# Influence of acid environment on the fatigue behavior of short glass fiber polyamide 6/6.6 (35% wt.) composites

Florent Alexis, Sylvie Castagnet, Carole Nadot-Martin, Gilles Robert, Peggy Havet

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## To cite this version:

Florent Alexis, Sylvie Castagnet, Carole Nadot-Martin, Gilles Robert, Peggy Havet. Influence of acid environment on the fatigue behavior of short glass fiber polyamide 6/6 (35% wt.) composites. International Journal of Fatigue, 2025, 200, pp.109084. 10.1016/j.ijfatigue.2025.109084. hal-05127868

HAL Id: hal-05127868

https://isae-ensma.hal.science/hal-05127868v1

Submitted on 6 Oct 2025

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# Influence of acid environment on the fatigue behavior of short glass fiber polyamide 6/6.6 (35% wt.) composites

Florent Alexis<fnref n="1" />, Sylvie Castagnet<fnref n="1" />*, Carole Nadot-Martin<fnref n="1" />, Gilles Robert<fnref n="2" />, Peggy Havet<fnref n="3" />

*Email: sylvie.castagnet@ensma.fr

## Abstract

This study investigates the impact of a low-pH acidic aqueous solution on the fatigue behavior of short glass fiber-reinforced polyamide composites (PA6 and PA6.6, 35% wt.). Mechanical tests were conducted under aging conditions (RH90, 60°C) with fixed parameters (f=1-3 Hz, R=0.1), considering anisotropy with three orientations (0°, 45°, and 90°). A specialized chamber was developed for in-situ fatigue testing in an acid-heated solution. The study tracked mass and dimensions during aging and compared them with data from demineralized water. Results indicate that acid exposure significantly degrades the fiber-matrix interface, leading to increased fiber pullout and reduced mechanical integrity. PA6 composites exhibited better hydrolytic stability compared to PA6.6. This research provides critical insights into the durability of SFRP composites in acidic environments, relevant for automotive applications such as EGR (Exhaust Gas Recirculation) systems.

## Keywords

Short-Fiber Reinforced Polymers (SFRP) ; PA6/66 ; Aging ; Lifetime ; in-situ fatigue tests

## 1. Introduction

Short fiber reinforced polymers (SFRPs) have gained significant attention over the last decades due to their excellent properties, lightweight nature, and ease of processing [1]. As such, they tend to be employed in more challenging applications where severed environments are encountered.

Among the wide variety of SFRP, glass fiber (GF) reinforced polyamides (PA) are known to be highly sensitive to environmental effects, both in the short-term, such as temperature or humidity changes [2–4], and in the long-term, such as aging. These factors, combined with the various load types sustained by the components during operation, such as fatigue and creep, can significantly impact their service life. Understanding how these behaviors are affected is vital for predicting the lifespan and ensuring the reliability of these materials.

Several aging phenomena for SFRPs are described in the literature. The main mechanisms include oxidation and hydrolysis, caused respectively by oxygen ($O_2$) and water ($H_2O$) molecules. These mechanisms, primarily affecting the matrix, are driven by temperature and it can lead to situations where both occur simultaneously. For instance, a temperature threshold of around 50°C has been identified for the PA6.6, above which hydrolysis predominates [5,6]. This has also been confirmed for the PA11 [7] and the PA6 [8]. While these studies are usually conducted with neutral solutions of demineralized water, few have explored the effect of acid compounds.

Serpe et al. [9] studied the effects of HCl at different concentrations (pH=0-7) on 0.5-2 mm PA11 thick plates with temperatures from 53 to 210°C. Chaupart et al. [10] studied the effects of HCl and potassium acidic solutions (pH=0-4) on 1 mm PA (6, 11, and 12) thick plates with temperatures from 80 to 120°C.

<footnote n="1">¹ Institut Pprime CNRS – ISAE-ENSMA – Université de Poitiers, UPR 3346, Département Physique et Mécanique des Matériaux – 1 avenue Clément Ader, F-86962, Futuroscope Chasseneuil, France</footnote>

<footnote n="2">² Polytechnyl sas (DOMO Chemicals) – Usine de Belle Etoile, Avenue Albert Ramboz, 69190, France</footnote>

<footnote n="3">³ Valeo Power – BG Material Laboratory – 8 rue Louis Lormand, 78322, Le Mesnil Saint-Denis, France</footnote>
Hocker et al. [11] studied the effect of small organic weak acids at low concentrations versus HCl ones (pH=2-6.2) on PA11 films with temperatures from 100 to 120°C. Maïza [12] studied the effects of CO₂ pressure injected water (pH=4) on PA11. These studies showed that acidic compounds accelerate hydrolysis depending on the type (weak-strong) and chemical structure (small-large) of the acid. However, they primarily focused on reaction kinetics and water sorption mechanisms. Among them, only Maïza [12] studied the effect on the mechanical aspect of degradation.

The random chain scission provoked by hydrolysis reduces the polymer chain mobility and promotes embrittlement, depending on the molar mass. Below a critical molar mass, the elongation properties are significantly reduced [7,12–14]. Conversely, water molecules create a plasticization effect, which promotes ductile behavior when stressed [15,16]. The introduction of fillers, such as GF, greatly improves the retention of mechanical properties by reducing the water content and preventing the critical molar mass transition [17]. However, GF also introduces new pathways for water molecules, depending on the fiber surface treatment [17], and reduces interfacial adhesion with the matrix [18], promoting different damage mechanisms [19,20]. To the authors' knowledge, this last point has not been addressed with non-neutral water-based solutions.

