Crack healing in cross-ply composites observed by dynamic mechanical analysis

Cross-ply composites with healable polymer matrices are characterized using dynamic mechanical analysis (DMA). The [90,0]s samples are prepared by embedding layers of unidirectional glass or carbon fibers in 2MEP4FS, a polymer with thermally reversible covalent cross-links, which has been shown to be capable of healing internal cracks and fully recovering fracture toughness when the crack surfaces are kept in contact. After fabrication, cracks in the composites' transverse plies are observed due to residual thermal stresses introduced during processing. Single cantilever bending DMA measurements show the samples exhibit periods of increasing storage moduli with increasing temperature. These results are accurately modeled as a one-dimensional composite, which captures the underlying physics of the phenomenon. The effect of cracks on the stiffness is accounted for by a shear-lag model. The predicted crack density of the glass fiber composite is shown to fall within a range observed from microscopy images. Crack healing occurs as a function of temperature, with chemistry and mechanics-based rationales given for the onset and conclusion of healing. The model captures the essential physics of the phenomenon and yields results in accord with experimental observations

Illicit and Counterfeit Drug Analysis by Morphologically Directed Raman Spectroscopy

Morphologically directed Raman spectroscopy (MDRS) is a novel tool for the forensic analysis of illicit and counterfeit drug samples. MDRS combines Raman microspectroscopy with automated particle imaging so that physical and chemical information about the components of a mixture sample can be obtained. Results of automated particle imaging are used to determine samples for Raman analysis. The use of MDRS for these types of samples can be employed for both forensic investigations and adjudications of cases. The method provides insight about the physical and chemical composition of the sample, as well as about manufacturing and sample history. Here, MDRS was used in four different illicit and counterfeit drug analyses: (1) examination of a multicomponent drug mixture where the results could be used for comparative source attribution, (2) the detection of low (or trace) concentration particles in a drug sample, (3) the analysis of synthetic cathinone samples (i.e., bath salts), and (4) a study of counterfeit pharmaceutical products