Degradation Of Material Properties Research Articles

Dynamic Fracture of Composite Overwrap Cylinders

The fracture behavior due to dynamic response in a composite cylinder subjected to a moving pressure has been investigated. The resonance of stress waves can result in very high amplitude and frequency strains in the cylinder at the instant and location of pressure front passage, when the velocity of the moving pressure approaches a critical velocity. The stress wave with high magnitude, while short in duration, might not cause structural failure immediately. However, it could accelerate the propagation in the cylinder with initial imperfection and shorten fatigue life of the cylinders. The fracture mechanism induced by dynamic amplification effects is especially critical for overwrapped composite cylinders because of the multi-material and anisotropic construction, thermal degradation in material properties, and a design goal inherent in lightweight structural applications.

Dynamic Fracture of Composite Overwrap Cylinders

The fracture behavior due to dynamic response in a composite cylinder subjected to a moving pressure has been investigated. The resonance of stress waves can result in very high amplitude and frequency strains in the cylinder at the instant and location of pressure front passage, when the velocity of the moving pressure approaches a critical velocity. The stress wave with high magnitude, while short in duration, might not cause structural failure immediately. However, it could accelerate the propagation in the cylinder with initial imperfection and shorten fatigue life of the cylinders. The fracture mechanism induced by dynamic amplification effects is especially critical for overwrapped composite cylinders because of the multi-material and anisotropic construction, thermal degradation in material properties, and a design goal inherent in lightweight structural applications.

Ultimate analysis of BWR Mark III reinforced concrete containment subjected to internal pressure

Numerical analyses are carried out by using the ABAQUS finite element program to predict the ultimate pressure capacity and the failure mode of the BWR Mark III reinforced concrete containment at Kuosheng Nuclear Power Plant, Taiwan, R.O.C. Material nonlinearity such as concrete cracking, tension stiffening, shear retention, concrete plasticity, yielding of reinforcing steel, yielding of liner plate and degradation of material properties as a result of high temperature effects are all simulated with proper constitutive models. Geometric nonlinearity as a result of finite deformation has also been considered. The results of the analysis show that when the reinforced concrete containment fails, extensive cracks take place at the apex of the dome, the intersection of the dome and the cylinder and the lower part of cylinder where there is a discontinuity in the thickness of the containment. In addition, the ultimate pressure capacity of the containment is 23.9 psi and is about 59% higher than the design pressure 15 psi.

Electron irradiation effects on photoconductive semiconductor switches (PCSSs) used in sub-nanosecond transient generators

Radiation-induced damage occurs in GaAs photoconductive semiconductor switches used in sub-nanosecond transient generators when subjected to 600 keV and 6 MeV electron irradiation. These switches are made from semi-insulating (SI) compensated material through a EL2/carbon compensation mechanism, and the liquid encapsulated Czochralski process. New defect levels are formed as a result of the non-ionizing energy loss (NIEL) process. The formation of new defect levels in the device alters the compensating balance between the existing deep level EL2 trap/donors and carbon acceptors, and changes the material properties. As a result, two important parameters of the device are adversely affected-the hold-off voltage of the switch at the pulse-charging (off) state, and the rise time during the conduction (on) state. The hold-off voltage shifts to a lower value since there are more trap-filled regions available that can fill up and alter the homogenous nature of the device material. Unstable filamentary conduction then occurs at a lower voltage and leads to premature breakdown. As with EL2 trap levels, new defect states induced by electron irradiation will further contribute to the delay in the rise time of the switch. The rise time determines the maximum energy transferred to the load. The electron damage mechanism and its effects on the switch characteristics depend on the material properties. Intrinsic material, or material made through compensation other than through the deep donor and shallow acceptor balancing process are not expected to behave similarly. Simulation results at higher bias show a marked degradation of material properties. The switch current-voltage (I-V) characteristic when the bias increases to the kilovolt range is similar to trap-dominated semiconductors. An initial sublinear current regime at low bias is followed by a super-linear regime of current flow at higher bias, and is in agreement with earlier observations.

