2.1. Toe Cap Planning and Mould Construction
Artec Studio (version 11.2) was the software used for 3D scanning and processing the toe cap data. It was developed by Artec Group [
8], a company known for its range of professional-grade portable 3D scanners. It has human vision with colour, making it easier to detect textures, as well as ultrasound and infrared that provide a resolution of 0.10 mm (100 μm). Before using the software, the composite toe cap of a safety boot was removed, first identifying the area where the toe cap was located and then carefully removing any external coating using an DeWalt X-acto knife (Towson, MD, USA). Finally, the fasteners holding the toe cap to the boot were located and loosened, taking care not to damage the toe cap.
Several captures were made, and after this process, they were aligned. Initially, an STL file was exported, as shown in
Figure 1a, and then transferred to Autodesk Fusion 360, a cloud-based 3D computer-aided design (CAD), computer-aided manufacturing (CAM), and computer-aided engineering (CAE) tool for product design and engineering. After importing the file, the shape was drawn and the views were drawn.
After drawing the views,
Figure 1b shows the result of using the Loft tool, which allows you to create a transition between two or more planar drawings. Next, a Shell was made, in which the material was removed from the inside, creating a hollow cavity with walls of a specific thickness of 2 mm. The Shell was then moved to provide a thicker toe cap (6 mm). After constructing the object, the mould had to be made. A parallelepiped was drawn and within it, the Boundary Fill tool was used, which allows objects to be filled with volumes using the selected boundaries. Next, four holes were designed to align and tighten the male and female moulds, as shown in
Figure 1c,d.
To create the mould for the toe caps, the additive manufacturing or 3D printing process was used, given its versatility, the availability of different materials used as raw materials, and the ease of access to this technology. The process involved several stages, starting with the design and ending with the final print. The first stage was to create the model using Autodesk AutoCAD 2023for the toe cap mould, considering the dimensions, clearances, and precise specifications. The design took into account the properties of the material and the limitations of the 3D printing process.
In this case, the toe cap mould was divided into five parts to facilitate the production of the toe caps, as shown in
Figure 2a. The blue, red, and orange parts were printed in polyethylene glycol terephthalate (PETG). The remaining parts, green and yellow, were printed in thermoplastic polyurethane (TPU) with a shore hardness of 95 A.
The printer, model Artillery Sidewinder X1, began printing the various parts of the toe cap mould layer by layer. The nozzle used was 0.6 mm, which expelled the printing material that is normally heated to a semi-liquid state. The polymer quickly solidified as it was deposited, forming a solid layer. The process was repeated, layer by layer, until all the parts of the toe cap mould had been formed, as shown in
Figure 2b. After printing, the mould required post-processing. This included cleaning the support material used during printing, smoothing the surfaces, and the curing process.
2.2. Experimentals Procedures
Using waste particles from the mechanical machining of rotor blades supplied by a large German manufacturer, a particle size analysis was carried out by dry sieving, as shown in
Figure 3a. This method is used to categorise granular material by the particle size distribution determined by maximum particle size.
The first step was to ensure that the sample was completely dry. Next, the particle size range of the sample was determined, and the appropriate sieves were selected. The sieves were arranged from top to bottom, with the largest opening at the top and the smallest at the bottom, in the following order: 2.36 mm, 1.18 mm, 850 µm, 600 µm, 300 µm, 150 µm, 75 µm, and <75 µm. A container was placed at the bottom to collect the finest particles and then a quantity of sample was placed on the top sieve. It was determined that the sieve shaking cycle should take place for 15 min, with a time interval of 10 s, to ensure that the time was sufficient for all the particles to have a chance to pass through the sieve openings until they were retained by their size on the respective sieve.
Figure 3a shows one of the particle sizes, in this case < 75 µm.
After sorting by size, the amount of material retained on each sieve was weighed separately. The percentage of the total sample weight, the fraction that each sieve contained, was determined, and a graph of the particle size distribution was constructed. The particle size distribution graph, as shown in
Figure 3b, is often used to represent this data, where the particle size is shown on the
x-axis and the cumulative percentage on the
y-axis. This graph shows the particle size distribution and the granulometric nature of the material.
In this initial phase of practical work, particles with smaller dimensions were selected due to the quantity available, as can be seen in the graph in
Figure 3b and as referenced in the literature. Smaller particles, when added to epoxy resin, are preferable for improving its mechanical properties than larger particles [
7].
To obtain an enhanced view and detailed characterisation of the particles, scanning electron microscopy (SEM) was used, using the Hitachi S-3400N model. The samples were coated with gold, a necessary measure due to the low conductivity of the particles under study,
Figure 4.
Given the production process and by observing
Figure 4a, the variation in geometric terms of the particles with a size of <75 μm is notable, which is also reflected in the dispersion in terms of size within this analysed range,
Figure 4b.
