Self-reinforced Polypropylene Composites Research Articles

Design and manufacture of thermoplastic carbon fiber/polyethylene terephthalate composites underbody shield to protect the lithium-ion batteries for electric mobility from ground impact

Recently, the electric mobility has emerged with great prospects as there has been a growing interest in and demand for eco-friendly energy. The electric vehicle uses a large number of lithium batteries as sources of power, and the lithium battery poses a risk of fire and explosion when the external impact is loaded. Therefore, in this study, an underbody shield (UBS) was designed and manufactured using carbon fiber reinforced thermoplastic composites for battery protection. The underbody shield is composed of two parts, which are the main shield plate and collision protection bar (CPB). Mechanical behavior against impact was analyzed through finite element analysis considering the position of the CPB and the shape of the underbody shield plate. For the UBS, the hybrid composites (Carbon fiber (CF)/polyethylene terephthalate (PET) and self-reinforced polypropylene (SRPP) composites) were used and the ratio between the CF/PET and SRPP was optimized. Through a finite element analysis, the optimal underbody shield was designed with a thickness of 2.4 mm and a hybrid ratio of CF/PET:SRPP as 1:2. Finally, the designed underbody shield was manufactured using a thermo-forming process and mounted to an electric vehicle, and then a vehicle crash test was performed with a concrete obstacle. The developed underbody shield could protect the battery against the impact damage successfully, showing that the deformation of a rear part was within 5 mm and no battery leakage occurred.

Self-Reinforced Polypropylene Composites based on Discontinuous Tapes - An Experimental and Numerical Study of the Influence of Tape Length

The creation of highly oriented, coextruded polypropylene (PP) tapes allows the production of recyclable self-reinforced polypropylene (SRPP) or all-PP composites, with a large temperature processing window and high volume reinforcement content (sim90%). The objective of this research is to assess the performance potential of SRPP composites based on discontinuous or short tapes. For this, the critical tape length for effective mechanical reinforcement of aligned discontinuous PP tapes was determined, while the stress transfer from PP matrix to PP tape was investigated in single tape model composites in combination with an optical strain mapping technique. Mechanical behaviour of both single tape as well as aligned tape model composites was evaluated using finite element analysis (FEA) and used to predict the properties of randomly oriented short tape composites. These discontinuous tape SRPP composites may be of interest from a manufacturing as well as recycling point of view. In terms of manufacturing, such systems are of interest as they may show improved formability during stamping or compression moulding operations while from a recycling point of view it would allow the re-use of production waste like cuttings of fabrics or tapes.

A Comparative Study of Structural Changes in Conventional and Unconventional Machining and Mechanical Properties Evaluation of Polypropylene Based Self Reinforced Composites

Abstract The present study addresses the mechanical behavior of polypropylene self-reinforced composites (SRC’s) considering polymeric structural changes after cutting. Self-reinforced polypropylene composite is fabricated using the HOT compaction method by maintaining the processing temperature at 164∘C. Conventional and unconventional cutting methods were used to cut the samples of standard dimensions. FTIR images revealed the formation of C=C, C-F, Halogen bonds after AWJ cutting initiated a decrease in the surface roughness value to 4.5μm (R a ). SEM analysis is performed to analyse structural integrity and damage of SRC’s. Structural changes formation after AWJ cutting leads to improve the ultimate tensile strength of the laminate by 20% compared to conventional cut samples. A similar trend is noticed for flexural properties and Shore –D hardness values for the SRC composite laminate correlated to polymeric changes with Conventional cutting due to the formation of C-N bond is observed after Laser cutting.

