# Exploring Thermal and Mechanical Strains During Laying Down Process in Fused Filament Fabrication

Boubakeur Mecheri, Jaianth Vijayakumar, Elodie Boller, Sofiane Guessasma

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Boubakeur Mecheri, Jaianth Vijayakumar, Elodie Boller, Sofiane Guessasma. Exploring Thermal and Mechanical Strains During Laying Down Process in Fused Filament Fabrication. Macromolecular Materials and Engineering, 2025, e00331, 10.1002/mame.202500331. hal-05490723

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RESEARCH ARTICLE

OPEN ACCESS

# Exploring Thermal and Mechanical Strains During Laying Down Process in Fused Filament Fabrication

Boubakeur Mecheri<fnref n="1" /> | Jaanth Vijayakumar² | Elodie Boller² | Sofiane Guessasma³

**Correspondence:** Sofiane Guessasma (sofiane.guessasma@inrae.fr)

**Received:** 11 August 2025 | **Revised:** 15 October 2025 | **Accepted:** 6 November 2025

**Keywords:** fused filament fabrication | infra-red temperature measurements | mechanical strain | thermal cycling

## ABSTRACT

Residual stress and internal strain in 3D printing can result in cracking and delamination. Here, we investigate the strain incurred during the deposition process in fused filament fabrication with a focus on polylactic acid (PLA) polymer. Specific geometries are employed to analyse thermal cycling and strain development, utilizing high-resolution infrared camera and strain gauges. We investigate various printing parameters such as printing temperature, base temperature, and printing speed. Both thermal cycling and strain development were characterized throughout the printing procedure. Our results show that strain arises from both mechanical pressure exerted by the nozzle and thermal transfer between layers. During heating, strain reached up to 0.25 με (microstrain), while cooling induced compressive behavior. The real strain, independent of thermal effects, peaked at 0.07 με, with the first five layers showing the largest variations. Increasing base temperature to 80°C reduced strain fluctuations (0.009–0.010 με). X-ray microtomography revealed 0.22% porosity, and thermal imaging measured heat penetration up to 4 mm, with cooling rates of –130°C/s slowing to –75°C/s. These findings highlight the critical role of printing parameters on residual stresses and structural integrity in 3D printed PLA.

## 1 | Introduction

Additive Manufacturing (AM) has emerged as a groundbreaking technology that has attracted considerable attention in recent decades [1, 2]. It holds significant potential as a manufacturing technique for creating highly intricate technical components [3]. A commonly accepted definition of AM involves the methodical layer-by-layer deposition of material, guided by a digital model [4]. One of AM's distinguishing characteristics is its ability to precisely control material deposition on a local scale [5], allowing for the full customization of parts with minimal reliance on traditional tooling [6]. This capability also facilitates the development of innovative material types, such as adaptive materials [7]. The inherent speed of fabrication in AM makes it an appealing technology across various industries, including bioengineering [8, 9], textile industry [10, 11], aerospace, construction, prototyp-

ing, automotive manufacturing, culinary arts, and artistry, among others [12–16]. In the biomedical field, Hasanzadeh et al. [17] developed and evaluated a hybrid auxetic stent combining arrow-head and missing rib geometries, fabricated via fused filament fabrication 3D printing. The authors demonstrated the auxetic stent's superior adaptability, structural integrity, and potential for advancing vascular implant design. In textile industry, Yang et al. [11] developed 3D-printed textiles using cotton fibres dissolved in a LiCl/DMAc solvent with hydroxyethyl cellulose to enhance printability. Garments and gloves fabricated from these materials demonstrated the potential of cellulose-based inks for producing comfortable, flexible 3D-printed textiles.

The widespread adoption of AM in multiple sectors can be attributed to the diverse processes encompassed by its definition, which enable the printing of a broad range of materials [18].

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). Macromolecular Materials and Engineering published by Wiley-VCH GmbH

<footnote n="1">¹Institut de Recherche IR, ESTP, Cachan, France | ²ESRF, Grenoble, France | ³Research Unit BIA UR1268, INRAE, Nantes, France</footnote>
For instance, fused Filament technology or FFF (Fused Filament Fabrication) is a notable and cost-effective method for producing polymeric structures [19]. Key performance aspects such as energy absorption [20, 21], fatigue behavior [22], and thermal stress mitigation [23] are among the trending topics in this field.

Polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) have been among the primary filament materials investigated in recent decades [24, 25]. For instance, initial research by Ahn et al. [26] emphasized the importance of part orientation in creating an anisotropic behavior in ABS printed material. Enhancing FFF involves advancing high-performance feedstock materials [27], such as ceramic-based composites [28], and carbon fibre-reinforced composites [29–31] to mitigate the typical loss in mechanical performance caused by process-generated porosity. There has also been considerable interest in metal-reinforced polymers [30]. Buj-Corral et al. [32] examined the effect of porosity in copper-reinforced PLA, while Martinez et al. [33] explored the 3D printing route to develop iron-filled PLA for microwave absorption applications. Török et al. [34] investigated the relationship between fused filament manufacturing parameters and the mechanical performance of various metal-reinforced PLA composites, including those with copper, steel, and iron fillers.

