A multilayered thick cylindrical shell under internal pressure and thermal loads applicable to solid propellant rocket motors

A solid propellant rocket motor can be considered to be made of various circumferential layers of different properties. A simple procedure is described here to obtain an analytical solution for the general case of multilayered thick cyclindrical shell for internal pressure and thermal loads. This analytical procedure is useful in the preliminary design analysis of solid propellant rocket motors. Since solid propellant material is of viscoelastic behaviour an approximate viscoelastic solution methodology for the multilayered shell is described for estimation of time dependent solutions of propellant grain in a rocket motor. The analytical solution for a two layer reinforced thick cylindrical shell available in the literature is shown to be a special case of the present analytical solution. The results from the present analytical solution for multilayers is found to be in good agreement with FEA results.     

短纤维增强橡胶复合材料热应力及热-结构耦合的数值研究

基于橡胶材料的非线性和不可压缩特性,建立细观数值模型,采用非线性有限元方法,在细观层面,通过对短纤维增强橡胶复合材料受热载荷和热-结构载荷时的应力传递分析,研究了温度对材料热弹性和失效形式的影响,探讨了对材料热应力影响的细观结构关键因素,揭示了材料的细观破坏机理。研究表明:当受热载荷时,界面处纤维受到的压应力加强了橡胶和纤维的粘合,而纤维的端部容易脱粘;材料受热-拉伸载荷时的应力是热应力及拉伸载荷产生应力的线性组合,且随着温度增大,界面脱粘失效的几率增大,纤维断裂失效的几率减小,温度的升高使复合材料的刚度急剧下降。     

Fracture behaviour of accelerated aged solid rocket propellants

The effect of temperature, strain-rate and ageing on the crack growth mechanism in a composite propellant has been examined to obtain an understanding of the fracture process under service life conditions. Both as-received and aged specimens were tested at each of three strain-rates and temperatures. The materials were aged by subjecting them to various thermal loads (accelerated ageing, thermal cycle and thermal shock) designed to expose them to conditions similar to that experienced by a rocket motor during its service life. The fracture behaviour of the propellant specimens were affected by changes in temperature whilst strain-rate had only a marginal effect over the range studied. It was found that as the material temperature decreased from 60 to – 40 °C the stiffening of the propellant caused increased hysteresis ratios and decreased crack velocities. The deterioration of the mechanical properties of the propellant depended on the severity of the thermal loads. In each case the accelerated aged specimens became harder and more brittle whilst the thermally cycled and thermally shocked specimens were only marginally affected. A distinct difference in the mechanism of crack growth was observed for the accelerated aged specimens, along with a marked decrease in hysteresis ratio, critical stress and critical strain and an increase in crack velocity.     

装药缺陷对固体发动机性能影响的实验分析

在积分形式 N-S 方程基础上, 建立了固体火箭推进剂裂纹燃烧过程的理论模型, 对含装药裂纹的翼柱固体火箭发动机内流场进行了数值模拟, 分析了推进剂中裂纹大小、位置、角度等多种因素对发动机内流场的影响, 为评估固体发动机含缺陷装药燃烧安全性提供了实验依据和分析方法.     

Effect of platinum on the oxide-to-metal adhesion in thermal barrier coating systems

An investigation was conducted to determine the role of Pt in a thermal barrier coating system deposited on a nickel-base superalloy. Three coating systems were included in the study using a layer of yttria-stabilized zirconia as a model top coat, and simple aluminide, Pt-aluminide, and Pt bond coats. Thermal exposure tests at 1,150 °C with a 24-h cycling period to room temperature were used to compare the coating performance. Additional exposure tests at 1,000, 1,050, and 1,100 °C were conducted to study the kinetics of interdiffusion. Microstructural features were characterized by scanning electron microscopy and transmission electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction. Wavelength dispersive spectroscopy was also used to qualitatively distinguish among various refractory transition metals. Particular emphasis was placed upon: (i) thermal stability of the bond coats, (ii) thickening rate of the thermally grown oxide, and (iii) failure mechanism of the coating. Experimental results indicated that Pt acts as a “cleanser” of the oxide-bond coat interface by decelerating the kinetics of interdiffusion between the bond coat and superalloy substrate. This was found to promote selective oxidation of Al resulting in a purer Al₂O₃ scale of a slower growth rate increasing its effectiveness as “glue” holding the ceramic top coat to the underlying metallic substrate. However, the exact effect of Pt was found to be a function of the state of its presence within the outermost coating layer. Among the bond coats included in the study, a surface layer of Pt-rich γ′-phase (L1₂ superlattice) was found to provide longer coating life in comparison with a mixture of PtAl₂ and β-phase.     

