Applications of Strain Gradient Plasticity: Modeling and Experiments

9610491许多工程应用(通常是宏观尺度)的效率和有效性受到潜在的微观尺度现象的显著影响。因此，工程产品和制造过程的宏观设计考虑需要详细的微观洞察。块体和板材成形中的织构形成、剪切带状(主要控制机械加工中的切屑形成特征)、颗粒/纤维增强复合材料的强度和韧性以及薄膜界面的力学性质都是这类问题的例子。为了有效地考虑这类工程问题，需要一个能够捕捉微观尺度效应并在其他宏观尺度模型的背景下综合其本质的多尺度表示。此外，近年来人们对几个微尺度工程问题的兴趣也显著增加，如MEMS、电子封装中的纳米和亚微米级表面改性技术以及微电子电路中的细线互连失效。实验观察(如微压痕、细丝扭转、薄膜强度)表明，材料在微观尺度上的行为与在宏观尺度上的明显不同。这些微尺度实验也显示了几何参数对材料性能的相当大的影响。基于传统本构模型的预测不能解释这种微尺度现象，这些现象可能对广泛的工程器件的设计和制造具有深远的影响。例如，传统的塑性理论不能解释与相同材料的宏观压痕相比，微压痕试验中硬度增加的原因。S在金属加工中的材料去除特性主要取决于产生的剪切带的厚度；传统的塑性理论无法提供直接的方法来估计这些参数。Fleck和Hutchinson(1993)最近提出的应变梯度塑性满足Clausius-Duem热力学对第二变形梯度本构关系的限制，有可能有效地探测微米和亚微米尺度的材料行为。它还可以为将微尺度效应纳入广义塑性框架提供一个有效的过渡。然而，在应变梯度塑性中引入了材料长度尺度，需要通过一组独立的微尺度实验来确定。因此，拟议的工作首先集中于开发两套独立的实验：(1)使用原子力显微镜来弯曲微米厚的铜梁和板，以及(2)使用光学、电子和原子力显微镜来测量剪切带厚度和亚结构(在压缩测试中)。从这些实验中观察到的结果，与适当的建模工作相结合，将为“材料长度尺度”提供独立的估计。将这些结果与其他结果进行比较，将有助于研究“材料长度标尺”作为材料参数的有效性。这些实验还将被用来(I)定量地估计单个材料的材料长度尺度，以及(Ii)建立特定材料的应变梯度理论的感兴趣区域。然后，我们将利用应变梯度塑性理论来探索不同的工程应用，其中微尺度现象起着重要的作用。在机械加工中，我们将直接估计不同FCC、BCC和HCP材料在不同切割条件下的剪切带厚度，并(I)根据拟议的加工实验来验证预测；(Ii)定量研究剪切带厚度对精加工表面质量和完整性的影响。对于复合材料，我们将研究颗粒/纤维尺寸对有效强度的影响。微机械传感器、执行器和换能器通常使用薄膜双层(薄膜上的薄膜)。我们将对这样的系统进行建模和实验。

9610491 Huang Efficiency and effectiveness of numerous engineering applications (typically macroscale) are significantly influenced by the underlying microscale phenomena. Consequently, macroscale design considerations of engineering products and manufacturing processes require detailed microscale insights. Texture formation in bulk and sheet forming, shear banding (that essentially governs chip formation characteristics in machining), strength and toughness of particle/fiber reinforced composite materials, and mechanical properties of thin film interfaces are examples of this class of problems, A multi-scale representation capable of capturing the micro-scale effects and synthesizing its essence within the context of an otherwise macroscale model is necessary for effective consideration of such engineering problems. Moreover, interests in several microscale engineering problems, e.g., MEMS, nano- and sub-micron scale surface modification techniques in electronic packaging and failure of fine line interconnects in microelectronic circuits, have also increased significantly in recent years. Experimental observations (e.g., micro-indentation, torsion of thin wires, strength of thin films) show that material behavior at the microscale is significantly different from that at the macroscale. These microscale experiments also exhibit considerable influence of geometric parameters on material properties. Predictions based on conventional constitutive models fail to account for such microscale phenomena that may have profound implications in design and manufacturing of a wide spectrum of engineering devices. For example, the conventional plasticity theory cannot explain the increased hardness in micro-indentation tests as compared to macro- indentation of the same material. Material removal characteristic s in machining of metals are crucially dependent on the thickness of the generated shear band; conventional plasticity provides no direct avenue to estimate such parameters. The strain gradient plasticity recently proposed by Fleck and Hutchinson (1993) satisfies the Clausius-Duhem thermodynamic restrictions on the constitutive law for second deformation gradients, and is potentially capable of effectively probing micron and sub-micron scale material behavior. It can also provide an effective transition for incorporating the microscale effects within a generalized framework of plasticity. However, a material length scale is introduced in the strain gradient plasticity, and needs to be determined from a set of independent microscale experiments. Accordingly, the proposed work first focuses on developing- two independent sets of experiments: (1) bending of micron-thick Cu beams and plates using Atomic Force Microscopy, and (2) measurement of shear band thickness and sub-structures (in compression tests) using optical-, electron- and atomic force microscopy. Observations from these experiments, in conjunction with suitable modeling efforts, will provide independent estimates of the "material length scale". Comparison of these results with others will facilitate investigations of the validity of the "material length scale" as a material parameter. The experiments will also be used to (i) quantitatively estimate the material length scale for individual materials, and (ii) establish the region of interest of strain gradient theory for particular materials. We will then utilize the strain gradient plasticity theory to probe different engineering applications where microscale phenomena play important roles. In machining, we will directly estimate the shear band thickness for different FCC, BCC and HCP materials under va rious cutting conditions, and (i) validate predictions against observations from proposed machining- experiments; (ii) quantitatively investigate the influence of shear band thickness on quality and integrity of the finished surface. For composites, we will investigate the influence of particle/fiber size on effective strength. Micro-mechanical sensors, actuators and transducers commonly use thin film bi-layers (thin film on thin film). We will carry out modeling- and experimentation on such systems.