Glass Fiber-reinforced Polypropylene Research Articles

Formability and Failure Mechanisms of Continuous Glass Fiber-Reinforced Polypropylene Composite Laminates in Thermoforming Below the Melting Temperature.

In this study, the thermoforming formability of continuous glass fiber-reinforced polypropylene (CGFRPP) laminates below the melting temperature were investigated. The forming limits of CGFRPP laminates were explored using flexural tests, Erichsen tests and deep drawing tests. The failure mechanism of CGFRPP in thermoforming was investigated by observing typical failure specimens using a microscope. The results show that the flexural performance and Erichsen performance are optimal at 130 °C and 2 mm/min. At 160 °C and 100 mm/min, the deep drawing performance is optimal. The restriction of fibers by the matrix is affected by the deformation temperature, and the creation of defects is affected by the deformation rate. During forming, the CGFRPP laminates undergo shear and extrusion deformations, resulting in wrinkles, delamination, and fiber aggregation.

Enhancing the Lap Shear Performance of Resistance-Welded GF/PP Thermoplastic Composite by Modifying Metal Heating Elements with Silane Coupling Agent.

Thermoplastic composites are gaining widespread application in aerospace and other industries due to their superior durability, excellent damage resistance, and recyclability compared to thermosetting materials. This study aims to enhance the lap shear strength (LSS) of resistance-welded GF/PP (glass fiber-reinforced polypropylene) thermoplastic composites by modifying stainless steel mesh (SSM) heating elements using a silane coupling agent. The influence of oxidation temperature, solvent properties, and solution pH on the LSS of the welded joints was systematically evaluated. Furthermore, scanning electron microscopy (SEM) was utilized to investigate the SSM surface and assess improvements in interfacial adhesion. The findings indicate that surface treatment promotes increased resin infiltration into the SSM, thereby enhancing the LSS of the resistance-welded joints. Treatment under optimal conditions (500 °C, ethanol solvent, and pH 11) improved LSS by 27.2% compared to untreated joints.

Influence of Processing Parameters in Injection Molding on the Properties of Short Carbon and Glass Fiber Reinforced Polypropylene Composites

Short-fiber reinforcement is a potent approach to improving the material properties of injection-molded parts. The main consideration in such reinforced materials is to preserve the fiber length, as this is the major influence on the properties of a given composite. The aim of this work was to investigate the different influencing parameters in injection molding processing on the properties of short carbon and glass fiber-reinforced polypropylene. We investigated parameters like melt temperature and back pressure, but also machine size and pre-heating regarding their influence on the tensile properties. We found that adjustments of melt temperature and back pressure only yield small improvements in the fiber length and the tensile properties, also depending on machine size, but a pre-heating step of the granules can significantly improve the properties.

Advancing laser transmission welding for additive manufacturing: A study of glass fiber reinforced polypropylene parts

Laser transmission welding (LTW) is a well-established technique for joining high-volume thermoplastic parts, such as automotive injection molded parts. For low-volume, prototype, and custom production, additive manufacturing is an emerging technology for producing complex thermoplastic parts. When compared to injection molding, the additive manufacturing process fused filament fabrication (FFF) results in an inhomogeneous structure with entrapped air within the volume. Additionally, the presence of short glass fibers in the polymer matrix leads to higher radiation scattering during the welding process. This paper presents a fundamental study of the weldability of additively manufactured fiber-reinforced parts. The specimens were fabricated using a FFF process with glass fiber-reinforced polypropylene (GF-PP). The study investigates the influence of layer thickness and line width of the FFF process on the optical transmittance. LTW-experiments were conducted using additively manufactured uncolored and black GF-PP samples. Lap shear test specimens were welded with a different energy per unit length. This research presents a process for welding additively manufactured GF-PP parts that can be used with optimized FFF-parameters to produce high-strength and durable prototypes or spare parts for the automotive industry. This research provides valuable insights into the process parameters and considerations required to achieve robust welds in additively manufactured thermoplastic parts, facilitating broader adoption of LTW in additive manufacturing contexts.

