A Virtual Simulation Experiment of Mechanics: Material Deformation and Failure Based on Scanning Electron Microscopy

Summary

This work presents a three-dimensional virtual simulation experiment for material deformation and failure that provides visualized experimental processes. Through a set of experiments, users can become familiar with the equipment and learn the operations in an immersive and interactive learning environment.

Abstract

This work presents a set of comprehensive virtual experiments to detect material deformation and failure. The most commonly used pieces of equipment in mechanics and material disciplines, such as a metallographic cutting machine and a high-temperature universal creep testing machine, are integrated into a web-based system to provide different experimental services to users in an immersive and interactive learning environment. The protocol in this work is divided into five subsections, namely, the preparation of the materials, molding the specimen, specimen characterization, specimen loading, nanoindenter installation, and SEM in situ experiments, and this protocol aims to provide an opportunity for users regarding the recognition of different equipment and the corresponding operations, as well as the enhancement of laboratory awareness, etc., using a virtual simulation approach. To provide clear guidance for the experiment, the system highlights the equipment/specimen to be used in the next step and marks the pathway that leads to the equipment with a conspicuous arrow. To mimic the hands-on experiment as closely as possible, we designed and developed a three-dimensional laboratory room, equipment, operations, and experimental procedures. Moreover, the virtual system also considers interactive exercises and registration before using chemicals during the experiment. Incorrect operations are also allowed, resulting in a warning message informing the user. The system can provide interactive and visualized experiments to users at different levels.

Introduction

Mechanics is one of the basic disciplines in engineering, as shown by the emphasis placed on the foundation of mathematical mechanics and theoretical knowledge and the attention given to the cultivation of students' practical abilities. With the rapid advancement of modern science and technology, nanoscience and technology have had a huge impact on human life and the economy. Rita Colwell, the former director of the US National Science Foundation (NSF), declared in 2002 that nanoscale technology would have an impact equal to the Industrial Revolution¹ and noted that nanotechnology is truly a portal to a new world². The mechanical properties of materials at the nanoscale are one of the most fundamental and necessary factors for the development of high-tech applications, such as nano-devices^3,4,5. The mechanical behavior of materials at the nanoscale and the structural evolution under stress have become important issues in current nanomechanical research.

In recent years, the development and improvement of nanoindentation technology, electron microscopy technology, scanning probe microscopy, etc., have made "in situ mechanics" experiments an advanced testing technique important in nanomechanics research^6,7. Obviously, from the perspective of teaching and scientific research, it is necessary to introduce frontier experimental techniques into the traditional teaching content regarding mechanical experiments.

However, experiments of microscopic mechanics are significantly different from macroscopic basic mechanics experiments. On the one hand, although the relevant instruments and equipment have been popularized in almost all colleges and universities, their number is limited because of the high price and maintenance cost. In the short term, it is impossible to purchase enough equipment for offline teaching. Even if there are financial resources, the management and maintenance costs of offline experiments are too high, since this type of equipment has high-precision characteristics.

On the other hand, in situ mechanics experiments such as scanning electron microscopy (SEM) are very comprehensive, with high operational requirements and an extremely long experimental period^8,9. Offline experiments require students to be highly focused for a long time, and misoperation can damage the instrument. Even with very skilled individuals, a successful experiment requires a few days to complete, from preparing qualified specimens to loading the specimens for in situ mechanics experiments. Therefore, the efficiency of offline experimental teaching is extremely low.

To address the above issues, virtual simulation can be utilized. The development of virtual simulation experiment teaching can address the cost and quantity bottleneck of in situ mechanics experimental equipment and, thus, allow students to easily use various advanced pieces of equipment without damaging high-tech instruments. Simulation experiment teaching also enables students to access the virtual simulation experiment platform via the internet anytime and anywhere. Even for some low-cost instruments, students can use virtual instruments in advance for training and practice, which may improve teaching efficiency.

Considering the accessibility and availability of web-based systems¹⁰, in this work, we present a web-based virtual simulation experimentation system that can provide a set of experiments related to fundamental operations in mechanics and materials, with a focus on the in situ mechanics experiment.

