2.1 Fiber-Reinforced Composites in Industrial Applications
Fiber-reinforced composites are engineered materials with high strength by weight and modulus by weight ratios compared to many metallic materials (Njuguna, Pielichowski, and Fan 2012). They are popular for their widespread industrial applications, especially in automotive, aviation, shipping, and other related sectors (Prashanth et al. 2017; Erden and Ho 2017). There are different types of fiber reinforcements, including glass fibers, carbon fibers, and Kevlar fibers, among many others. Carbon fibers are composed of a minimum of 92% by weight (mass fraction) carbon and can exist in the form of fibers, filaments, tows, yams, or rovings (Marsh and Rodríguez-Reinoso 2006). Carbon fibers also offer the highest specific modulus and strength in addition to their ability to retain tensile strength at high temperatures, independent of moisture (Prashanth et al. 2017). The market size for carbon fiber composite in 2021 was USD 18.4 billion and is expected to grow at a compound annual growth rate of 6% from 2022 to 2030 (Global Market Insights 2022). Because of its high industrial usage and economic importance, it is necessary to build a qualified and well-trained workforce to support production needs in the future. This research aims to solve this problem by adding a new training method using virtual reality, which has not been seen before for fiber-reinforced composite manufacturing.
2.2 Manufacturing Technique for Carbon Fiber Composite
Fiber-reinforced composites are manufactured by infusing the fibers in uncured epoxy resin and curing them (Sloan 2016). Curing is the chemical reaction between the epoxy and the hardener, resulting in a crosslinked polymer structure (Abliz et al. 2013). Prepregs are composite materials in which a high-strength reinforcement fiber is pre-impregnated with a thermoset or a thermoplastic resin (Bhatnagar and Asija 2016). Compression molding using prepregs is one of the most suitable manufacturing techniques for this type of fiber-reinforced composite (Yallew et al. 2020; Tapan Bhatt, Gohil, and Chaudhary 2018). Prepreg plies are stacked on top of each other to form a laminate, where the orientation and the number of plies depend on the desired strength and application of the material. The laminate stack is sandwiched between two mold plates and releases films to separate the epoxy from bonding to the metal mold. A release fabric known as peel ply creates a rough texture on the laminate that is often used for secondary bonding. The entire laminate layup is then placed inside a compression molding oven and cured by a recipe. The recipe can differ from machine to machine as recommended by the manufacturer. In common, there should be three segments in a typical recipe (TORAY 2020) – ramp up (increasing temperature and pressure), hold/ dwell, and ramp down (decreasing temperature and pressure). Once the recipe ends, the cured laminate is separated from the mold plates. The released films are removed and discarded, and the peel plies are peeled off. This research simulates this manufacturing technique in virtual reality training to enhance operators’ learning to perform this process successfully.
2.3 A Brief History of VR Training in Manufacturing
The use of VR in manufacturing-based industries is becoming increasingly popular (Renner et al. 2016; Doolani et al. 2020; Matsas and Vosniakos 2017; Jimeno and Puerta 2007; Matsas, Vosniakos, and Batras 2018; Choi, Jung, and Noh 2015; Salah et al. 2019; Monetti et al. 2022; Roldán et al. 2019). The design of VR training to simulate cooperation between industrial robotics manipulators and humans (Matsas and Vosniakos 2017) showed an excellent prospect of VR in human-robot applications. Similar research was conducted to observe the impact of VR training on operators of industrial robotics tasks (Monetti et al., 2022). The researchers found that participants using the VR first were quicker to complete the tasks than the others, at a higher passing rate. This training method is particularly viable because it is safe, engaging, and effective for vocational training. Doolani et al. (2020) emphasize that VR has been tested as the best immersive medium among augmented reality (AR), virtual reality, and mixed reality. Researchers have concluded in numerous examples that VR is also useful for training specific machinery. For example, in research conducted to learn the effectiveness of VR in the application for additive manufacturing process training, it was concluded that identifying and fixing a mistake in the VR is time and cost-efficient in achieving the desired quality (Renner et al. 2016). Complex hand movements to organize items in VR are also possible; for example, assembly operations. In a study conducted by Matsas and Vosniakos (2017) in prototyping proactive and adaptive techniques for human-robot collaboration, the user was able to collaborate on the lay-up of carbon fabric. Another research conducted by Matsas et al. (2018) showed that participants being trained to assemble a water pump suggested that VR, a 3D system, has a more intuitive learning approach than 2D systems. The user evaluations in VR training systems brought fruitful results in complex assemblies (Roldán et al., 2019). Users evaluated them better than a paper guide regarding mental demand, perception, and learning, improving their overall performance. VR is also a very inexpensive alternative to high-cost training. Choi et al. (2015) note that VR was previously used only in developing premier products for its low return on investment due to its high costs. However, VR has become more common in the industry because of its increased cost competitiveness. VR, having its practical applications in learning manufacturing, has inspired this research to evaluate its extended use in developing a workforce for fiber-reinforced composite manufacturing.
