Polymer composites with carbonaceous micro and nano-scale reinforcement have been extensively investigated due to their outstanding mechanical, electrical, and thermal properties. In particular, nanocomposites not only have remarkable properties that can be tailored for broad application in many fields, but their processability is also simple. Nanocomposites could help the development of light-weight structural materials and functional materials (Park and Seo, 2012; Mittal et al., 2015; Chen et al., 2018; Mohan et al., 2018). One interesting possibility offered by functional nanocomposites is to exploit their piezoelectric response for structural health monitoring (SHM) (Pegoretti, 2019). Sensing internal strain/stress can allow the detection of damage within a structural member during their lifetime (Georgousis et al., 2015; Moriche et al., 2016b; Saleh et al., 2019). This self-sensing with functional materials in fact, is a cost-effective method in comparison to other SHM methods such as acoustic emission or sonic infrared imaging.

Strain sensing with electrical conductive nanocomposites is based on the electrical resistance changes induced by deformation (piezoresistivity) and damages (loss of continuity) under loading conditions. The piezoresistivity behavior in nanocomposites is induced by the destruction of the conductive networks of nanofiller, the modification of tunneling resistance change in neighboring nanoparticles because of change of distance between them, and changes in piezoresistive of nanofillers themselves during applied deformation. Among these three factors, the first two are most likely the most influential on the electrical resistance upon mechanical deformation (Georgousis et al., 2015). The common matrices used for strain sensor materials can be thermosetting (Ku-Herrera and Avilés, 2012; Moriche et al., 2016b; Sanli et al., 2016), thermoplastics (Georgousis et al., 2015; Bautista-Quijano et al., 2016; Dawoud et al., 2018), and elastomers (Bautista-Quijano et al., 2010, 2013; Oliva-Avilés et al., 2011; Alsharari et al., 2018; Christ et al., 2019; Kim et al., 2019).

In recent years, increasing interest has focused on the producing of sensors through 3D printing technology or embedding 3D-printed components to traditional sensors. The main applicative areas are represented by electronics, force, motion, hearing, optics, etc… (Xu et al., 2017). 3D printing sensors have been achieved by several methods such as fused filament fabrication (FFF) (Alsharari et al., 2018; Dawoud et al., 2018), direct ink writing (DIW) (Muth et al., 2014), stereolithography (SLA) (Lee et al., 2015), laminated object manufacturing (LOM) (Park et al., 2012), selective laser sintering (SLS) (Ambrosi et al., 2016), photopolymer jetting (Polyjet) (Laszczak et al., 2015), and binder jetting (3DP) (Rivadeneyra et al., 2015). The frequently used conductive fillers for strain sensing applications are metal nanoparticles [e.g., silver (Lee et al., 2015), copper (Credi et al., 2016; Saleh et al., 2019), and Ti/Au (Cho et al., 2015)] and carbon-based fillers [e.g., carbon nanotubes (CNT) (Czyżewski et al., 2009; Bautista-Quijano et al., 2010; Oliva-Avilés et al., 2011; Pedrazzoli et al., 2012a; Zhao et al., 2013; Georgousis et al., 2015), carbon nanofibre (Pedrazzoli et al., 2012a), graphene (Moriche et al., 2016a,b; Alsharari et al., 2018), and carbon black (Dawoud et al., 2018; Zhao et al., 2018)]. In particular, however, only few reports are available on piezoresistive materials obtained through FFF technique, which is the dominated technique in 3D printing of polymers. Highly stretchable materials consisting of the blend of graphene-based polylactic acid (PLA) with thermoplastic polyurethane (TPU) were produced through the FFF process (Alsharari et al., 2018). The behavior of the obtained 3D-printed conductive composites was reversible until strain levels as high as 50%. Strain sensing of carbon black (CB) filled acrylonitrile butadiene styrene (ABS) nanocomposites was also investigated (Dawoud et al., 2018). The 3D-printed parts were produced with different raster angles and air gap parameters. PLA-carbon based nanocomposites derived from 3D printing have been also investigated on electrical and/or thermal conductivity (Guo et al., 2019; Ivanov et al., 2019). In order to produce strain sensor by using PLA conductive composites (Maurizi et al., 2019), the dependence of piezoresistive behavior on temperature should be properly considered. In particular taking into consideration the Tg of PLA matrix, a specific approach to compensate the temperature effect on resistivity of PLA conductive sample in the range 20–50°C has been discussed (Daniel et al., 2018; Coleman et al., 2019). The crucial effect of heating and distorsion in PLA conductive composites after voltage application has been recently detailed in dependence on the various process factors (extrusion and additive manufacturing); the authors compared the role of conductive filler (carbon black, carbon nanotubes, and nano copper wires) on resistivity in view of application for thermal sensors and piezo-resistive sensors (Watschke et al., 2019). Commercial graphene-PLA filaments with resistivity of 0.6 Ω.cm, are designed to be used for room-temperature operation, due to the low softening temperature (50°C), and for low-voltage and low-current only (lower than 12 volts and 100 mA, respectively)¹. The use of low-cost conductive composite (Carbomorph) based on polycaprolactone with resistivity of 9–12 Ω.cm is also limited for piezoresistive sensors at room temperature, or at lower temperature, due to melting temperature of polymer matrix (65°C) (Leigh et al., 2012). In order to enlarge the application fields with high performance properties, especially for high temperature, a different approach was recently proposed with the processing and 3D printing up to 380°C of thermoplastic polyimide (TPI) filled with CNT; the change of resistance under cyclic bending deformation were properly studied and considered for aerospace application (Ye et al., 2019). A commercial filament Proto-Pasta PLA filled with carbon black was 3D printed as a strain sensor (Munasinghe et al., 2019). The authors determined a nearly linear relationship between the electrical resistance (up to 6.05%) and the strain; in the same time, they concluded that a long term suitability of sensor materials for creep loading and a better understanding of their viscoelastic behavior would be object of future research.