The characterisation of the interfacial chemistry of adhesion of rigid polyurethane foam to aluminium

The interfacial interactions between a rigid polyurethane foam (RPUF) and aluminium have been studied to understand the mechanisms of adhesion. Three different blowing systems are used in the production of the foam: chemical blowing, physical blowing and a mix of chemical and physical blowing systems. In addition an unfoamed system has been examined for comparison of the catalysts behaviour with and without blowing agents and the surfactant. Peeled failure surfaces have been examined by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF–SIMS). To examine the intact interfacial regions of the RPUFs cured against aluminium, samples have been sectioned by microtomy. The failure surfaces of the aluminium sides exhibit relatively clean aluminium surfaces with RPUF residues observed for all three foamed systems; such thin RPUF layers (ca. 1 nm) indicate good adhesion (and a cohesive failure) between foam and substrate and that the interfacial adhesion is higher than the cohesive strength of the foam. The unfoamed system behaves in a similar manner but has a higher peel strength. A fragment indicative of covalent bond formation between isocyanate and aluminium (nominal mass at 102 u: AlCHNO₃ ⁻) is observed on the failure surface of aluminium side, where RPUF/aluminium interface region is present, for all foams. The catalyst used in these formulations, pentamethyldiethylenetriamine (PMDETA), is concentrated at the interface area. Whilst examination of the sectioned specimens shows that the silicone surfactant is concentrated within the cell area fulfilling its role on cell formation and stabilisation, and is not segregated at the RPUF/aluminium interface.     

Ablation,mechanical and thermal conductivity properties of vapor grown carbon fiber/phenolic matrix composites

The ablation, mechanical and thermal properties of vapor grown carbon fiber (VGCF) (Pyrograf III™ Applied Sciences, Inc.)/phenolic resin (SC-1008, Borden Chemical, Inc.) composites were evaluated to determine the potential of using this material in solid rocket motor nozzles. Composite specimens with varying VGCF loadings (30–50% wt.) including one sample with ex-rayon carbon fiber plies were prepared and exposed to a plasma torch for 20 s with a heat flux of 16.5 MW/m² at approximately 1650°C. Low erosion rates and little char formation were observed, confirming that these materials were promising for rocket motor nozzle materials. When fiber loadings increased, mechanical properties and ablative properties improved. The VGCF composites had low thermal conductivities (approximately 0.56 W/m-K) indicating they were good insulating materials. If a 65% fiber loading in VGCF composite could be achieved, then ablative properties are projected to be comparable to or better than the composite material currently used on the Space Shuttle Reusable Solid Rocket Motor (RSRM).     

火箭发动机装药包覆质量诊断的超声新技术

目的 研究火箭发动机装药包覆质量超声检测技术。方法 从发动机壳体外侧诊断内部推进剂包覆质量的超声技术。结果 研制了相应的检测设备并对实际物体实施了检测。结论 作者提出的板波诱发超声检测, 既可从发动机壳体外检测内壁涂敷的单层或多层包覆、绝热材料层厚度,也可识别各层界面的脱粘。     

Surface modifications to improve Ti–porcelain bonding

Investigations of surface modifications on cast titanium surfaces and titanium-ceramic adhesion were performed. Cast pure titanium was subjected to surface modification by preoxidation and introduction of an intermediate layer of SnO _x by sol–gel process. Surfaces only sandblasted with alumina were used as controls. Specimen surfaces were characterized by XRD and SEM/EDS. The adhesion between the titanium and porcelain was evaluated by three-point flexure bond test. Failure of the titanium–porcelain with preoxidation treatment predominantly occurred at the titanium-oxide interface. Preoxidation treatment did not affect the fracture mode of the titanium–ceramic system and did not increase the bonding strength of Ti–porcelain. However, a thin and coherent SnO _x film with small spherical pores obtained at 300 °C served as an effective oxygen diffusion barrier and improved titanium–ceramic adhesion. The SnO _x film changed the fracture mode of the titanium–ceramic system and improved the mechanical and chemical bonding between porcelain and titanium, resulting in the increased bonding strength of titanium–porcelain.     