While the tensile behavior under such conditions has been studied, fatigue lacks in comparison. While the effect of water content is well known [20–22], the effects of long-term aging were not tackled until recently by Lottier et al. [23] for PA66 composites aged (0-1000 h) in neutral water at 90°C. This study showed a significant decrease in fatigue life with aging time, but the specimens lacked the well-known shell-core-shell microstructure, and tests were conducted at 23°C which is far below the aging temperature.

This study aims to investigate the impact of a low-pH acidic aqueous solution on the fatigue behavior of SFRP composites with PA6 and PA6.6 matrices, for comparison purposes as PA6.6 is known for its better hydrolytic stability. The research compares the effects on these materials at three stages: before aging, after aging, and during aging. The goal is to understand how acid exposure affects the mechanical integrity and fatigue life of these composites. The chosen solution is relevant to direct applications in the automotive industry, specifically acidic condensates formed in SFRP parts after air-cooling in EGR (Exhaust Gas Recirculation) systems.

The experimental approach includes a detailed conditioning process for initial state and aging, the development of a specialized chamber for in-situ fatigue testing, and comprehensive mechanical testing under controlled conditions. The study examines various indicators such as mass, dimensions, and the effect of anisotropy with different orientations relative to the injection direction. The methodology used to discuss the influence of the different conditionings and media is shown in Figure 1.

<figure><img src="image_3.png" /><figcaption>Figure 1 Methodology of comparison for the different conditionings studied in the paper.</figcaption></figure>

The two lines on the left-hand side refer to the two states of the material: the reference state after conditioning at RH90 until saturation, and the aged state for which conditioned samples were further immersed into an acid solution for 1000h and brought back to RH90. These two series of samples were then tested in fatigue into two different environments (corresponding to the two columns): in a humid
environment (RH90; pH 7) and in the acid solution (RH100; pH 2.5). Results will focus first on the pre-
aging effect on fatigue tests in the humid medium (section 3.2; yellow square in Figure 1). Then, the two
environments will be compared, in the conditioned samples first (section 3.3.1; blue square in Figure 1)
and in aged ones (section 3.3.2; red square in Figure 1).

## 2. Materials and methods

2.1. *Materials*

Studied materials were two 35% wt. glass fiber reinforced polyamides with a PA6 or a PA6.6 matrix provided by DOMO Chemicals (Saint-Fons, France). They were prepared by loading short glass fiber-reinforced thermoplastic pellets in an injection molding machine to form 340 x 100 x 3 mm³ rectangular plates, the longest dimension being the injection direction. Dogbone-shaped samples were then machined out of each plate along three orientations (0°, 45°, and 90°) to the injection direction as shown in Figure 2. This process was done far away from the plate borders to avoid inconsistent microstructures but with fixed positions related to each orientation. The first component of the second-order orientation tensor (obtained by averaging the orientation of fibers computed from four different 3.5mm edge micro-tomography volumes (1.4 µm/voxel) and averaged within different slices through the sample thickness), is given in Figure 2 for both materialsIt is worth mentioning that the core layer of PA6 composites was wider than in PA6.6 ones.

<figure><img src="image_4.png" /><figcaption>Figure 2 Orientation of composite samples machined out of injected plates comparatively to the injection direction (x-axis); average of a component of the second-order orientation tensor across the thickness for both materials.</figcaption></figure>

2.2. *Conditioning: initial state and aging procedure*

2.2.1. *Initial conditioning.*

After machining, specimens were conditioned in a self-regulated climatic chamber at 90% Relative Humidity (RH90) and 60°C until complete mass stabilization (between 6 to 8 days). Masses and dimensions were monitored for every specimen at regular intervals, respectively, with a digital scale (resolution ±0.0001 g) and a digital caliper (resolution ±0.01 mm). Before each measurement, samples were quickly removed and wiped with paper. They were then immersed again. Airtight bags were used at the end of the stabilization procedure to keep a constant moisture content in samples before mechanical testing. This state is referred to as “unaged” in the following.
### 2.2.2 Preparation of the aging solution.

The aging medium was a replica of the EGR condensates mentioned in the introduction section. The solution was prepared in 8 L batches. The composition was set to 99% of demineralized water, 4 acids (sulfuric (H₂SO₄), formic (CH₂O₂), acetic (CH₃COOH), and lactic (C₃H₆O₃), and 3 salts (sodium sulfate (Na₂SO₄), sodium chloride (NaCl) and sodium nitrate (NaNO₃)). Each component was used in different concentrations to achieve an acid solution with pH=2.5 checked by a pH meter (Fisher® AE150). For confidential reasons, the precise recipe is not given in this paper. It can be underlined that the latter three acids were introduced in equal proportion, much higher than the first one.

### 2.2.3 Aging process.

As shown in Figure 3 aging was performed at 60°C in 2.25 L sealed beakers fitted with magnetic agitators and a specifically designed PTFE (polytetrafluoroethylene) holder to ensure maximum diffusion on each side of the samples. Around 20 specimens were immersed inside each beaker for durations up to 1000 h (42 days), except a few specimens exposed up to 3000 h (126 days) to extend the database. Beakers were refilled every 300 hours to compensate for solution evaporation. Both temperature and pH were followed throughout the entire process. The pH was monitored with a pH meter (Fisher® AE150). The temperature was monitored using the pH-meter integrated temperature probe and additional thermocouples taped on each beaker's external surface. Measurements are summarized thereafter (one can refer to Supplementary Materials (see Figure SUP-1) for detailed data. All indicators were stable during aging: pH was kept between 2.4 and 2.6, and the external temperature (glass-taped thermocouples) and internal temperature (temperature probe of the pH-meter) were approximately 52°C and 60°C with a variation of 1°C, respectively. After aging, specimens were brought back to RH90 60°C which led to a slight decrease in mass equivalent to -1% from the saturation levels. The same procedure was also conducted with demineralized water as a means of comparison. For clarity reasons, the “aged” term will be reserved for the specimens exposed to the acid solution although an extended stay in demineralized water can also induce chemical reactions.

