Damage characterization in SFRP using X-ray computed tomography after application of incremental and interrupted in situ quasi static tensile loading

The use of short fiber reinforced polymers (SFRP) is increasing steadily in automotive and aerospace industry due to its mechanical properties and light weight. The mechanical and physical properties of SFRP depend on the geometrical characteristics of the reinforcing material. Under tensile stress various defects are induced in SFRP composites. X-ray computed tomography (XCT) is a non-destructive method for damage characterization of SFRP. It helps to understand the material behaviour under different stress conditions and to evaluate the strength of the material. The study of the evolution of various defects in SFRP composite material was conducted for damage characterisation. The composite consisted of a polyamide matrix and 30 wt. % of short glass fibers oriented 90° relative to the direction of force. Damage was induced after application of predetermined tensile force in a quasi static method using an in situ tensile testing device. Quasi static tensile force was applied from 85% to 99 % relative to the pre-evaluated final fracture force of 462 N. Damages were analysed after every step of the force enhancement using XCT at the resolution of (4.5 μm)3 voxel edge length. The detected defects were classified into four types: 1) fiber pull-outs, 2) fiber fractures, 3) matrix fractures and 4) fiber/matrix debondings. Fiber pullouts and matrix fractures comprised of more than 90% of all the defects detected. The maximum number of separate defect voxels were detected prior to the final fracture post the application of 90% of final fracture force. The increase in tensile force from 90% to 96% showed a corresponding 66% increase in defect volume. Higher defect density was observed in the notched region. Fiber pullouts and fiber fracture together formed the prospective plane of final specimen fracture region. Small defects critical in the initiation of damage processes such as evolving pullouts and matrix inhomogeneities were observed from 85% to 90% of the final fracture force. The classification of defects at every step after applying force helped to understand evolution of damage mechanisms in the stressed region.