Every printed part is a lottery ticket. The prize in this lottery is not money but fatigue life: how many load cycles the metal can withstand before a crack initiates. The ticket number is set by the random arrangement of grains and pores that arise during laser powder bed fusion. Previously, engineers estimated this lifetime as a single number with a generous safety margin, but now there is a way to see the full spectrum of possible outcomes.
To run the lottery fairly, the researchers printed cylinders from 316L steel and cut out cubes with a side of 1.6 mm. Under an electron microscope, they observed the orientation of each grain, and X-ray tomography revealed the pores inside. Each cube was divided into 64 smaller subcubes, and for each they ran three simulations: one for the energy released during crack growth, another to find the easiest crack path, and a third for fatigue damage accumulation. The heaviest simulation took nearly 18 hours on a 28-core processor, so all calculations consumed more than 240 days of machine time.
By collecting the extreme values of the three quantities for each subcube, the scientists fitted distributions and derived an analytical formula for the probability of fatigue life. The distribution turned out to be right-skewed: most parts live around 29,300 cycles, but some stretch to 31,400 and beyond. If you plan inspections using an 80% margin from the mean, the interval is 4,200 cycles shorter than when using the full probability density. This means a part can be operated longer without risking sudden failure.
Previously, fatigue life was estimated using conservative factors or empirical correlations that did not provide a distribution. Now, from the measured microstructure, the probability of a given service life follows directly. This is the first approach that links grains and pores to an analytical formula, bypassing arbitrary safety factors. In aviation, energy, and medical implants, such a tool allows parts to be admitted to service based on probabilistic criteria rather than guesswork.
The lottery ticket of a printed part is no longer a mystery until the very end. Its number can be read in advance by calculating probabilities for each combination of grains and pores. The next step is to calibrate these probabilities against real fatigue tests, making the lottery even fairer. Then, perhaps, it will be possible to predict a part's reliability before it even leaves the printer.
The crystal plasticity simulation for a single subcube took almost 18 hours on a 28-core processor, whereas the elastic calculation finished in 355 seconds, and the graph search for the crack path in 91 seconds. For 64 subcubes and five cases, the total time for plasticity alone exceeded 240 days of machine time.
In Kim Stanley Robinson's novel 'Aurora', the interstellar ship suffers from fatigue cracks in printed structures that cannot be predicted in advance. This work offers precisely the tool the characters lacked: a probability distribution of fatigue life that allows repairs to be planned before a crack becomes critical.