Thermoplastic Laminates Research Articles

Analysis of fracture toughness in mode II and fractographic study of composites based on Elium® 150 thermoplastic matrix

One of the major challenges of composite materials is to increase their properties related to interlaminar fracture toughness and consequently delamination. This work shows a composite system based on a new thermoplastic Elium® 150 resin developed to be a competitive solution for the composites traditionally based on epoxy resins. Both composites were fabricated via VARTM using a 0/90° plain weave carbon fiber fabric. The thermoplastic composites presented 40% more resistance to interlaminar fracture in comparison to epoxy composites. These materials obtained superior performance in crack propagation resistance because it tends to absorb the energy associated with crack propagation in the form of plastic deformation. The unknown fractographic aspects of the fracture surfaces of the thermoplastic laminate was used as a complementary tool for the mechanical characterization. In this case, it was evidenced the presence of morphological features characteristic of pure mode II shear failure characterized by a failure directly in the interfacial fiber/matrix regions and shear cusps which are similar to the known aspects already observed in epoxy resin based matrices.

Applicability of mode II interlaminar fracture toughness testing methods for characterization of thermoplastic laminates with woven fabric reinforcements

The scope of both existing standardized methods for the determination of quasistatic mode II interlaminar fracture toughness, ASTM D7905/D7905M – 14 and ISO 15114:2014, is limited to testing laminates with unidirectional reinforcement. Furthermore the precision round-robins conducted as a part of the development of these procedures focused mainly on laminated with epoxy resin matrix. This paper compares the testing methods for quasistatic mode II interlaminar fracture toughness with the focus on their applicability to testing thermoplastic laminates with woven fabric reinforcement.Two standardized testing methods were used to determine the mode II fracture toughness of a laminate made of polyether ether ketone matrix reinforced with T300 carbon fibers in the form of 5 harness satin woven fabric. The testing methods were compared in terms of the set-up complexity, testing efficiency, the data reduction methods, the type of properties determined, the results scatter and the applicability to testing thermoplastic-based laminates with woven fabric reinforcement. The methods produced similar and consistent results. Both of them can be successfully applied to testing thermoplastic laminates with woven fabric reinforcement after including modifications discussed in the paper.

Comparison of damage resistance of thermoplastic and thermoset carbon fibre-reinforced composites

The research presented in the article concerns the resistance to damage of thermosetting and thermoplastic fabric carbon composites. A comparison of these materials resistance was made on the basis of the results of residual strength tests which include impact tests and static compression of samples after impact. The impact tests consisted of two impact criteria: specimens impacted with energy adjusted to the specimen thickness, determined on the basis of ASTM D7136 (energy criterion) and specimens impacted in such a way so as to obtain a specific depth of indentation (indentation depth criterion). The specimens struck according to the indentation depth criterion were additionally divided into two subgroups: with the indentation depth of 1.3 mm and the indentation depth of 2.6 mm. In addition, after impact tests, each specimen was subjected to ultrasonic phased array testing in order to obtain information about the extent of the damage. Experimental results showed that with less impact energy, the thermoplastic laminate has a higher residual strength than the thermosetting composite. This difference decreases as the impact energy increases.

A numerical analysis of an energy directing method through friction heating during the ultrasonic welding of thermoplastic composites

The ultrasonic spot welding of fiber-reinforced thermoplastic laminates received a wide interest from researchers mainly in the fields of aerospace and automotive industries. This study investigated a new technique for focusing the ultrasonic vibration energy at the desired spot between two mating thermoplastic composite laminates. In this investigated method, no additional energy directing protrusions between the mating laminates were required to focus the vibration energy. It was found that by welding the laminates amid an ultrasonic horn and an anvil in which the prior had a larger contact surface with the laminate as the latter, it was possible to generate a localized friction heating. In the initial phase of the welding, the friction heating softened the interfacial layers and thus caused the focusing of the majority of the cyclic ultrasonic strain energy in the weld spot center. The assumption for the presence of the friction and its influence on the heat generation was investigated by means of finite element method (FEM) mechanical dynamic analysis. Microscopic analysis of the weld spot eventually delivered the proof for the melt initiation by friction at a ring around the weld spot and subsequent spot growth by viscoelastic heating.

