This study designed a specimen that can simulate deformation and crack initiation in system, structure, and components (SSCs) of nuclear power plants (NPPs) under excessive seismic loads, and conducted ultimate strength tests using this specimen at room temperature (RT) and 316°C. The specimen designed was a compact tension (CT) type with a round notch, and both SA312 TP316 stainless steel (SS) and SA508 Gr.3 Cl.1 low-alloy steel (LAS) were used in the experiment. Displacement-controlled cyclic loads with constant and random amplitudes were applied as input loads for the test. One set of input loads consisted of 20 cycles, and the input amplitudes of load-line displacement (LLD) were determined to induce the maximum elastic stress of 6∼ 42Sm on the specimen, where Sm is allowable design stress intensity. The input LLD had a triangular waveform and was fully reversed for both types of amplitude. During the test, multiple sets of input cyclic loads, with increasing amplitude of input LLD, were applied to the specimen until a crack was initiated. The results demonstrated that the specimen used in this study adequately simulates the deformation and failure behaviors of SSCs under excessive seismic loads. In addition, the samples in both materials failed under cyclic load levels that were several times higher than those of design basis earthquake (DBE). The SA316 TP316 SS specimen had a greater safety margin under excessive seismic loading conditions than SA508 Gr.3 Cl.1 LAS specimen, regardless of test temperature.