A Damage Evolution Model for Thermal Fatigue Analysis of Solder Joints

Thermal fatigue of solder joints is critical to the performance and reliability of electronic components. It is well known that the fatigue life of solder joints is rather difficult to be estimated because of the complicated material behaviors and solder joint geometry. Conventional life prediction methods such as Coffin-Manson equation or its modifications usually over-estimate the thermal fatigue life. The main reason for this phenomenon is that the material properties are assumed constant during thermal cycling. In this paper, a damage evolution model is introduced for predicting the thermal fatigue life of solder joints. This method not only considers the degradation of material properties in the solder, but also saves substantial computational effort. In the present study, a damage function is determined by the hysteresis loops of creep shear stress-strain of solder joints in a double-beam specimen. The proposed model is then applied to investigate the solder joint reliability of a 272 PBGA package and a bottom-leaded plastic (BLP) package for model verification. The results from the present analysis seem to be encouraging. [S1043-7398(00)01003-3]

Statistical model for multiaxial fatigue behavior of unidirectional plies

A statistical model is developed for the prediction of the fatigue life of unidirectional composite laminae subjected to multiaxial fatigue loading. This model is based on the experimental data for the fatigue behavior of those laminae subjected to uniaxial fatigue loading. The method is based on the extension of the generalized residual material property degradation model previously developed to describe fatigue behavior of composite materials under a wide range of stress ratios, combined with the statistical nature of static and fatigue failure of fiber-reinforced composites. The distribution function of fatigue life is determined in terms of the distribution function of static strength of the composite laminae in different loading modes, which is considered to be stochastic in nature. The fatigue life and fatigue strength of unidirectional composite laminae under any multiaxial fatigue loading are evaluated statistically by using the fatigue behavior of the laminae under uniaxial loading in longitudinal, transverse and in-plane shear directions, which is determined experimentally through material characterization. The application of the statistical model to evaluation of fatigue life and residual strength of unidirectional composite laminae subjected to biaxial fatigue loading shows good agreement with the corresponding experimental data.

Damage model for flexural strength variation of ferroelectric materials induced by electric field

Ferroelectric material is being used widely in aerospace related applications, namely active control of large flexible space trusses, structure acoustics and helicopter rotary blades. Degradation of material properties affects the safety and reliability of these structural members. In particular, the flexural strength of ferroelectric materials tend to reduce when they are exposed to electric field. Experimentation cannot always explain the underlying reasons of these physical phenomena. To that end, a damage model is presented to study the switching process induced by applied electric field that might be the cause of damage. Furthermore, evolution of internal damage is considered using irreversible thermodynamics. Two relations are assumed to associate the flexural strength with the damage variable. Available test data for three-point-bend test are used to test the model for a PZT material exposed to electric field. The results reveal the validity of the present damage model while the critical strain energy density criterion led to a more reasonable correlation.

Why local damage increases the buckling load of circular plates

Local material damage is often characterised as degradation of material properties. A recent study has discovered a paradoxical phenomenon that clamped circular plates with a local axisymmetric damage may have a buckling load above that of a corresponding perfect plate. This paper describes a finite element investigation into the buckling behaviour of circular plates with a local axisymmetric damage. The paradoxical results of previous researchers are explained, and observations are made regarding the appropriate modelling of local damage.

Damage Development in Woven Fabric Composites during Tension-Tension Fatigue

Impacted woven fabric composites were tested in tension-tension fatigue. In contrast to results from static testing, the effects of low energy impact damage in a fatigue environment were found to be the critical element leading to failure of the specimen. This difference emphasizes the need to identify and understand the fatigue damage mechanism. A relatively new non-destructive inspection technique using infrared thermography was found to be a very useful tool in detecting damage initiation and growth. Furthermore, this technique supplies valuable information to the characterization of the operating fatigue damage mechanism(s). Fatigue leads to a degradation of material properties. Consequently, in connection with impact induced local stress raisers, fatigue produces continuously changing non-uniform stress fields because of stress redistribution effects. Other models addressing evolution of fatigue damage in composite materials have not been able to simulate evolving non-uniform stress fields. Therefore. in the second part of this paper. an analytical/numerical approach capable of addressing these issues is also proposed.

Dynamic response and fracture of composite cylinders

The fracture behavior caused by the dynamic response in a composite cylinder subjected to a moving pressure was investigated. The resonance of stress waves can result in very high amplitude and frequency strains in the cylinder at the instant and location of pressure front passage, when the velocity of the moving pressure approaches a critical velocity. The stress wave with high magnitude, while short in duration may not cause immediate structural failure; however, it could accelerate the propagation in a cylinder with an initial imperfection and shorten its fatigue life. The fracture mechanism induced by dynamic amplification effects is especially critical for overwrapped composite cylinders because of the multi-material and anisotropic construction, thermal degradation in material properties, and a design goal inherent in light-weight structures.