Shear Behavior and Modeling of Short Glass Fiber- and Talc-Filled Recycled Polypropylene Composites at Different Operating Temperatures

The present paper aims to broaden the field of application of the phenomenological model proposed by the authors in a previous study (ICP model) and to assess the shear properties of a recycled 30 wt.% talc-filled polypropylene (TFPP) and a recycled 30 wt.% short glass fiber-reinforced polypropylene (SGFPP), used in the automotive industry. The materials were produced by injection molding employing post-industrial mechanical shredding of recycled materials. In particular, Iosipescu shear tests adopting the American Standard for Testing Materials (ASTM D5379) at three different operating temperatures (−40, 23 and 85 °C) were performed. The strain was acquired using a Digital Image Correlation (DIC) system to determine the map of the strain in the area of interest before failure. Lower operating temperatures led to higher shear chord moduli and higher strengths. Recycled SGFPP material showed higher mechanical properties and smaller strains at failure with respect to recycled TFPP. Finally, the ICP model also proved to be suitable and accurate for the prediction of the shear behavior of 30 wt.% SGFPP and 30 wt.% TFPP across different operating temperatures.

High-temperature effect on continuous glass fiber reinforced polypropylene multilayer composite and corrugated sandwich panels

The high-temperature mechanical behaviors of Multi-Layer Composite Panels (MCP) and Corrugated Sandwich Panels (CSP) of Continuous Glass Fiber-Reinforced Polypropylene (CGFRPP) are critical for their application in aerospace fields, which have been rarely mentioned in previous studies. High-temperature quasi-static tensile and compression tests on CGFRPP MCP are conducted first. The results showed that the tensile and compression strength, stiffness, and tensile modulus of MCP decreased with increasing temperature. The Gibson model was found to be more suitable for predicting the high-temperature mechanical performance of MCP after comparing the calculated results of different theoretical models with experimental data. Secondly, high-temperature planar compression tests were conducted on the CGFRPP CSP, revealing that the main failure modes were corrugated core buckling and delamination between the face panel and core material, with delamination being intensified at higher temperatures. Therefore, we proposed a strength theoretical model that considers structural buckling failure and interface delamination failure, and introduced the influence factor to evaluate the effect of interface delamination on structural strength.

Influence of plasticisation during foam injection moulding on the melt viscosity and fibre length of long glass fibre-reinforced polypropylene

Abstract The processing of long glass fibre-reinforced thermoplastics results in considerable fibre damage, particularly during plasticising. By using thermoplastic foam injection moulding (FIM) with a constant blowing agent atmosphere, fibre damage during plasticisation can be reduced. This can be attributed to the reduction of the melt viscosity on the one hand and the influence of the melting behaviour on the other. Therefore, the influence of the FIM and the process parameters on the fibre length and the fibre length distribution are analysed and compared with the influence of the process parameters on the melt viscosity.

A methodology for direct parameter identification for experimental results using machine learning — Real world application to the highly non-linear deformation behavior of FRP

In this work, we demonstrate how Machine learning (ML) techniques can be employed to externalize the knowledge and time intensive process of material parameter identification. This is done on the example of a recent data driven material model for the non-linear shear behavior of glass fiber reinforced polypropylene (GF/PP) (Gerritzen, 2022). A convolutional neural network (CNN) based model architecture is trained to predict material modeling parameters based on the input of stress–strain-curves. The optimal model architecture and training setup are determined by hyperparameter optimization (HPO). Solely virtual data, generated using the target material model, is used throughout the training and HPO. The final CNN is capable of calculating model parameter combinations from experimental stress–strain-curves which lead to excellent agreement between experimental and associated model curve.

Assessing Intra-Bundle Impregnation in Partially Impregnated Glass Fiber-Reinforced Polypropylene Composites Using a 2D Extended-Field and Multimodal Imaging Approach.

