Mechanical properties and damping characteristics of Egyptian granite-epoxy composite material

Vibrations generated during the operations of machine tools, especially at high-speed operation impact several issues in machined parts such as imprecision dimensions and a poor surface finish. This prompts research and studies into alternative materials for machine tool structures to provide considerable damping performance and acceptable stiffness compared to traditional materials. This paper deals with the experimental study of a developed granite-epoxy composite, made from waste granite and local epoxy as an alternative material for machine tool structures. A waste of Egyptian Red Aswan granite was used as filler after being crushed and sifted into three sizes: fine (less than 1 mm), medium (1 to 5 mm), and coarse (5 to 8 mm). A local commercial epoxy resin kemapoxy 150 was added to a granite aggregate mixture having grain proportions 50:25:25 for fine, medium, and large, respectively. The influence of the variation of the epoxy weight ratio on the static and damping characteristics of a proposed granite–epoxy composite material was experimentally investigated. To ensure a coherent granite-epoxy composite, the required minimum resin content of 13.88 wt% was determined, and the granite/epoxy ratios were selected as 85:15 wt%, 80:20 wt%, and 75:25 wt%. The findings exhibit that the largest compressive strength of 76.8 MPa and the greatest flexural strength of 35.4 MPa is achieved at the highest epoxy weight ratio of 25%. The largest damping ratio of 0.0202 is observed at the epoxy ratio of 20% and it decreases to 0.015 when the epoxy ratio is increased to 25%. An Egyptian granite-epoxy composite, made from waste granite and local epoxy, is a promising alternative material for machine tool structures. It offers both economic and environmental benefits, along with high mechanical and damping properties compared to traditional machine tool materials.


Introduction
Vibrations of the structures of the machine tool, especially at high operating speeds, cause positional errors which affect the accuracy of the dimensions and surface finishing quality of the machined parts.Machine tool structures made from metallic materials such as cast iron and steel normally have neither high stiffness nor good damping properties.Consequently, it is vital to develop other materials that have good damping and stiffness properties.Polymer concretes using unsaturated polyester and epoxy resins have significantly higher compressive, split-tensile, and bending strengths compared to traditional cement concrete [1].The curing time has the highest impact on predicting the damping ratio of the polymer concrete beside to the volume fractions of the resin matrix [2].Due to its considerable damping ability and lower cost, it is a prospective material to enhance the damping performance for machine tool structures such as bases and columns [3][4][5][6][7].One of the preferred choices for materials of the machine tool structures is Granite-epoxy composite due to its high damping ability and lightweight compared to traditional metals [8,9].Arjun et al [10] enhanced the natural frequencies of the steel-reinforced epoxy granite column of the machine structure by 20% utilizing the Topology optimization approach and finite elements analysis.Shanmugam et al [11] investigated the effect of granite mass ratio and particles on static and dynamic performances of granite-epoxy composite using the Experimental analysis and optimization method (TOPSIS).Chinnuraj et al [12] concentrated on the design and investigation of a granite-epoxy CNC lathe bed reinforced with several types of steel instead of cast iron bed to improve the dynamic and static performance of the lathe.The reinforced granite-epoxy composite with steel for the lathe bed improved the dynamic characteristic by 4%-10% with improved stiffness and mass reduction of 22% compared to a cast iron bed.Selvakumar and Mohanram [13] investigated the damping performance of granite-epoxy composite structures.They reported that the damping ratio for the granite-epoxy structure increased by about 22% and 15% compared to cast iron and steel structures, respectively.Balakrishna et al [14] studied the characteristics of granite-epoxy composite and granite-epoxy with added 5, 10, and 15% cast iron particles as a filler.It was noticed that as the filler ratio increased, the damping ratio, tensile, and flexural strengths decreased, while the compressive strength rose.Shanmugam et al [15] studied the granite-epoxy composites with varying ratios of granite to epoxy and different aggregate sizes.The granite-to-epoxy ratio of 80:20 wt% with a 50:50 weight percentage of small and coarse granite aggregate was the ideal composition.Ubale et al [16] conducted an experimental study of granite-epoxy composite for particle size ranges from 0.5-3, 3-5, and 5-8 mm for coarse particles, and lower than 0.5 mm for fine particles were used with 90:10 wt% granite to epoxy.The values of compressive strength were 82 to 92 MPa, flexural strength was 21 to 27 MPa.Venugopal et al [17] introduced assorted designs of steel reinforcement in the epoxy-granite composite to enhance its stiffness and dynamic performance utilizing experimental and numerical methods.Krishna et al [18] investigated the variance of impact strength, tensile strength, and chemical resistance for various granite/epoxy ratios.The maximum impact strength of 56 MPa and the highest tensile strength of 26.13 MPa were recorded at a granite-epoxy ratio of 50%.Singh et al [19] investigated the use of marble dust as a filler in polylactic acid and recycled polyethene terephthalate composites.They found that adding marble dust improved the stiffness modulus and the hardness of both materials.Faizah and Aminullah [20] proposed an alternative approach for calculating the damping factor of concrete or mortar materials using a clear and easy-to-use method.Chuang et al [21] used the granite composite Material instead of cast iron for the machine tool beds.The tests showed increasing in the natural frequency, making it more resistant to vibrations and improving overall machine stability.Omar et al [22] investigated the influence of the epoxy content on the compression and flexural strengths of the granite-epoxy composite for granite/epoxy ratios of 80:20 wt% and 85:15 wt%.The maximum strengths of the material observed at 20 wt% epoxy ratios.Abdellah et al [23]developed a lightweight and cost-effective granite-epoxy composite utilizing low epoxy content and granite particles of different sizes.The results indicate the use of finer granite particles enhances vibration damping.The developed granite-epoxy composite materials exhibit mechanical properties and a specific density similar to that of light metals, such as aluminum and its alloys [24].Earlier research shows that the size of granite particles and the ratio of granite to epoxy are principal factors in determining the properties of a granite-epoxy composite.
This study deals with an experimental investigation into the static and dynamic characteristics of a graniteepoxy composite material made from waste Egyptian Red Aswan granite and local epoxy, as a substitute material for machine tool structures.The granite aggregate used in the study consisted of particles of varying sizes: 50% fine, 25% medium, and 25% coarse.To determine the optimal granite-to-epoxy ratio, specimens were fabricated with ratios of 85:15 wt%, 80:20 wt%, and 75:25 wt%.Compression, flexural, and free vibration tests were conducted on the specimens following ASTM standards.Moreover, SEM images were used to illustrate the various failure modes observed during the experiments.

