Investigations on the development of impact-resistant thermoplastic fibre hybrid composites from glass and steel fibre

With the increased use of fibre reinforced composites (FRP), the design of new generation of composite structures with high stiffness and a ductile material behaviour is required to cope with complex load scenarios and high damage tolerances. This can be achieved, in particular, by a combination of conventional fibre-reinforced composites (FRP), which possess high stiffness and strength with metallic materials characterized by their high ductility and associated higher energy absorption capacity. Currently, there are no solutions for hybridisation of high performance filament yarns, metal filament yarns and thermoplastic filament yarns on micro level. Therefore, the main objective of this study is to develop a fibre hybrid composite based on hybrid yarn consisting of glass, steel and polypropylene filament yarns and to compare its tensile and impact properties with those of the composite reinforced only with glass filament yarn. The tensile and impact properties of the unidirectional hybrid composites produced from the developed multi-material hybrid yarn consisting of steel, glass and polypropylene filament yarns are compared with those of a non-hybrid composite reinforced exclusively with glass filament yarn. The results show that by hybridising with steel fibres a characteristic post-failure behaviour can be achieved, so that the developed multi-material hybrid yarns have a high potential for use in composites with high crash and impact performance requirements, where safety demands are essential.


Introduction
Fibre reinforced composites, such as carbon fibre reinforced composites (CFRP) and glass fibre reinforced composites (GFRP), exhibit exceptional strength and stiffness.However a major disadvantage of these materials is their sudden brittle failure behaviour, which raises safety concerns and is often unpredictable. 1,2The lack of residual strength and their limited resistance to impact and crash scenarios contribute to these challenges. 3Overcoming these limitations is critical to improve the reliability and applicability of fibre-reinforced composites in various engineering applications. 4This can be achieved, in particular, by combining conventional fibre-reinforced composites (FRP), which have high stiffness and strength, with metallic materials, which are characterised by their pronounced ductility and the associated higher energy absorption capacity as well as high structural integrity.][10] At the macroscopic scale, a notable technique is laminate hybridisation, where alternating layers of metal sheets and FRP are systematically combined.These composites, known as fibre-metal laminates (FMLs), include metal sheet layers together with FRPs, as seen in examples such as CARALL (carbon fibre reinforced epoxy/aluminium foil laminate), ARALL (aramid fibre reinforced epoxy/ aluminium foil laminate) and GLARE (glass fibre reinforced epoxy/aluminium foil laminate). 8The focus of the studies was mostly on the effects of interlayer hybrid ratio and stacking sequence on the mechanical properties of laminated composites. 11,124][15] This phenomenon results from inherently poor interlayer adhesion due to geometrically constrained, small interfaces between aluminium foil and FRP layers, resulting in low maximum shear stress transferability. 16mproving adhesion or maximum shear stress transferability at the interfaces requires costly surface functionalisation of the aluminium foils and/or high-performance fibres, 17 such as sandblasting, etching, coating or plasmabased methods. 18Especially in forming processes with significant deformation, high shear stresses occur at the interlaminar interfaces, limiting the applicability of FMLs for complex component geometries.In addition, FMLs present economic challenges, including manual lay-up steps or the use of expensive robotic techniques, expensive autoclave processing, long processing times and significant joining effort.
In the case of mesoscale hybridisation (where the interweaving of different filament yarns within textile fabrics is possible), as in macroscopically hybridised composites, the mechanical performance of individual reinforcement components in the resulting composite cannot be fully exploited due to well-defined interlaminar interfaces and associated drawbacks. 19 potentially viable strategy to circumvent the layered configuration and create a substantial array of load-bearing interfaces is to uniformly integrate high-and low-elongation fibres at the microscale. 20This will result in a homogeneous microstructural blend that will reduce the stress concentrations that occuring at the interlaminar interfaces and thus decrease the tendency to delaminate.Compared to sequentially layered hybrids, highly dispersed fibrereinforced composites have a significant amount of interfacial regions between fibres and matrix.As energy can also be dissipated by delamination under loading conditions, 21 the abundance of these small interfacial regions can facilitate stress dissipation homogenously across the crosssection, for example, by pull-out mechanisms.In addition, the level of impregnation of thermoplastic composites can be significantly improved by reducing the flow paths during the thermoforming process as a result of micro-level mixing.This is particularly necessary when using thermoplastic materials, as these have a higher viscosity than thermoset matrix materials, which can result in air voids if the materials are not evenly distributed.Due to advantages in terms of recyclability, weldability and processability on a large scale, thermoplastics, in particular, are the focus of current research.As a result, there is a need for research into the development of thermoplastic fibre hybrid composites with improved impact properties made from high performance fibres such as carbon or glass fibre in combination with metal fibres.
Literature and research reports show that the production of hybrid yarns consisting of metal filament yarns and highperformance filament yarns is sparse compared to hybrid yarns consisting of high-performance filament yarns such as glass, carbon and aramid combined with thermoplastic filament yarns.In our previous paper, 22 the development and tensile properties of multimaterial hybrid yarns from glass, steel and polypropylene were reported.The main objective of this study is to develop a fibre hybrid composite based on hybrid yarn consisting of glass, steel and polypropylene filament yarns and to compare its tensile and impact properties with those of the composite reinforced only with glass filament yarn.Potential applications for the developed FRP structure include lightweight designs for large aircraft and automobiles that meet high safety standards, as well as for ballistic purposes.

