MOMENT-CURVATURE MEASUREMENT IN THIN SHEET - PART II - YIELDING AND KINKING IN AGED STEEL SHEET

The bending equipment described in Part I is used to study the behavio ur of a sample of galvanised steel sheet. The sheet is known to have a n internal stress distribution and to exhibit ageing. Tensile and bend behaviour is measured for the sheet in the ''as-received'' condition, after ageing at 170 degrees C and 300 degrees C and after ageing and stress relieving at 450 degrees C. In the bend test, attention is focu ssed on the imposed outer fibre bending stress at the limit of linear, elastic bending. In the ''as-received'' condition, this is substantia lly less than the tensile yield stress, but increases rapidly with age ing and for ageing at 300 degrees C it is approximately 200 MPa greate r than the lower yield stress in the tensile test. Samples treated at 300 and 450 degrees C showed maxima in the moment/curvature characteri stic and unstable ''kinking'' was observed in bending. The bending mom ent versus curvature diagrams presented indicate material characterist ics of importance in technological forming processes which are not eas ily identified in tensile tests. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.