How does one go about testing the robot once it is built? How does one predict the number of hours it can operate? I see most of the industrial robots for instance robotic arms have warranty of 12-18 months, how did they arrive at such an estimate, clearly testing for 12-18 months is not on option, so what is the procedure for determining the lifespan?
Testing of each individual robot produced (or any other product) is manufacturer and product specific. The tests will be small and quick, looking for unexpected problems in the manufacturing process.
Manufactures will test a few robots (that they don't sell) extensively so that they can make predictions about how the robot will perform over time, find unanticipated problems, learn how to improve future models, and how to manage customer expectations. Designs can be, and are, tested for years. The manufacturer will rely on good manufacturing practices and random sampling to insure that the actual device you receive has a performance similar to the tested devices.
Design engineers use lifespan calculations when designing. For many of the components like bearings, transmissions, and flexible wiring, the manufacturers provide data and calculations to estimate life based on the operating conditions. This data and calculations are derived from testing the manufacturer does on their own products.
In general, robotic arm is more mechanical than electrical. Manufacturers would use power law to increase vibration stress to accelerate life of their products. They will pick several product, estimate mean life and predict reliability and life of the model. It is called accelerated life test and can be used for failure analysis and life predict. Highly accelerated life testing (HALT) also accelerates life of product, but it cannot predict life range of tested unit. It is widely used to cause failures and thus improve design by exposing weakness of product.