Fiber-reinforced polymers have been used widely in aerospace, transportation, and other industrial sectors owing to their advantageous performance, such as high specific strength, design flexibility, and excellent resistance against fatigue and corrosion, which are superior to those of conventional metal materials. In particular, fiber-reinforced thermoplastic polymers (FRTPs) have a vast potential market and are receiving more attention because of their shorter production cycles, higher productivity, and lower production costs.
Compression molding is a typical forming process used to manufacture FRTPs in which temperature is a crucial parameter [1
]. The thermally induced residual stresses cause structural deformations after demolding, leading to material failure and even catastrophic failure of the whole structure [3
]. Thus, in situ real-time temperature and strain/stress monitoring during compression molding are eagerly desired for investigation of the forming process and optimizing the quality of final products. Traditionally, thermocouples (TCs) and strain gauges are used to measure the temperature and strain, respectively. However, the normal type of sensors is not suitable for embedded detection as severe initial damage of the composite may be induced by their relatively large sizes. Other additional issues, e.g., the electromagnetic interference, also hinder their practical application though there are advanced TCs with small sizes [4
]. Thus, both academia and industry are keen to develop a new measuring technique that can effectively monitor the composite-forming process without affecting its mechanical properties after the forming process.
In recent decades, fiber Bragg grating (FBG) sensors have been embedded into the composite to monitor the forming process, because of the advantages of small size, high-temperature resistance, and electromagnetic interference immunity. When an FBG serves as a temperature sensor embedded in composite, an encapsulating technique is usually necessary to eliminate its strain influence because of the cross-sensitivity of the FBG to the strain and temperature. Although FBG has been proven to measure temperature above 1000 °C [5
], its application of in situ temperature monitoring in the harsh environment of the composite forming process, that has high local pressure with uneven distribution, has not been thoroughly investigated yet. Takeda et al. [7
] monitored the temperature of a carbon/epoxy panel manufactured by vacuum-assisted resin transfer molding in its curing process under 120 °C with a measurement accuracy of 0.1 °C. Guo [8
] measured the temperature up to 180 °C during the forming of a laminate that was made of T300/HD03 (carbon/epoxy) prepregs and had an asymmetric cross-ply stacking sequence. There are more studies on temperature monitoring of thermoset composite using encapsulated fiber Bragg gratings (EFBGs) at temperatures lower than 200 °C [9
]. Mulle et al. [12
] monitored a glass fiber/thermoplastic matrix laminate by embedding EFBGs at different locations throughout the thickness of the laminate, and the temperature during fast or slow cooling rates was recorded during compression molding. The maximum forming temperature measured in his experiment was 220 °C. The Bragg wavelength shift was treated as linearly proportional to the temperature in this range, although the nonlinearity between the Bragg wavelength shift and the temperature was recognized by the authors. As these existing techniques restrict the measuring temperature lower than 220 °C and measuring object to unidirectional or cross-ply composite, they cannot satisfy some specific applications under harsh conditions. For example, polyetherimide (PEI)-matrix woven fabric thermoplastic composite raises high demands on both optical and mechanical robustness of the FBG sensor head. A high temperature of 332 °C is needed to achieve the sufficient melt index of the thermoplastic matrix and ensure the molding capability of the laminate. At such a high temperature, conventional FBG exhibits an irreversible wavelength shift of approximately −0.2 nm [13
] and shows a nonlinear relationship between the temperature and Bragg wavelength shift [14
], leading to measurement errors. Thus, the repeatable relationship of the Bragg wavelength shift and temperature, namely optical robustness, should be ensured. The mechanical robustness of the sensor head implies a stable sealant and strong capillary. The PEI at the rubbery stage is fluid, and thus, demands a stable sealability of sealant at high temperature to prevent resin from immersing into the capillary [15
]. The chosen prepregs are woven from perpendicular warp and weft yarns, and the applied pressure in the compression molding of FRTP is high, e.g., 2 MPa for PEI-matrix composite. As a result, the sensor head should be capable of withstanding high local pressure and prevent the inside FBG from distortion. All of the high processing temperatures, microwoven structure, and high pressure challenge the feasibility of the embedded sensor and obstruct the development of composite-forming process monitoring.
Besides temperature monitoring, the strain monitoring of composite materials using FBG also has been intensively researched. Michael et al. [16
] used bare FBGs to monitor strain in glass-fiber-reinforced thermoset composite laminates. Takuhei et al. [17
] monitored the residual strain in thick thermoplastic composite laminates using embedded FBG sensors. More research on in-plane strain monitoring can be found in references [18
]. Minakuchi [21
] monitored the out-of-plane strain during the curing process of unidirectional thermoset composite laminates by embedding an FBG in the through-thickness direction as the out-of-plane shrinkage is also a key deformation [22
]. However, it is very difficult to embed FBGs in woven fabric composites in the same way. In these existing techniques, strain measurement usually requires an additional sensor and/or instrument to decouple the temperature- and strain-induced Bragg wavelength shift, resulting in a redundant system and high cost. In addition, residual stresses that are usually calculated by multiplying the strain and the material modulus are not directly available owing to the sensing principle of the FBG. However, because residual stresses play a crucial role on influencing the structure deformation after demolding and onset of damage in service of the composite [3
], both the academia and industry eagerly prefer a single sensor that can reflect the change of residual stresses of the composite while does not increase the burden of equipment and investment.