Materials and specimens preparation
The granite rubble was obtained from the locally available granite industries at Aswan city in the south of Egypt.The crushed Egyptian granite grains were sifted into three different sizes, fine (less than 1 mm), medium (1 to 5 mm), and coarse (5 to 8 mm).A mixture of granite aggregate consisting of 50% fine, 25% medium, and 25% coarse sizes was used as a reinforcement.The matrix material is based on commercial epoxy (kemapoxy-150) resin with a density (1.11 g cm −3 ) at 25 °C.Kemapoxy-150 consists of two components solvent-free and liquid epoxy compound, and the curing agent used was cycloaliphatic polyamine.
Granite waste produced from quarrying and mining granite rocks is collected, crushed, and sifted to obtain granite particles.Firstly, a Stone Jaw Crusher is used to break down granite slabs into aggregates that are either less than or equal to 8 mm.Subsequently, these aggregates are crushed into smaller ones using a Denver Roll Crusher (Denver-Metso Roll Crusher -KLM Mining Inc.), while a Disk Mill (Roller Mill -DENVER -Single Pair-KLM Mining Inc.) is used to grind them into fine granite aggregates.Throughout each crushing step, the granite aggregates are sifted and classified into three groups based on their grain size: fine grain (0.150 to 1 mm), middle grain (1 to 5 mm), and coarse grain (5 to 8 mm).Finally, the particles are graded using sieve sizes ranging from #2½ to #16 on the Tyler mesh scale.The resulting aggregates are then washed thoroughly in water to remove any dust and foreign particles before being left out in the sun to dry for a day.It was shown that this process effectively produces high-quality granite aggregates with uniform grain sizes and few foreign particles.
Photographs, SEM images, and particle size distributions are illustrated in figure 1.An image analysis method is employed to characterize the particles, measuring their aspect ratio (W/L) and roundness factor (P 2 /4πA).Where, W is the particle width, L is the particle length, A is the projected area of the particle, and P is its perimeter.The statistical values for the parameters of the granite particles are listed in table 1.
The minimum resin content (MRC%) is calculated to identify the suitable epoxy weight ratio to prepare the granite-epoxy composite.Where; And the void volume percentage (VV)% can be calculated as: Dry apparent density of particles Density of granite 100 2 The utilized granite density is r granite = 2.653 g cm −3 , the utilized resin density is r risen =1.11 g cm −3 and the dry density of granite particles is equal to 1.915 g cm −3 .Consequently, the minimum risen content MRC% required to occupy the void volume is 13.88 wt%.
For a homogeneous and consistent blend of granite and epoxy and, the hardener and resin in a 1:2 ratio are mixed at a speed of 300 RPM for about 5 min.The mixture is further mixed for 15 min after adding the granite aggregates.The mixture was poured into an acrylic mold with a wall thickness of 10 mm.Two molds were fabricated using a CNC laser cutting machine: one for flexural specimens and the second for compressive specimens.The mold for flexural specimens has dimensions of 50 mm × 50 mm × 300 mm, the mold for compressive specimens has dimensions of 50 mm × 50 mm × 50 mm, and vibration damping test specimens utilize a mold measuring 10 mm × 50 mm × 450 mm.Each mold was designed to produce three standard test specimens at the same time.To remove any air voids, the mold is vibrated on a shaking table for approximately 20 min.The specimen is then cured for 24 h before taking from the mold and allowed to be set for an additional 7 days to achieve maximum hardness.The final fabricated granite-epoxy composite specimens according to ASTM standards are displayed in figure 2.

