Measurement of residual stresses in thick composite cylinders by the radial-cut-cylinder-bending method

Thick fabric composite cylinders for nozzle parts in solid rocket motors should be designed to endure the extreme temperature and pressure of combustion gas. As the thickness of the composite cylinder increases, fabricational residual stresses due to the anisotropic thermal expansion or shrinkage of fabric composites also increase, which induces inter-laminar failures. Therefore, the accurate estimation of the residual stresses is indispensable for the development of thick fabric composite cylinders.

In this paper, the residual stresses in thick cylinders made of carbon fabric phenolic composites were measured by a new radial-cut-cylinder-bending method. To obtain the residual stresses from the measured relative strains during the radial-cut operation, a bending test of the cylinder with the radial-cut was performed instead of measuring the material properties with respect to radial positions. The thermal residual stresses were also calculated by finite element method considering shear deformation of fabric layers, and compared with the measured residual stresses by the new method, from which it was found that the new simple method estimated the residual stresses pretty well. Also the inter-laminar tensile strength at the position of maximum radial residual stress could be obtained from the bending test.     

热压罐零吸胶工艺树脂压力在线测试及其变化规律

针对碳纤维缎纹布/环氧914预浸料热压罐零吸胶工艺,采用热压成型过程树脂压力在线测试系统监测树脂压力的大小与分布,分析了真空、外加气压对树脂压力的作用规律,通过显微观察研究了真空及外加气压对孔隙缺陷的影响。实验结果表明,所采用的在线测试系统可以定量分析真空在铺层内的作用程度和树脂压力的变化;零吸胶工艺树脂承担了大部分外压且沿层板厚度及面内方向分布均匀;真空通过铺层内的气路通道排出夹杂空气,其作用程度受到树脂黏流状态和铺层密实程度的影响;不同压力条件下复合材料层板孔隙状况与树脂压力的测试结果相吻合。     

Reduction of residual stresses in thick-walled composite cylinders by smart cure cycle with cooling and reheating

The nozzle parts of solid rocket motors must endure both the internal pressure generated by high temperature exhaust gas and the mechanical load generated by steering operation. Therefore, the nozzle parts of solid rocket motors are fabricated with thick carbon fiber phenolic resin composites. When the thick-walled phenolic composite cylinder is cooled down from the curing temperature of about 155 °C to the room temperature, thermal residual stresses are created due to the anisotropic thermal deformation of the composite structure.

In this paper, a smart cure method with cooling and reheating was developed to reduce residual stresses in thick-wound composite cylinders made of carbon phenolic woven composite. The optimal cure cycle was obtained to reduce the residual stresses without increasing processing time and applied to fabrication of the thick-walled composite cylinder. From the residual stresses measured by the radial-cut-cylinder-bending method, it was found that the residual stresses were reduced 30% by using the smart cure method.     

Internal stresses and voids in SiC particle reinforced aluminum composites for heat sink applications

Metal-matrix composites (MMC) are being developed for power electronic IGBT modules, where the heat generated by the high power densities has to be dissipated from the chips into a heat sink. As a means of increasing long term stability a base plate material is needed with a good thermal conductivity (TC) combined with a low coefficient of thermal expansion (CTE) matching the ceramic insulator. SiC particle reinforced aluminum (AlSiC) offers the high TC of a metal with the low CTE of a ceramic. Internal stresses are generated at the matrix-particle interfaces due to the CTE mismatch between the constituents of the MMC during changing temperatures. Neutron and synchrotron diffraction was performed to evaluate the micro stresses during thermal cycling. The changes in void volume fraction, caused by plastic matrix deformation, are visualized by synchrotron tomography. The silicon content in the matrix connecting the particles to a network of hybrid reinforcement contributes essentially to the long term stability by an interpenetrating composite architecture.     

