A new approach to predict the strength of high energy materials

This paper presents a new approach to predict the strength of energetic compounds in which there are important classes of high explosives including nitroaromatics, acyclic and cyclic nitramines, nitrate esters and nitroaliphatics. For C(a)H(b)N(c)O(d) compounds, the ratio of carbon to oxygen atoms and the predicted heat of detonation on the basis of the H(2)O-CO(2) arbitrary have been used to calculate the strength of an explosive. The new model can give good predictions for mentioned energetic compounds as determined by the Trauzl test. The novel correlation will be useful in predicting the strength or power of a new energetic compound that has significant potential in the field of explosives and propellants.     

Contact wear mechanisms of a dental composite with high filler content

The contact wear behavior of a dental ceramic composite containing 92 wt% silica glass and alumina filler particles in a polymeric resin matrix was examined. Because this composite is used for dental restorations, the tests were conducted under contact conditions that were relevant to those that exist in the mouth. Wear tests were conducted on a pin-on-disk tribometer with water as a lubricant. Results on wear volume as a function of load indicated two distinct regimes of wear. The wear volume increased slightly as the load was increased from 1 to 5 N. As the load was further increased to 10 N, the wear volume increased by one order of magnitude. At loads above 10 N (up to a maximum of 20 N), the wear volume was found to be independent of load. Examination of the wear tracks by SEM revealed that a surface film had formed on the wear tracks at all loads. Examination of these films by TEM showed that the films contained a mixture of small gamma-Al₂O₃ crystallites and glass particles. FTIR analysis of the adhered films indicated the presence of hydrated forms of silica and alumina, suggesting reaction of filler particles with water. Chemical analysis by ICP-MS of water samples collected after the wear tests confirmed the presence of Al and other elemental constituents of the filler particles. It is proposed that three simultaneous processes occur at the sliding contact: tribochemical reactions and film formation, dissolution of the reacted products, and mechanical removal of the film by microfracture. At low loads, wear occurs primarily by a tribochemical mechanism, i.e., formation and dissolution of the reaction products. At higher loads, wear occurs by a combination of tribochemical processes and mechanical detachment of the surface film.     

A new specimen geometry to determine the through-thickness tensile strength of composite laminates

The tensile strength in thickness direction is one of the dimensioning parameters for composite load introductions, which are exposed to complex three-dimensional stress states, like e.g. composite lugs. In the present paper a simple test setup which introduces the load into the specimen by a form fit was chosen to determine the through-thickness tensile strength of quasi-isotropic carbon/epoxy laminates. By means of detailed finite element analyses a new quadrilaterally waisted specimen geometry was developed and validated by mechanical testing. The influence of the manufacturing process on the location of failure was investigated and recommendations for future tests are made. Compared to alternative state of the art methods the proposed test method leads to higher accuracy and reproducibility of the determined through-thickness tensile strength.     

Variational approach development in analysis of matrix cracking and induced delamination of cross-ply composite laminates subjected to in-plane shear loading

Matrix microcracking and induced delamination propagating from the edge of microcracks in cross-ply composite laminates with [0_n/90_m]_s and [90_m/0_n]_s layups under in-plane static shear loading are investigated. An admissible stress field, which satisfies all of equilibrium equations, boundary conditions, and continuity of interfaces, is approximated. Then using the principle of the minimum complementary energy, the stress state is obtained from calculations of variation. The calculated stress state gives the stiffness reduction and the total strain energy of the laminated composite structure. Finally, the strain energy release rate of a general cross-ply laminate due to initiation and propagation of matrix cracking and induced delamination can be deduced. Results of the developed approach are in good agreement with experimental observations and finite element analyses, which confirms its accuracy.     

填料型高导热复合材料的研究现状与发展趋势

填料型导热复合材料是LED等半导体在封装及使用过程中一种常见的散热材料,它利用高导热填料填充具有密度小、可加工性能好、成本低廉等优点的高分子聚合物制备而成,对降低半导体器件的结温、增强其综合特性大有裨益。简要概述了近年来填料型导热复合材料的研究现状,并对其发展趋势进行了预测,以期为LED的实际散热需要提供技术参考,进而推动LED产业的发展。     

A comparative analysis of approaches to the strength assessment of composite laminates

We present a comparative analysis of the approaches to the strength assessment of composite laminates, which are based on application of different strength criteria to individual plies in a stack or to a stack as a whole. The Mises-Hill and Goldenblat-Kopnov strength criteria are considered. __________ Translated from Problemy Prochnosti, No. 4, pp. 36–42, July–August, 2008.     

