Fracture testing is a useful mechanical testing process to explore the properties and behavior of materials, one that has seen much development and re nement in recent decades. One of the most important steps in preparing samples for testing is the production of a sharp pre-crack to initiate crack propagation in a predictable way. While several methods have been developed for doing this, particularly for metals and brittle plastic materials, a quick and reliable method for more ductile materials is lacking. This technical note describes the design and veri cation of a simple desktop-sized pre-cracking device which safely uses a razor blade and hammer to quickly produce straight and sharp pre-cracks of consistent depth in ductile polymeric material samples. To verify its capability and consistency, a series of tests were performed using both molded and 3-D printed acrylonitrile butadiene styrene (ABS). First, a series of 40 notched 25 mm9:5mmABS bars were pre-cracked, and the distance under the crack measured on both sides of the bar. Several bars were then broken along the cracks to examine the quality of the pre-crack front. These tests were then repeated 20 times each for two print orientations of fused deposition modeling (FDM) ABS printed at 100% density. All 80 pre-cracks were found to be straight, sharp, and within 1% of the nominal distance under the crack for all samples. The consistency of the pre-cracks throughout the sample cross-section was also observed to be excellent, with all 80 tests showing less than 0:25mmof deviation, even on the highly-anisotropic FDM samples. Keywords: Fracture mechanics, polymer testing, pre-cracking 1. Introduction 1 To employ a standard plane-strain fracture theory to test materials which are not per- 2 fectly brittle, it is necessary to have the presence of a very sharp stress concentration in the 3 test specimen [1, 2, 3]. Impact tests usually do not require such sharp cracks [4, 5, 6], but 4 they are generally required for static tests such as 3-point and 4-point bending tests [7, 8, 9], 5 notched tensile tests [10, 11], and crack tip opening displacement (CTOD) tests [12, 13]. 6 The standard method for achieving this is to notch the specimens using a dedicated notch 7 Preprint submitted to TBD June 12, 2019 Preprintcutter, milling machine, or other devices, and pre-cracking the specimens. This is typically 8 done using fatigue cycling for metal and ceramic specimens [14], but for most polymer-based 9 materials it requires actually cutting the sharp cracks [1, 15, 3]. It is essential that the area 10 under the crack (the area between the bottom of the pre-crack and the bottom of the speci- 11 men) should be of a uniform length in order to perform a set of valid fracture experiments 12 with the samples. It is also very important to avoid strain-hardening around the cracked 13 area before the tests, as this may greatly distort the results of the fracture tests. 14 The standards typically followed for pre-cracking of polymeric materials, ASTM D5045 [16] 15 and ISO 13586:2018 [17], suggest notching the samples and then sharpening the cracks by 16 tapping or sawing the bottom of the crack using a new razor blade. ISO 13586:2018 provides 17 a tolerance, where the length under the crack is required to be the same on all specimens 18 within 0 :5mm. The report by Kuppusamy & Tomlinson [1] describes three methods (Fig- 19 ure 1) for accomplishing this for polymer-based samples, two of which use a traditional razor 20 blade and another which applies carefully-controlled compressive loads on the samples to 21 cause a local crack to grow in the material past the notch. Each method has its own advan- 22 tages and disadvantages, primarily driven by the brittleness of the material under test. For 23 very brittle polymer materials, the blade-base methods are generally best to avoid, as the 24 pre-crack may continue to grow on its own, making precision-length cracks impossible and 25 distorting fracture tests [18, 19]. 26 Figure 1: (a) Tapping/sawing with razor blade, (b) Hammered razor blade device, and (c) Ten- sion/compression device developed by Kuppusamy & Tomlinson (All gures from [1], published under CC-BY 4.0 license) In the rst method, the ASTM and ISO standard recommendations are followed exactly 27 and a razor blade is used to carefully sharpen the notch (Figure 1a). This method is easy to 28 apply and requires no special equipment, but is not very precise and raises safety concerns 29 around using hand-held razor blades. Extreme care must be taken to ensure that no hard 30 sawing pressure or heat from friction are present between the razor blade and the crack, as 31 this would harden the area around the crack