固体火箭发动机粘接界面变形破坏的细观试验与数值模拟

采用扫描电镜原位拉伸试验观察固体火箭发动机粘接界面试件在拉伸过程的变形和破坏过程,分析了载荷作用下界面失效模式和机理;依据粘接界面细观结构,建立了界面的细观数值模型,考虑了其细观损伤特点,在推进剂内部颗粒与基体之间以及推进剂/衬层之间引入界面元,对界面细观变形和破坏过程进行了数值模拟。结果表明,在外界应变5%时,表现为非均质材料内部应力分布不均,随应变的增加,推进剂内部脱湿形成的微孔洞不断扩展,最终导致界面破坏,界面拉伸失效过程表现为损伤的起裂和扩展,是推进剂内部脱湿和粘接界面脱粘共同作用的结果;界面元能有效地模拟粘接界面的脱粘过程,细观数值计算结果与试验结果吻合,正确反映了粘接界面在拉伸过程中细观损伤萌生与扩展的规律。     

NEPE推进剂的细观力学性能研究

采用原位拉伸扫描电镜技术对NEPE推进剂的单轴拉伸破坏过程进行了研究.结果表明,固体粒子与黏合剂基体的脱湿是破坏的主要因素.采用数字图像分析方法对此破坏过程进行定量化研究,对图像的分形维数进行了计算,发现随着拉伸破坏过程的进行,分形维数逐渐增大.采用此方法计算的细观结构分形维数可以作为研究NEPE推进剂细观损伤演化的定量指标.     

基于SEM与数字图像相关方法的HTPB推进剂裂尖扩展过程分析

为了从细观角度获得端羟基聚丁二烯(HTPB)推进剂裂纹的扩展特性并分析裂纹的细观破坏机理,通过原位扫描电镜(SEM)对HTPB推进剂三点弯试验裂纹尖端损伤及扩展过程进行了观察,获得了不同变形阶段的裂纹扩展变形形貌,并采用数字图像相关方法分析了图片序列,获得了推进剂裂纹尖端变形场。结果表明,随着推进剂裂纹的不断张开,当挤压位移达到1mm时,裂尖附近应变极值为0.3474,固体颗粒出现脱湿现象,颗粒周边基体受到了较大的应变作用;当挤压位移为2.5mm时,应变极值达0.4168,颗粒和基体界面产生的微裂纹与主裂纹汇聚导致裂纹的扩展。数字图像相关方法和扫描电镜相结合,可用于推进剂在细观尺度下的变形场测量与裂尖扩展过程的破坏机理分析。     

应变加载历史对推进剂力学性能的影响

对经不同应变幅值及不同次数往复拉伸后的复合固体推进剂试件进行单向拉伸试验,结合扫描电子显微镜断面观察与往复过程中原位观察结果,分析了应变加载历史对推进剂力学性能的影响。结果表明,颗粒与基体之间的脱湿程度由应变决定,往复拉伸应变幅值控制着损伤的范围,存在一个应变阈值,当往复拉伸应变幅值超出此阈值时,往复拉伸造成的损伤会影响推进剂的整体力学性能。     

基于材料基因工程的AP/Al/HTPB推进剂单轴拉伸性能及脱湿过程计算

为克服传统"经验型"材料研发模式和"唯象法"构建材料计算模型的不足,构建了一种基于材料基因工程的复合固体推进剂单轴拉伸性能预估方法;以高氯酸铵/铝/端羟基聚丁二烯(AP/Al/HTPB)推进剂为例,将填料堆积微结构定义为材料基因之一,建立了可反映推进剂配方的填料微结构最小代表性单元,分别确定了决定填料、基体、填料-基体界面力学性能的材料基因,并建立了材料基因与推进剂单轴拉伸性能之间的构效关系,获得了单轴拉伸条件下推进剂内部的损伤演变规律及应力—应变响应。结果表明,AP/Al/HTPB推进剂的应力—应变曲线可分为弹性段、黏弹性段和损伤段3个阶段;基体的黏弹性形变和填料脱湿会导致推进剂黏弹性模量下降;应变大于15.37%时,脱湿首先发生在粗粒径颗粒上方,随后以先快速、后平稳的趋势沿粗颗粒周向扩展;应变大于21.19%时,中等粒径颗粒开始脱湿;在0～25%的应变范围内细颗粒与基体始终黏接完好。     

