The effect of soil sodicity and phosphogypsum treatment on the structure of the crust of two soils (sandy loam and clay soil) was studied using a scanning electron microscope (SEM). The crust of the sandy loam with ESP 1.0, and the two soils at all levels of sodicity when treated with phosphogypsum, consisted of a compacted skin seal attributed to the mechanical impact of raindrops. Little clay movement or development of a “washed in” layer was observed under these conditions. Conversely, when the ESP > 1.6 (in the two soils) the crust consisted of naked sand and silt grains over a dense “washed in” layer. The compacted layer of disintegrated aggregates is not stable when soil and water conditions are favorable for clay dispersion (sodic soils and low electrolyte soil solutions). In soils with no exchangeable sodium, aggregate breakdown and surface compaction by raindrops are the dominant mechanisms for crust formation. In soils with ESP values above 1.5, chemical dispersion of the soil clays (enhanced by the sodicity of the soil, the low electrolyte concentration in the rain, and the stirring effect of the raindrops) and the downward movement of the clays into the “washed in” layer clogging the pores in this layer were the dominant mechanisms in crust formation. Soils with a “washed in” layer had a very low infiltration rate.

The objective of this study was to characterize the morphology of interlaminar fractures in unidirectional graphite/epoxy composites. The specific objective was to correlate characteristic fractographic patterns for different pure and mixedmode loadings with the loading parameters and fracture mechanisms. Two graphite/epoxy composite materials were investigated, one with a brittle matrix and the other with a rubber-toughened matrix. The tests were conducted with a specially designed loading system which allows the application of pure Mode I or Mode II loadings or any combination thereof. SEM fractographs, including stereo pairs, were taken of the specimens after failure at various points across the width of the crack and at various distances from the initial crack front. In the case of the brittle matrix composite, the fibers are, in general, covered with a film of matrix. The matrix fracture surface consists of hackles resulting from interfacial tensile or shear failure and tensile failure normal to the maximum tensile stress. The hackle density increases from Mode I, to Mode II, to mixed Mode I and II. In the case of the toughened matrix composite, the fibers are covered with matrix for Mode I and mixed Mode I and II fractures, whereas they are clean under Mode II and mixed Mode II with compression loading. Matrix fracture is granular or consists of hackles with a low degree of orientation and regularity increasing from Mode I to Mode II. Failure mechanisms include interfacial shear and high ductile or plastic elongation of the matrix.

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Scanning-electron microscope (SEM) investigation of the failure surfaces provides information on bidimensional state of stress under which failure of unidirectional fiber reinforced material occurred. The failure occurs by debonding between fibers and matrix for the cases of pure shear and shear + compression bidimensional state of stress. The multitude of small pieces of broken matrix is characteristic for the last case. The matrix failure is observed for the case of shear + tension.