Quasi-Static and Low-Velocity Impact Behavior of Intraply Hybrid Flax/Basalt Composites

In an attempt to increase the low-velocity impact response of natural fiber composites, a new hybrid intraply woven fabric based on flax and basalt fibers has been used to manufacture laminates with both thermoplastic and thermoset matrices. The matrix type (epoxy or polypropylene (PP) with or without a maleated coupling agent) significantly affected the absorbed energy and the damage mechanisms. The absorbed energy at perforation for PP-based composites was 90% and 50% higher than that of epoxy and compatibilized PP composites, respectively. The hybrid fiber architecture counteracted the influence of low transverse strength of flax fibers on impact response, irrespective of the matrix type. In thermoplastic laminates, the matrix plasticization delayed the onset of major damage during impact and allowed a better balance of quasi-static properties, energy absorption, peak force, and perforation energy compared to epoxy-based composites.

Effects of thermal gradient on failure of a thermoplastic composite pipe (TCP) riser leg

Thermoplastic composite pipe (TCP), consisting of a fibre-reinforced thermoplastic laminate fully bonded between homogeneous thermoplastic liners, is an ideal candidate to replace traditional steel riser pipes in deepwater where high specific strengths and moduli and corrosion resistance are advantageous. During operation, risers are subjected to combined mechanical and thermal loads. In the present paper, a 3D finite element (FE) model is developed to analyse stress state in a section of TCP under combined pressure, axial tension and thermal gradient, illustrative of a single-leg hybrid riser (SLHR) application. From the obtained stresses, through-thickness failure coefficient is evaluated based on appropriate failure criteria. The effects of increasing the internal-to-external thermal gradient are investigated considering temperature dependent material properties. The influence of varying the thickness of the isotropic liners with respect to the laminate is examined.

Novel reactive thermoplastic resin as a matrix for laminates containing phase change microcapsules

This work aims at developing multifunctional thermoplastic laminates combining structural and heat storage/management functions. The laminates are constituted by paraffin microcapsules as phase change materials (PCM), continuous carbon fibers, and a novel thermoplastic liquid methyl methacrylate resin (Elium), processable as a thermoset. The characterization aims to study how the paraffin microcapsules influence the thermo‐mechanical properties and thermal management performance of Elium and of the relative composite laminates. For the Elium/PCM systems, the phase change enthalpy increased with the experimental PCM concentration up to 101 J/g, but the mechanical properties decreased concurrently. The melting enthalpy of the laminates also increased with the microcapsule amount, up to 66.8 J/g, which indicates that the mild conditions applied in the processing of the liquid resin allow the integrity of the microcapsules to be preserved. This is also confirmed by the improved thermal management performance observed through thermal camera imaging measurements. Microscopy techniques showed that the PCM phase is preferentially distributed in the interlaminar region, which accounts for the observed decrease in the interlaminar strength and the flexural properties with an increase in the PCM content. These results show a potential for the future development of multifunctional thermoplastic composites with elevated thermal energy storage capabilities. POLYM. COMPOS., 40:3711–3724, 2019. © 2019 Society of Plastics Engineers

A Control Method for the Ultrasonic Spot Welding of Fiber-Reinforced Thermoplastic Laminates through the Weld-Power Time Derivative

It was found that the ultrasonic spot welding may serve as an efficient method to join relative large thin-walled parts made of fiber-reinforced thermoplastics. In this study, a new control method for the ultrasonic spot-welding process was investigated. It was found that, when welding fiber-reinforced thermoplastic laminates without energy directors, overheating and decomposition of the polymer at the weld spot occurred. The occurrence of the overheating took place at unpredictable times during welding. It was observed that the time trace of the consumed power curve by the welder follows a similar pattern as the time trace of the temperature in the weld spot center. Based on this observation, a control system was developed. The time derivative of the welder power was monitored in real time and, as soon as it exceeded a critical value, the ultrasonic vibration amplitude was actively adjusted through a microcontroller. The controlling of the ultrasonic welding process forced the temperature in the weld spot to remain in an adequate range throughout the welding duration for the polymer diffusion to occur. The results of the controlled welding process were evaluated by means of weld temperature measurements, computed tomography scans, and microscopic analysis of the weld spot fracture surfaces.

