Thermo-mechanical characterisation of a nanosized particle filled underfill
This paper describes work carried out as part of a larger project aimed at the development of nanosized particle filled underfill material. Several characterisation experiments have been applied to study the influence of these particles on the curing behaviour and the thermomechanical properties such as the coefficient of thermal expansion, geltime, glass transition temperature, and modulus. Constitutive models are assessed for their ability to describe the observed behaviour
Process Modeling of Composite Materials: Residual Stress Development during Cure. Part I. Model Formulation
In the production of composite parts the pertinent processing parameters are time, temperature, and pressure. A judicious choice of these three parameters pro duces composites which are fully cured, compacted, and of high quality. Slight deviations from the recommended processing conditions can result in unacceptable quality. One of the most significant problems in the processing of composites is residual stresses. Processing-induced residual stresses can be high enough to cause cracking within the matrix even before mechanical loading. This microcracking of the matrix can expose the fibers to degradation by chemical attack. Strength is adversely affected by residual stresses since a pre-loading has been introduced. The topics considered and discussion presented in this paper have been chosen to ad dress the issue of understanding how residual stresses develop during processing and how they can be predicted. A process model has been developed which can be used to predict the residual stress history during the curing of composite laminates. This model includes the effects of chemical and thermal strains and assumes the material to exhibit linear, vis coelastic behavior. A phenomenological model is used to predict the degree of cure history during the cure cycle. Mechanical properties are allowed to develop based on a functional dependence on the cure state (degree of cure) and the transverse compliance is taken as the only time-dependent compliance. Simultaneous application of the cure kinetics and a vis coelastic stress analysis yields the residual moments and curvatures for unsymmetric cross-ply laminates. An experimental correlation is provided in an accompanying paper.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.