Materials and methods
The composite manufacturing methodology used in this study is illustrated in Figure 1.It begins with the selection of raw filament yarns composed of glass, steel and polypropylene.This initial stage is followed by the use of a multilevel-intermixing system to blend the yarns, a process described in detail in reference 22.The yarns are then subjected to a winding process to create a unidirectional textile structure, which is then consolidated by a compression moulding technique.The following sections provide a detailed analysis of these process steps, from the initial selection of materials to a comprehensive evaluation of the final composite products.

Materials
For manufacturing of the fibre hybrid composite, stainless steel (StS), glass (GF) and polypropylene (PP) filament yarns were used.The characteristics of the filament yarns are detailed in Table 1.GF yarn (68 tex) and PP (33 tex) were used as received from the suppliers.However, 10 StS monofilaments (acid-resistant chrome nickel steel wire 1.4301), each having a diameter of 60 µm, were spooled together to make a yarn with a fineness of 225 tex.Glass filaments have been chosen because they offer high tensile strength per unit density at a lower cost 23 and require less energy for production than carbon fibres, 24 making them a more environmentally friendly option.They constitute over 90 % of composite materials, 25 attributed to their affordability and favorable mechanical characteristics.
Additionally, glass fibres are inert and resistant to corrosion. 26PP was chosen as the matrix material for the FRPs in this study due to its high stiffness and strength with low density, notable thermal stability, effective electrical and chemical resistance at an affordable cost, along with straightforward processing and recyclability. 27

Hybrid yarns production
Two different types of thermoplastic hybrid yarns have been produced.One was made from GF and PP filament yarns and the other from StS, GF and PP filament yarns.Both were produced by the Multi-level-intermixing process. 22By varying the material composition, the influence on the mechanical properties of the hybrid yarns and the composites based on them will be investigated.The proportions of different fibre components in the developed hybrid yarns are shown in Table 2.

Fabrication of composites
For the manufacturing of the composite specimens, the hybrid yarns were wound into unidirectional (UD) fabrics using the Multi-level-intermixing process.This step ensured a consistent and controlled fibre orientation, reinforcing the composite in a specific direction.The UD fabric samples were then consolidated using the P300 PV thermal Test specimens were cut from the consolidated composite plates to perform tensile and Charpy impact tests in the 0°direction (parallel to the fibre axis) using a precision table saw of type WOCO 50 from Uniprec Maschinenbau GmbH (Germany) with a galvanic coated blade of type TS322 from TSP Hildebrand (Germany) suitable for fibrereinforced composites.For the tensile tests of the composites, the specimens had dimensions of 250 ± 1 mm (length) x 25 ± 0.2 mm (width) x 2 ± 0.2 mm (thickness), following the guidelines of DIN EN ISO 527-5/A/2 standards.Additionally, composite specimens for the Charpy impact test were prepared, conforming to DIN EN ISO 179-1, with dimensions of 40 ± 1 mm (length) × 15 ± 0.2 mm (width) × 2 ± 0.2 mm (thickness).

