Analysis of the effects of matrix cracking on composite laminates is a well-known problem which has attracted considerable attention for the past decade. An approximate analytical solution is introduced in this thesis to study this type of problem. The subjects of primary concern are the degradation of effective laminate properties, such as axial stiffness, Poisson's ratio, shear modulus, and coefficient of thermal expansion, as a function of crack density and the axial stress redistribution due to the existence of matrix cracks. Both transverse cracks (2-D problem) and cross (transverse and longitudinal) cracks (3-D problem) are studied. Results for graphite/epoxy cross-ply laminates are presented and compared to those of other approaches. Some other materials, for instance, glass/epoxy, are also studied. The results and comparisons will appear where appropriate. In general, the agreement between the results of the present analysis and those of other approaches, in particular, the finite element method, is good for the lower crack density. The present study shows that Poisson's ratio may be a good indicator of the degree of damage for a cracked laminate. / Master of Science

This study reports on the development and application of the peninsula blister test to quantitatively measure the adhesion of various adhesively bonded joints. Analytical results reveal that this peninsula-like geometry benefits from both a constant strain energy release rate over the major portion of the debond length and a high strain energy release rate at any given pressure. Applications of this technique to several adhesion systems were conducted. Although some of these systems haven't been successfully tested due to various reasons, experimental results from systems of PSA tapes and thin polyimide films bonded on aluminum substrates were promising. The agreement of the bond strength in terms of strain energy release rates obtained from both experimental and analytical methods from the last two systems indicates the feasibility of this technique. Primary studies on the stress analysis for several thin film adhesion tests suggest that the high ratio of strain energy release rate to applied pressure offered by this modified blister geometry may not be able to overcome the tensile strength limitations of thin film adhesion testing. Further studies need to be conducted in order to understand if the strain energy release rate can be raised without the increase of membrane stresses by altering specimen geometries. In conclusion, although this modified blister is not an universal adhesion test for every adhesion system, the attractive nature of the constant strain energy release rate produced by the peninsula blister specimen warrants further investigations and wider applications. / Master of Science

The diagnostics of slight damage are extremely significant for providing the early warning damage information for in-service structures. This thesis presents the development of a systematic experimental method to identify structural damage by the experimental techniques. Three carbon fabric squared composite p1ates were used as the research objects. Two of them with light crack damage that can be classified as fiber breaking and matrix cracking were supposed to be identified through the dynamic experimental techniques. The tests of the frequency response functions (FRFs) of the investigated objects were conducted first to provide a general understanding of the dynamic properties of the material and the structures. Then the tests of the velocity fields at some specified frequencies are performed to acquire dynamic response data of the objects for the study purposes. A systematic method to process the experimental data has been developed first in this thesis. The best regressive mathematical models for the test velocity fields are built based on the linear polynomial regression procedures and statistical analysis. To perform damage identification, the correlation coefficient (CC) and spatial correlation coefficient (SCC) techniques based on the best-fitted models and the curvature models were used. Finally, the student t' statistical tests were applied to decide whether the two compared data sets are significantly different in statistical sense. / Master of Science

The bond performance and surface energy of hygro-thermal compression densified wood were studied using comparisons to hygro-thermally treated and control yellow-poplar (Liriodendron tulipifera). Bond performance was studied using opening mode double cantilever beam fracture testing and cyclic boiling of one half of all fracture samples. Phenol formaldehyde film (PF-film) and polymeric diphenylmethane diisocyanate (pMDI) were the two different adhesives used to bond fracture samples. Hygro-thermal samples bonded with PF-film had significantly higher fracture toughness than control samples, while no difference was found for densified samples. Densified samples bonded with pMDI had significantly higher fracture toughness than control samples while no change was seen for hygro-thermal samples. Boil cycling reduced fracture toughness of hygro-thermal fracture samples only, irrespective of adhesive type. Surface energy was studied using sessile drop contact angle measurement and the Chang model of acid-base, surface energy component calculation. Water, glycerol, formamide, ethylene glycol, and -Bromonapthalene were used as probe liquids. Densified and hygro-thermally treated yellow-poplar had significantly higher contact angles than control samples. The contact angle trends for densified and hygro-thermally treated wood were found to be the same. Total surface energy as well as the polar and acid components of surface energy decreased with hygro-thermal treatment. The dispersive and base components of surface energy increased with hygro-thermal treatment. / Master of Science

A study was conducted to evaluate the effect of specimen dimensions in slow strain-rate environmental effects testing. Tension tests of free machining brass were conducted in a mercuric nitrate solution at a constant crosshead displacement rate of 10⁻³(inch/sec). Thirty-six smooth round bar specimens with different dimensions were tested. It was shown that percent elongation to failure was inversely proportional to an effective ratio of length to diameter, ((D - 2a)L / D²), where D is the specimen diameter, L is the length of the reduced cross section of the specimen, and a is the environmentally induced crack depth. This effective length to diameter ratio correlates with the applied tearing modulus for a cracked round bar tension specimen as defined by P. C. Paris and co-workers in 1979. The results verify that the tearing modulus may be used as a parameter to evaluate tearing instability in terms of elastic-plastic fracture mechanics. More directly, these results show a possible source of error in evaluating the degree of susceptibility to environmentally induced cracking in a material-environment interaction. / Master of Science