Experimental and numerical studies on forming and failure behaviours of a woven self-reinforced polypropylene composite

This paper investigates forming and failure behaviours of a consolidated woven self-reinforced polypropylene (SRPP) composite through combined stamp forming experiment and finite element analysis. Mechanical properties of a woven SRPP composite were characterised. Constitutive equations were derived as functions of strains using a homogenised orthotropic material model. Specimens with novel geometries, different aspect ratios and fibre orientations were stretch formed in a custom-built press until catastrophic failure. Evolution of principal strains was captured using a real time Digital Image Correlation (DIC) system. A path-dependant failure criterion was developed as a function of deformation modes and invariants of strain tensor. Material and failure models were implemented into a finite element analysis using Abaqus/implicit. Strain path at the pole of specimens, evolution of surface strains, and onset of failure were predicted using a homogenised numerical scheme. Comparison with experimental outcomes demonstrated the high accuracy of the proposed numerical model in predicting deformation and failure behaviours of a thermoplastic composite under a wide range of deformation modes. The model demonstrates the potential to predict formability and failure behaviour of woven self-reinforced thermoplastic composites during manufacturing by eliminating the need to conduct expensive, time consuming trial and error procedures.

Progressive failure in interply hybrid composites of self-reinforced polypropylene and glass fibre

Hybridisation of two composites has the potential of tailoring properties to reach a composite structure with advantages of both constituents where the drawbacks are lessened. The current work reports this practice for self-reinforced polypropylene (SRPP) composites and glass fibre to create an interply hybrid composite with enhanced stiffness and ductility. The effect of glass fibre volume fraction, matrix adhesion, and layup, was studied as the parameters to affect progressive failure through pseudo-ductility. Hybrids with up to 6.5% glass fibre volume fraction showed pseudo-ductile behaviour. However, matrix adhesion and layup interfered with this effect. It was found that composites with low adhesion matrix exhibited higher pseudo-yield strain, and for the laminates with an interleaved layup, the maximum glass fibre volume fraction for progressive failure increased by 100%. The impact behaviour of the composite laminates also demonstrated that brittleness of the glass fabric reduces the energy absorption capability of the laminates compared to SRPP composite of the same thickness.

Strain Rate Dependent Behaviour of Self-Reinforced Polypropylene Composites and their Hybrids

Strain rate is one of the external parameters to affect the behaviour and response of polymer composites due to their viscoelastic properties. Therefore, understanding the strain rate effect on the materials’ properties is crucial. The current work demonstrates the strain-rate sensitivity of self-reinforced polypropylene composites (SRPP) and their interply hybrids with glass fibre in bending loading. Hybridisation of SRPP with the glass reinforcement changes the strain-rate dependence behaviour. It was concluded that increasing strain-rate resulted in increasing both the strength and the modulus of the structures but the modulus dependence on strain-rate was less pronounced. For self-reinforced hybrid composites, a yield point was observed in the stress-strain flexural curve where the low strain failure glass laminate fails. The failure strain of GRPP, as well as maximum flexural strength, showed an increase by increasing the strain-rate.

Engineering tensile behavior of hybrid carbon fiber/self-reinforced polypropylene composites by bio-inspired fiber discontinuities

Carbon fiber layer failure is vital for the tensile behavior of interlayer hybrid carbon fiber (CF)/self-reinforced polypropylene (SRPP) composites. Introducing cuts, termed here as discontinuities, into the carbon layer is a promising way to tailor its failure behavior. Inspired by structural features of biological composites, we designed and produced hybrid composites with hierarchical and polygonal arrangements of discontinuities in the carbon layer. Increasing the number of levels in the hierarchical patterns delayed the onset of carbon layer failure, hence improving the damage resistance of the hybrid composites. A progressive carbon layer failure was achieved with the polygonal patterns by creating a transition from fiber bundle pull-out to fiber bundle fracture. Spreading the polygonal patterns throughout the specimen resulted in a unique diffused delamination distribution that has not been reported in the literature. Pseudo-ductile behavior was achieved by creating dispersed fiber bundle pull-out with the fully dispersed polygonal patterns. The resulting hybrid CF/SRPP composites demonstrated a rare combination of stiffness (10 GPa) and ductility (~16% failure strain) with a pseudo-ductile strain over 14%. This paper delivers and proves the concept of utilizing discontinuities to engineer the tensile behavior of hybrid composites.