Protocol

In this work, the procedures of the microcantilever beam fracture experiment with cracks are discussed as follows, which is open for free access via http://civ.whu.rofall.net/virexp/clqd. All the steps are conducted in the online system based on the virtual simulation approach. Institutional Review Board approval was not required for this study. Consent was obtained from the student volunteers who took part in this study.

1. Accessing the system and entering the interface

1. Open a web browser, and enter the URL http://civ.whu.rofall.net/virexp/clqd to access the system.
   NOTE: The provided URL can be accessed through a mainstream web browser without a username and password.
2. Find the virtual simulation interface using the vertical scrollbar.
   NOTE: The virtual scene is embedded into the web.
3. Click on the FullScreen icon at the bottom-right corner to enable a full-screen interface.
4. Click on the Start Experiment button to start.
5. Click on the Enter button to follow the guidance for beginners, or click on the Skip button to skip this step.
   ​NOTE: The user can choose to follow (Enter button) or skip (Skip button). The guidance for beginners provides descriptions of the entire system. The interface also highlights the operation instructions step-by-step for performing the intended operations or equipment. Figure 1 shows the equipment used in the experiment, including seven types of equipment in the mechanical and material disciplines. Beginners are recommended to follow this guidance.

2. Preparation of the materials

1. Start the experiment after completing the beginner-level training. Follow the prompts on the interface to "walk" close to the laboratory table that contains the silicon wafers, review the differences between the normal-type and crack-type silicon wafers, and select the crack template.
   NOTE: Enter the experiment interface, and conduct experiments according to the highlighted pathway guidance. The highlighted guidance is provided throughout the process to offer clear guidance for experimentation.
2. Select a material from the provided materials list.
   NOTE: The provided material list includes gold, silver, PtCuNiP, ZrTiCuNiBe, polyether-ether-ketone (PEEK), and polymethyl methacrylate (PMMA).
3. Load the selected material onto the cutter clamp with a click on the highlighted material. Click on the highlighted ON/OFF button (on the right side) to turn on the cutter clamp, click on the Speed button (on the left side), and set the speed of the metallographic cutting machine in a pop-up interface.
   NOTE: The user can set a proper speed as they wish. Once the speed is set by the user, the cutter clamp will be activated, and the raw bar will be cut into thin slices.
4. Stack the mold, metal sheet, and cover sheet together in turn by clicking and dragging the highlighted object as guided in the user interface.
   ​NOTE: After cutting the material, this assembly step is necessary before nano-mold casting.

3. Molding the specimen

1. Walk virtually to the high-temperature universal creep testing machine following the guidance shown in Figure 2, and virtually place the stacked specimens between the plate clamps of the universal creep testing machine.
   NOTE: After this step, the virtual computer on the left side of the high-temperature universal creep testing machine will be highlighted.
2. Click on the Virtual Computer, and set the test scheme on the control computer of the universal creep testing machine.
   NOTE: After this step, the auxiliary equipment of the high-temperature universal creep testing machine for heating and vacuum pumping will be highlighted to provide guidance to the user.
3. Click on the highlighted Heating and Vacuum Pumping Equipment, and turn on the power supply. Open the virtual mechanical pump and the backing valve in the interface by clicking on the highlighted buttons.
   NOTE: This step completes the system vacuum control settings in the vacuum control system of the universal creep testing machine.
4. Click on the Clear button on the Control Panel of the universal creep testing machine to clear the data. Click on the Run button on the Control Panel of the universal creep testing machine to complete the experiment, which copies the pattern on the mold to the metal sheet using the parallel plate compression molding method.
   NOTE: After the mold casting is completed, remove the specimen, and close the backing valve and the mechanical pump, etc., of the heating and vacuum pumping equipment by clicking on the buttons in turn as required (in real heating and vacuum pumping equipment, the reverse order may cause the molecular pump to burn out).
5. Click on the Virtual Computer again, and check the experimental data on the control computer of the universal creep testing machine.
6. Open the cover plate on the metallographic specimen inlaying machine, and place the specimen.
   1. Click on the highlighted PMMA powder to pour the prepared powder, and click on the highlighted mold to place it on top of the PMMA powder.
   2. Click on the highlighted hand wheel to adjust the position of the mold, which will cover the cover plate automatically. Click on the ON/OFF button to turn on the inlaying machine. Take out the PMMA inlaid specimen after cooling.
      NOTE: The molded specimen should be mounted on the inlaying machine in the correct direction, as shown in Figure 3, in which the thermoplastic material PMMA is used in the experiment. Make sure the PMMA powder melts and adheres to the surface of the specimen. The bottom-left corner of Figure 4 illustrates the correct direction after the user confirms the selection shown in Figure 3.
7. Enter the room for polishing and corrosion following the pathway guidance, as shown in Figure 5. Find the highlighted polishing machine, and click on the gripper of the polishing machine to mount the inlaid specimen to the gripper. Set the speed to grind and polish the specimen to remove the molded material substrate.
   ​NOTE: Grind the mold on one side of the mold until the pattern on the mold is exposed.

