Method to produce pre-stressed composite laminates based on stress relaxation of the matrix thereof

Abstract

The present disclosure relates to a method to produce pre-stressed composite laminates comprising the following steps: providing a laminate composed of a matrix and reinforcement fibers; and increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, applying mechanical loading to the laminate, allowing the matrix to completely relax the stress induced by the mechanical loading applied, cooling down the laminate to room temperature, and releasing the mechanical loading; or applying mechanical loading to the laminate, increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, allowing the matrix to completely relax the stress induced by the mechanical loading applied, cooling down the laminate to room temperature, and releasing the mechanical loading; or simultaneously applying mechanical loading to the laminate and increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, allowing the matrix to completely relax the stress induced by the mechanical loading applied, cooling down the laminate to room temperature, and releasing the mechanical loading, wherein the matrix is capable of stress relaxation through heat treatment.

Description

[0001] METHOD TO PRODUCE PRE-STRESSED COMPOSITE LAMINATES BASED ON STRESS RELAXATION OF THE MATRIX THEREOF

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a method to produce pre-stressed composite laminates and to a pre-stressed composite laminate obtained by the method thereof.

[0004] BACKGROUND ART

[0005] Prestressed concrete has played a crucial role in the history of structures, revolutionizing the field of civil engineering. By introducing compression to concrete before applying external loads, this significantly improves the material's performance under tension, leading to longer spans, reduced material usage, and enhanced durability.

[0006] Similarly, in laminated composites, prestressing the laminate helps delay the initiation of matrix cracks by inducing a controlled compressive stress within the matrix, counteracting stresses imposed during loading. This approach can extend the lifespan of composite structures because, although matrix microcracking does not directly cause structural failure, it can facilitate more severe forms of damage, including fiber breakage and delamination. Furthermore, matrix cracks can act as conduits for the infiltration of moisture or other environmental agents, further degrading the material.

[0007] While there are two primary methods of prestressing concrete (pre-tensioning and posttensioning), prestressed fiber- re info reed polymer composites are produced primarily through two different pre-tensioning methods based on the behavior of the reinforcement material: i) elastically prestressed fiber- re info reed polymers (EPFRP) and ii) viscoelastically prestressed fiber-reinforced polymers (VPFRP). In both methods, the manufacturing process involves liquid resin impregnating the fibers and curing to form a solid composite.

[0008] On the one hand, EPFRP (i) employs a method similar to pre-tensioned prestressed concrete, where tensile stress (usually below the elastic limit) is applied to the reinforcement before matrix curing. After solidification, the tensile load on the reinforcement is released, causing the material to instantaneously return to its original size, thus generating compressive stress within the matrix. Consequently, producing EPFRP composites requires a method to apply tensile force to the reinforcement during matrix curing, which hinges on the feasibility of incorporating the appropriate mechanism or apparatus.

[0009] For instance, US5429693 discloses a method for forming a prestressed composite, comprising the steps of: (a) providing a source of fibers and a source of resin matrix material; (b) providing a mechanically expandable mandrel comprising a material of a known coefficient of thermal expansion greater than that of said fibers and matrix material; (c) applying said matrix material to said fibers; (d) selectively tensioning said fibers and winding said fibers with preselected tension onto said mandrel; (e) mechanically expanding said mandrel with said fibers wound thereon to impart a preselected tension in said fibers; and (f) heating said mandrel with said fibers and matrix material wound thereon to elevated temperature to thermally expand said mandrel and to cure said matrix material with said fibers under tension.

[0010] On the other hand, VPFRP (ii) involves applying a tensile load to viscoelastic fibers to induce creep strain. In this strategy, the tensile stress is released before incorporating the fibers into the matrix. Because of their viscous properties, upon stress removal, the material continues to contract temporarily before reverting to its original shape, thereby generating compressive stresses within the matrix. The production of VPFRP laminates offers more flexibility compared to EPFRP production because fiber stretching and molding are conducted as separate operations. However, this process relies on the viscoelastic behavior of the reinforcement fibers.

