Issue34

R.D. Caligiuri, Frattura ed Integrità Strutturale, 34 (2015) 125-132; DOI: 10.3221/IGF-ESIS.34.13 128 Figure 3 : Metallographic image from NTSB Final Report (Fig. 19a) that shows the single-side weld in Pup 1. Fractographic analysis of the Pup 1 fracture surface provides clear evidence of the ductile tear and the fatigue crack growth zone. A ductile tear is characterized by appreciable plastic deformation and energy dissipation, and is created by a single loading event. A schematic from the NTSB Final Report that highlights the regions of ductile tearing and fatigue cracking is shown in Fig. 4. Ductile fracture morphology, characterized by microvoid coalescence, was observed within the ductile tear region in a scanning electron microscope (SEM), as shown in Fig. 5, and is indicative of ductile tearing. As indicated in the NTSB schematic reproduced in Fig. 4, fatigue crack growth morphology was observed between the ductile tear and the final brittle cleavage fracture zone. Cleavage fracture morphology is characteristic of fast, unstable fracture in steel. Cleavage fracture morphology within the final rupture zone in Pup 1 is shown in Fig. 6. Figure 4 : Montage from NTSB Final Report (Fig. 21) that highlights approximate ductile tear (yellow) and fatigue crack growth (green). Figure 5 : Exponent SEM image that shows microvoid coalescence fracture morphology within the ductile tear (the yellow area in Fig. 3). Representative microvoid coalescence morphology is indicated within the white box.