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The Importance of Shear Testing in Process Design, Quality Control

February 7, 2012
KEYWORDS ndt / shear testing
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The choice of tooling, ease of processing and product quality are just a few of the factors driven by material shear strength.





Shear is defined as the force that causes two contiguous parts of the same body to slide relative to each other in a direction parallel to their plane of contact. Shear strength is the stress required to yield or fracture the material in the plane of material cross-section [1]. To the layman, this equates to “punching” or “chopping” a section or piece from a larger body of material and the strength required to accomplish this. Examples of shear methods to trim or remove material include punching shapes from sheet metal, cutting a piece of paper with scissors or even using a cookie cutter to shape cookie dough.

As the above examples suggest, there are many common applications that are affected by the shear strength of a material. The choice of tooling, ease of processing and product quality are just a few of the factors driven by material shear strength in these applications. Imagine, for example, trying to cut a disk out of sheet steel using a plastic cookie cutter or spending a lot of money to buy a diamond tip blade in order to cut paper. These are extreme cases of misunderstanding the requirements to shear the respective materials, but highlight the importance of knowing their shear properties. A more realistic example would be the careful selection of material for a shear pin such that the pin shears at a specific force or load in order to prevent a more catastrophic failure. A railroad crossing gate is one of the many exam-ples of such a design, requiring only a replacement shear pin after being impacted by an automobile rather than replacing the entire gate arm.

The importance of shear testing for a specific material depends on the type of material. Isotropic materials are those with consistent material properties in all directions. Most common metals are isotropic until they have been cold worked, at which point they become anisotropic. Anisotropic materials, such as cold worked metal, wood and composites, have varying material properties in specific directions. Testing for shear properties is especially important in anisotropic materials because of the complicated relationship between tensile stress and shear stress. The strength of a composite rod, for example, is much higher along the length of the rod than in a transverse direction. Similarly, the shear properties of a cold-rolled fastener, which may be subject to shear loads by design, can be significantly different than the shear properties of a hot-rolled fastener with the same tensile strength.



Pictured is the fracture surface of a 3/4-inch bolt that failed in shear overload.

The shear stresses in a component may be there by design or they may be caused by external forces. Shear pins, shaft keys and certain fasteners are designed to withstand shear forces. Similarly, torsional shafts and asymmetric cantilever beams must be designed to withstand shear stresses caused by torsional forces. Many manufacturing processes, such as punching and deep drawing, rely on materials failing as a result of shear forces. Whether you are designing material to withstand shear stresses or fail as a result of them, it is important to accurately know the shear strength of that material.

Shear characteristics are also important when characterizing the structural integrity of a bond between two surfaces. This bond could be a weld, an adhesive bond or a friction joint. In all of these cases, failure of the bond is primarily dependent on its shear strength, which can only be experimentally determined by a shear test.

Though the direction of force differs between tensile (axial) and shear stresses, there is a relationship between the two. Pulling in a single direction on a part or specimen will cause only an axial tensile stress in that direction. However, this same axial force causes the part to be stressed in shear in different directions. For isotropic materials, planes of maximum shear stress are oriented 45° from planes of maximum tensile or compressive stress. This is why components that fracture under torsional forces, which impart shear stresses on the component, commonly fracture on a plane or spiral oriented 45° from the torsional axis. The other implication of this relationship between tensile and shear stresses is that a component subjected only to axial tensile forces can still fail due to shear stresses in excess of the material shear strength.

There are a variety of factors that may affect the shear properties of a material. For metals, grain size, cold work, microstructure and impurities or contaminants will all affect shear strength. Shear strength of plastics will depend largely on the chemical makeup and percent crystallization. Composite materials will vary in shear properties depending on the relative volume of each constituent as well as their arrangement or structure. For all materials, the shear properties will vary depending on the tem-perature of the application.

There are several test methods for determining the shearing characteristics of a material. Some are more reliable or accurate than others. Shear testing is commonly performed by pulling on the part or joint arranged in a double shear configuration using a standard tensile test machine. This method works well for fasteners, friction joints and drive keys. Softer materials such as plastics or rubber can be easily tested using a punch die as detailed in ASTM D732. Testing the shear properties of adhesive bonds, composite laminates and wood structural panels is more difficult than the prior listed materials due to the small size and orientation of the bonding layer(s). Some of the methods developed to test the in-plane shear properties of these laminated materials are 3- and 4-point bend tests (ASTM D2344), notched shear specimen test (ASTM D3846), 45° tensile test (ASTM D3518), rail shear test (ASTM D4255) and Iosipescu double V-notched test [2]. Of these laminate shear tests, the Iosipescu double V-notched test is most highly recommended because it provides the most accurate and reliable results and even can be used to test brittle materials. The rail shear test also can provide accurate and reliable re-sults, but requires larger test specimens [3]. Though less accurate, the 3- or 4-point bend tests (ASTM D2344) can be very useful as a quality-control test because they are a good measure of quality of the laminate and there is an abundance of historical data for this type of test.



Pictured is the fracture of a keyed shaft under normal torsional stresses.

Shear testing of thicker materials or entire parts and assemblies can still be accomplished with custom test setups, but typically are performed similarly to one of the above listed tests.

Parts that are subject to cyclic shear stresses may be susceptible to fatigue cracking in a shear mode. Typical fracture properties of a material are generated by performing cyclic endurance tests in a mode of axial stress. The fatigue data obtained from these test methods are not necessarily consistent with what you would observe in a shear mode endurance test. Shear fatigue testing of materials can be accomplished using the methods for static shear testing, provided that the equipment and setup are capable of cyclically applying the required forces in the prescribed directions. ASTM C394 provides one such method for performing shear fatigue testing on sandwich core materials.

With current and evolving industry relying more heavily on composites and bonded materials, the need for reliable and accurate shear testing is at an all-time high. When parts or processes in a produc-tion are subjected to shear forces or stresses, quality control programs should include shear testing to ensure that processes are optimized and parts are safe. Fortunately, with the proper knowledge and test methods, shear testing can be performed accurately and reliably in a common laboratory setting. NDT





References

[1] Dictionary of Materials and Testing, William H. Cubberly, Society of Automotive Engineers, Inc., 1993.

[2] D. Walrath and D.F. Adams, “Iosipescu Shear Properties of Graphite Fabric/Epoxy Composite Laminates,” UWME-DR-501-103-1, University of Wyoming.

[3] Engineered Materials Handbook – Volume 1: Composites. ASM International, Metals Park, Ohio, 1987.

[4] "Standard Test Method for Shear Strength of Plastics by Punch Tool," D732, Annual Book of ASTM Standards, American Society of Testing and Materials.

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