[0011] For instance, US2013153121 A1 discloses a method for reducing the warping of a molded polymer-continuous fiber compound, the method comprising: pre-stressing a layer of continuous fiber; molding the layer of continuous fiber to a layer of polymer including a heating process; and cooling the layer of continuous fiber and the layer of polymer such that the resulting compound is substantially flat. In aviation, common fibers like glass, carbon and aramid are combined with epoxy-based resin matrices. Nowadays, glass fiber is preferred for critical components such as aircraft noses and pilot cabin doors, benefiting from properties like radar transparency and fire resistance. In contrast, carbon fiber, with stiffness and strength being 3-10 times greater than glass fiber, is indispensable for primary fuselage structures. Consequently, EPFRP stands as the sole viable method for producing prestressed fiber reinforced polymer (CFRP) for aeronautical applications, as the elastic properties of the commonly used fibers render them unsuitable for the VPFRP prestressing strategy.

[0012] However, due to the impracticality of stretching elastic fibers during curing, new methods are needed to produce prestressed laminates.

[0013] For instance, US4622086A refers to a method of fabricating a hollow body built-up with fiber- reinforced plastic material, and having at least two layers of fibers mutually differently oriented in relation to a longitudinal axis of the hollow body and the fibers of each layer extending at least partially in each such mutually different orientation over the entire hollow body and wherein the layers comprise a plastic which at least at a predetermined service temperature is hard-elastic, comprising the steps of: fabricating the at least two layers with the fiber-reinforced hard-elastic plastic to form at least two fiber-reinforced layers; mutually bonding the at least two layers by means of the hard-elastic plastic; subsequently heating at least said hard-elastic plastic bonding the at least two layers between the at least two layers; then subjecting at least one fiber- re info reed layer of said at least two fiber- re info reed layers to a least one force; and thereafter cooling said at least one fiber-reinforced layer to below the glass transition temperature and then relieving said at least one force thus providing a residual prestress in the hollow body.

[0014] EP1173323 refers to a homogenizing process for treating a structural composite having a shape wrapped with a multitude of fibres, the fibres being embedded in resin, the resin having been cured, the fibres acting to absorb tension when the composite is stressed, comprising: (a) elevating the temperature of the cured resin to enable the fibres to be more mobile therein; (b) imparting tension into the fibres; (c) maintaining the elevated temperature and fibre tension so as to permit the tensioned fibres to move sufficiently within the resin so as to permit disparate tension between individual fibres to homogenise; and (d) lowering the temperature of the resin and reducing the tension of the fibers.

[0015] However, there is still a need in the art of providing new methods to produce prestressed composites or laminates, e.g. methods not relying in shape memory and / or producing materials having improved mechanical properties.

[0016] SUMMARY OF THE DISCLOSURE

[0017] The present disclosure relates, in a first aspect, to a method to produce pre-stressed composite laminates comprising the following steps: a) a first step comprising providing a laminate composed of a matrix and reinforcement fibers; and b1) a second step comprising increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, c1) a third step comprising applying mechanical loading to the laminate, d1) a fourth step comprising allowing the matrix to completely relax the stress induced by the mechanical loading applied, e1) a fifth step comprising cooling down the laminate to room temperature, and f1) a sixth step comprising releasing the mechanical loading; or b2) a second step comprising applying mechanical loading to the laminate, c2) a third step comprising increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, d2) a fourth step comprising allowing the matrix to completely relax the stress induced by the mechanical loading applied, e2) a fifth step comprising cooling down the laminate to room temperature, and f2) a sixth step comprising releasing the mechanical loading; or b3) a second step comprising simultaneously applying mechanical loading to the laminate and increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, c3) a third step comprising allowing the matrix to completely relax the stress induced by the mechanical loading applied, d3) a fourth step comprising cooling down the laminate to room temperature, and e3) a fifth step comprising releasing the mechanical loading, wherein the matrix is capable of stress relaxation through heat treatment.

[0018] The present disclosure relates, in a second aspect, to a pre-stressed composite laminate obtained by the method as defined herein.

[0019] BRIEF DESCRIPTION OF THE FIGURES

[0020] The foregoing and other advantages and features will be fully understood from the following detailed description of an embodiment with reference to the accompanying drawings, to be taken in an illustrative and non-limitative manner, in which:

[0021] FIG. 1 schematically depicts an embodiment of the method disclosed herein disclosing five post-tensioning steps to produce an elastically prestressed fiber- re info reed composite laminate: a) curing a laminate under standard conditions; b) pre-heating to the posttensioning temperature (Tg or Tv); c) applying mechanical loading and facilitating stress relaxation; d) cooling down to room temperature; e) releasing the load.

