The characterisation of notched 3D woven composites using Thermo-elastic Stress Analysis and Digital Image Correlation

Although adhesive bonding of fabricated composite structures would be the preferred option from a design for manufacture point of view, there will always be a need to use mechanical fastening for highly loaded regions. Fatigue behaviour and notch sensitivity are two of the main concerns for fabricated composite structures, despite the excellent performance of the material in the former regard. For 3D woven composites previous research has concentrated on the un-notched fatigue behaviour with less research focused on the notched properties. In the present work, the quasi-static and tension-tension fatigue behaviour of notched orthogonal and angle-interlock 3D woven carbon/epoxy composites are investigated. Digital image correlation was used to obtain the full-field strain distribution during the quasi-static tensile tests, and thermo-elastic stress analysis was used to characterise the damage progression during the fatigue tests. The DIC results revealed that the tensile strength of the 3D woven composites was not sensitive to notch size, with the notched tensile strength being less than 17% lower than the un-notched tensile strength. Limitations of the test machine capacity meant that the fatigue specimens were loaded at 60% of the ultimate failure stress. The specimens were cyclically loaded for 5,000,000 cycles without complete fracture, although the TSA results revealed progressive surface cracking around the notch. The orthogonal weave was found to have a larger surface damage area than the angle-interlock weave.