Numerical and Experimental Study of the Bleeder Flow in Autoclave Process

In the autoclave process, resin flow is a primary mechanics for the removing of excess resin and voids entrapped in the laminate and obtaining a uniform and void free composite part. A numerical method was developed to simulate the resin flow in the laminate and the bleeder, and the effects of ‘bleeder flow’ on the resin flow and fiber compaction were conducted. At the same time, fiber distribution in the cured laminates was investigated by both experiments and simulations for the CF/Epoxy and CF/BMI composites. The data of the experiments and simulations demonstrated that fibers consolidated and reconsolidated in the laminate and it was impacted by the viscosity and gel time of the resin system. Compared to the post study in which only resin flow in the laminate are considered, these results will deepen the understanding of the consolidation process, resin pressure variation and void control during the autoclave process, which is valuable for the study of the performance of composite parts, provided that fiber distribution does affect some properties of composite material.     

孔隙对碳纤维/环氧复合材料层合板层间剪切疲劳性能的影响

研究了孔隙对碳纤维增强环氧树脂基复合材料层合板［(±45)/0₄/(0, 90)/0₂］_S的静态层间剪切强度和层间剪切疲劳性能的影响。采用不同的热压罐压力制备了孔隙率为0.4%~6.6%的试样。采用显微照相法和图像分析技术对孔隙率和孔隙的微观形貌进行了分析。研究结果表明, 随着热压罐压力的降低, 大孔隙(S>7.85×10-3mm2)所占的比例逐渐增加, 平均孔隙率增加。在孔隙率为0.4%~6.6%时, 每增加1%, 复合材料层压板的层间剪切强度下降2.4%。随着孔隙率的增加, 层压板的疲劳寿命降低。与静态试验相比, 孔隙率对层压板疲劳性能的影响比对静态性能的影响大。大孔隙的存在促进了疲劳裂纹的产生和扩展。      

Manufacture and mechanical characterisation of high voltage insulation for superconducting busbars – (Part 1) Materials selection and development

It is planned that the high voltage electrical insulation on the ITER feeder busbars will consist of interleaved layers of epoxy resin pre-impregnated glass tapes (‘pre-preg’) and polyimide. In addition to its electrical insulation function, the busbar insulation must have adequate mechanical properties to sustain the loads imposed on it during ITER magnet operation. This paper reports an investigation into suitable materials to manufacture the high voltage insulation for the ITER superconducting busbars and pipework. An R&D programme was undertaken in order to identify suitable pre-preg and polyimide materials from a range of suppliers. Pre-preg materials were obtained from 3 suppliers and used with Kapton HN, to make mouldings using the desired insulation architecture. Two main processing routes for pre-pregs have been investigated, namely vacuum bag processing (out of autoclave processing) and processing using a material with a high coefficient of thermal expansion (silicone rubber), to apply the compaction pressure on the insulation. Insulation should have adequate mechanical properties to cope with the stresses induced by the operating environment and a low void content necessary in a high voltage application. The quality of the mouldings was assessed by mechanical testing at 77 K and by the measurement of the void content.     

A non-local void filling model to describe its dynamics during processing thermoplastic composites

A model is developed to describe the void dynamics within thermoplastic composite tape during the tape placement process. The model relates the volatile pressure in voids, the applied compaction load, fiber bed response and the resin pressure due to squeeze-flow of resin from resin-rich regions to fill void regions. This model relies on some geometric simplifications, but incorporates the relevant physical phenomena.This void consolidation model was implemented in a numerical code which predicts the void development during the process. The initial void geometry can be introduced either manually, using a random generation algorithm or from actual processed tape micrographs.The model predicts that the final void content depends on the original void content but also on the initial void distribution. Presented results analyze the influence of void distribution on tape consolidation. Limitations of the consolidation process rate by the resin squeeze flow pressures are clearly demonstrated.     

Experimental characterization of voids in high fibre volume fraction composites processed by high injection pressure RTM

Replacing autoclave processes is a well-known industry drive in the composites community. One of the most recognized candidates for this replacement is high injection pressure resin transfer moulding (HIPRTM), because it is both an out of autoclave process and because the high processing pressures can, hypothetically, reduce the size of voids, thereby reducing void content. In order to clarify this issue, this paper presents our results on the size distribution and total void fraction of composites containing high fibre volume fractions (>60%) composites produced by HIPRTM. To substantiate this work we present a comparative study considering both autoclave and RTM at lower pressure/fibre volume fractions. Results show that HIPRTM is able to produce high fibre volume fraction parts at very low void content (<0.05%) and is comparable to autoclave results. Future work should study the mechanical properties of these laminates in order to clarify further the limits of HIPRTM.     

