August 3, 2015 at 8:19 PM by Dr. Drang
Over the weekend, my wife and I went on a bike ride to a nearby lake and wetland area. The park district built a wooden deck that extends out a bit into the wetland, and we went out onto the deck to look at the herons, frogs, and turtles. There are benches on the deck that use plastic instead of wood for the seat and back slats, and we noticed that several of the seat slats were broken. Vandalism, I thought, until I looked at the breaks more carefully.
“Go get your phone and take a picture,” my wife said. “You know you want to.”
So I did. You can click/tap on the photos to see bigger versions.
The first thing you should notice is that the breaks occur where the screws come up from below to secure the slats to the steel frame of the bench. You should also see that each fracture surface has an oval that’s a darker color than the outer edges. Let’s look closer at the fracture on the right.
A foaming agent was used to get those little bubbles in the plastic. It reduces the amount of resin necessary to fill the mold, which cuts down on the weight and the cost to produce the slat.
The fracture has a few important features:
- The area of plastic where the screw was. It’s exactly the size of the screw, indicating that the screw was driven directly into the slat with no predrilled hole.
- The radial lines that come out from the screw hole. These indicate that the fracture started at the screw hole and grew outward.
- The circumferential lines near the outer edge of the dark oval, especially in the upper right quadrant. These show the size of the internal fracture at earlier stages in its growth.
- The white “rind” outside of the dark oval. When polymers break through regular mechanical loading, they exhibit what’s called “stress whitening” as little tendrils of plastic are stretched and snapped.
The cracks started at the screws because of the high stresses induced by driving the screws directly into the slats. The cracks grew outward, probably through a process of creep rupture, which created the dark ovals. As the cracks grew, the stresses driving the crack growth lessened because the slat became more compliant. The final failure probably occurred when someone stepped on the slats, but possibly when someone sat out on the cantilevered end of the seat. The outer rind of the weakened slat stretched, leaving behind the whitened zone. I’ll bet most of the other slats have similar internal oval cracks and are just one step away from snapping off. I’ll also bet the manufacturer of these slats recommends that screw holes be predrilled to avoid this problem.
The frogs and turtles and herons were fun to look at, too.