Composite materials--Mechanical properties

Sandwich composites provide excellent structural integrity for a variety of
applications. In this study pristine and functionalized 30 nrn Silicon Carbide
nanoparticles are infused into a low density polyurethane foam used for the inner core
of the sandwich structure. The mechanical properties are characterized using
compressive, tensile, and flexural tests. A plane-strain fracture test and a TSD (Tilted
Sandwich Debond) test characterize the fracture properties of the foam and the coreskin
interface. Thermal characterization is carried out using Dynamic Mechanical
Analysis (DMA) and Thermo-Gravimetric Analysis (TGA). FTIR spectral analysis
reveals changes in molecular bonding due to pristine and functionalized nanoparticle
infusion. The fracture resistance of the foam is improved and the delamination
strength of the sandwich construction with nanophased cores is dramatically
improved. The TSD testing indicated that the G1c value rose from 0.14 kJ/m^2 in the
neat foam to 0.56 kJ/m^2 with just 0.1 wt% of SiC nanoparticle inclusion reflecting an
enhancement of almost 300%.

Degradation of the critical components of polymer matrix composites in marine
environments had been experimentally investigated. Water absorption behavior of neat
resin and composite specimens was examined. The tensile strength of fibers was
monitored using the single filament test. The mechanical properties of the resins were
monitored by tensile, flexure, and dynamic-mechanical tests. In addition, matrix
shrinkage during cure and matrix swelling after immersion in water were monitored. The
integrity of the fiber/matrix (F/M) interface of the composite systems was studied using
the single fiber fragmentation test (SFFT). Macroscopic composites were examined using
transverse tensile and transverse flexure tests to study the influence of the integrity of the
matrix and F/M interface on the macroscopic response. In addition, for characterization
of F/M debonding in the SFFT, a fracture mechanics model and modified test procedure
were developed.

The mechanical behavior of woven fabric composites is presented in this study through modeling of the elastic properties and experimental studies on the failure behavior and fracture analysis. A two-dimensional laminate theory based elastic model for the prediction of the elastic constants of satin weave fabric composites is developed. The predicted elastic constants are compared with results from other models and correlated with the experimental data. An experimental study is presented on mechanical response in tension, compression and shear and on damage development in tension of two woven fabric composite systems viz. carbon/epoxy and glass/epoxy. Damage inspection of the carbon/epoxy composite under tension revealed that the initial failure was cracking of pure matrix regions followed by transverse bundle cracking. Fill/warp debonding and longitudinal splits of the fill bundles occurred close to ultimate failure of the composite. The glass/epoxy composite displayed damage in the form of fill/warp debonding and longitudinal splits, but no transverse yarn cracking. Interlaminar fracture behavior of a five-harness satin orthogonal woven fabric carbon/epoxy composite laminate loaded in mode I, mode II and mixed mode has been investigated. Special emphasis was put on microscopic details of crack growth, and their relation to the fracture resistance. For all fracture mode combinations it was found that crack growth occurred in a nonplanar region of topology determined by the weave pattern and relative positioning of the plies adjacent to the crack plane. The woven fabric structure constrains fiber bridging, but partial debonding of transversely oriented fiber bundles led to occasional crack branching, stick-slip behavior leading to variations in the mode I fracture resistance. Slow stable crack growth occurred in the mode I and mode II fracture specimens prior to unstable fracture and resulted in nonlinear load-displacement response.