聚合物注塑残余应力研究进展

综述了聚合物注塑残余应力的研究进展,详细介绍计算残余应力所用的理论模型及其基本概念,注塑制品残余应力由两部分组成,并且有不同的测试方法,这些应力是造成制品曲变形的原因,对于残余流动应力,重点分析本构模型的发展和注塑控制方程的建立;对于残余热应力,则讨论了冷却过程中的相变问题,以及热粘弹性模型在应力分析中的应用;最后介绍测量残余应力的几种实验方法。     

ABS注塑制品的模内蠕变试验研究

不同于传统的热残余应力分析,本文着眼于注塑制品的模内蠕变研究。通过测试ABS注塑薄板在不同工艺条件下的收缩变形,比较基于弹性理论的热收缩,进而得到了模内蠕变量。分析结果表明,模内蠕变对于降低制品最终的收缩变形起着重要的作用。在研究了各种工艺条件对制件模内蠕变的影响后,发现较低的模腔表面温度或较长的成型周期会导致蠕变增大,而制件在模内的蠕变(或应力松弛)主要发生在固化后的高温区。本文同时探讨了保压压力对蠕变的影响。高注塑保压压力通常会减小注塑件的收缩,但同时会减小制件模内蠕变。     

高分子材料注塑固化阶段的残余应力分析

在结晶性高分子材料注塑过程的固化阶段，温度分布、材料细观结构和应力应变之间相互耦合，因而其变化规律非常复杂．本文在井上等人考虑材料细观结构变化的金属热加工工艺应力分析［３～６］的基础上，发展了一套用于高分子注塑固化阶段残余应力分析的本构描述和有限元分析方法．在本构模型中，同时考虑了温度变化、结晶和“冻结取向”对变形的贡献．     

一种经验型疲劳小裂纹扩展模型

金属材料疲劳寿命由裂纹萌生和裂纹扩展寿命两部分组成，其中对于萌生寿命中的小裂纹分析是精确描述裂纹萌生寿命的关键．而小裂纹在扩展过程中由于尺寸相对较小，导致传统线弹性断裂力学预测方法失效，需要对其进行改进，考虑裂纹尖端塑性区引起的残余压应力对小裂纹扩展速度的影响．本文针对此问题进行了初步分析，通过对塑性区引起的残余应力的量化，结合小裂纹门槛值特性，提出了一种经验型修正的小裂纹扩展模型，用于定量预测裂纹的萌生寿命．使用铝合金6082-T6缺口试样进行了疲劳实验，并与理论结果进行了对比，验证了所提模型的有效性．     

加工残余应力对海洋管道压溃性能的影响研究

为了研究加工制造过程产生的残余应力对海洋管道骨架层抗压性能的影响,首先介绍了骨架层的加工装备和成型原理,并利用MARC软件模拟了骨架层的整个冷弯成型过程,得到了加工残余应力等结果;然后将加工残余应力导入ANSYS有限元模型中进行分析,得到其对抗压溃性能的影响。计算结果表明,材料非线性和加工残余应力,最大可使骨架层的抗压溃能力下降约60%,因此在以后的抗压溃设计中必须要考虑材料非线性以及加工残余应力的影响。     

轴对称物体内三维残余应力场的确定

三维残余应力场的测量理论是国内外未能解决的问题.本文建立了新的轴对称空间力学模型,介绍了对45钢140×200圆柱体进行内剥层测量的实验.在分析中引入了罐载和非罐载应力的概念,为正确处理变动的边界条件给出了思路、使得空间力学模型真实地反映了轴对称体内残余应力的实质.在求解中提出了变应力函数及其解法,使得轴对称三维残余应力场的测量理论得到了真正解决.     

两种缓冲材料的应力松弛实验分析

 用逐次残余法分析了聚乙烯和聚苯乙烯缓冲材料的应力松弛试验数据．结果表明可用有限个并联的Ｍａｘｗｅｌ模型来表示这两种材料的应力松弛试验曲线．     

三维粘弹性复合材料层合板固化残余应力研究

应用改进的有限元算法,综合考虑了碳纤维复合材料固化过程中树脂的化学收缩、应力松弛、材料的热收缩,建立了一个基于三维线性粘弹性的积分模型对复合材料层合板热压罐成型工艺过程中产生的残余应力进行了研究。结果表明:本文提出的方法能够完整地模拟树脂的固化过程并计算层合板的固化残余应力;对于文中的正交对称层合板而言,热收缩是影响残余应力大小的主要因素;固化完成后,0°铺层内的中面中心区域层间残余应力值接近于0,边缘区域会迅速增加,容易发生分层破坏;任意点处,轴向应力比其他两个方向的应力高出1~2个数量级;残余应力值沿厚度方向有较大变化;树脂的应力松弛行为会在一定程度上降低残余应力。     