Butt-coupling loss of 0.1 dB/interface in InP/InGaAs multiple-quantum-well waveguide-waveguide structures grown by selective area chemical beam epitaxy

The lateral coupling of waveguiding structures in both and directions is studied using embedded selective area epitaxy by chemical beam epitaxy. All growth steps are carried out under the same growth conditions on InP substrates misoriented by towards . Both planar and selectively grown material exhibits bright luminescence and narrow PL linewidths (8 meV FWHM at 4 K), up to the lateral junction. Moreover, no degradation of the original material properties is observed after regrowth. SEM images show very flat layers and excellent lateral coupling for all four types of junctions. After reactive ion etching of waveguide ridges, the optical coupling losses have been determined using a Fabry-Pérot setup at 1530 nm (TE polarization). Values of 0.1 dB/interface with excellent uniformity are presented. From our results we conclude that, by optimization of the sample preparation prior to regrowth, values of 0.1 dB/interface can be obtained reproducibly for both perpendicular coupling directions.

Effect of novel paint removal processes on the fatigue behavior of aluminum alloy 2024

Conventional paint removal processes, based on application of chemicals and abrasion, are becoming inadequate for modern aircraft structures. In addition they are associated with severe environmental problems, mainly due to the production of hazardous waste. Several alternative novel techniques are being developed. However, the aspect of material property degradation due to the application of these new paint removal techniques has not been addressed adequately. The application of laser radiation (carbon dioxide and excimer), as well as plasma etching, has recently been associated with significant ductility deterioration and fatigue life extension. Residual stress measurements, roughness measurements and fractographic analysis were employed in order to rationalize the effect of these novel paint removal processes on the fatigue behavior of aluminum alloy 2024. The observed enhancement of fatigue life is attributed to the development of compressive residual stresses during paint removal processing. At low fatigue stresses, the magnitude of the residual stress correlates with the relative enhancement in fatigue life for the three processes investigated. The effect of surface roughening towards decreasing fatigue life is surpassed by the effect of residual stresses in extending fatigue life. Finally, the decrease of toughness and associated damage tolerance ability which follows the application of paint removal processes has been confirmed by fractographic measurements.

A model for predicting the damage and environmental degradation dependent life of SCS-6/Timetal®21S [0] 4 metal matrix composite

A method is developed herein for predicting the life of a continuous fiber titanium metal matrix composite. As a part of the research effort, the titanium metal matrix composite, SCS-6/Timetal®21S [0] 4, has been fatigue tested at 482°C and 650°C. Additional specimens have been environmentally degraded at 700°C and then fatigued at 482°C to failure. The research focuses on initial oxygen dissolution and its effect on the life of the material. The life-limiting physical mechanisms identified from the experiments are material inelasticity, surface embrittlement, and subsequent surface cracking, fiber/matrix debonding, fiber-bridging, and eventual fiber failure. A model incorporating all of these physical phenomena has been developed herein. The model utilizes the finite element method coupled with models for material inelasticity, surface embrittlement, and crack propagation. Material inelasticity is predicted using Bodner's unified viscoplastic model. Crack propagation is modelled via the inclusion of cohesive zones. Surface embrittlement is accounted for by degrading material properties. Both monotonic and fatigue loadings have been modelled at 482°C and 650°C for degraded and undegraded specimens. Results indicate that surface crack propagation rates are significantly slower when matrix viscoplasticity is included in the model instead of elasticity. Furthermore, surface cracking in environmentally degraded specimens enhances fiber stresses compared to undegraded specimens. This difference apparently leads to the premature failure of the degraded composite.