4. Specimen characterization

1. Register in the e-notebook before using a chemical. Open the chemical storage cabinet, and take out the solid KOH and acetone solution. Click on the highlighted beaker to use the acetone solution to clean the specimen. Click on another highlighted beaker and solid KOH for corrosion liquid preparation to prepare a 10% KOH solution. Click on the highlighted KOH solution and the specimen to corrode the specimen into a metallographic specimen.
   NOTE: In this experiment, to remove the silicon mold, a 6 mol/L KOH solution is usually prepared, the specimen is placed in the preparation solution, and the beaker containing the corrosion solution and the specimen is placed on a hot plate to heat up to accelerate the corrosion rate.
2. Clean the specimen after removing the silicon substrate, and run a characterized testing with the prepared specimen under an optical microscope.
   ​NOTE: Remember to determine the integrity of the specimen after the grinding and corrosion.

5. Specimen loading and nanoindenter installation

1. Load the specimen onto the sample stage of the nanoindenter. Choose the cone indenter to mount it on the driver of the micro- and nanomechanics testing system. Click on the highlighted drive to connect it with the nanoindenter.
   ​NOTE: The "Pin" must be inserted into the drive shaft when installing the indenter, and since the drive shaft is a slender bar, the latch avoids damaging the drive shaft when screwing the indenter with a threaded end into the drive.

6. SEM in situ experiment

1. Click the Vent button in the SEM control software after installing the indenter of the nanoindenter and loading the specimen as described in 5.1.
2. Open the SEM chamber after breaking the vacuum, install the nanoindenter on the SEM sample stage, and connect the wires (Figure 6 shows an example of connecting one of the wires).
3. Open the control software of the nanoindenter, and select Loaded Indenter Range > Select Experimental Protocol > Start Controller > Init* (Sample Stage Initialization).
   NOTE: The position initialization process of the nanoindenter sample stage must be carried out in the state in which the SEM cavity is open to avoid the initialization process of the nanoindenter sample stage hitting the pole of the SEM electron outlet port.
4. Close the SEM chamber, and click on the Pump button on the SEM control software.
5. Click on the Up or Down button in the SEM control software to adjust the position of the sample stage so that the sample to be measured falls into the SEM field of view. Click on the OK button to fix the position. Click on the highlighted EHT button to turn on the electron gun. Click on the Camera button, and switch to the electron microscopy observation mode.
   NOTE: The indenter of the nanoindenter should be controlled in observation mode to gradually approach the sample to be measured.
6. Click on the Run button on the control software of the nanoindenter.
   NOTE: During the experiment, it is necessary to observe and record the deformation characteristics and failure process during the loading process of the specimen and to open the original data of the experiment in the data analysis window after the experiment is complete for plotting and exporting the data.
7. Click the Stop button on the control software of the nanoindenter to terminate the experiment.
   NOTE: The virtual simulation experiment ends here. The user is asked to complete the online exam exercise in the virtual interface after the experimentation.

