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M A Y 2 0 1 4 | 1 5 TESTING TECH TESTING TECH Dr. Donald F. Adams is the president of Wyo- ming Test Fixtures Inc. (Salt Lake City, Utah). He holds a BS and an MS in mechanical engineering and a Ph.D in theo- retical and applied mechanics. Following a total of 12 years with Northrop Aircraft Corp., the Aero- nutronic Div. of Ford Motor Co. and the RAND Corp., he joined the University of Wyoming, directing its Composite Mate- rials Research Group for 27 years before retiring from that post in 1999. Dr. Adams continues to write, teach and serve with numerous industry groups, including the test methods committees of ASTM and the Composite Materials Handbook 17. S REVERSED LOADING FLEXURAL FATIGUE tatic fexural testing has long been conducted because it is a simple method for evaluating all types of materials. Static fexural testing of composite materials, in particular, was discussed in my March and May 2013 columns (see "Learn More"). In all stat- ic fexural testing, the load is applied monotonically. That is, the goal is to bend the specimen in just one direction. The specimen, therefore, remains in contact with the support and loading points at all times. Monotonic fexural static loading can be readily extended to monotonic fexural fatigue loading by taking into account what are only minor additional considerations. For one, it is typically necessary to constrain the specimen axi- ally, to prevent it from "walking away" from under the supports with repeated cycling. Likewise, any lateral movement must be constrained. This must be done because the specimen is being subjected to loads that cycle between a selected maximum load and a load as low as zero. As a result, the outer surfaces of the specimen lengthen or shorten (depend- ing upon whether that surface is the ten- sion or compression surface). This, in turn, causes the specimen to slide on the supports during each cycle unless some provision is made to prevent it. This sliding may or may not affect the specimen integrity and/or the test results, depending upon the type of material being tested, the specifed load levels and the magnitude of the speci- men defections achieved, but in any case, it will cause wear of the loading and support surfaces of the test fxture. (My March 2013 column discusses types of supports available for use.) However, the problems encountered here are not too diffcult to overcome, and for that reason, monotonic fexural fatigue test- ing is occasionally conducted. In contrast, reversed loading fexural fatigue introduces a signifcant addi- tional problem. Here, the specimen must be both pushed and pulled at each loading point, so it must be in contact with loading points on both of its sur- faces. Additionally, the specimen also must be constrained on both surfaces at the support points to prevent the speci- men from lifting off of the support points during half of each cycle. The specimen can be constrained at both the loading and the support points by clamping the specimen, with clamp faces positioned on directly opposing surfaces of the specimen at each point. Only a light clamping force is required to keep the fxture surfaces in contact with the specimen. However, the increased frictional resistance developed by using a greater clamping force will help prevent the "walking" action discussed above. An additional consideration is that, by defnition, the loading for reversed-cycle fatigue testing passes through zero during each cycle. And because the specimen is under no load at that instant, it is free to move if not constrained. Although friction forces developed by the clamps at the loading and support points might be suffcient to restrain this movement, any movement per load cycle, even if very small, will accumulate because a fatigue specimen might be subjected to a million or more load cycles. Thus, addi- tional, more positive, constraint might be an option. For example, a pointed screw can be lightly pressed into each side of the specimen at each of the load- ing and support points, at the speci- Fig. 1 Typical reversed loading flexural fatigue test fixture. Source: Don Adams 0514HPC_TestingTech-OK.indd 15 4/22/2014 2:54:21 PM