Days after Apple's first foldable iPhone went on sale, a small number of owners have reported cracks spreading across the device's inner screen. Independent teardowns point to three factors that may help explain why, though the exact cause has not been confirmed.

The $1,999 iPhone Duo launched Oct. 23 and quickly sold through its initial supply. Within hours, photos began appearing online showing damage along the center fold of the 7.6-inch inner display, often a crack or a bright line of stuck pixels running from the crease to one edge.

Apple has acknowledged the problem. In a statement to tech site 9to5Mac, the company said it was "aware of an issue impacting the inner display on a small number of iPhone Duo units."

"If users experience issues with their iPhone Duo display, they should contact AppleCare for a replacement," Apple said.

The company has not said what is causing the damage or how many phones are affected. But teardowns by repair specialist iFixit and independent testers point to three possible culprits.

1. A stress point at the hinge's center

The leading theory involves the phone's complex hinge, which uses gears, springs, interlocking metal joints and a "waterdrop" design that lets the display bend in a loop rather than folding flat.

iFixit built a 3D model of the hinge using CT scans, teardown footage and Apple's patent filings, then compared it with photos of damaged screens posted by owners. It found that the breaks tend to cluster in........

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