考虑夹杂相互作用的复合陶瓷夹杂界面的断裂分析

复合材料中夹杂含量较高时,夹杂间的相互作用能显著改变材料细观应力应变场分布,基体和夹杂中的平均应力应变水平也会发生较大变化,导致复合材料强度等力学性能发生显著变化.为修正单一夹杂模型运用在实际材料中的误差,基于相互作用直推估计法,建立一种考虑含夹杂相互作用的夹杂界面裂纹开裂模型.首先根据相互作用直推估计法,得到残余应力和外载应力共同作用下夹杂中的平均应力,再计算无限大基体中相同的夹杂达到相同应力场时的等效加载应力,将此加载应力作为含界面裂纹夹杂的等效应力边界条件,在此边界条件下求得界面裂纹尖端的应力强度因子,进而得到界面裂纹开裂的极限加载条件,并分析了夹杂弹性性能、含量、热残余应力、夹杂尺寸等因素对界面裂纹开裂条件的影响.结果表明,方法能够有效修正单夹杂模型运用在实际材料中的误差,较大的残余应力对界面裂纹开裂有重要的影响,夹杂刚度的影响并非单调且比较复杂;在残余应力较小时,降低柔性夹杂刚度或者增大刚性夹杂刚度都有利于提高材料强度;扩大夹杂尺寸将导致裂纹开裂极限应力显著降低,从而降低材料强度.     

硅基底多层薄膜结构材料残余应力的微拉曼测试与分析

针对MEMS器件制备中两种典型的硅基底多层薄膜结构的残余应力问题,本文提出了利用微拉曼光谱技术测量其残余应力的方法,分析并给出了硅基底多层薄膜结构中的残余应力分布规律。实验结果表明,在硅基底和薄膜内存在较大的工艺残余应力,残余应力在基底内靠近薄膜两侧部分呈非线性变化,在基底内主要呈线性变化,并引起基底整体翘曲。基于实验结果分析,提出了硅基底多层薄膜结构的分层结构模型。本文工作表明微拉曼光谱技术是测量与研究硅基底多层薄膜结构残余应力的一种有力手段。     

Prediction of residual flow and thermoviscoelastic stresses in injection molding

A general model for predicting the total residual stresses generated during filling and cooling stages of injection-molded parts has been developed. The model takes into account the phenomena associated with non-isothermal stress relaxation. The main hypothesis in our approach is to use the kinematics of a generalized Newtonian fluid at the end of the filling stage as the initial state for the calculation of residual flow stresses. These stresses are calculated using a single integral rheological model (Wagner model). The calculation of stresses developed during the cooling stage is based on a thermoviscoelastic model with structural relaxation. Illustrative results emphasizing the effect of both the melt temperature and the flow rate during the filling stage are presented.     

Residual stresses in gas-assisted injection molding

Residual stresses are a major issue in the mechanical and optical behavior of injection-molded parts. In this study, we analyze their development in the case of gas-assisted injection molding (GAIM) of amorphous polymers. Flow-induced residual stresses are computed within a decoupled approach, in which elastic effects are neglected in the momentum balance, assuming a generalized Newtonian material behavior. In a staggered procedure, the computed viscous flow kinematics are used to calculate normal stresses employing a compressible version of the Rolie-Poly model. For the computation of thermally and pressure-induced residual stresses, a linear thermo-viscoelastic model is used. A 3-D finite element model for GAIM is employed, which is able to capture the kinematics of the flow front and whose capabilities to predict the thickness of the residual material layer have been validated by Haagh and Van de Vosse (Int J Numer Methods Fluids 28:1355–1369, 1998). In order to establish a clear comparison, the development of residual stresses is analyzed using standard injection molding and GAIM for a test geometry.     

Laser surface-contouring and spline data-smoothing for residual stress measurement

We describe non-contact scanning with a confocal laser probe to measure surface contours for application to residual stress measurement. (In the recently introduced contour method, a part is cut in two with a flat cut, and the part deforms by relaxation of the residual stresses. A cross-sectional map of residual stresses is then determined from measurement of the contours of the cut surfaces.) The contour method using laser scanning is validated by comparing measurements on a ferritic steel (BS 4360 grade 50D) weldment with neutron diffraction measurements on an identical specimen. Compared to lower resolution touch probe techniques, laser surface-contouring allows more accurate measurement of residual stresses and/or measurement of smaller parts or parts with lower stress levels. Furthermore, to take full advantage of improved spatial resolution of the laser measurements, a method to smooth the surface contour data using bivariate splines is developed. In contrast to previous methods, the spline method objectively selects the amount of smoothing and estimates the uncertainties in the calculated residual stress map.     

