The aim of the current study is to examine the mechanical properties of pure sheep wool fabric through experiments. In our country, the fibers clipped from sheep grown are obtained from naturally produced material that is turned into fabric. The chopped sheep wool in fiber form is shown in Fig. 1a, and the sample made into fabric is shown in Fig. 1b. Composites were created by combining sheep wool fabric with epoxy-based materials.

The average technical characteristics of the wool of some sheep breeds raised in our country are shown in Table 1.

Sheep wool is a natural material used in various applications in the automotive industry. This soft, durable and insulating natural fiber is used to make some auto parts and interiors [34]. For this reason, it was preferred to be used in the study.

As the layer material, 20 mm thick XPS material complying with TS EN 13164 + A1 and En 13163 standards was preferred. They are foam-based materials, which are a closed-cell material with low density and high rigidity [41]. The properties of XPS foam are given in Table 2.

This material has low moisture absorption, compressibility, compressive creep, dimensional stability at a certain temperature, thermal conductivity and the ability to be shaped by heat. Additionally, its chemical resistance is quite good. Light and rigid layers of this type are commonly used as core materials. Layered foams are used in many areas, especially bumpers, in the automotive industry [44]. Safety is an important factor in automobiles. Thanks to their impact absorption properties, XPS foams can help protect passengers by dissipating energy during a crash [45].

The manual laying method is used in many areas in technical studies in the manufacturing of composite materials. Parts produced by hand-laying method have features such as high strength, low weight and chemical resistance. Reinforcement material in the form of fiber or fabric is placed on the mold by orientation. For sample production, MGS LR 285 epoxy resin and LH 285 epoxy hardener were mixed in appropriate proportions. The matrix material with polymeric resin is applied on top of the reinforcement material. After the application layers were completed, the resin was left in a vacuum atmosphere until it cured using the vacuum bagging method. The curing process was waited for approximately two hours at room temperature.

Vacuum bagging is an effective method used to optimize resin and air flow in the production of composite materials. This method allows the desired vacuum level to be created initially via the vacuum pump. Vacuum method: Thanks to the vacuum applied to the resin injection lines, the resin spreads homogeneously to fill all gaps. It removes air bubbles and allows them to come out. Once these steps are completed, the vacuum pump is turned off. Afterwards, the resin dispensing lines are clamped to ensure that the resin is kept in the desired areas [46,47].

Vacuum bagging method is a technique used in the production of composite materials. This method aims to achieve a higher material quality by eliminating air gaps between the resin and the sheepswool fabric. The application of the method is shown in Fig. 2. In sample production, the core material and fibers are combined with resin and a brush (Fig. 2a). In the study, the materials were cured at room temperature for 2 h under 20 bar pressure using the vacuum bagging method (Fig. 2b). The cured samples were prepared by cutting (Fig. 2c) the impact sample to 100 mm × 100 mm (Fig. 2d) using the precision cutting method using a jig saw.

Damage images on the upper and lower surfaces of single and double-layer sheep wool natural fiber test samples after impact are shown in the figure. In the figure, it is clearly seen that the damage area differs in samples with single and double layers under 10 and 20 J impact energy. When the post-impact damage areas for 10 and 20 J in single-layer XPS samples are compared, it is seen that the bottom and top surface damage areas of these two samples are the same. At 10 J impact energy, 16 mm damage is observed on the front surface of the monolayer and 19 mm damage is observed on the back surface of the fiber and core material. At 20 J impact energy, 12 mm damage is observed on the front surface of the single layer and 14 mm damage is observed on the back surface of the fiber and core material. It can be seen that under 10 J energy, smaller damage occurred on the upper surfaces of the double-layer XPS sample than the others, and there was no damage on the lower surface. However, at 20 J impact energy of the same sample, fiber breaks and core material damage are observed on the upper and lower surfaces. At an impact energy of 10 J, no damage is observed on the front surface of the double layer and 12 mm on the back surface. At an impact energy of 20 J, 20 mm damage is observed on the front surface of the double layer and 24 mm damage is observed on the back surface of the fiber and core material. Fig. 4 shows the damage caused to the samples as a result of the impact test.