The complexity inherent in measuring the forces applied by filament deposition in 3D printing presents significant technical challenges. This specialized field involves intricate parameters that defy conventional evaluation methods. Among the drawbacks of FFF is the buildup of internal residual stresses and strains during layer deposition, stemming from rapid transitions between physical states (such as repeated cycles of melting and rapid cooling). These transitions result in an uneven distribution of stresses and strains within the printed object, potentially compromising dimensional accuracy and leading to issues like delamination and cracking [35]. Conventional stress measurement techniques often fall short in adequately quantifying these challenges, highlighting the need for innovative approaches in stress assessment during filament deposition [36]. To accurately quantify the forces exerted by the molten filament and internal residual deformation stresses, advanced measurement techniques and a thorough understanding of the underlying mechanisms are required. Addressing this challenge requires integrating cutting-edge methodologies tailored to the unique characteristics of 3D printing, such as fibre Bragg gratting sensors (FBGS) [37]. The distinctive nature of this manufacturing process demands an innovative and specialized approach, leveraging advanced technologies to ensure optimal precision [38].

A variety of technologies with different levels of sensitivity, accuracy, and applicability are available for strain sensors measurement and the coefficient of the thermal expansion (CTE) [35]. In order to measure stress and temperature during and after the sample fabrication, they used fibre-optic sensors (FOS) based on FBGS embedded in samples [39]. The materials printed must have a melting temperature compatible with the temperature limits of the optical fibres. The materials tested include PLA, PET-G, ABS, HIPS, as well as composites based on carbon, glass fibre, aramid, and other fillers derived from these plastics.

Regarding strain gauges, the literature survey reveals two primary trends: sensors specifically designed for AM and the use of AM technologies for the development of sensors [40–43]. Shin et al. [42] used fused filament fabrication route to achieve crack-based strain gauges for healthcare monitoring application. The authors demonstrated larger sensitivity and reliability in comparison to flat sensors. Rahman et al. [41] considered the development of strain sensor for high temperature applications using a combination of aerosol jet-based additive printing route. The authors showed that nano-Ag-based sensors can be operated up to 500°C with a high gauge factor value. Stano et al. [43] investigated the impact of process conditions on developing electrical sensors using fused filament technology. They employed a design of experiment approach to examine the effects of ironing, nozzle temperature, and overlap in fabricating PLA/carbon black-based sensors. The study concluded that optimal selection of printing parameters improved electrical conductivity and tensile strength while reducing porosity in the manufactured electrical sensors.

Also, examples are reported for the use of gauge sensors to monitor stress and strain accumulation during AM. For instance, Arbogast et al. [44] studied thermal expansion in metal-based AM. Strain gauges placed on the surface of the parts showed thermal expansion properties of 3D printed part comparable to those of normal steel. In addition, the authors found a high coefficient of thermal expansion at the fusion zone between the steel and the Invar. The authors concluded that AM can be used as a route to decrease thermal expansion in multi-material part tooling. Chen et al. [37] used fibre Bragg gratting sensors to monitor the strain and stress in FDM printed lamellae. The authors found residual stresses in the range 0.5–3 MPa. In addition, the study concluded on the negative correlation between the residual stress and the layer thickness.

In this study, we combine thermal and strain gauge sensors to monitor the strain evolution during thermal cycling in PLA obtained under different printing conditions. The present study differs fundamentally from previous research through its integrated approach combining thermal and strain gauge sensors to monitor strain evolution during thermal cycling in PLA samples produced under different printing conditions. This dual-sensor setup enables simultaneous tracking of both thermal and mechanical responses, offering greater accuracy and temporal resolution than studies relying on single-sensor methods. The results are discussed with respect to complementary investigations, including X-ray microtomography and infra-red recording of the printing process. Both analyses are undertaken under high resolution, allowing microstructural details as small as 10 µm to be captured under thermal cycling thanks to synchrotron radiation experimental campaign conducted at the European Synchrotron Research Facility (ESRF)—Grenoble, France (beam line BM05) and high resolution bolometric thermal camera.

## 2 | Experimental Layout

### 2.1 | Materials and Processes

The filament employed as feedstock material for fused filament fabrication (FFF) is a 1.75 mm diameter PLA sourced from Polymaker company (Shanghai, China). PLA is chosen for its
<figure><img src="image_2.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>(b)</figcaption></figure>

<caption>FIGURE 1 | (a) Geometry of the stripline used for assessing thermal mechanical behavior during laying down process, (b) Toolpath trajectory extracted from G-code, (c) Experimental setup for measuring thermal cycling and strain induced by 3D printing.</caption>

biodegradability, availability, and cost-effectiveness, making it suitable for laboratory-scale studies. Its low glass transition and well-defined melting point enable precise thermal control and analysis of residual stress formation. Additionally, PLA's stiffness and dimensional stability provide reproducible mechanical behavior, distinguishing it from alternatives like ABS or PETG. The FFF equipment used is a commercial printer named Ender 3 from Creality 3D Technology Co., Ltd., a company (Shanghai, China). The tested specimen is a stripline exhibiting a specific shape composed of a large stipe (thickness equal to the nozzle size) connecting two blocks with typical dimensions of 80 mm

× 10 mm × 10 mm (Figure 1). The fixed printing parameters are selected as follows: A 0.4 mm nozzle was selected because it is the most commonly used nozzle dimension, which provides a good compromise between resolution and printing time. A 0.2 mm layer height (≈50% of the 0.4 mm nozzle) is chosen to maximise surface resolution without excessive print time. This ratio is a standard compromise that preserves geometric fidelity and ensures consistent interlayer bonding. A retraction speed of 60 mm/s and retraction distance of 4.5 mm are both selected for the retraction settings to minimize stringing and oozing while maintaining steady extrusion restart. The retraction
<figure><img src="image_4.png" /><figcaption>FIGURE 1 | (Continued)</figcaption></figure>

distance is tuned to the printer's filament path (the value reflects the Bowden-style filament run used in our setup), and the 60 mm/s retraction speed reduces the pressure build-up in the hotend without causing filament grinding. The shell thickness corresponds to three perimeters with a 0.4 mm nozzle (3 × 0.4 mm = 1.2 mm). This provides reproducible wall stiffness and local structural robustness while avoiding excessive mass. No bottom/top thickness is used to allow the same material structuring to be tested, especially for building the thermal gradient profiles. An average travel speed of 50 mm/s is used to reduce print duration while limiting travel-related vibrations and oozing. This speed mitigated undesired deposition during non-printing moves and improved dimensional consistency across samples. The no support, and no platform adhesion raft conditions are needed to ease the temperature and strain measurements.

