Gongcheng Kexue Yu Jishu/Advanced Engineering Science

Monocrystalline silicon is widely used in the field of photoelectric systems, and it is easy to cause its thermal damage and change of performance under the action of laser. For the urgent needs of high-precision laser weapons and laser fine processing industry, the thermal damage of monocrystalline silicon irradiated by pulse train of millisecond laser andthe relationship among the laser energy density, number of pulses and other important parameters of thermal damage were analyzed, and the damage law and mechanism were explored. Thermal damage of monocrystalline silicon by pulse train of millisecond laser was studied from both simulation and experimental aspects. Based on the heat conduction equation, a thermal damage model of monocrystalline silicon irradiated by pulse train of millisecond laserwas established, finite element and finite difference methods were used to solve temperature field of monocrystalline silicon treated by pulse train of millisecond laser. The equivalent specific heat capacity was introduced into the model to deal with the phase change after melting and vaporization, and the temperature rise of the model was corrected. The temperature measurement system of millisecond pulse laser damage monocrystalline silicon was constructed, and the high-precision spot temperature meter was used to measure the laser irradiation center point temperature in real time. Research indicated that when a pulsed laser is applied to monocrystalline silicon target, the center point of the laser irradiation and the radial and axial positions have a temperature accumulation effect, and the radial temperature rise range is much larger than the axial direction; With the increase of laser energy density, the temperature accumulation effect is significant; As the number of pulses increases, the melting time of monocrystalline silicon and the time from the melting point to the normal temperature are lengthened; when the number of laser pulses is increased to 90;The thermal damage threshold of monocrystalline silicon decreases to 73.8% of the single pulse damage threshold; When the number of pulses increases, the damage area of monocrystalline silicon increases. Comparing the experimental and simulation results, it can be seen that the laws of the two aspects are basically the same. The simulation model can reasonably describe the process of millisecond pulse laser damage to monocrystalline silicon.

Inverted planetary roller screw (IPRS) is a kind of linear transmission mechanism which has advantages of high load carrying capability, long fatigue life, high transmission accuracy and low noise. Due to these advantages, it has wide application prospect in the fields of aerospace, weapon equipment, CNC machine tools and petrochemical industry. Up to the present, there is little research about this mechanism. Therefore, a model was derived to calculate load distribution, axial deformation and contact fatigue life. Firstly, surface equation of screw, roller and nut was established. Based on Surface meshing theory, the meshing points of the roller with the screw and nut in a pitch were derived respectively. According to the surface equations and the location of contact points, an accurate method to calculate the elastic deformaion between the meshing surface was derived. Secondly, A model of IPRS's load distribution was obtained based on the geometric relationships between Hertz deformation, thread deformation and axial deformation. Based on its load distribution condition and motion principle, a fatigue life model was deduced using Lundberg-Palmgren equation. Then, a program was compiled in the MATLAB Environmet to calculate load distribution axial deformation and contact fatigue life. By comparing calculationg result with the expriments, the load distribution and axial deformation model was verified. The influence of the key parameters on the performance of IPRS was analyzed and the following results were derived. The load distribution was mainly influenced by the number of teeth, the number of rollers and the helix angle, and increased with the increases of these three factors. The axial stiffness was mainly influenced by the number of teeth, the number of rollers, the helix angle and the outer diameter of the nut, and increased with the increases of the number of rollers and the outer diameter of the nut, increased first and then decreased with the increase of the number of teeth and the helix angle. The contact fatigue life was mainly influenced by the number of teeth, the number of rollers, the contact angle and the radius of thread profile, and increased with the increases of the number of teeth, the number of rollers and the radius of thread profile, decreased with the increase of contact angle.