[0022] FIG. 2 is a schematic post-tensioning Time-Temperature-Transformation (TTT) plot, wherein the shaded region denotes the working area delimited by the four process design conditions.

[0023] FIG. 3 is a photography exemplifying the images obtained by submitting a laminate to the disclosed method.

[0024] FIG. 4 is a plot depicting the crack density (number of matrix cracks within 50 mm of the specimen edge) vs. strain applied for the six specimens tested (A, B no treated; C, D, E and F submitted to the method according to the present disclosure).

[0025] FIG. 5 is a plot depicting the modulus of elasticity of the tested specimens for the strain range from 0.1% to 0.3%.

[0026] DESCRIPTION OF THE DISCLOSURE

[0027] The present disclosure relates to a method to produce pre-stressed composite laminates comprising the following steps: a) a first step comprising providing a laminate composed of a matrix and reinforcement fibers; and b1) a second step comprising increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, c1) a third step comprising applying mechanical loading to the laminate, d1) a fourth step of allowing the matrix to completely relax the stress induced by the mechanical loading applied, e1) a fifth step comprising cooling down the laminate to room temperature, and e1) a sixth step comprising releasing the mechanical loading; or b2) a second step comprising applying mechanical loading to the laminate, c2) a third step comprising increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, d2) a fourth step comprising allowing the matrix to completely relax the stress induced by the mechanical loading applied , e2) a fifth step comprising cooling down the laminate to room temperature, and f2) a sixth step comprising releasing the mechanical loading; or b3) a second step comprising simultaneously applying mechanical loading to the laminate and increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix; c3) a third step comprising allowing the matrix to completely relax the stress induced by the mechanical loading applied; d3) a fourth step comprising cooling down the laminate to room temperature, and e3) a fifth step comprising releasing the mechanical loading; wherein the matrix is capable of stress relaxation through heat treatment.

[0028] Therefore, the present disclosure involves the following essential steps, as shown in FIG. 1 : providing a laminate composed of a matrix and reinforcement fibers, increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix, applying mechanical loading to the laminate, allowing the matrix to relax the stress induced by the mechanic loading applied, cooling down the laminate to room temperature and releasing the mechanical loading from the laminate. The steps of increasing the temperature and of applying mechanical loading to the laminate can be done sequentially, in either order, or the two steps can take place simultaneously.

[0029] The matrix herein disclosed needs to be capable of relaxing the stress posed by mechanical loading through heat treatment. Examples of such matrices include polymers (or polymeric systems) such as thermoplastics, and a new generation of thermoset resins based on covalent adaptable networks (CAN), which include vitrimers.

[0030] When mechanical loading is applied to the laminate and the temperature of the laminate is increased, the matrix is allowed to fully relax to the induced stress. This relaxation process occurs over a predetermined duration determined by the kinetics of stress relaxation. The selection of the proper temperature and time duration ensuring matrix stress relaxation while avoiding thermal degradation are conditioned to the type of laminate suitable for prestressing.

[0031] Once the stress in the matrix is relaxed, the laminate is cooled down to room temperature. At room temperature, the mechanical load is released, so that when the tension applied to the reinforcement fibers is released, the matrix compensates by resisting their return and thus, the elastically deformed reinforcement fibers do not completely return to their original shape because the matrix constrains them.

[0032] As a result, the equilibrium state of the system, composed of both fibers and matrix, is determined by the extent to which the fibers remain stretched after releasing the tension and the compression in the matrix balances the residual tension in the fibers.

[0033] Glass transition temperature (Tg) is herein defined as the temperature at which an amorphous polymer changes from a hard / glassy state to a soft / leathery state, or vice versa. Tg is directly related to a material’s strength and capabilities in any given end -use application.

[0034] Topology freezing transition temperature (Tv) is herein defined as a temperature in which a transition from viscoelastic solid to viscoelastic liquid occurs, or vice versa. This term is preferably applied to vitrimers, being a temperature above which dynamic covalent bonds allow for rapid stress relaxation, self-healing and shape reprogramming.

