It is well known that matrix failure in carbon/epoxy composites is influenced by multiaxial states of stress. However most of the experimental work to measure this interaction has not focused exclusively on matrix failure, and a general multiaxial stress criterion for matrix failure has not been established. To examine this question experimentally we have carried out a series of tests involving torsional shear combined with axial tension or compression of unidirectional hoop wound cylinders, using AS4/55A carbon/ epoxy lamina. The matrix failure stresses are seen to be well correlated with the Tsai-Wu quadratic polynomial. There is also a strong interaction between the strains at failure, with small amounts of transverse tension giving a significant reduction in the shear strain at failure. The effect of σ 2 on the nonlinear shear stress-strain curves is also presented.

Large uncertainty remains in the spatial distribution of deep soil organic carbon (OC) storage and how climate controls belowground OC. This research aims to quantify OC stocks, characterize soil OC age and chemical composition, and evaluate climatic impacts on OC storage from the soil surface through the deep critical zone to bedrock. These objectives were carried out at four sites along a bio-climosequence in the Sierra Nevada, California. On average, 74% of OC was stored below the A horizon, and up to 30% of OC was stored in saprock (friable weakly weathered bedrock). Radiocarbon, spectroscopic, and isotopic analyses revealed the coexistence of very old organic matter (OM) (mean radiocarbon age = 20,300 y BP) with relatively recent OM (mean radiocarbon age = 4,800 y BP) and highly decomposed organic compounds with relatively less decomposed material in deep soil and saprock. This co-mingling of OM suggests OC is prone to both active cycling and long-term protection from degradation. In addition to having direct effects on OC cycling, climate indirectly controls deep OC storage through its impact on the degree of regolith weathering (e.g. thickening). Although deep OC concentrations are low relative to soil, thick saprock represents a large, previously unrealized OC pool.

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Biotite content was positively related to regolith thickness in granitic terrain. Depth regulates regolith transformations by dampening subsurface temperature. The degree of soil development does not reflect deep regolith characteristics. We evaluated the effects of temperature and subtle differences in lithology (biotite content) on the degree of pedogenesis in regolith (soils + saprock) in granitic terrain of the southern Sierra Nevada. Deep regolith was sampled from summit and backslope landscape positions in two catchments representing the transition from rain‐dominated (1100 m elevation) to snow‐dominated (2000 m elevation) precipitation. Regolith thickness varied regardless of degree of soil development (e.g., Alfisols vs. Inceptisols), ranging from 1.4 to 7.6 m at the 1100 m site and 1.5 to 10.7 m at the 2000‐m site. Biotite content in fine sand fractions of saprock was positively correlated with regolith thickness at both sites. In the rain‐dominated catchment (1100 m elevation), particle grain size was negatively correlated with regolith thickness. Evidence of pedogenic transformations was evaluated based on clay content and dithionite extractable iron (Fed). The degree of regolith transformation was controlled by heat energy, which was related to the temperature mediated by depth. Annual sum of heat energy modeled from surface to hard bedrock showed a significant positive correlation with clay content and Fed concentration. The slopes of regression lines between annual heat energy load and clay and Fed were steeper at the 1100 m site, where the dominant form of precipitation was rain. This trend is a result of the greater degree of pedogenesis and higher temperatures in soils compared with saprock at 2000 m.

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