<figure><img src="image_5.png" /><figcaption>Figure 3 Aging setup for specimen immersion in acid and water baths.</figcaption></figure>

## 2.3. Experimental tests and techniques

### 2.3.1 Crystallinity ratio.

Differential Scanning Calorimetry (DSC, Q20 from TA Instruments®) was used to follow the evolution of crystalline ratio after aging. Samples of 10 mg extracted from dried (80°C in a vacuum chamber during 48 h) dogbone specimens (including a portion of the sample surface) were sealed in a standard non-hermetic aluminum pan (from TA Instruments®) and heated at 10°C/min from 10 up to 300°C. The crystallinity ratio was determined using Equation (1):

$$ \chi_c = \frac{\Delta H_f}{\Delta H_{f,100\%} \times (1-m_f)} \times 100\% \qquad (1) $$
where $\Delta H_{f,100\%}$ was the melting enthalpy for the 100% crystalline material (197 J/g for the PA6.6 [24] and 230 J/g for the PA6 [25]), $\Delta H_f$ the measured melting enthalpies and $m_f$ the mass ratio of fibers measured after sample pyrolysis (±2% from the theoretical 35% wt. content). Values of crystallinity ratio were averaged over two specimens for each condition.

### 2.3.2 Molar mass.

Size-Exclusion Chromatography (SEC, G7800A from Agilent Technologies®) was used to follow the evolution of average molar mass after aging. Tests were performed by the company DOMO Chemicals following the internal reference protocol developed by the company for polyamide-based materials [26]. For confidentiality reasons, the protocol and the initial molar mass of the unaged materials could not be detailed. It is however important to note that samples were extracted through the whole thickness of the specimens (3 mm) at regular intervals to evaluate the potential gradients of chain scission from the surface to the core of the sample.

### 2.3.3 Mechanical test procedures.

Two different fatigue machines were employed to conduct the mechanical tests under two moisture conditions: humid (RH90) (tensile and fatigue) or immersed (RH100) (only fatigue). It is important to note that both machines were comparable in terms of accuracy and reliability, considering the applied stress levels. The test types differ only from the environmental conditions and are described in the next paragraphs.

Humid tests were performed on a servo-hydraulic Intron® 8802 fatigue testing machine (5 kN load cell) equipped with a climatic chamber set at 60°C and RH90%. Both targeted values were stabilized for one hour and tests were performed in series without waiting for the cooling of the machine's loading train. All tensile tests were performed at constant crosshead displacement (equivalent to a $10^{-3}$ s$^{-1}$ conventional strain rate in the gauge length). All fatigue tests were conducted under load control at a fixed loading ratio ($R = \sigma_{max}/\sigma_{min} = 0.1$) with a sinusoidal waveform, constant amplitude, constant frequency (3 Hz) and a number of cycles to failure ($N_f$) ranging from $10^3$ to $10^6$. The cross-section dimensions of each sample were measured before testing to calculate the applied nominal stress considering the possible swelling of specimens during aging. For all the fatigue and tensile tests, the true strain was monitored from outside the chamber by following painted markers located on the sample surface in the gage area (10 mm length).

Immersed (in-situ) fatigue tests were performed on a servo-hydraulic Intron® 8874 fatigue testing machine (100 kN load cell) equipped with a chamber (Figure 4) specifically developed to perform mechanical tests during immersion in the heated acid solution, meaning 60°C to maintain the same test temperature and a humidity equivalent to RH100%. Structural components were chosen to minimize chamber degradation. For instance, the chosen base material for the chamber is the 316L stainless steel spray-coated by a thin layer of Teflon. Heating was ensured by a thermally controlled stainless 1000 W heat rod, located and parametrized to avoid boiling the liquid. All these steps were carried out to ensure homogeneous and stable conditions throughout the entire test duration. Two thermocouples and a pH probe were permanently immersed as a means of verification. Strains were measured through a borosilicate glass window at the front of the chamber. Prior calibration steps were undertaken to verify all the aforementioned conditions. Gathered data (see Figure SUP-2 in the Supplementary Materials) showed that the same medium (i.e., solution of 6.3L based upon liquid evaporation) could be used for several “short” fatigue tests or one “long” test of $10^6$ cycles at 1 Hz (≈12 days) before reaching a pH value of 3.5 (the chosen criterion for replacing the bath). This is due to the acid solution reaction directly with the chamber and devices like the clamping jaws, which inevitably change the solution composition over time. Finally, the reliability of the strain measurements was checked by comparison of several scenarios. The tests were conducted successively by adding possible sources of interference: the front glass, the circulating acid, and the temperature. The results showed a comparable accuracy in all situations (see Figure SUP-3 in the Supplementary Materials). During these tests, it was evidenced that air bubbles tend to appear on the specimens' surface in the first 2 hours of test preparation (chamber filling and heating). It was thus decided to wait one hour before each test to avoid any unwanted effect on the values (i.e., shift of the painted marker barycenter which affects the measurements).
<figure><img src="image_6.png" /><figcaption>Figure 4 In-situ fatigue testing setup: fatigue machine equipped with an acid-resistant chamber allowing fluid movement and heating.</figcaption></figure>

For all the tests, self-heating of the material was not considered due to the relatively low frequency (≤ 3 Hz), similar to the works on equivalent materials. Finally, the data were monitored until the complete failure of the specimens, and for confidentiality reasons, all stress variables (nominal stress and maximum stresses of the S-N curves) were normalized by a common arbitrary factor whereas all the other quantities (nominal strain and number of cycles) were left unchanged.

