Biaxial properties of individual bonds in thermomechanically bonded nonwoven fabrics

Abstract

Thermomechanically bonded nonwoven fabrics contain discrete bonds that are formed from melted and fused fibers. In these fabrics, the fibers are loosely organized although they lie predominantly in the machine direction (MD). Through a custom-built biaxial testing device and simultaneous image capture, the mechanical response of individual bonds in thermomechanically bonded nonwoven fabrics made of polyethylene/polypropylene sheath–core fibers was studied. Toward this end, cruciform specimens (n = 20) with bonds in the gauge areas and arms aligned in the MD and the cross-direction (CD) were subjected to displacement-controlled equi-biaxial tests. The biaxial force–displacement curves along the two loading directions were found to be different. The average maximum force and average stiffness were significantly higher in the MD than in the CD (p < 0.05). This difference was determined by the amount and orientation of fibers and size of the bonds in the two directions. By analyzing the images captured during equi-biaxial testing, the bonds were always observed to disintegrate into their constituent fibers. Digital image correlation was used to measure the local and average Eulerian strains of the bonds before their breakage initiated. The average axial strain experienced by the bond in the MD was always monotonically increasing with the axial load. The average axial strain in the CD, however, varied among bonds: it was monotonically increasing, monotonically decreasing, and increasing and decreasing with the axial load. Strain maps demonstrated the inhomogeneity in strain experienced by the bonds. These findings can guide the design and development of thermomechanically bonded nonwoven fabrics for applications in automotive, medical, consumer products, and civil engineering industries.