AU2019101006A4

AU2019101006A4 - Snap-Back Indirect Tensile Test

Info

Publication number: AU2019101006A4
Application number: AU2019101006A
Authority: AU
Inventors: Simon Golding; Murat Karakus; Giang Dinh Nguyen; Abbas Taheri; Rupesh Kumar Verma
Original assignee: University of Adelaide
Current assignee: University of Adelaide
Priority date: 2019-09-04
Filing date: 2019-09-04
Publication date: 2019-10-10
Anticipated expiration: 2027-09-04

Abstract

Abstract The patent presents an innovative methodology to stabilise instant cracking of circular disc specimens under diametrical compression. This is a challenging task and has never been done before (to the best of our knowledge) especially in testing simple circular shaped discs (e.g. Brazilian disc tests) where cracking occurs abruptly in a split-second timeframe. The invention uses lateral deformation as feedback to control the overall axial loading rate. Pairs of 3D printed plastic caps (or mounts) are designed to hold the Linear Variable Differential Transformer (LVDT) laterally across the vertical crack line of the specimen. Lateral strain feedback from the LVDT is coupled with the servo-controlled loading machine to enable capturing the post-peak response that exhibits both load and displacement reversals (snap-back). This overall framework stabilises the fracture process and allows time for strain measurements using advanced image based instrumentations. This also enables efficient measurements of intrinsic fracture properties of brittle materials such as rocks, concrete, ceramics, brittle metals. All instrumentations have been fabricated in the School of Civil, Environmental and Mining Engineering at the University of Adelaide. Tests were successfully conducted on concrete, and different rock types and results were analysed to prove the feasibility and expected performance of the invented methodology. 16 - ---- -- - -- - 17 --- - - - - - - -- - - - - - 1 MT~oadinfram 10 Ciruladisspeime 12 11 tm dga MS a s 09 9| Symbols Detail Symbols Detail 1 MTS loading frame 10 Circular disc specimen 2 Top/bottom loading plates 11 MTS-data acquisition 3 Plastic mount 12 DIC/AE data acquisition (Optional) 4 AE sensor (Optional) 13 Front side of set-up 5 Lateral LVDT 14 Back side of set-up 6 Cylinder for sample level 15 Feedback signal 7 Axial LVDT 16 Servo-control mechanism 8 DIC-camera(Optional) 17 Load control 9 DIC-light source(Optional) Figure. 1. Detailed overview of testing set-up

Description

Snap-Back Indirect Tensile Test

Objectives & Scope

Diametrical compression of circular shaped disc specimens is a widely used experimental approach developed to obtain indirect tensile strength. The ease in specimen preparation (i.e. because of simplistic circular shape) and low execution cost has made its applications popular in evaluating the tensile strength and fracture properties of rock, cement, concrete, ceramics, glass and even tablets (i.e.

o pharmaceutical industry). However, this type of compression test is very unstable with cracks occurring very abruptly in small fractions of a second due to excess strain energy stored in the specimen. Consequently, the load drops suddenly and immediately after peak, which makes it practically impossible to capture the actual post-peak response, besides making advanced image-based instrumentations impossible. The dynamic nature of this kind of failure makes direct displacement control impossible.

Thus, the invented methodology is the first attempt to enable capturing the usual snap-back behaviour (i.e. both load and displacement reversals) of the specimen. This snap-back control is essential for the correct measurement of not only tensile o strength but also surface energy released due to diametrical cracking and excess strain energy at peak. This invention also allows the applications of advanced experimental techniques like X-Ray Computer Tomography (X-Ray CT) and Digital Image Correlation (DIC) to obtain material property at local scales, thanks to the controllable post peak response that can allow sufficient time for these advanced image-based instrumentations. Overall, this invention has huge potentials to be used in a wide range of research based and commercial engineering/non-engineering organisations worldwide.

Invention descriptions

The following figures and illustrated practical excerpts have been used as references 30 to describe the invented methodology.

Figure 1: Detailed overview of testing set up along with the summary table to identify each constituting element. Here element named 1 indicates MTS loading frame (i.e. Model - C45.305E) with force capacity of 300 kN, maximum speed of 750mm/min and voltage 380-480 VAC. Element 2 signifies top and bottom loading plates. Element 3 is 3D printed plastic caps or mounts. Elements 5 and 7 are linear displacement transducers in lateral and vertical ^H directions. This special transducer has model number HS25 and uses a fully active 350 ohm strain gauge bridge to sense spindle displacement. This sensor of above HS25 model transducer allows user to directly connection it with element 1. For simplicity in further discussion, we referred this special displacement tranducer with more generic term i.e. LVDT (Linear Variable Differential Transformer). Elements 4, 6, 8, 9, 10 and 12 are advanced o experimental technique’s (including Acoustic Emission and Digital Image

Correlation) applications, which are optional depending upon the requirements. Element 10 is circular disc specimen while element 11 is MTS data acquisition system. This figure also presents a practical illustration setup via elements 13 and 14.