This study evaluates multimodal imaging for characterizing microstructures in partially impregnated thermoplastic matrix composites made of woven glass fiber and polypropylene. The research quantifies the impregnation degree of fiber bundles within composite plates manufactured through a simplified compression resin transfer molding process. For comparison, a reference plate was produced using compression molding of film stacks. An original surface polishing procedure was introduced to minimize surface defects while polishing partially impregnated samples. Extended-field 2D imaging techniques, including polarized light, fluorescence, and scanning electron microscopies, were used to generate images of the same microstructure at fiber-scale resolutions throughout the plate. Post-processing workflows at the macro-scale involved stitching, rigid registration, and pixel classification of FM and SEM images. Meso-scale workflows focused on 0°-oriented fiber bundles extracted from extended-field images to conduct quantitative analyses of glass fiber and porosity area fractions. A one-way ANOVA analysis confirmed the reliability of the statistical data within the 95% confidence interval. Porosity quantification based on the conducted multimodal approach indicated the sensitivity of the impregnation degree according to the layer distance from the pool of melted polypropylene in the context of simplified-CRTM. The findings underscore the potential of multimodal imaging for quantitative analysis in composite material production.

Effect of thermal aging on the long‐term dynamic and stress relaxation behavior of glass‐fiber reinforced polypropylene composites

AbstractThe increasing use of glass‐fiber reinforced polypropylene (GFPP) composites in a wide range of applications requiring long‐term service in challenging environments underscores the importance of its long‐term durability. This study aimed to investigate the effect of thermal aging on the long‐term dynamic durability and stress relaxation of GFPP composites. The time–temperature equivalence principle (TTSP) was used to assess the dynamic modulus at various temperatures and frequencies, as well as the relaxation modulus at different temperatures and relaxation times. To better understand long‐term durability behavior, the study also examined the impact of molecular structure on the durability of GFPP. Changes in chemical composition, crystallinity, melting point, and melt flow index of GFPP due to thermal aging were measured and analyzed using Fourier transform infrared spectroscopy, and differential scanning calorimetry, respectively. The results revealed that oxidation led to a decrease in the crystallinity and molecular weight of GFPP. The destruction of GFPP's molecular structure due to oxidation resulted in reduced long‐term durability. While TTSP can predict the long‐term durability of GFPP for decades or even centuries, its application in predicting the long‐term durability of GFPP is more suitable for nonaging conditions.Highlights Thermal aging reduces glass‐fiber reinforced polypropylene (GFPP's) long‐term durability. Time–temperature equivalence principle predicts GFPP durability under aging. GFPP's activation energy drops with thermal aging. Long‐term GFPP behavior aligns with Williams–Landel–Ferry model. Predictive models refine understanding of GFPP longevity.

Mechanical, Dielectric and Flame-Retardant Properties of GF/PP Modified with Different Flame Retardants.

With the rapid development of electronic information technology, higher requirements have been put forward for the dielectric properties and load-bearing capacity of materials. In continuous glass fiber-reinforced thermoplastic composites, polypropylene matrix is a non-polar polymer with a very low dielectric constant and dielectric loss, but polypropylene is extremely flammable which greatly limits its application. Aiming at the better application of flame retardant-modified continuous glass fiber-reinforced polypropylene composites (FR/GF/PP) in the field of electronic communication, the effects of four different kinds of flame retardants (Decabromodiphenyl ethane (DBDPE), halogen-free one-component flame retardant (MONO), halogen-free compound flame retardant (MULTI), and intumescent flame retardant (IFR)) on the properties of FR/GF/PP were compared, including the mechanical properties, dielectric properties and flame-retardant properties. The results showed that among the FR/GF/PP, IFR has the highest performance in mechanical properties, MULTI has better performance in LOI, DBDPE and IFR have better performance in flame retardant rating, and DBDPE and IFR have lower dielectric properties. Finally, gray relational analysis is applied to propose an approach for selecting the optimal combination (flame retardant type and flame-retardant content) of comprehensive performance. In the application exemplified in this paper, the performance of IFR-3-modified GF/PP is optimized.

