Abstract
In other studies dynamic fracture toughness has been measured using Charpy impact and modified Hopkinson Bar techniques. In this paper results will be shown for the measurement of fracture toughness using a new test geometry. The crack propagation velocities range from ∼0.15 mm/ st o 2. 5m /s. Digital image correlation (DIC) will be the technique used to measure both the strain and the crack growth rates. The boundary of the crack is determined using the correlation coefficient generated during image analysis and with interframe timing the crack growth rate and crack opening can be determined. A comparison of static and dynamic loading experiments will be made for brittle polymeric materials. The analysis technique presented by Sammis et al .( 1) is a semi-empirical solution, however, additional Linear Elastic Fracture Mechanics analysis of the strain fields generated as part of the DIC analysis allow for the more commonly used method resembling the crack tip opening displacement (CTOD) experiment. It should be noted that this technique was developed because limited amounts of material were available and crack growth rates were to fast for a standard CTOD method.
Highlights
Evaluation of the deformation data indicated that a linear elastic fracture mechanics (LEFM) solution could be used to find the unique solution for fracture toughness
Typical load displacement responses for the two materials in this study are shown Fig. 10. From these plots it can be concluded that the test results are reproducible with acceptable differences in their loading behavior for all loading rates. These results indicate that failure occurs at nominally the same load level independent of rate
The compression-fracture test geometry is suitable for measuring the fracture toughness of the materials in question and coordinated testing and evaluation techniques using high speed imaging, digital image correlation, and standard data acquisition from the load frames allowed for a calculation of fracture toughness using the semiempirical formula and LEFM with the only limitation of the analysis being subject to the capabilities of the equipment available
Summary
The primary reason for this investigation was to determine the most accurate means of measuring the effect of aging in polymeric materials and polymer bonded composites. The characterization technique used to measure displacement and strain in these experiments is the digital image correlation (DIC). Since the strain from the DIC technique does not have any machine compliance, it very accurately shows the early loading stages It is essentially like putting a strain gage of variable size or multiple strain gages on the test specimen. Further details on the stress analysis for the “compression-fracture” geometry will be described in the section on sample geometry Another key characteristic of the DIC technique is the ability to detect a breakdown in image quality. The definition of a crack in this study is defined as the value when “image correlation” begins to break down It is through this correlation breakdown that we can measure both the crack length and the crack growth rates. Both measurements are useful and necessary in in order to calculate fracture toughness
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