This paper investigates the roles Mode I (opening mode) and Mode II (inplane shear) strain-energy release rates (GI and GII, respectively), play in inducing delamination growth under static and fatigue loading. Double cantilever beam (DCB) specimens were used for pure Mode I tests. Cracked lap shear (CLS) specimens were used for mixed-mode tests. All specimens were fabricated using T300/5208 graphite/epoxy. Delaminations were introduced during fabrication by inserting folded Kapton films between selected plies. Delaminations were located between two 0° plies in an attempt to inhibit delamination growth across plies into adjacent interfaces. The critical Mode I strain-energy release rate, GIc, was obtained from static DCB tests. Static tests on mixed-mode CLS specimens measured the total strain-energy release rate, which was broken into GI and GII components using a geometrically nonlinear finite-element analysis. By incorporating these values of GI and GII, and GIc from static DCB test results, into an assumed failure criterion, the critical Mode II strain energy release rate (GIIc) was computed. Fatigue-induced delamination growth was characterized by conducting constant-amplitude fatigue tests at a minimum to maximum cyclic load ratio (R) of 0.05 and a frequency (ω) of 10 Hz. During the tests, the maximum and minimum strain-energy release rates (Gmax, Gmin) and the delamination growth rate (da/dN) were monitored. Fatigue test results on DCB specimens yielded a power-law relationship between da/dN and GImax. Similar results from CLS tests provided da/dN versus Gmax relationships. The contributions of GI and GII components to mixed-mode delamination growth were assumed to be additive. Hence, the power law for a pure Mode II delamination growth was derived from CLS test results by subtracting the contribution due to GI determined from DCB tests.
Using the Lagrangian multiplier technique, an analysis was developed to compute the natural frequencies and mode shapes of clamped orthotropic laminated plates. Transverse shear deformation effects were included using a higher-order plate theory. Predictions made with the developed analysis demonstrated excellent agreement with the available experimental test results and other analytical solutions. The validated free vibration analysis has also been used to develop an analysis to predict the dynamic response of clamped orthotropic plates subjected to low-velocity impact by a hard object. Nomenclature a = length of the plate AJJ =inplane stiffness b = width of the plate Dij = bending stiffness g = acceleration due to gravity h = thickness of the plate y =laminate weight density w = natural frequency Subscripts 9 x' 99 y 99 t = derivatives of ( ) dients. 1 ' 2 Also, the laminate is assumed to be midplane symmetric and in-plane (membrane) displacements are assumed to be negligible compared to flexural displacements.The total strain energy V in the laminate is expressed as Area
This paper discusses an experimental program that addressed the effect of low-velocity impact damage on the fatigue behavior of two laminates, fabricated from AS/3501-6 graphite/epoxy unidirectional tapes, with lay-ups representative of high-performance fighter aircraft wing skin lay-ups. Prior to testing, specimens from these laminates were subjected to one of two types of low-velocity impact damage. The first type was created using a blunt-tipped (1.6-cm tip diameter) impactor, and the impact energy was chosen to induce internal damage, approximately 5 cm in diameter, with no visible sign of damage on the outer surfaces. The second type of damage was created using a sharp (tetrahedral-tipped) impactor, with visible signs of damage on the impacted and opposite surfaces. Subsequent to introducing low-velocity impact damage, test specimens were subjected to static and constant amplitude fatigue loading. In the presence of compressive loads, lateral knife-edge supports were placed near the outer edge, for specimen stability, permitting unconstrained delamination growth elsewhere. Static tension and compression tests were conducted initially, and significant strength reductions due to the impact damages were measured. Fatigue load amplitudes were chosen to be fractions, S, of the static strengths. The behavior of impact-damaged specimens was investigated under tension-tension (R ≈ 0), tension-compression (R = −1), and compression-compression (R → −∞) fatigue loads. During fatigue, the growth in the impact damage was monitored by means of ultrasonic pulse-echo records. Fatigue life data for the various laminate-damage-loading combinations were obtained in the form of S-N curves. Interesting damage growth patterns were observed, and the criticality of each type of impact damage on the fatigue behavior of the tested laminates was assessed.
The present work experimentally investigates suppression of the sound level from an underexpanded jet of Mach number 2.8 by water injection. The jet is produced by a solid rocket motor being static test fired. Water is injected from a radial distance of 5.2 jet diameters, at different axial locations from the exit of the nozzle, at two different angles of injection relative to the downstream jet axis. The ratio of mass flow rates of water to the nozzle exhaust gas (referred to as the mass flow rate ratio) and the injection pressure are varied independently. Acoustic measurements are performed at a radius of 30 jet diameters, over angles in the range of 30-130 deg, relative to the downstream jet axis. Sound levels continuously decrease by 10 dB with the increase in the angle of observation. With water injection, higher levels of reduction in sound are observed in the upstream quadrant. Injection closer to the nozzle exit leads to better reduction, mainly due to suppression in the high-frequency range when observed from downstream, but it is almost in the entire frequency range as observed at the upstream locations. At intermediate mass flow rate ratios, an optimum injection pressure exists for maximum noise suppression, due to the penetration of water to the potential core and its evaporation there at high injection pressures. The results affirm that the validity of many past studies obtained on water injection to suppress noise levels on simulated jets can be extended to an actual rocket situation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.