异氰酸酯指数和湿度对丁羟衬层性能的影响

以端羟基聚丁二烯(HTPB)、甲苯二异氰酸酯、1,4-丁二醇、癸二酸二异辛酯等为原料,制备了聚氨酯复合固体推进剂丁羟衬层材料,研究了异氰酸酯指数(R)值和环境湿度对衬层性能的影响。结果表明,随着衬层R值增大,衬层的拉伸强度和邵A硬度增大,扯断伸长率减小,衬层/推进剂界面粘接强度增大;环境湿度增大,衬层发软,拉伸强度和扯断伸长率降低,内部和表面存在气泡生成。     

复合推进剂紧凑拉伸试件动态观察试验与变形场测量

对复合固体推进剂紧凑拉伸试件进行了拉伸动态观察试验,获得其变形过程的图片序列,采用数字图像相关方法得到了不同拉伸位移下裂纹尖端附近位移场及应变场。结果表明:采用数字图像相关方法可有效获取裂纹尖端的位移场和应变场以及它们的变化规律,为复合推进剂的变形破坏分析和数值模拟结果验证提供基础数据。     

干燥环境下混凝土内部损伤的量化

采用数字图像相关法(DIC)测试了C50混凝土在73％,55％和40％相对湿度环境下干燥1 d至60 d的非均匀变形.结合混凝土材料细观损伤模型,计算了混凝土内部的等效拉应变((ε)),用于获得混凝土内部损伤的分布特征,发现包裹粗骨料的砂浆拉应变较大,容易造成损伤,产生微裂缝.根据(ε)的大小和分布计算了混凝土内部损伤...     

温度和拉伸速率对NEPE推进剂力学性能影响研究

采用单轴拉伸和扫描电镜（SEM）法,研究了硝酸酯增塑聚醚（NEPE）推进剂在宽温域（–30～70℃）、宽拉伸速率（0.5～12 000 mm/min）下应力–应变曲线和拉伸断面形貌变化。结果表明,NEPE推进剂的力学性能曲线受温度和拉伸速率影响较大。随温度降低和拉伸速率升高,其应力–应变曲线从逐渐上升的曲线转变为阶跃两段式上升形状,推进剂拉伸断面颗粒脱离和基体撕裂越来越明显,高温和慢拉、低温和快拉的“偶合”作用,均加剧了推进剂内部损伤的发生。最大抗拉强度（最大拉伸应力）与拉伸速率具有较好的线性双对数关系,该变化规律可采用高分子链段的应力松弛理论进行解释,而最大伸长率（最大拉伸应变）受拉伸速率影响无明显规律。     

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。     

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.     

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.     

Effect of adhesive free-end geometry on the initiation and propagation of damaged zones in adhesively bonded lap joints

In this study the effect of adhesive free-end geometry on the initiation and propagation of damaged zones in adhesively bonded single- and double-lap joints was investigated considering the material non-linear behaviour of both adhesive and adherends and the geometrical non-linearity. The damaged adhesive and adherend zones exceeding the specified ultimate strains were determined based on the modified von Mises criterion for adherends and the failure criterion, including the effects of the hydrostatic stress states for the epoxy adhesives proposed by Raghava and Cadell. The stiffness of each finite element in the damaged zones was reduced to a negligible value, thus not contributing to the overall stiffness of the adhesive joint. This simple method provides useful information on the initiation and propagation of damaged zones in both the adhesive layer and adherends. The damaged adhesive zones due to a tensile load were observed to initiate around the rounded adherend corners inside the adhesive fillets and to propagate first towards both the free surface of the adhesive fillet and across the adhesive layer, and later along the adherend–adhesive interface. The damaged adhesive zones initiate at the left free-end of the adhesive-upper adherend interface and at the right free-end of the adhesive-lower adherend interface and propagate along these interfaces in the large adhesive fillets. In the bending test, the damaged adhesive zones appeared only at the left free-end in tension of the adhesive-upper adherend interface for the large adhesive fillets, but around the lower adherend corner for the smaller adhesive fillets. Later, it propagated with a similar mechanism as in the tensile load. In a double-lap joint subjected to a tensile load, the damaged zone appeared around the upper adherend corner inside the right adhesive fillet in tension, and propagated first towards the free surface of the adhesive fillet and through the adhesive layer towards the adhesive-middle adherend interface, and later along this interface. For all loading conditions, increasing the adhesive fillet size caused the damaged zone initiation to occur at a larger load level. The SEM micrographs of fracture surfaces around the adhesive fillets showed that the damaged zones initiated around the adherend corner inside the adhesive fillet and propagated through the adhesive fillets.     

