Design and repeatability analysis of desktop tool for rapid pre-cracking of notched ductile plastic fracture specimens
Introduction
To employ a standard plane-strain fracture theory to test materials which are not perfectly brittle, it is necessary to have the presence of a very sharp stress concentration in the test specimen [1], [2], [3]. Impact tests usually do not require such sharp cracks [4], [5], [6], but they are generally required for static tests such as 3-point and 4-point bending tests [7], [8], [9], notched tensile tests [10], [11], and crack tip opening displacement (CTOD) tests [12], [13]. The standard method for achieving this is to notch the specimens using a dedicated notch cutter, milling machine, or other devices, and pre-cracking the specimens. This is typically done using fatigue cycling for metal and ceramic specimens [14], but for most polymer-based materials it requires actually cutting the sharp cracks [1], [15], [3]. It is essential that the area under the crack (the area between the bottom of the pre-crack and the bottom of the specimen) should be of a uniform length in order to perform a set of valid fracture experiments with the samples. It is also very important to avoid strain-hardening around the cracked area before the tests, as this may greatly distort the results of the fracture tests.
The standards typically followed for pre-cracking of polymeric materials, ASTM D5045[16] and ISO 13586:2018 [17], suggest notching the samples and then sharpening the cracks by tapping or sawing the bottom of the crack using a new razor blade. ISO 13586:2018 provides a tolerance, where the length under the crack is required to be the same on all specimens within . The report by Kuppusamy & Tomlinson [1] describes three methods (Fig. 1) for accomplishing this for polymer-based samples, two of which use a traditional razor blade and another which applies carefully-controlled compressive loads on the samples to cause a local crack to grow in the material past the notch. Each method has its own advantages and disadvantages, primarily driven by the brittleness of the material under test. For very brittle polymer materials, the blade-base methods are generally best to avoid, as the pre-crack may continue to grow on its own, making precision-length cracks impossible and distorting fracture tests [18], [19].
In the first method, the ASTM and ISO standard recommendations are followed exactly and a razor blade is used to carefully sharpen the notch (Fig. 1a). This method is easy to apply and requires no special equipment, but is not very precise and raises safety concerns around using hand-held razor blades. Extreme care must be taken to ensure that no hard sawing pressure or heat from friction are present between the razor blade and the crack, as this would harden the area around the crack tip and distort the fracture tests. The second method (Fig. 1b) uses a device for more safely hammering a razor blade to form the pre-crack; as long as only one strike is made, sharp and consistent pre-cracks can be accomplished with this method. However, the precision of the pre-crack length and angle depend greatly on the ability of the user to deliver consistent taps to the material [1]. This mechanism design is also relatively compliant and top-heavy. The method for changing samples also appears to be slow (requiring disassembly for each sample), so it is limited for large sample batches. The third method (Fig. 1c), developed by Kuppusamy & Tomlinson [1], is to use a strategic tension-compression system to develop and grow a pre-crack in the material. As shown in their paper, it works best for brittle materials such as epoxy resin, for which the razor blade method is too hard to control. For more ductile materials, a variation of the razor blade method would be better to ensure a straight and consistent crack, one not subjected to obvious local plastic deformation that would result from the tension-compression system being used on a ductile material.
The device developed and demonstrated in this report is an improved version of the hammer-crack device described by Kuppusamy & Tomlinson (Method 2 above), with a more balanced and stiff design and adjustable end-stops to help ensure consistent crack length over numerous samples. Since the design allows pre-cracks to be made consistently with only a single tap or strike (depending on the needs of the user), the rate of work is much faster than other solutions available, offering a speed of about four specimens per minute for an experienced user. The specimen holder is also designed to be both stiff and facilitate rapid exchange of the samples during batch pre-cracking. A microscopic inspection station can easily be set up nearby to inspect and measure the cracks as they are made. The razor blade holder is designed for easy and regular changing of the blades to ensure good-quality pre-cracks.
To test the performance and reliability of the device, 80 tests were completed using molded and 3-D printed acrylonitrile butadiene styrene (ABS). A set of 40 molded ABS and 40 3-D printed bars ( were notched and pre-cracked, followed by careful examination under a microscope to find the reliability and consistency of the device. After examination and measurement of the cracks, they were broken as a strategy to study the consistency and straightness of the crack fronts. The razor-blade was changed every 25 samples to ensure that the edge of the blade is sharp during cracking. The details and results are presented in Section 3; the results showed that method may be used reliably for any ductile or semi-ductile polymeric materials, including both molded and 3-D printed samples. Due to the use of the razor blade and hammer to make the pre-crack, it is not recommended that this technique be used for very brittle materials.
Section snippets
Tool design
The tool described in this paper is designed to function in a similar way as the hammered razor blade device described by Kuppusamy & Tomlinson [1], with several improvements to make it suitable to produce fast and reliable pre-cracks for/in ductile materials. These include (in no particular order):
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The device is designed so that only a single strike from the hammer is used to create a pre-crack of the desired depth. This prevents the local strain hardening and crack non-verticality discussed in
Height adjustment and setup
The height of the end stops should be calculated prior to any use to ensure that they are adjusted to the correct height. The height can be measured with blocks or calipers, as desired by the user. Referring to the measurements shown in Fig. 4, the heights should be calculated in order to properly adjust the end-stop screws.
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The height of the material under the machined notch (non-cracked) is represented by D. The proper establishment of this measurement should be done by adjusting the springs
Repeatability analysis
When assessing the performance of pre-cracking devices, there are four essential evaluations that must be made. These are presented in this study to demonstrate the value and usefulness of this device. These are, in no particular order:
- 1.
Consistency of the crack length or length of material under the crack between the two sides of the sample; in a perfectly-tuned machine, they should be identical, but there will always be some variability. Compliance with the requirements of ISO 13586:2018 is
Conclusions
In this technical note, the design and consistency verification were completed on a simple, desktop device to rapidly and consistently pre-crack notched ductile plastic fracture testing samples. A series of 80 tests were completed with it, showing that it did an excellent job of producing consistent and straight cracks in both molded ABS and two orientations of 3-D printed ABS. This device will be useful in a commercial or lab setting where many fracture testing specimens are needed for static
Technical drawings and data
The raw data from the repeatability tests and all technical drawings and specifications for the described device are archived and available upon request from the corresponding author.
Acknowledgments, Conflicts of Interest, and Funding
All opinions and conclusions offered in this report are solely those of the authors. No external funding was used in the production of this device, its testing, or the publication of this technical note.
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