Characterization methods
Single filament.Single filament tensile testing is used to calculate the energy required to break the selected fibre materials, which is critical to understanding the fracture behaviour of the composite.For this purpose, a Vibromat ME (Textechno, Germany) was used to measure the stressstrain behaviour of single GF and PP filaments according to DIN EN ISO 5079.The single filament fineness of GF and PP was measured by determining the resonance frequency on the same instrument according to DIN EN ISO 1973 before measuring the stress-strain behaviour.Due to the high bending stiffness of the StS, the fineness of the filaments could not be determined accurately on the Vibromat ME.Therefore, the individual fibre fineness was determined gravimetrically and the tensile testing was performed on the Zwick type Z 2.5 tensile tester by following the standard DIN EN ISO 5079.The same test length (20 mm), crosshead speed (10 m/min) and load cell (100 N) were used to determine the tensile properties of StS filaments as for the GF-and PP filaments.The average values of StS, GF and PP filaments were taken from 50 randomly tested individual filaments.
Composites.The tensile tests on the composite specimens were conducted using a Zwick Z100 tensile testing machine from ZwickRoell (Germany) with a 100 kN load cell.The quasi-static tensile properties of the composites specimens were determined following the test method outlined in DIN EN ISO 527.One of the main objectives of the investigations is to determine the residual strength after the failure stresses have been exceeded.A characteristic feature of residual strength is the residual tensile behaviour of the fractured specimens.A ZwickRoell (Germany) Zmart.Pro tensile tester with a smaller load cell than that used in the initial tensile tests, with a maximum force of 10 kN, was used to test the residual strength specimens after the initial tensile and Charpy impact tests.All forces recorded are related to the initial cross-sectional area, while deformations are related to the initial gauge length of the specimen.Additonally, an auxiliary camera system was located adjacent to the composite specimens and aimed at the specimen surface.It acts as an auxiliary measurement system, based on the principles of digital image correlation, integrated with the tensile testing machine to capture the surface deformation of the specimens.The system provides a highly accurate, non-invasive measurement of deformation by continuously monitoring surface displacements on the specimen.It allows the detection of fractures or cracks and their progression over time.For the analysis of the captured digital image data, the Trackmate library 28 was utilized, and the OpenCV library 29 was used for visualization.The noncontact nature of the measurement reduces the risk of interference or damage.
The impact tests were performed using a Charpy pendulum impact tester CEAST 9050 from Instron GmbH (Germany), which is based on the Charpy impact test principle according to ISO 179.A pendulum hammer with a kinetic energy of 15 J was used.All testing equipment was housed in a temperature and relative humidity controlled laboratory, maintained at 20 ± 2°C and 65 ± 2%, respectively.
To analyse the mixing of components in hybrid yarn composites, the composite samples are embedded in an epoxy resin matrix and cured at room temperature.After polishing and cleaning, cross-sectional images of the composites are taken using an optical microscope, specifically the Axio Imager M1m model from Carl Zeiss (Germany).In order to understand the degree of intermingling between the filaments within the composite crosssections, a distribution analysis of specimen StS18_GF35_PP47 was carried out.This analysis involved the detection of filaments using the Difference of Gaussians (DoG) algorithm.Prior to the application of the DoG algorithm, the micrograph images underwent a series of preprocessing steps including normalisation, thresholding and blurring.These steps ensured that the images were prepared for filament detection.Once the filaments had been detected, the central coordinates of the filaments were extracted and used to carry out a point pattern analysis using the spatstat library in R. 30 To illustrate the spatial interactions between glass and steel filaments within a composite, the cross-type K-function was used.This statistical tool assesses the expected density of one filament type within a given radius r of another, with normalisation based on the intensity of the initial filament type.The result is an array of K-functions, which consists of graphs that articulate the different interactions between the filament types.Each graph within the array is normalised by the intensity of the initial filament type, facilitating comparisons across different filament densities and distributions.On the other hand, the elongation at break of GF is lower (4.2 ± 0.7 %) than that of StS 43.8 ± 0.7 %.The StS single filament has a higher Young's modulus of 202.8 ± 12.0 GPa compared to the glass filament which have a Young's modulus of 78.2 ± 32.4 GPa.

