Review of the Adhesively Bonded Interface in a Solid Rocket Motor

One of the most challenging requirements in a solid rocket motor (SRM) is the integrity of the charge structure which is a multilayer adhesive joint involving the propellant, liner, and insulation. The propellant/liner/insulation interface is considered to be the weakest part of the whole structure. This interface has some of the usual features of an adhesively bonded interface, as well as its own special characteristics: the co-cured process, ingredient migration between interfaces, and complicated damage mechanisms. We give a technical and critical review of the past 50 years of existing research on many aspects of the propellant/liner/insulation interface in terms of the adhesive properties and adhesive mechanisms, ingredients migration, damage determination, and fracture analysis. To present a comprehensive outline of this interface we also clarify some remaining problems which should be addressed in the future. With significant improvements in the theoretical and experimental studies of the propellant/liner/insulation interface, the problem of integrity failure of the charge structure in SRM will be well resolved.     

分子筛改性包覆层及与推进剂黏结机理初探

将分子筛添加于包覆层中,可显著提高包覆层的综合性能。从分子筛的结构特点,包括其大的比表面积和表面活泼的硅羟基等方面探讨了分子筛改性丁羟包覆层及其与硝酸酯增塑聚醚(NEPE)推进剂黏结性能提高的机理,发现分子筛改善包覆层的性能与其自身结构特点密切相关。     

Stress singularity in a rectangular bond specimen of a solid rocket motor: Effects and elimination

The propellant/liner interface is the weakest and most concerning part of the grain structure in a solid rocket motor. Rectangular bond specimen tests have gradually become one of the standard methods to measure the bonding abilities of propellant/liner/insulation joints. We performed a three-dimensional numerical study to give full knowledge of this new test, paying close attention to the stress singularity at the crack tip. The asymptotic stress field was presented to show the singularity at the crack tip on the steel/insulation interface. Subsequently the stress singularity was investigated numerically. Numerical results show that the stress singularity has a considerable effect on the stress distribution of the nearby propellant. Also we proposed some methods to eliminate these effects, such as inserting a cohesive zone model into the steel/insulation interface or increasing the thickness of insulation layer. Moreover, Mises stress and maximum principle stress have completely opposite distributions on the propellant/liner interface; thus the accurate failure criterion can be confirmed by the damage initialization observed in experiments.     

The peel ply surface treatment for adhesive bonding of composites: A review

This paper reviews existing literature related to the peel ply surface treatment of composite materials. A peel ply fabric is used as a removable layer in a composite lay-up and ripped off to modify the surface for adhesive bonding. A peel ply can affect the elemental composition of a surface and it is possible to distinguish between either a polyester or a polyamide peel ply treatment for a specific composite system and application. However, consistent and generalised relationships between the compositional modification or the surface energy and adhesive or resin adherence to treated surfaces are not probable. A compositional analysis and surface energy measurements are affected by the fractured matrix, peel ply fibre cavities and revealed reinforcements. Only a few recent studies have presented results for the regions of the matrix fracture and fibre cavities separately. Resin-impregnated peel plies, namely tear plies, offer additional tailorability for composite surface treatments, but related research is scarce.     

丁羟推进剂/衬层界面黏结性能劣化的动态力学表征

研究了高温加速老化过程中丁羟推进剂联合黏结试件的老化特性,探索了利用动态力学表征丁羟推进剂/衬层界面(P/L界面)黏结性能劣化的方法.结果表明,高温老化使黏结件P/L界面黏结强度增大,推进剂中可迁移组分向衬层的迁移量增加,衬层降解,这使得P/L界面两侧材料力学性能差异增大,缺陷增多,P/L界面黏结性能劣化.动态力学分析实验表明,衬层初始模量、界面推进剂和本体推进剂的储能模量与黏结件黏结强度具有相关性,可作为表征P/L界面黏结性能劣化的参数.给出了利用动态力学参量表征黏结件黏结强度的回归关系.     

In–situ Characterization and Cure Kinetics in NEPE Propellant/ HTPB Liner Interface by Microscopic FT‐IR

The content distribution of chemical groups and the kinetics of curing process in the micro‐region interfaces of nitrate ester plasticized polyether (NEPE) based propellant/hydroxyl‐terminated polybutadiene (HTPB) based liner were studied by in‐situ diffuse reflection FT‐IR spectroscopy. During the curing process, the content of –NCO groups showed little increase in the liner region toward the interface. It rises quickly through the interface layer and is then stable in the region of the propellant layer, while the content of –NH groups gradually increases from liner to propellant. In the micro‐region between liner and propellant, the –C=O decreases rapidly through interface and then has a slight increase in the propellant region. Migration of nitrate esters appears at the interface of the NEPE propellant/liner at early period of curing, and –O–NO₂ decreases from propellant to liner in the bonding interface micro‐region. A study of curing kinetics indicates that the second‐order reaction model can describe the curing reaction in the bonding interface at the early stage of curing process. The order of apparent curing reaction rate constant (k ) of liner (L point), intermediate point (I point) and propellant (P point) in the interface micron‐region is k L > k I > k P at the same curing temperature. The apparent reaction activation energy (E _a) at L, I, and P points are 39.96, 81.49, and 62.51 kJ mol^–1, respectively, based on the Arrhenius equation.     

