Abstract:Microcracking in composite honeycomb sandwich structure and its effect on mechanical properties are studied in this paper. A methodology is presented to study the extent of mechanical strength degradation of composite sandwich structure, subjected to thermal fatigue. The material under study is used for spacecraft structural applications. The test coupons were exposed to thermal cycling at elevated temperature as high as +150°C inside the oven and cryogenic temperature of −190°C by dipping in liquid nitrogen, … Show more
“…Designing the proposed structure in this research, which is the result of extensive studies in the field of load-bearing and flexibility of various structures [41,42], begins with applying some changes to the conventional sandwich structure. The basic structure consists of a honeycomb core with glass composite faces known in the composite industry as a strong, rigid, and light structure.…”
Section: Features Of the Proposed Load-bearing Sandwich Structurementioning
In this paper, a new flexible sandwich structure is introduced, which can be employed in morphing aircrafts capable of intelligently changing their shape in different flight conditions. To accomplish this goal, first, a review of the various ideas in the literature is presented. In the following, features of the proposed structure and its differences from other ideas are expressed. Then, the process of fabrication and the various stages of shaping the structure are described. In an aircraft with variable wings camber, the deformable section can be assumed to be a cantilever beam. Thus, samples of the proposed structure are fabricated as the cantilever beam and are tested as tip-loaded beams. Since the numerical analysis of the new structure involves the recognition of the mechanical behavior of its components, a comprehensive review of the mechanical behavior of each component of the structure is performed. Afterwards, the numerical method is utilized to model samples of the structure, and the changes in the samples’ deformation are examined under different loads. According to the observation of the broken samples, to arrive at more accurate numerical results, a distribution for the cavities, caused by the manufacturing process, is considered. Finally, with the same assumptions, another sample is analyzed, and it is shown that the results of the second model are consistent with experimental results.
“…Designing the proposed structure in this research, which is the result of extensive studies in the field of load-bearing and flexibility of various structures [41,42], begins with applying some changes to the conventional sandwich structure. The basic structure consists of a honeycomb core with glass composite faces known in the composite industry as a strong, rigid, and light structure.…”
Section: Features Of the Proposed Load-bearing Sandwich Structurementioning
In this paper, a new flexible sandwich structure is introduced, which can be employed in morphing aircrafts capable of intelligently changing their shape in different flight conditions. To accomplish this goal, first, a review of the various ideas in the literature is presented. In the following, features of the proposed structure and its differences from other ideas are expressed. Then, the process of fabrication and the various stages of shaping the structure are described. In an aircraft with variable wings camber, the deformable section can be assumed to be a cantilever beam. Thus, samples of the proposed structure are fabricated as the cantilever beam and are tested as tip-loaded beams. Since the numerical analysis of the new structure involves the recognition of the mechanical behavior of its components, a comprehensive review of the mechanical behavior of each component of the structure is performed. Afterwards, the numerical method is utilized to model samples of the structure, and the changes in the samples’ deformation are examined under different loads. According to the observation of the broken samples, to arrive at more accurate numerical results, a distribution for the cavities, caused by the manufacturing process, is considered. Finally, with the same assumptions, another sample is analyzed, and it is shown that the results of the second model are consistent with experimental results.
“…It is interesting to notice that a temperature of −170℃ is reached in 8 min compared to a couple of seconds in case of cold conditioning by means of direct dipping in LN 2 . 19 Figure 5.Temperature history during thermal cycling.…”
Section: Test Planmentioning
confidence: 99%
“…The data for contact cooling condition have been reported in Hegde and Hojjati. 19 It seems that there is a linear relationship between the crack area and strength with the number of thermal cycles up to saturation point. Similar method to study the effect of microcrack on mechanical property was conducted by subjecting the sample to accelerated cooling condition.…”
Effect of thermally induced microcracks on mechanical performance of a space grade laminated sandwich panel is investigated. A simple non-contact setup using liquid nitrogen is developed to subject the material to low temperature of −170℃ with cooling rate of 24℃/min. Then the samples are exposed to the elevated temperature of 150℃ inside oven. Microcracks formation and propagation are monitored through microscopic observation of cross-section during the cycling. Flatwise tensile test is performed after a number of cycles. A correlation is made between number of cycles and flatwise mechanical strength after quantifying the microcracks. It is observed that the crack formation gets saturated at about 40 cycles, avoiding the need to conduct more thermal cycles. Microcrack formation both at the free edge and middle of laminate was observed. The crack density at the middle was comparatively less than the ones found on the free edges. Results for non-contact cooling are compared with samples from direct nitrogen contact cooling. Microscopic inspection and flatwise test show differences between contact and non-contact cooled samples. Flatwise tensile strength for non-contact cooled samples shows 15% reduction, while the contact cooled samples have about 30% decrease in bond strength. A 3D finite element analysis is conducted to qualitatively identify the location of stress concentration which can be possible sites of crack formation. Good agreement is observed between the model and experimental results.
“…With subsequent thermal cycling, existing longitudinal cracks started to grow and new microcracks were formed. Cracks grow along the 90° tow until it touches the 0° tow and then jump to the adjacent 90° tow [20].…”
Section: Sample A: [( ±45)(0/90)core]s With 625 MM Thick Corementioning
confidence: 99%
“…Three point bending test on solid laminates was performed to indicate the effect of damage due to the microcracking. Thermal cycling of composite honeycomb sandwich structure is described more in detail by Hegde and Hojjati [8]. A particular configuration of sandwich construction was made using carbon fiber facesheet and Kevlar honeycomb core.…”
Sandwich panels made of polymeric composite materials used in hostile thermal fatigue environments are prone to microcracking due to internal stresses. The impact of micro-cracking as a result of thermal fatigue is more severe in sandwich structures as opposed to solid laminates.Four sandwich panel configurations are studied. They are quasi-isotropic panels made of polymeric carbon fiber reinforced skin bonded by adhesive to honeycomb Kevlar cores.Different facesheet and core thicknesses are investigated. Samples are subjected to thermal cycles from -195°C to 150°C. Microscopic inspection is performed at the sample cross-section for a number of cycles to observe the location and density of cracks. It is observed that cracks are formed mainly at the adhesive/composite interface. Also, microcracks are formed more in the core ribbon direction compared to the core transverse direction. For all samples, after 40 thermal cycles, the total crack length becomes saturated and remains almost constant and no more damage happens. To study the effect of microcracks on mechanical property, flatwise tensile test was performed at room temperature. By increasing the number of cycles, the crack area increases and the flatwise strength decreases. Experimental data indicates that the samples with higher core to facesheet thickness ratio has higher microcrack lengths and lower flatwise strength. Therefore, sandwich panels with thinner facesheet and thicker core are more susceptible to the damage if subjected to the thermal fatigue.
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