Several key printing parameters were systematically varied in order to evaluate their influence on the strain development during the fused filament fabrication process. The printing speed (Vp) was varied from 30 to 60 mm/s to investigate the effect of deposition rate on thermal gradients and layer bonding quality, slower speeds generally promote better interlayer adhesion, while

higher speeds reduce heat input and cooling time. The flow rate (F_L) was adjusted from 100% to 140% to examine the influence of material feed on dimensional accuracy and residual stress formation, as over-extrusion or under-extrusion can significantly alter the internal stress distribution. The printing temperature (T_p) was varied between 210 and 220°C, corresponding to the recommended processing window for PLA, to assess how the degree of polymer melting and interlayer diffusion affects strain accumulation. Finally, the bed temperature (T_B) was modified from 60 to 80°C to study its role in controlling the cooling rate and minimizing warping or delamination at the interface between the first printed layers and the substrate.

All slicing processes are conducted using Cura 3.6 from Ulti-
maker.

## 2.2 | X-Ray Micro-Tomography Analysis

The X-ray micro-tomography is carried out to obtain 3D information of the PLA samples. The measurements were performed at the ESRF beamline BM05 (Figure 2). We use polychoromatic X-ray beam with peak photon energy of 97 keV. Sample detector
<figure><img src="image_5.png" /><figcaption>**FIGURE 2** | The X-ray microtomography setup at the BM05 beamline in the ESRF.</figcaption></figure>

distance is kept at 1396 mm, pixel size at the detector is 3.04 µm. Projection images (5000) of the sample is acquired from 0° to 360° rotation angles, along with 101 flat field images, 100 dark images. The detector area is fixed to 2048 × 800 pixels, with an exposure time of 0.015 s, with a latency time of 0.0015 s. Tomography reconstruction was carried out using filtered back-projection algorithm implemented in Tomwer reconstruction software. Paganin filter is applied on projection images to determine phase shifts and enhance the phase contrast in the images.

Because of the limited field of view along the vertical axis, two successive acquisitions were performed along the height of the stripline. The two volumes are later stitched resulting in a total volume of 3805 × 3805 × 1600 voxels, corresponding to a volume of 649 mm³ of the sample. Image analysis were performed using the ImageJ software. Typical image processing procedure includes image brightness calibration, conversion to 8-bit images, segmentation by grey value thresholding, 3D rotation, and filtering using opening and closing operators. Additional processes involve background elimination, phase content analysis, anisotropy evaluation, and calculation of axial porosity levels.

## 2.3 | Strain Gauge Sensing

To obtain a precise understanding of the distribution of strain during thermal cycling of PLA-based stripline, we use wire-resistance strain gauges to enable both static and dynamic measurements during filament laying down process. To select an appropriate sensor, one needs to consider various criteria such as με-level strain resolution, non-invasive nature, integration within the 3D printing space. We find strain gauge KFH-1.5-120-C1-11L3M3R from Omega engineering company (Norwalk, Connecticut, USA) to be more suitable for our experiment. The main strain gauge specifications are summarized in Table 1. Temperature effects are corrected using the manufacturer's compensation polynomial (Equation 2).

The nominal resistance of the gauge $R_n$ is 120 Ohms, with a three-wire connection. These gauges are made of Constantin alloy (55% copper and 45% nikel), which is sensitive to thermal stress. The gauge is formed into a linear grid of 1.5 mm and mounted on a thin polyimide substrate.

The selected strain gauge is based on the principle of piezoresistivity, converting structural deformation variations into
<table><caption>TABLE 1 | Strain gauge technical specifications.</caption><thead><tr><th colspan="2">KFH-1.5-120-C1-11L3M3R</th></tr></thead><tbody><tr><td>Grid Resistance</td><td>121.2 Ω ± 0.35%</td></tr><tr><td>G<sub>F</sub> : Gage factor</td><td>1.98 ± 1.5%</td></tr><tr><td>Transverse sensitivity</td><td>0.3%</td></tr><tr><td>Thermal expansion coefficient</td><td>α = 10.8 [10<sup>-6</sup> /K]</td></tr><tr><td>Temperature coefficient of gage factor</td><td>93 ±10 [10<sup>-6</sup> /K] (-10°C ... +45°C)</td></tr><tr><td>Thickness Constantin</td><td>3.8 [μm]</td></tr><tr><td>Thickness Polyimide</td><td>45 ± 10 [μm]</td></tr></tbody></table>

measurable electrical signals (as voltages). Any expansion or contraction of the filament along the axis of the strain gauge causes a proportional change in the resistance of the strain gauge (ΔR<sub>SG</sub>). The total measured strain (ε<sub>T</sub>) is calculated from the change in resistance (ΔR<sub>SG</sub>) of the strain gauge using the equation:

$$G_F = (\Delta R_{SG}/R_n) / \varepsilon_T \quad (1)$$

where G<sub>F</sub> is the gauge factor and represents the ratio between the relative change in nominal resistance R<sub>n</sub> and the relative change in length, or strain.

