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.     

Compaction of thick carbon/phenolic fabric composites with autoclave method

Carbon/phenolic composites are used in the nozzle parts of solid rocket motors due to their heat-resisting, ablative, and high strength characteristics, which are required to endure the high temperature and pressure of combustion gas passing through the nozzle. But the thick axi-symmetric structure of the composite nozzle induces high thermal residual stresses due to the large difference of coefficient of thermal expansion (CTE) between the in-plane and the out-of-plane. In this work, in order to reduce the through-thickness CTE and the void content, a compression in the thickness direction was applied to the composite prepreg by a compressive jig during manufacturing of composite to supplement the low autoclave pressure. The through-thickness CTE of the fabric composite was calculated by a compaction model and compared with the measured one by thermo-mechanical analysis. The through-thickness CTE changed drastically with respect to the compaction amount, and the void content of the carbon/phenolic fabric composite laminate showed different characteristics from the ordinary fabric laminates with respect to the autoclave pressure and the jig pressure.     

CURE-DEPENDENT VISCOELASTIC RESIDUAL STRESS ANALYSIS OF FILAMENT-WOUND COMPOSITE CYLINDERS

The residual stresses induced during processing of [0/90] Tcross-ply composite cylinders is examined. A cure-dependent viscoelastic material model is used to describe the development of material behavior during cure. A finite-element model is developed using a recursive formulation in order to overcome the large memory storage requirements and lengthy calculations. Both chemical and thermal strains are modeled. The geometry modeled includes a mandrel and Teflon separation film between the mandrel and the cross-ply tube. The mandrel was shown to hare a profound influence on the level of residual stress during cure. For example, the maximum hoop stress during cure with a mandrel is 154 MPa. When no mandrel is used the maximum hoop stress is only 26 MPa. Chemical shrinkage was shown to increase the final residual stress in all cases analyzed, since both thermal shrinkage (during cool down) and chemical shrinkage (during cure) are additive. To some extent the mechanism of residual stress development in cylinders is much different compared to laminated composites. For cylinders the geometric constraint of the cylinder itself plays an important role. For example, the outer 90^° layers in a [0/90]T cylinder effectively prevent free expansion and contraction during curing. The effect is to induce radial and hoop stresses during cure.     

Interlaminar stress analysis in general thick composite cylinder subjected to nonuniform distributed radial pressure

Interlaminar stresses in thick a composite cylinder with general layer stacking subjected to uniform and nonuniform distributed radial pressure are studied. The layerwise theory of Reddy is employed for formulation of the problem. An analytical method is presented for solving the governing equations. To increase accuracy, interlaminar stresses are obtained by integrating the equilibrium equation of elasticity. After a convergence study, the accuracy of the layerwise laminate theory is investigated using the predictions of finite element method. Predictions of Hooke's law and integration method for interlaminar stresses are compared. Uniform and nonuniform internal and external loads are considered and a parametric study is done for various cylinders.     

Adhesive joining of composite journal bearings to back-up metals

Composite journal bearings are becoming popular for marine applications because they eliminate the possibility of seizure to steel journals, which is a drawback of white metal bearings. However, a reliable joining method for composite bearings to steel housings is required. In this work, hybrid composite journal bearings composed of carbon/phenolic and glass/epoxy laminated composites were manufactured with different stacking sequences and adhesively bonded to steel housings. The effect of deformations of the composite bearings due to thermal residual stresses on the adhesive joint performance was estimated with respect to stacking sequence by finite element method, and compared to the experimental results. From the measured and experimental results, it was found that the outward radial deformation of the composite bearings was beneficial to the adhesively bonded joint strength of the hybrid composite journal bearing.     