The effects of time and stress level were investigated in cross-ply laminates to gain more understanding on the damage events in composites. Analytical predictions of the effect of stress level were performed for the case of linear elastic materials. The predictions were based on energy methods and linear elastic fracture mechanics. Damage was simulated with a Monte Carlo numerical scheme. The predicted results corresponded well with experimental data in the literature. Experimental testing was performed on cross-ply laminates to gain a better understanding of the effect of time and rate on matrix cracking. The tests were performed on Kevlar/epoxy and graphite/epoxy [0/90₃]₈ laminates. The results indicate that the stress levels required for matrix cracking are a function of how fast the specimens were loaded. Also, significant time dependent damage was observed in cross-ply laminates which were subjected to sustained loads. / Master of Science

Two dimensional and quasi-three dimensional, linear elastic finite element models for the prediction of crack growth characteristics, including crack growth direction, in laminated composite materials are presented. Mixed-mode crack growth in isotropic materials, unidirectional and laminated composites is considered. The modified crack closure method is used to predict the applied load level for crack extension and two new failure theories, modifications of the point stress and the Hashin failure criteria, are proposed to predict the direction of crack extension in composites. Comparisons are made with the Tsai-Wu failure criterion and the Sih strain energy density criterion as well as with experimental results. It is shown that the modified versions of point stress and Hashin criteria compare well with experiment. / Master of Science

When bonds between copper and printed circuit board laminates are subjected to impulsive forces, the need arises to characterize fracture energies corresponding to related, high-speed failure events. Work (or energy) is required to create new surface area—with associated dissipation events—during fracture, and this energy (for a given material system) is dependent on the speed of crack propagation, the locus of failure, and the temperature of the bond when it is broken. Since the 90° peel test has been widely employed in quasi-static fracture testing of film adhesion for printed circuit board applications, this test was first used as a basis to which other test results could be compared. A test fixture was designed and built for quasi-static peel testing that accommodated peeling at different angles and temperatures. A similar test was then desirable for the direct comparison of dynamic fracture events to those quasi-static results. The “loop peel test” was thus developed to mimic the common 90° peel test and to quantify the time- and temperature-dependent fracture energies of peel specimens during low-velocity impact. This test has been successfully used to determine the apparent critical strain energy release rate of copper/epoxy bonds for low-velocity impact conditions (1-10 m/s), for a case of near-interfacial failure. The falling wedge test has also been adapted to estimate the apparent critical strain energy release rate at similar fracture conditions. Four types of printed circuit boards have been analyzed with the above impact test methods as well as with their corresponding quasi-static tests, and the fracture energies measured with the impact tests have been compared to those obtained using quasi-static tests. Fracture energies of the material systems considered were dependent on time (speed of fracture), temperature, and the amount of moisture migration, as determined via humidity conditioning parameters. / Master of Science

Environmental temperature changes produce cyclic thermal stresses in structures. Because temperature and hence thermal stresses are random and mechanical properties such as strength, relaxation modulus, and fracture toughness are time and temperature dependent as well as statistically variable quantities in rocket motor grains, crack growth will also be random. To include all these effects, the Forman crack growth rate relation will be used. The probability of failure, defined as the chance that a crack growing under the influence of such stresses exceeds the critical crack length, is calculated. A hazard function will be defined as the probability of failure during a single cycle after the cylinder has survived N previous cycles. The hazard function will then be integrated to yield the progressively increasing probability of fracture. The stress distribution through the cylindrical wall is not uniform; as a result, a step by step finite element analysis will be carried out to determine stress intensity and critical crack size as the crack travels through the varying stress field. The probability of failure is conditioned on the chance that a crack exists. / Ph. D.

Graphical simulation of crack growth using singular finite elements and interactive computer graphics is presented. The study consists of two main parts : (i) the formulation and application of an anisotropic singular element (ASE) for analyzing homogeneous anisotropic materials with cracks and, (ii) graphical simulation of crack growth in unidirectional composites. Lekhnitskii’s stress function method is used to formulate the traction-free crack boundary value problem with the stress function expressed in a Laurent series. The geometry of the element is arbitrary. The development of the stiffness matrix for general anisotropic materials is presented and it is shown how the singular element can be incorporated into a conventional displacement based finite element program. The anisotropic singular element (ASE) developed is implemented to analyze cracked anisotropic materials subjected to inplane loading. A 2-D, displacement based linite element code is used and center cracked on- and off-axis coupons under tensile loading are analyzed using the element developed. A general, interactive menu driven program is developed to track crack growth in composite materials. PHIGS (Programmers Hierarchical Interactive Graphics System) is used as the application program interface to integrate the finite element program with interactive graphics. Simulation studies are performed for center cracked on- and off-axis Iaminae using the normal stress ratio theory as the crack propagation criterion. The direction of crack propagation and values of the crack initiation stresses predicted are in reasonable agreement with the experimental values for the cases analyzed. / Ph. D.