Çelik sacların şekillendirme kabiliyeti

Abstract

- 2 - SUMMARY The old method of hand forming of sheet steel is today used primarily as a finishing operation to remove wrinkles left by forming machines. In the metalworking in dustries hand forming is primarily limited to experimental work where only a few identical pieces are required. Most high production volume sheet metal forming is done on a press, driven by either mechanical or hydraulic action. The basic tools used with a press are the punch and the die. Although there are many forming methods, two of them are more important: Stretch forming and deep drawing. St retch forming is the process of forming by the application of primarily tensile forces in such a way as to stretch the material over a tool or form block. Deep drawing is the metalworking process used for shaping flat sheets into cup shaped articles. This forming is done by pressing the metal sheet into a die with a punch. Because of the complexity of sheet forming ope rations, simple mechanical property measurements made from the tension test are of limited value. Over the years many laboratory tests have been developed to evaluate the forma- bility of sheet metals. A deep drawing test is developed in terms of limiting draw ratio. In Erichsen test the sheet is clamped between two ring dies while a punch, usually a ball, is forced agains the sheet until it fractures. The depth of the bulge before the sheet fractures is measured. This test subjects the sheet primarily to stretching. Howe ver, most practical sheet forming operations provide a com bination of both biaxial stretching and deep drawing. The limit strains are represented best by Forming Limit Diagrams (FLD). Hence, the FLD determines the limit of useful defor mation in sheet forming and controls failure.- 3 FLD is usually obtained by performing tension tests, deep drawing tests, Erichsen tests and bulge tests. Representation of fracture data as a plot of major versus minor strains at the fracture gives the limit curve. Frac ture occurs when the strain ratio at the end of the process is above the limit curve. The direction and magnitude of maximum strain are easily determined by a grid pattern of small diameter circles on the surface of the blank. The surface of the blank is covered by a grid of circles, pro duced by electrochemical marking before the tests. During forming, the circles are deformed into ellipses. The direc tion of maximum strain corresponds to the major axes of el lipses and the magnitude of that strain is determined by measuring the increase in length of the ellipses over the diameters of the circles. The minimum strains are also de termined by the same process. Formability is a function of both material pro perties (strain hardening exponent