The effects of Mandrel material and tow tension on defects and compressive strength of hoop-wound,on-line consolidated,composite rings

Even carefully controlled, on-line processing, which is superior to standard autoclave processing in a number of ways, is not immune to the development of process-induced defects. These include waviness, residual stresses and voids caused by poor process parameter selections. Several designed experiments and multivariate regression analyses were performed to formally examine the effects of mandrel material, tow tension, cooling rate and ring thickness (as well as their interactions) on the development of these defects and their effects on the compressive strength of thin and thick, hoop-wound rings. Cylinders were produced from T300/polysulfone powder-impregnated towpreg using on-line consolidation; they were wound using different mandrels (aluminum, steel), tow tensions (13.3 N, 44.5 N), cooling rates (natural and natural convection) and radius-to-thickness ratios (11, 5.5). Three or four rings per cylinder were failed using a new, compliant-ring-based, radial compression test method; the compressive hoop and radial stresses at failure were measured and compared based on either uniaxial longitudinal compressive failure (thin rings) or biaxial maximum work (or Tsai-Hill) theory (thick rings). In thin rings, independent changes in mandrel material and tow tension significantly affect the hoop and radial stresses at failure: decreasing with increasing mandrel coefficient of thermal expansion and decreasing tow tension. Void contents higher than certain threshold values (2–3% in this study) substantially degrade the rings’ compressive strengths by eroding its inter- and intralaminar shear strength. Changes in part thickness had a significant effect on the radial (but not hoop) stresses at failure. Waviness, which was isolated to the innermost six plies of any cylinder, was principally affected by mandrel material, with tow tension and thickness played lesser, moderating roles. Cooling rate had no discernable effect on waviness. Residual stresses were principally affected by tow tension and thickness, with the mandrel material's role difficult to de-convolve from its effect on waviness. Modest cooling rates did not appear to affect the residual stresses. Void contents decreased with increasing cooling rates and tow tensions in thin rings and with increasing radius-to-thickness ratios in thick rings. With the appropriate selection of process parameters, on-line processing can produce relatively ‘defect-free’ hoop-wound cylinders, which can match the compressive strengths of autoclave-consolidated, unidirectional plate laminates and be used to measure the laminates intrinsic compressive strength.     

多角度交替缠绕复合圆筒的剩余应力算法及水压试验

针对含薄壁钢内衬碳纤维增强聚合物基复合材料（CFRP）多角度交替缠绕复合圆筒的剩余应力计算问题,基于正交各向异性材料的厚壁圆筒理论和弹性叠加理论,提出了考虑卸去芯模影响的多角度交替缠绕下CFRP各层和钢内衬剩余应力的逐层叠加算法,研究了恒缠绕张力下,芯模厚度和螺旋层缠绕角对CFRP各层和钢内衬剩余应力的影响。计算表明：芯模厚度越大则CFRP层剩余应力越低,但芯模厚度过大将减弱缠绕张力对钢内衬的强化效应;螺旋层缠绕角约65°时,环向层剩余应力出现极小值,螺旋层剩余应力和内衬剩余应力均出现极大值。针对缠绕张力对钢内衬的强化效应,通过水压试验加载过程中钢内衬声发射特征与复合圆筒外壁应变测试,测得的钢内衬屈服载荷与理论预测值一致,基本证实了算法的有效性。为提高CFRP层缠绕质量,基于等剩余应力假设,提出了多角度交替缠绕张力制度优化设计思路,适用于内压管的张力制度优化。     

Thermal shock analysis and thermo-elastic stress waves in functionally graded thick hollow cylinders using analytical method

Transient stress field and thermo-elastic stress wave propagation are studied in functionally graded thick hollow cylinder under arbitrary thermo-mechanical shock loading, in this article. Thermo-mechanical properties of functionally graded (FG) cylinder are assumed to be temperature independent and vary continuously and smoothly in the radial direction. The governing dynamic equations are analytically solved in temperature and elastic fields. To solve the problem, Laplace transform is used respect to time in all constitutive equations and boundary conditions. At first, temperature field equation analytically solved using Laplace transform and series method. The dynamic behaviors of thermo-elastic stresses are illustrated and discussed for various grading patterns of thermo-mechanical properties in several points across the thickness of FG cylinder. Time history of temperature field and thermal stresses are obtained using the residual theorem and the fast Laplace inverse transform method (FLIT), respectively. Also, the effects of the cylinder thickness and convection heat transfer coefficient on dynamic response of FG cylinder are revealed and discussed. The presented analytical method provides a ground to study the time histories of radial and hoop stresses in FG cylinders with different thickness and various volume fraction exponents. The advantage of this method is its mathematical ability to support simple and complicated mathematical function for the thermo-mechanical boundary conditions. A reasonable agreement can be seen in comparison of obtained results based on the presented analytical method with published data.     