Figure 2: (i) Details of the 3D printed plastic caps (or mounts, i.e. element 3) along with specifications in terms of specimens’ dimensions summarised in the table, (ii) An illustration of how elements 3 and 5 are fitted to element 10.

Figure 3: Initial steps to keep element 3 clean using double-sided removable tape (i.e. element 18). This step is an optional example. Other methods to assure o clean surfaces of both plastic caps and specimen can also be taken.

Figure 4: Step to attach element 3 with element 10.

Figure 5: Ensuring smooth and uniform contact between element 2 and element 10.

Here, element 1 should have very high stiffness as compared to element 10.

Additionally, element 1 must also have an inbuilt closed-loop servo-control system (i.e. element 16) in the digitised form which can control the vertical loading as per the predefined axial or lateral strain rate. The intention here is to stabilise the instant cracking (or fracturing) process in disc specimen by maintaining a constant lateral strain rate. Element 2 must be flat and rigid, normally made up of steel. Both lateral and vertical strain across element 10 while undergoing diametrical compression should be recorded by elements 5 and 7, respectively.

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Further, element 3 has been designed and developed to rigidly hold element 5 across the horizontal diametrical axis of element 10. Screw tightening point (i.e. elements 3s) can also be incorporated to make it more user-friendly. As the time frame window for triggering servo-controlled response is very short, stronger bond 5 between elements 3 and 5 is a must. For this purpose, element 19 is suggested as a strong adhesive glue (i.e. figure 3) which has 8 MPa as the lap shear tensile adhesion strength on steel. It should have chemical resistance to acids, oils, alkalis, fresh and saltwater, high range of service temperature with negligible shrinkage. Any adhesives with the above specifications and strength may also be used. One o suggested example for such glue could be Plasti-Bond Heavy Duty BOG (i.e. type two part epoxy bonding builders filler).

The feedback received in the form of lateral displacement will be used to control the axial loading rate via element 16. When the lateral strain rate across the central vertical diametrical axis exceeds the predefined limit due to the cracking process, the 5 in-built element 16 triggers the reduction (or unloading if required) in axial loading rate. Consequently, stress relaxation across central diametrical axis accompanied with energy release takes place inducing snapping-back (or load and displacement reversals) characteristics of the load-displacement response.

Further, the following steps in a given order should be taken to conduct the new o methodology for indirect tensile testing using circular disc specimens:

Step 1: Prepare element 10 for testing as per ISRM or ASTM standard guidelines for indirect tensile testing.

Step 2: Clean lateral/side surface of element 10 properly and apply element 18 (optional, but strongly suggested) to provide temporary protection from dust or 25 any other sources, i.e. figure 4. There can be other different ways that can be used to avoid dust on the surface.

Step 3: Clean the inner surface of element 3 and apply element 18.

Step 4: Insert element 3cl (Figure 2i) in one part of element 3 (see Figure 3).

Step 5: Take approximately 2-3 gm of putty on a strip along with hardener (i.e.

suggested glue) as shown in Figure 4(a). The ratio between the putty and hardener, both of element 19 should be 1:3 approximately. You can also increase the hardener content slightly for a quick setting bond between elements 3 and 10.

Step 6: Thoroughly mix the putty and hardener for not more than 5 minutes.

Step 7: Remove element 18 and apply element 19 uniformly on this inner surface of element 3.

Step 8: Quickly remove element 18 from the outer surface of element 10 and manually attach element 3 on the outer surface of element 10 (see Figure 4b). Press this combination tightly for 2 minutes approximately to ensure strong contact between elements 3 and 10.

o Step 9: Keep the glued elements 3 and 10 undisturbed for 25-30 minutes.

Step 10: Repeat steps 5 to 9 on the second part of element 3 (another plastic cap) to attach it with the other surface of element 10 on the opposite side at 180°±2° (see Figure 2ii).

Step 11: Insert element 5 (lateral LVDT) in the remaining opening of element 3 attached with element 10 such that sensitive knob of element 5 must be sufficiently pressed against element 3cl (see Figure 2ii).

Step 12: If element 3 has a hole for the screw (element 3s; see Figure 2i), then tighten the screws. Otherwise additional masking tape can also be used inbetween element 3 and element 5 to ensure tight-fitting, to ensure there is no 20 slip between elements 3 and 5. This no-slip condition is essential for successful control of the post-peak response using feedback from the lateral LVDT (element 5).

