Fatigue testing of steel profiles filled with polymer concrete for machine tool body applications

Problem statement: The advancements in machine tools to maximize the production by increasing spindle speeds have caused vibration in machine tools. The two functional requirements of machine tool bed for machine tools are high structural stiffness and high damping, which cannot be satisfied simultaneously if conventional metallic materials such as cast iron are employed. Hence there is a need to replace cast iron with alternate materials. Approach: The objective of this study is to improve the stiffness, natural frequency and damping capability of machine tool bed using a composite material containing welded steel and polymer concrete. Welded steel material has high stiffness but low damping and polymer concrete has high damping but low stiffness. So in this study, a machine tool bed made of sandwich structures of welded steel and polymer concrete is designed and manufactured. Modal and static analyses were conducted numerically and experimentally to determine the modal frequencies, damping ratio, deformation and strain. Results: The results at first mode showed that the natural frequency improved by 24.7% and damping ratio was 2.7 times higher than cast iron. The comparison of strain and deformation also showed significant improvement. Conclusion: This study proposed a hybrid welded steel bed as a replacement for cast iron as a machine tool bed material and the results showed that the static and dynamic characteristics were superior to cast iron.

Modelling and dynamic simulation of polymer concrete material for machine tool structure of a lathe

Polymer concrete (PC) overlays are typically used in infrastructure applications, specifically bridges and parking structures, to provide durable protection to the structural system. However, PC suffers from cracking and crack propagation during its service life mostly due to fatigue. Fatigue cracking of PC results in limiting the service life of PC considerably. Monitoring of fatigue damage in PC can help extend PC service life.In this paper, we demonstrate the possible use of carbon nanotubes to monitor damage initiation and propagation in PC under fatigue loading. PC prisms were produced using epoxy polymer concrete with varying contents of multi-walled carbon nanotubes (MWCNTs). The percolation level of MWCNTs necessary to produce conductive PC was first determined. Fatigue testing using an AASHTO modified test set-up was conducted. Electrical conductivity of PC overlay was continuously measured during fatigue testing. Damage initiation and propagation in PC incorporating MWCNTs overlays can be detected and monitored.

With its low thermal conductivity and high thermal capacity, polymer concrete has excellent thermal properties that when used in machine tools ensure increased accuracy of the manufactured parts. Polymer concrete has been used successfully in machine tool construction for many years in the form of machine beds. The aforementioned thermal properties in combination with a low density, high damping and a lower primary energy requirement also make polymer concrete interesting for use as a structural component. However, the comparatively low tensile strength and creep tendency of the material pose a challenge here. One approach to increase the tensile strength of the material is the integration of prestressed carbon fibres into the material. In order to clarify the suitability of this hybrid material, its temperature behaviour is investigated in this paper. The focus is on the investigation of residual stresses that arise during heating, which result from the combination of the positive thermal expansion coefficient of polymer concrete with the negative thermal expansion coefficient of the carbon fibres. In addition, the flexural properties of pure polymer concrete and of pre-stressed fibre-reinforced polymer concrete are determined at different test temperatures within the scope of this paper. It is shown that the prestressing of fiber-reinforced polymer concrete leads to an improvement of the flexural strength and the stiffness of polymer concrete.

Optimization of the polymer concrete used for manufacturing bases for precision tool machines

Identification of stiffness distribution of fatigue loaded polymer concrete through vibration measurements

Energy efficiency and resource economizing are the drivers for the development of new types of material-hybrid design approaches for machine tools. Polymer concrete has been used for machine beds in machine tool design for many years. The good thermal and dynamic properties of the material are particularly convincing in this context. The good damping properties for structural components are also interesting, as this reduces for example tool wear and at the same time the high damping compared to steel structures has a positive effect on the surface quality of the machined workpiece. Current research in the field of structural dynamics is dealing with the substitution of steel and cast components with hybrid, actively preloaded polymer concrete parts. This allows the use of the positive damping properties of polymer concrete and the positive tensile strengths of the integrated fiber-reinforced structures for dynamically loaded machine components such as machine arms or machine stands. The focus of the study is to replace the arm of a bed-type milling machine, which is currently a welded design, with a component made of prestressed carbon fiber-reinforced polymer concrete. Based on the first results of the volume ratios of the structures, conclusions are drawn about the life cycle assessment (cradle to gate) of the components. The results will contribute to a design recommendation for the carbon fiber reinforcement in the polymer concrete arm to achieve a better structural efficiency on the one hand and a better life cycle assessment on the other.