Microdebonding investigation of the effects of thermal residual stress on the bond strength of a graphite/polymide composite

The microdebonding test was used to investigate the effects of thermal residual stresses resulting from different lay-ups in fabrication on the fiber/matrix bond strength of a graphite-fiber-reinforced polyimide composite. This was accomplished by comparing the results of a cross-plied laminate with those of a unidirectional laminate. The results indicated that the measured interfacial bond strength of the unidirectional composites was greater than that of the cross-plied laminate. The thermal radial stress distribution around the fiber for the unidirectional and the [0₂, 90₂]_s laminates were estimated, to explain this reduction of the interfacial bond strength.     

Thermal residual stresses in metal matrix composites: A review

Recently, metal matrix composites (MMCs) have generated a considerable interest in the materials field because of their attractive physical and mechanical properties. However, during the fabrication of MMCs, thermal residual stresses are reportedly developed in the matrix as a result of the mismatch of the thermal expansion coefficients between the reinforcement and the matrix. It is well established that these residual stresses have a significant effect on the composite properties. For example, due to the presence of thermal residual stresses, it is almost never possible to achieve the maximum elastic response of the composites. In addition, yield stress and fracture toughness of the composites are significantly affected by thermal residual stresses. In this paper, a critical review of the published literature on thermal residual stresses in MMCs and their effect on composite properties are presented. Also, experimental and numerical techniques that are currently available to measure and estimate thermal residual stresses are reviewed and discussed.     

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.     

Thermal Residual Stresses in W Fibers/Zr-based Metallic Glass Composites by High-energy Synchrotron X-ray Diffraction

Thermal residual stresses in W fibers/Zr-based metallic glass composites were measured by in situ high energy synchrotron X-ray diffraction(HEXRD). The W fibers for the composites were 300,500,and 700 m m in diameter,respectively. Coaxial cylinder model(CCM) and finite element model(FEM) were employed to simulate the distribution of thermal residual stress,respectively. HEXRD results showed that the selected diameters of W fiber had little influence on the value of thermal residual stresses in the present composites. Thermal residual stresses simulated by CCM and FEM were in good agreement with HEXRD measured results. In addition,FEM results exhibited that thermal residual stress concentrated on interface between the two phases and area where the two W fibers were the closest ones to each other.     

Polar shear-lag analysis of a composite of concentric cylinders with longitudinal cracks: Application to internal checking in wood

Composites of concentric cylinders can fail by microcracks that span the thickness and length of one cylinder but arrest at the boundary to adjacent cylinders. This failure mode was analysed by developing a new polar shear-lag analysis for stresses in a cross-section of a concentric-cylinder composite. The analysis can accurately calculate stresses in each cylinder in the presence of microcracks. These stresses were used to calculate the energy release rate for formation of a new microcrack as a tool for predicting the microcracking process due to any source of applied or residual stresses. The target problem was to model internal checking in certain species of trees. In a tree, the alternating regions of early- and late-wood that form the growth rings are the concentric cylinders. Internal checking is microcracking in the early-wood layers. The shear-lag analysis was used to predict the propensity of growth rings to form internal checks.     

石墨化参数对四向炭/炭复合材料残余应力的影响

依据四向编织炭/炭复合材料的结构特点,建立了能反映其编织方式和空间构型的单元体胞模型.在单胞几何模型和介观计算力学基础上,采用有限元方法研究了在不同石墨化温度、降温梯度和界面刚度情况下四向编织炭/炭复合材料的残余热应力分布.研究表明:低温石墨化复合材料的残余热应力比高温石墨化时小.界面刚强度低时石墨化过程中残余热应力比界面刚强度高时小.冷却速率越快,残余热应力越大.     