Shock Wave Fabricated Ceramic-Metal Nozzles

Shock compaction was used in the fabrication of high temperature ceramic-based materials. The materials' development was geared towards the fabrication of nozzles for rocket engines using solid propellants, for which the following metal-ceramic (cermet) materials were fabricated and tested: B₄C-Ti (15 vol.-%), B₄C-Al, and TiB₂-Al, with an Al content typically between 15–20 vol.-%. Here, the B₄C-Ti was only shock-compacted, while the other two cermets were shock compacted followed by melt infiltration with Al.The materials were subjected to gradually more severe testing conditions. Slabs of the materials were first tested for thermal shock resistance in an acetylene flame, followed by testing in the exhaust gas stream of a rocket propellant, and thereafter as a cylindrical insert in a nozzle of TZM alloy. The B₄C-Ti composite showed erosion and cracking after the first test in the propellant flame, while the B₄C-Al composite failed the insert tests. The TiB₂-Al composite performed well under all conditions. A venturi nozzle of that material was formed during compaction. This real, shaped nozzle was shown to function well, even during repeated 3–6 s tests. This could be explained by the resistance of TiB₂ to molten Al, the high thermal conductivity of the TiB₂-Al cermet and the in situ formation of a protective layer, consisting mainly of Al₂O₃.     

低温推进剂火箭发动机泵出口密度、温度计算的一种新方法

对补燃循环和使用低温推进剂的液体火箭发动机，需要考虑泵出口温度、密度变化对发动机动、静态特性的影响，目前计算泵出口温度、密度的方法存在一定的局限性，本文提出了一种新的计算方法，它根据泵出口推进剂的压力和比焓，利用推进剂的有关热力状态方程来迭代计算泵出口推进剂的密度与温度，具有使用方便，适用范围广和计算精度高的优点，采用最小二乘法拟合了低温推进剂火箭发动机泵出口温度、密度计算所需的热力状态方程，利用本文的方法对低温发动机泵出口密度、温度进行了计算，计算结果与实测值吻合很好。     

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.     

Microwave method for locating inhomogeneities in cured rocket propellant samples

A simple microwave NDT technique for locating the exact position of a localized flaw or asymmetrical portion within a solid rocket propellant motor is presented. Sample observations and probable errors involved in the method are indicated.     

Failure mechanism of a thermal barrier coating system on a nickel-base superalloy

An investigation was carried out to determine the failure mechanism of a thermal barrier coating system on an Ni-base superalloy. The coating system consisted of an outer layer of yttria-stabilized zirconia (top coat), and an inner layer of Pt-aluminide (bond coat). Specimens were exposed at 1010 and 1150 °C with a 24-h cycling period to room temperature. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used in microstructural characterization. Spallation of the oxide scale developed by the bond coat was found to be the mode of failure. Experimental results indicated that the breakdown of oxide was affected by internal oxidation of Hf diffusing from the alloy substrate into the bond coat surface developing localized high levels of stress concentration at the oxide–bond coat interface. It was concluded that the cause of failure was degradation of thermal stability of the bond coat accelerating its oxidation rate and permitting outward diffusional transport of elements from the substrate.     

Effects of the interface on the mechanical response of CF/EP microcomposites and macrocomposites

The aim of this study was to investigate the effects of the interfacial bond quality on the mechanical response of composite laminates, for example an epoxy matrix reinforced by continuous carbon fibres of varying surface coating. The fibre/matrix adhesion was characterized by determining the interfacial shear strength τ_i in single-fibre fragmentation and microdroplet pull-off tests. The failure mechanisms were deduced from the stress birefringent patterns (fragmentation test) and from fractographic analysis in a scanning electron microscope (microdroplet pull-off test). Selected interface-relevant properties were evaluated in mechanical tests on laminates. The present paper highlights the problems related to the micromechanical characterization and the interface relevance of data resulting from transverse tensile, transverse flexure and interlaminar shear tests. Furthermore, the effects of the interface on the impact performance of unidirectional and cross-ply laminates were studied. Attempts were made to correlate the macroscopic mechanical response with the interface-related characteristics (τ_i and failure mechanisms).     

