纤维缠绕复合材料固化过程残余应力/应变的三维数值模拟

采用有限元分析软件ABAQUS,对具有金属内衬的纤维缠绕复合材料圆筒在固化过程中残余应力及应变的变化规律进行了模拟计算。采用FORTRAN语言编制了用以分析固化过程中残余应力的子程序,该子程序考虑了固化过程中复合材料力学性质的变化和由于树脂固化收缩产生的化学收缩应变。算例结果表明：复合材料和金属内衬的残余应力在初始阶段均接近于零,当固化到一定阶段,残余应力迅速增加并且很快达到最大值,在降温阶段释放了部分的残余应力;在整个固化过程中,金属内衬受到压应力,而纤维缠绕层受到拉应力。本文中的三维有限元模型可以得到任意时刻复合材料的温度及固化度分布,通过数值模拟可以有效地优化复合材料固化工艺参数,提高制件的质量。     

固化度与固化收缩对非对称复合材料 层合板固化变形的影响

采用三维有限元方法研究复合材料非对称层合板在热载荷和固化收缩载荷下的固化变形情况, 建立了材料力学特性、 固化体积收缩量和温度与固化度之间的函数关系, 考察了层合板变形曲率与温度和固化度之间的关系。数值计算结果表明: 非对称层合板变形曲率与固化终止时固化度有密切关系; 固化变形主要发生在降温阶段; 固化收缩对层合板变形曲率影响很小, 主要发生在第二个保温平台的前半段。      

底充胶及其材料模型对倒装焊热循环可靠性的影响

对倒装焊电子封装可靠性进行了热循环实验和有限元模拟,结果表明,有底充胶（underfill)时,SnPb焊点的热循环寿命可提高约16倍,并确定了Coffin-Manson半经验方程的参数,采用3种底充胶材料模型,亦即定常弹性模型,温度相关弹性模型和粘弹性材料模型,描述了底充胶U8347－3的力学性能。模拟结果表明,材料模型影响计算得到的SnPb焊点的塑性应范围,封装形变以及底充胶／芯片界面应力,采用弹性材料模型可能过高估计了SnPb焊点的热循环寿命和界面应力。     

Measurement of effective chemical shrinkage and equilibrium modulus of silicone elastomer used in potted electronic system

A fiber Bragg grating sensor technique is employed to measure two critical properties of elastomers for residual stress evaluation (total effective chemical shrinkage and equilibrium modulus). A novel experimental procedure is developed to cope with technical challenges associated with the extremely low modulus of elastomers. A constraint-free mold is first developed to ensure “free” movement of the elastomer during curing. Then, a special optical fiber with a diameter smaller than the standard fiber is employed in the experiment to make a test configuration unaffected by the free-edge effect. Two critical assumptions made for measurements are verified by an extra specimen with the standard fiber and a conventional tension test. The measured properties are used as input variables to analyze the stress distribution in a potted electrolytic capacitor system using FEM.     

复合材料层合板的固化残余应力和变形分析

本文采用非线性有限元方法研究了复合材料层合板在固化后期降温过程的残余应力和变形问题。考虑了材料的热物理与力学性质随温度的变化以及变化率和应力间耦合的影响。给出了一些有意义的结果。     

Residual stresses formation during the manufacturing process of epoxy matrix composites: resin yield stress and anisotropic chemical shrinkage

A study of residual stresses during the curing process of thermosetting resin composites is presented. A methodology is proposed for predicting the formation and the development of the manufacturing residual stresses, this approach is based on the determination of the resin yield stress. A self-consistent model is used to determine the cure-dependent effective mechanical properties, thermal expansion and chemical shrinkage coefficients of the composite material. This model allows considering for the composite material behaviour an anisotropic chemical shrinkage, which is not represented by a classical linear model. Finally, a one dimensional cure simulation and a modelling of residual stresses formation in composite plate are made by using a finite elements code. The effects of the cure-dependent material properties and of the resin yield stress on the residual stress formation are studied.     