2.4 Effective VR System Design
Designing an effective VR system involves several key considerations to ensure that the system is user-friendly, immersive, and provides a positive user experience. The choice of hardware and the development platform is essential to ensure these features in designing and developing a virtual reality system. HTC Vive hardware system is widely used for its proven performance in precision and tracking quality (Ikbal, Ramadoss, and Zoppi 2021). Compared to similar head-mounted displays from Oculus (Currently Meta), Valve, and Samsung, the HTC Vive Pro showed balanced qualities in the main display and tracking (Angelov et al. 2020). This hardware system has been used numerous times in developing successful VR programs (Quevedo et al. 2017; Matsas and Vosniakos 2017). Unity is the most favorable development platform because of its previous application in scientific research (Dai 2022; Gabajová et al. 2021; Deb et al. 2017; Rahman et al. 2022) and provides free access for non-commercial use. Dai (2022) proposed a system to develop a simulation system for dancing using Unity. Deb et al. (2017), in their study of determining the efficacy of VR in pedestrian safety, also used Unity as the development platform. Gabajová et al. (2021) also acknowledged the effectiveness of Unity in their design and development of a 3D virtual workspace.
Different interaction methods are available in a virtual reality system, e.g., teleportation, grabbing and realizing, recasting, etc. The form of teleportation to move a player from one location to another inside the virtual environment is very effective and has a low possibility of inducing simulation sickness (Moghadam, Banigan, and Ragan 2020; Prithul, Adhanom, and Folmer 2021; Bozgeyikli et al. 2016). Using controllers to grab and release objects induces a natural feeling among users (Lin & Schulze, 2016). Ray-casting is a widespread pointing and selecting object technique that can be used in in-game menu design (Lee et al. 2003). These are the common types of interaction methods also used in this research to simulate the best user experience in attending the training.
2.5 Evaluation of Virtual Systems
Evaluation of virtual learning platforms is crucial to confirm the developed system’s usability, users’ safe and easy navigation and interactions with less workload, users’ accessibility, and realistic and practical functionality of the system. The usability of a system refers to how easy it is for users to learn, understand, and use the system to achieve their goals effectively and efficiently. Usability is an essential aspect of user experience design and development, as it directly impacts the satisfaction and productivity of users. The System Usability Scale (SUS), developed by Brooke (1996), is a robust and versatile tool for measuring the perceptions of usability (Bangor et al., 2008). It is a popular subjective measurement tool comprising a 10-item questionnaire with a five-point Likert scale, ranging from "strongly agree" to "strongly disagree." SUS can be useful for assessing the usability of the VR platform’s interface, controls, and overall experience. Participants rate their experience with a system under study on each item, and the results are combined to generate a single SUS score ranging from 0 to 100. A score of 68 is considered average, with higher scores indicating better usability (Sauro 2011). The SUS has been previously used to investigate different virtual reality systems (Webster and Dues 2017).
Workload refers to the available physical and cognitive resources required to perform or complete tasks. It can be affected by various factors, such as the job’s complexity, the amount of information presented, the level of experience and expertise of the individual, and the level of stress or fatigue. In the context of training and learning, the workload can have a significant impact on the effectiveness of the training or learning experience. If the workload is too high, individuals may become overwhelmed, leading to decreased performance and retention of information. On the other hand, if the workload is too low, individuals may become bored or disengaged, leading to decreased motivation and effort. To optimize workload during training and learning, it is essential to carefully design the learning materials and activities to ensure they are appropriately challenging for the intended users. This can involve breaking down complex concepts into smaller, more manageable pieces of information, providing clear and concise instructions, and incorporating interactive activities and assessments to help individuals actively engage with the material. Virtual training will likely support these requirements to optimize users’ workload in training. The NASA Task Load Index (NASA-TLX), developed by Hart (1986), is a subjective workload assessment tool used to measure a task or system's perceived mental, physical, and temporal demands. This tool has also been used to determine the perceived work effort inside a VR module in much past research (Dayarathna et al. 2020; Kournaditis, Chinello, and Venckute 2018; Armougum et al. 2019; Marucci et al. 2021).
To confirm users' safe exposure to virtual reality, researchers assess the simulation sickness induced in a participant from exposure to VR. One great tool to evaluate is the Simulation Sickness Questionnaire or SSQ (Kennedy et al. 1993). The questionnaire asks participants to rate sixteen symptoms of sickness from the simulation on a four-point scale (0–3). A total score of 5 or more indicates unsafe health conditions for VR exposure and does not allow users to continue using VR. This can be a limitation of VR training which can only be addressed by improving graphics quality and enhancing the realism of the immersion experience. However, safe VR exposure will always be limited, even with an optimized quality build. The VR platform’s realism can be tested by selecting important features used in the development. A realism score for these features can be collected on a 5-point Likert scale, with 1 being the least realistic and 5 being the most realistic. These scores can be used for the face validity of the VR platform by using an informal review of VR elements from non-experts. Face validity refers to the extent to which each selected feature is close to reality. Many past studies used face validity to confirm the realistic presentation of objects within a VR platform (Deb et al., 2017;Sethi et al.,2009).Carruth (2017) noted that the objective of VR is to facilitate the transfer of knowledge from VR to the real world. To evaluate the effectiveness of the virtual system, it is essential to expose novice trainees to the VR training, collect data on their performance, and transfer it to the real-world task to compare the real-world performance with those who only had traditional training. A great example would be the research conducted by Thomsen et al. (2017), where a high correlation between virtual reality and real-life performance for complex tasks like cataract surgery was observed. This research also emphasized forming groups with and without VR training to compare their performance on a virtual reality simulator and motion-tracking metrics from real-life cataract surgery.
This study used a virtual training platform for the compression molding process. Two operator groups were compared in their in-situ compression molding tasks: one group received virtual training while the other did not. Both subjective and objective measures were collected to investigate significant differences between the two groups’ performance and workload. The findings shed light on the potential of virtual training as a workforce development tool for improving worker performance and reducing their workload in real-world industrial training settings while effectively transferring knowledge from virtual tasks to real-world tasks.