tip and distort the fracture tests. The second 32 method (Figure 1b) uses a device for more safely hammering a razor blade to form the pre- 33 crack; as long as only one strike is made, sharp and consistent pre-cracks can be accomplished 34 with this method. However, the precision of the pre-crack length and angle depend greatly 35 2 Preprinton the ability of the user to deliver consistent taps to the material [1]. This mechanism 36 design is also relatively compliant and top-heavy. The method for changing samples also 37 appears to be slow (requiring disassembly for each sample), so it is limited for large sample 38 batches. The third method (Figure 1c), developed by Kuppusamy & Tomlinson [1], is to use 39 a strategic tension-compression system to develop and grow a pre-crack in the material. As 40 shown in their paper, it works best for brittle materials such as epoxy resin, for which the 41 razor blade method is too hard to control. For more ductile materials, a variation of the razor 42 blade method would be better to ensure a straight and consistent crack, one not subjected 43 to obvious local plastic deformation that would result from the tension-compression system 44 being used on a ductile material. 45 The device developed and demonstrated in this report is an improved version of the 46 hammer-crack device described by Kuppusamy & Tomlinson (Method 2 above), with a more 47 balanced and sti design and adjustable end-stops to help ensure consistent crack length 48 over numerous samples. Since the design allows pre-cracks to be made consistently with 49 only a single tap or strike (depending on the needs of the user), the rate of work is much 50 faster than other solutions available, o ering a speed of about four specimens per minute 51 for an experienced user. The specimen holder is also designed to be both sti and facilitate 52 rapid exchange of the samples during batch pre-cracking. A microscopic inspection station 53 can easily be set up nearby to inspect and measure the cracks as they are made. The razor 54 blade holder is designed for easy and regular changing of the blades to ensure good-quality 55 pre-cracks. 56 To test the performance and reliability of the device, 80 tests were completed using 57 molded and 3-D printed acrylonitrile butadiene styrene (ABS). A set of 40 molded ABS 58 and 40 3-D printed bars (25 mm9:5mm) were notched and pre-cracked, followed by 59 careful examination under a microscope to nd the reliability and consistency of the device. 60 After examination and measurement of the cracks, they were broken as a strategy to study 61 the consistency and straightness of the crack fronts. The razor-blade was changed every 62 25 samples to ensure that the edge of the blade is sharp during cracking. The details and 63 results are presented in Section 3; the results showed that method may be used reliably 64 for any ductile or semi-ductile polymeric materials, including both molded and 3-D printed 65 samples. Due to the use of the razor blade and hammer to make the pre-crack, it is not 66 recommended that this technique be used for very brittle materials. 67 2. Tool Design 68 The tool described in this paper is designed to function in a similar way as the hammered 69 razor blade device described by Kuppusamy & Tomlinson [1], with several improvements to 70 make it suitable to produce fast and reliable pre-cracks for/in ductile materials. These 71 include (in no particular order): 72 The device is designed so that only a single strike from the hammer is used to create 73 a pre-crack of the desired depth. This prevents the local strain hardening and crack 74 non-verticality discussed in the ASTM and ISO standards. 75 The center of gravity is made as low as possible to make it stable during hammer 76 tapping (Figure 2a). 77 3 PreprintThe device is designed with long guide rods to allow pre-cracking of tall specimens if 78 needed, without disassembling the hardware (Figure 2a). 79 The device is designed to use a 680-gram plastic dead-blow hammer for better operator 80 control (Figure 3b). 81 The device is designed to be sti to enable accurate and repeatable operations (Fig- 82 ures 2a and b). 83 The device uses adjustable end-stops to ensure that the distance between the bottom 84 of the blade and the bottom of the sample remain at a constant height throughout the 85 tests (Figure 2a). 86 The device can successfully use either a calibrated-weight balancing bar or adjustable 87 nuts on the guide rods to ensure that the razor blade is always kept vertical (in spite 88 of any wear or compliance in the moving parts), while also ensuring that the blade is 89 held just above the machined notch and without putting pressure on it (Figure 3d). 