Test to the Application, Not the Specification

When it comes to the correct testing for welded parts, the answer lies not in the specification but in the application. Too much testing can be costly and unnecessary. Likewise, too little testing, or testing only to the standard, can leave you with parts that don’t perform in the application. Keep the requirements of the application in the forefront, and the test will follow.

The production of precision welded parts and assemblies demands a significant investment of time and effort, with much of it centered around quality control and meeting the particular requirements of the application. Design engineers spec and scope every step of production toward meeting those requirements, including exact welding parameters. The more demanding the application, the more demanding the specifications.

In aerospace, biomedical engineering, and other precision industries, application requirements can, and should be, stringent, and no less so for welding requirements. When every part has to function flawlessly, then every weld needs to be laid perfectly and precisely.

Weld certifications are, of course, a gold standard, but not necessarily a one-size-fits-all solution to controlling the quality of welds. As an example, one of the most common weld standards is AMS 2681, which defines the procedures and requirements for joining metals and alloys using the electron-beam welding process. But for some requirements in aerospace, AMS 2681 doesn’t go far enough, and parts must be welded to the even tougher AMS 2680. But for another application, perhaps in a different field, welding to AMS 2681 is overkill and ultimately wasteful.

When does a standard go too far? When is a standard not enough?

Weld certification and standards are, by nature, broad and all-inclusive, and aren’t tailored to specific applications. In many cases, standards were developed long before new technologies or applications were even created. Is a standard welding for cold war era aircraft an appropriate gauge for the performance of your part?

For many modern applications, the solution is to test to the application, and not simply the standard.

An Inspector At Work
An Inspector At Work

Testing to Set the Standard

Testing can offer a clear understanding of the structural viability of a part’s design, and how it will hold up to the rigors of its application. More than a broad, general spec, intelligent testing it can help determine if the design and production will meet the requirements. But how much testing is too much, and how much is too little? How does a designer, and a parts manufacturer, find the “sweet spot,” where the design parameter is met and exceeded, but the testing process doesn’t add excessive or unneeded cost and time to the process?

The key is to have a clear understanding of the test/inspection process in order to choose the right combination to meet the needs of the design, without belaboring the process.

To begin to establish this balance, first one must understand the different testing methods, their advantages and disadvantages, and how to use that knowledge to develop a provable, traceable way to test to your application requirements.

While the methods of weld testing are as varied as the parts themselves, generally they fall into two broad, main categories – Nondestructive and Destructive.

Nondestructive Testing

Nondestructive testing (NDT) involves inspecting, testing, and evaluating materials and assemblies for flaws without destroying the part. One of the most significant advantages of this method is that the part can still be used or sold after the inspection is completed. One of the most common methods used is visual inspection.

Visual weld inspection is the process in which inspectors look at welds either with the naked eye or, more commonly, under a microscope. At EB Industries, our standard is to inspect parts under 20x magnification to identify surface defects such as cracks, pinholes, underfill and other conditions. This is only performed by technicians who have been certified to our internal procedure, including completing a test using sample hardware visually inspected by a third party NADCAP approved metallurgical lab. The main limitation to this process is that you’re only limited to surface defects and it’s virtually impossible to know what occurred beneath the surface. For a look at the weld below the surface, Radiography or Computed Tomography (CT), is often used in NDT.

Radiography can reveal the presence and nature of internal defects in a weld, such as cracks, holes, slag, and incomplete fusion zones. A CT scan produces a 3D volumetric density map. The 3D volume is generated by the reconstruction of a high number of 2D x-ray images. Many 2D projection images can be combined using intuitive software to produce a 3D volume of practically any part, object, or product.

Although visual inspection is the most used Nondestructive method, there are other ND methods, including:

  • Dye Penetrant Inspection
  • Mass Spectrometer Helium Leak Testing

Disadvantages of Nondestructive Testing include the need for specialized equipment and the properly trained inspection personnel. Allowances must also be made for the make-up of different metals being scanned, how they are read in the inspection process, and the extra time needed to perform these inspections. There are also certain assembly geometries that can make it difficult to perform some ND inspections.

Engineering staff examine welds.
Engineering staff examine welds.

Destructive Testing

Destructive Testing (DT) involves the physical destruction of the completed weldment to evaluate its characteristics and the overall quality of the process being used. Common Destructive tests include

  • Metallographic Cross Section and Weld Analysis
  • Bend Testing
  • Burst Pressure Testing

The most common destructive test method that EB Industries performs is metallographic cross section and weld analysis. This process begins with a visual inspection, usually via a microscope, to identify any outward issues. Next, a location on the part is selected, typically near the middle of the weld, where the part may be be cut in order to measure and inspect, among other things, the depth of penetration, the presence of any inclusions or flaws, and the overall quality of the weld. The part and associated test results are then sent to the customer for approval.
But this method does have some potentially serious drawbacks. The inspector only sees a 2-dimensional plane of the weld. Flaws, voids, cracks, or other defects could be a few millimeters distant from the cut location and go unnoticed.
That’s where a 360° sample can provide a much more complete picture. The part is machined to the centerline of the weld to provide a better glimpse of what’s happening in the deepest part of the penetration. Artifacts that may have been hidden in 2D are brought to light by this type of sample. It’s important to note, though, that not all welding vendors and inspectors offer this service. So, for parts where the finished application requires this extra step, look for vendors who are familiar with this process.

electron beam butt weld
Cross section of a weld – destructive testing in action.

Reliability vs. Liability

Choosing the right vendor for your welding production and testing is as important as the test itself. That’s because it could mean the difference between reliability and liability. It’s best to work with a welding expert who is knowledgeable about the process and the various applications involved, and dedicated enough to form a partnership with you. The right welding provider can not only meet your specifications during manufacture, but bring their own experience to bear in setting up the right testing methods to prove all those requirements.

Your weld vendor should also be up-to-date on new and developing technologies. As an example, it is now possible to monitor electron beam welding in real-time, with nine parameters automatically recorded and verified, ensuring there’s no arcing, that the beam isn’t out of focus , and that the piece is being welded in precisely the right spot, among other requirements. Not all vendors are investing in this technology — many use manual or semi-automated procedures for applications that are really beyond the scope of a human operator. When the part and application require it, it’s crucial to find one who does.

Methodology Matters

A suitable testing method is critical to the success of the end product. Merely testing to the certification may get the part made, but if you’re not also testing to the specific demands and requirements of the application the part will be integrated into, there may be consequences down the road. Developing a testing methodology that keeps the application in mind will ensure long-term performance while not negatively affecting production time and budget. Meeting the specifications of a certification should take a back seat to ensuring the part performs, and that you can prove that performance through the right testing protocol.

In future posts, we will review different testing methods, and make clear their advantages and disadvantages. Links to those stories will appear here as they are published. If you’d like more information now, please feel free to contact us.