Applied Research and Development of Adhesives for Bonding Filled Carboxyl Terminated Polybutadienes to Various Substrates

A one-year applied research and development program was conducted on the bonding of carboxy-terminated polybutadiene (CTPB) propellant to various substrate materials encountered in solid propellant rocket motors. Under this program, in addition to CTPB liners, liners were also prepared from polyesters, polyethers, polyurethanes, polyacetal polymers, and epoxy resins. The use of various crosslinkers, emulsifiers, wetting agents, fillers, and stabilizers was also evaluated.

Four optimized liner formuations with the best all-round properties were fully characterized. The optimized formulations represented an HC liner formulation with two Ievels of glycerol additive, an HC formulation with a sorbitol additive, and a Butvar polyacetal-type liner. A standard HC-polymer liner formulation, designated as TL-H-304, was used as a control.

Unaged liner peel and shear properties were measured at -65°F, 77°F, and 160°F. Samples, aged for 30 days at 160°F, were tested at 77°F only.

The liners were tested against propellant, steel, aluminum, magnesium, titanium, epoxy-fiberglass, phenolics, polyisoprene, and butadiene-acrylonitrile as substrate materials. The steel, titanium, and polyisoprene rubber substrates gave the best adhesive results.

The substitution of asbestos and Cab-O-Sil for the Thermax filler in the liner gave comparable adhesive results while the substitution of clay fillers gave poor results.

This program was performed while at the Elkton Division of the Thiokol Chemical Corporation in fulfillment of the requirement of Contract N123 (60530-53329A) U.S. NAVAL ORDNANCE TEST STATION, China Lake, California, reported previously in U.S. Naval Report NOTS-TP4283.     

NEPE推进剂拉伸断裂行为研究

通过单向拉伸力学性能实验,考察了不同测试温度和不同拉伸速率条件下NEPE推进剂力学性能的变化情况。采用扫描电镜(SEM)和原位拉伸SEM观察了推进剂拉伸断面形貌。结果表明,在低温测试条件下,NEPE推进剂最大伸长率较常温条件下显著降低,最大抗拉强度较常温和高温条件下显著升高,NEPE推进剂的破坏主要表现在黏合剂的撕裂和固体颗粒的断裂;在高温、慢拉伸速率的测试条件下,推进剂断裂时结构被破坏的程度较大,NEPE推进剂的破坏首先发生在固体颗粒堆积处,再到黏合剂网络结构。推进剂断裂的过程是推进剂拉伸取向与裂纹扩展之间的竞争过程。     

Adhesive Joint Durability Assessed Using Open-faced Peel Specimens

This paper presents an investigation of the durability of two aluminum-epoxy adhesive systems by means of open-faced peel specimens. A peel analysis model was used to determine the fracture energy from the peel data. Both wet and dry peel tests were conducted in order to distinguish between the reversible and the permanent effects of water. The effects of water on the cohesive properties of the adhesives were also assessed by tension tests. It was found that, for the two-part epoxy adhesive, which plasticized to a large extent, the peel testing should be carried out in a dry state to assess the interfacial weakening. It was also observed that the two-part adhesive was much stiffer in the dry, degraded state, and it was important to take account of such permanent changes in the cohesive properties associated with water uptake when determining the fracture energy from the peel data. In contrast, the one-part epoxy system did not suffer from appreciable cohesive changes, either reversible or permanent. In this case, both wet and dry failure loci were interfacial, and some of the interfacial damage was found to be reversible. Finally, surface analyses of the peel failure surfaces were carried out, and the formation of micro-debonds was identified as a possible mechanism of degradation for the two-part system.     

