An Examination of Binder Systems and Their Influences on Burn Rates of High‐Nitrogen Containing Formulations

The examination of Laminac 4116/Lupersol and Epon 813/Versamid 140 binder systems, and their influences on burn rates of pyrotechnic formulations is described. Laminac 4116/Lupersol‐based formulations had shorter burn times and larger heat of explosion values compared to their Epon 813/Versamid 140 counterparts. An understanding of the chemical structures and approximate oxygen balances of these binder systems served to explain their burn time differences when used in pyrotechnic formulations. Bomb calorimetry of Laminac 4116/Lupersol‐ and Epon 813/Versamid 140‐based formulations provided heat of explosion values to further explain the burn time differences of the two binder systems used.     

腰果壳油改性酚醛胺环氧树脂固化剂性能研究

对比了酚醛胺NX-2003D及聚酰胺Versamid140与环氧树脂Epon828胶粘剂的力学强度,及在室温、低温、潮湿面的粘接强度。通过设计的建筑胶配方,研究了其室温及低温的粘接强度增长速度,及耐湿热老化性能。结果表明,在常温固化时,NX-2003D体系具有与Versamid140体系相近的良好力学强度及粘接性能;但在低温条件下,NX-2003D体系表现出优于聚酰胺140体系的固化性能,在0℃固化7 d及14 d后,NX-2003D体系的拉伸剪切强度分别达到5 MPa及11 MPa以上,可以满足使用要求。NX-2003D体系的耐湿热老化性能及对潮湿面的粘接强度,也明显优于聚酰胺140体系。     

Fatigue of glass and carbon fiber reinforced engineering thermoplastics

Cyclic tension fatigue S-N curves are given for injection moleded Nylon 6/6, polycarbonate, polysulfone, polyphenylene sulfide, and poly(amide-imide) matrices with glass and carbon fibers as well as for unreinforced material. The S-N curves for most composites appear linear, with no evidence of a fatigue limit up to 10⁶ cycles. Some nonlinearity is evident with the Nylon 6/6 composities, and these appear to fail at a cumulative strain similar to the ultimate static strain. The remainder of the composites appear to fail by a crack propagation mechanism. The glass reinforced materials all degrade at a similar rate in fatigue, while the carbon reinforced materials with brittle matrices degrade more slowly than do those with ductile matrices. The latter effect may be due to greater integrity of the cracked regions for brittle matrix systems.     

Effect of residual stresses on fatigue crack growth behavior of aluminum substrate repaired with a bonded composite patch

Composite patches bonded to cracked metallic aircraft structures have been shown to be a highly cost-effective method for extending the service life of the structures. The fatigue crack growth behavior of pre-cracked 7075-T6 aluminum substrate with the 12.7-mm V-notch crack repaired with boron/epoxy composite patches was investigated. 1-ply, 2-ply, 3-ply and 4-ply composite patches were studied. The residual stresses due to mismatch of the coefficients of thermal expansion between the aluminum plate and boron/epoxy composite patch were calculated based on the classical equation. The effects of the residual stresses and patch layers on fatigue lifetime, fatigue crack growth rate, and fatigue failure mode of the repaired plates were examined experimentally. A modified analytical model, based on Rose's analytical solution and Paris power law, was developed for this research. This model considered the residual stress effect and successfully predicted the fatigue lifetime of the patched plates. Results showed that the composite patch had two competing impacts on the structure. The composite patch could cause residual tensile stress in the aluminum substrate, which could consequently increase the crack growth rate. Moreover, reinforcement with the composite patch could also retard the crack propagation in the aluminum plate. If a 4-ply composite patch was used, it resulted in high residual stresses and effectively would not extend the fatigue lifetime of cracked aluminum plates.     

Mechanical and thermal properties of epoxy resins with reversible crosslinks

The tensile properties: Young's modulus, ultimate tensile strength, ultimate elongation, the glass transition temperature, and the dynamic mechanical properties (dynamic shear modulus (G'), loss tangent (Tan δ)), of three epoxy resins (Epon 828, Epon 836, Epon HPT 1071) cured with the disulfide-containing crosslinking agent—4.4-dithiodianilme (DTDA) have been characterized. The results show that DTDA is a satisfactory crosslinking agent for the epoxide resins that have been studied as compared to the well-known curing agent methylene dianiline (MDA). There are no significant differences between the properties of Epon 828 cured with DTDA at stoichiometric ratio (2:1) and Epon 828 cured with DTDA at small amine excess ratio (1.75:1). The glass transition temperature of the cured tetrafunctional epoxy resin Epon HPT 1971 (235°C) is significantly higher than that of difunctional epoxy resins such as Epon 828 (T_g–175°C), but the product is too brittle to be used without plasticizer.     

