<p>Quarter-circular corner cracks are typical flaws in three-dimensional (3D) structures. For curved 3D cracks, the application of a plane-strain fracture criterion often leads to inaccurate fracture predictions. To enhance prediction accuracy, a 3D fracture assessment method incorporating an equivalent thickness model based on the <i>T</i>_z factor is employed to evaluate the fracture strength of a quarter-circular corner crack within a finite body. Analysis reveals that as the ratio of crack depth to plate width (<i>c/W</i>) increases, the plane of strongest <i>T</i>_z constraint shifts away from the 45° plane normal to the crack front. Through systematic finite element analyses, a new equivalent thickness equation for quarter-circular corner cracks in finite bodies is derived. Additionally, a modified stress intensity factor equation is developed based on Newman’s original formulation, with the applicable upper limit of <i>c/W</i> extended from 0.2 to 0.95. To validate the proposed 3D fracture prediction method, a new fracture test involving a quarter-circular corner crack is conducted. Experimental results demonstrate that the 3D method provides significantly improved accuracy compared to traditional two-dimensional fracture criteria. For a specimen containing a quarter-circular corner crack under tensile loading, the prediction error is reduced from 35.3% to 2.2%.</p>