基于ESPI技术的混凝土单轴拉伸断裂特性研究

为了研究混凝土的拉伸断裂特性,对带有预制切口的C80高强混凝土棱柱体进行单轴拉伸试验,采用电子散斑干涉(ESPI)技术测量棱柱体表面的场位移等信息,分析加载过程得到混凝土的断裂参数以及断裂特性等。为进行对比分析,本次试验还采用线性位移计和夹式位移计测量试件的变形程度。对比结果表明ESPI技术的测量结果与夹式位移计结果吻合较好,证实了ESPI技术用于测量混凝土表面位移场的精确性和可行性。分析得到:初裂点应力约为峰值应力的82%;峰值下裂缝口张开位移(CMOD)为11 μm,结合公式得到混凝土在单轴拉伸下的断裂韧度约为0.41 MPa·m^1/2,断裂能约为24.71 N/m;利用两种拟合公式对试验应力-应变关系进行拟合,得到了较好的拟合结果。最后,通过不同加载步骤下ESPI技术测得的位移云图分析裂缝口张开位移随试件宽度的变化规律,得到了单轴拉伸下的裂纹扩展规律。     

The adhesion of natural rubber to steel and the use of the peel test to study its nature

The peel test has been used to study the adhesion achieved between a simple natural rubber compound and mild steel by injection moulding in the presence of a commercial bonding system. Peel energies were calculated from the peel force results by allowing for the elastic energy stored in the detached rubber strip. The peel energy was found to be insensitive to the changes of peel rate evaluated in this work. Peel energy increased by a factor of about five as the peel angle increased from 30° to 90°. The locus of failure was cohesive within the rubber, but moved more deeply within the rubber and produced a severely torn fracture surface.     

Micromechanisms of high-energy peel failure in polyester–aluminum–polyester laminates

Symmetrical polyester-aluminum-polyester laminates are prepared using poly(ethylene terephthalate) film subjected to a glow discharge pretreatment. Extremely high peel energies (several hundred J m^?2) are achieved, and the fracture path is principally cohesive: the laminate performance thus fully exploits the bulk mechanical properties of the PET. The peel surface exhibits extensive plasticity on a scale of tens of microns, with finer ductile tearing on a scale of order 1 μm. The mechanism of peel propagation and energy dissipation is discussed with regard to the optimisation of peel strength.     

THE DEVELOPMENT OF A MANDREL PEEL TEST FOR THE MEASUREMENT OF ADHESIVE FRACTURE TOUGHNESS OF EPOXY–METAL LAMINATES

A mandrel peel test is established for measuring the adhesive fracture toughness of a metal/rubber-toughened epoxy laminate system. By adopting an energy balance analysis it is possible to determine directly both adhesive fracture toughness and plastic work in bending the peel arm around the mandrel. The suitability of the procedure is examined for various types of metal peel arms, which are classified in terms of their ability to deform plastically during the test. The plastic work is also predicted theoretically, and comparisons are made between the measured and calculated values. The fracture energies determined from the mandrel tests are compared with those obtained from 90° fixed-arm peel tests. For the calculations of plastic work in bending in the fixed arm test, various options are used when modelling the tensile stress-strain behaviour of the peel arm material. In addition, the adhesive layer thickness is considered in terms of its influence on the calculation of adhesive fracture toughness.     

THE DEVELOPMENT OF A MANDREL PEEL TEST FOR THE MEASUREMENT OF ADHESIVE FRACTURE TOUGHNESS OF EPOXY-METAL LAMINATES

A mandrel peel test is established for measuring the adhesive fracture toughness of a metal/rubber-toughened epoxy laminate system. By adopting an energy balance analysis it is possible to determine directly both adhesive fracture toughness and plastic work in bending the peel arm around the mandrel. The suitability of the procedure is examined for various types of metal peel arms, which are classified in terms of their ability to deform plastically during the test. The plastic work is also predicted theoretically, and comparisons are made between the measured and calculated values. The fracture energies determined from the mandrel tests are compared with those obtained from 90° fixed-arm peel tests. For the calculations of plastic work in bending in the fixed arm test, various options are used when modelling the tensile stress-strain behaviour of the peel arm material. In addition, the adhesive layer thickness is considered in terms of its influence on the calculation of adhesive fracture toughness.     

Adhesive Joint Durability Assessed Using Open-faced Peel Specimens

This paper presents an investigation of the durability of two aluminum-epoxy adhesive systems by means of open-faced peel specimens. A peel analysis model was used to determine the fracture energy from the peel data. Both wet and dry peel tests were conducted in order to distinguish between the reversible and the permanent effects of water. The effects of water on the cohesive properties of the adhesives were also assessed by tension tests. It was found that, for the two-part epoxy adhesive, which plasticized to a large extent, the peel testing should be carried out in a dry state to assess the interfacial weakening. It was also observed that the two-part adhesive was much stiffer in the dry, degraded state, and it was important to take account of such permanent changes in the cohesive properties associated with water uptake when determining the fracture energy from the peel data. In contrast, the one-part epoxy system did not suffer from appreciable cohesive changes, either reversible or permanent. In this case, both wet and dry failure loci were interfacial, and some of the interfacial damage was found to be reversible. Finally, surface analyses of the peel failure surfaces were carried out, and the formation of micro-debonds was identified as a possible mechanism of degradation for the two-part system.     

