Experimental Mechanics

Palpation is an economical, safe and effective method to detect breast cancer among other expensive and sometimes limited diagnostic tools such as mammography and magnetic resonance imaging (MRI). To understand the mechanics of palpation, a rigid inclusion was embedded in a phantom gel to simulate the lesion. An array of indents using a rigid indenter was made over an assigned area of the phantom surface, while the applied load, F, was measured as a function of instantaneous indentation depth, w. When the local stress field interacted with a sufficiently shallow inclusion, the mechanical response F(w) yielded an augmented apparent stiffness C. A 2-dimensional spatial map of C shows the presence, depth, and geometry of the simulated lesion. A camcorder was used to capture the in-situ movement of the inclusion during indentation, which showed consistency with the finite element (FEA) prediction. Results provide preliminary confirmation that mechanical indentation is a good tool to complement existing imaging techniques and has the potential to guide design and fabrication of an automatic palpation device.

A digital image measurement (DIM) system is used to study the plastic damage accumulation around a notch under conditions of low-cycle fatigue. This system incorporates a contrast correlation method to evaluation the level of plastic damage at each point of the studied area from two images acquired before and after the introduction of fatigue deformation. A compact tension specimen of 304 stainless steel with a notch radius of 1 mm is analyzed during the stages of fatigue crack initiation and growth. The results obtained using this measurement system are compared with those attained by means of a recrytallization technique.

In nature, several species use flexible probes to actively explore their environment and acquire important sensory information, such as surface topology, texture, and water or air flow velocity. For example, rats and other rodents have an array of facial vibrissae (or whiskers) with which they gather tactile information about the external world. The complex mechanisms by which mechanical deformations of the probe lead to neuronal activity in the animal’s nervous system are still far from being completely understood. This is due to the intricacy of the deformation mechanics of the flexible sensors, the processes responsible for transforming the deformation to electrical activity, and the subsequent representation of the sensory information by the nervous system. Understanding how these mechanosensory signals are transduced and extracted by the nervous system promises great insight into biological function, and has novel technological applications. To understand the mechanical aspect of sensory transduction, here we monitored the deformation of a rat’s vibrissa as it strikes rigid objects with different topologies (surface features) during locomotion, using high-speed videography. Motivated by our observations, we developed detailed numerical models to study the mechanics of such flexible probes. Our findings elucidate how active sensation with vibrissae might provide sensory information and in addition have direct implications for several technological areas. To put this in perspective, we propose strategies in which flexible probes can be used to characterize surface topology at high speeds, which is a desirable feature in several technological applications such as memory retrieval.

We present our efforts to measure the tensile strength of clock-rolled pure zirconium in the through-thickness (TT) direction of the plate. Although the plate is too thin to produce standard ASTM tensile samples in the TT orientation, such measurements are relevant to benchmarking our constitutive models of hardening and texture evolution. We have designed a fixture and sample to perform tensile tests on our 9 mm thick plate. The sample is a double-ligament mini-tensile sample: 8 × 8 × 1 mm overall; each ligament has a gage section of 1 × 1 × 3 mm. In contrast, our standard “macro” tensile sample is a flat dogbone with a gage section of 3 × 1.5 × 25 mm. We validate our design by comparing the results of mechanical tests performed on samples of both geometries. Although the hardening response is nearly identical, the flow stress of the miniature samples is offset by +25 MPa at the onset of plastic yield. We present our efforts to resolve the origin of this offset.

Bronze foil of 0.1 mm thickness was placed between faying surfaces of two plates to be butt-welded as marker material to reveal the flow behavior of weld metal during friction stir welding of 7075-T651 aluminum alloy. By tracing the bronze foil fragments in the weld after welding, the metal flow behavior during the welding process was revealed. Besides, the tool forces in the welding process were measured by the octagonal loop resistance turning dynamometer to expound the periodic variation of metal flow pattern. Results show that the flow behavior of the weld metal is different along the thickness direction. The flow pattern presents a periodic variation, and a formula has been proposed to calculate the periodicity of the metal flow. In addition, the weld nugget zone presents a “spoon” shape and the fine grains at the spoon handle and those at the spoon bowl are originated from different zones. A plastic metal flow model in FSW was proposed based on the results. Furthermore, the formation of defects was explained by researching the weld metal flow behavior.

A machine has been developed for studying the static and dynamic triaxial constitutive behavior of large specimens of geologic and construction materials. Test specimens can also contain a cylindrical tunnel cavity to permit study of tunnel-reinforcement structures and rock-structure interaction. The specimens are 0.3 m in diameter and 0.3 to 0.45 m high; the model tunnels can be up to 50 mm in diameter. Static and dynamic triaxial loads can be applied with maximum pressures of 200 MPa in static tests and 100 MPa in dynamic tests. Dynamic loading can also be superimposed on a static preload as large as 20 MPa. To facilitate study of tunnel reinforcement, the tunnel is maintained at ambient pressure, with access at both ends for instrumentation and photography. Example results show the influence on tunnel deformation of loading rate as well as the presence of joints and their orientation. For a given allowable tunnel closure, substantially greater pressures can be sustained under dynamic loading than under static loading, and substantially greater pressures can be sustained by an intact specimen than by a jointed specimen.

A set of similarity relationships is developed and discussed for use in the study of transient and steadystate thermal displacements, strain and stresses between model and prototype. The similarity relationships are shown to be dependent upon the existing state of stress in the thermally loaded member. Their utilization in the design or analysis of data for such experimental techniques as photoelasticity and the moiré method is cited.