Composite nozzle dam in the steam generator of a nuclear reactor

The period of normal shutdown and maintenance for a nuclear power plant can be remarkably shortened when the examination and maintenance of the tubes of a steam generator are simultaneously carried out with refueling. The nozzle dam in a steam generator is a closure to block the flow of coolant from the inlet-outlet nozzles in a steam generator. Recently, the installation and removal operations of the nozzle dam were attempted using a robot rather than the manual operation because of the radioactive hazard. For the operation using the robot the weight of the dam must be reduced because the robot's payload for assembling operations is usually small.

In this work, a lighter nozzle dam was designed and manufactured using a carbon-epoxy composite, a glass-epoxy fabric composite, an aluminium plate and honeycomb. The carbon-epoxy composite was used for the main structural material and the honeycomb was used to increase the bending stiffness, while the aluminium plate and the glass-epoxy fabric composite were used to reduce the stress concentration around bolt holes. Also, the variation in mechanical properties of composites with respect to radiation emission was investigated.

In order to verify the structural integrity of the nozzle dam installed on the nozzle ring with bolts, the stress analyses of the nozzle dam under hydrostatic pressure were performed using commercial finite-element software and the pressure test was performed. The stiffness of the composite nozzle dam was measured and compared to that obtained by finite-element analysis.     

The effects of void contents on the long-term hygrothermal behaviors of glass/epoxy and GLARE laminates

In this paper the effect of voids formed in both the glass/epoxy and GLARE laminates on their long-term hygrothermal behaviors are evaluated. By adopting different autoclave pressures during a cure cycle, the specimens with different void contents ranging from 0.5% to 2.0% were obtained for both materials. Two types of hygro and thermal aging tests were then carried out according to water absorption in distilled water and thermal cyclic fatigue up to 1500 cycles, respectively. The resulting behaviors of degradation for both materials due to two types of aging tests were evaluated by the test for interlaminar shear strength which is highly affected by those types of aging. Experimental results indicate that the autoclave pressure enhances a certain extent of the GLARE’s interfacial bonding with the low void contents. It therefore leads to an increase in the resistance to the hygro and thermal agings.     

Carbon fiber-reinforced cyanate ester/nano-ZrW2O8 composites with tailored thermal expansion

Fiber-reinforced composites are widely used in the design and fabrication of a variety of high performance aerospace components. The mismatch in coefficient of thermal expansion (CTE) between the high CTE polymer matrix and low CTE fiber reinforcements in such composite systems can lead to dimensional instability and deterioration of material lifetimes due to development of residual thermal stresses. The magnitude of thermally induced residual stresses in fiber-reinforced composite systems can be minimized by replacement of conventional polymer matrices with a low CTE, polymer nanocomposite matrix. Zirconium tungstate (ZrW(2)O(8)) is a unique ceramic material that exhibits isotropic negative thermal expansion and has excellent potential as a filler for development of low CTE polymer nanocomposites. In this paper, we report the fabrication and thermal characterization of novel, multiscale, macro-nano hybrid composite laminates comprising bisphenol E cyanate ester (BECy)/ZrW(2)O(8) nanocomposite matrices reinforced with unidirectional carbon fibers. The results reveal that incorporation of nanoparticles facilitates a reduction in CTE of the composite systems, which in turn results in a reduction in panel warpage and curvature after the cure because of mitigation of thermally induced residual stresses.     