Single filament tensile test
To assess the energy absorption potential of these filaments, the mechanical work during tensile testing was determined by calculating the areas under the average stress-strain curves (i.e.work W) using Formula (1): Where, ε st = start of elongation, ε break = elongation at break and σ = tensile stress.This region (Figure 3(b)) embodies the ability of the filaments to absorb and dissipate energy through mechanical work in the form of deformation and load absorption in case of structural stress.A greater capacity for energy dissipation is expected when the work performed by the filaments is higher, indicating a greater potential for damage tolerance in the composite materials.
In Figure 3(c), the calculated mechanical work performed by each filament variant derived from the single filament tensile assessments is illustrated.This observation suggests that the StS filaments has significantly enhanced energy dissipation capabilities (7.46 ± 0.15 GPa*mm) within composites, in contrast to GF (1.19 ± 0.34 GPa*mm).This supports the hypothesis that the incorporation of StS filaments has the potential to improve impact strength.

Effect of hybridisation on the composite tensile properties
Composite tensile tests.The results of the tensile tests of the composite samples manufactured with hybrid yarns are shown in Figure 4.The stress-strain behaviour of the composites (Figure 4(a)) can be divided into three primary regions (I, II and III).In the initial section (I), all specimens show an almost linear response.Within this initial zone, specimen StS18_GF35_PP47, which contains StS, shows a more pronounced rise in the stress-strain curves.This phenomenon is due to the higher Young's modulus of StS compared to GF.The stress-strain behaviour of composites containing steel filaments shows a slight degressive trend.This phenomenon can be attributed to the increased ductility of the steel fibres.Consequently, this ductility leads to the development of a force with strain-dependent characteristics similar to those observed in single fibre tests.As a result, this force-strain response superimposes the glass fibre behaviour observed in previous single fibre tests.
The second region (II) is characterised by the initial failure of the composites, which occurs at a strain range between 1.8% and 2.3%.Within this particular strain range, the hybrid composite shows a tensile strength of 631.2 ± 21.3 MPa (StS18_GF35_PP47) when containing StS and 705.5 ± 21.3 MPa (GF53_PP47) when not containing StS (Figure 4(b)).This difference is due to the higher proportion of GF, which has superior mechanical strength, resulting in a higher proportion of high strength components within the hybrid composites.The substitution of a GF component from 53 % to 35 % with StS (equivalent to 18 %) reduces the overall strength of the composite.The third section (III) describes the post failure behaviour of the composite StS18_GF35_PP47.In specimen without StS, no residual strength is observed after initial failure.Conversely, specimens containing StS exhibit well defined post-failure properties and residual tensile strength.After failure of the high-stiffness GF, the steel fibres remain intact and support the applied loads up to strains of 4.5 %.Even at these critical strain levels, a subset of fibres retain their structural integrity.The results of the investigations of these residual tensile strengths are explained in 3.2.3.This behaviour highlights the potential of hybridising high strength fibre reinforced composites with high ductility metal fibres, giving safety benefits over brittle high-strength composites.
In order to improve the understanding of the stress-strain behaviour of the composites, the results of the deformations measured with the auxiliary measuring system are presented below.Figure 5 shows a stepwise representation of the surface deformation during the tensile test.The relative deformation rates are represented by a colour space in which the colour steps correspond to the measured deformation.Since the deformations in the pre-fracture section are almost identical for both specimens and have a homogeneous distribution of strain rates, only the deformation characteristics after the end of the initial section are shown here (I).
The results of the surface deformation analysis reveal a further subdivision of the previously identified fracture zone (II) into two distinct subzones.In the first subzone (II.1), ranging from 1.8% to 2% strain, the first instances of localised deformation occur along the longitudinal axis of the fibres.These localised deformations represent the initial fractures of the fibres, followed by debonding of either the glass or steel fibres.In particular, the stress concentrations are more pronounced in the GF53_PP47 sample compared to the steel composites.This difference can be attributed to the increased ductility introduced by the presence of steel fibres.
The second sub-zone (II.2) is characterised by the deformations observed in the composites after almost complete breakage of all the glass fibres.In both specimens, there are significant concentrations of deformation along the longitudinal orientation of the fibres.The fracture of the glass fibres results in the release of energy which in turn causes deformations on the specimen surfaces.In the case of the StS18_GF35_PP47 specimen during initial fibre fracture events, particularly in glass fibres, which have a lower elongation at break than steel fibres, local stress concentrations occur and redistribute to immediately adjacent steel fibres.This redistribution process is enhanced by shortened load paths resulting from superior component mixing, bypassing the glass fibres and transferring stress directly to the load-bearing steel filaments.Consequently, such frequent load redistribution reduces the stiffness of the hybrid composite, leading to failure at larger deformations.Conversely, areas of glass fibre agglomeration show significant deformation due to the absence of intact steel fibres which would otherwise prevent rapid deformation.