An investigation on the initiation and propagation of damaged zones in adhesively bonded lap joints

In this study, the initiation and propagation of damaged zones in the adhesive layer and adherends of adhesively bonded single and double lap joints were investigated considering the geometrical non-linearity and the non-linear material behaviour of the adhesive and adherends. The modified von Mises criteria for adherends and Raghava and Cadell's failure criteria (J. Mater. Sci. 8, 225 (1973) [1]) including the effects of the hydrostatic stress states for the epoxy adhesive were used to determine the damaged adhesive and adherend zones which exceeded the specified ultimate strains. The stiffness of all finite elements corresponding to these zones was reduced so that they could not contribute to the overall stiffness of the adhesive joint. This approach simplifies to observe the initiation and propagation of the damaged zones in both the adhesive layer and adherends. A tensile load caused first the damaged adhesive zones to appear at the right free end of the adhesive-lower adherend interface and at the left free end of the adhesive-upper adherend interface, and then to propagate through the adhesive regions near the adhesive-adherend interfaces (interfacial failure). In the bending test, the damaged zone initiated at the left free end of the adhesive-upper adherend interface in tension, and similarly propagated through the adhesive regions close to the adhesive-adherend interface (interfacial failure). In the double-lap joint subjected to a tensile load, the damaged adhesive zones initiated first at the right free end of the adhesive-middle adherend interface and then propagated through the adhesive region near the adhesive-adherend interface. After the damaged zone reached a specific length it also grew through the adhesive thickness, and the adhesive joint failed. The SEM micrographs of fracture surfaces around the free edges of the overlap region indicated that the failure was interfacial. An additional damaged zone growth was observed in the side adhesive regions due to lateral straining, called the Poisson effect.     

丁羟推进剂/衬层粘接界面材料力学性能研究

比较了目前测试推进剂/衬层粘接界面性能的方法。针对丁羟推进剂/衬层粘接界面,采用国内外关于推进剂与金属材料粘接界面性能的微型拉伸试验方法,设计并开展了微型拉伸试验,得出该推进剂材料在粘接界面处的受影响区域范围及相应的材料性能特征。     

A new test method to obtain biaxial tensile behaviors of solid propellant at high strain rates

A new test method was proposed and applied for studying the biaxial tensile behaviors of hydroxyl-terminated polybutadiene (HTPB) propellant at high strain rates. The biaxial tensile stress responses of the propellant at room temperature and at different strain rates (0.40–85.71 s^?1) were obtained through the use of biaxial tensile strip samples, a new designed aluminum apparatus and a uniaxial Instron testing machine. A high-speed camera and scanning electron microscop (SEM) were employed to observe the biaxial tensile deformation and the damage of HTPB propellant under the test conditions. The results indicated that strain rate could remarkably influence the biaxial tensile behaviors of HTPB propellant. The effect of strain rate on the characteristics of stress–strain curves, mechanical properties and fracture mechanisms was consistent with that in uniaxial tension. However, the biaxial weakening of HTPB propellant was obvious. The strain at biaxial maximum tensile stress was between 10 and 30 % lower than that at the corresponding uniaxial case. Finally, the correlations between the fracture mechanisms and the mechanical properties of HTPB propellant, stress state and the damage of HTPB propellant were discussed. The damage of the propellant under the biaxial tensile test was less serious than that under uniaxial tension at the same strain rate. In addition, continuously increasing strain rate could change the fracture mechanism of the propellant under the biaxial and uniaxial tensile tests. In this investigation, the dominating fracture mechanism of HTPB propellant changed from the dewetting and matrix tearing at lower strain rate to the particles fracture at higher strain rate.     

