Thermal modeling of solid nonfocusing pump-light collectors used for diode-pumped Nd:YAG lasers

We have developed a thermal model for the determination of the temperature distribution of a diode side-pumped Nd:YAG laser in which the laser rod is fixed in a solid nonfocusing (prismatic) pump-light collector. The model permits the temperature to be determined as a function of both spatial and temporal parameters for a wide range of boundary conditions and different collector materials. Interferometric measurements were carried out to obtain the averaged rod temperatures for comparison with results from the model and to fix a convective-cooling rate for ambient air that best fits the experimental results. Two cases were studied both theoretically and experimentally with artificial sapphire and BK7 as prism materials, and good agreement was achieved between model and experimental results. The use of artificial sapphire as the prism material reduces by a factor of ~7 both the rod temperature and the warm-up time compared with BK7 glass. Peltier cooling of the underside of the BK7 glass prism yields thermal Characteristics of the device that are similar to those devices with sapphire prisms. Calculations also show that the thermal properties of the fixant are not critical for moderate fixant thermal diffusivities (i.e., k(f) > 10(-8)-10(-7) m(2) s(-1)), thus the choice of an appropriate fixant can be based on its mechanical and index-matching properties alone.     

Theoretical modeling of a diode-pumped Nd:YAG laser with a solid nonfocusing pump light collector

We report theoretical modeling for a diode-side-pumped Nd:YAG laser in which the laser rod is fixed in a solid nonfocusing (prismatic) light collector. The geometry provides for pumping the rod from four sides, which gives a relatively uniform gain profile across the transverse section of the rod and enables a high tolerance of the laser output to resonator and pump diode misalignment. The numerical model is developed to illustrate how the pumping uniformity and the transfer efficiency are affected when changes in the collector and lasing materials are made. We use small-signal gain measurements to test the predictions of the model and to examine the extent to which surface scattering from the rough rod barrel further spatially averages the deposited pump energy. The effects of the different refractive indices of the rod, collector, and fixant and the absorption properties of the laser material on optical transfer efficiencies are discussed.     

Encapsulated rod for efficient thermal management in diode-side-pumped Nd:YAG lasers

Theoretical calculations of energy deposition, temperature, and thermally induced birefringence are presented for a diode-side-pumped Nd:YAG laser that has the laser rod optically fixed into a right-rectangular prism with a square cross section. The design provides uniform cooling of the rod and allows for a number of conduction-cooling techniques to be used. Average output powers >5 W are feasible at optical-to-optical efficiencies of ~20% for long-pulse operation and ~10% for Q-switched operation for quite simple and convenient cooling arrangements (e.g., Peltier cooling).     

Relations between cold drawing behaviour and optomechanical properties of vestan (polyester) fibres

A modified stress-strain device is used to investigate the dynamical behaviour of optomechanical properties. The optical properties and strain produced in vestan fibres by different stresses have been measured at room temperature interferometrically. It has been found that the relation between strain and birefringence is linear up to strain of 12%. For greater strain the rate of change of birefringence with strain is cut off due to breaking. An empirical formula is suggested to represent the variation of the cross-sectional area of vestan fibres with draw ratio and the constants of this formula are determined. An expression has also been suggested for the birefringence related to the strain. The strain optical coefficient is determined. Poisson's ratio, Young's modulus, elastic shear modulus and the compressibility are calculated over different strain values. Microinterferograms are given for illustration.     

Second order shear lag theory

The standard shear lag theory for elastic aligned short-fibre composites is extended to allow for a gradient of overall strain. The result is a one-dimensional strain gradient theory of the Toupin–Mindlin type. All parameters are the same as in the standard theory, and in the limits of weak strain gradients, large fibre aspect ratios or low elastic modulus ratios, the standard theory is recovered. The gradient effect is illustrated by a simple one-dimensional boundary value problem: a vertical composite rod fixed at both ends and loaded by gravity. The fibre length significantly affects the solution when the fibres are rigid and their length is near the rod length; but otherwise the effect is weak.     

