Huanchun Chen

Polymer Nanofabric Interleaved Composite Laminates

The concept of electrospun polymer nanofiber fabric interleaving to enhance dynamic properties, impact damage resistance, fracture toughness and resistance, and delamination onset life was evaluated. Polymer nanofabric interleaving increased the laminate thickness and weight by an order of 1%, and its impact on in-plane mechanical properties of the composite laminate would be statistically zero. On the other hand, its influence on interlaminar fracture toughness and resistance, impact damage resistance, and damping is substantial. Results of this study showed that interleaving AS4/3501-6 composite laminate increased the damping by 13%, reduced the impact damage size to one-third, increased fracture toughness and resistance by 1.5 times and one-third, respectively, significantly increased delamination onset life, and increased the fatigue threshold energy release rate by two-thirds. These improvements are comparable to that of the commercial T800H/3900-2 composite but with no thickness increase penalty, loss of in-plane properties, or multiple glass transition temperatures.

An Evaluation of Data Reduction Methods for Opening Mode Fracture Toughness of Sandwich Panels

The interfacial bond toughness between the face sheet and the core continues to be a limiting parameter in the design of sandwich structures. An accurate method of measurement and data reduction of interfacial fracture toughness is essential to minimize the potential for errors and uncertainty. The cracked sandwich beam (CSB) specimen along with compliance calibration (CC) method is commonly used to measure the opening mode fracture toughness of sandwich panels. In this paper two alternative methods, namely modified beam theory (MBT) and modified compliance calibration method (MCC) are validated for data reduction of the CSB specimen. Both MBT and MCC were found to be valid and gave almost identical results. The simplicity of MBT method compels it to be recommended for data reduction of CSB test.

Effect of Thermal Fatigue on Tensile and Flexural Properties of Carbon/Cyanate Ester Pultruded Composite

The effect of thermal cycling on tensile and flexural properties of Cytec T650 carbon fiber/Lonza Primaset PT-30 cyanate ester pultruded composite rods proposed to be used for brush seals in gas turbine engines was evaluated. The thermal cycle consisted of 4min each of heating and cooling with 28min of hold at 315°C and 24min of hold at room temperature. Tension and three-point bend flexure tests were conducted after thermal cycling for 100, 200, 400, 600, and 800 cycles. Thermal cycling reduced the tensile strength and fracture strain almost linearly with the number of cycles while the tensile modulus remained unchanged. The flexural modulus did not change for the first 100 cycles and then decreased as much as 28% after 800 thermal cycles. Thermal cycling increased the Tg of the composite rods from 408 to 468°C. Optical and scanning electron microscopy analysis showed fiber/matrix interfacial separation and matrix shrinkage and oxidation due to thermal cycling.

Structural Performance of Eco-Core Sandwich Panels

Eco-Core, a fire resistant core material for sandwich composite structures developed under the US Navy (ONR) program, was used to study its performance as a sandwich beam with glass/vinyl ester face sheet. Performance of Eco-Core was compared with balsa and PVC core sandwich panels. Test specimens were designed to simulate shear, flexural, and edgewise compression loadings. These tests were conducted on Eco-Core as well as balsa and PVC sandwich composite specimens. Failure loads and modes were compared with each other and the analytical prediction. Both Eco-Core and balsa cored sandwich beams had similar failure modes in all three test conditions. In the case of transversely loaded (four-point) beams Eco-Core specimens failed by core shear for span/depth (S/d) ratio less than 4 and the failure mode changed to core tension for S/d >4. This is attributed to weak tensile strength of the core material. An expression for core tension failure load based on beam theory was derived. On the other hand, ductile materials like PVC failed by core indentation. Under edgewise compression, face sheet microbuckling and general buckling are the two potential failure modes for Eco-Core and balsa core sandwich composites. For specimen length/depth ratio L/d 13 the failure is by general buckling. Predictions from the existing equations agreed well with the experiment for both core materials. For PVC core, wrinkling/shear buckling and general buckling are the potential failure modes. For L/d ≤8.5 the failure is wrinkling and for L/d >8.5 the failure is general buckling.

Effect of Geometric Constraint on Fracture Toughness of PVC Foam Core Sandwich Beams

Purpose of this study was to understand the effect of core material thickness (tc) on the core deformation constraint and the associated mode I fracture toughness in Double Cantilever Sandwich Beam (DCSB) specimens. Specimens were made from woven roving glass fiber/vinyl ester composite face sheet with PVC core, whose thickness ranged from 3.18 mm to 40.6 mm. The specimens were tested in mode I loading and measured fracture initiation (GIC) and resistance (GIR) toughnesses. The GIC was found to be practically same for core thicknesses from 3.18 to 40.6 mm. The GIR was found to be 1.02, 0.88 and 0.91 kJ/m2 for tc’s 3.18, 6.25, and 12.7 mm. For tc ≥ 25.4 mm, the crack grew by only few mm’s before it deflected to face sheet. Larger GIR for tc = 3.18 mm is probably due to resin densification of foam cells in the co-cure processing of panels. Three dimensional, material nonlinear finite element (FE) analysis very well simulated the test data. The JIC integrals from FE analysis agreed well with GIC from the test. The analysis revealed that the deformation constraint was nearly the same for all core thicknesses considered and thus resulting in nearly identical fracture toughness.

Thermal Fatigue Effects on Mechanical Properties of T650 Carbon/Cyanate Ester Pultruded Composites

The effect of thermal cycling on tensile and flexural properties of Cytec T650 carbon/Lonza Primaset PT-30 cyanate ester pultruded composite rods used for brush seals for turbine engines was evaluated. Each thermal cycle consisted of 4 minutes each of heating and cooling with 28 minutes of hold at 600°F and 28 minutes of hold at RT. Tensile and three-point bend flexure tests were conducted after thermal cycling for 100, 200, 400, 600 and 800 cycles. Thermal cycling reduced the tensile strength and fracture strain at almost linear rate with the number of cycles while the tensile modulus remained unchanged. The flexural stiffness did not change for the first 100 cycles and then decreased as much as 28% after 800 thermal cycles. Optical and SEM microscopy analysis showed signs of thermal degradation due to thermal cycling.