Issue 29

A Fortini et alii, Frattura ed Integrità Strutturale, 29 (2014) 74-84; DOI: 10.3221/IGF-ESIS.29.08 80 the transformation of the residual multi-variant martensite into austenite. This front is denoted z m in Fig. 6. From the point z 0 (A s ) two transformation fronts nucleate, denoted z + and z 0 in Fig. 6, which shows the evolution of the fronts with the temperature calculated using the material data listed in Tab. 2. In the region between z + and z 0 , both multi- and single- variant martensite transform into austenite. At the temperature T 1 = 90.4 °C, the fronts z m and z + coalesce, thus for T>T 1 multi-variant martensite transforms into austenite between the origin and the front z 0 , and single-variant martensite transforms into austenite between z + (=  ) and z 0 . At T =A f , the transformation of both martensite variants into austenite finishes and the beam recovers its original (memorised) curvature   . As the fronts evolve, the curvature of the beam also evolves along the continuous curve plotted in Fig. 7. Correspondingly, the single-variant martensite fraction and stress distributions also change inside the cross-section and Fig. 8 shows the calculated evolutions. The details for calculating the curves shown in Fig. 6, 7 and 8 are given in [20]. Figure 5 : Calculated stress ( ߪ ௥ ) and single-variant martensite (  sr ) residual distributions after the first bending. For symmetry reasons, only the distributions in the upper half of the cross-section are shown. Figure 6 : Calculated evolution of the phase transformation fronts with the temperature; z indicates the coordinate along the thickness of the cross-section. At the temperature A s , single-variant martensite is present only below ߣ (cfr. Fig. 5) and multi-variant martensite is present throughout the cross-section. For temperatures between A s and T 1 , multi-variant martensite transforms into austenite in the region between the origin and the front z m (dot-dashed line), and both multi-variant and single-variant martensite transform into austenite in the region between the fronts z + (dashed line) and z 0 (continuous line) . At the temperature T 1 , the fronts z m and z + coalesce, thus for T>T 1 multi-variant martensite transforms into austenite between the origin and the front z 0 and single-variant martensite transforms into austenite between z + (= െ ߣ ) and z 0 .