Joining of Cf/SiC composite to GH783 superalloy with NiPdPtAu-Cr filler alloy and a Mo interlayer

With assistance of Mo interlayer, joining of C_f/SiC composite to GH783 superalloy was carried out using NiPdPtAu-Cr filler alloy. Under the brazing condition of 1200 °C for 10 min, the maximum joint strength of 98.5 MPa at room temperature was achieved when the thickness of Mo interlayer was 0.5 mm. Furthermore, the corresponding joint strength tested at 800 °C and 900 °C was even elevated to 123.8 MPa and 133.0 MPa, respectively. On one hand, the good high-temperature joint strength was mainly attributed to the formation of the refractory Mo-Ni-Si ternary compound within the joint. On the other hand, the residual Mo interlayer as a hard buffer, can release the residual thermal stresses within the dissimilar joint. The C_f/SiC-Mo bonding interface was still the weak link over the whole joint, and the cracks propagated throughout the whole reaction zone between the C_f/SiC composite and the Mo interlayer.     

Optimum content of copper slag as a fine aggregate in high strength concrete

This study investigated the mechanical properties of high strength concrete incorporating copper slag as a fine aggregate and concluded that less than 40% copper slag as sand substitution can achieve a high strength concrete that comparable or better to the control mix, beyond which however its behaviors decreased significantly. The workability and strength characteristics were assessed through a series of tests on six different mixing proportions at 20% incremental copper slag by weight replacement of sand from 0% to 100%. The results indicated that the strength of the concrete with less than 40% copper slag replacement was higher than or equal to that of the control specimen and the workability even had a dramatic growth. The microscopic view demonstrated that there were limited differences between the control concrete and the concrete with less than 40% copper slag content. It also suggested that the determination of the copper slag replacement level should consider with the desired compressive strength of concrete.     

A new filler metal with low contents of Cu for high strength aluminum alloy brazed joints

The effect of Cu with low contents of 10, 12, 15 wt.% on the microstructure and melting point of Al–Si–Cu–Ni alloy has been investigated. Results showed that low-melting-point properties of Al–Si–Cu–Ni alloys with low contents of Cu were attributed to disappearance of Al–Si binary eutectic reaction and introduction of Al–Si–Cu–Ni quaternary reaction. With raising Cu content from 10 to 15 wt.%, the amount of complex eutectic phases formed during low temperature reactions (Al–Cu, Al–Si–Cu and Al–Si–Cu–Ni alloy reactions) increased and the melting temperature of Al–Si–Cu–Ni filler metals declined. Brazing of 6061 aluminum alloy with Al–10Si–15Cu–4Ni (all in wt.%) filler metal of a melting temperature range from 519.3 to 540.2 °C has been carried out successfully at 570 °C. Sound joints can be obtained with Al–10Si–15Cu–4Ni filler metal when brazed at 570 °C for holding time of 60 min or more, and achieved high shear strength up to 144.4 MPa.     

A model of polymer composite structure with intermediate layer

In real structures isothermal planes may be distorted, and it is necessary to take this into account while carrying out the calculations of thermal conductivity λ_k of a material. The distortion depends on thermal conductivities of constituent particles and the matrix. With computer speeds already available, we can calculate temperature fields of the structural elements and to evaluate the thermal conductivity of the material. A structural model of polymeric compositions with solid particles with a shell and with microspheres is proposed. It is possible to evaluate the influence of a particle distribution order and particle spacing on the properties of compositions using structural elements. The model is suitable for the estimation with the use of corresponding coefficients of general regularities in compositions which are different by physical nature and yet are represented by analogous mathematical expressions (Fourier, Fick and Ohm laws). Solutions of integral equations for the structural element provide relative magnitudes and are, therefore, simple to use when evaluating other physical parameters of a composition. The application of the model is illustrated by examples.     

Reliability modeling of a hard real-time system using the path-space approach

A hard real-time system, such as a fly-by-wire system, fails catastrophically (e.g. losing stability) if its control inputs are not updated by its digital controller computer within a certain timing constraint called the hard deadline. To assess and validate those systems’ reliabilities by using a semi-Markov model that explicitly contains the deadline information, we propose a path-space approach deriving the upper and lower bounds of the probability of system failure. These bounds are derived by using only simple parameters, and they are especially suitable for highly reliable systems which should recover quickly. Analytical bounds are derived for both exponential and Weibull failure distributions encountered commonly, which have proven effective through numerical examples, while considering three repair strategies: repair-as-good-as-new, repair-as-good-as-old, and repair-better-than-old.     

A new approach of vibration isolation analysis of periodic composite structure based on phononic crystal

It is well-known that phononic crystal has a good effect of vibration isolation within some frequency range. The transmission performances of elastic waves propagating in three-dimension composite structure, based on phononic crystal, embedded periodically with spherical scatterers with different mass density and Lame coefficients are analyzed by the transfer matrix method in this paper. The transfer matrix of monolayer structure and the entire structure are deduced by the wave equations considering boundary conditions of displacement and stress continuity between layers, the reflectivity and transmissivity of vibration isolation of the structure are obtained by calculating its effective impedance, and some useful conclusions are drawn after numerical simulation is carried out.     

