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1 6 | H I G H - P E R F O R M A N C E C O M P O S I T E S TESTING TECH As the tensile force is applied, a force vs. loading-point displacement curve is obtained. This displacement can be measured between the two loading pins, using an extensometer, clip gage or Lin- ear Variable Differential Transformer (LVDT). However, simply using the test- ing machine crosshead displacement is usually acceptable. In any case, testing machine compliance and local deforma- tion of the specimen at the loading pins must be accounted for, as necessary. If the force vs. displacement plot ob- tained is an approximately straight line, with an abrupt drop of force to zero at the instant of crack growth initiation, in- dicating a material response reasonably close to linear elastic material behavior, a trial value of the critical stress intensity factor, K 1c , termed K Q , can be calculated based on the maximum force attained, P Q . The value of K Q is calculated as a func- tion of W , B , and the maximum force P Q (the formulas are given in ASTM D 5045). If the specimen was designed according to the previously discussed guidelines for selecting W , B , and a, this calculated value of K Q is almost certain to be valid, that is, K 1C has been obtained. The check for validity is that B , a, and ( W – a) must all be greater than 2.5( K Q /σ y ) 2 , where σ y is the yield stress of the material, which is defned in the standard. Of course, if the test proves invalid, then one or more of the values ( W , B , a) must be modifed and the test repeated. If the force-displacement plot is not linear, the specimen initial compliance (the reciprocal of the initial slope of the plot) is determined from the best- ft straight line for the initial portion of the curve. Then a second straight line is drawn, with a compliance 5 percent greater than the frst line. The intersec- tion of this line with the force vs. dis- placement curve is used to determine P Q . The remainder of the process to obtain K IC is the same as given above for the lin- ear material response. The strain energy release rate, G IC , can be calculated, having obtained the value of K IC , using G IC = (1-μ 2 ) K IC 2 / E where μ is the Poisson's ratio and E the elastic modulus of the material being tested. However, it is usually more reli- able to determine G IC directly, by inte- grating the area under the experimentally determined force vs. displacement curve. As this summary indicates, the proce- dure for determining fracture toughness and strain energy release rate using the compact tension test is relatively straightforward, even though the basic concepts of fracture mechanics some- times seem abstract. ASTM D 5045 has a well-written tutorial on the process. Fig. 2: A pair of compact tension tensile clevises and pins. Source: Don Adams High Density Urethane Tooling Board and Core Material (800) 845-0745 • www.precisionboard.com • Closed cell structure • No out-gassing • 15 standard densities • Exceeds aviation flammability standards Make it Precision Board Plus 0714HPC TestingTech-OK.indd 16 6/17/2014 10:18:16 AM