### 2.3.4 Observations before and after testing.

Specimens (unaged and aged) were observed using Scanning Electron Microscopy (SEM, VEGA3 from Tescan®) with a 10 kV acceleration voltage and a 6 to 10 mm working distance. Before the observations, surfaces were coated by a thin platinum nanometric layer (≈ 10 nm), using a sputtering coater device from Cressington®.

## 3. Results and discussion

For clarity reasons, this section first presents all results regarding the PA6.6 composite and ends with a shorter overview of the differences with the other material, the PA6 composite.

### 3.1. *Physical characterization of aged PA6.6GF35 before testing*

The effect of aging, that is the 1000h exposure to the acid solution after initial conditioning at 60°C and RH90%, was investigated first through three physical indicators: the mass evolution during aging, the crystallinity ratio, and the surface aspect after aging (1000h).

Compared with the initial conditioning, the 60 specimens (20 per orientation) exhibited an average mass gain of 0.9±0.08% after 1000 h of immersion into the acid bath. This result was consistent with the humidity increase from the RH90 of the conditioning atmosphere to the RH100 of the acid solution. A comparative batch was also made using demineralized water. Both measurements were done on nearly dry specimens for a better definition of the absorption kinetics.

Figure 5 compares the first 600 h of immersion into demineralized water and acid solution. In both media, the material exhibited a typical absorption curve with a rapid initial uptake (matching a linear Fick model in $\sqrt{t}$) followed by a slow convergence toward a stabilized saturation level. Data recorded up to 3000 h showed no further evolution of the saturation level, confirming a fully stabilized state after 200h. The stabilized mass gain after water uptake from the dried state (≈ 4.1% on average) was similar in demineralized water and acid solution although the absorption rate was slightly accelerated for the latter solution (around 18% faster) (Figure 5b). The curve shape seemed to be consistent with the absence of external degradation which usually tends to be expressed as cracks and mass diminution [27]. The absence of such a trend could be due to the lower temperature used in this work. During the procedure, the cross-section area was also increased by 3.8% on average (considering all three orientations) for both media, with
no further variations after saturation. No differences could be identified when comparing the orientations because the curve differences were close to the scatter and ranged differently depending on the considered batches. Nevertheless, the other observed trends (saturation level and absorption rate) were consistent.

<figure><img src="image_7.png" /><figcaption>Figure 5 Mass gain during immersion of PA6.6 composite from the dried state: (a) demineralized water and (b) acid solution.</figcaption></figure>

Figure 6 and Figure 7 show, respectively, the typical SEM images and the DSC thermograms obtained for both unaged and aged PA6.6 composites. The SEM images (Figure 6) showed that the surface asperities visible on the unaged surface were no longer present after aging. Additional investigations revealed that they were limestone residues that could be fully dissolved by the acid. Such residues were formed during the initial conditioning despite all the care taken in specimen preparation. The analysis of the thermograms (Figure 7) showed that, at the macroscale, no significant evolution of the crystalline structure occurred during aging and the crystallinity ratio stayed relatively constant (33.8±1%). This could be different at the microscale, especially near the sample surface in direct contact with the liquid. It was reported that acid media could impact the crystallization process of polyamides [28]. It doesn't seem to be the case here. Chemical aging (hydrolysis and/or oxidation) could also modify the molar mass distribution [8,29]. In our case, the SEC measurements showed that the average molar mass decreased during aging (0 vs. 1000 h) in the acid solution (see Figure 8; normalized values with the same arbitrary factor for both curves), which is a sign of hydrolysis and a decrease in polymer chains' mobility. The value appeared stabilized through the whole specimen's thickness with no significative difference between the shell and the core. They were close to those obtained for water-aged specimens for the same aging time (i.e., in the ±5% measurement error band). It means that the chosen acid conditions had a relatively low impact on the matrix chemical degradation. At least, if there was an acidity effect, it could not be pointed out from the present characterization means. Further investigations would be interesting, such as Atomic Force Microscopy (AFM) which was recently used by [29] to investigate the T<sub>g</sub> gradient near the oxidized surface of a thermally aged PA6.6.

<figure><img src="image_8.png" /><figcaption>Figure 6 Typical SEM images of the PA6.6 composites surfaces: (a) unaged and (b) aged.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 7 DSC thermograms of unaged and aged PA6.6 composites: representation centered on the fusion peak.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 8 Differential molecular weight obtained from SEC measurement in the skin layer (300µm to the surface) before and after aging (normalized values for confidentiality reason; same factor for both curves).</figcaption></figure>

## 3.2. Effect of aging on the mechanical behavior of PA6.6GF35 in the conditioning conditions (RH90)

To evaluate the consequences of aging on mechanical behavior (monotonic tension and fatigue), the unaged and aged (1000 h in acid solution) specimens were tested first in the same environment as initial conditioning, that is in the humid environment (RH90). It is reminded that aged samples were unsealed and stabilized again at RH90 before mechanical testing. The impact of acidity will be addressed in section 3.3.

