Bend Testing Fixtures
Overview:
Flexural properties are analyzed by testing a material using a 3 or 4 point bend fixture. These jigs have 2 bottom support spans and one or two loading nose points on top. Most three point flex tests such as ASTM D790 require that the third loading point be centered on the sample. 4 point tests such as C1609 require that the 2 top loading points bisect the sample into thirds or fourths. Other tests include the guided bend test and the more obscure serpentine fixture for tensile bend testing.
3 Point Bend Fixtures:
Three point bend testing fixtures use center loading to flex the sample. Common 3 point bend tests include ASTM D790 for plastics, and ASTM D143 for wood. The 3rd point loading nose is attached directly to the top arm of the universal testing machine by use of a clevis pin adapter. The bottom two supports are adjustable and can accomodate samples of different length. The concentric gear crank custom bend fixture is a nice solution if you are changing between multiple specimen sizes.
Four point flexural testing fixtures have two base supports and two top loading points. There are two types of 4 point bend tests based on where the load contact points are on the sample. The first type divides the sample into three parts and the loading points are placed at 1/3(L) apart. The second test splits the sample into four parts and the two top points are at 1/4(L) and 3/4(L).
Support Spans and Loading Noses:
The radius of the contact points is important and this measurement is usually specified in the relevant ASTM test overview. The standard radius tends to be 5mm. The bottom spans on most of these jigs are reversible, so they actually have two usable radii. Universal Grip also offers self-adjusting roller spans which will automatically react to the normal force during the test.
Adjustable and Interchangeable Rollers:
Interchangeable Rollers allow the bend fixture to have much more versatility in terms of the contact radius. Small rubber O-rings are used to secure the roller into place. The rollers can also be specified to allow for extra wide samples or regular sizes.
Fully Articulated Bend Fixtures:
Articulating contact points are used for four point loading to ensure that each contact point receives an equal amount of force. This is critical for measuring Modulus of Rupture (MOR) and fracture toughness. These types of measurements are most commonly used when testing extremely brittle materials such as ceramics, glass, and composite materials. Common flexure tests for ceramics include ASTM C1161 and the most widely used test for glass bending is ASTM C158. Spring loaded ball bearings allow the rollers to adjust evenly as the load is applied. The picture below shows the 25mm rubber pad specified in ASTM C393.