Fig. 5 shows the force-deformation graph of single and double layer samples with XPS core at 10 J energy. It was determined that at an energy value of 10 J, the upper layer of the double-layer sample was pierced, it could not advance to the lower layer and it bounced back as the foam absorbed the energy. In the single-layer sample, it completely pierced the top layer, damaged the core foam and remained stuck in the foam. While it resists a force of approximately 1500 N in the two-layer sample, it is around 800 N in the single-layer sample. The deformation was determined to be approximately 9 mm in the double-layer sample, and the deformation reached up to 12 mm in the single-layer samples.

Fig. 6 shows the force-deformation graph of single and double-layered samples with XPS core as a result of 20 J energy. As a result of the impact test, the first layer of the double-layer samples was completely pierced and the striking tip reached the second layer and stuck into the sample. The downward descent in the force graph is a result of the foam absorbing energy. Approximate deformation is around 16 mm and the maximum force reached up to 2500 N. In single-layer samples, the striking tip penetrated both layers. The advancement amount of the striking tip inside the sample is approximately 25 mm, and the maximum force resistance is around 500 N, as seen in the graph in Fig. 8.

In Fig. 7, the deformations of single-layer samples with XPS foam core under 10 and 20 J energy are compared. When the graph was examined, it was determined that the striking tip penetrated the single-layer samples under 20 J energy. The maximum force is around 600 N and the striking tip completely pierces the sample and the advancement amount is determined as 25 mm. In the test samples performed under 10 J energy, the tests resulted in stabbing. In the experiments, the maximum force was around 900 N and the deformation was determined as 12 mm.

In Fig. 8, the deformations of double-layer samples with XPS foam core under 10 and 20 J energy are compared. In XPS 20 experiments, the test resulted in stabbing in the sample. It was determined that at an energy value of 10 J, the upper layer of the double-layered sample was pierced, it could not advance to the lower layer and it bounced back as the foam absorbed the energy. In 20 J experiments, the maximum force was around 2500 N and the deformation was determined as 16 mm. In 10 J experiments, the sample resisted a force of approximately 1500 N and the deformation was determined to be approximately 9 mm.

ASTM C393-00 standard was used when applying this test method. According to this standard, the load applied to the samples advances 1 mm/min with the moving jaw [52,53]. Samples prepared in accordance with the standard were subjected to compression testing along their edges in the SHMADZU AUTOGRAPH AG-100 universal testing device. Images during sample testing are shown as in Fig. 9.

Compression testing of porous materials is a testing method used to evaluate how the material behaves under pressure. This test helps determine the amount of energy the material absorbs and its mechanical properties. The area under the compression test graph is an indicator of energy absorption. Similar to the data obtained in the impact tests, it was observed that the compressive strength of the two-layer samples was better than the single-layer ones.

Fig. 10 shows the XPS single and double layer force-deformation graph. When the graph is examined, when the compression test results on XPS core composite materials are examined, it is seen that the plastic destruction values are approximately 2200 N in XPS double layer samples and 1300 N in XPS single layer samples. Although the curves continued parallel to each other, they decreased slightly and continued horizontally, similar to the compression curves of porous structures. Due to the closure of the pores after approximately 50% deformation, the force increased and the experiment was terminated.

Three-point bending tests were examined in a single category, namely layer thickness. The samples were prepared in dimensions of 180 mm × 30 mm × 23 mm and 1 mm/min was selected as the device advance speed. In order to detect the breakage of the substrate in the experiments, a deformation of approximately 30 mm was applied to the samples (ASTM D790) [54]. Three samples from each layer were tested and the average of the results was taken and force-deformation graphs were drawn. Depending on the wool single-layer double-layer configuration shown in Fig. 11, the behavior of the beam under bending is shown in phase 3. When Fig. 11 is examined, separations and breaks in the core material and fiber layers were observed during the tests.

Fig. 12 shows the force-deformation graphs obtained from three-point bending tests of XPS single layer and XPS double layer For XPS-filled sheepwool, three separate test samples were used for each two layers. XPS single layer samples experienced first layer fracture with a deformation of approximately 8 mm and withstood a force of approximately 200 N. On the other hand, XPS double layer samples experienced first layer fracture with a deformation of approximately 12 mm and withstood a force of approximately 360 N. Based on these data, it is seen that XPS double layer samples have higher strength than XPS single layer samples.