Mathematical model and experimental results for cryogenic densification and sub-cooling using a submerged cooling source

Among the many factors that determine overall rocket performance, propellant density is important because it affects the size of the rocket. Thus, in order to decrease the size of a rocket, it may be desirable to increase the density of propellants. This study analyzes the concept of increasing the propellant density by employing a cooling source submerged in the liquid propellant. A simple, mathematical model was developed to predict the rate of densification and the propellant temperature profile. The mathematical model is generic and applicable to multiple propellants. The densification rate was determined experimentally by submerging a cooling source in liquid oxygen at constant positive pressure, and measuring the time rate of change in temperature with respect to vertical position. The results from the mathematical model provided a reasonable fit compared to experimental results.     

A simple method for measuring tensile and shear bond strength of ceramic-ceramic and metal-ceramic joining

A simple method for measuring the ceramic-ceramic and metal-ceramic bond strength was presented, by which uniaxial tensile stress normal to the interface or shear stress in the interface can be produced using uniaxial compression load on a cross-bonded sample. Both tensile and shear bond strength were obtained by this testing technique for Ti₃SiC₂–TiO₂ and Ti₃SiC₂–Al₂O₃ composite as well as for glued steel samples, respectively. The novel method provided a solution for determining bond strength in solid (especially brittle) materials, and it is also demonstrated as a useful method for evaluating the tensile and shear strength of various glues. Electronic Publication     

Adhesion of epoxy resins to metals

The adhesion of an epoxy resin above its glass transition temperature to aluminium, steel and gold surfaces has been studied using the methods of fracture mechanics. The results are compared with those of a previous study of elastomeric adhesives by Andrews and Kinloch, and the “intrinsic failure energies”,θ ₀, for the epoxy-metal bonds are deduced by similar methods. Correspondence, within a factor of two, is found betweenθ ₀ and the thermodynamic work of adhesion,W _A, for most cases of interfacial failure, indicating both the absence of specific or chemical interactions at the interface and a purging of surface contaminants by the epoxy. An exception occurs when an excess of epoxy groups exist in the uncured resin. Here, for steel and aluminium but not for gold, the interfacial bonding is stronger than the cohesive strength of the resin due probably to the formation of strong bonds with the metal oxide surface layer.     

Structure and properties of diffusion bonded transition joints between commercially pure titanium and type 304 stainless steel using a nickel interlayer

The solid-state diffusion bonding was carried out between commercially pure titanium and Type 304 stainless steel using nickel as an interlayer in the temperature range of 800–900 °C for 9 ks under 3 MPa load in vacuum. The transition joints thus formed were characterized in the optical and scanning electron microscopes. The inter-diffusion of the chemical species across the diffusion interfaces were evaluated by electron probe microanalysis. TiNi₃, TiNi and Ti₂Ni are formed at the nickel–titanium (Ni–Ti) interface; however, the stainless steel–nickel (SS–Ni) diffusion interface is free from intermetallic compounds up to 850 °C temperature. At 900 °C, the Ni–Ti interface exhibits the presence of α-β Ti discrete islands in the matrix of Ti₂Ni and λ + χ + α-Fe, λ + FeTi and λ + FeTi + β-Ti phase mixtures occur at the SS–Ni interface. The occurrence of different intermetallics are confirmed by the x-ray diffraction technique. The maximum tensile strength of ∼276 MPa and shear strength of ∼209 MPa along with 7.3% elongation were obtained for the diffusion couple processed at 850 °C. At the 900 °C joining temperature, the formation of Fe–Ti base intermetallics reduces the bond strength. Evaluation of the fracture surfaces using scanning electron microscopy and energy dispersive spectroscopy demonstrates that failure takes place through Ni–Ti interface up to 850 °C and through the SS–Ni interface of the joint when processed at 900 °C.     

STRESS CONCENTRATIONS IN A RECTANGULAR PLATE WITH CIRCULAR PERFORATIONS ALONG ITS TWO BONDED EDGES AND SUBJECTED TO RESTRAINED SHRINKAGE

Two-dimensional photoelasticity is used for minimising stress concentration associated with a fillet present at the end of a solid propellant rocket grain subjected to restrained shrinkage. For this purpose, successive holes were added along the bonded interface of case and propellant, but no decrease was obtained. The photoelastic method used illustrates two uncommon techniques of interest to experimental stress analysts: (1) restrained shrinkage loading and (2) a parametric stress-analysis solution obtained with a single model.