No description available.

No description available.

A numerical method is developed to determine the dynamic behavior of delamination and transverse cracks in multilayered plates. The plate is subjected to a time dependent antiplane shear stress field which is acting on the plate surfaces. The interaction of waves diffracted at the crack tip with those reflected at the plate boundaries and transmitted at the material interface makes the problem very complicated, so analytical study of this problem cannot be carried out with our present state of knowledge; hence the problem is solved numerically. The finite element equations are obtained by variational calculus applied in the frequency domain. Thus time intregration schemes are avoided, but time dependent response can still be obtained after inverting the frequency dependent response spectra numerically by Fast Fourier Transform (FFT) routine. Another advantage of the frequency domain analysis is that the resonance frequency can be easily detected from the sharp peaks of the response spectra. The numerical difficulty associated with the singular behavior of the stress field near the crack tip has been avoided by using quarter point elements. The numerical results obtained from this investigation are compared with analytical results to verify the accuracy of the method.

The purpose of this study was to gain a better understanding of the parameters affecting crack growth direction in unidirectional comµosite materials. To achieve this, the effect of anisotropy and biaxial loading on the direction of crack growth in unidirectional off-axis composite materials were investigated. Specific emphasis was placed on defining the crack tip stress field and finding a consistent criterion for predicting the direction of crack growth. Two models are presented to predict the crack tip stress field, an anisotropic elasticity solution and a singular isoparametric finite element formulation. After defining the crack tip stress field, three crack extension direction criteria, the Normal Stress Ratio, the Tensor Polynomial and the Strain Energy Density Criterion, were applied to predict the direction of crack extension. The theoretically predicted crack extension directions were then compared with experimental results. After comparison, it was determined that only the Normal Stress Ratio Criterion correctly predicts the direction of crack extension. / Master of Science

Baigiamajame darbe išnagrinėtos kompozitnės medžiagos jų charakteristikos ir ypatybės. Išnagrinėtas kompozitinių medžiagų panaudojimas aviacinių konstrukcijų gamybai.Išnagrinėti galimi defektai bei jų atsiradimo priežastis. Aptarti ultragarsinės neardomosios kontrolės principai, bei pritaikymas kompozitinių medžiagų ir konstrukcijų tyrimui. Išanalizuotos frontalinio echoskopinio vaizdo formavimo ypatybės ir jų pritaikymas medžiagos struktūrinei analizei. Išnagrinėti veiksniai įtakojantis ultragarsinio vaizdo sudarymo kokybę, bei sudaryta FVE struktūrinė schema. Sudarytas defekto atpažinimo algoritmas kurio realizavimas parodytas pasitelkiant Matlab programinį paketą. Pateikti pasiūlymai sistemos panaudojimui bei tolimesniam jos vystymui. / In this work examined composite materials, their characteristics and features. Examined the use of composite materials to aircraft structures manufacturing. Examined the potential defects and their occurrence causes. Discuss ultrasonic non-destructive control principles and applications of composite materials and structures research. We analyzed the formation of frontal acoustic scenery features and adaptability for material structural analysis. Examine the factors influencing the quality of ultrasound imaging and created structural diagram of acoustic scenery formation. Created the defect detection algorithm and demonstrated through the realization of the Matlab software package. Proposals for the use of the system and its further development.