m/s E= 125J H= 3150mm Mass Min= 4.43kg Mass Max = 40.43kg V1= 5m/s V2= 4.43kg E= 647J Remark: H=Machine Height, V1= Max Velocity without acc, V2= Max Velocity with acc, E= Max Energy H= 3270mm Mass Min= 9.27kg Mass Max= 29.42kg V1= 4.4m/s V2= 9.27kg E= 1126J H= 3870mm Mass Min= 9.27kg Mass Max= 29.42kg V1= 5.42m/s V2= 9.27kg E= 2044J 35

27 Energy, speed and mass in DWT The available potential energy is generated by the mass and the drop height. The height determines the impact speed. E pot = m g H 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. H E pot = E kin m g H = ½ m v² E kin = ½m v² The impact speed only depends on the drop height. v = (2gH) 36

28 Impactor Energy, speed and mass in DWT Standards require that the speed drop during impact remains less than 20% of the initial impact speed 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 23kg giving 230 J of potential Energy, it is possible to test specimen where the consumed energy is up to 83 J. Specimen speed 4,4 m/s Typical values for 2 mm plastics are below 50 J. travel > 0.8 * 4,4 m/s 37

29 2,2 m/s 4,43 m/s 10 m/s Energy in J Energy, speed and mass in DWT For a given specimen we need a defined minimum kinetic energy at a fixed speed. 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 Min required Energy ENERGY / Impact SPEED Both axes are logarithmic! Speed, m/s 38

30 Instrumented Drop-Weight Testers, HIT230F The Amsler HIT230F are single purpose drop-weight testers. Purpose: Multiaxial impact tests Standards: ISO , 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 Impactor and clamps Brittle and ductile specimen Amsler HIT230F in multiaxial impact configuration 40

31 Instrumented Drop-Weight Testers, HIT600F The Amsler HIT600F is a universal drop-weight tester for materials testing, covering many standards and methods. 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 Instrumented Charpy and Izod tests CAI tests at variable and low impact work including second impact prevention 43

32 Instrumented Drop-Weight Testers, HIT600F The HIT600F provides a large working range covering different test methods and automotive specifications. Multiaxial impact, Charpy, Izod CAI configuration Working range Working range, CAI 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. 44 HIT600F in CAI configuration The work range covers the needed low energy range from less than 10J@2.2 m/s up to over 125J@5 m/s

33 Instrumented Drop-Weight Testers The Amsler HIT1100F and HIT2000F cover both, materials testing and testing of components. 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 Amsler HIT1100F Amsler HIT2000F 45

34 Agenda Why impact testing? Typical DWT standards DWT versus other impact methods Energy, speed and mass in DWT Results evaluation acc. standards 47

35 Force in kn Results evaluation acc. standards Polymers designed for impact applications are typically quite ductile. Examples are PC, PMMA, PP Ductile PC specimen of 2 mm thickness tested in multiaxial impact Indentation travel in mm 48

36 Force Results evaluation acc. standards The surface below the stress strain curve represents the energy being consumed by the specimen. 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 / Work Energy up to FM Energy up to 50% FM 49 Travel

37 lp Stable cracking Force, F lm Results evaluation acc. standards Several characteristic points in the force-travel diagram are defined as the result of this test. FM maximum force 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 FM ½ FM EM EP 50 Deflection, l

38 lp lm Force, F Results evaluation acc. standards Oscillations of the signals may have some influence on the test results, but in many cases they are not possible to avoid. These oscillations mainly result from the natural frequency of the measurement system. FM maximum force is by half the amplitude too high lm deflection at maximum force lp puncture deflection may strongly scatter FM ½ FM ½ Amplitude The influence is only little for EM Energy at maximum force EP Energy at puncture deflection EM EP 51 Deflection, l

39 Force in kn Results evaluation acc. standards Brittle materials show low deformation and thus the test time is often less than 1 millisecond. Sensor oscillations may become visible and relevant. Brittle PBT GF specimen tested in multiaxial impact Indentation in mm 52

40 Thank you for your attention! 53