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by John B. Student
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The Iosipescu shear test fixture was actively developed for use with composite materials during the late 1970's and early 1980's by the Composite Materials Research Group at the University of Wyoming. A large number of fixtures have been provided to research groups around the world since that time and the test method is now in widespread use. ASTM Standard D 5379 governing the test method was first issued in 1993 (Reference 1). General testing procedures are provided in that standard. Detailed user instructions are provided with each fixture supplied by Wyoming Test Fixtures, Inc. In addition, considerable additional discussion of results typically obtained is contained in the References listed here.
The standard test specimen (as shown installed in the front view photograph) is 3" long, 0.75" wide, and of any thickness up to 0.50". Typically, a specimen on the order of 0.1" thick is used. The fixture will also accommodate shorter and narrower specimens, if desired. A 90° notch is machined into each edge of the specimen, and a strain gage can be mounted on the specimen surface in the notched region to monitor shear strain. Either a standard ±45° biaxial gage or a special Iosipescu gage is commonly used. The small diameter rod projecting out of the fixture below the lower notch in the specimen (front view photograph) is lifted up with a finger while installing a test specimen, to center the specimen left to right. The specimen is pushed up against the flat back of the fixture to locate it fore and aft. The tapered wedges are not intended to be clamps; they are adjusted with finger knobs to accommodate specimens of slightly varying widths. Wedges of alternate thickness can be provided to accommodate specimens outside this small range of thickness variation.
The moving half of the fixture incorporates a linear ball bearing (see rear view photograph) that moves on a hardened steel vertical post mounted in the base of the fixture. This ball bearing eliminates guidepost binding and minimizes frictional resistance. The tightness of fit of the bearing can be adjusted via the nut visible on the backside of the moving half of the fixture.
In use, the fixture rests unconstrained directly on the base of the testing machine. An adapter of the required configuration, such as the two-piece standard Instron adapter shown here, can be provided. This particular adapter pins together and is secured via a lock ring. Whatever adapter is used, it threads into the top of the moving half of the fixture itself, as shown. The upper end of the adapter attaches to the crosshead of the testing machine, being configured to the type of machine the fixture is to be used with, as specified by the purchaser. Special adapters can be provided at nominal additional cost.
1.1 Background
An assembled short beam shear test fixture is shown in the above photograph, but without a test specimen mounted in it. This fixture is commonly used to test composite materials in interlaminar (through-the-thickness) shear, in general accordance with ASTM Standard D 2344.
ASTM Standard D 2344, in existence since 1965, now recommends a specimen span length-to-thickness ratio of four. A typical specimen on the order of 0.1" thick would thus be tested at a span length of 0.4".
However, to accommodate various types of materials, and various laminate thicknesses, the span length of the fixture is infinitely adjustable over the full range of span lengths up to approximately 4.5". Scale markings are engraved on the base to aid in setting the desired span length, by sliding the specimen supports to the required positions. These specimen supports are then clamped in place using the Allen head screws shown.
Two hardened steel alignment posts, each 0.5" in diameter, are fixed in the base. The top plate, containing linear bearings, moves freely up and down on these alignment posts, thus maintaining alignment between the two lower supports and the upper loading cylinder [[1]].
1.2 Fixtures
The two supports and the loading head are hardened steel cylinders, each 2.5" wide, held in vee-grooves by small springs on each end. This permits them to be rotated, and also to be easily replaced if desired. Loading heads and support blocks incorporating cylinders of any diameter can be supplied. Unless specified otherwise, 1/8" diameter supports and a 1/4" diameter loading head will be supplied, as specified in ASTM Standard D2344. However, supports and loading heads of any other diameters can be provided, if desired. Adjustable mechanical stops for centering the specimen in the fixture can also be added as an option.
1.2.1 End Loaded
This fixture can also be supplied in a four-point loading configuration, or combined three- and four-point loading, although only the three-point loading configuration is included in the ASTM standard.
This fixture is also frequently used as a flexure fixture for testing small specimens per ASTM D 790, and to test ceramics.
The fixture is designed to rest unconstrained directly on the base of the testing machine. A compressive loading is typically applied by the loading head of the testing machine directly onto the top plate of the short beam shear test fixture. Alternatively, a 0.50"-13 UNC threaded hole is provided in the top plate, to mount an optional adapter which in turn can be attached to the crosshead of the testing machine.
(1.1)
1.2.2 Shear Loaded
The IITRI Compression Test Fixture, introduced by the Illinois Institute of Technology Research Institute (IITRI) in 1977 (see Reference 1), became part of ASTM Standard D 3410 in 1987 (see Reference 2). The IITRI test fixture configuration incorporates flat wedge grips, as shown, which make full surface contact with the mating blocks, independent of the tabbed specimen thickness.
FIGURE 1.1
IITRI compression text fixture.(Courtesy of Wyoming Test Fixtures.)
Alignment rods in linear bearings are used, rather than an external alignment sleeve as in the Celanese fixture. This minimizes frictional binding between the fixture halves.
The Wyoming version includes an upper bracket attached via an adapter to the crosshead of the testing machine. This permits easy slip-in installation and removal of the heavy upper block. The lower block normally rests freely on the base of the testing machine, although threaded attachment holes are also provided [[2]].
The Wyoming version of the IITRI Compression Test Fixture is designed to accommodate tabbed specimens up to 5.5" long, and of any width up to 1.5", as specified by the ASTM standard. For example, the fixture can accommodate a Celanese specimen, which is also 5.5" long, but only 0.25" wide.
The Wyoming IITRI fixture is supplied with three sets of wedge-shaped spacers, which can be interchangeably inserted in the cavities of the upper and lower blocks, to accommodate tabbed specimens of a wide range of thicknesses. The approximate thickness ranges are (assuming the wedges are flush with the blocks when at the lower end of each range, and protruding a maximum of 0.50" at the upper end of each range): Low Range: 0.17"-0.35"; Medium Range: 0.29"-0.47"; High Range: 0.42"-0.60". That is, each set of spacers provides a tabbed specimen thickness range of about 0.18", with an approximately 0.05" overlap between ranges.
The standard gage length (distance between specimen tabs) per ASTM Standard D 3410 is 0.50". The fixture can be used to test nonstandard gage length specimens also, from zero to approximately 2". (With longer alignment rods, specimens of longer gage lengths can be used.)
The Wyoming IITRI fixture includes three sets of end-loading bars, and an installation jig with centering bars for four different specimen widths.
All accessory parts for each fixture are supplied in a foam-lined, wooden storage case, fitted with brass hinges, latch, and side handles. This provides an attractive and protective case for the various accessories when not in use.
References
[1].ASTM Standard D 2584-94, Test Method for Ignition Loss of Cured Reinforced Resins, American Society for Testing and Materials, West Conshohocken, PA, 2001.
[2].R.B. Pipes, J.R. Vinson, and T.W. Chou, On the hygrothermal response of laminated composite systems, J. Compos. Mater., 10, 129-148, 1976.