In situ micro-scale high-speed imaging for evaluation of fracture propagation and fracture toughness of thermoplastic laminates subjected to impact

Measuring parameters related to each damage mode of composites subjected to impact is very challenging because of the complex damage phenomenology. Here, we developed an experimental methodology for evaluating the micro-scale fracture characteristics of two principal damage modes, i.e., transverse crack and delamination, and providing the corresponding fracture toughness. We demonstrated the capability of the method by comparing and providing additional insights about two materials, namely homopolymer-based (ductile) and copolymer-based (less-ductile) glass/polypropylene thermoplastic composites. We found that (i) transverse crack behavior of both composites is similar as indicated by a small difference in their fracture toughness, (ii) delamination growth in copolymer-based composites is slower than in homopolymer-based composites, (iii) the fibrillation induced by rubber particles in copolymer-based composites is responsible for decelerating the delamination growth and improving its fracture toughness during delamination. This method is deemed useful and quick for determining the micro-scale fracture behavior of composite laminates under impact in order to support the material selection process.

Internal damage evaluation of composite structures using phased array ultrasonic technique: Impact damage assessment in CFRP and 3D printed reinforced composites

The application of non-destructive techniques for the evaluation of internal damage in composite materials is a challenge due to their complexity. The aim of this study is to analyse the ability of ultrasonic technique to identify and evaluate manufacturing defects and internal damage in composite laminates. Artificial object inclusions of different shapes, sizes and materials were embedded in carbon fibre reinforced epoxy (CFRP) laminates. Comprehensive investigation of non-destructive evaluation using phased array ultrasonic testing to trace and characterize the embedded defects is presented. Phased array ultrasonic technique was able to precisely locate most of the artificial inclusion in the composite laminates. Nerveless, the shape and size of the inclusions were not accurately determined due to the high signal attenuation and distortion characteristics of the carbon fibre epoxy composite.Furthermore, a major concern affecting the efficient use of composite laminates is their vulnerability to low velocity impact damage. The dynamic behaviour of composite laminates is very complex as there are many concurrent phenomena during composite laminate failure under impact loading. Thus, the practicality of the previous results is demonstrated by the evaluation of impacted carbon fibre reinforced epoxy laminates using ultrasonic testing. The influence of laminate thickness and impact energy on impact damage is also investigated. The performance of phased array ultrasonic testing for the inspection of composite laminates with barely visible impact damage is evaluated. In addition, fibre-reinforced thermoplastic composites are becoming more significant in industrial applications due to their excellent mechanical performance and potential recycling. In this study, impact damage in 3D printed continuous glass fibre reinforced thermoplastic laminates is also analysed. Phased array ultrasonic testing is performed and the amount of damage is quantified in terms of delaminated area.

Revealing the effects of matrix behavior on low-velocity impact response of continuous fiber-reinforced thermoplastic laminates

Matrix behavior is expected to widely influence the impact response of composites, but detailed conclusions in the case of thermoplastic laminates are still needed. In this paper, we investigated the effect of using either ductile homopolymer PP or less-ductile impact copolymer PP matrices on the low-velocity impact responses of continuous glass fiber-reinforced polypropylene (PP) laminate. These PP types represent two variants in the same family of thermoplastic matrix. A thorough experimental campaign was first performed to provide the tensile properties (for PP and glass/PP) and fracture toughness (Mode-I and Mode-II, glass/PP only) of the employed materials. Then, low-velocity impact tests where the energy levels are ranging from 12 to 30 J were performed. Using ductile PP in glass/PP laminates reduces the energy dissipated during impact as well as the impact damage area. The effect of selected stacking sequences on the resistance to impact was also studied as a way to reveal the difference between ductile and less-ductile glass/PP. Stacking sequence with thin plies shows better impact properties than other sequences regardless of the matrix ductility, which can be explained by micromechanics for both grades of material. Finally, as quasi-static indentation (QSI) is usually used to quickly access the resistance of laminates towards out-of-plane impact, we systematically compared our impact results with QSI results. We found that the prospective use of QSI in forecasting impact properties and damage is very limited in glass/PP composite due to strain-rate sensitivity.

Damage Modelling in Thermoplastic Laminates Reinforced with Steel and Glass Fibres under Quasi-Static Indentation Loading at Low-Velocity

This paper deals with experimental and numerical investigations of the composites damages with ductile and fragile reinforcement under quasi-static indentation loading. The main goal of the work is to increase the post-damage residual strength and ductility of thermoplastic composite. Two types of composite laminates with polypropylene (PP) matrix are tested: glass fibre laminate (GFPP) and steel fine wire mesh laminate (SWPP). The specimens are [0° 90°]2s stacking sequence and prepared by using a compression moulding technique. Quasi-static indentation tests were performed with two distinct penetration scales under low velocity (1.2 mm/min). The diameter of the hemispherical indenter is 16 mm. The failure mechanisms of composite layers were examined by the field emission scanning electron microscope (SEM). The results show that the failure mode of SWPP laminates is principally dominated by the plastic deformation component. In contrast, the GFPP laminate exhibits a fragile behaviour which is related to the fragile failure of glass fibres. In addition, the SEM shows that matrix cracking, fibre breakage, debonding and fibre pull out are the major damages observed around the indentation area. A model based on the combined use of plasticity, damage and fracture, was developed and applied to simulate quasi-static indentation behaviour and predict the resulting damage.

