The final tensile fracture of any composite structure is primarily due to the failure of its constituents, namely fibers and matrix in the present case. To date, no experimental data exists, to the author’s knowledge, to define the behavior of constitutive fibers of a composite structure throughout its life span. The prime candidate for a fiber-based investigation is unidirectional zero-degree composite coupons. But unidirectional coupons do not demonstrate any significant loss of stiffness during fatigue cycling compared to other lay-ups. Even if stiffness degradation was significant, due to the nature of damage in this material system it would be impossible, practically, to monitor that change using conventional techniques (e.g. an extensometer) because the damage and failure process destroys the integrity of the contact between those devices and the material, under cyclic conditions. This investigation presents the findings of a fiber-based investigation of unidirectional composite material systems. In particular, a unidirectional graphite/epoxy system was studied, and the influence of applied load level on fiber fractures, and their influence on damage growth documented. A damage monitoring technique (patent pending) was developed to accurately record the state of damage in this material system without the usage of extensometers or strain gages. Following this method, two new damage norms were introduced, namely, “percent phase damage” and “percent gain damage”. Fiber fracture, strength degradation, and the life of unidirectional specimens were investigated and recorded as a function of various load levels. Fiber fracture, in general, showed no definitive growth pattern during fatigue cycling. It appears that the majority of the broken fibers that occur over nearly 90% of the life are due to the initial applied load cycle. This is one of the key findings of this investigation. “Proof testing” which is a common practice in industry for “verifying” the integrity of a structure, could very well be causing significant subsequent reductions in life. With these findings as a base, it is now possible to postulate the first well-founded mechanistic model of fiber-dominated fatigue degradation under tensile loading. / Ph. D.

No description available.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERO / Includes bibliographical references. / by Michael Jeffrey Graves. / Ph.D.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

The results of an experimental investigation into the compressive behavior of T300-5208 graphite/epoxy plates measuring 12.70 cm by 25.40 cm and containing circular cutouts ranging to 3.81 cm are reported. Two thicknesses, 24 ply and 48 ply were chosen to differentiate between stability failures due to buckling and strength failures due to the presence of the cutout. The critical load of the 24 ply panels was found to be independent of hole size with bifurcation occurring in the two halfwave mode longitudinally and one halfwave laterally in the quasi-isotropic panels. For the orthotropic 24 ply panels, bifurcation began in the one halfwave shape in each direction but changed to the two halfwave shape before failure. Consistent post-buckling strength was exhibited by both laminate configurations. The 48 ply specimens displayed decreasing ultimate load with increasing hole diameter to approximately 50% of the no-hole panel ultimate for a diameter-to-width ratio of 0.30. The 48 ply orthotropic panels displayed a strength/stability threshold between cutout diameters of 0.3175 cm and 0.635 cm. Panels with cutout diameters below 0.3175 cm displayed buckling before failure and those with cutouts larger than 0.635 cm experienced material failure around the hole before panel collapse. The 48 ply quasi-isotropic panels showed no sign of such a threshold with failure due to the hole occurring for all hole sizes. / Master of Science

Thesis (M.S.)--University of Michigan, 2001. / Includes bibliographical references.

The iso- and non-isothermal crack problems of layered fiber-reinforced composite materials are investigated within the framework of linear anisotropic thermoelasticity and under the state of generalized plane deformation. The crack is assumed to be parallel to the layer bounding surfaces. By employing the Fourier integral transform technique and the flexibility/stiffness matrix formulation, the current mixed boundary value problems are reduced to solving a set of simultaneous singular integral equations with Cauchy-type kernels. The crack·tip stress intensity factors are then defined in terms of the solution of the integral equations. Numerical results are presented addressing the salient and unique features for a class of crack problems involving highly anisotropic fibrous composite materials. Specifically, the cases of a crack embedded i) within a homogeneous and anisotropic slab, ii) between two bonded dissimilar anisotropic half-spaces and iii) within the matrix-rich interlaminar region of a generally laminated anisotropic slab are considered. The effects of relative crack size, crack location and fiber volume fraction on the stress intensity factors are examined as a function of über angle. For the case of layered composites, the matrix-rich interlaminar region is modeled as a separate interlayer. As the interlayer thickness approaches zero, the interlaminar crack model illustrates no smooth transition to the ideal interface crack model of zero interlayer thickness which exhibits oscillatory stress singularities. The mixed-mode crack tip response is shown to involve the simultaneous presence of three fracture modes. It is demonstrated that the corresponding values of stress intensity factors are strongly influenced by the laminate stacking sequence and layer orientation. In addition, the partially insulated crack surface condition is observed to alleviate the severity of thermally-induced stress fields near the crack tip. / Ph. D.