Design, preparation, and mechanical properties of glass fiber reinforced thermoplastic self‐anchor plate cable exposed in alkaline solution environment

AbstractThe reliable anchorage and long‐term durability of high‐performance fiber reinforced polymer composites were the decisive factors for engineering applications owing to the anisotropy of materials and the complexity of service environment. In this paper, an innovative glass fiber reinforced polypropylene (GFRPP) self‐anchor plate cable was developed to utilize the high toughness/durability of thermoplastic resin and self‐anchor load‐bearing system. The effects of different reinforcement methods and cable arc angles (10°, 20°, and 30°) on mechanical properties of plate cable were investigated. Water absorption behavior and mechanical properties immersed in alkali solution were tested to evaluate its long‐term service behavior. The thermogravimetric and surface morphology analysis were conducted to reveal the performance evolution mechanism. The results showed through the combination of secondary melting, carbon fiber winding confinement and epoxy reinforcements, the splitting failure of transition zone and interlaminar cracking failure of straight zone for plate cable were effectively avoided. The tensile strength retention of plate cable for arc angles of 10°, 20° and 30° were 45.6%, 41.3%, and 34.0% of GFRPP plate, respectively. Larger arc angle increased the curvature of arc‐straight transition zone and stress concentration near the straight section, leading to interlaminar splitting failure of plate cable at weak transition zone. The tensile strength of plate cable with the immersion time deteriorated obviously until the lowest retention of 29.7%. The degradation mechanism was mainly due to the etching of glass fibers in alkali solution and the formation of pores and internal defects, including fiber/resin interface debonding and resin swelling. The research results were of significance for solving the anchoring problems and promoting the long‐term service reliability in bridge applications.Highlights The mold can realize reliable molding of glass fiber reinforced polypropylene cables after the reinforcements. The tensile strength retention of plate cable was up to 34.0%–45.6% of plate. Larger arc angle increased the stress concentration near the straight section. Non‐polar polypropylene plate was proved to have better hydrophobic performance. The degradation mechanism of plate cable in alkali solution was revealed.

Long-term properties evolution and life prediction of glass fiber reinforced thermoplastic bending bars exposed in concrete alkaline environment

Glass fiber reinforced polypropylene (GFRPP) bending bars have many advantages, such as high toughness, high durability, repeated molding and recycling, which can be used in concrete structures to replace steel stirrup for solving the problems of corrosion and service life. However, the changes in the properties of GFRPP bending bars during long-term use in alkaline concrete environments require further investigation. In the present study, GFRPP bending bars were immersed in alkaline solution at 20 °C, 40 °C, and 60 °C for as long as 180 d. The water absorption, hydrolysis, mechanical properties, and microscopic properties of the bending bars were evaluated to understand the long-term performance and mechanism of degradation. The bending bars had a higher saturated water absorption percentage and time along with a lower diffusion rate than the straight bars due to the presence of pores and micro-cracks formed during the bending process. The dissolution of the glass fibers in alkaline solution led to the partial mass loss of the GFRPP bars. Compared to the control, the tensile strength retention of the bending bars was 20.6%–56.0 %. The tensile failure mode changed from fiber bursting before immersion to shear fracture after immersion due to the decreased interfacial bonding strength and increased glass fiber brittleness. The primary degradation mechanisms were the etching of glass fibers, interface debonding, and increased void volume (∼23.7 %). The bending bars were more seriously eroded than the straight bars due to the twisting of the fibers and imperfect interfaces. The predicted degradation times of the GFRPP bending bars ranged from 1.7 years (Haikou) to 3.9 years (Harbin), corresponding to a stable tensile strength retention of 22.13 %. The results provide a basis for evaluating the durability and service life of GFRPP bending bars and promote the application of GFRPP composite stirrups in concrete structures.

Mechanical, Flame-Retardant and Dielectric Properties of Intumescent Flame Retardant/Glass Fiber-Reinforced Polypropylene through a Novel Dispersed Distribution Mode.

The application of continuous glass fiber-reinforced polypropylene thermoplastic composites (GF/PP) is limited due to the inadequate flame retardancy of the polypropylene (PP) matrix. Apart from altering the composition of the flame retardants, the distribution modes of flame retardants also impact material performance. In this study, an alternative approach involving non-uniform distribution is proposed, namely, dispersed distribution, in which non-flame-retardant-content layers (NFRLs) and/or low-flame-retardant-content layers (LFRLs) are dispersed among high-flame-retardant-content layers (HFRLs). The mechanical, flame retardant and dielectric properties of GF/PP with intumescent flame retardant (IFR/GF/PP) are investigated comparatively under uniform, gradient, and dispersed distributions of the flame retardants. The results demonstrate that non-uniform distribution exhibits superior flame retardant performance compared to uniform distribution. Dispersed distribution enables IFR/GF/PP to attain enhanced mechanical properties and reduced dielectric constants while maintaining excellent flame-retardant properties.