High damping and high static stiffness are essential to improve the static and dynamic characteristics of machine tools. Traditionally, the structural parts of machine tools are usually made of cast iron and steel, which may lead to poor surface finish and inaccurate dimensions of finished products. Therefore, many scholars have investigated other alternatives of structural material for machine tools, such as concrete, polymer concrete and epoxy granite. Although epoxy granite has better damping properties, its structural stiffness (as measured by Young’s modulus) is only 1/3 of the gray cast iron. Thus, this study would introduce several steel bar designs into epoxy granite to improve the static stiffness and quantitatively review the performance by finite element analysis. The results showed that the equivalent static stiffness and natural frequency were about 12 to 20% higher than the structural material of cast iron. Therefore, the proposed finite element model of vertical machining center (VMC) column in epoxy granite could serve as a feasible alternative to achieve higher damping or static stiffness, as it was also more environmentally friendly to the manufacturing process.

Polymer concrete (PC) has been used successfully as bridge deck overlays and in machine foundations. In such applications, PC is subjected to cyclic loading and thus is prone to fatigue failure. In this paper, we introduce a new method for improving the fatigue strength of PC using carbon nanotubes. To overcome the challenge of fatigue testing of concrete, a four point flexural displacement control test borrowed from Asphalt standards in AASHTO was used. In this test the displacement was ramped up from zero to 1.2 mm then the PC specimen (25 × 25 × 200 mm) was cycled between zero and 1.2 mm using a sinusoidal signal with a frequency of 0.5 Hz. Guided by AASHTO specifications, failure is defined as 50 % reduction in stiffness. Four PC mixes were tested. These mixes incorporated neat epoxy, and epoxy including 0.5, 1.0 and 1.5 % multi-walled carbon nanotubes (MWCNTs) by weight of epoxy resin. Damage in PC due to fatigue was evaluated with time. The experiments showed the ability of MWCNTs to improve fatigue strength by 61 and 100 % for PC incorporating 0.5 and 1.0 % MWCNTs respectively. PC incorporating 1.5 % MWCNTs reached 50,000 cycles without experiencing fatigue failure showing improvement above 520 %. Microstructural analysis of PC was conducted using scanning electron microscope (SEM) to explain the ability of MWCNTs to significantly improve fatigue strength of PC.

Designing machine tool structures in polymer concrete

To maximize the productivity of precision products such as molds and dies, machine tools should be operated at high speeds without vibration. As the operation speeds of machine tools are increased, the vibration problem has become a major constraint of manufacturing of precision products. The two important functional requirements of machine tool bed for precision machine tools are high structural stiffness and high damping, which cannot be satisfied simultaneously if conventional metallic materials are used for bed structure because conventional high stiffness metals have low damping and vice versa. This paper presents the application of hybrid polymer concrete for precision machine tool beds. The hybrid polymer concrete bed composed of welded steel structure faces and polymer concrete core was designed and manufactured for a high-speed gantry type milling machine through static and dynamic analyses using finite element method. The developed hybrid machine tool bed showed good damping characteristics over wide range of frequency (η = 2.93–5.69%) and was stable during high speed machining process when the spindle angular speed and acceleration of slide were 35,000 rpm and 30 m/s2, respectively.

Polymer concrete is reported to have better mechanical properties than its counterpart, Ordinary Portland Cement (OPC) concrete. It is gaining increased popularity as a new construction material due to its high compressive, tensile and flexural strengths, short curing time, impact resistance, chemical resistance and freeze-thaw durability. It can be used to repair concrete structures, build slabs and beams of small cross sections and sleepers. A research program has been initiated to improve fundamental understanding of this material and to provide the knowledge required for its broad utilization. In this experimental program, two types of resins (vinylester and epoxy resin) combined with fly ash and sand were used to make polymer concrete mortar. The weight percentages used in the mix designs were selected after analyzing volumetric properties of sand. This paper presents and discusses the results from an investigation of uniaxial compressive stress-strain relationship of polymer based concrete. The effect of resin (binder), and fly ash contents on the compressive strength, flexural strength, split tensile strength and modulus of elasticity of vinylester and epoxy resin based polymer filler is reported. It has been found that epoxy resin based polymer concrete and vinylester based polymer concrete can achieve compressive strengths of 75MPa and 113MPa respectively. Vinylester polymer concrete showed 4% ultimate strain, while that for epoxy polymer concrete was 8%. Tensile strengths were as high as 15MPa for both types of polymer concrete. The results show that the polymer based filler materials are suitable for both compression and tensile loading situations.