Representative Results

The system provides clear guidance for the user's operations. First, beginner-level training is integrated when a user enters the system. Second, the equipment and the laboratory room to be used for the next-step operation are highlighted.

The system can be used for several different educational purposes for different levels of students. For example, Figure 1 includes seven of the most commonly used types of equipment in the mechanical and material disciplines, namely, the metallographic cutting machine, high-temperature universal creep testing machine, metallographic specimen inlaying machine, polishing machine, optical microscope, SEM, and micro- and nano-mechanics testing system. In the guidance for beginners, the user can learn about the descriptions of all the equipment used in the experiment. Then, all the equipment is used one by one to complete the experiment. The students can choose the equipment for repetitive experiments until they master the operating skills.

Figure 3 and Figure 4 also demonstrate that the system can enhance the design of the experimental scheme combined with the experimental operations, which can provide instant validation. In Figure 3, the user should choose to place the specimen in the correct direction to create a molded specimen. Figure 4 shows the interface for using the metallographic specimen inlaying machine, which also shows the results (as indicated at the bottom-left corner of Figure 4) of the previous step after the user confirms the selection, as shown in Figure 3. Figure 7 illustrates the in situ mechanics experimental results of the micro-cantilever beam with preset cracks. Through the analysis of the results, the user can determine how the results were obtained.

This protocol simulates the scenario in which the students are required to evaluate the load size and loading time of the rheological experiment of the parallel plate according to the length-to-diameter ratio of the specimen to be prepared. The experimenter needs to analyze the relation of the length-to-diameter ratio of the viscous fluid flowing into a cylindrical hole mold, the pressure p₀, and the time t with a diameter of d under the action of constant pressure p₀. This relation is shown below:

where L is the length, d is the diameter of the cylindrical hole mold, p₀ is the constant pressure, η is the material viscosity, and t is the loading time. Once p₀, η, and L/d are given, t can be calculated. If L/d doubles, the loading time will be four times larger than before. Figure 8 illustrates the relationship between the length-to-diameter ratio of metal glass flowing into the mold hole and time.

In real-world experiments, it was found that students often used a trial-and-error approach-that is, constantly adjusting the load size or loading duration until the required sample was finally made. In this protocol, an interactive interface is provided to validate the theoretical knowledge, and the loading time is determined according to the provided parameter values (material viscosity, initial sample size, and load size). A guiding question is provided as follows: "Metal glass is a Newtonian fluid with a viscosity of η = 10⁷ Pa·s at the die casting experimental temperature. The fluid has no slip at the mold contact boundary. It is necessary to prepare a cylindrical specimen with a length-to-diameter ratio of 5. If the experiment can apply a large amount of pressure of 100 MPa, how long should the loading time be? If the length-to-diameter ratio is increased by 1x, how many times does the loading time increase by?" The students should figure out the answers, set the test scheme accordingly, and then conduct their experiments.

After the experiment, the students are asked to answer a few questions of different types, such as fill-in-the-blank questions and single-answer/multi-answer multiple-choice questions (MCQ), which focus on the key steps during the virtual simulation experiment to enhance their theoretical knowledge and experimentation. Table 1 shows the question examples for the online exam exercise after the experimentation. With integrated exercises, users can systematically review the entire process of the experiment and connect the theory with the experimentation.

The set of experiments offered by the implementation of the proposed virtual simulation mean that the following visualized and interactive knowledge-enhanced and skill-enhanced experiences can be provided: 1) an immersive virtual learning environment where users can "walk" through and understand the layout of the laboratory rooms and the details of each piece of equipment; 2) operations on different typical pieces of equipment in the mechanical and material disciplines to master operating skills; 3) safety awareness enhancement through wrong operations and warnings; 4) repetitive experiments and shorter time experiments instead of the duration of experiments; 5) following the protocol of conventional laboratories as closely as possible so that users can be familiar with the procedures and the "dos" and "do nots" even in the virtual environment.

