Bar impact testing

ASTM E23-93, "Notched Bar Impact Testing of MetaUic Materials," Annual Book of ASTM Standards, ASTM Puhhcations, Philadelphia, 1993. ASTM D256-92, "Impact Resistance of Plastics and Electrical Materials," Annual Book of ASTM Standards, ASTM Puhhcations, Philadelphia, 1993. A. N. Atiuri and T. Nishioka, Int. / fract. 27, 245 (1985). 

The bar impact test, which is a variation of the plate impact test, produces a one-dimensional compressive square stress pulse which neglects the effects of lateral inertia caused by Poisson s effect. This testing procedure was developed in the laboratory of the author and his colleague,30 and is described here in some detail. 

Some specifications require a minimum Charpy v notch impact requirement of 15 ft lb energy absorbed at the minimum expected service temperature. However, this does not mean that a test specimen exhibiting 60 ft lb is four times tougher than the minimum. The main value of notched bar impact testing is as a criterion for acceptance of materials where reliable correlation with service behavior has been obtained. 

The Charpy U-notch test refers to the ASTM E23 [12], Standard Test Methods for Notched Bar Impact Testing of Metallic Material. The test was used to determine fracture energy. A Tinius Olsen Change-O-Matic impact testing machine was used in this test. 

ASTM E23, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials(2009)... 

ASTM (2007a), Standard Test Methods for notched bar impact testing of metallic materials, E 23-07, American Society for Testing and Materials International, West Conshohocken, PA. 

Impact resistance is determined usiag flyer plate impact tests, long rod impact tests, Hopkinson bar tests (50), and the Hquid jet technique (51). Impact damage resistance is often quantified by measuring the postimpact strength of the ceramic. 

The study on commercial HDPE samples could not provide a correlation of the izod impact test with the field performance test, i.e., drop impact resistance on moulded products [113]. It was found that the sample of highest density and lowest izod impact strength passed the drop impact test, but other samples of lower density and higher izod impact strength could not withstand shock loading by drop impact and failed in brittle manner. This may be due to the fact that velocities and modes of loading vary widely in different impact tests. It has been reported that even the qualitative agreement between the different impact tests is poor because the test bars and moulded products often have different orientation characteristics, particularly near the surface [115]. 

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). 

L. R. Deobald and A. S. Kobayashi A Bar Impact Tester for Dynamic Fracture Testing of Ceramics and Ceramic Composites, Experimental Mechanics, 32, 109-116 (1992). 

In the notched Izod impact test, with the exception of some rubber modified plastics, the fracture surface of a specimen that has failed ductilely exhibits significant inward collapsing on the sides of the impact bar near the notch, indicating that a large amount of plastic flow has... 

PC/PE. In the case of PC/PE, plane strain alone does not produce significant changes in the yield stress and the deformation behavior. Its yield locus in the tension-tension quadrant is therefore either very nearly a quarter circle or similar to a Tresca locus. The exact shape of the locus can be determined only by much more elaborate biaxial tests. This material is not very notch sensitive compared with PC. The energy to break in a notched Izod impact test is 15 ft-lb/inch for Vs-inch thick bars and 11 ft-lb/inch for 4-inch bars whereas for PC the latter figure is about 2 ft-lb/inch. This reduction in notch sensitivity over pure PC appears to be related to the material s ability to void internally, probably relieving the plane strain. 

Impact and Tensile Testing. A number of mechanical tests were carried out on test bars cut from the molded sheets. For the impact tests six sample bars were cut for each mixing and composition condition while for the tensile tests four test specimens were prepared. The reported results are average values as determined from that number of test bars. The tensile tests were run at a crosshead speed of 0.2 inch/min. Table I lists the results for the impact and tensile tests of the dry blended mate-... 

Izod. An impact-testing device of the notched-bar type. 

From the physical point of view, the Izod equipment is equivalent to that of Charpy. However, the main differences between the two are the clamping system, the notch, the hammer speed and its weight. Charpy adopted the keyhole form of notch. At such an early stage in the history of impact testing, Charpy found that correlations between static and dynamic tests were obtainable provided a notched bar was used. In 1925, the Izod and Charpy tests were extended to plastic materials and many results on plastics were pub-hshed a year later [Werting, 1926]. 

The growth of fracture mechanics has placed greater emphasis on tests that use sharply notched specimens. These results were found to provide more fundamental information. Instrumented impact testing is a recent development that provides information on force-deflection curves. In addition to these notched bar tests, extensive use of falling dart tests is also being made. However, interpretation of the data from the latter is far from straight... 

The test may be carried out on plain rectangular bars, but most often a carefully defined notch is molded or machined into the face to be struck (Figure 12.8). The impact tests are often regarded as a means of assessing the resistance of a material to shock where notches or stress raisers generally are present. The ratio of impact strength of unnotched to that of notched specimen is sometimes regarded as a measure of the notch sensitivity of a material. 

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