Prior to the ultimate failure of the composite, there is a notable development of microcracking on the fibre surfaces in the case of StS18_GF35_PP47 specimen.This can be attributed to the relatively low interface strength between glass fibre (GF)/steel (StS) and polypropylene (PP).These microcracks propagate along the fibre surfaces resulting in filament debonding from the matrix and finally leading to the distinct appearance of longitudinal cracks.
Closer examination of the composites tested in the tensile tests reveals significant fibre debonding in all specimens.This is evidenced by the appearance of fibre strands on the specimen surface (see Figure 6).It is assumed that the interface strength between the glass fibre (GF)/steel (StS) and polypropylene (PP) is relatively low due to the low  polarity and consequently low surface free energy, which contributes to the tendency for filament debonding.This debonding effect is enhanced by the energy released during fibre breakage.When the fibres break, there is an abrupt retraction of the broken fibre segments against the direction of tension.This highly dynamic process increases the debonding of the fibres from the matrix.In contrast to the GF53_PP47 test group, which does not contain steel fibres, the StS18_GF35_PP47 specimen shows less damage.This is partly due to the lower proportion of high dynamic fracture GF and partly due to the remarkable ductility of the steel fibres, which are characterised by a ductile fracture behaviour resulting in a less pronounced dynamic unloading of the fibres at fracture.Microscopic analysis.Figure 7 shows microscopic images of composite cross-sections, providing a visual inspection of the internal structure.In these images, the distribution of fibres across the composite cross-section is observed to be homogeneous, indicating a uniform dispersion throughout the material.However in the specimen referred to as GF53_PP47, the micrographs show the presence of a few matrix-rich areas in between clusters of glass filaments.This suggests areas of higher concentration of binding matrix.The specimen labelled StS18_GF35_PP47 shows that the steel filaments are evenly distributed across the composite cross-section.
The array of Cross-type K-function resulting from the fiber spatial point pattern analysis of central coordinates of filaments is illustrated in Figure 8.The benchmark value K = πr 2 (shown as a red line) serves as a reference for assessing the spatial randomness or independence between filament types, with actual observations marked by a black line.Graphs (a) and (d) in Figure 8 correspond to the conventional K-function, examining interactions within the same filament type (GF-GF and StS-StS, respectively).These interactions provide insight into the internal clustering or dispersion patterns of each filament type, with the GF-GF interaction indicating slight clustering, 30 as the observed values slightly exceed those predicted under complete spatial randomness.Conversely, the StS-StS interaction suggests a random distribution of StS, with observed values close to theoretical expectations.
Plots (b) and (c) in Figure 8 show the cross-type K-functions that capture the inter-filament interactions (GF-StS and StS-GF).The GF-StS analysis reveals a marginal dependence of the glass on the steel filaments, as evidenced by the observed values slightly lagging behind the Poisson process benchmark at closer ranges.However, the StS-GF plot shows a lower observed value compared to the benchmark, possibly due to the location of steel filaments in matrixrich areas, suggesting a spatial separation between filament types.Using spatial point pattern analysis with the cross-type K-function has effectively quantified spatial interactions between fibre types in composites.However, future research must focus on improving segmentation techniques for accurate fibre detection.The implementation of AI-based segmentation methods offers a promising approach to increase detection accuracy and efficiency.In addition, the investigation of alternative methods for assessing the degree of blending of composites is crucial.These improvements are essential to deepen our understanding of composites and expand their application possibilities.Therefore, the refinement of detection methods and the evaluation of mixing assessment methods will be central in future research efforts.
In order to understand the damage mechanisms present in the fibre composite specimens, the specimens were encapsulated to allow microscopic analysis of their cross-sectional characteristics.Examination of the micrographs shown in Figure 9 provides a detailed inspection of the fracture topographies specific to the tensile specimens tested, observed perpendicular to the longitudinal axis of the fibres.These micrographs reveal the remarkable presence of fracture regions where the StS and GF fibres are debonded from the matrix.The presence of a crack passing through the entire thickness of the specimen indicates the occurrence of stress concentration in this region.This phenomenon is attributed to the initial initiation of a crack, its subsequent propagation and eventual matrix fracture along the fibre constituents.The results presented in the cross-section of the composites show evidence of the longitudinal cracks observed in the images of the specimens shown in Figures 6 and 9.