Variation of part thickness and compaction pressure in vacuum infusion process

In vacuum infusion (VI), it is difficult to manufacture a composite part with small dimensional tolerances, since the thickness of the part changes during resin injection. This change of thickness is due to the effect of varying compaction pressure on the upper mold part, a vacuum bag. In this study, random fabric layers with an embedded core distribution medium is used. The thickness of the composite part and resin pressure are monitored using multiple dial gages and pressure transducers; the results are compared with the model developed by Correia et al. [Correia NC, Robitaille F, Long AC, Rudd CD, Simacek P, Advani SG. Analysis of the vacuum infusion molding process: I. Analytical formulation. Composites Part A: Applied Science and Manufacturing 26, 2005. p. 1645–1656]. To use this model, two material characteristics databases are constructed based on the process parameters: (i) the thickness of a dry/wet fabric preform at different compaction pressures, and (ii) the permeability of the preform at different thicknesses. The dry-compacted preform under vacuum is further compacted due to fiber settling in wet form after resin reaches there; the part thickens afterwards as the resin pressure increases locally. The realistic model solution can be achieved only if the compaction characterization experiments are performed in such a way that the fabric is dry during loading, and wet during unloading, as in the actual resin infusion process. The model results can be used to design the process parameters such as vacuum pressure and locations of injection and ventilation tubes so that the dimensional tolerances can be kept small.     

Influence of functional graphene as filler on the tribological behaviors of Nomex fabric/phenolic composite

Graphene and polystyrene functionalized graphene (PS-graphene) had been synthesized, and were employed as fillers to improve the anti-wear property and load-carrying capacity of Nomex fabric/phenolic composites. Pin-on-disk type wear tests show that the friction coefficients and wear rates for both graphene and PS-graphene filled fabric/phenolic composites were reduced, when compared with unfilled fabric composite. Moreover, it was found that the 2 wt% PS-graphene filled Nomex fabric/phenolic composites exhibited the optimal tribological properties. The enhancement on the wear property of graphene and PS-graphene filled Nomex fabric composite was mainly due to the self-lubrication of graphene and the easy-formed transfer film on the counterpart pin. We also investigated the effect of filler content, applied load, and sliding speed on the tribological properties of the Nomex fabric/phenolic composites.     

Effect of the smart cure cycle on the performance of the co-cured aluminum/composite hybrid shaft

In this work, a smart curing method for the co-cured aluminum/composite hybrid shaft which can reduce the thermal residual stresses generated during co-curing bonding operation between the composite layer and the aluminum tube was applied. In order to reduce the thermal residual stresses generated during co-cure bonding stages due to the difference of coefficients of thermal expansions (CTE) of the composite and the aluminum tube, a smart cure cycle composed of cooling and reheating cycles was applied. The heating and cooling operations were realized using a pan type heater and water cooling system. The thermo-mechanical properties of the high modulus carbon epoxy composite were measured by a DSC (differential scanning calorimetry) and rheometer to obtain an optimal time to apply the cooling operation. Curvature experiment of the co-cure bonded steel/composite strip was performed to investigate the effect of cure cycle on generation of the thermal residual stress. Also, the thermal residual stresses of the aluminum/composite hybrid shaft were measured using strain gauges with respect to cure cycles.

Finally, torsional fatigue test and vibration test of the aluminum/composite hybrid shaft were performed, and it has been found that this method might be used effectively in manufacturing of the co-cured aluminum/composite hybrid propeller shaft to improve the dynamic torque characteristics.     

Electrical behavior of carbon fiber polymer-matrix composites in the through-thickness direction

The electrical behavior of continuous carbon fiber epoxy-matrix composites in the through-thickness direction was studied by measuring the contact electrical resistivity (DC) of the interlaminar interface in the through-thickness direction. The contact resistivity was found to decrease with increasing curing pressure and to be higher for unidirectional than crossply composites. The lower the contact resistivity, the greater was the extent of direct contact between fibers of adjacent laminae. The activation energy for electrical conduction in the through-thickness direction was found to increase with increasing curing pressure and to be lower for unidirectional than crossply composites. The higher the activation energy, the greater was the residual interlaminar stress. Apparent negative electrical resistance was observed, quantified, and controlled through composite engineering. Its mechanism involves electrons traveling in the unexpected direction relative to the applied voltage gradient, due to backflow across a composite interface. The observation was made in the through-thickness direction of a continuous carbon fiber epoxy-matrix two-lamina composite, such that the fibers in the adjacent laminae were not in the same direction and that the curing pressure during composite fabrication was unusually high (1.4 MPa).     

