Breaking bad (and sometimes good)

Nothing counts except for your thirst, and you’re cracking open a cold can of cola to deal with the problem when… arrrrrgggggh – the little opening key breaks off.

So annoying.

But what if the thing that cracks or breaks isn’t as trivial as a drink can? What if it’s a hinge in an airlock in an aircraft that’s thousands of feet in the air?

These are examples of fracture by design or design flaw, and a fundamental test of nearly every manufactured object is fracture testing: how easily do things break?

Tim Briggs of Sandia National Laboratories, in Albuquerque, New Mexico, spends a lot of time breaking things, and that’s what helped him build a device that can fracture test, well, almost anything.

Briggs points out that this applies to almost anything that is made of more than a single unit bonded together with glue or other bonding agents – a vast array of things including devices and machines for aerospace, defence, automatics (things that start, operate or move independently), energy, medicine, sporting and communications. All are subject to simple, fracture-based failure.

Sometimes things break and it isn’t really a big deal – going back to that cola-can opener – but there are times when a seemingly small fracture can be downright deadly. It’s almost certainly not the most glamorous area of engineering, but it is utterly essential.

“Think about critical applications like a pressurised aircraft at 30,000 feet with 300 or more souls on board relying on bonded surfaces as part of a critical load path,” Briggs explains. “That can never fail. But people also don’t want their very benign carbon-fibre hockey stick, or mountain bike that they paid hundreds or even thousands of dollars for, to break.”

The bright side of breaking

Not all fractures are bad, either. Some are deliberate, and in some cases if something’s too difficult to break, it’s no longer useful.

There are countless examples in our everyday lives when things fracture both intentionally by design and unintentionally by flaws

Tim Briggs

“Think of a survival glow-stick that activates when the internal tube is intentionally cracked open to allow the necessary chemical mixing for the lighting effect,” says Briggs.

“There are countless examples in our everyday lives when things fracture both intentionally by design and unintentionally by flaws, misuse, accident and lack of sufficient design.”

A fracture test on the new Mode I Fracture Testing Apparatus. Credit: Courtesy Sandia National Laboratories

In so many of these engineering cases, subunits are stuck together with something like metal or glue, which forms a chemical bond between the two subunits. Sometimes there’s no glue at all, and it is just bonded together by its own molecules – for example, a sheet of metal. But it can still crack.

Chemical bonds range from the weak (such as whiteboard-marker ink sticking to a whiteboard), to the stronger (permanent-marker ink), to the severe – think of the bonds that hold diamonds together.

Building for better breaking

Making sure a product is safe or practical all comes down to finding the right balance of effort that goes in to breaking a chemical bond.

Traditionally, this testing involved aligning and bonding hinges on the fracture testing machine to a material, and then pulling it apart in a controlled manner. Simple? Not quite.

“I can’t tell you how much work it was for us to cut hinges, abrade and clean all the bonded surfaces, mix adhesives, precisely align the hinge to the specimen face, glue the hinge to one side of the specimen, allow it to cure, clean up the mess… then do it all again to the other side,” says Briggs.

Clearly weary of the old process, Briggs invented a device: the “Mode I Fracture Testing Apparatus”. You have to give credit to the name, because it describes exactly what it does.

Sandia defines this device as an “apparatus that uses load frames to conduct Mode I Fracture Testing of bonded structures without surface prep, adhesive mixing, alignment jigs, or prolonged wait times”.

The key thing is that it doesn’t have to use any glue in sample preparation – instead, it uses a table-top drill-hole jig and a rotating pin block to load the material onto the testing machine.

The so-called “drill and go” method takes out all of the former procedure’s middle steps – of waiting for things to dry and later dissolving adhesives – and its rotating pins allow for ‘wiggle room’ because they don’t need to be custom fitted and aligned in the way the hinges did. 

Cool. Things have now been sped up in the fracture-testing caper. So what does fracture testing measure?

Think of a piece of tape stuck to a table. How would you pull the tape off? Instinctively, it’s easier to pull the tape up and off, instead of trying to slide it off horizontally, or to remove it by pulling it along the table in the same plane and direction.

The “up and off” technique uses the least amount of energy to break the glue bonds between the surface and the tape – because it is easier to break things when they are wrenched in the opposite direction.

This type of fracture is called “Mode I”. It’s the most common type and is often referred to as the “weakest link”, so it’s generally the first type to be tested with a new object.

Now, imagine you were testing different types of tape. Some would only require only a little bit of work to pull off, and some would require a fair amount.

You don’t want to put much work into opening a soft-drink can, but for something like the airlock in an aeroplane, you want your bonds to require so much work to separate that they just don’t fracture at all, under any circumstance.

That comes down to the amounts of energy referred to earlier. Fracture testing isn’t just about asking “does this break?”. It is, after all, a science, and science is all about quantifying things.

Essentially, and very simplistically, you can calculate how much work (or energy) is needed to break chemical bonds.

Briggs’ device tests this amount of work by simulating the tape on the table: numerically, how much work is needed to tear the material apart – like a wishbone – when it is pulled in the opposite direction?

The material being tested is put on the device and wrenched apart. Two things are considered here: the displacement, or distance moved, of material to reach the point it which fractures; and how much force is applied to get it to that point.

These two numbers can be used to calculate the work done (energy) to pull the material apart, which can then be compared what kind of conditions the material will normally be subjected to.

The bond strength is also informed by adhesion chemistry (certain molecules are going to bond differently), the geometry of the material (some shapes are harder to break than others), and elasticity (a springy glue behaves differently to a rigid glue).

“We pull the fracture specimens apart in a very controlled manner,” says Briggs. “Then, we’re able to measure the response of the material and quantify the relevant fracture properties, which informs us how cracks might actually grow when used in finished products under various loading conditions.”

Pieces together

A cola can’s hole is not shaped solely for safety and comfort (when you put the can to your lips); it also allows less work to be used to pop it open.

So, despite being pervasive and necessary, fracture testing wasn’t all that easy to do until now.

“There’s a beauty and simplicity here,” Briggs explains. “Now you can completely abandon the old, laborious process of bonding hinges to the surfaces of the specimen.”

Making this necessary process faster and easier should have a trickle-down effect to customers, Briggs says. When less labour is needed in development, products can be made and sold more cheaply.

“Seeing the interest from other Sandia researchers for this device is exciting,” Briggs says.

“I hope this new approach, and the work it could enable for others, can have a broad reach and impact beyond Sandia’s national security mission, touching people’s everyday lives more visibly in their day-to-day activities.”

Named for the mountain range just east of Albuquerque, New Mexico, Sandia National Laboratories provides science and technology solutions to the US Government and private industry in areas including nuclear weapons and power, national security and energy.