An epoxy resin SR 8100 was combined with hardener SD 8824, both supplied by Sicomin (Chateauneuf les Martigues, France) and initially cured for 24 h at room temperature and then post-cured for a further 24 h at 40 °C, according to the supplier’s instructions [
9]. For the curing process used in this study, the main mechanical properties of the pure cast resin are modulus of elasticity (tension/flexion) around 2.90/3.00 GPa, resistance at break (tension/flexion) around 60/108 MPa, resilience around 52 kJ/m
2, and glass transition temperature (
Tg) of 63 °C.
According to the established guidelines and for samples with 1 wt.% of particles, 1.22 g of particles were mixed for every 100 g of resin. Dispersion in the resin was performed simultaneously using a 1000 rpm high-speed mixer and sonication using an ultrasonic bath with a frequency of 40 kHz for 3 h at room temperature. After 3 h of stirring under an ultrasonic bath, 22% by weight of the hardener was added, i.e., 22 g. Bearing in mind that variables such as stirring time, stirring speed, and the use or not of sonication directly influence the final mechanical properties of the composite, a method previously used in other studies was applied and proved promising [
10].
This procedure continued for another 10 min at a rotation speed of just 200 rpm to mix the hardener into the system. The purpose of the low rotation speed was to minimise the formation of air bubbles and promote homogeneous mixing of the hardener in the system. When the mixture had a uniform appearance, it was subjected to a degassing process in a vacuum pan for 10 min at a pressure of 0.9 ± 0.1 bar. Finally, the mixture was poured into a cardboard mould with dimensions of 200 × 120 × 3 mm3, previously manufactured and protected by a technical plastic for this purpose. Finally, all the moulds produced were cured as recommended by the resin and hardener supplier.
After the matrices had cured, they were cut using a Struers Accutom 2 (Copenhagen, Denmark) cutting machine. A water-cooled diamond blade was used to prevent the composite from heating up, considering the cutting speed. The dimensions of these specimens were established by BS EN ISO 178:2019 [
10], which describes a procedure for assessing the flexural properties of a thermosetting material [
11].
The standard states that the span, referred to as the distance between supports (
L), must comply with
L = (16 ± 1) ×
t, with
t representing the average thickness of the respective material [
11]. To normalise the average thickness of the specimens, a constant thickness of 2.76 ± 0.50 mm was considered. Thus, the distance between supports was 50 mm. The static three-point bending (3PB) tests were carried out at room temperature using 80 × 10 × 2.76 mm
3 specimens cut from the samples produced, as shown in
Figure 5a. For each condition, at least five samples were tested, with a displacement rate of 2 mm/min.
The laminate production process began with the selection of glass fibres, chosen from various types provided by the manufacturer, and a type E glass fibre, in order to reproduce the toe cap. The properties associated with these materials can be seen in
Table 1. The choice of fibre directly influences the mechanical properties of the final composite according to the specific requirements of the application, optimising the performance of the composite to meet the desired mechanical requirements.
The glass fibre laminates were manufactured using the hand lay-up method, with the control laminate being the first to be produced. The SR 8100 epoxy resin was mixed with the SD 8824 hardener in the exact proportions described above. This stage is crucial, as the proportion and homogeneity directly influence the quality and properties of the final laminate.
Once the different layers of glass fibre had been prepared, a layer of epoxy resin was applied to the release agent. Next, the first layer of type E glass fibre was emplaced. The fibre was then impregnated with more resin, always controlling the amount of resin between layers to 15 ± 10 mL. This sequence—resin application, fibre placement, impregnation—was repeated four times (one layer of type E fibre, two layers of higher density fibre, and one layer of type E fibre) until the desired thickness of the laminate was reached, as shown in
Figure 5d.
This sequence was established in order to reproduce the laminate of a protective toe cap taken as a reference,
Figure 5d, for mechanical characterization and optimization, followed by manufacturing the toe cap in the laboratory, for subsequent characterization
Figure 5e. One of the challenges during this process was to ensure the elimination of air bubbles and perfect consolidation between the fibre and resin layers. A strip of felt fabric was placed around the laminate to create an air circulation channel, helping to absorb excess matrix during vacuum application.
Subsequently, the additive laminates were produced, i.e., a quantity of particles < 75 μm in size was added to the resin solution with the hardener. The sequence—resin application, fibre placement, impregnation—was repeated four times.
Once the last layer of release agent had been applied, each system was placed inside a vacuum bag and a 2.50 kN load was applied for 24 h to maintain a constant fibre volume fraction and a uniform laminate thickness. During the first 4 h, the bag remained attached to a vacuum pump to eliminate any air bubbles existing in the laminate. Finally, the post-cure was performed according to the manufacturer’s datasheet in an oven at 40 °C for 24 h. This procedure was used to produce laminates with overall dimensions of 200 × 150 × 2.10 mm3.
The samples used in this study were cut and tested using the same equipment used for the epoxy matrix, from the previously produced boards with dimensions of 60 × 10 × 2.10 mm
3. The 3PB tests were carried out at room temperature with a displacement rate of 2 mm/min and, for each condition, at least five samples were tested in accordance with European standard BS EN ISO 178:2019 [
10]. The distance between supports used for all configurations was 35 mm.