In our previous works, highly conductive ABS/CNT nanocomposites with 6 wt.% of nanofillers were successfully 3D-printed through FFF process and their extensive characterization including tensile, thermal and electrical properties was reported (Dul et al., 2018a). The composition percentage of 6 wt.% of CNT was properly selected in the tested range 2–8% wt.%, as an adequate compromise between the improvement of some properties after addition of the filler, and the correspondent reduction of composite processability, as evidenced by the critical decrease of melt flow (Dul et al., 2018b) and melt viscosity (Ecco et al., 2018). Electrical and magnetic properties of both graphene and CNT nanocomposites were studied and compared in view of EMI-SE applications (Ecco et al., 2018). Moreover their hybrid 6 wt.% nanocomposites with composition of graphene and CNT were also considered, in order to define suitable composites with relatively easy flowability for low CNT content, and adequate electrical and magnetic properties for high CNT content (Dul et al., 2020). Another recent work reported about piezoresistivity of ABS filled with 5% CNT nanocomposites through 3D printing with resistivity of about 100 Ω.cm. The authors studied and compared a balanced effect of filler content and process on the fracture properties and on the samples conductivity (Thaler et al., 2019). On the other hand, a much lower resistivity in the range of about 29 and 0.9 Ω.cm was obtained for CNT/ABS plates with composition between 2 and 8 wt.% of nanofiller, respectively (Dul et al., 2018b).

Therefore, in the actual scenario of the additive manufacturing for the fabrication of conductive and functional parts, the present work has been also devoted for a specific evaluation of various effects, such as temperature, applied strain rate and loading/unloading cycles. In particular, the strain monitoring of ABS nanocomposites at 6 wt.% of CNT prepared by FFF technique has been investigated. Two different raster angles and different testing conditions cyclic test were evaluated and compared to test the piezoresistive behavior of 3D-printed parts, not only in short term testing (i.e., temperature effect, fracture test, and applied strain rate), but also in long term testing such as creep experiments and cycling test, for the evaluation and evolution of gauge factor.

Before studying the piezoelectrical behavior, morphological analysis of filaments, conductivity/resistivity measurements at various temperature and dynamical mechanical thermal analysis of 3D printed specimens were preliminarly considered.

An initial morphological analysis was performed on composite filament as extruded by twin screw extruder. Figure 3 shows TEM microanalysis of parallel surface of CNT/ABS 6 wt.%, and various carbon nanotubes appear almost randomly dispersed with some point of contact, with evidence of a connective path. Supplementary Figures are also showing the distribution and connection of CNT in both perpendicular and parallel cross-section. It should be noted that the percolation threshold of CNT in ABS has been determined at 0.42 vol.% and no electrical conductivity has been observed for CNT content lower than 2 %wt. (Dul et al., 2018b).

Then, in order to confirm the conductivity of 3D-printed samples, their electrical resistivity was assessed at values of 17.2 ± 1.5 and 31.2 ± 4.9 Ω.cm for HC and H45 items, respectively. As expected these values are higher than the values previously determined on compression molded plates (1.5 Ω.cm) (Dul et al., 2018b), and on the extruded filaments (4.1 Ω.cm) (Dul et al., 2018a). The increase of resistivity of extruded specimens could be attributed to the higher level of anisotropy and alignment in nanocomposite filaments, with respect to the molded plates where conversely CNT are isotropically distributed, as documented in literature (Georgousis et al., 2015; Sanli et al., 2016; Dul et al., 2018b). Moreover, the processing effect is further evidenced in 3D printed specimens and it is directly attributed to the internal structure of FFF samples. In fact, the resistance R₀ of 3D-printed samples has been found to be dependent on the manufacturing infill pattern, as summarized in Table 1. In particular, significant different average resistances were observed, being 387 ± 34 Ω for HC and 671 ± 106 Ω for H45 samples, respectively. The relatively low standard deviation confirms the quality and the good homogeneity and reproducibility of the manufacturing process.

