Actually, they are not Chinese, I think they were made in the UK and are about 20years old at a quick guess.
Looking at the fracture line, it looks like a crack started a long time ago, could even have been there from day one.
Obviously today was the day to finally let go!
On edit:
I am sure they were not quite expensive enough for Lindstrom, looking carefull there appears to be the name Wilkinson engraved on one side of the joint.
I do have some Lindstrom cutters, cost me about £30+ a couple of years ago.
An "impulse purchase" at the RS trade counter!
Weird! The exact same thing happened to me a week or so back - maybe even the same day - my favorite pair of needlenose pliers suddenly snapped like that. WTF? Sunspots? Cosmic rays?
It could be a manufacturing fault (unlikely if they're good quality) but pliers don't normally fail like that unless they've been stressed in some way.
I've never had pliers break like that. Mine just disappear entirely. I think there's another dimension in the universe where all our missing pens, pliers, and socks wind up.
A copper wire breaks with metal fatigue if you bend it back and forth many time.
Steel is stonger but still breaks with metal fatigue if it is bent back and forth many times.
Pliers bend a little when squeezed then unbend when released. Back and forth until failure occurs.
Good steel has a sort of fatigue lower limit. Up to a specific point it acts like spring. Past that point is just starting to bend and hold that bent position. Sort of the point of no return in a way. If that point is passed enough times it will eventually break. How far and how often that point is passed is what will determine the limit of fatigue before it breaks.
All materials have similar properties but their point of no return can vary greatly and even the shape of the materials can affect that point.
In high shock load applications its well known that by having no sharp edges on a part can greatly increase its durability and resiliency towards stress fatigue and cracking.
The durability of one tool VS another from the same batch on an assembly line may just be the difference in which part was in the deburring process a little longer than the other. The one with sharp edges on its corners will break before the one with rounded edges on its corners will even they both may have identical working characteristics and conditions.
Look at the close-up photos.
The busted pliers have been machined with sharp corners in the joint and the surface in the joint is not much to shout about, very coarse finish from a fine pointed tool.
Sharp corners are bad in stressed components, the highest stress is at the corner and cracks will start right there.
Exactly what has happened in this case.
Compare the Lindstrom cutters, rounded corners and fine finish on the surface.
There are no high stress points to initiate cracking.