Charpy method

Charlie CHARMm Charpy Izod impact Charpy method Charpy test CHARTEK59 Charybdotoxin Chaser mill Chatecholates... 

There are two principle methods for measuring impact and notched impact strength, which in practice differ only in the way in which the test bar is held. In the Charpy method the test piece is suspended at both ends and is struck in the center by a weighted pendulum. In the Izod method the test piece is clamped at one end only and is struck at the free end by the pendulum. 

Fig. 6.22 Illustration of Charpy method to measure the notched impact strength...

FIGURE 24.20. Relation between the stress concentration factor Kf and the impact transition temperature 7 in K for LDPE. Circles represent experimental values obtained by the Charpy method and crosses those calculated from Eq. (24.37). 

ISO 9854-1 1994 Thermoplastics pipes for the transport of fluids - Determination of pendulum impact strength by the Charpy method - Part 1 General test method. 

Undoubtedly, impact strength must feature in the selection process. The problem is that the commonly used test methods have little relevance to real situations. The Izod and Charpy methods as normally used call for specimens which are severely notched (the sort of notches which would disgrace any designer who inadvertently included them in a component ). 

The following discussion is based on results obtained from the notched Izod or Charpy methods, as these are the most widely used tests to determine impact behavior in plastics. Even with the above-stated limitations, these protocols provide a useful method for discriminating between materials with different degrees of toughness. 

The Izod test requires a specimen to be clamped vertically as a cantilever beam. The specimen is struck by a swing of a pendulum released from a fixed distance from the specimen clamp. A similar setup is used for the Charpy test except for the positioning of the specimen. In the Charpy method, the specimen is supported horizontally as a simple beam and fractured by a blow delivered in the middle by the pendulum. The obvious advantage of the Charpy test over the Izod test is that the specimen does not have to be clamped and, therefore, it is free of variations in clamping pressures. Charpy test is now preferred for testing polypropylene. 

Figure 1. Plane stress deformation in pipe-grade PE tested at 1 m/s, 23 °C using the Reversed Charpy method.

Materials selection for low-temperature service is a specialized area. In general, it is necessaiy to select materials and fabrication methods which will provide adequate toughness at all operating conditions. It is frequently necessaiy to specify Charpy V-notch (or other appropriate) qu fication tests to demonstrate adequate toughness of carbon and low-aUoy steels at minimum operating temperatures. 

Methods employed to determine the impact resistance of plastics include pendulum methods (Izod, Charpy, tensile impact, falling dart, Gardner, Dynatup, etc.) and instrumented techniques. In the case of the Izod test, what is measured is the energy required to break a test specimen transversely struck (the test can be done either with the specimen notched or unnotched). The tensile impact test has a bar loaded in tension and the striking force tends to elongate the bar (Chapter 5, Impact Strength). 

The Charpy test and the Izodtest are both pendulum-type impact tests. The difference between these two tests is essentially the orientation of the sample. In the Charpy test, or simple-beam method, the sample is supported at both ends, but is not held down, as shown on the left in Figure 15.30. In the Izod test, or cantilever beam method, the sample is supported on one end in a vice, as shown on the right in Figure 15.30. In both tests the sample is at the bottom of the pendulum arc and the amount of energy absorbed by the sample is measured. 

The test method may specify either a notched sample or an unnotched sample. If a notched specimen is specified, the dimensions of the notch are also given. Figure 15.31 is an example of these dimensions for a metal sample. In the Charpy test the notched sample is struck from behind the notch. In the Izod test the notch is facing the hammer. The notch, called a stress raiser, concentrates the stresses applied by the impact load. It makes the material brittle and increases the elastic limit in the notch area. Without the notch, many materials will bend without fracture, and the capacity to absorb energy will not be accurately determined. 

Gibson(Ref 1) investigated both methods for testing steels, such as used in manuf of shells gun barrels and gave preference to Charpy Test... 

It has been shown that fracture is a very complex process and the fracture performance depends on both the initiation and the propagation of a defect [6-10] in the material. Under impact, most polymers break in very distinct manners. Several types of fracture have been identified depending on the amount of plastic deformation at the crack tip and the stability of crack propagation. For each type, an appropriate analysis has been developed to determine the impact fracture energy of the material. These methods have also been verified in various plastics [11,12]. The different fracture behaviors in most polymers are illustrated in Figure 27.1, which shows a schematic drawing of the load-deflection diagram of Charpy tests on HIPS [13] under an impact velocity of 2 m/s at various temperatures. 

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