Staggered ply discontinuities for tailoring the tensile behavior of hybrid carbon fiber/self-reinforced polypropylene composites: A study of pattern parameters

This work explores the potential of discontinuities combined with hybridization in controlling the failure mechanisms of composite materials. Laser cuts, termed as discontinuities, with a predefined staggered pattern were introduced into the carbon layer of hybrid carbon fiber (CF)/self-reinforced polypropylene (SRPP) composites. Three geometrical parameters were used to define the staggered pattern: cut fraction, cut length and step length. The effects of these three parameters on the failure mechanisms of the hybrid CF/SRPP composites under tensile loading were investigated. The three geometrical parameters can be successfully used to: (1) trigger more diffused damage; (2) delay onset of carbon layer fracture; and (3) promote a transition between fiber bundle pull-out and fiber bundle fracture. Comparing to hybrids with continuous fibers, hybrids with the staggered ply discontinuities exhibited a rich diversity of failure mechanisms. The enrichment of failure mechanisms is beneficial for expanding the design space for tailoring the mechanical response of interlayer hybrid composites.

The influence of consolidation temperature on in-plane and interlaminar mechanical properties of self-reinforced polypropylene composite

In this work, the mechanical properties of self-reinforced polypropylene (SRPP) composite were investigated regarding their manufacturing process. The self-reinforced composite composed of polypropylene fiber and matrix were fabricated according to the consolidation temperature. Then, tensile and compressive tests were conducted to measure the mechanical properties such as strength and failure strain. Mode-I and Mode-II fracture toughness tests were performed to confirm the influence of consolidation temperature on interlaminar properties of SRPP composite. To evaluate crystal structure and thermal transition of the polypropylene fiber reinforcement, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses were carried out according to the consolidation temperature. Digital microscope and scanning electron microscope (SEM) were used to observe failure modes of the fractured SRPP composite specimens. Finally, the optimal consolidation temperature was suggested for improvement of in-plane and interlaminar mechanical properties of SRPP composite. It was also found that the superior impact characteristic of the optimally fabricated SRPP could be achieved under drop weight impact test.

Failure mechanisms in unidirectional self-reinforced biobased composites based on high stiffness PLA fibres

Biobased self-reinforced polymer composites using polylactic acid (PLA) were developed. To create PLA based self-reinforced composites (SR-PLA) with improved properties, first high stiffness PLA fibres with high melting temperature were produced by optimising the melt spinning process parameters. Unidirectional SR-PLA composites with enhanced stiffness were obtained, comparable to the commercially available self-reinforced polypropylene composites. The failure strain of the composites was several times higher than the failure strain of the compact PLA. An increased strength, compared to the compact PLA, was obtained but the increase was less pronounced. To understand the behaviour of unidirectional self-reinforced PLA composites, in-situ tensile tests were performed inside the chamber of an Environmental Scanning Electron Microscope. The damage initiation and evolution under tension was monitored. It was found that the primary limiting strength factor is the high brittleness of the compact (matrix) PLA.

Strain-rate sensitivity and stress relaxation of hybrid self-reinforced polypropylene composites under bending loads

The strain-rate sensitivity and stress relaxation of self-reinforced polypropylene (SRPP) and hybrid carbon fibre/SRPP composites were analysed under bending loads. For this purpose, three point bending static tests were performed at room temperature and displacement rates of 200, 20, 2, 0.2 and 0.02 mm/min. Tests of stress relaxation were also carried out, where a fixed strain was applied and the stress was recorded during the loading time. The study concluded that, for all materials, the bending stress and strain at the maximum bending stress were sensitive to the strain rate. The bending modulus, however, was found to be insensitive to the strain rate for hybrid composites, and sensitive (with a linear increase) for SRPP composites. From the stress relaxation tests and for both materials, the stress decreased with time and this decrease was more significant for higher strains. The results were fitted following the Kohlrausch-Williams-Watts model, evidencing good accuracy of the model for the stress relaxation time.