Compression and flexural tests
According to ASTM-579-01 Method B, the compressive tests have been performed out on the Universal Testing Machine (UTM-Machine model 15376/1956, with a maximum load of 40000 kg).The test specimens have flat, smooth faces, a cubic shape, and a side length of - + 50 3.0 1.5 mm.All faces are perpendicular to each other with a tolerance of ±0.5°.The granite-epoxy specimens are continuously and gradually loaded at the rate of 41 MPa min −1 without any shocks until the failure takes place thereby the compressive strength is calculated.
The Universal Testing Machine (UTM-Model type 17449, with a maximum load of 10,000 kg) was used to conduct the flexural tests in accordance with ASTM-C580-02 -Test Method B. The test specimens have a square cross-section with a side of - + 50 3 3 mm and a length of 300 mm.They are loaded using the three-point bending method with a span of 254 ±3 mm.The crosshead speed of the UTM is determined by equation (3) to achieve a  Where; L represents the span (mm), and d denotes the depth of (mm) of the granite-epoxy specimen being tested, Therefore, from equation (3) the applied cross speed of the head is equal to 2.15 mm min −1 .
The average flexural strength for each epoxy ratio is obtained by testing three specimens and applying the following equation.
Where; S represents the flexural strength (MPa), and F denotes the maximum load at or before the moment of break or crack (N).
The flexural modulus E bend of the granite-epoxy composites can be calculated as: Where; d = The maximum deflection of the beam (mm).

Damping test
The vibration-damping characteristics of the granite-epoxy composite are assessed via a free vibration test performed following the ASTM E756-05 method [25].In this case, the test specimen was 450 mm in long, 50 mm wide, and 10 mm thick set as a cantilever beam by fixing one end on a rigid base using a fixation clamp and the other was kept free.An accelerometer with a sensitivity of 101.1 mV g −1 was placed at the free end of the cantilever beam where the highest vibration occurs.To initiate the oscillation a small static initial displacement is applied on the free end of the beam, bending it from its equilibrium position.Once released, the specimen oscillates about its equilibrium position without any external force.The process is repeated multiple times to ensure accuracy During the experiment, the data acquisition (DAQ) card collects signals from the accelerometer and the specimen response is analyzed using Fast Fourier transform (FFT) graphs by LabVIEW. Figure 3 depicts the experiment and the measurement setup for the free vibration test.
The damping ratio (ζ) is used to analyze the damping characteristics of the granite-epoxy composite, which is calculated by obtaining the logarithmic decrement.The data of specimens recorded during a free vibration test is utilized to determine the logarithmic decrement (δ) as following [26]: In which X 0 is any amplitude of the granite-epoxy specimen response, and X n is the amplitude of the specimen repose wave after N of complete oscillation cycles.Thereby, the damping ratio (z ) can be determined as following [27]:

Results and discussions
Three specimens are tested at each epoxy weight ratio for compression and flexural tests.Average stress is calculated, and any specimen with deviation greater than 15% is excluded.Figure 4 displays the compression test results for specimens with various granite-epoxy weight ratios.It can be observed that the curves follow a smooth trend, this is an indication of high consolidation and bonding of the composite components.When the epoxy ratio is low (15% wt), the bonding between the granite grains is deficient, leading to the reduction of the material strength.In this case, most of the load is conveyed by granite.Increasing the epoxy weight ratio to 20% and 25% by weight enhances the adhesive properties of the granite grains, leading to a significant improvement in the material's strength.Referring to figure 5, it can be observed that the mean compressive strength value reaches 61 MPa when the granite/epoxy ratio is 85:15 wt%.Moreover, there is an increase of about 16.8% in the mean compressive strength value when the epoxy weight ratio is raised from 15% to 20% and a about 26% increase when the ratio is further raised to 25%.This is due to the improved bonding between the granite particles at higher epoxy ratios while specimens with only 15% epoxy had loosely bonded particles, resulting in a reduction of the maximum compressive strength.Figure 6 displays the failure modes of the compressive specimens.It is noticeable that the failure model closely resembles the semi-failure mode [28,29].Cracks initially appear at the middle heights of the specimens and parallel to the load direction.Subsequently, they spread towards the corners at an angle of approximately 60 degrees to the horizontal until the specimen fractures.
The variance of the deflection of the Egyptian granite-epoxy composite with the flexural stress for different ratios of granite-epoxy is shown in figure 7. Flexural stress and displacement variation for Egyptian graniteepoxy composite for different granite/epoxy ratios.It is evident that the trend of specimens with epoxy weight ratios of 15 wt% and 20 wt% is the same and the specimens fail rapidly with a little deflection due to the less bonding between granite grains.On the other hand, the specimens with the epoxy ratio of 25 wt% exhibit the highest breaking deflection and flexural strength.Figure 8 illustrates the variation of flexural strength with a weight ratio of epoxy.The highest flexural strength of 35.4 MPa is attained when the specimens have a granite/epoxy ratio of 75:25 wt%, which is an improvement of about 63.6% and 78.2% compared with the flexural strength of about 21.6 MPa and 19.8 MPa achieved at a granite/epoxy ratios of 85:15 wt% and 80:20 wt% respectively.The flexural modulus was calculated using equation (5) and its values at different granite/epoxy ratios were listed in table 2. The minimum flexural modulus value of 2.3 GPa occurs for the specimens having the granite/epoxy ratio of 75:25 wt% and those with a ratio of 85:15 wt% and 80:20 wt% have the flexural modulus values of 3.8 GPa and 4.4 GPa respectively, where the flexural modulus is proportional to both strength and deformation.This indicates that epoxy resin with strong adhesive properties can enhance strength with a slight deformation change.Therefore, increasing the epoxy content to 25 wt% in a granite-epoxy composite leads to the highest strength but does not reduce deformation.A brittle fracture mode appears for all specimens in the flexural test as demonstrated in figures 9 and 10.The crack starts from the tension face (lower face) and progressively spreads upward to the point where the load is applied.The failure takes place in the middle third portion of the specimens.
Figure 11 illustrates a comparison of the compressive and flexural strengths of the proposed Egyptian granite-epoxy composite material with cement concrete [30], and polyester concretes [1].The comparisons demonstrate that the Egyptian granite-epoxy composite exhibits superior mechanical properties compared to cement concrete and higher strengths than polyester concretes.This suggests that the proposed granite-epoxy composite has a low occurrence of pores, and the granite particles and epoxy resin have a strong interaction.Figure 12 illustrates the measured signal of the free vibration response of the granite epoxy specimen over time.The unwanted noise present in this measured signal is removed using a digital-filtering process and the resultant time-acceleration matrix is converted into the frequency domain using FFT available in Origin software as depicted in figure 13.
The displacement response over time is obtained by double integrating the filtering acceleration data using Origin software.To ensure accuracy, the experiment is repeated five times for each specimen.Figures 14-16 show the free vibration responses of the granite-epoxy composite in the time domain for different granite/epoxy ratios of 85:15 wt%, 80:20 wt%, and 75:25 wt% respectively.The logarithmic decrement and damping ratio were determined according to equations (6) and (7).Finally, the average damping ratio was calculated for each epoxy weight ratio.Figure 17 demonstrates the variation of the average damping ratio on the granite-epoxy composites with the granite-to-epoxy ratio.It can be noticed that the damping ratio value reaches 0.016 when the granite/epoxy ratio of 85:15 wt%.It increases to 0.0202 when   the epoxy weight ratio is raised to 20%.Moreover, there is a decrease of about 24.5% in the damping ratio value when the epoxy weight ratio is raised to 25% and reaches 0.0152.
Figure 18 illustrates the damping ratio of the proposed Egyptian granite-epoxy composite material compared to common materials used in machine tool structures such as cast iron [3] , steel [31] , and polyester concrete [6].The comparison indicates that the proposed Egyptian granite-epoxy composite exhibits superior damping performance compared to steel and cast iron, and higher damping compared to polyester concrete.This demonstrates the potential advantages of using the proposed granite-epoxy composite to attenuate the machine tools' vibration, eventually leading to improved quality of the machined parts.
Referring to figure 19 it can be noticed that, with the epoxy ratio of 15 wt% the particles of the granite form a homogeneous and bonded firmly to the matrix.By increasing the epoxy resin content in the mixture to 20% and 25% weight resin content, the aggregate particles were less densely packed, and the resin formed a puddle, that is a thick layer of resin was found between the particles.Also, the granite particles were noticed to be well separated by the resin as shown in figures 20 and 21.

