Characterization Of Damage Evolution Research Articles

Performance of integrated active fiber composites in fiber reinforced epoxy laminates

Active fiber composite (AFC) composed of lead zirconate titanate (PZT) fibers withinterdigitated electrodes (IDEs) has been integrated into orthotropic glass fiber reinforcedplastic (GFRP) laminates to characterize the performance of AFC as a smart materialcomponent in laminated materials. Monotonic cyclic tensile loading was performed onintegrated specimens at different strain levels. The AFC output was monitored todetermine the effect of applied strain level on the AFC performance. It was found that theAFC sensitivity degraded beyond strains of 0.20% and approached a minimum at 0.50%strain. The degradation in the AFC performance appears to be attributed tothe dominating effect of PZT fiber fragmentation during testing, as opposed todepolarization. Acoustic emission (AE) monitoring was used to detect damage inlaminates during testing and was correlated with crack evidence from microscopyobservations during testing to characterize damage evolution in response to strain levels.

アルミニウム合金中の粗大な第二相粒子の損傷が力学的性質に与える影響の実験的評価

The present work is aimed at experimental evaluation of the effects of damage evolution at coarse intermetallic particles in Al–Li–Cu–Mg alloy, with particular attention on fracture toughness. To vary volume fraction of the particles, various thermo-mechanical treatments have been applied. Strength, ductility and fracture toughness decrease with the increase in volume fraction from 1.2 to 2.2%. Especially, σ0.2 and crack propagation resistance are significantly affected. In-situ SEM observation of the fracture toughness test has also been used to characterize damage evolution at the particles and crack propagation behavior. Some types of particles remain intact even adjacent to the fracture surface, while others are extensively fractured far ahead of a crack-tip. A combination of the crack-tip singularity and micromechanics is used to estimate in-situ strengths of the particles. It is concluded that the particle strengths have strong dependence on diameter, and that, due to the existence of the damaged particles, crack deflection and formation of secondary cracks occur in the case of high particle volume fraction. Fracture mechanical analysis reveals that reduction in the crack propagation resistance is attributed to the amplification of mode I crack driving force due to the existence of microcracks. Finally, measured fracture toughness is interpreted utilizing a computer simulation constructed in the previous study. Although individual microstructural factors have only modest effects, competitive and synergistic effects are attributable to the measured large drop in the fracture toughness.

Damage evolution in low velocity impacted unreinforced vinyl ester 411-350 and 411-C50 resin systems

Damage evolution in plaques made of vinyl ester resin systems was investigated as a function of specimen thickness, impact energy level and matrix material. Dow DERAKANE vinyl ester 411-350 and 411-C50 resin systems, which have low viscosity and are ideally suited for low-cost liquid processing techniques like vacuum assisted resin transfer molding (VARTM), were considered for the low velocity instrumented impact testing. Characterization of damage evolution was undertaken using optical microscopy and analysis of impact load histories recorded during the impact event. Radial cracking, perforations at the point of impact (in the form of a truncated cone), and damage resulting from the support constraints were identified as the dominant failure characteristics in both resin systems. Radial cracking, which originated from the bottom surface, was operative in all failed specimens and was attributed to the catastrophic failure due to extensive flexural tensile strength losses. For specimens that could deflect significantly, radial cracking and support-constraint-induced damage were the operative failure mechanisms. Radial cracking and through-thickness shearing led to failure in stiffer plaques. The DERAKANE 411-350-vinyl ester resin system was found more damage resistant than the 411-C50 system.

Permeability of Natural Rock Salt from the Waste Isolation Pilot Plant (WIPP) During Damage Evolution and Healing

The US Department of Energy has developed the Waste Isolation Pilot Plant (WIPP) in the bedded salt of southeastern New Mexico to demonstrate the safe disposal of radioactive transuranic wastes. Four vertical shafts provide access to the underground workings located at a depth of about 660 meters. These shafts connect the underground facility to the surface and potentially provide communication between lithologic units, so they will be sealed to limit both the release of hazardous waste from and fluid flow into the repository. The seal design must consider the potential for fluid flow through a disturbed rock zone (DRZ) that develops in the salt near the shafts. The DRZ, which forms initially during excavation and then evolves with time, is expected to have higher permeability than the native salt. The closure of the shaft openings (i.e., through salt creep) will compress the seals, thereby inducing a compressive back-stress on the DRZ. This back-stress is expected to arrest the evolution of the DRZ, and with time will promote healing of damage. This paper presents laboratory data from tertiary creep and hydrostatic compression tests designed to characterize damage evolution and healing in WIPP salt. Healing is quantified in terms of permanent reductionmore » in permeability, and the data are used to estimate healing times based on considerations of first-order kinetics.« less

The effect of high pressure hydrogen on the creep fracture of notched ferritic-steel components