Mitigation of Single‐Point‐of‐Failure: Development of M127A1 White Star Illuminant Compositions Containing an Epoxy Binder System

A replacement for the M127A1 hand‐held signal illuminant was developed to alleviate concerns associated with single‐point‐of‐failure. In addressing single‐point‐of‐failure, Laminac 4116/Lupersol binder system were replaced with Epon 813/Versamid 140 binder system. Powdered sodium nitrate was replaced with prilled sodium nitrate in the disclosed formulations to minimize hygroscopicity concerns associated with this oxidizer. The performance of the prilled sodium nitrate‐based formulations, their burning behaviors, and the sensitivities of the best performing illuminant toward various ignition stimuli are also described in detail.     

PE100管及其热熔接头的蠕变裂纹扩展行为

聚乙烯(PE)管性能优异,广泛应用于城市水及燃气供应系统。PE管的主要破坏形式是长期静压载荷下的慢速裂纹扩展失效。在蠕变条件下,采用光滑试样和裂纹圆棒试样对PE100管及其热熔接头进行了测试,得到了基于蠕变断裂参数C*的蠕变裂纹扩展动力学关系式。利用扫描电子显微镜(SEM)分析了裂纹圆棒试样的断面形貌,对比分析结果发现,蠕变裂纹扩展失效模式对应的最大应力为15.05 MPa,热熔接头熔合面分布有约11个/mm^2、直径范围为1~5μm的微气孔,热熔接头断裂微纤平均长度比母材约小20%~45%。当热熔对接时,熔合面存在的微气孔以及系带分子的浅渗透是导致PE100热熔接头蠕变裂纹扩展抗力降低的主要原因。     

The effect of time and temperature on the mechanical behavior of epoxy composites

A crosslinked epoxy resin consisting of a 60/40 weight ratio of Epon 815 and Versamid 140 and composites of this material with glass beads, unidirectional glass fibers and air (foams) were tested in tension, compression and flexure to determine the effect of time and temperature on the elastic properties, yield properties and modes of failure. Unidirectional continuous fiber-filled samples were tested at different fiber orientation angles with respect to the stress axis. Strain rates ranged from 10^?4 to 10 in./in.-min and the temperature from ?1 to 107°C. Isotherms of tangent modulus versus strain rate were shifted to form master modulus curves. The moduli of the filled composites and the foams were predictable over the entire strain rate range. It was concluded that the time-temperature shift factors for tangent moduli and the time-temperature shift factors for stress relaxation were identical and were independent of the type and concentration of filler as well as the mode of loading. The material was found to change from a brittle-to-ductile-to-rubbery failure mode with the transition temperatures being a function of strain rate, filler content, filler type and fiber orientation angle, indicating that the transition is perhaps dependent on the state of stress. In the ductile region, an approximately linear relationship between yield stress and log strain is evident in all cases. The isotherms of yield stress versus log strain rate were shifted to form a practically linear master plot that can be used to predict the yield stress of the composites at any temperature and strain rate in the ductile region. The time-temperature shift factors for yielding were found to be independent of the type, concentration and orientation of filler and the mode of loading. Thus, the composite shift factors seem to be a property of the matrix and not dependent on the state of stress. The compressive-to-tensile yield stress ratio was practically invariant with strain rate for the unfilled matrix, while fillers and voids raised this ratio and caused it to increase with a decrease in strain rate. The yield strain of the composites is less than the unfilled matrix and is a function of fiber orientation and strain rate.     

A crack layer approach to fatigue crack propagation in high density polyethylene

Fatigue crack propagation (FCP) in high density polyethylene (HDPE) is observed to occur with an accompanying layer of damage ahead of the crack tip. The crack layer theory, which accounts for the presence of both the damage and the main crack, is applied to the problem. It is observed that the kinetic behavior of HDPE under fatigue consists of three regions: initial acceleration, constant crack speed (“deceleration”), and reacceleration to failure. Within the first two regions, crack propagation appears “brittle,” while in the third region “ductile” behavior is manifested. Ultimate failure occurs via massive yielding of the unbroken ligament. Two damage mechanisms are found to be responsible for HDPE failure: formation of fibrillated voids and yielding. Both mechanisms are present throughout the entire lifetime of the crack, but the former dominates the “brittle” crack propagation region, while the latter is more prominent in the “ductile.” Throughout the analysis the resistance moment R_t is approximated as the total volume of transformed material associated with crack advance. Crack layer analysis produces a satisfactory fit of the experimental data and yields a specific enthalpy of damage, γ^*, value in the 1–2 cal/g range.     