[0035] Mechanical loading is defined herein as a physical stress on a mechanical system or component leading to strain.

[0036] The method as disclosed herein relates to a method to produce pre-stressed composite laminates wherein the laminate is preferably fiber-reinforced.

[0037] Preferably, the matrix according to the present invention is a polymeric matrix.

[0038] Even more preferably, the matrix is composed of a vitrimer.

[0039] Vitrimers are herein defined as a class of polymeric materials that possess dynamic covalent bonds, allowing them to undergo reversible chemical reactions while maintaining their structural integrity. Therefore, vitrimers are a type of polymers that can be reshaped, reprocessed, and repaired multiple times without losing their properties or performance. Vitrimers are formed by introducing a dynamic covalent bond in a polymer matrix like epoxy resins, but other polymers might be used, such as polyurethanes, poly(urea-urethane), aromatic polyesters, polylactic acid, polyhdyroxyurethanes, polybutadiene, polyimine or polybenzoxazines.

[0040] In an embodiment, the vitrimer may be epoxy-based. In a preferred embodiment, the vitrimer may comprise epoxy groups and aromatic disulfide moieties. In the present disclosure and even more preferably, the vitrimer may comprise epoxy groups and a hardener based on aminophenyl disulfide.

[0041] The reinforcement fibers according to the present disclosure are preferably carbon fibers, glass fibers, aramid fibers, natural fibers such as cellulose fibers and / or combinations thereof. Preferably, the reinforcement fibers are carbon fibers, glass fibers or combinations thereof.

[0042] The reinforcement fibers may preferably show elastic behavior.

[0043] In the method according to the present disclosure, the mechanical loading applied preferably produce a tensile stress in the reinforcement fibers. The mechanical loading may be tensile loading. Moreover, the mechanical loading may be applied simultaneously to the laminate in multiple directions, and not limited to the same direction.

[0044] The present disclosure also relates to a pre-stressed composite laminate obtained by the method as defined herein.

[0045] The present disclosure relates to the obtention of a laminate comprising an elastically prestressed fiber-reinforced polymers (EPFRP) from a fiber-reinforced polymer (FRP).

[0046] This disclosure, therefore, discloses a new post-tensioning method to produce prestressed composite laminates. Utilizing matrices that can relax stresses, the operations of curing and prestressing are separated. Hence, the method herein disclosed facilitates the production of laminates under conventional processing conditions, followed by post-curing thermal treatment to achieve post-tensioned prestressed composite components with elastic reinforcement fibers.

[0047] To evaluate the feasibility of the post-tensioning process, a practical approach is to visually represent the influence of key variables such as time, temperature, or viscosity. Time- Temperature-Transformation (TTT) plots are suitable for optimizing thermally driven processes. These diagrams have previously been employed to optimize curing cycles, and to accurately define the conditions necessary for achieving complete curing of the polymer matrix herein disclosed while avoiding degradation.

[0048] For the post-tensioning method herein disclosed, there are four process design conditions that limit the working area of the post-processing TTT map (as shown in FIG. 2):

[0049] 1) From a practical standpoint, the glass manufacturing industry defines the softening point (4 106Pa s) as the minimum viscosity that prevents glass from deforming under its own weight during typical working times. Post-tensioning conditioning should be performed above this viscosity, as viscosities below this threshold can cause the resin to melt, leading to loss of laminate shape, structural integrity, and proper dispersion of fibers within the matrix.

[0050] 2) For the prestressing treatment disclosed herein, it is advisable to select a temperature and duration ensuring a minimum of degree of the tensioning stress relaxation in the matrix. Stress relaxation is a characteristic time-dependent behavior of certain polymers typically evaluated by subjecting a specimen to a fixed deformation and measuring the load needed to maintain it constant at a given temperature. The stress relaxation time, Tsr, exhibits temperature dependence usually described by an Arrhenius relationship:

[0051] Equation 1 : Arrhenius relationship

[0052] When plotted in a TTT diagram, the Tsrcurve defines the minimum thermoforming time required to achieve 63.2% stress relaxation. Similarly, curves representing 2rsrand 3rSr, correspond to 86.4% and 95% stress relaxation, respectively. Alternatively, the inventors have recently introduced the application of isoconversional methods to the modeling of stress relaxation kinetics of a disulfide-containing epoxy vitrimer assuming that the degree of conversion of the exchange reaction is proportional to the applied load.