### 3.2.1 Tensile behavior

Figure shows the normalized tensile curves for the three orientations of both the unaged and aged PA6.6 composite. For each orientation, the strain-at-break decreased (≈ -20% on average) after aging while the tensile modulus stayed the same. The latter observation was consistent with the stable crystallinity ratio of the matrix measured from DSC (Figure 7). The strain-at-break decrease could result from the hydrolysis reaction, which is known to promote embrittlement as discussed in the introduction. This was mainly supported by the overall decrease in molar mass observed with SEC for the aged specimens.
<figure><img src="image_11.png" /><figcaption>Figure 9 Normalized tensile curves of PA6.6 composite at RH90 60°C: unaged (plain-) and aged (dotted-line) specimens.</figcaption></figure>

### 3.2.2 Fatigue behavior

Figure shows the normalized S-N curves (one curve per orientation) for both the unaged and aged PA6.6 composite represented by plain and empty markers, respectively. For each curve, a regression line was fitted with the optimal power-law set of parameters ($\bar{\sigma}_{max} = \beta \cdot N_f^{\alpha}$) within the range of $10^3$ to $10^6$ cycles. The fracture surface was always located in the gauge area for 45° and 90° specimens and at the extremity of it in 0° samples, whatever in unaged or aged materials. Table 1 displays the slope of each curve and a dispersion coefficient calculated as follows. The distance to the average curve of each experimental point was first calculated as the difference between Log N<sub>exp</sub> -Log N<sub>average curve</sub>, for a given stress level. The dispersion coefficient was the standard deviation of this series of values.

From a global perspective, the fatigue curves appeared more dispersed than the monotonic ones, regardless of the conditioning. In particular, the 0° curve displayed the most dispersion, which was more pronounced at higher stress values (corresponding to shorter fatigue lives). This aspect was further investigated by considering the specimen position on plates (see Figure SUP-4 in the Supplementary Materials). It was observed that similarly located specimens tended to exhibit more consistent fatigue lifetimes at higher stresses, unlike those at other orientations (45 and 90°). No clear explanation was found to clarify these differences, they could be related to plasticity-related components being more pronounced due to the microstructure, the higher load, or a combination of both. This could explain why the other orientations seemed less dispersed, as they are loaded at lower stresses. Nevertheless, consistency was observed with the results for the other composite, as shown in section 3.4.

For the aging impact, the fatigue lifetimes appeared slightly decreased, with the 90° curve displaying the minimal deviation from the unaged curve. For a more decisive comparison, the 45° curve could have benefited from more than two stress levels for the unaged curve and ultimately have a different fitted curve. Nevertheless, this could not be invoked for the other orientations, confirming an impact of aging, potentially originating from the hydrolysis and/or other fatigue mechanisms. Regarding hydrolysis, a shift of S-N curves towards shorter lifetimes has been reported once in PA6.6 composites [23], with a higher sensitivity than here. This was difficult to discuss because of several experimental differences between the two works. The higher aging temperature in their case (80°C) could explain a more pronounced shift. But the pre-conditioning (RH62; 70°C) and fatigue testing conditions were also different (samples were stabilized for one week at 23°C and RH50 for fatigue testing) and could have led to a variable activation of fatigue mechanisms. Finally, the orientation of fibers was not specified in [23]. Since samples were injected, the skin/core effect could be minimized; samples could be rather mono-oriented and thus closer to the 0° series here, which exhibited the more pronounced effect.
<figure><img src="image_12.png" /><figcaption>Figure 10 Normalized fitted S-N curves of PA6.6 composite at RH90 60°C: unaged (plain-) and aged (empty-marker) specimens.</figcaption></figure>

<table><caption>Table 1 Dispersion measurements and slopes of the S-N curves plotted in Figure 10.</caption><thead><tr><th rowspan="2"></th><th colspan="2">UNAGED</th><th colspan="2">AGED</th></tr><tr><th>dispersion</th><th>slope</th><th>dispersion</th><th>slope</th></tr></thead><tbody><tr><th>0°</th><td>0,237</td><td>-0,0588</td><td>0,189</td><td>-0,0783</td></tr><tr><th>45°</th><td>0,122</td><td>-0,0987</td><td>0,141</td><td>-0,0724</td></tr><tr><th>90°</th><td>0,0987</td><td>-0,0844</td><td>0,103</td><td>-0,082</td></tr></tbody></table>

An important issue was to investigate whether fatigue lifetime was still governed by damage initiation in aged materials. To accomplish this, the evolution until failure of the secant modulus (determined from the fatigue loop: $E_s = \Delta\sigma/\Delta\varepsilon$, where $\Delta=\kappa_{max} - \kappa_{min}$) for the aged materials was plotted against that of the unaged ones in Figure . To facilitate comparison, the x-axis was scaled as a percentage of the fatigue lifetime, with darker colors indicating longer test durations. For clarity reasons, only three tests with different fatigue lives (short, mid, and long-range) were shown for each type of specimen, which were representative of the full set.