R-Curve Behavior and Stable Crack Growth at Elevated Temperature (1500°–1650°C) in a Si3N4/SiC Nanocomposite

The crack growth resistance behavior and the stable crack growth regime of a Si₃N₄/SiC composite have been examined at high temperature (1500°–1650°C). SENB specimens were used and the load/unloading technique, with high deflection rates to ensure an elastic behavior, has been employed to estimate the crack lengths. Rising R -curves have been obtained with a maximum crack growth resistance almost twice as high as the initial value. Above the T _g of the intergranular glassy phase, the behavior changes from brittle to visco-plastic and, consequently, the fracture characteristics become strongly rate dependent. It is observed experimentally that in the enhanced ductile region the crack extension velocity during the stable crack propagation from a preexisting flaw decreases rapidly with time. This phenomenon has been tentatively attributed to dynamic crack-tip stress relaxation resulting from the rapid flow of the glassy intergranular phase in the process zone. Thus, the rheological properties of the composite appear to be of major importance to gain insight into the mechanical behavior at such elevated temperatures.     

环氧树脂增韧固化剂固化反应的FT-IR研究

使用环氧-三乙烯四胺缩聚物G(Epon828-TETA)及其环氧油酸辛酯改性物(G1)作为环氧树脂(Epon828)的固化剂,通过傅立叶变换红外光谱(FT-IR)法,比较研究了G/Epon828和G1/Epon828两个固化体系的动态等温固化过程,并能进一步对两个固化体系固化初期的固化反应进行了动力学研究.结果表明:两个固化体系的环氧基转化率都在较短时间内达到较高水平,随时间的延长,转化率上升缓慢并逐渐趋于恒定;固化温度越高,转化率越高;固化反应的反应速率在开始时很快达到最高,随反应时间的延长,反应速率逐渐下降并趋于稳定,并且固化温度越高,反应速率越高;但在相同的固化温度下,G/Epon828固化体系的转化率始终高于G1/Epon828固化体系.前者的固化反应速率在初期也大于后者;G/Epon828和G1/Epon828固化体系的固化反应均属一级反应,在相同的环氧基转化率时,前者的反应活化能和指前因子均小于后者.     

CRACK GROWTH IN ADHESIVELY BONDED JOINTS SUBJECTED TO VARIABLE FREQUENCY FATIGUE LOADING

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

High-Temperature Failure of an Alumina-Silicon Carbide Composite under Cyclic loads: Mechanisms of Fatigue Crack-Tip Damage

Experimental results are presented on the mechanisms of tensile cyclic fatigue crack growth in an A1₂0₃-33-vol%-SiC-whisker composite at 1400°C. The ceramic composite exhibits subcritical fatigue crack propagation at stress-intensity-fator values far below the fracture toughness. The fatigue characterized by the stressintensity-factor range, ΔK, and crack propagation rates are found to be strongly sensitive to the mean stress (load ratio) and the frequency of the fatigue cycle. Detailed transmission electron microscopy of the fatigue crack-tip region, in conjunction with optical microscopy, reveals that the principal mechanism of permanent damage ahead of the advancing crack is the nucleation and growth of interfacial flaws. The oxidation of Sic whiskers in the crack-tip region leads to the formation of a silica-glass phase in the 1400°C air environment. The viscous flow of glass causes debonding of the whisker-matrix interface; the nucleation, growth, and coalescence of interfacial cavities aids in developing a diffuse microcrack zone at the fatigue crack tip. The shielding effect and periodic crack branching promoted by the microcracks result in an apparently benefcial fatigue crack-growth resistance in the A1₂0₃—SiC composite, as compared with the unreinforced alumina with a comparable grain size. A comparison of static and cyclic load crack velocities is provided to gain insight into the mechanisms of elevated temperature fatigue in ceramic composites.     

Crack propagation in biaxially oriented polypropylene at low temperature

The crack growth behavior of polypropylene biaxially oriented by cross-rolling was studied at low temperature. Single edge notch testing produced a stable tearing type of crack growth in both 50% and 80% biaxially oriented polypropylene at ?40°C, in contrast to the brittle fracture of unoriented polypropylene. The crack growth in the two oriented materials began slowly and accelerated to a constant rate that was higher in the 80% oriented material than in the 50% oriented material. The main difference between the crack growth behavior of the two was the longer period of initial slow growth in the case of 80% orientation. This period of slow growth corresponded to crack growth through the notch tip damage zone. Residual strength diagrams were used to present the crack growth data obtained when the stress state was intermediate between plane stress and plane strain. Fractography revealed large differences among the fracture surfaces of the three materials with the unoriented polypropylene showing a grainy appearance from the brittle fracture. The two oriented materials showed considerable ductility. The 50% oriented material showed many voids in the fracture surface, indicating that voiding during the fracture process contributed significantly to the toughness improvement. The 80% oriented polypropylene showed delamination crazing on the fracture surface with layered material and fibrils bridging the crazes.     