Moreover, in order to better understand the performances at various temperatures, the results of dynamic mechanical analysis on 3D-printed specimens is briefly presented and discussed. The storage modulus and loss factor values of ABS/CNT nanocomposites as a function of temperature are reported in Figure 4. Two transitions can be observed as loss tangent peaks which can be attributed to the glass transition of butadiene phase (B-phase; T_g1 = −84°C) and the glass transition of styrene–acrylonitrile phase (SAN phase; T_g2 = 125°C), respectively. This means that at room temperature, the behavior of ABS is depending on the concurrent effects of two different phases, rubbery and glassy, respectively. DMTA thermogram reveals that in the range −45/+75°C between two main transitions both storage modulus and loss factor evidence only a slight linear dependence without any other secondary transition. Consequently, taking also into account that the maximum using temperature is 83°C (determined by HDT at 1.8 MPa according to technical data sheet), precautionally the interval −25–60°C evidenced in Figure 4 has been selected for investigating electrical resistance and mechanical response.

Two parallelepiped specimens built-up along different orientations, HC and H45 with resistance of 321 and 685 Ω, respectively, were selected for monitoring their electrical behavior in various isothermal conditions. The results are showed in the inset of Figure 4. No significant change in resistance was observed at 60°C; and only a very low variation of resistance of about +0.7 and +1.6% for HC and H45 samples, respectively, were evidenced at the lowest temperature. This behavior suggested that the performance of samples is stable in the selected range of temperature in direct dependence on the absence of transitions, as indicated by DMTA analysis. On the other hand conductive PLA devices evidenced significant variations of resistivity with temperature and some specific corrections have been properly suggested for analysis up to 50°C (Daniel et al., 2018). For these findings, ABS conductive nanocomposites appears more suitable for application at least up to 60°C, being the polymer matrix in the glassy state until about 90°C.

This work shed more light on the application of conductive nanocomposites for strain monitoring, evidencing the potential of piezoresistive behavior of 3D-printed CNT/ABS composites in short term and long term testing. In particular the strain sensing capabilities of electrically conducting samples of ABS containing 6 wt.% of CNT produced by FFF with two different infill patterns (HC and H45) were evaluated. The electrical resistance of the 3D-printed specimens was observed to be dependent on the manufacturing design and to maintain almost constant values in the temperature range between −25 and 60°C. Electrical resistance changes were also monitored during short mechanical testing under different loading conditions. An initial gauge factor has been determined for each 3D printed item, and it was found directly dependent on the infill pattern. In particular, the higher sensitivity of the 3D-part within H45 pattern in comparison to HC, according to the measurement of the gauge factor. A change of the strain rate in the range of 0.3–10%/min has no significant effect on gauge factor, which is about 5.0 for HC and 7.0 for H45 part. An abrupt variation of resistance of the specimens was detected at higher deformation when damage starts to develop approaching the failure of specimen. Long time testing of conductive behavior of thermoplastic glassy polymers has been also presented for the first time, according to the authors' knowledge. Both creep experiments and cycling tests evidence the reduction of resistivity and gauge factor. The CNT conductive network path seems to be reformed during creep and fatigue test with a progressive reduction of gauge factor till an almost constant of about 2.5 for all samples. This behavior indicates a sort of rearrangement of the composite material, independently on the infill pattern, and directly related to the CNT content. In conclusion, the selection of material composition and the manufacturing of products confirmed the positive results in terms of potential applications in the field of strain monitoring.

In particular ABS conductive systems evidence a much larger interval of temperature application, with respect to commercial type composites based on PLA matrix. Moreover the composition of 6 wt.% of CNT in ABS can be relatively easy processed in quite common conditions, much more convenient with respect to high performance conductive products, such as thermoplastics polyimide, that require very high processing temperature and specific equipments for production, and in the same time guarantee a high temp application conditions.

The limits of the tested 3D printed for strain monitoring applications are the maximum using temperature (no more than 80°C) and the maximum deformation of strain 2–3%.

Each manufactured item requires to be properly calibrated and tested for the determination of its intrinsic gauge factor, and to evaluate any stabilization level depending on time and/or temperature. Some further research developments could be devoted to the long time aging up to 1–3 years, and to the evaluation of accelerated temperature aging.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.

SD, AP, and LF conceived and designed the research work, analyzed the data, and wrote the paper. SD performed the experiments.