Influence of prepreg conditions on the void occurrence and tensile properties of woven glass fiber-reinforced polyimide composites

The influence of prepreg solvent content on void occurrence in woven glass fiber-reinforced polyimide composites and their tensile properties was studied. A precursor solution of SKYBOND 703 was diluted in an additional solvent (n-methyl pyrrolidone) and the glass woven fabric was immersed in about 40 wt.% polyamic acid, in solvent. Prepregs were dried at 373 K for different time intervals, ranging from 2 to 24 h. Prepregs with varying residual solvent content under each condition were laid up, and their [(0/90)]₄ composite laminates were formed by autoclaving at a hydrostatic pressure of 0.7 MPa. The relationship of drying time with the amount of residual prepreg solvent, as well as with the volume fractions of fiber and voids was investigated. The void geography and content for each composite laminate, and the tensile strength and modulus at room temperature were also evaluated. The results clearly indicated that, depending on the altering residual solvent content in the prepreg, the void geometry and location influenced reduction of the tensile properties of woven fabric composite laminate. An appropriate prepreg resin viscosity during curing, which avoids reduction of the tensile properties, was revealed.     

Experimental investigation of high fiber tow count fabric unsaturation during RTM

High fiber tow count fabrics have been developed by fibers and fabrics suppliers to meet automotive cost and performance requirements when manufacturing structural automotive composite parts at high production rates by RTM. Impregnation of these large fiber clusters may lead to local incomplete saturation of fabrics. Mechanical softening, early failure, or part rejection because of high voids content may be expected. A new experimental method has been proposed to measure the air volume entrapped within the wetted part of the fabric at any given time and to quantify air entrapment kinetics. An important observation is that the whole unsaturation grows linearly with time for 1D flow. The modified capillary number has been correlated to the amount of air entrapped during the injection process. However, results of this study show that it cannot account for void mobilization and elimination. A critical pressure for the onset of void mobilization has been identified for one fluid/preform combination. This experimental work carried out with proper calibration provided the evidence that for high modified capillary number, a decrease in void content is to be expected.     

Deformation behavior of cement treated demolition waste with recycled masonry and concrete subjected to drying and temperature change

Cement treated materials are widely used as road bases in pavements. Shrinkage of these materials due to moisture and temperature changes is a critical issue for determining shrinkage cracking in pavements. This paper presents the influence of four mixture variables (masonry content, cement content, water content and degree of compaction) on drying shrinkage and coefficient of thermal expansion (CTE) of cement treated demolition waste with recycled masonry and concrete (CTM_iG_r). The experimental results showed that the masonry content was the dominating factor affecting dry shrinkage and CTE of CTM_iG_r. Increasing the masonry content can not only lead to an obvious decrease of dry shrinkage of CTM_iG_r, but also a low CTE level. Dry shrinkage of CTM_iG_r increased as the increase of cement content as well as degree of compaction and water content. The CTE of CTM_iG_r was between 7.58 × 10⁻⁶/°C and 10.22 × 10⁻⁶/°C, which was mainly determined by the masonry and cement content.     

Glass bead filled Polyetherketone (PEK) composite by High Temperature Laser Sintering (HT-LS)

Thermal expansion behaviour and mechanical performance of high temperature laser sintered PEK (LS-PEK) and glass bead filled PEK composite (LS-GB/PEK) are reported and discussed for the first time. The laser sintered PEK and its composite show anisotropic thermal expansion properties related with their multilayer structure created by the laser sintering method. Compared with the PEK manufactured by conventional injection moulding (IM-PEK), LS-PEK has a 10% lower tensile strength but a higher hardness due to its greater crystallinity. Addition of glass bead to PEK increases the hardness of PEK without affecting the ultimate tensile strength and also improves the thermal stability of laser sintered parts. The rule of mixtures is applied to simulate the coefficient of thermal expansion (CTE) of the LS-GB/PEK composite structures and compare with experimental results. The measured CTE values match the calculated results below Tg and deviate slightly from the simulated trend line above Tg.