The newly developed Allvac 718Plus alloy has shown mechanical properties superior to alloy 718 and comparable to Waspaloy at the temperature up to 704 C. Previous results showed that fatigue crack propagation (FCP) resistance without holding time has no significant difference between three alloys with 718Plus being the best and 718 the lowest. During the holdtime FCP tests, 718Plus shows comparable results to those of Waspaloy and better than Alloy 718. In this paper, the effect of various thermal-mechanical treatments, including direct aging, pre-treatment, and long-term exposure, on the hold-time fatigue crack propagation behavior of 718Plus alloy were investigated and the results are summarized as follows:(1) The hold-time fatigue crack propagation rates (FCPRs) of DA sample are almost the same as that of the alloy after conventional solution plus age heat treatment, while the FCP of DA sample is slower than conventional 718Plus alloy under the 650 C, 3S loading condition; (2) the longterm exposure tests show that the alloy's hold-time fatigue cracking resistance is improved after exposed at 760 C for 350 hours; (3) the fine-grain alloy shows better hold-time FCP resistance than that of coarse-grain alloy, which is attributed to the delta-phase effect; and (4) it is indicated that after pre-treatment at 857 C for up to 24 hours, the alloy's fatigue cracking resistance is improved because of delta-phase at grain boundaries.

No abstract

Delamination is the critical damage type for integrated carbon fiber structures, which are employed in primary aircraft structures. In order to minimize the danger, one common solution is to install fasteners, clamping the laminate together and partially arresting the delamination. Current research has indicated that multiple fasteners installed in series do not fully arrest delaminations, instead merely slowing and redirecting the propagation of the crack, first by compressing the lamina together and second by transferring load via shear engagement of the fastener. Additional work has found that fastener spacing can be increased beyond the typical spacing of 5 fastener diameters; experimental and computational studies have indicated that the failure mode can be shifted away from delamination under both static and fatigue loading even with a significantly greater fastener spacings. NomenclatureD = Fastener Diameter = Crack growth per cycle = Total Strain Energy Relase Rate = Critical Total Strain Energy Release Rate = Mode I Strain Energy Release Rate = Mode II Strain Energy Release Rate = Critical Mode I Strain Energy Release Rate = Critical Mode II Strain Energy Release Rate

Delamination is the critical damage type for carbon fiber structures, which are increasingly being employed in primary aircraft structures. One common solution to prevent the delamination from propagating is to install fasteners, clamping the laminate together and partially arresting the delamination. Current research has indicated that multiple fasteners installed in series do not fully arrest delaminations, instead merely slowing the propagation of the crack, first by compressing the lamina together and second by transferring load via shear engagement of the fastener. It has also been found that fastener spacing can be significantly larger than the common spacing of 5 fastener diameters; experimental and computational studies have indicated that the failure mode is shifted away from delamination even with a fastener spacing of 2 inches, or 8 times the fastener diameter. NomenclatureD = Fastener Diameter = Total Strain Energy Relase Rate = Critical Total Strain Energy Release Rate = Mode I Strain Energy Release Rate = Mode II Strain Energy Release Rate = Critical Mode I Strain Energy Release Rate = Critical Mode II Strain Energy Release Rate