Indentation of functionally graded beams and its application to low-velocity impact response

The problem of contact between a rigid cylindrical indenter and a functionally graded (FG) beam is studied. The elastic modulus of the material varies in an exponential fashion across the thickness of the beam. For the sake of comparison indentation of a homogeneous beam is also considered. In the case of FG beams indentation of both soft and hard sides of the beam are studied. Results are presented for contact force–contact length relations and contact stresses in the three types of beams. Maximum normal strains and stresses and maximum transverse shear stresses are plotted as a function of strain energy (work done by the indenter) in the beam. The results are extended to low-velocity impact problems. It is seen that for a given impact energy in low-velocity impacts, the maximum stresses and strains are significantly lower in FG beams when the impact occurs on the softer side of the beam.     

Bimodular behaviour and crack closure in compression in a brittle material

A vibrating beam method was used to determine the elastic modulus of graphite rods. The frequency and apparent modulus were determined as a function of compressive end-loading. Following fracture of the rod, the frequency and apparent modulus were decreased. At a compressive end-loading of about 0.83 MPa (120 p.s.i.), crack closure was sufficient for the fractured rod to behave similarly in vibration to the unfractured rod. Thus, the fractured material behaves in a bimodular fashion and crack closure can be achieved to enable unimpeded stress transfer across the fracture surface during vibration.     

Interpreting strain bursts and size effects in micropillars using gradient plasticity

Size effects and strain bursts that are observed in compression experiments of single crystalline micropillars are interpreted using a gradient plasticity model that can capture the process of sequential slip and heterogeneous yielding of thin material layers. According to in situ experiments during compression sub-grains and significant strain gradients develop, while deformation occurs through slip layers in the gauge region. In the multilayer strain gradient model, the higher order stress is discontinuous across the interface between a plastic layer and an elastic layer, but it becomes continuous across the interface between two plastic layers. Strain bursts occur when two neighboring layers yield. Based on this hypothesis the experimental stress-strain curves with strain bursts observed in micropillars can be fitted by properly selecting the number of layers that yield and the ratio of the internal length over the specimen size; the modulus and the yield stress are obtained from the experimental curves while the hardening modulus evolves during deformation based on the dislocation mechanisms.     

Thermal Compensation of a Cw-Pumped Nd:YAG Laser

Thermally induced lensing and birefringence modify the transverse laser profile and may eliminate any global polarization state in systems utilizing Nd:YAG as a gain medium. This creates fundamental difficulties in obtaining a high-power, polarized output beam. Although abundant literature exists regarding thermal lensing, only one birefringence compensation scheme is prevalent in the literature. A modification of this scheme is given that eliminates residual birefringence. Experimental data verify the model's validity. A theoretical model is then presented that modifies the birefringence-compensated amplifier as a single power-dependent lens. After showing that solutions exist for a power-independent resonator consisting of a power-dependent lens between two flat mirrors, this amplifier is inserted into the resonator solution to produce, to first order, a resonator that is insensitive to thermally induced fluctuations in the rod focal length.     

Linear-superelastic metals by controlled strain release via nanoscale concentration-gradient engineering

The elastic strain limit of most metals are less than 0.2% except for whiskers or freestanding nanowires whose elastic strain limit could reach 4–7%. Ferroelastic metals such as shape memory alloys (SMAs) do exhibit giant recoverable strains (up to ∼13%). However, the strong non-linear pseudo-elasticity of SMAs leads to mechanical instability. By taking advantage of the strong composition-dependent critical stress for stress-induced martensitic transformation (MT) in NiTi SMA, this work demonstrates a novel design approach to achieve linear-superelasticity (∼4.6%) and ultralow modulus (8.7 GPa) of a NiTi single crystal. These unprecedented properties are realized through precisely controlling strain release during the MT via nanoscale concentration-gradient engineering. The computer simulation results and theoretical analyses reveal that the stress–strain behavior of NiTi and other SMAs can be regulated effectively by fine-tuning the concentration gradient. This may open a new avenue for the design of next generation ferroelastic materials.     

