Laser-ultrasonic evaluation of damage in unidirectional ceramic matrix composites

Ceramic matrix composites (CMCs) have attracted great attention because of their potential for high temperature structural applications. Among these materials, calcium aluminosilicate (CAS) glass ceramic and similar composites reinforced by Nicalon{trademark} SiC fiber with carbon-rich interface have been under active investigation because of their {open_quotes}notch-insensitivity{close_quotes}: stress near holes and notches can be redistributed by inelastic deformation in the form of multiple matrix cracking. Therefore, stress concentration is alleviated near these sites. Understanding the damage mechanism in these composites is very important for the development of constitutive modeling. To achieve this goal, monitoring damage initiation and accumulation in-situ are especially critical. In most of the previous work, the change of elastic modulus along loading direction was used to characterize the damage. However, the overall anisotropic damages such as fiber-matrix debonding or shear deformation were unknown. In this study, we have pursued an in-situ nondestructive laser-ultrasonic technique to assess the overall anisotropic stiffness degradation under loading. When a laser pulse is brought to sample surface, high frequency acoustic waves can be generated by thermal or ablation mechanisms depending on the incident power intensity. The propagation of the elastic waves through anisotropic media is characterized by the well-known Christoffel equation

The influence of fiber/matrix interface on the mechanical behavior of Nicalon SiC fiber reinforced glass-ceramic composites

Mechanical properties of unidirectional Nicalon SiC fiber reinforced Ca aluminosilicate (CAS/SiC) and Mg aluminosilicate (MAS/SiC) glass-ceramic composites were investigated by tensile testing and nondestructive laser-ultrasound technique. The Ba-stuffed MAS was either undoped or doped with 5% borosilicate glass. Degradation of elastic stiffness constant C{sub 11} in transverse direction due to interface damage was monitored in situ by measuring the laser- generated ultrasound wave velocity. The three composite materials show different characteristics of macroscopic deformation behavior, which is correlated strongly to interface degradation. A stronger reduction trend of the elastic constant C{sub 11} is associated with a larger degree of inelastic deformation. The fracture surfaces also reveal the close relation between fiber pullout length and interfacial characteristics. Interfaces of these composites were studied by TEM; their influence on inhibiting and deflecting matrix cracks is discussed

Dynamic compression of foam supported plates impacted by high velocity soil

The response of back-supported buffer plates comprising a solid face sheet and foam core backing impacted by a column of high velocity particles (sand slug) is investigated via a lumped parameter model and coupled discrete/continuum simulations. The buffer plate is either resting (unattached) or attached to a rigid stationary foundation. The lumped parameter model is used to construct maps of the regimes of behaviour with axes of the ratio of the height of the sand slug to core thickness and the normalised core strength. Four regimes of behaviour are identified based on whether the core compression ends prior to the densification of the sand slug or vice versa. Coupled discrete/continuum simulations are also reported and compared with the lumped parameter model. While the model predicted regimes of behaviour are in excellent agreement with numerical simulations, the lumped parameter model is unable to predict the momentum transmitted to the supports as it neglects the role of elasticity in both the buffer plate and the sand slug. The numerical calculations show that the momentum transfer is minimised for intermediate values of the core strength when the so-called “soft-catch” mechanism is at play. In this regime the bounce-back of the sand slug is minimised which reduces the momentum transfer. For high values of the core strength, the response of the buffer plate resembles a rigid plate with nearly no impulse mitigation while at low values of core strength, a slap event occurs when the face sheet impinges against the foundation due to full densification of the foam core. This slap event results in a significant enhancement of the momentum transfer to the foundation. The results demonstrate that appropriately designed buffer plates have potential as impulse mitigators in landmine loading situations

Ductile-Phase Toughening of Brazed Joints

A heat treatment is presented that uses ductile-phase toughening to mitigate the effect of brittle intermetallics in a Ni-based braze alloy. The fracture resistance has been enhanced by creating a microstructure containing elongated ductile γ-(Ni) domains that align, preferentially, across the joint. The development of this beneficial microstructure is based on an understanding of the transient dissolution, isothermal solidification, and coarsening phenomena. Due to slow kinetics, the elimination of intermetallics by diffusion is avoided in favor of ductile domain formation through solidification control. The toughening has been attributed to a combination of bridging and process zone dissipation, enabled by the ductile phase

Quantitative acoustic emission studies during deformation and fracture A review

SIGLELD:9091.9F(AERE-R--10353). / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Dynamic elastic displacement due to pulsed laser absorption at the surface of an infinite plate

SIGLELD:9091.9F(AERE-R--10401) / BLDSC - British Library Document Supply CentreGBUnited Kingdo