During the manufacture of the toe cap, also using the hand lay-up method, five small glass fibre layers (35 × 60 mm) were prepared and laminated to reinforce the tip of the toe cap, according to the model followed,
Figure 5c.
Regarding interlaminar shear strength (ILSS) tests, the short beam shear method is the simplest and most widely applied. The interlaminar shear tests were carried out in accordance with ASTM D2344/D2344M-22 [
11] at a speed of 1 mm/min [
12]. In this study, at least five samples were considered valid. Standard-sized samples of the composites with a rectangular shape were prepared, i.e., length of 12 mm, width (
w) and height (
h) of approximately 4 mm and 2 mm, respectively, and distance between the supports in the specimens of 10 mm, all as a function of the height (
h) of the laminate.
Figure 5b shows the schematic view of the tests and the respective dimensions of the samples used in the experimental tests. In this method, the specimen is placed between two supports and a load is applied in the centre at a third point, causing the layers to slide relative to each other until failure occurs.
The failure mode that was sought was a shear failure between the laminate layers. The maximum load at which the failure occurs was recorded and, using the dimensions of the specimen, the ILSS was calculated.
ILSS is a critical mechanical property of composite materials that measures the material’s resistance to interlaminar shear forces. It is an important parameter that characterises the bond strength between adjacent fibre layers and the polymer matrix in a composite laminate. If the value is low, it may indicate poor bonding between layers or other problems with the composite manufacturing process. It should be noted that ILSS is a critical parameter for the practical case, as the toe caps will be subjected to high loads and there are concerns about the potential for delamination under service conditions.
2.3. Characterization of the Matrix, Laminates, and Toe Cap Properties
The dimensions of the specimens used in the static characterization 3PB tests were based on the BS EN ISO 178:2019 [
10] standard, which specifies a method for determining the flexural properties of a thermoset with a 50 mm matrix span and 35 mm laminate span (see
Figure 5a). A Shimadzu universal testing machine, model Autograph AGS-X, equipped with a 10 kN load cell was used.
The flexural strength was determined by calculating the rated stress in the mid-span section. This was achieved by using the maximum load value and applying Equation (1).
where
P is the load,
L is the distance between supports,
b is the width, and
h is the thickness of the specimen. The stiffness modulus was calculated using the linear elastic bending beam theory relationship:
Here,
I represents the moment of inertia of the cross-section, while ∆
P and ∆
u represent the load change and mid-span bending displacement change, respectively, for an interval in the linear region of the load versus displacement graph. The stiffness modulus was obtained by performing linear regression on the load–displacement curves, considering the interval in the linear segment with a correlation factor greater than 95%. Finally, the flexural strain was calculated according to the European Standard BS EN ISO 178:2019 [
10] by the following equation:
where
S is the deflection.
The short-beam shear (SBS) method is the simplest and most widely used for measuring ILSS. In accordance with the ASTM D2344/D2344M-22 [
11] standard, interlaminar shear tests were conducted using the same Shimadzu equipment. The ILSS value was obtained using Equation (4) with a crosshead speed of 1 mm/min on the Autograph AGS-X.
The maximum load is represented by Pc, while the width and thickness of the beam are represented by w and h, respectively. Five samples were tested under each condition at room temperature. The specimen’s nominal dimensions for ILSS tests are a length of 12 mm, with a width (w) and height (h) of approximately 4 mm and 2 mm, respectively. The distance between the supports in the specimens is 10 mm, which is dependent on the height h of the laminate.
The low-velocity impact (LVI) tests were performed using a drop-weight testing machine IMATEK-IM10, in accordance with EN ISO 20346:2022 [
13] and EN ISO 20344:2021 [
14] standards. An impactor with a diameter of 20 mm and total mass of 5.65 kg was used for the tests. The selected impact energy for the tests was 100 J. This specific energy level was chosen according to the standard to avoid inducing perforation in the specimens.
The equipment comprises two guiding columns, an impactor (guided by bearings), and a mechanism designed to prevent subsequent impacts. Gravity is the sole supplier of impact energy, which is regulated by adjusting the falling height, capable of reaching up to 3.50 m. Data acquisition is managed by a computer system, and the outcomes are processed and presented using Impact software (version 1.3; IMATEK). Impact force is measured through a piezoelectric load cell embedded within the impactor, capable of capturing 32,000 data points, with a precision level of 1% of the peak force. Deflection of the specimen is derived from double integration of the acceleration versus time curve. Each experiment involved testing five specimens at room temperature.
The Shimadzu model AG-IC universal testing machine with a 50 kN load cell was used for the compression tests. The main aim of this test is to assess the ability of glass fibre to withstand progressively increasing compression loads, thus guaranteeing the safety, durability, and structural integrity of the toe cap. During the test, the toe cap was subjected to an increasing force until it reached 10 kN, according to the EN ISO 20346:2022 [
13] and EN ISO 20344:2021 [
14] standards,
Figure 6.