In order to find out the damage evolution rule of the concrete with initial damage caused by early freezing under the coupling of freeze-thaw cycle and sulfate erosion, initial damage to concrete specimens by freezing and thawing them at the 7th and 14th day during the preparation of concrete specimens was carried. Based on this, different initial damage degree on macro performance and internal structure was researched. Then according to the principle of damage mechanics, the dynamic modulus of elasticity and compressive strength were defined as damage variables, by regression fitting analysis of damage process of variable, and the damage evolution equation of different initial damage specimen caused by early freeze-thaw was established in corrosion and freezing environment. The results indicated that two kinds of concrete specimens with different initial damage are affected by early freeze-thaw in the decay process of their mechanical properties and their form are " lead effect", which is closely related to the degree of initial damage. At the same time, the comparison and analysis of the change process of ultrasonic pulse propagation velocity and internal defect zone of concrete inferred that early freeze-thaw accelerates the propagation speed of cavity and crack, and the initial value of macroscopic mechanical properties of concrete and the ability of resisting salt and freezing in later stage are affected. The dynamic modulus of elasticity and the damage evolution equation of compressive strength can be used as a general formula for the attenuation of mechanical properties of concrete with similar damage degree, which can provide a theoretical basis for evaluating the service status and predicting the life of structures in similar environments. At the same time, the damage relationship between dynamic modulus of elasticity and compressive strength can be deduced from each other, which can reduce the limitation of data acquisition due to the shortage of instruments.

In mountain rivers with coarse sediments, the ratio of water depth to sediment diameter (h/d) is relatively small especially under the condition of shallow water depth. Therefore, the vertical velocity distribution along the river cannot be described accurately with traditional logarithmic or logarithmic-wake law. In this paper, flume experiments were carried out with two kinds of sediments (glass beads d=1.4 cm and pinpong d=4 cm) to test the influence of coarse sediment on vertical velocity distribution under different h/d. In the presence of coarse sediment bed, one of the problems encountered is how to determine the reference level y0. For practical purpose, the present study assumes that y0 does not varies with h/d, namely, y0=0.2d. The measured data were first converted to dimensionless and then compared to theoretical results calculated by logarithmic-wake law. In addition, the coefficient B and another wake coefficient Π in in logarithmic compensation formula were calibrated to fit for the measured data. The results indicate that when the value of h/d is small, the coefficient B decreases with the increase of h/d while the wake coefficient Π presents an opposite trend, when the value of h/d becomes larger, both B and Π tend to a constant. Finally, a modified logarithmic-wake law is derived to describe the velocity distribution in mountain river flows with small h/d. The modified model agrees well with the experimental data conducted in the present laboratory flume. Furthermore, it also has a good adaptability to riverbed with coarse sediments.

In order to perfect the salt expansion theory of coarse-grained saline soil and explore the influencing factors and mechanism of the salt−frost heaving force, 96 groups of coarse-grained sulphate saline soil samples are prepared artificially then indoor model tests have been carried out by self-designed test device. Test results showed that the sensitive range of temperature on the salt−frost heaving force is −0.2℃～−1.0℃. The salt content had no influence on the rules that the salt−frost heaving force increasing with temperature and determines the maximum salt−frost heaving force together with water content. For samples with same void ratio, the increasing water content led to an increased amount of the salinity which resulted in the maximum salt−frost heaving force. When the salt content kept constant, however,the increase of water content caused the decrease of maximum salt−frost heaving force. The maximum salt−frost heaving force achieved the maximum and minimum values ​​when the void ratio was 0.58 and 0.55, respectively, which was related to the crystallization position of salt in the soil frame. Furthermore, based on the test results, a mathematical expression of frost heaving limit depth of coarse-grained sulphate saline soil is given by the hierarchical synthesis method, which is expected to provide guidance for the quantitative evaluation and design of salt− frost heaving deformation of coarse-grained sulphate saline soil.which was related to the crystallization position of salt in the soil frame. Furthermore, based on the test results, a mathematical expression of frost heaving limit depth of coarse-grained sulphate saline soil is given by the hierarchical synthesis method, which is expected to provide guidance for the quantitative evaluation and design of salt−frost heaving deformation of coarse-grained sulphate saline soil.which was related to the crystallization position of salt in the soil frame. Furthermore, based on the test results, a mathematical expression of frost heaving limit depth of coarse-grained sulphate saline soil is given by the hierarchical synthesis method, which is expected to provide guidance for the quantitative evaluation and design of salt−frost heaving deformation of coarse-grained sulphate saline soil.