[0053] 3) The thermal stability of the matrix is at risk during post-tensioning. Stress relaxation of polymers requires high temperatures to activate the mechanisms involving the movement and reorganization of polymer chains, but at these temperatures, these mechanisms compete with the thermal degradation of the polymer. Like thermoplastics repeatedly melted for recycling, thermal degradation can lead to detrimental changes in the properties of posttensioned polymers. These changes can significantly reduce material performance, potentially making it unsuitable for certain applications. Therefore, understanding the kinetics of thermal degradation is crucial for safe post-tensioning conditioning.

[0054] 4) Finally, from an industrial perspective, there may be a maximum desired post-tensioning time that balances the cost of the conditioning process with the benefits gained from the improved mechanical properties of the material. This balance is crucial to ensure a positive return on investment for the conditioning process.

[0055] Tuned residual stress to avoid matrix cracking

[0056] Composite materials are suitable for aeronautical structures due to their high specific strength stiffness, and low coefficients of thermal expansion (CTEs). However, matrix cracking poses a challenge in composite aeronautical structures. The matrix in a composite material helps to transfer loads between fibers. When cracks form in the matrix, this load transfer is disrupted, leading to an uneven stress distribution and potentially overloading certain fibers, causing them to fail. Moreover, initial matrix cracks can propagate through the laminate, leading to larger cracks and delamination, which significantly reduces the loadbearing capacity and can result in catastrophic failure. This is particularly critical in the cryogenic storage of liquefied hydrogen in propellant tanks, a key enabler for reducing CO2 emissions in aviation. As microcrack density increases, these cracks may propagate through the tank's thickness, potentially leading to leakage and compromising the structural integrity and safety of the aircraft.

[0057] Residual stresses have been identified as the most important cause of micro-cracking. A primary cause behind the residual stresses is the mismatch in thermal expansion between the matrix and embedded fibers, which can induce microcracks in the polymer matrix. In cryogenic environments, the mismatch in thermal expansion between the matrix and fibers is exacerbated by the extreme temperature differences, increasing the risk of thermal stresses and microcracks. Another primary cause of matrix cracking is curing-induced volume shrinkage. Upon cure, the matrix typically undergoes a shrinkage of 3% to 7%, while the fibers do not. Other sources of residual stresses in composites include moisture absorption, variations in fiber volume throughout the laminate, non-uniform cure degrees, and tool-part interaction.

[0058] The proposed post-tensioning conditioning as disclosed in the method according to the present disclosure involves heating the laminate to a temperature that enables stress relaxation, exceeding the glass transition temperature (Tg) or the topology freezing transition temperature (Tv). While the method according to the present disclosure effectively relaxes residual stresses arising during curing, it also addresses issues caused by mismatched CTEs between fibers and the matrix. By tuning the residual stress field in the matrix, the post-tensioning technique induces stresses that counteract thermal residual stresses, thereby reducing the likelihood of microcrack formation and propagation. Additionally, posttensioning can be adjusted to counteract in-service loads that induce cracking in the matrix. Consequently, prestressing can extend the lifespan of the structure without increasing its dimensions or overall mass.

[0059] Load application

[0060] At the laminate level, prestressing the fibers produces a compressive stress in the matrix of perpendicular plies, delaying the appearance of matrix cracks. This can be achieved across multiple ply orientations by simultaneously tensing the laminate in the longitudinal direction of several plies. Existing pre-tensioning EPFRP methods require specific apparatus for holding separate fibers, making this process challenging. In contrast, the post-tensioning method allows the load to be introduced by gripping the solid laminate with standard testing fixtures, making fiber tensioning much more convenient.

[0061] For instance, a large fuselage testing fixture, such as the Full-Scale Aircraft Structural Test Evaluation and Research (FASTER) fixture developed at the FAA William J. Hughes Technical Center, could apply pressurization, longitudinal, and hoop loads to perform controlled post-tensioning of curved fuselage panels after manufacturing. The direction and magnitude of these loads can be adjusted to counteract in-service loads. Similarly, in the case of propellant tanks, pressurization can introduce the necessary tensioning hoop stress while allowing matrix relaxation. After post-tensioning conditioning, a compressive hoop stress would be achieved, enhancing the overall durability of the structure when in-service internal pressure is applied.