The influence of distinct fiber arrangement on the thermo-mechanical behavior of steel fiber-reinforced thermoplastic matrix composites

The effect of different fiber arrangements on mechanical behavior was investigated by using both experimental study and finite elements analyses. In particular, this study examined resultant residual stresses and plastic strains of steel-fiber reinforced thermoplastic composite discs under constant convective air cooling conditions. Three composite discs were manufactured with an identical concentration of woven, circular and radial arrays. The thermal and mechanical properties of the composite discs were measured. The numerical and experimental cooling curves were converged to correctly describe the convective cooling condition of the finite element analyses. After the cooling, the residual stresses and plastic strains in each disc were compared with one another and the results were analyzed. No thermal residual stress or plastic strain was observed for the woven fiber array. Residual stress and plastic strain found in the circular fiber array was twice as high as those in the radial fiber array. It is concluded that the reinforcement fiber array of thermoplastic composites is an effective parameter to describe their thermo-mechanical properties for the formation of thermal residual stresses and plastic deformation.     

General analytical solution for elastic radial wave propagation and dynamic analysis of functionally graded thick hollow cylinders subjected to impact loading

An analytical method is proposed for the dynamic response analysis of functionally graded thick hollow cylinders under impact loading. The wave motion equation is solved using an analytical method that is based on the composition of Bessel functions. The mechanical properties are considered as power functions of the radius across the thickness of FG cylinder. The FG cylinder is excited by an impact loading at the inner surface of the cylinder, and the plane strain and axisymmetry conditions are assumed for the problem. The time histories of radial displacement and radial and hoop stresses are presented. Also the dynamic response of the FG cylinder is obtained and discussed for various kinds of power function exponents.     

T250马氏体时效钢旋压薄壁圆筒变形

通过金相、透射电镜、XRD分析及应力测试,对T250马氏体时效钢旋压薄壁圆筒时效后的变形进行研究。结果表明时效后板条马氏体的基体中弥散析出强化相,位错密度下降,残余应力有所消除,同时有少量逆变奥氏体生成。时效引起的应力消除是薄壁圆筒径缩的主要原因,位错密度的下降,逆变奥氏体及析出相也对变形有明显影响。     

Influence of thermal residual stresses on the composite macroscopic behavior

The influence of the thermal residual stress on the deformation behavior of a composite has been analyzed with a new micromechanical method. The method is based on secant moduli approximation and a new homogenized effective stress to characterize the plastic state of the matrix. It is found that the generated thermal residual stresses after cooling and their influence on the subsequent deformation behavior depends significantly on the aspect ratio of the inclusions. With prolate inclusions, the presence of thermal residual stresses generate a higher compressive hardening curves of the composite, but it is reversed with oblate inclusions. For particle reinforced composite, thermal residual stresses induce a tensile hardening curve higher than the compressive one and this is in agreement with experimental observations.     

自增强厚壁筒残余应力和温度应力下应力强度因子的计算

本文用权函数法计算了自增强残余应力和温度应力下厚壁筒的应力强度因子,导得了适用于各种材料、自增强度、裂纹深度、温度和尺寸的厚壁筒应力强度因子的计算公式,编制了计算程序,方便于工程应用。     

Micromechanical analysis of layered systems of MMCs subjected to bending––effects of thermal residual stresses

A three-dimensional finite element micromechanical model is presented to study the effects of manufacturing process thermal residual stresses on the mechanical behavior of layered systems of metal matrix composites subjected to four point bending. The presented model contains layered systems, consisting of layers of monolithic titanium alloy (IMI834) and unidirectional fiber reinforced titanium metal matrix composite (SiC/Ti). A representative volume element (RVE) was defined and appropriate boundary conditions were imposed to apply bending and temperature change simultaneously on the model. In an agreement with experimental data, the model is able to predict asymmetric behavior of the composite in tension and compression on the bottom and top surfaces of the beam. This is due to the existence of a high level of thermal residual stresses arising from cool-down from manufacturing temperature. As a result of this asymmetric behavior, the neutral axis of the beam during bending moves from the mid-surface through the compressive part of the beam.