Residual tensile strength after tensile fracture.Even after the initial tensile tests, a subset of fibres retain their structural integrity.To gain a more accurate understanding of the residual tensile properties, addtional tensile tests were carried out using a separate tensile tester (Zmart.Pro) with a reduced load cell capacity of 10 kN.The smaller load cell allows to determine small forces more accurate.Figure 10(a) shows stress-strain curves derived from tensile tests, which reveal notable differences in the residual tensile strengths of composite specimens containing steel fibres.The initial portion of these stress-strain curves, spanning the strain range of 0-0.5 %, shows an almost linear increase due to the reorientation at the start of the tensile test.After reaching the maximum stress, the stress-strain curve exhibits a continuous decline within the elongation range.This phase is defined by two main phenomena: the fracture of previously undamaged fibres and the concurrent pull-out mechanisms of fibres that have separated from the matrix.Importantly, the specimens reinforced with steel fibres exhibit a much higher residual tensile strength of 59.6 ± 2.6 MPa, in contrast to the specimens reinforced solely with glass fibres, which record a lower value of 6.9 ± 2.2 MPa (as shown in Figure 10(b)).Elongations at break of up to 10 % can also be observed in the steel fibre group of specimens.In contrast, the elongation of GF53_PP57 is only up to 5 %.These results illustrate the outstanding post-failure characteristics of the composite specimens hybridised with steel fibres.
For a better understanding of the fracture behaviour of the tensile test specimens, scanning electron microscope (SEM) images of the fracture surfaces were taken.Figure 11(a) shows a fracture surface from specimen group StS18_GF35_PP47.In this specimen, predominantly fractured steel fibres are observed.The steel fibres show necking at the fibre end region, indicating ductile fracture behaviour.Conversely, as shown in Figure 11(b), the glass fibres exhibit plane fracture surfaces, suggesting a brittle fracture behaviour.The differences observed in the fracture characteristics of glass fibres and steel fibres are clearly consistent with the results of the tensile tests.The remarkable ductility of the steel fibres is also demonstrated by the presence of constricted regions at the fibre ends.Importantly, even after the brittle failure of the glass fibres, the steel fibres remain partially intact and continue to expand after fracture, allowing a distinct post-failure response.
Residual tensile strength after impact.The results of Charpy impact test performed with the composites are shown in Figure 12.From the force-deformation curves illustrated in    Figure 12(a), the difference between GF53_PP47 composite and StS18_GF35_PP47 composite can be observed.It can be seen that the extent of deformation in the StS18_GF35_PP47 specimens covers a significantly larger area when compared to the GF53_PP47 composites, indicating increased impact strength.Additionally, the curves of GF53_PP47 composites show a pronounced initial force followed by a rapid and significant decrease in force magnitude in contrast to the StS18_GF53_PP47 specimen.Therefore, the inclusion of steel filament in the composite faciliates an efficient distribution of forces in the composite.
The composite impact strength calculated from the measured breaking energy based on the Charpy impact test is illustrated in Figure 12(b)).The average impact strength of the GF53-PP47 is 128 ± 10 kJ/m 2 .The impact strength of the StS18-GF53-PP47 hybrid composite is 139 ± 6 kJ/m 2 .The slightly higher impact strength can be attributed to the increased stiffness exhibited by the metallic fibres and the wider distribution of the deformation area as shown in Figure 3(a).The modest plateau from 0.9-1.4ms after the initial failure between 0.4-0.6 ms is due to plasticity of StS, where localised load paths remain unobstructed, facilitating the continued transfer of load to adjacent regions.This in turn allows the neighbouring regions to absorb energy.However, due to the significantly increased ductility of the steel fibres, a much greater increase in impact strength was expected.It is suggested that the limited adhesion properties of the steel fibres result in poor force transfer, leading to the occurrence of localised stress concentrations and subsequent microcrack formation.In addition, it can be suggested that the predominant stresses in Charpy pendulum impact tests are associated with bending.In particular, the tensile stress component is minimal.This limits the comprehensive representation of the impact of metal fibres in these assessments.Further research is essential to improve the adhesion properties of the steel fibres.
From images of the specimens after Charpy impact testing (Figure 13), a visual representation of the larger deformation area in the cross-section of the StS18-GF53-PP47 specimens compared to the GF-PP specimens can also be seen.In both the GF53_PP47 and StS18_GF35_PP47 specimen groups, shear buckling is apparent, accompanied by the formation of kinkbands at the edges of the impacted areas.This formation of kink-bands is a noteworthy indicator of pronounced ductility, as it signifies that the specimens do not exhibit brittle fracture behaviour even under substantial deformations.Notably, when comparing the two groups, it is evident that a larger proportion of defective fibres detached from the matrix is observed on the specimen surfaces of GF53_PP47 specimens compared to those in the StS18_GF35_PP47 group.
In order to determine the residual tensile strength of impacted specimens equivalent to the residual tensile strengths obtained in section Residual tensile strength after tensile fracture, the specimens from the Charpy test were subjected to the same tensile test setup as described in that  section.The results of these tensile tests are shown in Figure 14.The specimens with steel fibres had a higher residual tensile strength of 83.2 ± 15.3 MPa compared to the specimens without steel fibres which had a residual tensile strength of 62.7 ± 9.7 MPa.The increase in residual tensile strength due to the presence of steel fibres is more pronounced in the specimens tested under tension in section 3.2.3compared to the impacted specimens.The observed difference in residual tensile strength can be explained by differences in fibre damage due to the different loads specific to the test methods (tensile, impact).In the tensile tests, a greater number of fibres are destroyed as their breaking strain is usually exceeded.In contrast, the impact tests show a lower incidence of apparent defects in the glass fibres, resulting in a comparatively higher residual strength in the tested specimens.

