Ball screws are robust and economical linear positioning systems widely employed in high-speed and high-precision machines. Due to precision and stability requirements, the preload force is ...considered one of the main parameters defining the axial stiffness and the maximum axial load of the ball screw feed drives. In high-speed motions, thermal effects are also considerably relevant regarding positioning precision and dynamic stability of the machine. The temperature increase and the thermal gradient between the screw, the balls and the nuts result in geometrical variations and, consequently, variations in the preload force. This paper presents a numerical modelling strategy to predict the preload variation due to temperature increase using a thermo-mechanical 3D finite element method (FEM)-based model for double nut-ball screw drives. Two different thermo-mechanical coupling strategies are compared, and the obtained results are validated with experimental measurements for different initial preload and linear speeds. In the mechanical analysis, the nut-screw ball contact interface, the offset-based preloading and the restrictions of the ball bearings are included in the model, while the thermal analysis considers heat generation and heat diffusion. The causes of the thermal preload variation are discussed considering the ball load distribution and the axial and radial thermal displacements of the contacting points.
The mechanical behavior of an AZ31
Mg sheet has been investigated at high strain rate (10
3
s
−1) and compared with that observed at low rates (10
−3
s
−1). Dynamic tests were carried out using a ...Hopkinson bar at temperatures between 25 and 400
°C. Tensile tests were carried out along the rolling and transverse directions and compression tests along the rolling and the normal directions in both strain rate ranges. The tension–compression yield asymmetry as well as the yield and flow stress in-plane and out-of-plane anisotropies were investigated. The microstructure of the initial and tested samples was examined by electron backscatter diffraction. The dynamic mechanical behavior is characterized by the following observations. At high temperatures the yield asymmetry and the yield anisotropies remain present and twinning is highly active. The rate of decrease in the critical resolved shear stress of non-basal systems with temperature is smaller than at quasi-static rates. Rotational recrystallization mechanisms are activated.
Magnesium alloys are an ideal candidate due to their low density in comparison to aluminum and steel alloys when designing a vehicle with lower weight and therefore, reduced fuel consumption. It is ...important to characterize the strain rate sensitivity of any material that will be used in a structure which can undergo high rate deformation (as in an automobile crash) as well as during high velocity forming processes such as electromagnetic or electrohydraulic forming. Tensile tests for AZ31B magnesium alloy sheet at different strain rates were carried out using different testing techniques: (i) quasi-static strain rates tests were conducted in a range between 10
−3 and 10
−1
s
−1 using a conventional electro-mechanical tensile testing apparatus; (ii) intermediate strain rates tests at 4.0
×
10
1 to 10
2
s
−1 using an instrumented falling weight apparatus; and (iii) high strain rates at 0.5
×
10
3 to 1.5
×
10
3
s
−1 using a tensile split Hopkinson bar. Furthermore, quasi-static and high strain rate tests were also performed for different temperatures, from room temperature up to 250
°C. Strain rate and temperature effects are also discussed for rolling and transverse direction, to identify the variation of sheet properties with loading direction. Finally, the constitutive fitting of the stress–strain curves to the widely employed Johnson–Cook material model equation is evaluated and also a new model is proposed based on a modified J–C model to account for the variation of strain hardening with strain rate.
Radial fretting is defined as the damage caused by the relative motion of a ball under a variable normal load. The conducted experimental tests proved the existence of this damage even in contacting ...bodies with similar materials and elastic properties; however, the reviewed analytical methods do not predict damage under these conditions, and the numerical methods are time-consuming, especially when the investigation requires the analysis of several loading cases or during large number of cycles. The main objective of this work was to develop a fast analytical method to analyse radial fretting wear. For this purpose, a novel formulation for contact displacements under a normal variable load was built and merged to different contact mechanic formulas. For validation purposes, false brinelling in thrust bearings was used as a representative industrial example. The results were compared with a FEM model showing a relative difference under 9% and a massive reduction of the calculation time of more than 30 000 times, moreover experimental tests were carried out, showing a good agreement between the density of friction energy and the obtained fretting damage what endorse the use of this formulation for the analysis of a large number of cycles and loading cases.
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•An analytical new formulation for radial fretting wear simulation.•An experimental and numerical validation of the proposed methodology.•An industrial application to false brinelling in stationary thrust bearings under variable loading conditions.•An alternative fretting method to FEM simulations with a massive time reduction of 30 000 times.
