Applications of Strain Gradient Plasticity: Modeling and Experiments

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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.