Rupture Stress The rupture stress or rupture strength is the stress at which failure occurs. For structural steel the nominal rupture strength is considerably lower than the ultimate strength because the nominal rupture strength is computed by dividing the load at rupture by the original cross- sectional area. The true rupture strength is calculated using the reduced area of the cross section where the fracture occurred. The difference in the two values results from a phenomenon known as necking. As failure approaches, the material stretches very rapidly, causing the cross section to narrow, as shown in Fig. 2.6. Because the area where rupture occurs is smaller than the original area, the true rupture strength is larger than the ultimate strength. However, the ultimate strength is commonly used as the maximum stress that the material can carry.