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Abstract

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.

Keywords

Hybrid yarns commingling steel filament yarn glass filament yarn thermoplast composites

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.^1,2 The lack of residual strength and their limited resistance to impact and crash scenarios contribute to these challenges.³ Overcoming these limitations is critical to improve the reliability and applicability of fibre-reinforced composites in various engineering applications.⁴ This 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. The hybridisation of FRP with metals not only serves to advantageously complement the respective mechanical properties but also to generate or increase additional functional properties, such as electrical conductivity, thermal conductivity, temperature resistance and permeability.^5–7

The hybridisation of high-performance fibres with metals has so far been mainly limited to the macroscopic scale, involving layered combination of metal and high-performance fibres as hybrid fibre composites.^8–10 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).⁸ The focus of the studies was mostly on the effects of interlayer hybrid ratio and stacking sequence on the mechanical properties of laminated composites.^11,12

However, under loading conditions, the macroscopic layer-by-layer arrangement of FMLs can lead to stress concentrations at the interfaces between the aluminium foil and the fibre-reinforced composite layers, resulting in layer delamination and premature failure of the composite components.^13–15 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.¹⁶ Improving 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 plasma-based methods.¹⁸ Especially 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

A 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.²⁰ This 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 fibre-reinforced 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 cross-section, 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 high-performance 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 multi-level-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.

Figure 1.

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

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 cost²³ 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.²⁶ PP 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

Table 1.

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

	Stainless steel (StS)	Glassfibre (GF)	Polypropylene (PP)
Notation	Acid resistant chrome nickel steel wire 1.4301	EC9- 68tex- Z28 302P	Prolen HP 330/66, white, untwisted
Manufacturer	Heinrich stamm GmbH	P-D glasseiden GmbH oschatz	Chemosvit fibrochem, s.r.o
Acronym	StS	GF	PP
Diameter of single filament (µm)	60±0.2	7.2±0.9	15±1.5
Fineness filament yarn (tex)	225	68	33
Number of filaments in a yarn	10	302	66

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.²² By 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.

Table 2.

Mixing ratio of different fibre component in the developed hybrid yarns.

	Fibre component			Linear density of hybrid yarn (tex)	Hybrid yarn designation
	StS	GF	PP	Linear density of hybrid yarn (tex)	Hybrid yarn designation
Linear density (tex)	-	204	66	270	GF-PP- HY
Weight (%)	-	75.6	24.4
Volume (%)	-	52.7	47.3
Linear density (tex)	225	136	66	422	StS-GF-PP-HY
Weight (%)	52.4	31.9	15.7
Volume (%)	18.2	34.5	47.3

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 laboratory press from COLLIN 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 Figure 2.

Figure 2.

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

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 fibre-reinforced 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 stress-strain 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 library²⁸ was utilized, and the OpenCV library²⁹ was used for visualization. The non-contact 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 cross-sections, 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 pre-processing 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.

Results

Single filament tensile test

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). 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.

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.

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): $W = \int_{ε_{s t}}^{ε_{b r e a k}} σ d ε$ (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.

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.

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).

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.

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.

Figure 6.

Microscopic images of the cross-sections of composite specimens: (a) GF_PP47 and (b) StS18_GF35_Pp47.

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.

Figure 7.

Array of Cross-type K-function graphs depicting spatial interactions within composite materials: (a) GF-GF interaction; (b) GF-StS interaction; (c) StS-GF interaction; (d) StS-StS interaction.

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 = πr² (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.

Figure 8.

Images of specimens after tensile test.

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.

Figure 9.

Microspopic crossection images of tensile tested composite (fracture area marked red).

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.

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.

SEM-images of fibres from tensile tested composites.

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.

Figure 12.

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

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/m². The impact strength of the StS18-GF53-PP47 hybrid composite is 139 ± 6 kJ/m². 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 0.9–1.4 ms 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 kink-bands 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.

Figure 13.

Images of specimens tested in Charpy impact test.