90 The specimen holder was replaced by a machined slot in the base that was of the 91 same width as the specimens to be pre-cracked; this holds the samples more securely, 92 while also speeding up the exchange of samples signi cantly during batch-processing 93 (Figures 2a and b). 94 An ecient system for exchanging the blades at periodic intervals is also developed 95 (Figure 3c). 96 4 Preprint Figure 2: (a) Basic tool design with parts shown, (b) Parts list, and (c) Working prototype The basic design of the tool is shown in Figure 2, along with a parts list. The tool 97 consists of four main parts: the base, the blade holder, the balancing weight, and the dead- 98 blow hammer. Figure 2c shows the working prototype of the device and all of its parts 99 except for the dead blow hammer. The standard hardware (nuts, etc.) were purchased by 100 the authors, with the rest fabricated from scratch. It was manufactured primarily out of 101 scrap materials and required approximately six hours to machine, assemble, and tune at a 102 cost of about $35 US for materials. Those costs were primarily the springs, hammer, sliding 103 rods/nuts, end-stop and blade-holder bolts, razor blades, and various drill bits and taps that 104 were not already available in the lab tool set. Tools needed for production included a milling 105 machine with a 3 =8-inch TiN end mill, a drill press with bits, a tap-and-die set, pliers, and 106 an Allen-key set. 107 5 Preprint Figure 3: (a) Typical specimen example, (b) Dead blow hammer, (c) Operation of the blade holder, and (d) System with stabilizer bar mass installed The razor blade holder was designed to facilitate steady and consistent pre-cracking of 108 samples, while also allowing fast and easy changing of the blades with minimal recalibration 109 requirements. It was noted in preliminary tests that the steel razor blades wore into the 110 aluminum sample holder, distorting the results over time and damaging the holder. This 111 was solved by installing a 6 mm steel back plate (Figure 3c) for the blade to rest against 112 during operation and blade changes, with the blade clamp then being installed over the 113 blade. This provided a strong base for the blades and allowed fast blade exchange without 114 needing to recalibrate the end stops for the holder. The backstop and clamp should be M6 115 square steel stock cut to t into the cut groove and to hold the hole pattern as shown in 116 the drawing. The technical drawings of the rig can be obtained upon request by contacting 117 corresponding author. 118 It was also noted that there was some compliance in the device, especially between the 119 blade holder and its guide rods, which could result in the blade not being applied perfectly 120 vertical during the hammering. While this was addressed primarily by adjusting the guide 121 rod nuts, as shown above; this was found to be relatively slow. It was discovered, after 122 several iterations of the design, that a balancing mass (Figure 3d) did an excellent job of 123 preventing this, as it kept the moving parts centered and levelled during the operation and 124 absorbed vibration in the system. Note that this required the use of sti er springs, as it 125 6 Preprintincreased the mass of the blade holder. The mass was calibrated so that the blade sat above 126 the base of the sample notches without directly putting pressure on them, as required by the 127 ASTM and ISO standards. The design presented here is very exible and can be modi ed 128 as needed without changing the fundamental design. For example, 129 The guide rods selected were 7 =16inthreaded rods, but these may be exchanged for 130 any size that is appropriate for the desired testing abilities of the device. Smooth rods 131 may be used as well, but they will likely be more dicult to install and calibrate. 132 The machined-groove sample holder was made for samples or a certain size, but the 133 groove can be cut to be any size desired by the experimenter (both width and depth 134 of cut). It should be noted, however, that e orts should be made to ensure that the 135 milling is precise and that the groove is not wider than the sample, as this will result 136 in less stability for the samples. The depth selected for the prototype shown here was 137 10mm, but it could have been made deeper; it is not recommended that less than 25% 138 of the sample be in the groove, but it is also important to expose as much material as 139 possible to the blade. 