Carbon nanotube-induced localized laser heating toward simultaneously enhanced laydown efficiency and fracture toughness of in-situ consolidated glass fiber thermoplastic composites

Thermoplastic composites suffer from weak inter-tape bonding during high-rate laser-assisted automated fiber placement (L-AFP). Here, we propose a strategy to simultaneously enhance the laser heating efficiency and fracture toughness of glass fiber-reinforced polypropylene prepreg tape for L-AFP process by depositing carbon nanotubes (CNTs) on the tape surface by a low-cost bath coating process. CNTs function as efficient light absorbing and photothermal coating to promote molecular inter-diffusion and interfacial healing between successive tapes, as well as toughen the weak interface by CNT pull-out and rupture. A 56 % increase in the near-infrared laser absorption of tapes results in a 946 % improvement in the laydown rate. CNTs also remarkably improve transverse temperature uniformity. Furthermore, double cantilever beam and pants tearing tests show that 0.5 wt% CNTs improve the mode I interlaminar fracture toughness and mode III fracture strength by 17 % and 52 %, respectively, attributed to the enhanced molecular inter-diffusion and CNT bridging effect.

Design, preparation and mechanical properties of novel glass fiber reinforced polypropylene bending bars

Fiber reinforced thermoplastic composite bars offered the advantages of repeated molding, high toughness, and recyclability, making them a promising alternative to steel bars. For the stirrups applications in concrete structures, it was essential to develop efficient bending technologies for producing high-quality thermoplastic bending bars. In the present paper, the bending properties of glass fiber reinforced polypropylene (GFRPP) composite bars were investigated by experimental and theoretical methods. A novel bending fixture of GFRPP bars was specifically designed for GFRPP bars to ensure the reliable bending of the bars. Finite element simulation was employed to analyze the stress distribution of GFRPP bars, providing valuable insights into their mechanical behavior. Additionally, a bending strength model was developed based on the material mechanics, offering a theoretical framework for understanding and predicting the bending strength of GFRPP bars. Finally, the bending mechanism of bars was further revealed according to experimental and theoretical findings. The results of the study demonstrated that the specially designed bending fixture was successful in minimizing damage to the bending section of the GFRPP bars, leading to a significant improvement in bending strength retention of up to 43.2 %. Furthermore, the theoretical results and experimental test have a good agreement in terms of stress distribution and failure mode. The maximum strain in the transition zone between the straight section and the bending section of the GFRPP bars was nearly double that of the straight section at the same stress levels. With the increase of bending radius to diameter ratio from 3.0 to 7.0, the tensile strength of GFRPP bending bars increased by ∼7.9 %. The study identified the transition area between the bending section and straight section of GFRPP bars as the most critical failure point. The strength degradation mechanism during the bending were attributed to the shrinkage and torsion of inner fibers owing to the compression caused the local stress concentration, which caused the inner resin matrix to crack and interface debonding of fiber/resin, leading to the premature tensile failure.

Performance analysis of fiber-reinforced polypropylene composite laminates under quasi-static and super-sonic shock loading conditions for impact application

In the present research, glass and basalt fiber-reinforced polypropylene composites’ behaviour in quasi-static and dynamic conditions is studied. Composites were fabricated by vacuum assisted Compression molding method. Composites failure under quasi-static tension and compressive conditions was studied along with its failure behaviour under low-velocity impact and super-sonic shock loading under dynamic conditions. The study results showed that basalt fiber-reinforced polypropylene (Basalt/PP) composite’s tensile and compressive strength is higher than glass fiber-reinforced polypropylene (Glass/PP). The Basalt/PP showed no penetration against low velocity impact (LVI) with negligible deformations till 50 J. However, the Glass/PP perforated at 50 J with various failure patterns occurring at back side. The fiber-matrix interface adhesion plays an important role in super-sonic shock loading by absorbing shock wave energy due to ductile nature of polypropylene and the two composites absorbed energy via matrix and fibers failure, no brittle failure of laminates occurred under shock loading.