As the grid of the strain gauge is made from a stress-sensitive alloy, it produces a resistance change proportional to the thermally induced deformation with a coefficient of thermal expansion named α<sub>G</sub>. This phenomenon is observed when the gauges are installed on a different material with a different coefficient of thermal expansion α<sub>s</sub>.

The minimization of the sensitivity to temperature variations during dynamic filament deposition is handled by considering the apparent deformation induced by temperature (ε<sub>m</sub>(T)). The selected gauge sensor has a residual potential error of approximately 1με/°C. To further refine the precision of the measurements and eliminate additional resistance variations, correction is performed by measuring the surface temperature and applying a specific correction curve provided by the supplier using a third-order polynomial approximation.

$$\varepsilon_m (\mu\text{m/m}) = 31.18 + 2.629 \times T (^{\circ}\text{C}) - 5.78 \times 10^{-2} \times T^2 (^{\circ}\text{C}) + 2.41 \times 10^{-4} \times T^3 (^{\circ}\text{C}) \quad (2)$$

The real strain ε<sub>r</sub> independent of the temperature effect is determined as follows:

$$\varepsilon_r = \varepsilon_T - \varepsilon_m(T) \quad (3)$$

Uncertainty u associated to strain measurement in Equation (2) can be evaluated from temperature-sensor accuracy, which in this case is close to 0.2 °C. When the base temperature is close to 60°C, the temperature-to-strain sensitivity is |δε<sub>m</sub>/δT| = 1.76 με/°C, hence the temperature contribution to uncertainty is u(ε<sub>T</sub>) ≈ 0.35 με.

The evolution of the real strain during thermal cycling is monitored for several targeted processing parameters, such as nozzle travel speed, filament extrusion rate, as well as the printing temperature and printing base temperatures.

Strain gauges were bonded to the 3D-printing area using an ethyl-cyanoacrylate adhesive rated for -54 to 82°C. During nozzle crossings on the first layer, brief over-spec peaks are observed (≈89°C, <1 s). At these excursions, the adhesive shear modulus may drop transiently (shear-lag), leading to a short under-reporting of strain, their average bias is negligible (≤0.1%-0.3%). In addition to intrusive measurements, K-type micro-thermocouples (Omega Engineering, Stamford, Connecticut, USA) are used with a diameter of 50 μm, with calibrated temperature range from -200°C to +1350°C, to assess the temperature in the PLA / building platform interface. These thermocouples are positioned at various locations, such as near the strain gauges to allow thermal correction (Figure 1), and at first layer to measure variations in the unidirectional thermal flux between the specimen and the building platform (Figure 2).

As strain measurements typically involve deformations rarely exceeding the micro-strain range, strain gauges are integrated into a Wheatstone quarter bridge (Figure 3) configuration to measure small resistance variations while compensating for temperature sensitivity.

The strain measurement setup comprises a single active strain gauge and three fixed external precision resistors (R<sub>1</sub>, R<sub>2</sub>, and R<sub>3</sub>). It represents the simplest deformation measurement configuration with the lowest measurement sensitivity. The output voltage generated by the resistance change ΔR<sub>SG</sub> across the Wheatstone bridge circuit is measured by connecting its conductor wires to a dedicated signal conditioning circuit using NI 9237.

In order to minimize the heating of the strain gauge and related drift due to Joule effect from the current passing through it, a nominal resistance of the strain gauge R<sub>n</sub> = 120 Ohm, is used. The voltage used to power up the Wheatstone bridge is V<sub>e</sub> = 2.5 Volt.

The resistance of each connecting wire, denoted as R<sub>wire</sub> = 4.8 Ohms, is significantly lower than that of the strain gauge. This allows the circuit shown on the left side of Figure 3 to be simplified to the one on the right, given that the wire resistance accounts for less than 5% of the strain gauge resistance.
<figure><img src="image_6.png" /></figure>

Data acquisition system

(a)

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>FIGURE 3 | Strain measurement setup detailing (a) strain gauge and thermocouple connection diagram in the data acquisition chain, (b) two-wire connections of quarter-bridge circuit used in this study.</figcaption></figure>

The relationship between the resistances ($R_1, R_2, R_3$, and $R_{SG}$), and output voltage ($V_O$) is governed by the equation:

$$V_O = \left( \frac{R_3}{R_3 + R_{SG}} - \frac{R_2}{R_1 + R_2} \right) V_e \quad (4)$$

where $V_O$ and $V_{ex}$ are the output and excitation voltages. For $V_O$ to be zeroed, the following condition is satisfied, indicating that the bridge is in balance:

$$R_1/R_2 = R_{SG}/R_3 \quad (5)$$

The output voltage ($V_O$) of the quarter-bridge circuit can be expressed in terms of $V_e$, $G_F$ and the measured strain ($\epsilon_m$) as indicated in the following equation:

$$V_0/V_e = -(G_F \times \epsilon_m(T)/4) \times (2/(2 + G_F \times \epsilon_m(T))) \quad (6)$$

The factor $2/(2 + G_F \times \epsilon_m(T))$ in Equation (6) accounts for slight nonlinearity. Over the operating range $|\epsilon_m(T)| < 10^3 \mu\epsilon$ with $G_F \approx 1.98$, its deviation from unity is < 0.1%, so its effect on $\epsilon_r$ is negligible in the selected range.