A numerical methodology for optimizing the geometry of composite structural parts with regard to strength

The increased use of composite materials in lightweight structures has generated the need for optimizing the geometry of composite structural parts with regard to strength, weight and cost. Most existing optimization methodologies focus on weight and cost mainly due to the difficulties in predicting strength of composite materials. In this paper, a numerical methodology for optimizing the geometry of composite structural parts with regard to strength by maintaining the initial weight is proposed. The methodology is a combination of the optimization module of the ANSYS FE code and a progressive damage modeling module. Both modules and the interface between them were programmed using the ANSYS programming language, thus enabling the implementation of the methodology in a single step. The parametric design language involves two verifications tests: one of the progressive damage model against experiments and one of the global optimization methodology performed by comparing the strength of the initial and the optimum geometry. There were made two applications of the numerical optimization methodology, both on H-shaped adhesively bonded joints subjected to quasi-static load. In the first application, the H-shaped joining profile was made from non-crimp fabric composite material while in the second from a novel fully interlaced 3D woven composite material. In the optimization of the joint’s geometry, failure in the composite material as well as debonding between the assembled parts was considered. For both cases, the optimization led to a considerable increase in joint’s strength.     

Simplified ultimate strength analysis of compressed composite plates with linear material degradation

Simply supported, rectangular, composite plates subjected to in-plane compressive load have been investigated for ultimate strength. An efficient, semi-analytical method has been established based on large deflection theory and first order shear deformation theory. After damage initiation, linear degradation of the material properties has been applied to the affected region of a failed ply. Two different displacement fields have been examined for their influence on the strength predictions. The approach is validated against earlier advanced finite element calculations, and can be readily applied in specific design situations or to generate parametric design curves.     

Ultimate strength of pin-loaded composite laminates: A limit analysis approach

A simplified approach to determining the ultimate strength of a pin-loaded composite laminate (PLCL), based on limit analysis theory, is presented. The composite laminate is treated as a multilayered deformable three-dimensional solid. The upper bound theorem of limit analysis and kinematic fields that include discontinuities in plies and interfaces are used to predict the bearing strength of pin-loaded composite laminate. An analytical estimation of the ultimate failure load is obtained. The theoretical predictions are compared to available experimental results. The effects of the geometry and the number of interfaces on PLCL strength are also examined.     

A fracture mechanics approach to the residual strength behavior of ballistically damaged high hardness laminar composite steel

Welded and unwelded specimens of an air-melted laminar composite steel armor were tested for degradation of strength from ballistic impact. Specimens were impacted with cal. 0.30 AP and ball projectiles at various velocities and 0-degree obliquity. During impact, specimens were tensile loaded from 0 psi to the preload that would result in specimen failure at impact. Specimens that did not fail upon impact were pulled to failure to determine residual strength.

Damage due to cal. 0.30 impact maximized at approx. 2.25 in. laterally for ball and 1 in. for AP at a velocity near the ballistic limit. Preload increased the damage slightly. Impacts near the weld produced no detectable degradation of the weld.

Damage near the maximum resulted in residual strengths near 20% of the original σ_M. Fracture mechanics analysis showed that residual strengths could reasonably be predicted by assuming that all damage could be modeled as a center notch through the hard face.     

复合材料层合板单钉双剪连接挤压强度的一种工程估算方法

将上限理论应用到复合材料层合板单钉双剪连接挤压强度分析中, 把连接结构的位移场划分为动态区域(层合板)和静态区域(紧固件), 并认为失效发生在位移可动场和不动场之间的钉孔边受挤压部分。由于受挤压孔孔边各层应力状态不一样, 受挤压孔边各层的失效区域和失效模式也各不相同。从宏观上研究复合材料层合板单钉连接孔边的失效区域和失效模式, 结合上限理论提出了一种估算复合材料单钉连接挤压强度的工程算法。通过与试验结果对比, 发现该方法能较好地预测出复合材料单钉双剪连接挤压强度。     

A 3-D fracture mechanics approach to the strength of composite materials

The first ply failure in composite laminates are studied by 3-D fracture mechanic approach. The fracture model is based on a crack embedded in one of the inner layers. Numerical results for the case of “angle ply” laminates are based on a general 3-D finite difference program.Stress intensity factors and fracture energy are evaluated by the $\text{J-}\text{integral}$ 2-D and 3-D methods as well as by the stresses and energy balance. It was found that the $\text{J(2-}\text{D}\text{)}$ method can be applied to 3-D problems while the $\text{J(3-}\text{D}\text{)}$ method is not suitable for the numerical computation.The influence of geometrical properties of the crack and the layers on the various stress intensity factors is presented and discussed. Change in failure mechanism of the weakest ply was proved by experiments to fit with numerical results. It was found that the weakest layer is strengthened by the others, a fact which has been verified experimentally and hasn't been explained by other approaches.     

Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 μ and their mixture with Si of ∼ 4.83 μ particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si₃N₄ phase and blunting crack front. Formation of Si₃N₄ whisker was also observed. The strength of the mixture composition was maximum.