不同温度环境中沥青混凝土动态抗压性能试验研究

为研究沥青混凝土在不同温度环境中的动态力学特性,该研究在-20~30℃和10-5~10-2 s-1条件下对其进行了动态抗压试验研究。试验结果表明:温度和应变速率对沥青混凝土的力学性能有显著影响,降低温度或增加应变速率导致抗压强度和弹性模量增加,峰值应变减小;当温度大于20℃或小于-10℃时,应变速率由10-5 s-1增加到10-2 s-1,温度对抗压强度和弹性模量的影响逐渐减小,该研究提出的温度影响因子经验公式较好地反映了抗压强度和弹性模量随温度变化的规律。在-20~0℃温度区间,抗压强度和弹性模量的动态增强因子随应变速率呈线性增长;在0~30℃温度区间,抗压强度和弹性模量的动态增强因子随应变速率呈非线性增长。在此基础之上,基于时温等效原理,建立了沥青混凝土抗压强度和弹性模量的计算模型。该模型考虑了温度和应变速率对沥青混凝土的共同作用,与试验结果吻合较好。     

Triaxial compression deformation for artificial frozen clay with thermal gradient

In order to study the deformation characteristics of artificial frozen soil with thermal gradient, such as the stress-strain relationship, a series of triaxial compression tests for frozen clay had been conducted by K₀DCGF (K₀ consolidation, freezing with non-uniform temperature under loading) method and GFC (freezing with non-uniform temperature, isotropic consolidation) method at various consolidation pressures and thermal gradients. Stress-strain curves in K₀DCGF test present strain softening during shearing process and the elastic strain is approximately 0.001;but which present the strain hardening characteristics in GFC tests and the elastic strain is approximately 0.01. The elastic modulus and peak stress for frozen clay decrease as the thermal gradient increased at different consolidation pressure both in K₀DCGF test and GFC test. The peak stress and elastic modulus in K₀DCGF test are significant independent on the pressure melting and crushing phenomena occurring in GFC test. To describe the shear deformation characteristics for frozen clay with thermal gradient, the exponent and power equations considering the correction equation on thermal gradient and model parameters from frozen clay with uniform temperature are developed .The results indicated that the proposed equations can reproduce the shear deformation well both in K₀DCGF test and GFC test.     

Diode-laser-to-waveguide butt coupling

A diode-laser-to-waveguide butt-coupling model is described. The model takes into consideration the Fabry-Perot reflection and transmission of the étalon formed by the laser front facet and the waveguide entrance facet. The model predicts coupling efficiency and the coupled-power fluctuations that occur versus the separation between the laser and the waveguide. Calculations performed for Fabry-Perot-laser-to-KTP waveguide coupling show that the transverse and the angular alignment tolerances of the waveguide can be increased when the waveguide entrance facet is antireflection coated. The longitudinal alignment tolerance of the waveguide can be increased by use of an index-matching gel between the laser and the waveguide.     

Elastic solutions of a cylindrical rod containing periodically distributed inclusions with axisymmetric eigenstrains

Summary. In this paper, we give the elastic solution for a special type of microstructure – a circular cylindrical rod containing periodically distributed inclusions along its axial direction. Each inclusion has the same uniform axisymmetric transformation strain (eigenstrain). Analytical elastic solutions are obtained for the displacements, stresses and elastic strain energy of the rod. The effects of microstructure and its evolution (growth of inclusions) on the elastic stress and strain fields as well as the strain energy of the rod are quantitatively demonstrated. As a result of such microstructure evolution nominal stress-strain relation with strain softening is derived for a rod under uniaxial tension.     

Temperature dependence of the elastic constants of niobium and lead in the normal and superconducting states

The temperature dependence of the elastic constants of niobium and lead was measured in the normal and superconducting states. Both a continuous wave and a coherent gated-carrier technique were used to measure the velocity changes associated with the superconducting transition. The elastic constants were found to exhibit the same temperature dependence as the free energy in either state. The discontinuity in the modulus associated with longitudinal strains and the slope discontinuities at the transition temperature were used to calculate the strain dependence of the transition temperature. The strain dependence in all cases is mainly quadratic.Research supported by a NSF Grant and in part by the Office of Naval Research.     

