tailieunhanh - Experimental Strain Analysis

Như đã thấy trong các mô-đun trước đó, phân tích căng thẳng thậm chí đơn giản xuất hiện hình học có thể dẫn đến vận động toán học phức tạp. Điều thực tế - crankshafts động cơ, bộ phận giả y tế, vợt tennis, vv - có hình dạng ranh giới có thể không dễ dàng được mô tả toán học, và ngay cả khi chúng nó sẽ là vô cùng khó khăn để phù hợp với các giải pháp của các phương trình quản lý cho họ. . | Experimental Strain Analysis David Roylance Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge MA 02139 February 23 2001 Introduction As was seen in previous modules stress analysis even of simple-appearing geometries can lead to complicated mathematical maneuvering. Actual articles engine crankshafts medical prosthe-ses tennis rackets etc. have boundary shapes that cannot easily be described mathematically and even if they were it would be extremely difficult to fit solutions of the governing equations to them. One approach to this impasse is the experimental one in which we seek to construct a physical laboratory model that somehow reveals the stresses in a measurable way. It is the nature of forces and stresses that they cannot be measured directly. It is the effect of a force that is measurable when we weigh an object on a spring scale we are actually measuring the stretching of the spring and then calculating the force from Hooke s law. Experimental stress analysis then is actually experimental strain analysis. The difficulty is that strains in the linear elastic regime are almost always small on the order of 1 or less and the art in this field is that of detecting and interpreting small displacements. We look for phenomena that exhibit large and measurable changes due to small and difficult-to-measure displacements. There a number of such techniques and three of these will be outlined briefly in the sections to follow. A good deal of methodology has been developed around these and other experimental methods and both further reading1 and laboratory practice would be required to put become competent in this area. Strain gages The term strain gage usually refers to a thin wire or foil folded back and forth on itself and bonded to the specimen surface as seen in Fig. 1 that is able to generate an electrical measure of strain in the specimen. As the wire is stretched along with the specimen the wire s electrical .