Large engineering structures made of various materials, including concrete (e.g., bridges, dams, buildings, and multilevel car parks), steel (e.g., power towers, ships, and wind turbines), or others, are often subjected to severe vibration, dynamic, and cyclic loads, which lead to crack initiation, crack growth, and finally structural failure. One of the effective techniques to increase the fatigue life of such structures is the use of reinforced materials. In the meantime, environmental factors, such as corrosion caused by corrosive environments, also affect the fatigue behavior of materials. Therefore, the main purpose of this paper is to study the influence of corrosive environment on the high-cycle fatigue (HCF) behavior of concrete reinforced by epoxy resin. For this purpose, five corrosive environments with different intensities, including fresh air, water: W, sea water: SW, acidic: AC, and alkaline: AL, were considered and the laboratory samples of conventional concrete (CC) and polymer concrete (PC) were immersed in them for one month. Next, axial fatigue tests were performed under compressive-compressive loading with a frequency of 3 Hz on cylindrical specimens. Moreover, to achieve reliable results, for each stress amplitude, the fatigue test was repeated three times, and the average number of cycles to failure was reported as the fatigue lifetime. Finally, the stress-life cycle (S-N) curves of different states were compared. The results showed that polymer concrete can resist well in corrosive environments and under cyclic loads compared to the conventional concrete, and in other words, the epoxy resin has performed its task well as a reinforcer. The results of fatigue tests show that the load bearing range of 10 tons by CC has reached about 18 tons for PC, which indicates an 80% increase in fatigue strength. Meanwhile, the static strength of samples in the vicinity of fresh air has only improved by 12%.

Thin-walled, box section, steel beams are the basic components of machine tools, vibratory machines, production lines and other structures subjected to time-varying loads. Therefore, it is desirable to shape their dynamic properties. It can be achieved by filling these components with polymer concrete. The polymer concrete filling increases stiffness and damping of the steel structure. However, it also causes a significant increase in its weight. Therefore, it is reasonable to fill such beams only in selected regions in order to improve its dynamic properties while maintaining relatively low mass. The paper presents the finite element modeling of dynamic properties of thin-walled, box section, steel beams partially filled with polymer concrete. Obtained numerical results i.e., natural frequencies, mode shapes and frequency response functions were verified experimentally, showing the good agreement.

In the construction and infrastructure sector, efforts are being made to find faster and more efficient materials. Polymer concrete (PC) challenges traditional concrete with its fast setting, durability and abrasion resistance. While studies on PC strength are abundant in the literature, studies on the effects of resin amount on damping capacity are fewer than mechanical performance. In this paper, the effect of resin proportion on damping capacity is investigated by modal tests. PC mixtures in the production with different resin proportions (11‒19%) were poured into molds of 10x25x500 mm, using aggregates of up to 3.15 mm in size. After 14 days, the natural frequency and damping ratios of the specimens up to 1000 Hz were determined in modal tests. While the damping ratio (DR) decreased in resin contents up to 17%, the results of the specimens with 19% resin ratio increased. However, when the products with the same resin ratio are analyzed, the random distribution of the aggregate affects the damping capacity. The main reason of negative correlation between resin amount and DR is the filler amount in the mixture. Because of the production consistency, fluidization of all the mixtures is prevented by adding fillers. Therefore, the impact of the resin amount on DR is limited or even negative. Besides that, to compare measurement results finite element method (FEM) analyzes are conducted. It can be said that the natural frequencies are not suited well especially in high frequency ranges due to frequency dependent properties (visco-elastic) of PC.