Dispersion effects in elliptical-core highly birefringent fibers

Modal birefringence and its sensitivity to temperature and hydrostatic pressure were measured versus wavelength in three elliptical-core fibers and one fiber with stress-induced birefringence. We carried out the measurements in the spectral range from 633 to 843 nm by using interferometric methods. In fibers with elliptical cores all the measured parameters showed high chromatic dependence, whereas in fibers with stress-induced birefringence this dependence was weak. We modeled the dispersion characteristics of two elliptical-core fibers by using the modified perturbation approach first proposed by Kumar. The modification consists of introducing into the expression for the normalized propagation constants an additional perturbation term that contains information about stress-induced birefringence. The results of modeling show that the temperature and pressure sensitivity of elliptical-core fiber are associated primarily with variations in stress induced by these parameters. The agreement between measured and calculated values of sensitivity in the worst case was equal to 20% for modal birefringence and temperature sensitivity and 50% for pressure sensitivity. Lower agreement between measured and calculated values of pressure sensitivity is most probably associated with uncertainties in the material constants used in modeling.     

Multichannel Strain and Temperature Measurement System for Structural Elements

he paper describes a 32-channel system for studying full-scale structures developed with the use of the state-of-the-art components. It provides continuous long-term measurement of strains and temperatures up to + 350°C and accumulation and storage of measurement data. The system was used to record the elastic strains and temperatures of a welded joint of the hot collector to the PGV-1000 steam generator nozzle in the period between the preventive maintenances. The measurement results reveal a complex pattern of strain and temperature distribution different from the axisymmetric one.     

Strain Measurement Using Embedded Fiber Bragg Grating Sensors Inside an Anchored Carbon Fiber Polymer Reinforcement Prestressing Rod for Structural Monitoring

Results are reported from a study carried out using a series of Bragg grating-based optical fiber sensors written into a very short length (60 mm) optical fiber network and integrated into carbon fiber polymer reinforcement (CFPR) rod. Such rods are used as reinforcements in concrete structures and in tests were subjected to strain through a series of cycles of pulling tests, with applied forces of up to 30 kN. The results show that effective strain measurements can be obtained from the different sensors mounted along the rod. Additionally, the tests show that close agreement with the results obtained from the calibrated force applied by the pulling machine and from a conventional resistive strain gauge mounted on the rod itself is obtained. Calculations from strain to shear stress show a relatively uniform stress distribution along the bar anchor used. The results give confidence to results from various methods of in situ monitoring of strains on such CFRP rods when used in different engineering structures.     

Characterization of microstructural damage during plastic strain of a particulate-reinforced metal matrix composite at elevated temperature

The evolution of damage in a SiC-reinforced 2618 AI alloy during plastic strain has been investigated by elastic modulus reduction and direct observations of the microstructure at room temperature and temperatures up to 220 °C. Particle fracture increases as a function of strain at all temperatures but the total number of fractured particles at any given strain is lower at higher test temperatures. The dependence of fracture on particle size and aspect ratio was recorded. Normalized elastic modulus measurements decrease as a function of strain at the same rate for tests at 25,110 and 220 °C with an anomalous set of measurements at 165 °C showing a reduced damage rate. There is no universal correlation between the number of damaged particles and reduced modulus with each test temperature showing a different relation. This indicates the different temperature dependence of void nucleation and subsequent growth. The results are used to interpret different models of load sharing between reinforcement and matrix during straining.[/p]     

Strain mechanisms in grey cast iron

Grey cast iron exhibits a continuous stress-strain curve on which elastic and plastic strains cannot be identified. Tensile straining is analysed here in the case of a grey cast iron solidified by continuous casting. It is demonstrated that the main damaging processes are directly related to strain components which can be macroscopically identified. The amount of microcracking is evaluated by the decrease in elastic stiffness. On complete unloading, a small increase in the elastic stiffness is recorded; however, the initial value is not recovered. Deformation due to localized plastic strains is evaluated by recording the tangent modulus. For stresses inducing negligible creep strains the tangent modulus decreases linearly for increased applied stress. Under low stress amplitude, however, an elastic range is clearly observed. Both mentioned strain mechanisms are fully responsible for the non linear behaviour of the material. Furthermore, it is demonstrated that as a consequence of these strain mechanisms, the stress-strain curve of a prestrained grey cast iron is directly related to the initial stress-strain curve of the same material.