In order to study hydrogen attack under near-service conditions, an experimental technique for examining the multiaxial behaviour of model-sized tubular components was modified to allow exposure to high pressure hydrogen gas, at a temperature in the creep range for the material chosen. Components with both axial and circumferential internal notches were loaded using the internal pressure of hydrogen, at a temperature of 600°C. The material under study was a low-alloy ferritic steel, 2 1 4 Cr–Mo, used widely in pressure vessels and piping in the petro-refinery industry, where high pressure hydrogen is encountered. Analysis of the results takes the form of metallographic investigation, correlation of rupture life with base-line data, and characterisation of damage evolution. The distribution of inter-granular voids and cavities observed by S.E.M. confirms the stress-assisted nature of hydrogen-induced degradation. The hydrogen attack caused a dramatic change in the multiaxial creep deformation mechanism from von Mises control to maximum principal stress, a factor very important in the design of plant components. The effect of this change on the reference-stress calculations for both notch types is examined in terms of the stress rupture time correlation with base-line data. C* values are compared with standard data for the material. Factors incorporated in the method of C* calculation, such as the reference stress equations and Norton constants, which may influence the value of the results are considered.

An endochronic damage model for three-dimensional ductile failure analysis of double-edge notched thick-tension specimens

A three-dimensional ductile damage model based on the endochronic plastic theory is developed and employed to characterize the failure process of double-edge notched thick-tension specimens. A ductile damage evolution equation is derived with a new intrinsic time-scale specifically defined to characterize damage evolution. Two damage failure criteria are proposed and employed to predict the failure loads and crack initiation sites which correspond well with those measured experimentally.

Complete p-type activation in vertical-gradient freeze GaAs co-implanted with gallium and carbon

High-resolution triple-axis x-ray diffractometry and Hall-effect measurements were used to characterize damage evolution and electrical activation in gallium arsenide co-implanted with gallium and carbon ions. Complete p-type activation of GaAs co-implanted with 5×1014 Ga cm−2 and 5×1014 C cm−2 was achieved after rapid thermal annealing at 1100 °C for 10 s. X-ray diffuse scattering was found to increase after rapid thermal annealing at 600–900 °C due to the aggregation of implantation-induced point defects. In this annealing range, there was ∼10%–72% activation. After annealing at higher annealing temperatures, the diffuse scattered intensity decreased drastically; samples that had been annealed at 1000 °C (80% activated) and 1100 °C (∼100% activated) exhibited reciprocal space maps that were indicative of high crystallinity. The hole mobility was about 60 cm2/V s for all samples annealed at 800 °C and above, indicating that the crystal perfection influences dopant activation more strongly than it influences mobility. Since the high-temperature annealing simultaneously increases dopant activation and reduces x-ray diffuse scattering, we conclude that point defect complexes which form at lower annealing temperatures are responsible for both the diffuse scatter and the reduced activation.

A new representation for the strain energy of anisotropic elastic materials with application to damage evolution in brittle materials

In this paper, we develop a new representation of the strain energy of an elastic material using twenty-one scalar measures of strain. These measures are associated with material line elements which are directed along the six axes of symmetry of a regular icosahedron, and they include: six measures of axial strain and fifteen measures of the angular strain. When the strain energy is a quadratic function of strain, this representation yields twenty-one elastic moduli which naturally separate into only two physically different types. This is in contrast to the five physical types of moduli associated with the common rectangular Cartesian representation of the stiffness tensor. By formulating evolution equations for the elastic moduli, we describe general changes in the elastic state of a brittle material. Due to the physical nature of these moduli, we anticipate that the evolution equations will have simplifying features when expressed in terms of the new representation of the strain energy presented here. In general, these evolution equations are restricted by sufficient conditions which ensure that the change in the elastic state is a dissipative process in the sense that the second law of thermodynamics is satisfied. It is shown that not all changes of the elastic state can be interpreted as damage evolution, even if they are dissipative and cause reductions of the magnitudes of the twenty-one moduli. To ensure damage evolution, we propose an additional restriction on the strain energy. Furthermore, a simple modification of the strain energy is introduced to model the effect of crack closure. Specific equations for damage evolution which satisfy all necessary restrictions are presented. These equations characterize damage evolution in a material that exhibits a transition from an isotropic to an anisotropic elastic state. Examples are considered which show physically reasonable brittle material response.

On the low endurance fatigue of rubber-toughened adhesives and its implication on characterization of damage

A high sensitivity test method has been developed for axial strain con trolled fatigue testing of flat sheet specimens. The low strain fatigue behaviour of a high performance epoxy film adhesive (FM73) and a cast lab-made rubber-toughened epoxy resin has been established over a specified range of applied total strain amplitude. This has aided the characterization of damage evolution and the determination of related cyclic and monotonic stress/strain properties.

On the Low Endurance Fatigue of Rubber-Toughened Adhesives and Its Implication on Characterization of Damage

A high sensitivity test method has been developed for axial strain con trolled fatigue testing of flat sheet specimens. The low strain fatigue behaviour of a high performance epoxy film adhesive (FM73) and a cast lab-made rubber-toughened epoxy resin has been established over a specified range of applied total strain amplitude. This has aided the characterization of damage evolution and the determination of related cyclic and monotonic stress/strain properties.