Step 13: Thoroughly clean the surface of element 2 to ensure its smoothness for uniform contact with the specimen or element 10. This step is important to avoid any potential stress concentration which may have significant effects on the lateral strain recordings, and thus the success of the post-peak control.

Step 14: Place this sample set up with elements 3, 5 and 10 on the flat surface of bottom loading platen in element 1 (loading machine).

Step 15: Again, ensure the smooth contact between the top and bottom loading 30 platens (i.e. 2) with element 3 as shown in Figure 5.

Step 16: Also, ensure the angle of inclination of the horizontal axis for elements 3 and 5 must not vary more than ±2° (i.e. Θ < 2° in Figure 2ii).

Step 17: Connect element 7 with the loading machine (element 1) to start the test.

Step 18: Attach element 4 at the backside of specimen for Acoustic emission data recording and calibrate DIC set-up (this step is optional, depending on the type of instrumentation needed).

Step 19: In the beginning, vertical compression of the disc sample is applied gradually by increasing the vertical displacement of top platen (while keeping o the bottom plate fixed) to maintain a constant vertical strain rate. The rate of vertical displacement must follow ASTM or ISRM standards for quasistatic strain rate (i.e. strain rate <10^-4 /sec). In case of rock, vertical displacement loading rate of 0.2 mm/min for element 10 (of 42mm diameter or 10a) is recommended.

Step 20: Switch the control to lateral displacement as feedback from element 5 (lateral LVDT). This switch must be well below the expected maximum load that the specimen (element 10) can take. The recommended load at which control switching should be is 50%-70% of the maximum load. Switching before 40% of the maximum load may delay the test and sometimes induce o unloading, especially in case of hard rocks.

Step 21: Load element 10 using lateral displacement control, taking the lateral displacement from element 5 to adjust the load so that the lateral displacement is always increasing at the specified rate. The specified rate of lateral displacement from element 5 must be determined/calibrated depending 25 on the types of materials and the diameter of the specimen (element 10). The recommended approach is to start with a very low rate (e.g. 0.1 micrometre/minute) and observe the response. If unloading takes place after switching to lateral displacement control, the test must be re-started with lateral displacement rate increased by +0.1 micrometre/min.

Step 22: For rocks with element 10a (specimen diameter; Figure 2ii) of 42mm, the recommended rate that can be specified for the control is 0.4 micrometre/minute.

Step 23: Keep loading using lateral displacement control from element 5 (lateral LVDT) until the specimen (element 10) fails. This post-peak loading can be stopped at any time, depending on the users’ requirement.

Note: Element 3 can be re-used after each test provided that its internal surface is 5 properly cleaned for effective bond between elements 3 and 10.

An improvement over any existing methodology

There is no such technology/methodology available to control the dynamics of the disc cracking in existing testing methods for indirect tensile strength (Brazilian disc tests). Therefore, as mentioned earlier, the invented methodology is the first to o provide an efficient platform to estimate various material properties, including tensile strength, and fracture energy. Furthermore, it provides an insight into the material’s energy storing and releasing characteristics under indirect tension. In combination with advanced image based experimental techniques, i.e. DIC or X-Ray CT, it will allow observing the failure progression and determining properties at a scale much lower than the size of the specimen.

In addition to the above benefits, the invented methodology is based on the simple disc shaped specimens that are very easy to prepare. The overall experimental setup requires only LVDT for capturing the sample’s lateral expansion which itself is a cheap and commonly available experimental equipment. The remaining components o of overall testing setup are also standard and largely available in any engineering laboratories. Therefore, the invented methodology can provide an economical and viable approach in terms of specimen preparation and logistics for applications to a wide variety of material types.

Illustration, applications and disclosure

Several experiments have been conducted in the Mining engineering laboratory at the University of Adelaide in 2018 and 2019 to demonstrate the feasibility and finalise the details for the proposed methodology. The potential applications of invented methodology have been illustrated on different brittle materials including concrete and rocks (ranging from soft to hard rocks, e.g. Sandstone, Iranian granite and

Bluestone). Advanced experimental techniques, including Digital Image Correlation (DIC) and Acoustic Emission (AE), have also been used to illustrate the benefits of the invented methodology. These successful attempts with expected results ensure

2019101006 04 Sep 2019 the applicability and validity of the proposed method. It indicates the potential applicability of the invented method in mining engineering, structural engineering, materials sciences, ceramic engineering and other non-engineering fields where circular discs are widely used for measurements of strength and fracture properties 5 of brittle materials. Details of the invented methodology have not been disclosed to anyone, neither previously nor currently.