Conventionally, due to the limited amount of equipment and the occupation of graduate students for research purposes, undergraduate students rarely have the chance to conduct experiments with physical equipment. The virtual simulation system that integrates different types of equipment can help provide concurrently accessible and repeatable experiments to enhance their laboratory skills. After its deployment, the virtual system was applied in the autumn semesters of the 2020 and 2021 academic years for students with engineering mechanics backgrounds. Table 2 shows the results of the experiment, which include the mean completion time, the standard deviation of the completion time, and the average scores of the different years. The average score (100 in total) is calculated based on the evaluation of the experiment (70%, evaluated by the system) and the laboratory report on the web (30%, evaluated by the teacher). The results demonstrate that students can, on average, complete the experiment in ~73 min using a web browser, which is time efficient and verifies the efficiency of the web-based system based on the virtual simulation approach. In 2022, we performed a study to demonstrate the efficiency of the proposed protocol. Students from two classes with engineering mechanics backgrounds (two classes with the same teacher and the same class modules, divided into two classes for class size reasons) were divided into two groups (one class for each group). The students from Group 1 attended the physical laboratory to learn the theoretical knowledge and watch the operations from the teacher, while the students from Group 2 used the virtual interface that was developed based on the physical laboratory (including the layout and the equipment) for their experiment. Table 3 shows the online exam results (with a total score of 10) for the students without (Group 1) and with (Group 2) the virtual interface experience. It can be concluded that the students with the virtual interface experience performed better than those without the experience.

Figure 1: The developed three-dimensional equipment used during the experiments. It can be concluded that through this virtual simulation experiment, the user can be trained to be familiar with the most commonly used equipment in the mechanical and material disciplines. Please click here to view a larger version of this figure.

Figure 2: Highlighted high-temperature universal creep testing machine in the virtual simulation laboratory room. After completing the previous step (cutting the specimen), the next step is generated automatically, which either highlights the machine (when the machine is nearby) or the pathway leading to the machine (when the machine is not nearby). Please click here to view a larger version of this figure.

Figure 3: The interface for choosing the placement direction of the specimen. The user should choose the correct placement direction of the specimen to continue with the next step. Please click here to view a larger version of this figure.

Figure 4: The interface for using the metallographic specimen inlaying machine. The results of the previous step after the user confirms the selection (in Figure 3) are shown in the bottom-left corner. Please click here to view a larger version of this figure.

Figure 5: The interface with a highlighted pathway guidance. The user is guided to enter a room for the polishing and corrosion of the specimen. Please click here to view a larger version of this figure.

Figure 6: Wiring for the SEM machine. The user should connect the wires to continue with the experiment. Please click here to view a larger version of this figure.

Figure 7: In situ mechanics experimental process results of the micro-cantilever beam with preset cracks. The two curves show an example of the in situ mechanics experimental results of amicro-cantilever beam with preset cracks. (A) Displacement-time curve, (B) stress-strain curve. Please click here to view a larger version of this figure.

Figure 8: Calculation based on theoretical knowledge. The relationship between the length-to-diameter ratio of metal glass flowing into the mold hole and time. Please click here to view a larger version of this figure.

Figure 9: The warning shows that a wrong operation has damaged the scope. Users can click the button to level up/down the SEM detector. However, if they level up too much, the SEM detector will be damaged. Please click here to view a larger version of this figure.

Figure 10: The e-notebook for the online registration before using a chemical. Before the corrosion process, the user must register it in the notebook, which is the same as the procedure in the physical laboratory. Please click here to view a larger version of this figure.

