Testing Charpy impact strength of polymeric materials.

Nita, Alexandra ; Opran, Constantin

1. INTRODUCTION

Charpy test on polymeric material is an impact test to a suddenly applied force, which measure the resistance to failure of a material. The Charpy test measures the impact energy or the energy absorbed prior to fracture. We measure the impact energy for a test specimen made by molded polymeric material, called polypropylene (PP). In this study, the authors want to remark the impact performance and the behaviour of the molded parts.

The most commonly Charpy test is used by other researchers on metals, but it is also used on polymers, ceramics and composites. The Charpy test evaluate the relative toughness or impact toughness of materials and as such is often used in quality control applications where it is a fast and economical test (Opran et al., 2004).

We used the method described in ASTM Standard D 6110 to study the impact behaviour on specimen made by molded polymeric material. When the striker impacts the specimen, the specimen absorbed the energy until it yields. At that point, the specimen began to undergo plastic deformation. The test specimen continued to absorb energy and worked hardens at the plastic zone. At the moment when the specimen couldn't absorb more energy, fracture occurred.

We observe that materials behave very differently at high rates of loading and for that we cannot use static strength tests to predict impact behaviour. Regarding the author's further research is to measure the impact on different polymeric material and to make a comparative study between them.

2. MATERIALS AND TESTING EQUIPMENT

Impact performance can be one of the most important properties for a component designer and also the most difficult to quantify. In our case, tests were done on polymeric material made from PP and the specimens measure 80 [+ or -] 0,2, 10 [+ or -] 0,2, 4,9 [+ or -] 0,1 (mm), with respect to ASTM D 6110 or SR EN ISO 179 Charpy plastics testing (Hylton, 2004).

The testing machine that we used is an Instron Dynatup Impact System with Data Acquisition and Control, model 8200. The Dynatup Model 8200 Impact Test Instrument meets the need for a small mass drop weight impact test instrument with a wide range of adjustable energies and velocities. The 8200 is ideal for low energy testing of thin section or brittle plastics, composites, ceramics, and metals. Designed for use with Dynatup tups and data acquisition systems, the model 8200 can be equipped with the appropriate options to perform dart-penetration ASTM D 6110 Charpy plastics testing.

[FIGURE 1 OMITTED]

The system includes a support table for testing large or odd-sized specimens. The impact testing system have: maximum gravity mode velocity up to 5,0 m/s, maximum spring assisted high velocity up to 20 m/s, maximum physical drop height of 1,25 metres, self-id load cell for measuring drop mass (figure 1).

The method is applicable to specimen without notch. The specimen is placed horizontally on two supports, and it is the subject to disruption by a striker one-shot, applied at a distance equal supports (Opran et al., 2008).

3. EXPERIMENTAL RESULTS

The system used is a fully-integrated electronics and software package that increases impact testing productivity through automated data acquisition, analysis and reporting. Impulse utilizes an impact force transducer and falling mass velocity detector to capture load vs. time information from instrumented impact tests. The table 1 presents the experimental results collected for twelve specimens from molded polypropylene material.

In the table 1 you can observe that the break type is evident for the height of impact, which in our case is from h=30mm on specimen 1 to h=85mm on specimen 12.

The Impulse console is designed to provide intelligent test setup and control with a very flexible interface. This controller displays real-time data while providing access to test set-up controls. Digital displays tell the user exactly what the current settings are, including test drop height, velocity, and impact energy (Instron, 2006).

Data collected by the Impulse system was organized, analyzed and displayed both graphically and numerically based on PC software. Analysis options include automatic yield and failure point calculations, as well as digital filtering to screen out load cell resonances and noise. Test data could be exported to spreadsheets and charts, as you can see in table 2. In the charts presented in the table 2 the blue variation is the energy, E in [Kgm] and the red variation is the load, F in [kN].

The figure 2 presents the photograph taken with the polypropylene specimens with thickness 4,9 [+ or -] 0,1 (mm) used to determine the resistance to impact, after we made the Charpy tests.

[TABLE 2 OMITTED]

[FIGURE 2 OMITTED]

4. CONCLUSION

Break type of evidence is amended according to the drop height as to h = 30 to 80, mm no breakage, for h = 85 mm hinge breaking. Standard test method such as Charpy is an important tool for raw material research and quality control.

In the next figure 3, you can observe the breaking occurrence for polypropylene specimens tested to Charpy impact, where W is the total energy used to break the polymeric specimen and h the drop height. The total energy, W [J] accumulates over time and increased to achieve a level of constancy, then it is absorbed in the polymeric material.

[FIGURE 3 OMITTED]

5. ACKNOWLEDGEMENTS

The research performed for this paper vas financed by the research project Nr.237/12.10.2004, Politehnica University of Bucharest, Acronym CELAPCOM, National programme INFRAS. (Opran et al, 2004-2006).

6. REFERENCES

Hylton D. C. (2004). Understanding Plastic Testing. Hanser Publishers, Munich Hanser Gardner Publications, Cincinnati, ISBN 1-56990-366-2, Munchen

Opran, C; Blajina, O & Marinescu, A (2008). Researches concerning the behaviour at impact of the polymeric composite structures type omega, Proceedings of "Advanced composite materials engineering and advanced in human body protection to vibrations", Vol. 1A, pp 272-278, ISSN 1844-0336, Transylvania University of Brasov

Opran, C; Marinescu, A & Blajina, O (2008). Researches concerning the behaviour at impact of the polymeric composite sandwich structures type plaque, Proceedings of "Advanced composite materials engineering and advanced in human body protection to vibrations", Vol. 1A, pp 266-271, ISSN 1844-0336, Transylvania University of Brasov

Opran, C; Vasile, N; Racicovschi, V; Pencioiu, P; Pauna, I; Casariu, M & Mohan, G (2004). Biostructuri polimerice degradabile in mediu natural, Vasile Goldis University Press, ISBN 973-664-041-8, Arad

Instron (2006). Dynatup Drop Weight Impact Test Machine, Impact Testing Solutions Brochure, pod_8200_rev5_0606, Available from http://www.instron.com, 2006-06-06

Tab. 1. Experimental results

 Specime Peak Deflection Energy
 n drop load at peak to max
 height [kN] load load
 h [mm] [mm] [kgm]

30 0,1524 10,1548 0,1033
35 0,1475 10,5005 0,1010
40 0,1523 11,8337 0,1232
45 0,1499 11,2487 0,1179
50 0,1690 10,2506 0,1168
55 0,1690 9,7446 0,1083
60 0,1716 9.6870 0,1117
65 0,1765 9,5657 0,1136
70 0,1692 9,2399 0,1076
75 0,1789 10,0323 0,1260
80 0,1741 9,5603 0,1179
85 0,1790 9,8975 0,1205

 Specime Impact Total Total
 n drop velocity energy energy
 height v [m/s] E [kgm] W [J]
 h [mm]

30 0,8273 0,1167 1,1441
35 0,8402 0,1177 1,1539
40 0,8877 0,1397 1,3696
45 0,9890 0,1652 1,6196
50 1,0238 0,1971 1,9324
55 1,0416 0,2012 1,9725
60 1,1090 0,2278 2,2333
65 1,1357 0,2404 2,3569
70 1,2068 0,2545 2,4951
75 1,2275 0,2727 2,6735
80 1,2821 0,2827 2,7716
85 1,3299 0,3061 3,0010