聚合物注射成型流动残余应力的数值分析

建立了可压缩黏弹性聚合物熔体在薄壁型腔中充模/保压过程中非等温、非稳态流动 的数学模型,用数值方法实现了注射成型过程中流动应力和取向建立及松弛过程的模拟,研 究了熔体温度、模具温度和注射速率等工艺条件对分子冻结取向的影响,取得了与实验相符 的结果.     

3D numerical simulation of drilling residual stresses

Drilling can affect the integrity of the surface of a mechanical component and reduce its fatigue life. Thus, drilling parameters such as lubrication or drilling velocity must be optimized to ensure a satisfactory residual mechanical state of the hole surfaces. Unfortunately, experimental tests are time consuming and it is not easy to observe the cutting process because of the confinement of the drill zone. The literature does not exhibit any numerical simulation capable of simulating 3D thermomechanical phenomena in the drill zone for large depth holes. Therefore, residual stresses cannot be easily simulated by means of the sole drilling parameters. The aim of this article is to propose a new numerical approach to compute drilling residual stresses for large-depth holes. A first simulation is developed to simulate heat transfer by means of a 3D thermoviscoplastic simulation in a new Rigid-ALE framework allowing the use of large calculation time steps. Then, a time interpolation and a spatial projection are implemented to rebuild the Lagrangian thermal history of the machined component. Finally, a thermo-elastoplastic simulation is carried out to compute residual stresses in the final workpiece. In this paper, the method is applied to a 316L austenitic stainless steel in the case of an unlubricated hole. The computed residual stresses are compared to experimental measurements.     

Calculation of residual stresses in injection molded products

Both flow- and thermally-induced residual stresses which arise during the injection molding of amorphous thermoplastic polymers are calculated in the filling and post-filling stage. To achieve this, a compressible version of the Leonov model is employed. Two techniques to calculate flow-induced residual stresses are investigated. First, a direct approach is developed where the pressure problem is formulated using the viscoelastic material model. Second, generalized Newtonian material behavior is assumed in formulating the pressure problem, and the resulting flow kinematics is used to calculate normal stresses employing the compressible Leonov model. The latter technique gives comparable results, while reducing the computational cost significantly.     

An Improved Computational Strategy to Estimate Residual Stresses from Crack Compliance Data

The successive cracking (crack compliance) method is a destructive technique aimed at determination of residual stresses in various structural members. The laboratory measurements performed during extension of a crack are followed by a computational analysis. We propose a modification of the numerical approach in order to simplify the method and improve its accuracy. The basic idea of the proposed modification is to approximate plastic strains rather than the residual stresses directly. Furthermore, we use the goal oriented adaptive finite element method that generates optimal meshes for evaluation of strains at specific points.     

各向同性材料中微裂纹的最大屏蔽效应

通过对微裂纹屏蔽不同来源的分析及计算,发现在各向同性脆性材料中,残余应力释放引起的微裂纹对主裂尖产生最大屏蔽效应时该微裂纹的倾角与最大张应力的方向没有明显的对应关系．在Hutchinson[1]所指出的屏蔽效应的第二个来源中,还应计及微裂纹形成引起的远场应力在微裂纹处产生的应力场的释放从而导致应力场的再分布．     

Realistic Cold Expansion Finite Element Model and Experimental Validations for Aluminium Alloys

Accurate understanding of the residual stresses produced by cold expansion (CX) of rivet holes is required for fatigue considerations, as well as for better addressing the loading problems in aerospace parts. This paper presents a physically based finite element (FE) model for predicting the residual stresses in aluminium parts submitted to CX. The model relies on experimental measurements for both construction and validation purposes. The model predictions were compared against the experimental results delivered by three different complementary techniques: Digital Image Correlation (DIC) (provided strain fields on the surface of the workpiece); X-Ray Diffraction (XRD) (provided residual stresses measurements on the surfaces); Microscopy measurements (measured the out-of-plane displacement on the hole bore). The FE model proves to be in good agreement with the experimental data. Further considerations on the simulation method regarding geometries, boundary conditions and friction effect are also discussed.