Non-destructive damage sensing and cure monitoring of carbon fiber/epoxyacrylate composites with UV and thermal curing using electro-micromechanical techniques

Interfacial properties and cure monitoring of single-carbon fiber/thermosetting composites by thermal and different ultraviolet (UV) curing processes were investigated using electro-micromechanical test and electrical resistance measurement. During curing process the residual stress was monitored in terms of the electrical resistance and then they were compared to various curing conditions. In thermal cure the tensile strength and modulus of epoxyacrylate matrix were higher than those of UV cure, whereas the failure strain was lower. Interfacial shear strength (IFSS) increased gradually with elapsing UV exposing time and then saturated. For thermal cure the IFSS was significantly higher than that of UV cure, and cure shrinkage was observed due to matrix shrinkage and residual stress due to the difference in thermal expansion coefficient (TEC). The difference in electrical resistance, ΔR during thermal curing was larger than that of UV cure. In thermal cure apparent modulus indicating embedding matrix modulus and interfacial adhesion was highest and reaching time up to same stress was shortest. Thermal cure showed the strong durability against the IFSS deterioration after boiling test compared to UV cure.     

基于黏弹性本构模型的热固性树脂基复合材料固化变形数值仿真模型

针对热固性树脂基复合材料热压罐成型工艺过程,采用广义Maxwell(麦克斯韦)黏弹性本构模型建立了残余应力和固化变形的三维模型。模型考虑了复合材料固化过程中的热-化学效应、材料的热胀冷缩效应、基体树脂黏弹性效应以及材料的各向异性。通过与文献中实验结果的比较,证明了所建立的模型具有较高的可靠性。对复合材料C型制件的固化过程进行了数值模拟和实验对比,比对结果表明该数值模型具有较高的准确性。     

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.     

Impact of the non-homogenous temperature distribution and the coatings process modeling on the thermal barrier coatings system

The present study deals with a numerical investigation of the residual stresses arising during the plasma-sprayed coatings process and their effects on the final stress state of the thermal barrier coatings system (TBCs) during service. A new thermo-mechanical finite element model (FEM) has been designed to function using a non-homogenous temperature distribution. Several phenomena are taken into account in the model such as: residual stresses generated during the spraying of coatings, morphology of the top-coat/bond-coat interface, oxidation at the top-coat/bond-coat interface, thermal mismatch of the material components, plastic deformation of the bond-coat and creep of all layers during thermal cycling. These phenomena induce local stresses in the TBCs that are responsible of micro-crack propagation during cooling and thermal cycling, specifically near the ceramic/metal interface.     

不锈钢球壳无模液压胀形过程的有限元模拟

用弹塑情限元对不锈钢球壳液胀形过程进行了数值模拟，给出了胀形过程中壳体外形和焊变化规律，分析了应力应变及残余应力分布规律，并与试验结果进行了比较。     

基于Morris方法的纤维复合材料结构件固化均匀性的全局灵敏度分析

纤维增强树脂基复合材料结构件的残余应力问题是制约其在航空航天、汽车和建筑领域大规模应用的关键问题。复合材料固化过程中温度场和固化度场的非均匀性是引起残余热应力和固化收缩应力的重要因素。为了探讨纤维复合材料结构件在固化成型过程中固化工艺温度、热传导系数、对流换热系数及结构件厚度对固化均匀性的敏感程度,采用数值模拟分析了这4个关键参数对温度场和固化度场均匀性的影响规律。模拟结果表明:升高固化工艺温度,复合材料温度场的非均匀性增大,固化度场的非均匀性减小;增大对流换热系数和热传导系数,复合材料温度场和固化度场的非均匀性减小;增加复合材料结构件的厚度,复合材料温度场和固化度场的非均匀性增大。在此基础上,应用Morris全局灵敏度分析方法对4个关键参数对复合材料固化均匀性的影响程度进行定量分析,得到固化均匀性的影响因素按灵敏程度由大到小的顺序为:结构件厚度、热传导系数、固化工艺温度、对流换热系数。     

Impact of packaging materials on reliability test for low-K wire bond-stacked flip chip CSP