Conclusion
The results presented in this paper provide insight into the mechanical properties and energy absorption capabilities of glass fibre (GF) and steel filament (StS) based thermoplastic unidirectional composites.The single fibre tensile tests have shown that while GF has a higher tensile strength than StS, StS has significantly improved energy dissipation capabilities, which can potentially improve the impact strength of composites.This suggests that StS fibres have the ability to absorb and dissipate a larger amount of energy during deformation and failure, making them preferable for applications requiring improved damage tolerance and postfailure behaviour in fibre-reinforced composites.The higher energy dissipation potential of StS fibres can contribute to enhancing the overall mechanical performance and resilience of composites, making them suitable for applications where impact resistance and toughness are critical considerations.Consequently, the inclusion of StS fibres in composite materials may offer significant advantages in engineering applications that demand superior energy absorption capabilities and damage tolerance.
In addition, detailed fracture analyses, including scanning electron microscope (SEM) images, provided insight into the fracture behaviour of these composites.The remarkable ductility of the steel fibres and their ability to remain structurally intact even after the brittle failure of the glass fibres was a notable observation.This ductility, coupled with a wider distribution of the deformation area, contributed to a increase in the impact strength of GFreinforced composites when steel filament yarn was incorporated.The post-failure behaviour of composite structures hybridised with steel fibres, as described in the studies presented, shows considerable potential.An existing disadvantage of the investigated hybrid composites is the low interface strength, which is caused by the low polarity of the PP, between the matrix material PP and the reinforcing fibres GF and StS.In further investigations, an additional sizing of the reinforcing fibres should improve the interphase properties between the reinforcing fibres and PP.In addition, it should be investigated whether a better bonding of the fibres to the matrix can be achieved by adjusting the degree of mixing of the reinforcing fibres.

Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)-441549528.

Figure 1 .
Figure 1.Schematic overview of the experimental works required for the composite production and characterizations of composite specimen.

Figure 2 .
Figure 2. Pressing parameters (temperature, pressure) used for consolidation of composites.

Figure 3 a
Figure 3 a shows the representative stress-elongation curve of GF and StS single filaments.GF has a higher tensile strength (2525 ± 355 MPa) compared to StS (922 ± 2 MPa).

Figure 3 .
Figure 3. Results of single filament tensile tests with (a) average stress-strain curves, (b) schematic representation of the calculation of the areas under the curves and (c) work required to break.

Figure 4 .
Figure 4. Results of composite tensile tests with (a) stress-strain curves and (b) tensile strength.The shaded region in (a) illustrates the 95 % confidence interval.

Figure 5 .
Figure 5. Stress-strain curves and images of composites with coloured scheme that indicates the surface deformation of the composites.The shaded region in a) illustrates the 95 % confidence interval.

Figure 8 .
Figure 8. Images of specimens after tensile test.

Figure 10 .
Figure 10.Results of the tensile tests to determine the residual tensile strength with (a) stress-strain curves and (b) tensile strength.The shaded region in (a) illustrates the 95 % confidence interval.

Figure 11 .
Figure 11.SEM-images of fibres from tensile tested composites.

Figure 12 .
Figure 12.(a) force-deformation curves (b) and calculated impact strength of composites.The shaded region in a) illustrates the 95 % confidence interval.

Figure 13 .
Figure 13.Images of specimens tested in Charpy impact test.

Figure 14 .
Figure 14.Residual tensile strength of impacted specimens with (a) stress-strain curves (b) and calculated impact strength of composites.The shaded region in a) illustrates the 95 % confidence interval.

Table 1 .
Characteristics of filaments yarns used for the development of hybrid yarns.

Table 2 .
Mixing ratio of different fibre component in the developed hybrid yarns.laboratorypressfromCOLLIN Lab.& Pilot Solutions GmbH (Germany).The consolidation process was carried out at 220°C with a pressure of 188 MPa.The curve of temperature and pressure during the production of the composite is shown in Figure2.