Difference Between Seam Strength and Seam Slippage
Both the parameters measure the performance of seam. Seam strength referes to the strength when seam finally ruptures or when the fabric breaks.
However before rupturing there is an unacceptable opening in the seam which makes the seam 'failed' commercially even when there is no visible rupture. Seam slippage measures that.
Seam strength depends upon stitch type, thread strength, stitches per inch, thread tension, seam type and seam efficiency of the material.
Seam slippage depends upon the stitch rate, the weave structure of the fabric and the width of the seam allowance.
There is another term called 'yarn slippage' which measures the shifting of warp yarn over weft yarn to render the garment unusuable.
Yarn slippage depends upon a low number of warp or filling yarn, two shallow seam allowance, too tight a fit and improper seam construction.
Find Pictures of Seam Quality Defects here.