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 3.2.3 compared 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.

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.

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 post-failure 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 GF-reinforced 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.

Footnotes

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.

ORCID iDs

Matthias Overberg

Mir Mohammad Badrul Hasan

Data availability statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study. *

References

Striewe

Reuter

Sauerland

K-H

, et al. Manufacturing and crashworthiness of fabric-reinforced thermoplastic composites. Thin-Walled Struct 2018; 123: 501508, DOI: 10.1016/j.tws.2017.11.011.

Bin

Yuan

Juhyeong

, et al. In-plane compression response of woven CFRP composite after low-velocity impact: modelling and experiment. Thin-Walled Struct 2021; 158: 107186, DOI: 10.1016/j.tws.2020.107186.

Bambach

Elchalakani

Zhao

. Composite steel–CFRP SHS tubes under axial impact. Compos Struct 2009; 87: 282–292, DOI: 10.1016/j.compstruct.2008.02.008.

Swolfs

De Cuyper

Callens

, et al. Hybridisation of two ductile materials Steel fibre and self-reinforced polypropylene composites. Compos Appl Sci Manuf 2017; 100: 4854, DOI: 10.1016/j.compositesa.2017.05.001.

Textile materials for lightweight constructions: technologies - methods - materials - properties. New York, NY: Springer Berlin Heidelberg; 2015.

Kühn

Rehra

May

, et al. Dry fiber placement of carbon/steel fiber hybrid preforms for multifunctional composites. Adv Manuf Polym Compos Sci 2019; 5: 37–49, DOI: 10.1080/20550340.2019.1585027.

Hannemann

Backe

Schmeer

, et al. Hybridisation of CFRP by the use of continuous metal fibres (MCFRP) for damage tolerant and electrically conductive lightweight structures. Compos Struct 2017; 172: 374–382, DOI: 10.1016/j.compstruct.2017.03.064.

Costa

RDFS

Sales-Contini

RCM

Silva

FJG

, et al. A critical review on fiber metal laminates (FML): from manufacturing to sustainable processing. Metals 2023; 13: 638, DOI: 10.3390/met13040638.

Khalid

Arif

Ahmed

, et al. Evaluation of tensile properties of fiber metal laminates under different strain rates. Proc IME E J Process Mech Eng 2021; 236: 095440892110530, DOI: 10.1177/09544089211053063.

10.

Swolfs

Verpoest

Gorbatikh

. Recent advances in fibre-hybrid composites: materials selection, opportunities and applications. Int Mater Rev 2019; 64: 181–215, DOI: 10.1080/09506608.2018.1467365.

11.

Hynes

NRJ

Vignesh

Jappes

JTW

, et al. Effect of stacking sequence of fibre metal laminates with carbon fibre reinforced composites on mechanical attributes: numerical simulations and experimental validation. Compos Sci Technol 2022; 221: 109303, DOI: 10.1016/j.compscitech.2022.109303.

12.

Zareei

Geranmayeh

Eslami-Farsani

. The effect of different configurations on the bending and impact properties of the laminated composites of aluminum-hybrid basalt and jute fibers-epoxy. Fibers Polym 2019; 20: 1054–1060, DOI: 10.1007/s12221-019-1148-2.

13.

Alderliesten

. Fatigue crack propagation and delamination growth in glare. Delft: Delft Univ. Press, 2005.

14.

Kuhtz

Hornig

Richter

, et al. Increasing the structural energy dissipation of laminated fibre composite materials by delamination control. Mater Des 2018; 156: 93102, DOI: 10.1016/j.matdes.2018.06.039.

15.

Wisnom

. The role of delamination in failure of fibre-reinforced composites. Philos Trans A Math Phys Eng Sci 2012; 370: 1850–1870, DOI: 10.1098/rsta.2011.0441.

16.

Zimmermann

Specht

Spieß

, et al. Improved adhesion at titanium surfaces via laser-induced surface oxidation and roughening. Mater Sci Eng, A 2012; 558: 755760, DOI: 10.1016/j.msea.2012.08.101.

