It may seem like a very simple test to perform, but compression testing is actually one of the least understood, and often yields error-filled results. It seems straight-forward to perform a compression test between two flat platens, but testing from brittle specimens to soft rubber, each type of specimen presents its own unique challenge.
The first fundamental challenge is alignment. The platens may not be parallel with the specimen surfaces, consequently making the initial contact a point contact on the edge of the specimen. This creates a stress concentration that can initiate failure, or induce a bending moment into the specimen and cause premature failure. This challenge is most severe with brittle materials and is less critical for softer, ductile materials.
Our recommended solution is to use platens with spherical seating. The intent is for the platen to realign itself onto the surface of the specimen so that it will provide an even contact. However, how the platen re-aligns is critical. To do so correctly, the center of rotation of the platen must be on its surface in contact with the specimen. When the center of rotations are above or below the surface, the act of realignment will induce lateral movements that will either limit the ability of the anvil to properly align or apply an undesirable side load on the specimen. We generally recommend that only one platen with spherical seating is used.
The importance of this is reflected in various compression testing standards, which explicitly describe the above requirement. For example, ASTM E9 states: “the spherical surface of the block shall be defined by a radius having its point of origin in the flat surface that bears on the specimen."
The above notwithstanding, the use of spherical seating is not without its caveat. In effect, it changes the end conditions of a column in compression. The difference is usually minor except where the specimen is long (a high length to diameter ratio) or positioned offset from the center axis of the platen. To address this, it is usually preferable that the platen is locked to prevent rotation after it is initially seated on the specimen. Locking should be “finger tight” only to prevent user induced misalignment, but rigid to ensure significant platen rotation is not possible.
The second fundamental challenge is friction. Under compression, the test specimen will want to expand laterally elastically (Poisson’s effect) or plastically (maintaining constant volume). However, the platens are usually rigid and do not expand - being in contact with the specimen will, through friction, constrain the specimen from expanding. The net result is the specimen becomes barrel shaped, and transverse stresses are introduced that increase the apparent axial stiffness of the specimen. All this contributes to measurement error. The severity of this issue is the inverse to alignment, being more significant in ductile materials, although it cannot be strictly ignored even for brittle materials.
To minimize friction, it is important that the compression platens be hard (at least 55HRC) and smooth; they should not have concentric rings engraved into the surface as that will exacerbate friction. And often, it is good practice to oil or grease the surface.
Large platens with a smooth surface create a new challenge - specimen centering. Any offset in the specimen’s position relative to the central axis of the load string will result in bending moment being induced. If the load string – the platens, spherical seating, load cell and test frame – is laterally stiff, the induced moments will be restrained and less severe. Since brittle specimens are inherently stiff, the induced bending moment may not be ignored, and careful specimen centering is important. The most common solution to this is to provide concentric rings on the platen for visual positioning. Recognizing still the need for a smooth finish, micro-textured surface marking is the only way to solve this and yet be indelible.