In the present paper a numerical model for predicting the crushing behaviour of semi-hexagonal E-glass/polyester composite structures has been developed. Qualitative and quantitative analysis have ...shown that the results of the simulation are accurately predicted comparing with the experimental data. The peak force has been predicted with 7.5% of error while the mean force of the crushing process, the total amount of absorbed energy and the specific energy absorption capability have been simulated within 1% of error. Moreover the effect of the wall angle of the semi-hexagonal section and the effect of the overall size of the semi-hexagonal section have been numerically analyzed. The crushing process becomes stable when the wall angle is higher than 50° and the highest specific energy absorption values are obtained using the wall angle of 60° and wall length of 10 mm. Higher wall angles and wall lengths increases the stress concentration in the edges of the semi-hexagonal section and in consequence, the load carrying capability of the structure decreases dissipating less energy.
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•E-glass fibres have shown the highest specific energy absorption values; 30kJ/kg.•Bidirectional E-glass fibres dissipate more energy by axial splitting.•Unidirectional E-glass fibres ...dissipate more energy by delamination.•Bidirectional basalt fibres have shown unstable collapse due to buckling.•Hybrid E-glass/basalt configuration has shown stable and progressive collapse.
Feasibility of basalt fibre composites for automotive crash applications has been analysed and compared with E-glass fibre composites in the present study. Semi-hexagonal composite structures with different stacking sequences of E-glass and basalt fibres have been tested in quasi-static crush conditions and specific energy absorption (SEA) capability has been calculated. Samples with unidirectional and bidirectional E-glass fibres have shown the highest SEA values, around 30kJ/kg. However different deformation mechanisms of energy absorption have been observed. On the other side, basalt/polyester samples have exhibited unstable collapse due to buckling effect during crushing process. Therefore the crush behaviour of the present basalt/polyester configuration is not suitable for automotive crash applications. Regarding the hybrid configuration using E-glass and basalt fibres with polyester, it has been shown that unstable collapse problems of basalt/polyester have been avoided. However the SEA values for this configuration are lower comparing with those values obtained by E-glass/polyester configurations. Adhesion between fibre plies of different material has caused delamination with low energy absorption capability.
Semi-hexagonal glass/polyester composite structures with different fibre volume fractions have been studied for automotive crash applications. Interlaminar shear strength and specific energy ...absorption capability of the material have been characterised in order to analyse the effect of the fibre content. Samples with different fibre content among 40% and 60% have shown similar interlaminar shear strength values, around 35–40 MPa. It has been found that by increasing the fibre percentage from 40% to 47% the specific energy absorption values of the material increased to 56 kJ/kg. For specimens with fibre volume fraction above 47%, the total amount of energy dissipated is similar. Increasing fibre content increases the linear density of the material and in fact, the same value of dissipated energy quantities with a higher linear density implies a decrease in the specific energy absorption values.
The microstructural evolution of an AZ31 rolled sheet during dynamic deformation at strain rates of ∼10
3
s
−1 has been investigated by electron backscatter diffraction, X-ray and neutron ...diffraction. The influence of orientation on the predominant deformation mechanisms and on the recovery processes taking place during deformation has been systematically examined. The results have been compared with those corresponding to the same alloy tested quasi-statically under equivalent conditions. It has been found that strain rate enhances the activation of
{
1
0
1
¯
2
}
extension twinning dramatically, while contraction and secondary twinning are not significantly influenced. The polarity of
{
1
0
1
¯
2
}
extension twinning is even reversed in some grains under selected testing conditions. Significant grain subdivision by the formation of geometrically necessary boundaries (GNBs) takes place during both quasi-static and dynamic deformation of this AZ31 alloy. It is remarkable that GNBs of high misorientations form even at the highest strain rates. The phenomenon of recovery has been found to be orientation dependent.
In this work, the impact behaviour of an AZ31B-H24 magnesium-based fibre metal laminate (FML) and a 2024-T3 aluminium-based FML with self-reinforced polypropylene (SRPP) is analysed. The study ...focused on determining the impact properties of the FMLs and explaining the differences between their impact responses. Additionally, in the comparative study, a single SRPP was included as reference base material. Low-velocity impact tests were performed by using a drop-weight machine. The failure energy for cracking of metal sheets, onset of fibre breakage in the composite constituent and total perforation of the laminate were identified by correlating force–time curves measured from impacts with images of impacted specimens obtained by a macroscope. Moreover, the ‘energy profile method’ was used to identify the penetration and perforation thresholds enabling the definition of the no-penetration and the penetration phases. The results revealed that the Al-FML presented more specific resistance and energy dissipation capacity under impact than the Mg-FML.
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