For data acquisition, the thermocouples were connected to an NI 9213 data acquisition module, while the strain gauges were linked to NI 9237 modules. These two modules were integrated into an NI cDAQ-9174 data acquisition system (National Instruments France, 92700 Colombes, France), which was connected to a dedicated computer responsible for collecting and storing all measurements taken during the experiments in a coordinated manner. Prior to the start of the experimental tests, specific parameters for each variable factor, such as nozzle movement speed, filament extrusion rate, layer height of PLA filament deposition, as well as the heating temperature of the printing bed and nozzle, were defined in the design files prepared for printing.
<figure><img src="image_10.png" /><figcaption>**FIGURE 4** | Thermal cycling and strain levels monitored during the heating of the printing bed without material deposition (fixed nozzle position).</figcaption></figure>

Furthermore, thermal cycling of the stripline is monitored using a bolometric infrared camera VarioCam HD 980 (InfraTec GmbH, Dresden, Germany) with a full-frame resolution of 1,024 × 768 Infra-Red pixels and a pixel size between 7.5 and 14 µm. The thermal sensitivity of the thermal camera is less than 30 mK and the frame rate is fixed to 30 Hz for all experiments.

# 3 | Results and Discussion

## 3.1 | Analysis of Thermal Cycling

Figure 4 shows the effect of heating the printing bed on the generated strain. The thermal cycling corresponds to a nearly linear increase in the temperature up to 62°C followed by a steady state regime for about 50 s before cooling down. During this cycling there is no nozzle motion, which allows measuring the thermal effects related to the adhesive through the strain gauge deformation with respect to the printing bed. The overall strain exhibits a positive trend with respect to the base temperature. After subtracting the thermal effects related to the strain gauge, the true trend related to the material on which the strain gauge is bonded is obtained. It shows negative correlation between the base temperature and the adhesive. We observe strain magnitudes reaching up to 25 µε during the heating stage, which decrease during the cooling stage. This indicates a relative compressive behavior of the adhesive, which contributes to the origin of residual stresses.

Figure 5 shows data extracted from the g-code of the stripline printing at 200°C. Figure 5a shows the active and inactive positions of filament extrusion. The corresponding extrusion flow rate highlights variations in material deposition and the presence

of two connecting filaments composing the central line with a typical overlap of 0.3 mm. In addition, the presence of an inactive path distant from the stripline by 1.2 mm allows the study of the possible cooling effect by analysing temperature gradients and solidification dynamics in the surrounding material. This inactive path serves as a reference to assess how heat dissipation influences the mechanical properties and adhesion of adjacent extruded filaments. Figure 5b shows the X- and Y- components of the nozzle displacement speed. The travel speed mostly in Y-direction reaches 50 mm/s while the printing speed is maintained at 40 mm/s. In between, a large variation in acceleration and deceleration are observed. This fluctuation in acceleration and deceleration may induce variations in extrusion flow dynamics, potentially affecting filament uniformity and layer adhesion. Additionally, these variations can contribute to localized stress accumulation, influencing the overall structural integrity of the printed part. The change in travel affects the thermal dynamics of the deposited material by influencing heat accumulation and cooling rates. Rapid travel movements reduce local heating, while slower printing speeds allow more time for heat conduction, potentially impacting filament bonding and overall print quality.

Figure 6 presents a typical infrared (IR) recording of the printing sequence, showcasing several key features. Figure 6a highlights an instance of unsuccessful printing caused by the accumulation of molten feedstock material on the nozzle. This buildup originates from an excessive extrusion rate combined with insufficient adhesion of the initial layers to the printing bed. As the filament fails to anchor properly, the molten material curls upward and adheres to the nozzle tip instead of being deposited on the substrate. This phenomenon leads to improper filament extrusion, resulting in the filament sticking to the nozzle instead of adhering to the printing surface. Such a defect can disrupt
<figure><img src="image_11.png" /><figcaption>**FIGURE 5** | G-code data associated with the printing process of PLA using a printing temperature of 200°C. (a) trajectory, (b) speed.</figcaption></figure>

the printing process, affecting layer deposition and overall print quality. Figure 6b shows a successful printing process, where the filament is smoothly extruded from the nozzle and properly adheres to the printing surface. The infrared recording captures a uniform thermal distribution, indicating stable extrusion conditions and optimal adhesion. This successful deposition ensures consistent layer formation, reducing the likelihood of defects such as warping, delamination, or poor interlayer bonding. The clear distinction between the heated nozzle, the extruded filament, and the cooling layer further illustrates the controlled temperature gradient necessary for achieving high-quality print.

