Modeling of the progressive failure behavior of multilayer knitted fabric-reinforced composite laminates

This paper deals with the characterization and modeling of the failure behavior of laminates with application to multilayer plain weft-knitted-fabric-reinforced composites. Experiments have been performed to measure the uniaxial tensile strengths of eight knitted-fabric laminates with lay-ups of [0/0/0/0], [0/+30/−30/0], [0/45/−45/0], [0/+60/−60/0], [0/90/90/0], [+30/−30/−30/+30], [+60/−60/−60/+60], and [90/90/90/90], where 0 refers to the fabric wale direction and 90 to the course direction. A theoretical modeling procedure for the progressive failure process in the laminate is presented. The simulation, made at the lamina ply level, is based on the classical thin-laminate theory and a recently developed bridging micromechanics model. Only material properties of constituent fiber and matrix as well as fabric geometric parameters are necessary. The internal stress increments generated in the fiber and the matrix of each lamina are directly related to the overall applied load increment on the laminate. Whenever any constituent material attains its ultimate stress state, according to the maximum normal stress criterion, the corresponding lamina fails, and a stiffness discount is applied to the remaining laminate. An angle-ply laminate subjected to in-plane biaxial load combinations together with the eight knitted-fabric laminates under uniaxial tension has been analyzed. The predicted ultimate failure strengths or strength envelope of the laminates agree well with our or available experimental data. Furthermore, the influence of different fabric lay-up configurations on the first and the last-ply failure strengths of the knitted-fabric laminates has been investigated, and the failure envelopes of several knitted-fabric laminates subjected to bi-axial loads are also shown in the paper.