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where ΔL is the change in gauge length, L 0 is the initial gauge length, and L is the final length. The force measurement is used to calculate the engineering stress, σ, using the following equation: [5] = where F is the tensile force and A is the nominal cross-section of the specimen.
Advantages of three-point and four-point bending tests over uniaxial tensile tests include: simpler sample geometries; minimum sample machining is required; simple test fixture; possibility to use as-fabricated materials [6] Disadvantages include: more complex integral stress distributions through the sample
The resulting slip bands appear at the lower yield point and propagate along the gauge length, at constant stress, until the Lüders strain is reached, and deformation becomes uniform. Beyond the Lüders strain, the stress increases due to strain hardening until it reaches the ultimate tensile stress.
The length of the crack is measured. The specimen is then loaded monotonically. A plot of the load versus the crack opening displacement is used to determine the load at which the crack starts growing. This load is substituted into the above formula to find the fracture toughness .
The specimen is placed on the work table and a notch is created exactly at the centre. The crack should be generated such that the defect length is about half the depth. The load applied on the specimen is generally a three-point bending load. A type of strain gauge called a crack-mouth clip gage is used to measure the crack opening. [3]
The stress intensity factor at the crack tip of a compact tension specimen is [4] = [() / / + / / + /] where is the applied load, is the thickness of the specimen, is the crack length, and is the effective width of the specimen being the distance between the centreline of the holes and the backface of the coupon.
The ultimate tensile strength of a material is an intensive property; therefore its value does not depend on the size of the test specimen.However, depending on the material, it may be dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material.
Nevertheless, this strain is a meaningful indication of the “ductility” of the metal – more so than the commonly-used “nominal strain at fracture”, which depends on the aspect ratio of the gauge length of the tensile test-piece [3] – see the article on ductility.
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