A mixed-mode decohesion finite element is proposed for simulating delamination in composite materials. The element models delamination initiation and growth at the interface between solid elements. A single relative displacement-based damage parameter is used in a softening law to track interface damage and prevent restoration of the cohesive state during unloading. The softening law can be applied to any mode interaction criterion, such as the two-parameter power law or the three-parameter Benzeggagh-Kenane criterion. The accuracy of the predictions and the irreversibility of the constitutive law are demonstrated through simulations of steady-state delamination growth for quasi-static loading-unloading cycles of various single-mode and mixed-mode delamination test specimens. The element formulation includes a quadratic interaction between tractions to predict softening onset and a criterion to capture mixed-mode fracture toughness under different mode ratios for delamination propagation. The proposed formulation is validated by simulating double cantilever beam (DCB), end-notch flexure (ENF), and mixed-mode bending (MMB) test specimens, and comparing the predictions with experimental data. The decohesion element is implemented in the ABAQUS finite element code as a user-written element subroutine. The element is used to simulate DCB, ENF, and MMB tests for different composite materials, including T300/977-2 and PEEK/APC2. The simulations show excellent agreement between experimental data and numerical predictions, validating the unloading behavior of the constitutive equation. The B-K interaction law is used to predict delamination propagation under mixed-mode loading conditions. The B-K criterion requires an additional material parameter, η, determined from standard mixed-mode tests. The criterion is shown to provide accurate predictions for mixed-mode delamination in composite materials. The proposed method allows for the simulation of delamination growth without prior knowledge of the crack location and propagation direction. The method is validated through simulations of DCB and ENF tests, which represent cases of single-mode delamination, and MMB tests, which simulate mixed-mode delamination under various loading conditions. The results show good agreement with experimental data, indicating that the proposed mixed-mode criteria can predict the strength of composite structures with progressive delamination.A mixed-mode decohesion finite element is proposed for simulating delamination in composite materials. The element models delamination initiation and growth at the interface between solid elements. A single relative displacement-based damage parameter is used in a softening law to track interface damage and prevent restoration of the cohesive state during unloading. The softening law can be applied to any mode interaction criterion, such as the two-parameter power law or the three-parameter Benzeggagh-Kenane criterion. The accuracy of the predictions and the irreversibility of the constitutive law are demonstrated through simulations of steady-state delamination growth for quasi-static loading-unloading cycles of various single-mode and mixed-mode delamination test specimens. The element formulation includes a quadratic interaction between tractions to predict softening onset and a criterion to capture mixed-mode fracture toughness under different mode ratios for delamination propagation. The proposed formulation is validated by simulating double cantilever beam (DCB), end-notch flexure (ENF), and mixed-mode bending (MMB) test specimens, and comparing the predictions with experimental data. The decohesion element is implemented in the ABAQUS finite element code as a user-written element subroutine. The element is used to simulate DCB, ENF, and MMB tests for different composite materials, including T300/977-2 and PEEK/APC2. The simulations show excellent agreement between experimental data and numerical predictions, validating the unloading behavior of the constitutive equation. The B-K interaction law is used to predict delamination propagation under mixed-mode loading conditions. The B-K criterion requires an additional material parameter, η, determined from standard mixed-mode tests. The criterion is shown to provide accurate predictions for mixed-mode delamination in composite materials. The proposed method allows for the simulation of delamination growth without prior knowledge of the crack location and propagation direction. The method is validated through simulations of DCB and ENF tests, which represent cases of single-mode delamination, and MMB tests, which simulate mixed-mode delamination under various loading conditions. The results show good agreement with experimental data, indicating that the proposed mixed-mode criteria can predict the strength of composite structures with progressive delamination.