Micromechanisms of Creep–Fatigue Crack Growth in a Silicide-Matrix Composite with SiC Particles

An experimental study has been conducted to examine the cyclic fatigue crack growth characteristics in 1200^oC air of a MoSi₂-50 mol% Wsi₂ alloy the unreinforced condition and with 30 vol% SiC particles. For comparison purposes, crack growth experiments under sustained loads were also carried out in the silicide-matrix composite. Particular attention is devoted to developing an understanding of the micromechanisms of subcritical crack-tip damage. The results indicate that enhanced viscous flow of glass films along interfaces and grain boundaries imparts pronounced levels of subcritical crack growth in the composite material; the composite exhibits a higher fatigue fracture threshold and a more extended range of stable fracture than the unreinforced alloy. The effects of glass phase in influencing fatigue crack growth in the silicide-based material are compared to the influence of in situ -formed and preexisting glass films on high-temperature cyclic fatigue crack growth in ceramics and ceramic composites. The paper concludes with a comparison of present results with the high-temperature damage tolerance of a variety of intermetallic alloys and ceramic materials.     

Initial slow crack growth behavior followed by rapid brittle fracture in a viscoelastic solid

Initial dynamic crack propagation behavior in a viscoelastic solid just after crack initiation was investigated by newly devised instrumentation at different temperatures. It was observed that initial slow crack growth precedes rapid brittle fracture. The very initial slow crack growth first appears as ductile fracture and successively as brittle crack propagation, and the latter only exists within very short crack passage. It is also recognized that this slow crack growth in a brittle manner greatly depends on the temperature.     

Crack growth in adhesively bonded joints subjected to variable frequency fatigue loading

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

Scanning electron fractography of lap-shear joints

Fracture surfaces of Epon 901/B-3 bonded aluminum alloy joints in the lap-shear configuration were studied using scanning electron microscopy. Major differences in the appearance of the fracture surface from those reported (8) for tensile loaded joints at 23°C are produced either by cyclic loading at 23°C or a change in test temperature to ?196°C. Fracture in tensile loaded joints at ?196°C is a brittle single step process in the opening mode in which rapid crack extension occurs throughout the joint with very little adhesive flow. Tensile fatigue fracture at 23°C is in the opening mode but crack extension is complicated by extensive adhesive flow throughout the entire joint.     

In situ observation of stable crack growth in freestanding diamond film samples under low cycle fatigue loading

Laser notched and pre-cracked freestanding diamond film samples were tested under cyclic loading in three point bending using a fatigue machine incorporated with the SEM(550)-SERVO scanning electron microscope. Stable crack growth was observed in situ. Crack growth was found to be discontinuous; there was generally a break after each growth. It was found that the minimum cyclic load at the first stable crack growth was approximately 60% of the fracture load for the notched sample under static loading. The minimum cyclic load for the ultimate brittle fracture was about 74% of the fracture load for the notched sample. Mechanisms for the observed stable crack growth in freestanding diamond film samples under cyclic loading are discussed. The columnar grain feature of the freestanding diamond films and the grain boundary may be responsible for the stable crack growth under cyclic loading.     

Molecular network development and evolution of nanoscale morphology in an epoxy‐amine thermoset polymer

Epoxy‐amine thermoset polymers exhibit a complicated, highly crosslinked network structure. The connectivity of this network drives material parameters such as mechanical properties and solvent permeation. Understanding the molecular network architecture is also an important aspect of the developing realistic network topologies for use in molecular dynamic simulations. Here, the evolution of network connectivity in a typical crosslinked epoxy‐amine network (Epon 828/3‐aminophenyl sulfone) is monitored as a function of cure time. Special attention is paid to nanoscale variation in the crosslink density of the network. Submicron atomic force microscope images of sample fracture surfaces revealed three distinct types of crack tip propagation. Near‐infrared spectroscopy, rheological and thermal characterization were used to correlate each type of fracture propagation behavior to a different stage of network development. Monitoring changes in the nanoscale fracture behavior reveals information regarding changes in the network architecture during cure and provides insight into the final structure of the epoxy‐amine network. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012