140 Di erent types of hammer, springs, and razor blade are all usable with this design, but 141 it is recommended that the ones shown here be used. 142 The amount of strike or tap force on the sample is dependent on the material used, 143 the strength of the operator, and the desired precrack depth. E ort should be made to 144 make the pre-crack using only one strike or tap, so this should be taken into account 145 when selecting the force to use. 146 The frequency of blade changes is a user decision and dependent on the material used. 147 3. Height Adjustment and Setup 148 The height of the end stops should be calculated prior to any use to ensure that they 149 are adjusted to the correct height. The height can be measured with blocks or calipers, as 150 desired by the user. Referring to the measurements shown in Figure 4, the heights should 151 be calculated in order to properly adjust the end-stop screws. 152 The height of the material under the machined notch (non-cracked) is represented by 153 D. The proper establishment of this measurement should be done by adjusting the 154 springs and balancing mass. 155 The height of the blade is represented by B. Any standard blade can be used, but the 156 height protruding from the holder should be known and recorded. 157 The height of the sample holder is represented by Cand should be known to the user 158 before testing 159 Arepresents the height of the retaining nuts above the end stop screws. To ensure 160 ecient operation of the device, this should be set high enough to allow the rapid 161 exchange of samples while also being low enough to maintain a low center of mass. If 162 7 Preprintthe nuts are used for each sample, their end height can be set to speed up the operation; 163 if the nuts are not used and the balancing bar is, the nuts may be set in a xed location 164 that will ensure fast operation while preventing the blade holder from ying o . This 165 setting of this is subjective and up to the user. 166 Finally, Hrepresents the needed height adjustment for a particular material length 167 under the pre-crack, and C+Hrepresents the length under the crack. 168 The length Lof the pre-crack itself can be calculated by L=D(C+H) 169 If the parameters are known, the machine can be tuned for a particular notch height 170 Z(from the base of the sample to the bottom of the machined notch) by calculating 171 Z=D+BH 172 Figure 4: Setup and height adjustments to be made before operation (shown with balancing bar, but nuts can be used as well to ensure straight and e ective operation) 4. Repeatability Analysis 173 When assessing the performance of pre-cracking devices, there are four essential evalua- 174 tions that must be made. These are presented in this study to demonstrate the value and 175 usefulness of this device. These are, in no particular order: 176 1. Consistency of the crack length or length of material under the crack between the 177 two sides of the sample; in a perfectly-tuned machine, they should be identical, but 178 8 Preprintthere will always be some variability. Compliance with with the requirements of ISO 179 13586:2018 is typically considered sucient in practice. 180 2. How straight the cracks are relative to the side view of the sample (viewed parallel to 181 the blade) 182 3. The consistency of the crack front inside of the material; the straighter the crack front, 183 the more reliable the test is from a theoretical fracture mechanics perspective. 184 To address the rst two points, a series of 80 trials was completed. No sample replacement 185 was performed and every pre-crack specimen was counted, ensuring that a true dataset could 186 be found. A set of 40 molded ABS samples with a cross section of 25 mm9:5mm was 187 used, as well as a set of 20 FDM-printed samples printed in a horizontal orientation and 20 188 printed in a at orientation (Figure 5a). The FDM samples were printed from Hatchbox 189 Blue ABS using an XYZ Printing Da Vinci Pro machine (with full enclosure) with a 0 :4mm 190 steel nozzle, a layer height of 0 :2mm, nozzle temperature of 230C, and a print speed 191 of 50 mm=s . The printing was completed on 3 :5mm epoxy-coated tempered glass sheets 192 heated to a temperature of 90C. Notching was done for all samples using a milling machine 193 with a 60V-groove router bit with a 0 :2mmtip. Only one strike was used for each sample, 194 ensuring that the surface would not be strain-hardened by pressure, heat, or multiple hits. 195 As previously suggested, the blades were exchanged every 25 samples. 