17.

Mamalis

Obande

Koutsos

, et al. Novel thermoplastic fibre-metal laminates manufactured by vacuum resin infusion: the effect of surface treatments on interfacial bonding. Mater Des 2019; 162: 331–344, DOI: 10.1016/j.matdes.2018.11.048.

18.

Monden

. Adhäsion zwischen epoxidharzbasiertem CFK und oberflächenmodifiziertem Stahl: grenzschichtversagen von Hybridlaminaten unter Mode I, Mode II und Mixed-Mode Belastung. Germany: Universität Augsburg, 2016.

19.

Montinaro

Cerniglia

Pitarresi

. Evaluation of interlaminar delaminations in titanium-graphite fibre metal laminates by infrared NDT techniques. NDT E Int 2018; 98: 134–146, DOI: 10.1016/j.ndteint.2018.05.004.

20.

Swolfs

McMeeking

Verpoest

, et al. The effect of fibre dispersion on initial failure strain and cluster development in unidirectional carbon/glass hybrid composites. Compos Appl Sci Manuf 2015; 69: 279–287, DOI: 10.1016/j.compositesa.2014.12.001.

21.

Sayyar

Balachandra

Soroushian

. Energy absorption capacity of pseudoelastic fiber-reinforced composites. Sci Eng Compos Mater 2014; 21: 173–179, DOI: 10.1515/secm-2013-0021.

22.

Overberg

Hasan

MMB

Rehra

, et al. Development of multi-material hybrid yarns consisting of steel, glass and polypropylene filaments for fiber hybrid composites. Textil Res J 2023; 93: 00405175231179759, DOI: 10.1177/00405175231179759.

23.

Radulović

. Hybrid filament-wound materials: tensile characteristics of (aramide fiber/glass fiber)-epoxy resins composite and (carbon fibers/glass fiber)-epoxy resins composites. Sci Technol Rev 2020; 70: 36–46, DOI: 10.5937/str2001036R.

24.

Duflou

Deng

Van Acker

, et al. Do fiber-reinforced polymer composites provide environmentally benign alternatives? A life-cycle-assessment-based study. MRS Bull 2012; 37: 374–382, DOI: 10.1557/mrs.2012.33.

25.

Thomason

Yang

Meier

. The properties of glass fibres after conditioning at composite recycling temperatures. Compos Appl Sci Manuf 2014; 61: 201–208, DOI: 10.1016/j.compositesa.2014.03.001.

26.

Ryvolová

Svobodová

Bakalova

. Validation of an image analysis method for evaluating the chemical resistance of glass fibers to alkaline environments. Materials 2021; 15: 161, DOI: 10.3390/ma15010161.

27.

Van Den Oever

Peijs

. Continuous-glass-fibre-reinforced polypropylene composites II. Influence of maleic-anhydride modified polypropylene on fatigue behaviour. Compos Appl Sci Manuf 1998; 29: 227–239, DOI: 10.1016/S1359-835X(97)00089-4.

28.

Ershov

Phan

M-S

Pylvänäinen

, et al. TrackMate 7: integrating state-of-the-art segmentation algorithms into tracking pipelines. Nat Methods 2022; 19: 829–832, DOI: 10.1038/s41592-022-01507-1.

29.

Bradski

. The OpenCV library. The United States: Dr Dobb’s Journal of Software Tools, 2000.

30.

Baddeley

Rubak

Turner

. Spatial point patterns: methodology and applications with R. Boca Raton ; London ; New York: CRC Press, Taylor & Francis Group, 2016.

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

Abstract

Keywords

Introduction

Materials and methods

Materials

Hybrid yarns production

Fabrication of composites

Characterization methods

Single filament

Composites

Results

Single filament tensile test

Effect of hybridisation on the composite tensile properties

Composite tensile tests

Microscopic analysis

Residual tensile strength after tensile fracture

Residual tensile strength after impact

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

Data availability statement

References