Seam slippage is the other phenomenon that may occur when seams are put under load. As previously described, when the seam itself is strong (resistant thread, high stitch density) the fabric may fail first, either by slippage (behaviour type 2 or 3 as classified by Gribaa et al., 2006), or by rupture (type 4 behaviour). Assuming that the seam does not fail, the occurrence of seam slippage depends both on the seams as well as on the fabric properties. Seam slippage decreases with increasing stitch density, since there is a better distribution of the load. It depends also on the sewing thread’s material and structure.
The fabric properties, namely the frictional properties of the yarns and the fabric density, cover factor and weave type also play an important role. The frictional properties of the yarns are influenced by the yarn’s twist, size, type of fibre and density. The width of the fabric edge (the seam margin) is also relevant: the fabric has more tendency to slippage if the margin is narrow (Gribaa et al., 2006; Yildirim, 2010; Uçar, 2002). Many researchers have reported these factors as being the most influential to the propensity of a fabric to seam slippage. Miguel et al. (2005) related slippage to the fabric’s material and type of finishing (which influence the frictional interaction between the yarns in the structure).
The interaction between yarns in the fabric is also influenced by the structure of the fabric yarns and the weave type. Filament yarns have been found to cause higher slippage due to their more slippery surface, and weave type has an important influence on slippage (Kalaoglu & Meric, 2005). Regarding weave type, the latter work also describes the influence of the direction of stress in combination with the directional properties of the fabric structure. The analysis of seam slippage is of interest not only in static but also in dynamic conditions. Yildirim (2010) has found that seam slippage is generally worse under dynamic than under static loads, and the importance of the causal factors varies depending on the type of load.