ID	Exam question type	Question details	Provide choices
1	Fill-in-the-blank question	In this experiment, __ solution was used to corrode the silicon wafer.	None
2	Single-answer MCQ	When the high temperature universal creep testing machine is used for the experiment, which of the following materials can be regarded as Newtonian fluid?	A.    Conventional metal
			B.    Amorphous alloy
3	Single-answer MCQ	If a specimen is estimated to withstand the maximum force of 60mN, then in the range selection, choose InForce 50 or InForce 1000?	A.    InForce 50
			B.    InForce 1000
4	Multi-answer MCQ	Nanoindenter can be used to measure?	A.    Hardness
			B.    Modulus of elasticity
			C.    Fracture toughness
			D.    Viscoelasticity
5	Single-answer MCQ	SEM is an abbreviation for	A.    Optical microscope
			B.    Scanning electron microscopy
			C.    Transmission electron microscopy

Table 1: Question examples for the online exam exercise after the experimentation. Users are required to complete different types of questions so that they can systematically review the entire process of the experiment and connect the theory with the experimentation.

Year	Number of students	Mean completion time	Standard deviation of the completion time	Average score
2021	58	71 min and 46 s	11 min and 39.5 s	79.83
2020	77	73 min and 3 s	11 min and 15.4 s	80.21

Table 2: The results of experiments in different years. Students with engineering mechanics backgrounds completed the experiments in two different academic years.

Group ID	Number of students	Average score	Standard deviation of the score
1	18	5.56	1.15
2	22	8.09	1.27

Table 3: The online exam results (with a total score of 10) for students without (Group 1) and with (Group 2) the virtual interface experience. Students with engineering mechanics backgrounds were divided into two groups in 2022 to demonstrate the efficiency of the protocol.

Discussion

One of the advantages of virtual simulation experiments is that they allow users to conduct the experiments without concerns regarding damaging the physical system or causing any harm to themselves¹¹. Thus, users can conduct any operations, including either correct or wrong operations. However, the system gives the user a warning message that is integrated into the interactive experiment to guide them to conduct the experiments correctly when a wrong operation is conducted. In this way, users can learn the correct operations. For example, when a user conducts operations on the SEM, as shown in Figure 9, they may level up the SEM detector too much and damage it by accident.

Similar to hands-on experiments in physical laboratories, users who conduct virtual experiments should also follow correct procedures, which can potentially enhance their experimentation and safety awareness. For example, as illustrated in Figure 10, when preparing a KOH solution for the corrosion process of the specimen into a metallographic specimen, the user should register in a notebook before using the chemical.

Although this system provides a complex and comprehensive virtual environment for material deformation and failure experimentation, the main limitation is that it currently lacks user customizations. Users follow the steps to conduct experiments, and they rarely have a chance to implement their ideas. However, the system can be improved to provide students with more freedom to implement their ideas and create their own designs and implementations.

Three-dimensional virtual simulation has been an important topic throughout the world during the past decade in terms of providing immersive interfaces for engagement and learning^12,13. Studies regarding virtual simulation have been conducted in various disciplines, such as in control engineering¹⁴ for safety considerations¹⁵ and in chemical engineering for production practice¹⁶. In the materials and mechanics discipline, the system can be used for the training of students regarding experimental protocols, the use of equipment, and the verification of theoretical knowledge. With respect to existing methods, the proposed virtual simulation approach can be accessed by users at any time from anywhere as long as internet and a web browser are available, meaning this approach is cost-effective and highly efficient. By providing seven different types of costly equipment, the online system allows users to repeatedly enhance their operations and laboratory skills in this single online system.

The system can be used in combination with traditional teaching and learning in future applications of the technique. For example, the system could be combined with hands-on experiments. Students could conduct virtual simulation experiments before they conduct hands-on experiments in conventional laboratories. Compared with conventional methods, the system is interactive and immersive. Further to the benefits provided by traditional education, virtual simulation-based experimental teaching provides a full range of auxiliary functions, which can exercise students' ability to use the knowledge they have learned to solve practical problems. Additionally, this type of teaching also cultivates students' research interests and sense of innovation by training them to master the testing techniques, methods, and principles of advanced micro- and nano-scale mechanical experiments and effectively helps students improve their professional and comprehensive qualities.

Materials

Name	Company	Catalog Number	Comments
Virtual interface	None	None	http://civ.whu.rofall.net/virexp/clqd