As the consumer electronics market keeps expanding, the system-in-package (SiP) has become more and more popular. The SiP has the benefits of space efficiency and an integrated flexible function by stacking different function dies in a single package. With die stacking, bi-material interfacial delamination becomes a primary concern of reliability testing. This work evaluates the impact of encapsulated molding compound (EMC) and underfill material on the reliability test for the low-K wire bond-stacked flip chip CSP (WB-sFC CSP). The CSP packaging size is 16 × 16 × 1.2 mm with a 4 × 5 × 0.15 mm low-K wire bond die stacked on a low-K 8 × 8 × 0.15 mm flip chip die. The reliability testing condition is JEDEC MSL 2a (reflow peak temperature is 260 °C) followed by the temperature cycling test (TCT, −55 to 125 °C) of 1000 cycles. The severe pre-conditioning environment makes the packaging materials selection a challenge. Delamination between the interface of the underfill and other packaging components leads to premature failure during preconditioning. TCT stress also induces delamination between the interface of the backside of the bottom die and the encapsulated molding compound. Initially, the EMC selection was decided by the first stage experimental result based on the assembly yield. A DOE was performed to study the effect of underfill property on the packaging stress using the finite element analysis (FEA). According to the FEA results, the trend of underfill property selection for solving the underfill delamination and the backside of the bottom die delamination were in conflict. The low coefficient of thermal expansion (CTE) and the high Young’s modulus (E) underfill can reduce the stress located at the backside of the bottom die corner but it will increase the underfill stress. Plasma cleaning was applied to improve the underfill and the packaging components interface bond strength prior to dispensing the underfill. In the second stage, three underfills were evaluated based on the three different reasons, low modulus for preventing underfill delamination, high modulus for reducing the backside of the bottom die stress, and low moisture absorption by reducing steam pressure during preconditioning. The experimental results in this work demonstrate that only a high modulus underfill can pass the reliability test.     

Thermomechanical Analysis of Preheat Effect on Grade P91 Steel During GTA Welding

Effect of preheat on the thermal cycles, residual stress, and distortion during autogenous GTA welding of Grade P91 steel has been analyzed using FEM. Phase transformation effect on the residual stresses is also analyzed. Thermomechanical analysis has shown that preheat reduced the peak temperatures, cooling rate, and the distortion values. The phase transformation involving martensite during cooling resulted in typical “M” shaped residual stress profile with a reduction in peak tensile residual stress value and a wider distribution of residual stress. There is good agreement between the predicted and measured thermal cycles, residual stresses, and distortion of the P91 steel weld joint.     

SOLID-STATE DIFFUSION BONDING OF INCONEL ALLOY 718 TO 17-4 PH STAINLESS STEEL

Solid-state diffusion bonding of Inconel® alloy 718 to 17-4 PH® stainless steel was studied. Mechanical tests and metallographic examinations were used to evaluate the joint quality. The effect of bonding pressure on the joint integrity was assessed. In all joints, failure occurred nearly without plastic deformation. The thermal residual stress generated from cooling to room temperature was calculated by finite element modeling. The result showed that the poor joint ductility was not caused by the residual stress, but by the formation of continuous intermetallic films along the interface.     

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.     

Novel high performance no flow and reworkable underfills for flip-chip applications

 Flip-chip interconnect is the emerging technology for the high performance, high I/O (Inputs/Outputs) IC devices. Due to the thermal mismatch between the silicon IC (CTE=2.5 ppm/⁰ C) and the low cost organic substrate such as FR-4 printed wiring board (CTE=18–22 ppm/°C), the flip chip solder joints experience high shear stress during temperature cycling testing. Underfill encapsulant is used to couple the bilayer structure and is critical to the reliability of the flip-chip solder joint interconnects. Novel no-flow underfill encapsulant is an attractive flip-chip encapsulant due to the simplification of the no-flow underfilling process. To develop the no-flow underfill material suitable for the no-flow underfilling process of flip-chip solder joint interconnects, we have studied and developed a series of metal chelate latent catalysts for the no-flow underfill formulation. The latent catalyst has minimal reaction with the epoxy resin (cycloaliphatic type epoxy) and the crosslinker (or hardener) at the low temperature (<180° C) prior to the solder reflow and then rapid reaction takes place to form the low-cost high performance underfills. The effects of the concentration of the hardener and the catalyst on the curing profile and physical properties of the cured formulations were studied. The kinetics and exothermic heat of the curing reactions of these formulations were investigated by differential scanning calorimetry (DSC). Glass transition temperature (Tg) and coefficient of thermal expansion (CTE) of these cured resins were investigated by using thermo-mechanical analyzer (TMA). Storage moduli (G′) and crosslinking density of the cured formulations were measured by dynamic-mechanical analyzer (DMA). Weight loss of these formulations during curing was investigated by using thermo-gravimetric analyzer (TGA). Additionally, some comparison results of our successful novel generic underfills with the current commercial experimental no-flow underfills are reported. Additionally, approaches have been taken to develop the thermally reworkable underfill materials in order to address the non-reworkability problem of the commercial underfill encapsulants. These include introducing the termally cleavable blocks to thermoset resins, and adding additives to thermoset resins. For the first approach, five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed with the hardener and the catalyst. Then the properties of these mixtures including Tg, onset decomposition temperature, storage modulus, CTE, and viscosity were studied and compared with those of the standard formulation based on the commercial epoxy: ERL-4221D. These mixtures all decompose at lower temperature than the standard formulation. Moreover, one mixture – Epoxy5 – showed acceptable Tg, low viscosity, and fairly good adhesion. For the latter approach, two additives were shown that after added to typical cycloaliphatic epoxy formulation, do not interfere with epoxy curing, and do not affect the typical properties of cured epoxy system, yet provide die removal capability to the epoxy. Furthermore, the combination of the two approaches showed positive results. Received: 28 September 1998 / Reviewed and accepted: 1 October 1998     

