Investigation On Elastic Constants Measurement For Limited-Size Samples Using Acoustic Microscopy Technology

Along with the emergence of new materials unceasingly, it is an urgent demand of the measurement for material mechanical and other physical properties. However, due to prepared technology, some new materials, such as bulk nano-materials and metal glasses, are hard to process large size samples to satisfy the requirement of common static tension experiment, furthermore, many materials exhibits anisotropic characteristics in their mechanical behaviors. Therefore, it has very important scientific values and wide application foreground for the investigation of characterization method and measurement technique to evaluate prepared technology and mechanical properties including elastic behaviors of these types of materials or structures.In this dissertation, aimed at the existing problem of elastic constant measurement of limited size bulk materials, the main technical contributions of the research work are shown as following.(1) Ultrasonic measurement method for material elastic constants based on low frequency acoustic microscopy technology. Based on acoustic characterization method of material mechanical properties, elastic constants characterization method and measurement technique of limited size bulk materials is fully developed by the relation between the velocities of longitudinal and leak surface waves and material elastic constants.(2) Ultrasonic system for elastic constant measurement of limited size materials is wholly established. In this developed system, high frequency non-lens large aperture line focus cylindrical PVDF ultrasonic transducer is designed and manufactured. In this system, NI PXI bus embedded controller is the core and a four-axis moving precisely framework is used as measurement platform. Ultrasonic wave signals are excited and received by line focus cylindrical PVDF ultrasonic transducers driven by ultrasonic pulser/receiver. With the help of high velocity digital analyzer, automatic collection and analysis of ultrasonic signals can be achieved. The software program including measurement of the velocities of longitudinal and leaky surface waves simultaneously in spatial domain and waveform processing algorithm is developed. With the help of the combination of the method and software, automatic measurement of limited size materials can be achieved. Experimental results show that the system provides reliable guarantee of accurate measurement of limited size materials and the echoes with high signal-noise ratio can be obtained to satisfy the need of signal analysis and processing.(3) Common metal materials, such as aluminum, copper, carbon steel, titanium alloy, are measured by using this developed ultrasonic system for limited size material elastic constant measurement. In these experiments, elastic constants of these materials are obtained and measurement error factor is analyzed. Primary experimental results show that this measurement method with high precision and low error can satisfy the need of the engineering and scientific research.(4) Elastic constants of uncommon materials such as amorphous materials, nano-materials and non-metal materials are experimentally measured. Furthermore, the measurement value of Young's module of amorphous materials by using ultrasonic method is agreement well with that by using other methods. Non-destructive Ultrasonic testing can overcome the shortcoming of limitation of indentation method which fails to measure Poisson's ratio of materials and its intrinsic destructive nature.(5) Special property materials or areas such as cold rolled steel sheet and electronic beam welding are measured by using ultrasonic measurement system experimentally. Experimental results show that the method can be used for the measurement of cold rolled steel sheets which exhibit anisotropic characteristics induced by machining process. Furthermore, material mechanical properties of micro areas can be measured to evaluate their gradient.(6) When mechanical properties of gadolinium with relatively low surface wave velocity are measured by ultrasonic method, a coupling liquid with a low wave velocity is employed for conquering the effect of the velocity of coupling liquid and its restriction to measurement range of leaky surface wave velocity. Therefore, the singularity problem induced by excessively low surface wave of measured samples when water immersion method is adopted can be solved and extend the range of material applications.