Interfacial Interpretation of Autohesion of Ethylene/1-Octene Copolymers by Atomic Force Microscopy

The T-peel fractured surfaces of bonded films of ethylene/1-octene copolymers with different 1-octene contents were characterized using atomic force microscopy (AFM) and analyzed by fractal analysis. The AFM images showed strong dependence on the bonding temperature, peel rate, and the 1-octene content visually. This dependence has been demonstrated quantitatively by the fractal analyses which quantified an irregular surface by fractal dimensions and characteristic sizes. Two regimes showing fractal features were identified for each surface. In Regime I (higher magnifications) the welding and the following T-peel fracture procedures did little to change the fractal dimensions compared with the original surfaces before welding. But there were significant changes in Regime II (lower magnification) before welding and after T-peel fracture tests. The length scale that separated these two regimes is of the same order as that of polyethylene lamellar crystal structures. This suggests that the amorphous chains interdiffused across the interface while unmelted interfacial crystal structures remain essentially unaltered during the autohesion process. A “stitch-welding” autohesion mechanism was proposed to describe the bonding process in which only chains in the amorphous portions could interdiffuse. During the T-peel fracture tests, a crystal structure on the interface is either pulled over to the other side of the interface due to the interdiffused chains, remains unchanged, or is used as an anchor to pull a crystal structure from the other side of the interface. The characteristic sizes at which the fractal characteristics emerge were shown to be larger for the surfaces fractured at higher peel rates, which corresponds to higher fracture energy. This suggests that the appearance of fractal behavior at larger scales requires higher fracture energies. The characteristic sizes and fractal dimensions were also shown to depend on the molecular structure.     

The effect of adherend alignment on the behaviour of adhesively bonded double lap joints

For an adhesively bonded double lap joint, end mismatch between the two outer adherends can not be removed completely although it can be controlled within a manufacturing tolerance. This paper shows that the end mismatch introduces local bending and, consequently, results in a significant effect on the surface normal displacement. Furthermore, the end mismatch also affects the shear and peel stresses in the adhesive. To include the end mismatch effect, a modified equation is developed to characterise the peel stress in the adhesive layer in terms of the surface normal displacement measured using the holographic interferometry technique. The surface normal displacement predicted by the FEM is validated experimentally. A good correlation is also noted between the adhesive peel stress computed using the FEM and that calculated using the modified equation and the measured surface normal displacement.     

Significance of Peel Test Speed on Interface Strength in Cohesive Zone Modeling

The analysis of the experimental peel test data for obtaining the adhesion fracture energy of an adhesively laminated polymer to the sheet metal surface is considered. The experimental results of the 180° peel test on two types of polymer laminated sheet metal at three different peel speeds are analyzed by two methodological approaches in cohesive zone modeling. These approaches are linear-elastic stiffness approach and critical maximum stress approach. Comparing the results of these two approaches reveals the significance of the peel test speed on the interface strength determination for cohesive zone modeling. It is concluded that a “reference” peel speed may exist at which the interface strength is equal to the yield strength of the peel arm material. A constitutive equation has been proposed which relates the interface strength to the peel test velocity by using the reference peel speed and its corresponding peel arm yield strength.     

衬层预固化程度对衬层/推进剂界面粘接性能的影响

研究了壳体粘接式固体火箭发动机装药中衬层的固化程度对推进剂和衬层间界面粘接性能的影响,采用红外光谱技术对衬层固化过程中微观结构的变化进行了表征,初步探讨了衬层固化程度影响界面粘接强度的机理。结果表明,衬层固化0~8h,-NCO含量迅速下降,衬层与推进剂界面的粘接强度随衬层固化时间的增加而增大;固化8~40h时,衬层中的-NCO含量下降速度减小;衬层与推进剂界面的粘接强度随衬层固化时间的变化不大。     

湿度对HTPB复合推进剂力学性能的影响

通过常温湿度试验,研究了HTPB复合推进剂力学性能随试验时间的变化规律.试验证明,湿度使HTPB推进剂的力学性能大幅度下降;经干燥后,其力学性能能够得到部分恢复.用扫描电镜对常温湿度试验前后推进剂的表面状态和拉伸断口进行了对比分析,结果表明,试验后推进剂表面的AP粒子形状有明显改变,拉伸断口上的AP粒子裸露面增大,粒子脱落坑表面光滑、规整.由此得出HTPB推进剂吸湿后,通过干燥方法不能使其力学性能恢复到原始状态.