碳纳米管增强镁基复合材料热残余应力的有限元分析

为了探寻Ni层厚度对镀镍碳纳米管增强AZ91D镁基复合材料(Ni-CNTs/AZ91D)中热残余应力的影响, 在实验基础上, 建立不同Ni层厚度时Ni-CNTs/AZ91D复合材料的有限元模型, 模拟了Ni-CNTs/AZ91D复合材料中热残余应力的分布。研究发现: 在碳纳米管表面镀镍能够明显降低Ni-CNTs/AZ91D复合材料中的热残余应力。Ni-CNTs/AZ91D复合材料中, 热残余应力在Ni层厚度为6nm时最小; Ni层厚度由2nm增至6nm时, 热残余应力随着Ni层厚度的增加而减小; 当Ni层厚度超过6 nm时热残余应力随着Ni层厚度的增加而增大。复合材料中热残余应力的最大值随碳纳米管表面Ni层厚度的增加向Ni层与基体的界面移动。      

常温养护型超高性能混凝土的圆环约束收缩性能

研究了HCSA膨胀剂3种掺量(0%、3%、6%)下常温养护型超高性能混凝土(Ultra high performance concrete,UHPC)的圆环约束收缩性能,包括:(1)UHPC轴拉应力应变曲线测试;(2)根据GB/T50082的UHPC自由收缩实验;(3)根据ASTM C1581的UHPC圆环约束实验。结果表明,3种UHPC的极限拉伸应变均高于3 000με,28d总收缩值分别为1 005.6με、600.0με、462.2με,并且在圆环约束作用下转化为残余应变、弹性拉应变和塑性拉应变,其中塑性拉应变分别为700.4με、437.9με、389.9με。3种UHPC在拉伸应变达到1 000με时及圆环约束实验中均未发现0.01mm以上的可检测裂缝。基于拉伸实验和声发射损伤分析方法对UHPC进行应变分析,可知具有应变强化段的3种UHPC在圆环约束实验中的塑性变形以小于0.01mm的多点分布微裂纹形式存在。通过添加HCSA膨胀剂对常温养护型UHPC进行收缩补偿,可有效地降低UHPC自身的拉应力以及对原有结构的影响。     

Numerical calculation of residual stress development of multi-pass gas metal arc welding under high restraint conditions

During welding, residual stresses build-up created by the steep thermal gradient that occurs in the weld zone from localized heating and cooling, and phase transformations appearing in low-alloyed structural steel is inevitable. Welding of rather simple test plates do not cover the actual structural effects, which have to be considered during real component welding. However, the resulting welding-induced residual stress state is highly influenced by the structural characteristics, i.e. restraint conditions, of the welded construction. Therefore, a unique large-scale testing facility providing a specific shrinkage restraint while welding and subsequent cooling was used for the present investigations. Hereby, a six bead multi-pass gas metal arc weld of 20 mm thick structural steel S355J2 + N was welded under shrinkage restraint. The residual stresses were experimentally and numerically investigated, and compared to an analysis of plates welded under force-free support and free shrinkage conditions.The experimentally determined and calculated residual stresses using both 2D and 3D numerical models are in a good agreement. Furthermore, the influence of a shrinkage restraint on the residual stress distribution is both experimentally and numerically shown for the present test set-up.