Three successive regimes were observed whatever the conditioning: a rapid decrease at the beginning (first 200 cycles) followed by a long stationary strain rate regime (from 10 to 90% of $N_f$), and a rapid decrease at the end (from about 90% of $N_f$). For unaged fiber-reinforced polyamides, such a shape was shown to be typical of spread-out damage initiation and microscale propagation. It is related to several well-identified mechanisms (debonding, cavitation, fibrillation, fiber breakage ...) that eventually end up coalescing into a mesocrack in the very final stage of the fatigue life [20,30]. In aged materials here, a similar evolution of the secant modulus pleaded for similar fatigue mechanisms. At least, it suggested that the fatigue lifetime of aged materials was not governed by the continuous propagation of one or a few mesocracks. To confirm this statement and further investigate fatigue mechanisms, failure surfaces were observed by SEM for each type of specimen.
<figure><img src="image_13.png" /><figcaption>Figure 11 Evolution of the normalized secant modulus for short- (8 000); mid- (60 000) and long-range (150 000 cycles) fatigue tests of PA6.6 0° specimens at RH90 60°C: (a) unaged and (b) aged. A darker line corresponds to a longer test.</figcaption></figure>

### 3.2.3 SEM observations after testing

Figure presents a side-by-side comparison of failure surfaces of unaged (left) and aged (right) fatigued 0° specimens, and more precisely of the corner (up) and of the edge with a higher magnification (bottom). Indeed, the only significant differences between unaged and aged failure surfaces were observed in these areas: in the aged sample, most fibers were pulled out with a clear lack of interfacial adhesion to the matrix whereas cohesive debonding was observed in the unaged sample. Except for these external layers, the failure surfaces closely resembled those of the unaged specimens in both the aged and unaged materials, characterized by a cohesive debonding of the matrix around fibers.

As mentioned in the introduction, an increase in water concentration is known to weaken interfacial strength [18]. This typically occurs to some degree without completely eliminating adhesion between the phases. Fibers usually remain bonded to the matrix as observed in our case for the unaged but wet specimens. The debonding observed close to the surface of the aged specimen could originate from the acid compounds in the solution. This was confirmed below by a comparison of ruptured specimens in other testing conditions. To discriminate the effect of moisture, aged specimens were dried before testing and observed thereafter by SEM.

The front of the external “embrittled” layer was sharp enough to be delimited on the failure surfaces. It was drawn with a dotted line in Figure. The external layer encircled the aged specimen’s border with a fairly constant width of 150±20 µm (over 2 samples). This deepness slightly increased above 200 µm in the vicinity of the four corners, consistently with a diffusion-like phenomenon due to a doubled contact surface area.

All these features were consistent across all 1000 h aged specimens, including those oriented at 45 and 90° (see Figure SUP-5 in the Supplementary materials). Nevertheless, some slight width variations were observed in these cases, such as 200±57 µm for the 90° specimens. Similar measurements were performed in sets of 0° and 90° samples with much longer lifetimes, as illustrated in Figure. It is important to note that the layer width remained fairly constant across tests with short and long fatigue durations, indicating that it was not heavily impacted by the applied mechanical stresses. Moreover, similar observations were made with 0° and 90° samples broken during tensile test, plotted also in Figure. This illustrated that failures occurring at higher stress levels than fatigue displayed similar features in the border regions.
<figure><img src="image_14.png" /><figcaption>Figure 12 SEM fractography of (left) unaged ($N_f$ = 20 527 cycles) and (right) aged ($N_f$ = 12 800 cycles) 0° PA6.6 composites tested in fatigue at RH90 60°C. The dotted line drawn over the aged specimen images is used for separation purposes of the external layer exhibiting adhesive decohesion of fibers.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Figure 13 Summary of the external layer thickness measurements for fatigue and tensile tested PA6.6 specimens. Note that only one specimen was used for the tensile data.</figcaption></figure>

As mentioned above, failure surfaces of dried aged specimens were analyzed in the same way. As shown by the example depicted in Figure 13, a privileged area of debonding could be seen near the specimens' borders, supporting the interfacial degradation hypothesis.

<figure><img src="image_16.png" /><figcaption>Figure 14 Typical SEM fractography of the shell of a dried aged specimen of 90° PA6.6 composite after tensile failure.</figcaption></figure>
Finally, observations were also performed on 3000 h aged specimen failure surfaces. Only one sample shown in Figure 1 with a 0° orientation was observed after tensile testing, which showed a slightly larger “degraded” layer of at least 200 µm on the edges (i.e., high density of fiber pullouts). This could not be verified statistically with more samples, but overall still indicated that the diffusion kinetics of acid compounds is far slower than for water molecules.

<figure><img src="image_17.png" /><figcaption>Figure 15 SEM fractography of 3000 h aged-specimen of 0° PA6.6 composite after tensile failure.</figcaption></figure>

As a conclusion, failure surface observations evidenced that, despite hydrolysis of the matrix in the volume, the fatigue lifetime of the aged material still obeyed an initiation regime and that the 100-200µm “degraded” layer due to acid environment did not play any role in fatigue mechanisms.

### 3.3. Effect of acid medium on the mechanical behavior of PA6.6GF35

The last questions addressed in this work were the impact of acidity on the fatigue properties of unaged samples (section 3.3.1), and the interest of in-situ fatigue testing in the acid solution in the case of already aged samples (section 3.3.2).

#### 3.3.1 Impact of acidity on the fatigue properties of the unaged material

As compared with tests in the same environment as conditioning RH90 (section 3.2), in-situ testing introduced two differences. The first was the rise of humidity ratio (RH100 here while conditioning was at RH90) and the second was acidity.
Figure shows the normalized S-N curves (one curve per orientation) of unaged PA6.6 composites for tests performed in humidity (RH90) and acid solution (RH100), respectively, represented by plain and empty markers. The fatigue life was slightly but systematically shorter for specimens immersed in acid. The decrease was similar to that observed between unaged and aged specimens tested at RH90 shown in Figure although the exposure to the aging solution (≈130 h for the longest in-situ tests) was much shorter than the pre-aging time (1000h).

