Analysis of the Charpy and Izod Impact Tests Using Fracture Mechanics

3 ANALYSIS OF THE CHARPY AND IZOD IMPACT TESTS USING FRACTURE MECHANICS 

The analysis of fracture mechanics has been applied to the Charpy impact test by Williams, and Adams and Wu. ° They use the compliance C in their analysis. We use the stiffness k, where k = l/C. 

Note that 9 is a function of stiffness (or compliance) which in turn is a function of the material and geometry. Experimental results show that when for the Charpy test and for the Izodtestis plotted against 2b Q for ductile polymers like PE and PP a linear relation is found where the slope is G. This shows that the fracture mechanics relations apply to these tests even for polymers exhibiting some ductile yielding. Why is this It is because the stiffness k of the specimens is largely dependent on the elastic portion of the beam specimen and much less dependent on the small yield region near the crack. 

Results also show for the Charpy test that unstable brittle fracture (dG/da 0) occurs for crack lengths from initiation to a/h = 0. 2 where h is the depth of the beam specimen. Unstable fracture with arrest at alh = 036 occurs for 0.13 a/ft 0.36 and stable fracture occurs for a/h 0.36. (This is an important result for small a/h. Recall that for NDE and quality control, inspection for flaws and voids is important. From Equation 5.22, K =ovra so that more stress can be applied for smaller cracks, but that the results here show that the failure will be more brittle and catastrophic which is bad for pressure vessels or piping under pressure.) 

Impact fatigue tests have also been conducted on some polymers namely Nylon 66, HIPS acrylonitrile butadiene styrene (ABS), and rubber-toughened Nylon. The impact energy was 98% of the crack initiation energy. Specimens withstood 100-200 cycles of loading before failure. 

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