Cutting Mechanical Study of Pick Cutting Coal Seams with Coal and Rock Interface

Shearer provide an effect solution for mining coal, and the cutting performance of pick largely accouts for the ability of a shearer and mining performance. We conducted pick cutting coal experiments in different seam forms on the coal and rock cutting teasted. According to the rotary cutting mechanical model of single pick cutting coal seam, combined with the strength condition of coal seam and coal-rock interface, the rotary cutting mechanical model of pick cutting coal seam with coal-rock interface is established. The stress strain and strength condition of the area in and around the interface are analyzed based on the coal-rock interface crushing theory, which provides basis for further research on the cutting mechanical model of single pick crossing the coal-rock interface. According to the analysis on the ampulitude domain, the phenomenon that force increment between the pick cutting rock and uniform coal seam linearly increases with the increase of compressive strength difference between coal seam and coal-rock interface, and the load fluctuation keep a positive correlation with the compressive strength. Analysis on the signals of the pick cutting coal seam with coal and rock interface at different conditions shows that some basic properties of the cutting load changes over times. In addition, the coal seam with coal-rock interface appears larger impact load and other time-domain characteristics.

Extended Residual-State Creep Test and Its Application for Landslide Stability Assessment

This paper contains the results of a newly developed residual-state creep test performed to determine the behavior of a selected geomaterial in the context of reactivated landslides. Soil and rock creep is a time-dependent phenomenon in which a deformation occurs under constant stress. Based on the examination results, it was found that the tested clayey material (from Kobe, Japan) shows tertiary creep behavior only under shear stress higher than the residual strength condition and primary and secondary creep behavior under shear stress lower or equal to the residual strength condition. Based on the data, a model for predicting the critical or failure time is introduced. The study traces the development of the limit state based on the contact model corresponding to Blair’s body. The time to occurrence of the conditions necessary for unlimited creep on the surface is estimated. As long-term precipitation and infiltrating water in the area of the landslides are identified as the key phenomena initiating collapse, the work focuses on the prediction of landslides with identified surfaces of potential damage as a result of changes in the saturation state. The procedure outlined is applied to a case study and considerations as to when the necessary safety work should be carried out are presented.

Predictive Variables of Adolescents’ Intention to Be Physically Active after Graduation. Is Gender a Conditioning Factor?

The acquisition of physical activity habits during adolescence is fundamental to guarantee its adherence throughout life. However, these levels decrease during this stage, with women experimenting a more significant decrease. This paper aims to analyze if there are significant differences in the variables related to the intention to be physically active between men and women and if there is a moderating effect of gender on the variables that predict this intention. The sample is composed of 256 adolescents, aged between 16 and 19 years, 55.50% of whom are men, and 44.50% women. The results show that there are statistically significant differences (p < 0.05), with boys showing higher scores in the intention, athletic identity, and in the strength, condition, and attractiveness. About the predictive variables of the intention, the gender moderates the relationships between the physical attractiveness, condition and strength with the perceived behavioral control (pcfmale-pcfmale = 0.44; −0.48 p < 0.001; 0.27, p < 0.05, respectively), and between the subjective norm and the intention to be physically active (pcmale-female = 0.33, p < 0.01). These results highlight the importance of considering gender when designing specific policies for the promotion of physical-sports practice among adolescents to reduce the existing gender gap.

Generalized mechanical-mathematical model for optimization of two-bearing shafts of minimal material consumption

A universal theory of solving the original problems of applied mechanics was developed to determine the rational geometric parameters of two-bearing three-stage shafts of the smallest volume with two nozzles, covering three modifications of the design under consideration. The optimization process model is brought to the calculation algorithm and is illustrated by a characteristic numerical example. Keywords: optimization, strength condition, minimum volume, extremum, objective function. [email protected]

Effects of post-activation potentiation exercises on kicking frequency, fatigue rate and jump performance in taekwondo athletes: a case study (Efectos de los ejercicios de potenciación post activación sobre la frecuencia de pateo, tasa de fatiga y saltabi