Self-reinforced biobased composites based on high stiffness PLA yarns

In self-reinforced polymer composites (SRPC) the same polymer material forms both the reinforcing fibre and the matrix phase. The current project aims to develop a biobased alternative for these composites using polylactic acid (PLA). Both the development of the reinforcing fibres, the modelling and production of the composites are being studied. The stiffness of the PLA filaments could be increased to 9 GPa by optimising processing parameters during extrusion. After consolidation to composites, promising results are obtained showing that the stiffness of the PLA SRPC can match the requirements of currently used commercial self-reinforced polypropylene composites (ca 4GPa).

Viscoelastic Behaviour of Self-reinforced Polypropylene Composites under Bending Loads

The viscoelastic behaviour of self-reinforced polypropylene (SRPP) was investigated under bending loads. For this purpose, an overfed Twill 2/2 weave pattern was used, which in the hot compacted form is known under the trade name of Curv®. The areal density of the SRPP fabric is 130 g/cm3 and the density of this grade polypropylene is 0.92 g/cm3. Three point bending (3PB) static tests were performed with a span length of 20 mm, according to the EN ISO 178:2003 recommendations. The strain rate effects on the flexural properties were obtained by the 3PB static tests carried out at room temperature with a displacement rate of 200, 20, 2, 0.2 and 0.02 mm/min. Finally, tests of stress relaxation were also performed, where a fixed strain was applied and the stress was recorded during the loading time. It was possible to conclude that higher strain rates promote higher maximum bending stresses and bending modulus. For both cases, a linear model fits successfully the data and the strain-rate effect on the bending strain at a maximum bending stress showed that SRPP composites are strain rate sensitive. The stress relaxation tests evidence a decrease of the stress with time, and this tendency further persists with the increase of the strain values. Maxwell and Kohlrausch-Williams-Watts (KWW) equations were used to fit the data obtained from the stress relaxation tests and, while the Maxwell model was not good enough to predict the stress relaxation time, the KWW model could fit the data with good accuracy.

Hybridisation of two ductile materials – Steel fibre and self-reinforced polypropylene composites

Hybridisation of self-reinforced polypropylene (SRPP) with ductile steel fibres is proposed as a way to create composites with a high stiffness, failure strain and toughness. The stiffness and yield stress of SRPP were significantly increased without sacrifice of toughness, even after normalisation to the composite density. The measurements revealed a higher stress level and slope after yielding than the predictions by the linear rule-of-mixtures. This effect was attributed to differences in the Poisson contraction of the two constituents and was supported by modelling predictions. The absolute penetration impact resistance of SRPP was increased, whereas the specific impact resistance remained the same, despite the high density of steel fibres. Placing steel fibre layers on the outside of the laminate maximised the penetration impact resistance, as it allowed the steel fibres to exploit their full plastic potential. These results can guide the optimisation of other hybrid composites with two ductile fibres.

Effect of Fiber Orientation on the Wrinkling Behavior of Thermoplastic Composite

In this work, experiments were conducted to examine the impact of fiber orientation of self-reinforced polypropylene on wrinkling phenomenon. Defects due to wrinkling are major quality issues in rapid forming of sheet materials. This article examines the influence of two fiber orientations [0°/90° and 45°/-45°] on wrinkling initiation of a self-reinforced polypropylene composite (Curv®) material system. It was found that 45°/-45° specimen wrinkles at smaller axial displacement compared to 0°/90° specimen. In both specimens, there was an abrupt change in strain increment ratio that corresponded to the onset of wrinkling. This phenomenon validates the robustness of the wrinkling indictor based on strain increment ratio concept.

Multivariable dependency of thermal shrinkage in highly aligned polypropylene tapes for self-reinforced polymer composites

Self-reinforced composites offer a unique combination of properties such as high specific strength, high impact resistance, and recyclability by incorporating highly aligned fibers within a random matrix of the same polymer. However, high temperatures will shrink the system to recover randomness in the aligned segments, compromising the composite thermal stability during processing as self-reinforced tapes are consolidated into the final composite through heating and pressure. Hence, the dynamic nonlinear multivariable (i.e., time, temperature, stress) shrinkage exhibited by self-reinforced polypropylene (SRPP) tapes was measured and modeled at the maximum shrinkage limit achieved in the proximity of the composite processing temperature [∼140 to160°C]. At high stress (∼7.5MPa) the thermal shrinkage of the SRPP tapes was reduced and a parallel creep mechanism was activated. The modeling, and prediction of the main factors governing the thermal shrinkage expand and diversify the dynamic design window for new SRPP composites.

