Practical TIG Welding: Destructive Testing and Penetration
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Destructive Testing and Penetration
05.18
| 00:00 | - When we compare TIG welding to other welding operations we can see that there isn't really anywhere to hide. |
| 00:05 | The weld bead usually gives a pretty good indication of the quality of the weld and this will also give us some understanding of the strength characteristics of the joint. |
| 00:14 | In the majority of motorsport applications, it's very rare for any inspection to be more thorough than a visual inspection and this is typically what you'd expect before a roll cage structure is signed off for the first time for example. |
| 00:27 | The problem with a visual inspection is that we can't see how much penetration there is and this penetration is the key to the strength of the completed weld. |
| 00:36 | The term penetration refers to the depth of the weld and the inclusion into the parent metal. |
| 00:41 | Full penetration refers to the weld being the same depth as the material welded which means it will be present on the back side of the material and easily visible. |
| 00:50 | In theory, a TIG weld that has full penetration, correct filler material and sufficient shielding gas will be stronger than the heat affected material to each side of the weld. |
| 01:00 | To test this theory, we need to perform some destructive tests and this is something that you won't need to do a lot of in motorsport TIG welding but it can assist you in understanding the strength of your welds as well as what lies beneath the surface. |
| 01:14 | There are a few methods of testing welds that we can easily do and all of them involve completing a test part that will reflect the material, settings and technique that you plan on using for your project. |
| 01:27 | The first test we can perform is the bend test, by running a series of test welds on a range of different materials you will not only spend some valuable time practicing your welds but you'll also be testing them and getting a greater understanding for the strengths and weaknesses that these will have. |
| 01:43 | The bend test uses a 90° fillet weld performed on one side of the material and then it's bent to breaking point in order to expose the weld and its failure points. |
| 01:53 | To demonstrate this, we've welded up a 3 mm test piece in mild steel in a single sided fillet weld arrangement using 120 amps using our recommended amperage of 40 amps per millimetre rule. |
| 02:06 | You can perform this test on other materials too if you wish and the process is identical. |
| 02:11 | We've chosen 3 mm thickness because it's a common material size used in motorsport for load bearing structural chassis modifications and it's also a regulation mounting material used for the footings in a rollover protection system. |
| 02:24 | Testing material under this thickness can be challenging as the nature of the materials mean that they'll tend to bend and deform without breaking the weld or the heat affected zone. |
| 02:34 | For the purpose of comparison, we've also welded a section of 5 mm steel on the same 120 amp setting that we used for our other test piece to demonstrate what happens when we have a lack of penetration. |
| 02:46 | As you can see on the back side of the material there's absolutely no weld penetrating through. |
| 02:51 | This isn't necessarily a bad thing though as we don't have any shielding gas in this region and if it was to penetrate through, this may draw in contaminants from the atmosphere into our weld pool. |
| 03:03 | Before we perform the bend test, we'll cut off a section for another test we can perform called the macro etch test but more on that in a moment. |
| 03:11 | To perform the bend test, we need to apply a load to the unwelded side. |
| 03:15 | Typically this is done in a press or a vice and the load is increased until the weld fails. |
| 03:21 | The fillet weld bend or break test provides a good indication of failure points within the weld. |
| 03:26 | This type of weld inspection can detect problems such as a lack of penetration, internal porosity within the weld and overall poor weld strength. |
| 03:35 | Although the fillet weld bend or break test is often used on its own, it's best used in conjunction with the macro etch test as the two methods compliment each other by providing information on similar characteristics but with different detail. |
| 03:48 | This method requires cutting a sample from the welded join which is then polished at the cross section and etched using a mild acid mixture. |
| 03:57 | In this case we've used hydrochloric acid to etch the cross section and this then reveals a clear visual indication of the weld's internal structure. |
| 04:05 | When we look at the etched sample it shows us the profile of the weld and the depth of the penetration into what's called the root of the joint. |
| 04:13 | This is the intersecting point of our mating halves and the furthest our weld could possibly travel. |
| 04:19 | A full penetration weld will go all the way into the root of the joint and create the perfect weld depth. |
| 04:25 | Any small holes in the weld are known as porosity and may indicate you need to clean your material a little more thoroughly or check your gas flow rates. |
| 04:33 | We can also look at the weld uniformity to the two parts. |
| 04:37 | Ideally, we want the same amount of penetration into both halves of the material we're welding together. |
| 04:43 | In most cases, this has a lot to do with your torch position and angle so make sure that your torch angle is even between the two halves in order to maintain even heat input. |
| 04:53 | Destructive testing isn't something you need to do every time you break out the TIG welder, but particularly when you're getting started in dialling in your settings and techniques, it can be a powerful way to understand the quality of your completed weld. |
| 05:07 | This is also a great way of giving you the confidence to know that your weld is going to stand up to whatever punishment you can throw at it. |
