Chair Failure Analysis, Plastic Lawn Chair

A failed plastic chair was submitted to our failure analysis laboratory for examination. The right rear leg had broken off; however, the initial chair failure occurred on the left rear side. The left rear leg separated from the chair body due to poor bonding and molding porosity at the top of the leg. The failure resulted from manufacturing defects. On the upper right are photographs of the initial failure. There is porosity in the region where the leg is attached to the chair. In addition, the bond line is very narrow. Upper left is a photograph of the same region on an exemplar chair. One can see that the where the leg meets the chair there is severe undercutting. this creates a "stress riser" that reduces the joint strength.