196 Figure 5: (a) Print orientation of the FDM samples and (b) the notching and cracking con guration, shown with measurement points The results are shown in Figures 6 and 7. The basic con guration of the samples is shown 197 in Figure 5b, including the location and con guration of the notch and the measurement 198 location. Note that the actual depth of the pre-cracks varied somewhat, as there was some 199 variability in the height of the raw samples; therefore, the distance under the crack was 200 measured for consistency. Figures 6a and 6b demonstrate the results of the molded ABS 201 samples, showing the measured values of d1 and d2, as well as the percent error. The FDM 202 printed ABS samples behaved in a very similar way, as shown by Figures 6c through 6f. 203 Finally, Figure 6g presents a comparison between the three types of samples based on the 204 mean crack distance (i.e., the average of d1 and d2). Note that, even though the FDM 205 samples were highly anisotropic, there was very similar variation between them and the 206 molded samples. All errors were observed to be less than 1%, well within the 2 :33% error 207 allowed by ISO 13586:2018. 208 9 Preprint Figure 6: Collected repeatability analysis data for (a) molded ABS, (c) at print FDM ABS, (e) horizontal print FDM ABS, along with observed percent errors for (b) molded ABS, (d) at print FDM ABS, and (f) horizontal print FDM ABS and (g) comparison of the mean under-crack length for the cases (average of both side measurement points). All observed error in the length under the notch was well within the 2 :33% error allowed by ISO 13586:2018 10 PreprintTo examine the straightness of the cracks, the series of cracks from the three sets of 209 samples were examined under a microscope. Typical examples of cracks in the molded ABS 210 (Figure 7a), the at-printed ABS (Figure 7b), and horizontally-printed ABS (Figure 7c) are 211 shown below. Note the sharpness of the cracks and that they did not branch o after the 212 strike. This behavior is expected only in ductile or semi-ductile materials however, as these 213 kinds of cracks may continue to grow uncontrollably in a more brittle material. Most of the 214 other cracks studied look very similar and it was observed that none of the cracks deviated 215 from the vertical by more than 1. Therefore, it is clear that the presented device generates 216 straight and reliable pre-cracks. 217 Figure 7: Randomly selected ABS samples showing pre-crack front after breaking (crack front locations and precision determined by examination under a microscope) Finally, three of the pre-crack samples from each of the cases (nine samples total) were 218 selected randomly, clamped, and broken. This was done to expose the inside of the specimens 219 11 Preprintand examine the straightness of the crack fronts. While it was expected that there would 220 be some variance on how well-behaved the crack fronts were within the material, all were 221 observed to be straight and well-de ned. The molded ABS crack fronts were extremely 222 straight and uniform (Figure 7d), as expected for an isotropic material. Those for the FDM 223 samples (Figure 7e and Figure 7f) were less well-de ned, but still far superior to those 224 observed in previous work for plastic materials (e.g., those shown in [1] and [20]). The crack 225 fronts were easily observable and any deviation was localized and predictable based on the 226 obviously anisotropic structure of the material. This indicates that the presented method is 227 very reliable for pre-cracking of ductile and semi-ductile polymeric materials. 228 5. Conclusions 229 In this technical report, the design and consistency veri cation were completed on a 230 simple, desktop device to rapidly and consistently pre-crack notched ductile plastic fracture 231 testing samples. A series of 80 tests were completed with it, showing that it did an excellent 232 job of producing consistent and straight cracks in both molded ABS and two orientations 233 of 3-D printed ABS. This device will be useful in a commercial or lab setting where many 234 fracture testing specimens are needed for static and dynamic fracture tests requiring pre- 235 cracks. The production of good-quality pre-cracks in ductile materials will also help to 236 generate more consistent and reliable fracture testing results for these materials. Design 237 details were provided so that the device can be further tested, reproduced, and improved 238 in future research e orts. The design is very exible and can be adapted for a variety of 239 environments and materials. 240 Acknowledgments, Con icts of Interest, and Funding 241 All opinions and conclusions o ered in this report are solely those of the authors. No 242 external funding was used in the production of this device, its testing, or the publication of 243 this technical note. 244 Technical Drawings and Data 245 The raw data from the repeatability tests and all technical drawings and speci cations 246 for the described device are archived and available upon request from the corresponding 247 author. 248