Bond Strength of a Silicone Soft Lining Material to Poly(methyl methacrylate) Resin Treated with Maleic Anhydride and its Terpolymers

This study investigated the effectiveness of surface treatment of Poly (methyl methacrylate) (PMMA) denture base resin on tensile bond strength between PMMA/silicone-based soft liner. A total of 25 specimens were fabricated and assigned into five groups (n = 5). The surfaces of PMMA were treated with maleic anhydride, maleic anhydride-styrene-vinyl-acetate, n-butylmaleate-styrene-vinyl-acetate, or n-pentamaleate-styrene-vinyl-acetate prior to Primo adhesive primer application and silicone liner placement. The Primo adhesive primer on applied group untreated dentuse base resin served as control. The tensile test was performed using a universal testing machine. Fractured surfaces were observed under Scanning Electron Microscopy (SEM) and spectroscopic interpretation of the interfaces was done by Fourier Transform Infrared (FTIR). Test results showed that surface treatment increased interfacial strength giving the highest value for n-butylmaleate-styrene-vinyl acetate treated group. SEM micrographs revealed that the specimens with n-butylmaleate-styrene-vinyl-acetate and n-penta maleate-styrene-vinyl-acetate terpolymers underwent cohesive failure. FTIR analysis indicated secondary interactions such as hydrogen bonding, possibly on acrylic resin surfaces, caused by the use of maleic anhydride and its terpolymers, and the adhesive.     

Tensile properties and damage behaviors of glass‐bead‐filled modified polyphenylene oxide under large strain

Based on Continuum Damage Mechanics (CDM), a damage model for glass‐bead‐filled modified polyphenylene oxide (GB/PPO) has been proposed to describe its damage behavior at various levels of tensile strain by considering the reduction of effective loading area. Hence, an equation for prediction of effective elastic modulus of the damaged GB/PPO composites in terms of the three principal true strains was derived. The tensile properties and damage behaviors of the GB/PPO composites with different volume percentages of glass beads were investigated using standard tensile tests and load‐unload tests, respectively. The addition of glass beads increases Young's modulus of PPO but has a weakening effect on its tensile strength. A maximum value of tensile work to break and tensile strain at break was found when 5 vol% of glass beads with a mean diameter of 11 μm was blended with PPO. These results were justified through microscopic examination of the fracture surfaces of the tensile specimens by using a scanning electron microscope (SEM). In‐situ observations of the strain damage processes were made through the SEM equipped with a tensile stage to determine the strain at fully debonding of glass beads. The volumetric strain of GB/PPO composites increases because of microcavitation during strain damage. In general, the prediction for the effective elastic modulus of the damaged GB/PPO composites at different true strains is slightly higher than the experimental results. The damage evolution rates after fully debonding of glass beads from the matrix are close to those predicted by the proposed damage model.     

Bond Strength of a Silicone Soft Lining Material to Poly(methyl methacrylate) Resin Treated with Maleic Anhydride and its Terpolymers

This study investigated the effectiveness of surface treatment of Poly (methyl methacrylate) (PMMA) denture base resin on tensile bond strength between PMMA/silicone-based soft liner. A total of 25 specimens were fabricated and assigned into five groups (n = 5). The surfaces of PMMA were treated with maleic anhydride, maleic anhydride-styrene-vinyl-acetate, n-butylmaleate-styrene-vinyl-acetate, or n-pentamaleate-styrene-vinyl-acetate prior to Primo adhesive primer application and silicone liner placement. The Primo adhesive primer on applied group untreated dentuse base resin served as control. The tensile test was performed using a universal testing machine. Fractured surfaces were observed under Scanning Electron Microscopy (SEM) and spectroscopic interpretation of the interfaces was done by Fourier Transform Infrared (FTIR). Test results showed that surface treatment increased interfacial strength giving the highest value for n-butylmaleate-styrene-vinyl acetate treated group. SEM micrographs revealed that the specimens with n-butylmaleate-styrene-vinyl-acetate and n-penta maleate-styrene-vinyl-acetate terpolymers underwent cohesive failure. FTIR analysis indicated secondary interactions such as hydrogen bonding, possibly on acrylic resin surfaces, caused by the use of maleic anhydride and its terpolymers, and the adhesive.