[0062] Moreover, the method as disclosed herein allows for applying different levels of pre-stress to various regions of a composite structure during multiple post-tensioning operations. This introduces an additional variable for optimizing the design of composite structures. For example, when a positive bending moment is applied to a beam -shaped structure, the tensioned part can be locally heated and post-tensioned. In a subsequent operation, a negative bending moment can be applied to post-tension the previously compressed region. Vitrimeric EPFRP

[0063] While other matrices capable of relaxing stress can be taken into account, the present disclosure preferably discloses the use of vitrimeric resins. As disclosed hereinabove, vitrimers possess covalent cross-links -like thermosets-, but they can undergo reversible changes in the molecular structure. This characteristic enables stress relaxation, a thermally activated process that occurs above two critical temperatures: the glass transition temperature (Tg), and the topology freezing transition temperature (Tv). Furthermore, exploiting the characteristic exchange reaction, vitrimer-based composites offer reprocessability, reparability, and recyclability. Recent studies demonstrate their potential as aeronautical-grade structural materials, with mechanical properties comparable to traditional thermosets. It is worth noting that, among vitrimers, epoxy-based formulations stand out for their abundant availability of monomers and straightforward synthesis processes, thereby facilitating their scalability in industrial settings.

[0064] EXAMPLES

[0065] Validation experiments

[0066] Tests were done on specimens cut from a panel manufactured by resin infusion. Fiber reinforcements were unidirectional carbon fabric layers (24 K and 300 g / m2) supplied by Castro Composites. The stacking sequence was [O2, 902]s. The matrix was an epoxy-based vitrimer supplied by Fundacion Cidetec (Donostia-San Sebastian, Spain), extensively described in EP3149065B1. It consists of commercially available functional epoxy groups DGEBA (Araldite LY1564) and a hardener (also commercially available) with dynamic crosslinking based on aromatic disulfide species (4-Aminophenyl disulfide). For the mixing, the NH equivalents were set as 1.1 per epoxy group. The laminate was cured at 140°C for 3 hours. The curing cycle was chosen based on the optimal processing conditions of this epoxybased vitrimer to prevent vitrification and thermal degradation while ensuring complete resin curing. The resulting panel had an area of 300 x 300 mm2and was 2.3 mm thick. Specimens cut from the panel were 250 mm long and 20 mm wide. Before testing, the specimens’ sides were polished with a medium grit (350 mesh) followed by fine grit (1200).

[0067] Specimens were mechanically loaded with two-column tabletop MTS lnsight50 electromechanical testing machine with a load cell of 50 kN. Tests were conducted on six specimens. Post-tensioning conditioning was applied to four of them following the method described herein. Prestressed specimens were loaded under different constant force conditions (refer to Table 1). Thermal treatment was carried out at 165°C for 1 hour, a duration exceeding 3rSr, as determined by the re-processing TTT-like diagram of the vitrimeric resin, thus ensuring a minimum 95% relaxation of the initial stress. Moreover, the treatment time duration was selected to prevent thermal degradation.

[0068] Table 1. Prestressing loading conditions of the specimens tested

[0069] After conditioning, specimens were subjected to a tensile test up to a maximum load of 20 kN. The strain was measured by a contact clip-on mechanical extensometer. The matrix cracking at the 90° middle layers was monitored in real time. A Canon digital camera (EOS 550D) with macro-lenses (100 mm f / 2.8 Macro L IS USM) capturing 18 Megapixels was employed to monitor a 50 mm section of the specimen edge (midway between the grips). Photos were taken every 5 seconds using the camera's software, connected to a PC via a USB port. The high image resolution facilitated the identification of any transverse cracks. FIG. 3 provides an example of the captured images.

[0070] The specimen was loaded at a loading rate of 0.5 mm / min, and this process persisted without interruption until the applied load reached 20 kN. Images obtained were correlated with load readings from the testing machine over time. The number of cracks within the gauge length on the specimen edge was counted throughout the loading history, and the crack density (number of cracks per unit length) was subsequently calculated.