Assessing the Feasibility of Fabricating Thermoplastic Laminates from Unidirectional Tapes in Open Mold Environments

The automation of the manufacturing processes of thermoplastic composite laminates has become dependent on open mold processes such as automated tape placement (ATP), which couples tape layering with in situ consolidation. The manufacturing parameters of ATP open mold processes, which comprise processing time, consolidation pressure and temperature, affect the bond strength between the plies and the quality of the laminates produced. Therefore, the effect of the manufacturing parameters should be characterized. This work experimentally evaluates the feasibility of fabricating thermoplastic laminates using an open mold process that reasonably models that of ATP. Glass fiber-reinforced polypropylene laminates are fabricated from unidirectional tapes under different consolidation periods, pressures, and temperatures. The bond quality in the produced laminates is assessed by measuring their interlaminar shear strength, which is measured using a short beam standardized shear test in conjunction with digital image correlation. Results show that consolidation can occur at temperatures slightly below the composite tapes’ complete melting temperature, and consolidation times between 7 and 13 min can result in acceptable bond strengths. The results confirmed the feasibility of the process and highlighted its limitations. Analysis of variance and machine learning showed that the effect of process parameters on interlaminar shear strength is nonlinear.

Optimization of Thermoplastic Pultrusion Parameters of Jute and Glass Fiber-Reinforced Polypropylene Composite.

Thermoplastic pultrusion is a suitable process for fabricating continuous unidirectional thermoplastics with a uniform cross-section, high mechanical properties due to continuous fiber reinforcement, low cost, and suitability for mass production. In this paper, jute and glass fibers were reinforced with a polypropylene matrix and fabricated using the thermoplastic pultrusion process. The volumetric fraction of the composite was designed by controlling the filling ratio of the reinforcing fiber and matrix. The effects of molding parameters were investigated, such as pulling speed and molding temperature, on the mechanical properties and microstructure of the final rectangular profile composite. The pulling speed and molding temperature varied from 40 to 140 mm/min and 190 to 220 °C, respectively. The results showed that an increase in molding temperature initially led to an increase in mechanical properties, up to a certain point. Beyond that point, they started to decrease. The resin can be easily impregnated into the fiber due to the low viscosity of thermoplastic at high temperatures, resulting in increased mechanical properties. However, the increase in molding temperature also led to a rise in void content due to moisture in jute fiber, resulting in decreased mechanical properties at 210 °C. Meanwhile, un-impregnation decreased with the increase in molding temperature, and the jute fiber began to degrade at high temperatures. In the next step, with varying pulling speed, the mechanical properties decreased as the pulling speed increased, with a corresponding increase in void content and un-impregnation. This effect occurred because the resin had a shorter time to impregnate the fiber at a higher pulling speed. The decrease in mechanical properties was influenced by the increase in void content and un-impregnation, as the jute fiber degraded at higher temperatures.

Effect of nozzle aperture on glass fiber reinforced polypropylene fabricated by DFFIM

Fiber length and interfacial bonding properties are important factors in determining the mechanical properties of the composites. As an emerging fiber-reinforced thermoplastic molding method, DFFIM can reduce the fiber breakage during the molding process and thus improve the mechanical properties, while the fiber length and interfacial bonding properties of the composites fabricated by DFFIM are mainly affected by the equipment structure. This paper takes nozzle as the research object to investigate the effect of nozzle on the properties of glass fiber reinforced polypropylene (GF/PP) fabricated by DFFIM. First, GF/PP composites at three positions of nozzle flange, nozzle air-injection outlet and mold cavity were selected for fiber length detection, and it was found that the nozzle would lead to more severe fiber breakage in GF/PP composites; POLYFLOW was subsequently used to analyze the shear stress of GF/PP mixed flow fields in different aperture nozzles; DFFIM experiments were carried out to analyze the fiber length, micromorphology, and mechanical properties of GF/PP composites using the nozzle aperture as the test factor, and the results were consistent with the simulation results, as the nozzle aperture increased, the fiber length and interfacial bonding properties of GF/PP composites gradually increased, hence improving the mechanical properties.