Glass Coffee Pot Failure Analysis: Thermal Shock Failure Of Borosilicate Glass Coffee Carafe - Example of Glass Fatigue

Result Summary

The fatigue failure initiated on the outside bottom where the carafe had been scratched with the abrasive scrub pad. As a result of cyclical thermal shock (between 195°C and 15°C), the cracks grew progressively till it reached a critical length. Glass thickness at the origins is approximately 3 mm. One failed after 12.

Introduction

In order to simulate normal, use, coffee carafes were scratched with a chosen abrasive and thermal cycled. In this case a commercial scrub pad was used. It is a sponge with embedded 320-400 grit aluminum oxide.  This scratching is to simulate both cleaning and sliding the carafe on hard counter tops. In this test, the bottom of a new borosilicate coffee carafe was scrubbed with a wet abrasive sponge and then thermal cycled numerous times till a visible crack formed. A carafe that failed after 51 cycles is examined. The progressive crack initiated at an abrasive scratch.

Results
51 Cycle Failure

Figure #1: Photograph of the failed carafe bottom. The “wavy” nature of the crack indicates that it was driven by thermally generated stress.

Figure #2: Photomicrograph of the fracture origin on the fracture surface. The fracture originated at an abrasive scratch. As a result of thermal shock, the crack grew progressively till it reached a critical length. Glass thickness at the failure origin 2.8mm (Mag. 10X).

Figure #3: Higher power photomicrograph of the fracture origin on the fracture surface. The crack initiates on the outside bottom where the carafe had been scratched with the abrasive scrub pad. As a result of stresses from thermal shock, the crack grew progressively till it reached a critical length. Glass thickness at the origin 2.8mm (Mag. 40X.