In this research, a hybrid temperature and stress monitoring of a woven fabric composite laminate that has glass fiber-reinforcement and PEI matrix in its compression molding process was achieved using a single FBG based sensor. The problems of high temperature, high pressure, and the influence of microwoven structure to conventional FBG-based in situ monitoring technique were solved by properly designing a pre-annealed sensor head and a data-processing method. The experimental results demonstrated the reliability of the novel sensing technique through a comparison of the temperature and strain measured by the TCs and strain gauges. The temperature curve of the composite laminate during heating, dwelling and cooling phases was precisely described, and the out-of-plane residual stress was also revealed through the micro-bending-caused optical loss of the fiber pigtail without the use of an additional instrument, which helps to understand the different states of the woven fabric thermoplastic composite in the forming process.
3.1. FRTP Laminate
The composite laminate was manufactured from the prepregs (TC1000 Design, TenCate Cetex®
, Nijverdal, The Netherlands) that consist of PEI as matrix and plain woven glass fibers (Glass 7628) as reinforcement. The properties of PEI and glass fiber at room temperature are shown in Table 1
. The ply thickness of the Cetex®
prepreg is 0.16 mm. Twenty plies of prepregs with the same layup direction were used in this experiment.
3.2. Forming Procedure
In the temperature-monitoring experiment, two EFBG sensors and a TC were embedded in the middle of the laminate with a size of 200 × 200 mm, as shown in Figure 5
. Both the EFBG sensors and TC were laid 52.5 mm away from the center. The 40-mm embedding length of the pigtails of the EFBG sensors ensures sufficient reflectivity. EFBG1 and EFBG2 were placed along the warp and weft, respectively. The head of the reference-sheet TC has a size of 7 × 7 × 0.2 (L × W × H) mm, and its lead wire has a diameter of approximately 2 mm. It is highly possible that embedding TC with large-diameter wires into the laminate will cause initial material damage, and then affects the mechanical performance of the manufactured laminate. In contrast, EFBG has better minimal invasion capability due to its diameter of 500 μm.
In the strain-monitoring experiment, two strain gauges (KFRP-5, KYOWA, Obu, Japan) were embedded parallel to the EFBG1 to monitor the local in-plane strain in the warp directions to clarify the mechanism of reflectivity change during the entire forming process. The lead wires of the strain gauges were glued using ethyl α-cyanoacrylate, whereas the sensor head was not fixed to the target material.
The woven fabric composite with the sensors was placed in a hot press (3690, CARVER, Wabash, IN, USA). An interrogator (sm130, Micron Optics, Atlanta, GA, USA) with a sampling frequency of 1 kHz and a resolution of 0.1 pm was used to record the Bragg wavelength shift. The temperature and strain were collected using a data logger (NI9219, National Instrument, Austin, TX, USA) and static strain meter (XL2101B, XIELI, Qinhuangdao, China).
The forming protocol of the TC1000 prepregs is provided by TenCate and can be divided into three phases. In the heating phase, the target temperature was set to 332 °C. In the dwelling phase, a pressure of 2 MPa was applied to the prepregs. In the cooling phase, the laminate was naturally cooled to room temperature together with the molds.
3.3. Manufactured Laminate
presents the manufactured laminate. The embedded EFBG was barely visible to the naked eye, as the surface of the composite was flat. In contrast, the embedded TC was obvious, even observed from outside. It is believed that the TC induces initial damage to the composite.
After the composite was formed, a laminate was cut, and the cross sections of the head and pigtail of the EFBG sensor were observed under a microscope. Figure 7
shows that the sensor head did not deform under the forming pressure, and the composite does not have voids, cracks, or delamination. It has been reported that resin-rich areas resulting from an embedded bare optical fiber do not affect the mechanic performance of composite significantly [31
]. Therefore, it is believed that a slightly larger resin-rich area existing around the EFBG sensor head has a limited impact on composite performance. Moreover, the resin-rich area can be reduced if the size of the sensor head is optimized in the future.
In this research, the hybrid temperature and stress monitoring of a woven fabric thermoplastic composite could be achieved by using a newly designed FBG based sensing technique. The robust sensor head was manufactured using a steel capillary and high-temperature resistant sealant, and was then pre-annealed at a temperature of 350 °C, leading to the capability of isolating both the axial and transverse strains and withstanding high temperature and high pressure. The nonlinear relationship between the Bragg wavelength shift and the temperature higher than 150 °C was fixed with the cubic polynomial fitting. Then, it is demonstrated that the forming temperature of the woven fabric composite within the heating, dwelling, and cooling phases can be measured by the sensor in both the weft and warp directions, showing only 2.92 °C difference in the temperature measured by the reference TC. It is also found that the microwoven structure of the composite will induce extra loss of the light power along with the optical fiber pigtail when the temperature is lower than the glass-transition temperature of the resin, which can be reversely used to indicate the forming residual stresses according to the reflectivity decrease of the FBG spectrum. In addition, except for a small resin-rich area, the sensor head does not induce severe initial defects in the formed composite due to its small size. The experiments were repeated three times to ensure the reliability of the newly developed technique.
This experiment explores the changes of temperature and stresses of the woven fabric thermoplastic composite during the forming process, underpins the in situ monitoring technique of embedded sensors in harsh conditions, helps to understand the relationship between the set temperature and actual temperature within a composite laminate during forming, and finds the regularity of residual stresses and reflectivity changes. This new sensor may be also multipliable. For example, EFBG with both lead-in and lead-out lines can be manufactured by bonding both ends of a slightly-bent FBG to the capillary. Multiples of this kind of EFBG sensors can be cascaded to form a sensing network and measure the temperature at multiple points. This EFBG sensor after further development could be used in process monitoring of thermoplastic composite fan blade with high processing temperature and complex residual stresses, and process monitoring of hydrogen composite vessel that has a thick composite layer and complex microstructure.