For the sake of comparison, a few in-situ tests were conducted in neutral demineralized water instead of the acid solution. Lifetimes were plotted with black stars in Figure. They were very close to those measured in acid solution, revealing that the impact of acidity was negligible compared to that of moisture content.

<figure><img src="image_18.png" /><figcaption>Figure 16 Normalized fitted S-N curves of unaged PA6.6 composite: humid (plain-) and immersed (empty-marker) tests.</figcaption></figure>

All samples plotted in Figure were systematically weighed before and after the fatigue test to confirm the major effect of the additional water uptake during immersion testing. Full square marks in Figure show the corresponding mass gain as a function of the square root of the total testing time, which includes the time for preliminary stabilization and cooling at the end of the test. Data could be fitted by a Fickian law characterized by an almost perfectly linear initial regime and a saturation mass gain of +0.8%. This value closely aligned with the average one obtained with the sixty aged specimens (see section 3.1) suggesting a similar absorption process. A few comparison specimens were immersed in the same chamber during fatigue tests without being mechanically loaded. Their mass gain was plotted with circle marks in Figure. Roughly the same amount of water was absorbed in both cases. This indicates that stress coupling was negligible during this additional sorption stage.

<figure><img src="image_19.png" /><figcaption>Figure 17 Mass gain for all specimens tested in in-situ fatigue in acid solution (Fig. 15 immersed during in situ testing). Round markers represent not fatigued control specimens and triangle-shaped markers pre-aged specimens.</figcaption></figure>
Such an increase in water concentration certainly affected T<sub>g</sub> and thus the mechanical response, which could explain the decrease in fatigue life. On one hand, the effect may be limited in this case since the material was almost fully saturated at RH90 before aging in the acid solution. On the other hand, aging may occur during immersion testing, especially for the longer tests.

### 3.3.2 Relevance of in-situ fatigue testing in pre-aged samples

Figure shows the same comparison as Figure (i.e., between tests performed in humidity (RH90) and acid solution (RH100)) but for aged samples. Due to the limited number of tests conducted in acid solution in this case, the corresponding regression lines for each orientation were not fitted. The decrease in fatigue life due to the change of environment was less pronounced for the aged specimens. This could be related to a reduced sensitivity to changes in water concentration for the long-term aged specimens. The mass gain from in-situ fatigue tests was also plotted for aged samples in Figure as empty triangle marks. During immersion testing, these specimens exhibited roughly similar water uptake as unaged ones. It could be argued that the curve for the aged specimens seemed slightly shifted upward, but this would have needed more measurements to confirm. Finally, when tested in the acid solution, the unaged and aged materials exhibit similar differences as in humid conditions. There is no gain from in-situ testing in this acid solution once samples have been previously aged with the present protocol.

<figure><img src="image_20.png" /><figcaption>Figure 18 Normalized fitted S-N curves of aged PA6.6 composite: humid (plain-) and immersed (empty-marker) tests.</figcaption></figure>

## 3.4. Comparison with PA6 specimens

This last section was dedicated to the direct comparison with PA6 composites tested in the same conditions. The after-aging data (mass, surface aspect, and crystallinity) were only briefly described. Figure displays an overview of the mechanical data, that is the tensile (Figure a) and fatigue (Figure b) curves, as well as SEM images of the border region of aged failure surfaces (Figure c).

As already mentioned in the introduction, PA6 tends to absorb more water than PA6.6 in any given environment. This was also observed here with a saturation level (respectively diffusion coefficient) increased by 12% (respectively 25%) at the end of aging (up to 3000 h), regardless of the type of immersion solution (acid-free demineralized water or acid). This amount was consistent with those obtained in the literature and was reflected similarly in the other specimens' characteristics (cross-section area increased by 15%). Like in the other material, no changes were detected on the external surface or for the initial crystallinity ratio (30.5±1%). Furthermore, a drop in the average molar mass was also measured in a similar proportion relative to the before-aging value and without any distinctive gradient through the thickness. Figure 20 shows the differential molecular weight distribution of the unaged and aged PA6 samples in the skin layer (300µm to the surface). Values were normalized with the same factors as previously done in Figure 8 for the PA6.6 samples.
Regarding the tensile and fatigue curves (Figure a-b), a less pronounced effect was observed after aging. Indeed, both the strain at failure for the tensile tests and the fatigue lifetimes slightly decreased in value. The 0° orientation seemed to still be the most impacted, in particular in the case of the fatigue curves. It is to be noted that more data points for the aged results could have permitted the fitting of these curves. As a means of comparison, the few in-situ fatigue tests performed with this composite (see Figure SUP-6 in the Supplementary Materials) demonstrated similar fatigue lifetimes, even in the case of the longest test (632 438 cycles) which was consistent with the other material's results. The SEM observations performed on failure surfaces of aged specimens (see an example for the 90° specimens in Figure c) revealed again the presence of a degraded band near the surface periphery. This was not the case for in situ specimens. The measurements showed that for a given orientation, the bands were always smaller in the case of the PA6 composite as shown by the inset graph on the right side of Figure c for the 90° orientation. This graph also shows the similarities in the bandwidth for both tensile and fatigue tests. Overall, the degradation induced by the acid environment was found to be limited for the PA6 composite when compared to the other material.