Figure 7 depicts the evolution of the total strain ($\epsilon_m(T)$) for different printing conditions. Figure 7a shows the change in the measured strain at the centre position from the stripline for a reference printing condition. Positive strain values are obtained along the thermal cycling. This indicates that the material experiences tensile strain at the centre position throughout the thermal cycling process. This cycling starts after a short preheating stage of less than 50 s, followed by fluctuations due to material expansion and contraction phases, however, the overall trend still remains under tension. Both thermal expansion and nozzle compression effects explain the observed trend. The cycling ends when the last pass is performed, which is reflected
<figure><img src="image_12.png" /><figcaption>**FIGURE 6** | Infrared recording of laying down sequence of PLA printed at 200°C. (a) unsuccessful printing due to surface tension inducing filament dragging, (b) successful printing after nozzle cleaning.</figcaption></figure>

by the descending trend in strain starting from 800 s of the
laying down process. This trend reflects the retraction effect
due to cooling cycle. Figure 7b shows the same strain evolution
profile for a large base temperature (T<sub>B</sub> = 80°C). Despite the
overall similar strain profile, two main differences prevail: the
duration of each cycle as influenced by a rapid printing process,
and the limited levels in strain. A high base temperature during
thermal cycling reduces thermal gradients, leading to the low
strain levels in the range 0.009–0.010 με. The cooling stage after
400 s is similar to Figure 7a, which suggests a uniform cooling
process. Figure 7c depicts the effect of a high printing temper-
ature on the thermal cycling and strain evolution. As material

fluidity increases, low mechanical compression from the nozzle is
expected, which reduces the strain amplitudes. However, greater
thermal expansion effects are also expected during deposition,
followed by more pronounced contraction upon cooling, which
can result in high strain alternation as observed between 300 and
600 s. The increased heat input prolongs the time the material
stays at elevated temperatures at the first cycles and decreases
after a few passes.

Figure 7d shows the effect of a higher material flow rate on the evolution of the total strain. A large material flow rate increases as well the heat input, leading to greater thermal gradients between the deposited material and the underlying layers. Large variations in strain due to rapid expansion and contraction are observed between 100 and 200 s. At the end of the printing sequence starting from 300 s, additional high alternations in strain levels are observed, which may be due to excessive material deposition that causes overheating, and prolonged cooling times.

Figure 8 shows the same monitored quantities namely the recorded temperature, measured (ε<sub>m</sub>(T)) and real (ε<sub>r</sub>) strain during the printing process of PLA stripline. The preheating stage is clearly identified from the first 300 s of recording. The skirt follows immediately, which causes fluctuations in the temperature reading up to -9°C. Temperatures as low as 52°C are obtained close to 300 s due to the fan crossing the location of the strain gauge. It should be noted that the skirt is located 1.2 mm laterally away from the strain-gauge position (Figure 1). The thermal cycling due to the crossing of the nozzle on top of the thermocouple occurs at 312 s where the first peak is recorded at a maximum level of 89°C. The second-largest peak occurs at 318 s, which culminates at 80°C. This peak corresponds to the formation of the second filament composing the track (Figure 5). This peak is smaller compared to the first one because it is not fully centred on the position of the thermocouple. Thermocouple wire diameter is 50 μm.

With regards to the adhesive strain transfer, a single first-layer
excursion (~89°C, <1 s) is observed at the gauge. During this brief
over-spec transient, the adhesive may momentarily reduce strain
transfer. This interval is flagged in Figure 8 and not used for inter-
pretation; all subsequent cycles at this location remain < 82°C
and attenuate as layers accumulate. Consistency with bonded-
gauge strain transfer. This qualitative handling is consistent
with analyses for bonded resistance gauges, where short thermal
transients primarily cause temporary under-transfer rather than
a lasting bias [45].

The elapsed time between the two successive passages is 6 s, and the duration of a thermal cycle is 16.8 s. This duration corresponds to the nozzle travel time at the same position. The total duration of the sequence is 336 s, which corresponds to 20 thermal cycles. The sequence ends with a cooling stage starting from 648 s. Comparison with prior IR studies. The localized first-layer peaks we observe (~89°C, <1 s) and their strong spatio-temporal confinement agree with in-situ IR thermography of FFF, which reports brief nozzle-crossing transients and sensitivity to process settings and layer index [46–49]. The subsequent increase of the temperature is related to the thermal conduction from the top layer to the bottom layers while the bed temperature decreases due to the cooling. After 678 s, a clear decreasing trend
<figure><img src="image_13.png" /><figcaption>**FIGURE 7** | Total measured strain monitored during the laying down process for different conditions (a) reference parameters, and effect of (b) base temperature, (c) printing temperature, and (d) material flow rate.</figcaption></figure>

is observed for the temperature. The “cool-down followed by diffusion-driven reheating” signature aligns with thermal-history models and experiments for FFF/MEX that capture interlayer reheating and subsequent equilibration [50–52].

The measured strain ($ε_m(T)$) data reveal the development of negative strain levels as low as -30 $με$ during the heating stage, resulting from differences in thermal expansion between the

adhesive and the building platform (Figure 8). At the start of the first cycle, the strain level increases, reaching a peak of -90 $με$ due to the compression exerted by the nozzle, which induces lateral expansion within the construction plane. As the nozzle moves away from the strain gauge position, the strain decreases, reaching -60 $με$ during the second cycle. This cyclic strain behavior continues, gradually diminishing in magnitude until it vanishes after only a few cycles. The cooling stage seems
<figure><img src="image_14.png" /><figcaption>**FIGURE 8** | Thermal cycling and strain levels monitored during the laying down process.</figcaption></figure>

to oppose the thermal cycling in Figure 4, where the difference between the two cases is the heat effect induced by the PLA laying down. Instead of the reduction in the measured strain, the trend opposes the temperature decreasing profile. In order to further understand the mechanical effects behind, the real strain profile is explored. A positive strain is observed with peaks as large as 0.07 με. The decrease in peak levels for larger times indicate a lesser effect of the nozzle compression on the development of the residual strain. Most of the effect is reduced to the five first layers between 300 and 400 s. At the cooling stage, a subsequent increase in the strain is observed which can be related to the heat diffusion within the PLA from the top layers to the bottom ones. Cooling and solidification of the extruded material further reduces strain levels as small as 0.01 με.

