CompositesWorld
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10 CompositesWorld DESIGN & TESTING JULY 2015 to the tabbed specimen, and the assembly is loaded between fat platens. Te amount of shear loading is controlled by the torque applied to the bolts that connect the fxture blocks, with a goal of providing sufcient shear loading to avoid crushing at the specimen ends. Te specimen dimensions are the same as those specifed in ASTM D 3410, although specimen widths up to 30 mm can be accommodated. Advantages of the CLC test method include the relatively small, simple and inexpensive test fxture and the potential to obtain compressive strength test results with less data scatter — a beneft of combined shear and end loading. Regardless of which test method is selected, the most impor- tant consideration is proper specimen design. As in tensile testing, relatively thin specimens are of interest, and bonded tabs are required. Unlike tensile testing, however, the use of a shallow tab taper angle leading into the gage section isn't possible due to the risk of buckling. In general, a thicker specimen and shorter test section are needed to prevent specimen buckling prior to compressive failure. Typically, the shortest practical test section length is selected, and the required minimum specimen thickness to prevent buckling is determined. Te two popular ASTM test methods for compression testing of composites, ASTM D 3410 and D 6641, provide an equation for calculating the required specimen thickness, h, to prevent bucking of an orthotropic specimen of rectangular cross section, which can be written as where l g is the specimen gage length, E x is the axial modulus of elasticity in the loading direction, G xz is the interlaminar shear modulus, and σ cu is the estimated compressive strength of the specimen. Without having performed tests, values for these three material properties may not be known and, thus, will need to be estimated. Although it might be tempting to neglect the last term in the denominator, 1.2/G xz , especially when a value of G xz is not readily available, doing so will result in a signifcant underpre- diction of the required specimen thickness and the likelihood of specimen buckling. Since this formula estimates the minimum FIG. 3 Buckling, detected using back-to-back strain gages (left) and out-of-plane bending (right) detected using back-to-back strain gages. Strain Strain Stress Stress H i g h T e m p e r a t u r e B o n d T o o l s • BMI • Epoxy M i l l F i x t u r e s P r e s s u r e I n t e n s i f i e r s / C a u l s B a c k u p S t r u c t u r e M a t e r i a l s • Panels, Tubes, Angles B a c k u p S t r u c t u r e K i t s C O M P O S I T E T O O L I N G 6262 W. 34th Street South ● Wichita, KS 67215 Phone: 316-946-5900 ● Email: Sales@BurnhamCS.com There's Still Time To Save Big On Advanced Composite Training www.abaris.com +1 (775) 827-6568 Gain the Abaris Advantage Save Up To $800. On 3 Courses DIRECT SERVICES: • ENGINEERING • ONSITE TRAINING • CONSULTATION TRAINING IN: • ENGINEERING • MANUFACTURING • REPAIR