Shear and tensile joint strengths of carbon fiber-reinforced thermoplastics using ultrasonic welding

The shear and tensile strengths of carbon fiber-reinforced thermoplastic (CFRTP) laminate joints by ultrasonic welding were investigated by lap shear tests (LSTs) and cross tensile tests (CTTs). The adherends were the cross-ply and the twill woven CFRTP laminates, and the flat energy director (FED) was used to clarify its effectiveness. A modified jig and testing method for CTTs were newly proposed. In each test, the strengths were calculated in two ways: by indicators of “welding efficiency” and “welding quality.” From the experimental results, the strengths of the cross-ply laminate joints were saturated at high welding energy, and the “welding quality” was improved by FED. On the other hand, higher strengths of the twill woven laminate joints were obtained as the welding energy increased, and the effectiveness of FED was confirmed mainly for the “welding efficiency.” The fracture surfaces were then observed to clarify the fracture mechanisms of the joints.

Assessment of failure toughening mechanisms in continuous glass fiber thermoplastic laminates subjected to cyclic loading

Tensile fatigue behaviour of glass fiber/polyamide composites, including unidirectional ([0]8, [90]8) and cross-ply ([02/902]s, [04/904]s and [904/04]s) laminates, was studied and compared to that of similar glass fiber/epoxy composites. The fatigue resistance of cross-ply glass/polyamide was greater than that of glass/epoxy while also exhibiting lower stiffness reduction. To explain this key observation, residual stiffness and residual strength fatigue tests were performed on cross-ply laminates, while optical microscopy was used to measure ply crack density during the different stages of cycling. Testing of the cross-ply laminates at lower peak stresses of 50% of the ultimate tensile strength (i.e., high cycle fatigue regime) revealed partial cracks that did not propagate completely through the width and thickness of plies due to high matrix toughness and other observed toughening mechanisms such as matrix bridging. A micromechanical finite element model with explicit ply cracks was also used to predict laminate stiffness degradation corresponding to observed ply crack densities, revealing that stiffness degradation was overpredicted when cracks were assumed to span the entire specimen width. Additional finite element simulations with partial cracks showed notably less stiffness reduction. These observations suggest glass/polyamide is inherently more damage tolerant than glass/epoxy and may be a suitable replacement for fatigue critical structures.

Assessing residual stress redistribution during annealing in thick thermoplastic composites using optical fiber sensors

In thick thermoplastic composite laminates, nonuniform temperature and cooling rate distribution arises in the through-thickness direction during cost-effective high-rate manufacturing processes. Annealing is often carried out after molding to homogenize degree of crystallinity (DOC) and to reduce residual stress. Even though the change in the residual stress/strain distribution occurring inside thick laminates by this heat treatment is practically important, the changing process and the detailed mechanism are not sufficiently clarified. This present study addresses development and redistribution behavior of residual stress through both molding and annealing using multiple optical fiber sensors deployed in the thickness direction. This article begins by explaining about process monitoring of thick laminates to discuss process-induced strain distribution depending on cooling conditions during molding. Next, strain monitoring is performed during annealing, and the strain change caused by cold crystallization is clarified. Finally, the residual stress distribution is evaluated by a transverse three-point bending test, and the validity of the redistribution mechanism deduced from the strain measurement is confirmed.

An experimental approach that assesses in-situ micro-scale damage mechanisms and fracture toughness in thermoplastic laminates under out-of-plane loading