The amplitudes and trends of (ε<sub>m</sub>(T)) (-30 to -90 με in the early cycles, then decaying) and the small positive peaks of ε<sub>r</sub> (~0.07 με) fall within the orders of magnitude reported for polymer FFF systems, where measured or simulated residual strain/stress typically spans tens to a few hundreds of με in early layers [53].

Adhesive over-spec note (Figure 8). Brief *T* > 82 °C excursions (~89°C, <1 s) occur only on the first layer. These windows are shaded in Figure 8 and excluded from aggregate metrics (or

shown with enlarged uncertainty). Their average impact on strain metrics is negligible compared with other uncertainties.

Figure 9 presents snapshots from the IR recordings taken throughout the construction of 20 layers. The moments when the nozzle reaches the strain gauge are indicated close to 318, 487, 538, 555, and 571 s. Excessive heat is generated at the connecting blocks, particularly after 348 s after the laying down process. A temperature gradient forms along the deposition path at 487 s, which gradually diminishes in thickness until the retraction stage at 668 s, where the thermal gradient appears to dominate in the building direction.

Figure 10 shows the temperature profiles recorded by the IR camera after 20 passes at various positions along the stripline. The time taken to reach the centre position—where the strain gauge is attached—is indicated in Figure 10a as ‘time to centre’. The figure also indicates the distance from the center for each recorded temperature peak. The jagged behavior, particularly noticeable at the tail end of each thermal cycle, is attributed to resolution limitations. Specifically, the IR camera has a pixel size of 185 µm, which affects the accuracy of temperature measurements within each layer. The temperature profiles show that the area affected by the heat is as large as 8 mm and extends along the tail. As the heat source moves along the stripline, the temperature profile follows
<figure><img src="image_15.png" /><figcaption>**FIGURE 9** | Infra-red recording of thermal cycling during the laying down process of the stripline.</figcaption></figure>

a characteristic transient distribution. At the centre position, the temperature gradually rises from 39°C, and in 1.3 s up to 130°C, with a rate of nearly 70°C/s. The temperature reaches its peak when the nozzle is directly over the centre position, after which it begins to decrease as the source moves away. The cooling process is initially rapid (-130°C/s) due to conduction into the surrounding material, followed by a slower decay as heat dissipates (-75°C/s). This results in an asymmetric temperature profile, with a steeper rise ahead of the nozzle and a more gradual cooling behind it. There is, however, a severe alteration of the temperature profiles at 0 and 70 mm, where lower peaks and different profile forms are observed. This is due to nozzle-path modifications near the connecting blocks.

In order to check the depth of the zone affected by the nozzle motion, the same profiles are built along the depth of the

specimen after 20 passes. The centre position where the strain gauge is attached is selected as a reference. Figure 10b shows temperature profiles captured in a time window of 2.6 s. The heat penetrates the PLA material over time, creating a temperature gradient. At the surface, the temperature rises rapidly as the heat source passes, reaching a peak of 140°C before gradually decreasing as the nozzle moves away. Beneath the surface, heat diffusion leads to a more gradual temperature rise with a rate of 124°C/s, with temperature peak occurring later and at lower magnitudes (peak at 72°C after 0.5 s) compared to the surface. The depth of heat penetration is as large as 4 mm. However, a slightly linear increase in temperature from a depth of 5 mm to the ground position with 0.75°C/mm.

To check the occurrence of overlapping thermal cycles, which can potentially lead to cumulative heating effects, the thermal
<figure><img src="image_16.png" /><figcaption>**FIGURE 10** | Temperature profiles as recorded using IR camera along the stripline after 20 passes (a) parallel and (b) perpendicular to the stripline main line.</figcaption></figure>

cycling is monitored at a fixed position to avoid the influence of the overall left-over heat in the system. Additionally, limitations in measurement resolution, such as the pixel size of an infrared camera, may introduce jagged fluctuations in the recorded profile, particularly at the trailing edge of each thermal cycle.

Figure 11 depicts the rapid cooling and solidification of PLA filament due to exposure to the surrounding environment and active cooling system, i.e., two fan system. This figure shows particularly that the filament temperature at the nozzle exit does not exceed 100°C. The thermal gradient along the length of the filament is close to 22°C/mm under nearly quasi-static conditions. These conditions correspond to material flow triggered by residual pressure in the nozzle, which causes molten filament to ooze out even after extrusion stops. When compared to the same material flow during laying down process, the thermal gradient as evidenced from Figure 9 is lesser with only 5°C/mm within the first 35 mm from the temperature peak value.

Overall, our thermal and strain observations corroborate prior reports: brief, localized IR peaks in the first layers; thermal histories governed by interlayer reheating and diffusion; strain magnitudes of the same order as reported for polymer FFF; and adhesive-mediated strain-transfer consistent with shear-lag theory. Our added value is (i) the experimental isolation of bed-

only heating (no-printing) to quantify purely thermal terms, and (ii) a quantified upper bound on adhesive-induced attenuation during brief over-spec peaks, showing a negligible average impact on aggregate metrics.

## 3.2 | Analysis of 3D Microstructure

Figure 12a presents standard orthogonal views of a part of the stripline with a layer height of 0.2 mm and a base temperature of 60°C. The region of interest spans 1392 × 1980 pixels, corresponding to a physical area of 4 mm × 6 mm. The measured dimensions of the sample are 4.076 mm in height and 1.718 mm in width, with the height closely matching the specified value.