Conclusions
Static and dynamic behaviors of the Egyptian granite-epoxy composite were experimentally investigated.The granite-epoxy composite specimens were fabricated with a mixture of granite grain sizes of 50% fine, 25% Medium, and 25% coarse.The compressive, flexural strengths and damping ratio were evaluated at epoxy resin content of 15%, 20%, and 25% in weight.The findings can be concluded as follows: 1.The greatest compressive strength of 76.8 MPa occurred at the epoxy weight ratio of 25 wt% increasing by about 26% and 7.7% than the recorded values at the epoxy ratio of 15 wt% and 20 wt% respectively.
2. The highest flexural strength of 35.4 MPa is attained at the epoxy ratio of 25 wt%, increasing by about 63.6% and 78.2% to the recorded values at the epoxy ratio of 15 wt% and 20 wt% respectively.
3. The Egyptian granite-epoxy composite demonstrates greater strength than other traditional materials used in machine tool structures 4. The recorded vibration damping ratios range from 0.015 to 0.0202, exceeding those reported for traditional materials used in machine tool structures.This superior performance can be attributed to the internal friction between the granite aggregate and the epoxy resin, which effectively dissipates vibration energy.
5. The good damping characteristics of the proposed Egyptian granite-epoxy composite demonstrate the potential advantages of attenuating the vibration of the machine tool's structures, eventually leading to improved quality of the machined parts.
6.The results indicate that the size ratio of granite grains archives good interfacial contact and dense packing of the granite aggregate.Additionally, the fine particles of granite fill the void spaces between the bigger particles.
7. The proposed granite-epoxy composite material produced with Egyptian granite residue and local epoxy resin can render Egyptian granite waste a valuable, potentially profitable material, and help in reducing environmental pollution.

Figure 1 .
Figure 1.Photographs, SEM image and particle size distributions.

Figure 2 .
Figure 2. The manufactured specimens of Egyptian granite-epoxy composite for conducting compression and flexural tests.

Figure 5 .
Figure 5. Variation of the maximum compressive strength with the epoxy weight ratio.

Figure 6 .
Figure 6.Failure mode of the Egyptian granite-epoxy composite specimens from the compression test.

Figure 7 .
Figure 7. Flexural stress and displacement variation for Egyptian granite-epoxy composite for different granite/epoxy ratios.

Figure 8 .
Figure 8. Variation of the maximum flexural strength with the epoxy weight ratio.

Figure 9 .
Figure 9. Failure of the Egyptian granite-epoxy composite specimen under three-point bending.

Figure 12 .
Figure 12.The measured signal from free vibration test of the granite-epoxy specimen.

Figure 13 .
Figure 13.Fast Fourier Transform (FFT) spectrum of the filtered signal.

Figure 14 .
Figure 14.Free vibration decay at granite for epoxy ratio 15 wt%.

Figure 17 .
Figure 17.Variation of damping ratio for granite-epoxy composite with epoxy weight ratio.

Table 1 .
The statistical values of particle size and shape for utilized Egyptian granite.
strain rate of 0.01 ±0.001 per minute at the bottom and top of the testing beams.