Some simple tests can be done in the shop to provide support for day-to-day operations

December 2012 - The pursuit of form, fit and function of welded workpieces in modern engineering is accompanied by the desire to reduce weight while maintaining high strength. How do we meet those goals in manufacturing structural weldments and welded components? One common but expensive solution is using failure modes and effects analysis (FMEA). However, by keeping in mind the type of information necessary to attain the desired performance goals, we can inexpensively and effectively test materials  on the shop floor.

There are two basic types of applicable tests:

• Material testing consists of ascertaining the strength of materials and determining the effects of external influences upon them.

• Technological tests are performed to determine the characteristics of materials when being cut, reshaped and welded.

The major function of mechanical testing is to evaluate the resistance of the material to deformation and breaking. In terms of the nature of the applied force, one can choose between tension, compression, bending and shear.

In evaluating the stress resulting from each of these forces, a distinction is made between strength relating to a gradually applied force and the value for sudden stress.

Let’s look at a simple experiment. A steel rod is subjected to tensile stress that is gradually increased. The rod stretches and tears when the force becomes too large. If it is subject to sudden stress, it tears as if cut by a knife.

There are various forms of stress testing, as shown in the accompanying table. When testing materials on the shop floor, these tests do not show numerically quantified results but will give an idea of how the material behaves when processed.  

Additional tests

Forgeability: After heating, a steel flat is forged out just to the point when cracks begin to appear on the edges.

The flat should broaden to twice or three times its original dimensions before cracks appear.

Cold reshaping: A piece of flat mild steel 1⁄8-in. thick is bent in a bench vise until cracks appear. A steel that deforms well can be folded through a 180-degree arc. The alternate bending test uses a metal rod that has been clamped in a vise and bent back and forth several times through a 180-degree arc. High-strength steels are difficult to bend and can stand only a few flexures. The number of flexures necessary to break the piece is an indication of the strength, malleability and deformability of the material.

Deep drawing: This test uses a clamped plate that gradually increases the force on a pin, which applies a vertical force on to the surface of the plate. An indentation is observed until the first crack happens on the test piece.

These types of material testing, although they may be simplified, are very important in supporting quality control and continuous improvement in parts production. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.