The 3D-printed structure consists of two adjacent filaments (Figure 1b), each exhibiting different aspect ratios due to the combined effects of lateral overlap and mechanical pressure exerted by the nozzle during the deposition process. This results in an asymmetric structure with varying filament aspect ratios. The aspect ratio of the deposited filament is defined as the ratio between the measured layer height and the effective lateral flattening of the filament. In Figure 12a, the filament on the left, which experiences the greatest flattening, has an aspect ratio of 0.38, while the more compact filament on the right has a higher aspect ratio of 0.70. Both values are above the ratio of layer height (0.2 mm) to nozzle diameter (1.2 mm). Despite the asymmetric structure, there is no difference in width between the bottom and top, ensuring consistent extrusion and layer adhesion throughout the print.

The filament forming the skirt in Figure 12a has an overall aspect ratio of 0.28. Along with the built structure and the skirt, thin dragged filament structures with a thickness of 50 µm extend along the length of the strip. The dragged filaments come from the travelling between the connecting blocks represented by the diagonal and straight lines at the bottom and top positions in Figure 1b. The cross-sectional views in the YZ and XZ planes reveal a fully dense printed material with minimal porosity. The pores have an elongated morphology, measuring approximately 3 mm in length and 20 µm in width.

Figure 12b presents a rendering of the 3D-printed PLA stripline, which has a length of 4.25 mm. This view highlights variations in compactness resulting from an uneven layer height, ranging from 130 to 370 µm. The same figure also reveals residual roughness at the contact surface between the sample and the building platform, influenced by the adhesive used to enhance the cohesion of the first layer to the platform.

Figure 13 presents the same orthogonal views for a sample printed with a base temperature of 80°C, which is 20°C higher than in Figure 12. The resulting structure is more compact, with a height of 3.812 mm and a width of 1.784 mm. The 3D rendering reveals a significant height variation in a specific region due to the presence of the strain gauge. This creates a step with an overall height of 279 µm, leading to substantial compression of the layer above and affecting the layer structure, particularly at a height of 1.2 mm. The comparison between the process-induced porosity for the two cases (Figures 12 and 13) shows a limited difference. For both conditions, the porosity generated by the printing process
<figure><img src="image_17.png" /><figcaption>FIGURE 11 | IR recording of filament cooling.</figcaption></figure>

represents only 0.22% (Figure 14). It is discontinuous along the building direction and mostly fully connected perpendicularly to the same direction.

4 | Conclusions

In conclusion, this study investigated the thermal and mechanical
strain evolution during fused filament fabrication (FFF) of PLA
using high-resolution strain gauge sensors and infrared thermal
imaging. The results demonstrate that strain levels vary signif-
icantly depending on printing conditions. For instance, during
the heating stage, strain levels reached up to 0.25 x 10<sup>-6</sup>, while
cooling resulted in a reduction in strain, indicating compressive
behavior due to adhesion effects. The real strain, independent
of thermal effects, exhibited peak values of 0.07 x 10<sup>-6</sup>, with
notable variations occurring during the first five deposited layers.
Higher base temperatures, such as 80°C, led to reduced strain
fluctuations (0.009–0.010 με) compared to lower base temper-
atures. X-ray microtomography confirmed an overall porosity
of 0.22%, primarily in the form of elongated pores (~3 mm in
length, 20 μm in width). Thermal imaging revealed that the heat
penetration depth reached 4 mm within the PLA structure, with
a rapid cooling rate of -130°C/s, transitioning to a slower decay

of -75°C/s. The thermal gradient along the extruded filament was
measured at 22°C/mm under quasi-static conditions, decreasing
to 5°C/mm during actual deposition. These findings highlight
the influence of thermal cycling and nozzle-induced mechanical
pressure on strain development, emphasizing the importance
of optimizing printing parameters to minimize residual stresses
and improve structural integrity in FFF-printed parts. In future
research, we plan to extend this methodology to PLA blends
incorporating textile residues, allowing a systematic investigation
of their impact on melting temperature, mechanical behavior, and
strain.

This study is conducted at a laboratory scale to provide a
fundamental understanding of residual stress formation in 3D-
printed PLA under thermal cycling. Some advanced techniques,
like high-resolution infrared thermography and sensitive strain
gauges, could be adapted for industrial use, while synchrotron
radiation experiments and high-resolution micro-CT remain less
transferable due to accessibility and cost. The findings establish a
scientific basis for process optimization and stress management
in additive manufacturing. In industry, these insights can guide
the development of simplified, scalable monitoring systems for
real-time process control.
<figure><img src="image_18.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**FIGURE 12** | Synchrotron X-ray micro-tomography results showing (a) orthogonal views (b) 3D rendering of the stripline printed sample.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**FIGURE 13** | Synchrotron X-ray micro-tomography results showing the effect of the base temperature on the microstructure of 3D printed strip (a) orthogonal views (b) 3D rendering of the printed samples.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**FIGURE 14** | Comparison between the process-induced porosity as a function of the base temperature.</figcaption></figure>

## Acknowledgements

This research was funded by the French National Research Agency (ANR) through the projects MAGNELIN4D ANR-22-CE10-0010 and by the European Union through the UPWEARS project (Grant Agreement No. 101130741). Views and opinions expressed are however, those of the author(s) only and do not necessarily reflect those of the European Union or HADEA. Neither the European Union nor the granting authority can be held responsible for them. We acknowledge the European Synchrotron Radiation Facility (ESRF) for the provision of beam time on BM05.

## Conflicts of Interest

The authors declare no conflicts of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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