Claims

Claims

1. The new methodology proposed for indirect tensile testing using circular disc can effectively control the post-peak response ofthe specimen under diametrical compression.
2. The new methodology as claimed in claim 1 comprises the loading control based on lateral displacement obtained from the LVDT attached to the disc specimen, via two plastic caps, to successfully control the post-peak response ofthe specimen.
3. The new methodology, as claimed in claim 1 comprises the design and development of 3D printed plastic caps (or mounts) to hold a Linear Variable Differential Transformer (LVDT) across the vertical crack line. The size of the plastic caps is determined based on the diameter and thickness ofthe circular disc.
4. The designed plastic caps (or mounts) as claimed in claim 2 and 3 also comprise the steps to attach them to disc specimen using strong glue (shear strength of 8MPa recommended).
5. The new methodology as claimed in claim 1 and claim 2 also comprises specifications for the lateral strain rate to successfully control the post-peak response.
6. A holder for circular disc testing where said holder includes;

a) two 3D printed plastic caps (or mounts) to hold the Linear Variable Differential Transformer (LVDT). The size of the plastic caps are determined based on the diameter of the circular disc; each plastic cap has a hole to fit the LVDT and a 3D printer plastic rod. One plastic cap is used to hold the LVDT and the other to hold the plastic rod. The purpose of this plastic rod is two folds: (i) to help in horizontal alignment ofthe plastic caps ensuring the deviation of LVDT axis within the limit of ±20 from the horizontal axis, (ii) to enable the functioning of LVDT to record the cracking induced lateral displacement of the circular disc.

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b) a 3D printed plastic rod that fits into the hole of the plastic cap. This plastic rod must be tightly fitted to the hole and there should be no slippage between it and the hole so that the LVDT can correctly record the lateral displacement of the circular disc.

c) strong glue (shear strength of 8 MPa recommended) to attach the plastic caps to the circular disc. The inner surface of the cap must be thoroughly cleaned before the application of glue.
7. A method for circular disc testing where the said method includes;

a) prepare the circular disc specimen as per ISRM or ASTM standards for indirect testing.

b) prepare the plastic caps, plastic rod and arrange permanent glue (recommended in manuscript), and LVDT.

c) attach the plastic caps to the specimen using the above-recommended glue, followed by fitting the LVDT and plastic rod (that the LVDT rests on) to the caps. As mentioned earlier, the alignment of lateral LVDT must not deviate from the horizontal axis of the specimen by ±20

d) mount the circular disc (with plastic caps, LVDT and plastic rod attached) to the loading machine, in between two steel loading platens. The surface of the steel loading platens must be cleaned and smooth. Uniform contact between both (top and bottom) loading platens with disc specimen surfaces is a MUST to avoid any potential of stress concentration points.

e) connect the LVDT to the loading machine. The loading machine must allow taking lateral displacement from the LVDT to adjust the load accordingly during the test. The loading machine must allow displacement controlled loading.

f) at the beginning, axial displacement controlled loading approach is adopted where we gradually increase the vertical displacement to induce vertical load on the circular disc specimen. The rate of vertical displacement must follow ASTM or ISRM standards of quasi-static strain rate (i.e. strain rate <

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10e-4 strain/sec). In case of rock, axial displacement loading rate of 0.2mm/min for circular discs of 42mm diameter is recommended.

g) switch the control to lateral displacement as feedback from the lateral LVDT. This switch must be well below the expected maximum load the circular disc can take. The recommended load at which control switching should be is 0.5-0.7 maximum load. Switching before 0.4 maximum load may delay the test and sometimes induce unloading especially in case of hard rocks.

h) load the circular disc using lateral displacement control, taking the lateral displacement from the LVDT to adjust the load so that the lateral displacement is always increasing at the specified rate. The specified rate of lateral displacement from the LVDT must be determined/calibrated depending on the types of materials and the diameter of the specimen. For rocks, the recommended rate that can be specified for the control is 0.4 micrometre/min. If unloading takes place after switching to lateral displacement control, the test must be re-started with lateral displacement rate increased by +0.1 micrometer/min.

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Symbols Detail Symbols Detail 1 MTS loading frame 10 Circular disc specimen 2 Top/bottom loading plates 11 MTS-data acquisition 3 Plastic mount 12 DIC/AE data acquisition (Optional) 4 AE sensor (Optional) 13 Front side of set-up 5 Lateral LVDT 14 Back side of set-up 6 Cylinder for sample level 15 Feedback signal 7 Axial LVDT 16 Servo-control mechanism 8 DlC-camera(Optional) 17 Load control 9 DIC-light source(Optional)

Figure. 1. Detailed overview of testing set-up