Practical MIG Welding: Destructive Testing

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Destructive Testing

04.18

00:00	There really isn't much room to hide when it comes to the quality of a completed MIG weld and once we're familiar with the process and the completed weld, we can do a reasonable job of understanding how good or bad the weld is and hence how strong the weld is going to be, just by looking at it.
00:14	On this basis, for the majority of motorsport applications, it's very rare for any inspection to be more thorough than a visual examination.
00:21	Taking a look at your sanctioning body's rulebook should give you a few hints on what they expect from your welds.
00:26	In Motorsport New Zealand's case, most of these guidelines relate to safety structures such as rollover protection.
00:32	These guidelines discuss a visual inspection of the weld joint and stipulate that there must be sufficient penetration.
00:39	Now, this is a little contradictory as we can't visually inspect the weld penetration without performing what's known as destructive testing.
00:45	The term penetration refers to the depth of the weld and the inclusion into the parent metal.
00:51	Full penetration refers to the weld being the same depth as the material being welded.
00:54	This means it will be present on the backside of the material and easily visible.
00:59	In theory, a MIG weld that has full penetration, correct filler wire and sufficient shielding gas will be stronger than the heat affected material each side of the weld.
01:08	To test this theory, we need to perform some destructive tests.
01:11	This is something that we won't need to do a lot of in motorsport but it can assist in understanding the strength of our welds and what lies beneath their appearance, particularly when you're relatively new to MIG welding and building up your understanding of what good or bad weld looks like.
01:27	There are a few methods of testing our welds that we can easily do and all of them involve completing a test part that will reflect the material, settings and technique that we plan on using for our actual project.
01:38	The first test we can perform is called the bend test which uses a 90 degree fillet weld performed on one side of the material and then it's bent to the breaking point to expose the weld and its failure point.
01:48	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.
01:56	To perform the bend test we need to apply a load to the unwelded side.
02:00	Typically this is done in a press or a vice and the load is increased until the weld fails.
02:05	The fillet weld bend test provides a good indication of failure points within the weld.
02:10	This type of inspection can detect such items as a lack of fusion, internal porosity and overall strength of the weld.
02:16	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 complement each other by providing information on similar characteristics but with different detail.
02:30	The macro etch test requires cutting a sample from the welded joint.
02:33	The sample is then polished at the cross section and etched using a mild acid mixture.
02:38	In this case we've used hydrochloric acid to etch the cross section and reveal a clear visual of the weld's internal structure.
02:44	When we look at the etched sample, it reveals the profile of the weld and the depth of the penetration into what's called the root of the joint.
02:52	This is the intersecting point of our mating halves and the furthest our weld could possibly travel.
02:58	A full penetration weld will go all the way into the root of the joint and create the perfect weld depth.
03:04	As we've already learned, any small holes in the weld are porosity and a more thorough clean of our material or a recheck of our gas flow rates may be required.
03:12	We can also look at the weld uniformity of the two parts.
03:14	Ideally we need the same amount of penetration into both halves of the material we're welding together.
03:20	In most cases this has a lot to do with our torch position and angle so we need to make sure that the angle is even between the two halves to maintain even heat input.
03:29	Both the macro etch test and the bend and break test are excellent methods that we can perform with very simple tools.
03:35	If you're curious as to what lies beneath your welds, or maybe you're a little concerned about the strength of them, then spend some time running some test welds and completing these destructive tests on them.
03:44	Let's cover off the main points found in this module.
03:47	Destructive tests are crucial for understanding our weld strength.
03:50	One method is the bend test involving a 90 degree fillet weld on one side of the material which is then bent to failure.
03:56	Another method is the macro etch test where a sample from the welded joint is cut, polished and then etched with an acid mixture to display the weld's internal structure.
04:05	A full penetration weld reaches the joint's root while holes indicate porosity, often requiring more material cleanliness or an adjustment to our gas flow rate.