The aim of the present study was to establish the effects of three conditions of post-activation potentiation (PAP) exercises on kicking frequency, fatigue rate (FR) and jump performance in novice and advanced taekwondo athletes. Secondarily, to establish if the PAP strength plus plyometrics exercises produced a significantly higher increase with respect to exercises of isolated strength and plyometrics. Eight university taekwondo athletes (n=4 novices and n=4 advanced) were randomized into four (one control and three experimental) intervention conditions. Kicking frequency and FR were evaluated with the Frequency Speed of Kicks Test (FSKT), and countermovement jump test (CMJ). Effect size (ES) was calculated and the significance level was stabilized at p<0.05. Advanced taekwondo athletes obtained significantly higher results when compared to novices in the control condition for the fifth FSKT-10s (p=0.019; ES=2.382); in the strength condition for the second FSKT-10s 2 (p=0.028; ES=2.590); and in the strength plus plyometrics condition for the first FSKT-10s (p=0.037; ES=1.805) and third FSKT-10s (p=0.027; ES=2.117). Furthermore, the control condition showed a difference when compared to strength plus plyometrics: on the first (p=0.040; ES=0.552) and second FSKT-10s (p=0.032; ES=0.687), respectively. The FR and CMJ did not significantly differ between the athletes nor between the intervention conditions. In conclusion, the PAP exercises did not improve kick frequency, FR and jump performance in the taekwondo athletes evaluated. However, these exercises did not produce adverse effects in the FR; thus, they could be used in taekwondo athletes with previous strength training.Resumen. El objetivo del presente estudio fue establecer los efectos de tres condiciones de ejercicios de potenciación post-activación (PAP) sobre la frecuencia de pateo, tasa de fatiga (TF) y saltabilidad en atletas de taekwondo novatos y avanzados. Secundariamente, establecer si los ejercicios de PAP de fuerza más pliometría producen un aumento significativamente mayor respecto a los ejercicios de fuerza y pliometría aislados. Ocho atletas universitarios de taekwondo (n=4 novatos y n=4 avanzados) fueron asignados aleatoriamente a cuatro condiciones de intervención (una control y tres experimentales). La frecuencia de pateo y TF se evaluaron con el Frequency Speed of Kicks Test (FSKT) y la saltabilidad con el salto contramovimiento (CMJ). Se calculó el tamaño del efecto (TE) y se estableció un nivel de significancia de p<0,05. Los atletas de taekwondo avanzados obtuvieron resultados significativamente mayores en comparación con los novatos en la condición control para el quinto FSKT-10 (p= 0,019; TE=2,382); en la condición de fuerza para el segundo FSKT-10s 2 (p=0,028; TE=2,590); y en la condición de fuerza más pliometría para el primer FSKT-10 (p=0,037; TE=1,805) y el tercer FSKT-10 (p=0,027; TE=2,117). Además, la condición control mostró diferencias al compararla con la condición de fuerza más pliometría en el primer (p=0,040; TE=0,552) y segundo FSKT-10s (p=0,032; TE=0,687), respectivamente. La TF y CMJ no presentaron diferencias significativas entre los atletas ni entre las condiciones de intervención. En conclusión, los ejercicios de PAP no mejoran la frecuencia de pateo, TF y saltabilidad en los atletas de taekwondo evaluados. Sin embargo, estos ejercicios no produjeron efectos adversos en la TF, lo que podría favorecer su uso con entrenamiento previo en fuerza.

DYNAMIC STRENGTH CONDITION OF CONCRETE UNDER PLANE STRESS STATE

Modern calculation models must take into account the dynamic deformation of reinforced concrete. Currently, the main regularities of nonlinear dynamic deformation of reinforced concrete under uniaxial stress state are theoretically analyzed in detail along with a wide range of experimental studies. Properties of concrete under plane stress -strain state and dynamic loading are examined to a lesser extent. This paper proposes the dynamic strength condition for the concrete strength which allows for changing the strain -hardening coefficient of concrete depending on the type of stress -strain state, the ratio of the primary stresses and the deformation rate.