Aluminium is a fatigue-prone material but metal fatigue is not a time-dependent phenomenon, so to recommend replacement of handlebars after so many years (or so few!) is nonsensical. What matters is how much they have been used, and how hard. Don’t ride the bike and its handlebar will last forever. Or if a bike is ridden mainly on smooth roads, by a lightly-built person, who seldom stands on the pedals but sits and twiddles low gears uphill and is careful that his bike and particularly his handlebar never gets scratched, this component may likewise last indefinitely. But if it’s ridden on rough roads by a broad-shouldered brute of a rider who likes to heave big gears uphill and doesn’t take any special care of their bike, the handlebar may endure only a few thousand miles. And that much punishment may well accumulate in only three years. So any limit, miles or years, is no better than a guess.
Any scratching of the handlebar, particularly near or coincident with the stem, must be avoided. Because a scratch is a ready-made crack, that focuses stress, which if repeatedly reversed (like when you bend an old credit card to and fro to weaken and tear it) will gradually grow through the metal, concentrating stress ever more strongly as it becomes deeper and wider until there’s not enough unbroken material remaining and it snaps.
When handlebars are simply clamped in a traditional stem by one bolt, they sometimes have to be pushed hard and swivelled to and fro to persuade them through the tight-fitting clamp. Then it can be hard to avoid scratching them and failures are not uncommon. Some bars are formed into a bulge in the centre with another, shorter piece of tube inside to reinforce the centre. But sometimes this reinforcement can be misplaced, off-centre, and I recall a spate of breakages of one model of handlebar made like that.
Modern stems are mostly ‘front opening’, the front clamp being removable, so there’s no risk of scratching a bar when fitting it or adjusting the angle, provided the bolts are loosened enough! But careless handling and ill-fitting metal brackets (for lamps etc.) can always scratch a bar. And it seems likely that manufacturers have taken advantage of the new, less-damaging method of clamping bars to stems, plus the move to ‘oversize’ clamp diameter, to reduce metal thickness and weight. This makes any slight damage more significant and reduces the handlebar’s ability to shrug off small episodes of abuse. So take care of your handlebar.
This was first published in the April / May 2015 edition of Cycling UK's Cycle magazine.