The image above is a "coital model" at SSL International's Cambridge Technical Centre. SSL International manufactures Durex condoms. The machine was used in a scientific study published last year by SSL in which they studied why condoms break. From the paper, published in the journal Contraception:
Evidence combined from examining returns, questionnaire responses and the coital model strongly suggests a single predominant mechanism of failure we named "blunt puncture," where the tip of the thrusting male penis progressively stretches one part of the intact condom wall until it ultimately breaks."Male condoms that break in use do so mostly by a "blunt puncture" mechanism." (via NCBIROFL)
CONCLUSIONS: Blunt puncture appears to be the mechanism of breakage responsible for more than 90% of condom breakage not attributable to misuse. Knowledge of the main mechanism of breakage should help develop better user instructions, better test methods and, ultimately, better condoms.