Studying the response of laminated composites under out-of-plane loading routinely involves mechanical tests, such as quasi-static indentation or impact. The phenomenology during these tests is so complex that it is difficult to identify different material properties related to each failure mechanism (damage mode). We aim at providing an experimental approach, which is practical and fast, for assessing the in-situ micro-scale damage mechanism and extracting the fracture toughness in thermoplastic laminates under out-of-plane loading. To this end, we developed a dedicated, micro-scale, three-point bending (micro-3PB) test fitted inside a scanning electron microscope (SEM). In a single experiment, we were able: (i) to assess the initiation of a transverse crack, the transverse crack-to-delamination transition, delamination growth, development of shear-induced microcracks during delamination, and fibrillation, and (ii) to evaluate the effective fracture toughness during transverse cracking and delamination under a representative out-of-plane loading. We used this approach to rank two types of glass fiber-reinforced polypropylene cross-ply laminates, i.e., based on either homopolymer PP (ductile matrix) and copolymer PP (less-ductile matrix), according to their relative fracture parameters. We also performed short edge notch bending (SENB), double cantilever beam (DCB) and end-notch flexure (ENF) to obtain the standard fracture toughness values. We found that the relative fracture toughness values obtained by SENB, DCB and ENF are comparable with that of micro-3PB results. Furthermore, ENF results showed that the delamination process during micro-3PB is dominated by Mode-II fracture.

Effects of misalignments on thermally induced shapes of woven fabric reinforced laminates

Fabric misalignment leads to unsymmetry in fibre reinforced composite laminates. This unsymmetry induces out-of-plane deformation after processing. This paper evaluates the influence of fabric misalignment on the out-of-plane deformation of flat press-consolidated thermoplastic laminates. The results of this analytical and experimentally validated study shows a strong relation between deformations and length/width ratio. In particular narrow laminates show high twisting values. The results obtained are of importance for the prediction of deformations of stamp formed thermoplastic laminates.

A new approach for preparation of metal-containing polyamide/carbon textile laminate composites with tunable electrical conductivity

Multiscale thermoplastic laminate composites based on polyamide 6 (PA6) dually reinforced by carbon fiber woven textile structures (CFT) and different micron-sized metal particles are prepared for the first time by microencapsulation strategy. In a first step, activated anionic ring-opening polymerization (AAROP) of e-caprolactam is carried out in suspension, in the presence of different metal particles, to produce shell-core PA6 microcapsules (PAMC) loaded with 13–19% metal. In a second step, the loaded PAMC are distributed between CFT plies with fiber volume fractions V f = 0.25 or V f = 0.50 and then the ply arrays are consolidated by compression molding. Separately, metal-loaded PA6 hybrid composites are prepared by direct compression molding of PAMC and used to compare their properties to the CFT-metal laminates. Light- and scanning electron microscopy are used to study the morphology and the interfaces between the fillers and the polymeric matrix. These structural results are related to the mechanical behavior in tension and the electrical properties. A notable increase of the d.c. electrical conductivity in 7 orders of magnitude is observed for the CFT-metal laminates with respect to the neat PA6. This increase is accompanied by a 2.5–3.0 times growth of the Young’s modulus and of the strength at break. It is concluded that the microencapsulation strategy can be applied to produce multifunctional CFT-metal-PA6 thermoplastic composites with tailored electrical and improved mechanical properties for advanced applications.

Effect of in situ treatment on the quality of flat thermoplastic composite plates made by automated fiber placement (AFP)

Composite structures used as aerosurfaces in aerodynamic applications are required to have a certain surface finish quality. In manufacturing of thermoplastic composites using automated fiber placement (AFP) for aerodynamic applications, it is not only desirable to achieve good consolidation by using AFP alone and avoiding secondary treatment in an autoclave, but also to achieve acceptable surface smoothness required for aerosurfaces. In this study, an in situ treatment called “repass” was implemented to achieve surface finish quality required for aerodynamic applications. Moreover, the effect of this in situ treatment on the quality of the thermoplastic laminates, including void content and crystallinity was investigated. Autoclave-treated samples were used as references for comparing surface quality and other quality indicators.

Assessment of the thermomechanical performance of continuous glass fiber-reinforced thermoplastic laminates

The effects of temperature on the static tensile behavior of continuous E-glass/polyamide laminates were studied in order to assess the feasibility of using the material system for structural applications. Uniaxial tensile tests were conducted on [0]8, [90]8, [02/902]s and [04/904]s laminates at multiple temperatures above and below the glass transition temperature, which was measured using different methods. Optical and scanning electron microscopy were performed on the tested samples, and the effects of temperature on failure modes were investigated. The [0]8 and [90]8 laminates displayed three reduction stages in modulus versus temperature, where the largest reduction was in the glass transition region as a result of notable softening of the polyamide matrix, as confirmed by fractographic analysis. However, the [02/902]s and [04/904]s laminates displayed the largest modulus reduction prior to the glass transition temperature with little reduction beyond, which was attributed to matrix softening coupled with in situ ply constraining effects.