[0071] The modulus of elasticity was determined according to the ASTM D3039 / D3039M-17 standard. However, instead of utilizing the chord modulus as recommended in the standard, the modulus was calculated as the slope of the linear fitting of stress-strain data for the strain range from 0.1% to 0.3%.

[0072] FIG. 4 shows the crack density within the 90° layer. The results from six specimens are shown: two identical specimens without post-tensioning conditioning (square markers), two identical specimens post-tensioned at a constant load of 7.5 kN (cross and triangular markers), and two identical specimens post-tensioned at a constant load of 10 kN (round and dash markers). It is noteworthy that neither delamination nor fiber breakage occurred during the tests.

[0073] The results depicted in FIG. 4 illustrate a delay in matrix cracking with post-tensioning conditioning. Specimens subjected to 10 kN of post-tensioning force exhibit a doubling of strain at the same crack density compared to untreated specimens. This phenomenon is attributed to the generation of compressive residual stresses within the matrix. These stresses effectively enhance the apparent strength of the prestressed matrix, resulting in transverse cracks initiating at higher levels of laminate strain. It is worth noting that good repeatability was observed among the specimens within each series.

[0074] FIG. 5 illustrates the modulus of elasticity of the tested specimens calculated as the slope of the linear fitting of stress-strain data. In all cases, the correlation coefficient exceeded 0.999. It can be observed that the mean modulus of elasticity increased by 4% and 5% in specimens post-tensioned with forces of 7.5 kN and 10 kN, respectively, compared to untreated specimens. This improvement could be attributed to the increased straightness of fiber yarns due to tensile post-tensioning. During the fabrication of composite laminates, thermal residual stresses induce compressive axial loads on fibers, leading to microscale buckling or waviness. It is proven that subjecting fibers to tensile load reduces this waviness, resulting in an increase in the tensile modulus of elasticity. However, in the present case, the differences in modulus between specimens within the same series are comparable to the increases observed in mean values across different series.

Claims

CLAIMS1. Method to produce pre-stressed composite laminates comprising the following steps: a) a first step comprising providing a laminate composed of a matrix and reinforcement fibers; and b1) a second step comprising increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix; c1) a third step comprising applying mechanical loading to the laminate; d1) a fourth step comprising allowing the matrix to completely relax the stress induced by the mechanical loading applied; e1) a fifth step comprising cooling down the laminate to room temperature; and f1) a sixth step comprising releasing the mechanical loading; or b2) a second step comprising applying mechanical loading to the laminate; c2) a third step comprising increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix; d2) a fourth step comprising allowing the matrix to completely relax the stress induced by the mechanical loading applied; e2) a fifth step comprising cooling down the laminate to room temperature; and f2) a sixth step comprising releasing the mechanical loading; or b3) a second step comprising simultaneously applying mechanical loading to the laminate and increasing the temperature of the laminate up to the glass transition temperature or the topology freezing transition temperature of the matrix; c3) a third step comprising allowing the matrix to completely relax the stress induced by the mechanical loading applied; d3) a fourth step comprising cooling down the laminate to room temperature; and e3) a fifth step comprising releasing the mechanical loading; wherein the matrix is capable of stress relaxation through heat treatment.

2. The method according to claim 1, wherein the matrix is a polymeric matrix.

3. The method according to claim 1 or 2, wherein the polymeric matrix is selected from the group consisting of thermoplastics, thermoset resins based on covalent adaptable networks and vitrimers.

4. The method according to claim 3, wherein the polymeric matrix is composed of a vitrimer.

5. The method according to claim 4, wherein the vitrimer is epoxy-based.

6. The method according to claim 5, wherein the vitrimer comprises epoxy groups and aromatic disulfide moieties.

7. The method according to claim 6, wherein the vitrimer comprises epoxy groups and a hardener based on aminophenyl disulfide.

8. The method according to any one of the previous claims, wherein the reinforcement fibers are carbon fibers, glass fibers or combinations thereof.

9. The method according to any one of the previous claims, wherein the reinforcement fibers show elastic behavior.

10. The method according to any one of the previous claims, wherein the mechanical loading applied produces a tensile stress in the reinforcement.

11. The method according to any one of the previous claims, wherein the mechanical loading in the laminate is applied simultaneously in multiple directions.

12. A pre-stressed composite laminate obtained by the method defined in claims 1 to 11.

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