<figure><img src="image_21.png" /><figcaption>Figure 20 Overview of the PA6 composite aging results: (a) tensile curves, (b) fatigue curves and (c) example of failure surface. A graph showing the band width comparison for the 90° specimens is shown for both tests (fatigue and tensile).</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>Figure 20 Differential molecular weight obtained in the PA6 samples from SEC measurement in the skin layer (300µm to the surface) before and after aging (normalized values for confidentiality reason; same factor for both curves and same as for PA66 sample in Figure 8).</figcaption></figure>

**4. Conclusions**

The objective of this work was to investigate the effect of a low-pH acid aqueous aging solution mimicking EGR condensates on the fatigue behavior of SFRP. Two composites with different matrices (PA6, PA6.6) and 35% glass-fiber in mass were compared at three stages: prior, after, and during aging. Mechanical tests were performed in aging conditions (RH90, 60°C) with fixed parameters ($f=1-3$ Hz, $R=0.1$) while considering the effect of anisotropy with three orientations relative to the injection direction (0, 45 and 90°). A specific chamber was developed, built, and validated to perform the fatigue tests while immersed in the acid heated solution. Several indicators (mass and dimensions) were tracked during aging and compared with the data obtained with pure demineralized water solution.

The introduction of such acid compounds into water was found to deeply affect the fiber-matrix interface through a thin layer, about 100 to 200 µm, near the surface of the specimens. In this zone, fibers were easily detachable from the matrix when the specimen was loaded, creating a higher density of fiber pullouts observable with SEM. The degradation process seemed to directly depend on the contact surface with the solution, the immersion time, and the matrix nature, all affecting the band width. Such characteristics resembled those of a diffusion-like phenomenon but with a rate seemingly much slower than the one of water absorption. The analysis of the matrix properties (crystallinity, molar mass) at the macroscale showed that acidity did not promote changes in the polymer structure. These tendencies were found to be consistent with the tensile behavior evolution, showing signs of embrittlement attributed to hydrolysis from the water molecules. A similar decrease in the fatigue life for a given stress level was observed with both pre-aged and in situ specimens. The comparison of failure surfaces showed that similar damage mechanisms occur despite the interface degradation from the acid, which is not visible for in situ specimens. Composites with PA6 matrix were found to be more stable than those with PA6.6, resulting in less degraded properties and a smaller apparent diffusion of acid for the former. This last item is not fully understood yet and further works are still needed in this area, especially as these materials are increasingly used in specific challenging applications. Extrapolation of these results to any other low-pH acid solution must be considered very carefully since chemical degradation processes could depend on the chemical structure of the acid. Hocker at al. showed for instance that PA11 was more sensitive to butanoic acid than to hydrochloric one at the same pH [31]. To the authors' knowledge no such comparative analysis has been reported in PA6 or PA6.6.

**Supplementary materials**

Figures referred to as SUP-xxx are joined as Supplementary materials of this article.
## Authors Statement

**Florent Alexis:** Data curation, Results analysis, Methodology, Writing – original draft. **Sylvie Castagnet:** Conceptualization, Results analysis, Funding acquisition, Methodology, Supervision, Writing – review and editing. **Carole Nadot-Martin:** Conceptualization, Results analysis, Funding acquisition, Methodology, Supervision, Writing – review and editing. **Gilles Robert:** Conceptualization, Formal analysis, Funding acquisition, Methodology, Writing – review and editing. **Peggy Havet:** Conceptualization, Formal analysis, Funding acquisition, Methodology, Writing – review and editing.

## Acknowledgments

Pprime Institute gratefully acknowledges «Contrat de Plan Etat - Région Nouvelle-Aquitaine" (CPER) as well as the "Fonds Européen de Développement Régional (FEDER)" for their financial support to the reported work. Authors would like to thank the ANRT (French National Association for Research and Technology) for its financial support and grant of F. Alexis' PhD (CIFRE).

This work pertains to the scope of the French Government program "Investissements d'Avenir" LABEX INTERACTIFS (reference ANR-11-LABX-0017-01) and EUR INTREE (reference ANR-18-EURE-0010).

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SUPPLEMENTARY MATERIALS

<figure><img src="image_23.png" /><figcaption>Figure SUP-8 pH and temperature measurements acquired during the aging process of PA6.6 specimens.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>Figure SUP-9 pH evolution across 9 consecutive fatigue in-situ tests starting from the initially fabricated EGR solution. Each test is separated by vertical dotted lines. The test indicated by the symbol (*) is the only one during which data acquisition failed after some time.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>Figure SUP-10 (a) Effect of the ambient environment on strain measurements from the surface-painted markers during tensile loading. (b) Corresponding crosshead displacement for each test. Four different configurations are compared, each with the addition of a potential perturbator (Glass-Water-Flow) introduced one at a time.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>Figure SUP-11 Alternative representation of Figure 9 located in the main document showing the relative position on plates of each fatigued PA6.6 specimen.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>Figure SUP-12 SEM fractography of (a) unaged and (b) aged PA6.6 composites tested in fatigue at RH90 60°C. The dotted line drawn over the aged specimen images is used for separation purposes of the external layer exhibiting adhesive decohesion of fibers.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>Figure SUP-13 Normalized fitted S-N curves of PA6 composite tested in-situ: unaged (plain-) and aged (empty-marker) specimens. The gray markers displayed in the background represent the test conducted at RH90 60°C.</figcaption></figure>