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Bag Types & How They Fail

Bags are designed to support the load of their own weight and the weight of objects placed inside them. Even with a small collection of bags, you will probably find three broad types: bags with No Handles, those with Attached Handles, and ones with Cut-Out Handles. Attached handles can be made of paper, plastic, string or cord. How these handles are attached and reinforced determines their strength -- via glue, tape, thread or heat sealing. Cut-out handles can take different shapes and locations on the bag -- on the side or front.

Getting students to focus their attention on where and how different bag designs start to give way and fail is key to their doing thoughtful redesign work. Click on the bag icons above to see videos with slow-motion shots of bags being loaded to failure with things like 2-lb and 5-lb bags of sugar, bottles of water (1 pint = 1 pound), and rolls of pennies (can be borrowed from and returned to any bank at no cost).

Bags fail when forces get concentrated and exceed the strength of their materials. Failure can be predicted by noticing where stress lines gather. Failure can occur in places where the amount of bag materials is reduced (Figure 1) or when a pointy object such as a box or shoe is placed with its sharp end pressed against the bag (Figure 2). Square-shaped, cut-out handles are weaker than rounded ones (see Figure 3) because forces concentrate more in cut-out corners than curves in bag material.

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