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Knuckle threads with a flat 30 degree flank thread angle are standardized in DIN 405 for inch pitches and diameters ranging from 8 mm to 200 mm. [3] A more recent standard DIN 20400 uses metric thread pitch and lists diameters from 10 mm to 300 mm. [4] As DIN is a German organization, many instances of the DIN thread charts [5] write numbers with a comma as the decimal marker.
Many crack propagation equations have been proposed over the years to improve prediction accuracy and incorporate a variety of effects. The works of Head, [6] Frost and Dugdale, [7] McEvily and Illg, [8] and Liu [9] on fatigue crack-growth behaviour laid the foundation in this topic.
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.
For example, according to ASTM E399, the maximum stress intensity K max should be no larger than 0.6 during the initial stage and less than 0.8 when crack approaches its final size. [ 14 ] In certain cases grooves are machined into the sides of a fracture toughness specimen so that the thickness of the specimen is reduced to a minimum of 80% of ...
For the situation where the plate is large compared to the size of the crack and the location of the force is relatively close to the crack, i.e., , , , , the plate can be considered infinite. In that case, for the stress intensity factors for F x {\displaystyle F_{x}} at crack tip B ( x = a {\displaystyle x=a} ) are [ 11 ] [ 12 ]
Size and geometry also plays a role in determining the shape of the R curve. A crack in a thin sheet tends to produce a steeper R curve than a crack in a thick plate because there is a low degree of stress triaxiality at the crack tip in the thin sheet while the material near the tip of the crack in the thick plate may be in plane strain.
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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]