Key Role of Reinforcing Structures in the Flame Retardant Performance of Self-Reinforced Polypropylene Composites.

The flame retardant synergism between highly stretched polymer fibres and intumescent flame retardant systems was investigated in self-reinforced polypropylene composites. It was found that the structure of reinforcement, such as degree of molecular orientation, fibre alignment and weave type, has a particular effect on the fire performance of the intumescent system. As little as 7.2 wt % additive content, one third of the amount needed in non-reinforced polypropylene matrix, was sufficient to reach a UL-94 V-0 rating. The best result was found in self-reinforced polypropylene composites reinforced with unidirectional fibres. In addition to the fire retardant performance, the mechanical properties were also evaluated. The maximum was found at optimal consolidation temperature, while the flame retardant additive in the matrix did not influence the mechanical performance up to the investigated 13 wt % concentration.

Nonlinear creep response of oriented polypropylene tapes

The creep response of a highly oriented polypropylene tape used for the manufacture of self-reinforced polypropylene or all-polypropylene composites was studied over a range of stresses and temperatures. Similar to oriented polyethylene, the creep compliance is linear viscoelastic at short loading times, whereas pronounced stress dependence is observed at longer loading times. A mathematical model is proposed, where the total deformation of the fibre is regarded as being composed of a stress-linear delayed elastic component and a nonlinear plastic flow contribution. Model predictions are in good agreement with the experimental data.

Wrinkling Phenomenon during Uniaxial Extension of a Woven Thermoplastic Composite

In the current research, wrinkling behaviour of a pre-consolidated woven self-reinforced polypropylene (SRPP) composite is investigated. Specimens possessing different aspect ratios were uniaxially extended at room temperature. Principal strains were recorded by a 3D optical photogrammetric system. Continuous measurement of out-of-plane displacements and induced in-plane strains elucidated the effect of specimens’ aspect ratios on wrinkling behaviour of SRPP composite. Investigating the evolution of meridian strains offered a valuable insight into development of principal strains before and after wrinkling onset. Plotting major and minor strains in a 2D strain space clarified the role of different deformation modes in the evolution of wrinkles. It was shown that common wrinkling predictors used on metals cannot effectively predict wrinkling initiation in this class of composites and a more reliable measure, based on evolution of local strains, should be employed. The current study investigates the wrinkling behaviour of a woven thermoplastic composite for the first time through a modified Yoshida buckling experiment. The outcomes illustrated effective parameters on the wrinkling evolution in SRPP specimens.

Stretch Forming Simulation of Woven Composites Based on an Orthotropic Non-Linear Material Model

Characterisation experiments have been conducted on a woven self-reinforced polypropylene composite (SRPP) including uniaxial and bias extension tests. Outcomes of these experiments were employed to develop a non-linear orthotropic material model within an incremental deformation framework. The material model of the woven composite was implemented into a finite element simulation to predict stretch forming behaviour of SRPP specimens. The predicted strain paths at the pole of specimens were verified against experimental outcomes. It was shown that specimens possessing different aspect ratios deform under a wide range of deformation modes from uniaxial extension to biaxial stretch modes. Finally, the effect of different forming parameters on the strain path evolution of the woven composite was elucidated through numerical simulations. It was shown that the aspect ratio of the samples plays an important role in forming behaviour of woven composites. Development of a reliable and accurate numerical model for predicting forming behaviour of woven composites and understanding their main forming mechanisms promote and encourage the extensive application of these materials systems in a wide range of mass producing industries. Adopting woven composites in manufacturing industrial components facilitates addressing environmental concerns such as recyclability and sustainability issues.