Butt-coupling loss of 0.1 dB/interface in InP/InGaAs MQW waveguide-waveguide structures grown by selective area chemical beam epitaxy

The lateral coupling of waveguiding structures in both [0 1 1] and [0 1 ̄ 1] directions is studied using embedded selective area epitaxy by chemical beam epitaxy. All growth steps are carried out under the same growth conditions on (1 0 0) InP substrates misoriented by 0.5° towards (1 1 1)B. Both planar and selectively grown material exhibits bright luminescence and narrow PL line widths (8 meV FWHM at 4 K), up to the lateral junction. Moreover, no degradation of the original material properties is observed after regrowth. SEM images show very flat layers and excellent lateral coupling for all four types of junctions. After reactive ion etching of waveguide ridges, the optical coupling losses have been determined using a Fabry–Perot setup at 1530 nm (TE polarization). Values of 0.1 to 0.2 dB/interface with excellent uniformity are presented for both perpendicular coupling directions. From our results we conclude that by optimization of the sample preparation prior to regrowth, values of 0.1 dB/interface can be obtained reproducibly.

Inhibited Carbon-Carbon Composites: Isothermal and Fatigue Exposure

The oxidation process is examined under isothermal, cyclic thermal and thermomechanical fatigue conditions for inhibited carbon-carbon composites. Mass loss and material property degradation assessment is undertaken with subsequent exploratory nondestructive testing utilizing DMA and PUCOT techniques. An analytical diffusion model is developed to evaluate oxidation degradation for selected exposure conditions. The results are then compared with the experimental data generated. Nondestructive testing techniques introduced along with the diffusion model successfully trace the shear and axial modulus degradation of inhibited carbon-carbon composite as oxidation progresses. These results when compared with thermomechanical data establish the significance of the combined temperature and mechanical load on the overall material characterization of carbon-carbon laminates.

Carburization of high temperature PM-materials

Carburization of powder metallurgically processed materials for high temperature use is interesting for several reasons: Production of carbide powders e.g. for hard metals, formation of carbide layers on bulk materials to improve corrosion and/or wear resistance and degradation of material properties like strength and ductility by metal-carbon reactions. Molybdenum, TZM (Mo-0.5Ti-0.07Zr-0.05C) and tungsten are finding wide application as construction material in high temperature furnaces which are operated under high vacuum or inert/protective gas conditions. If grafite is used in the same system reactions of the refractory metal components with carbon containing species have to be considered. Therefore a variety of examinations was performed on the carburization characteristics of molybdenum and tungsten (and alloys), especially at temperatures above 1200°C where carburization rates become technically relevant. High temperature oxidation resistant alloys like steels or Ni- and Co-based superalloys can withstand severe carburization as long as their surfaces are covered with tight, protective chromia scales. In case of porous or cracked scales or conditions where chromia is not stable anymore a front of carburization proceeds through the materials - frequently along the grain boundaries. The PM-materials Ducropur (pure chromium) and PM 2000 (Fe-19Cr-5.5Al -0.5Ti-0.5Y 2 O 3 ) show distinctly lower carburization rates: Ducropur forms tight chromium carbide layers, whereas PM 2000 is nearly unaffected up to 1100°C because of its tight and stable alumina scale.

Fracture and material degradation properties of cortical bone under accelerated stress.

The fracture stress and material property degradation of bovine cortical bone specimens were investigated experimentally under accelerated cyclic tensile stress testing. The fracture stress of a typical specimen was found from a static tensile test, and the cyclic loading/unloading was calculated as a percentage of this fracture stress. The results of accelerated cyclic stress tests were compared to monotonically increased static tests to determine if loading/unloading has an effect on the damage mechanism in bone. It was found that fracture stress of the bone increases due to accelerated stress cycling whereas the modulus decreases in a logarithmic fashion with increasing cyclic stress.

Strength Prediction for Mechanical Joints in Laminated Composite Plate Using Progressive Failure Analysis

An investigation into the pinned joint strength of laminated composite plate was performed. Numerical prediction of joint strength using ply material properties and geometrical parameters, such as width and edge distance to hole diameter ratios has been made. Six failure criteria with material properties degradation rules were proposed to model the joint failure behaviour. The technique of progressive failure analysis was used to simulate the ultimate failure load of mechanical joints in laminated composite plate. Numerical results were compared with experimental data. They showed excellent agreement.

Strength Prediction for Mechanical Joints in Laminated Composite Plate Using Progressive Failure Analysis

An investigation into the pinned joint strength of laminated composite plate was performed. Numerical prediction of joint strength using ply material properties and geometrical parameters, such as width and edge distance to hole diameter ratios has been made. Six failure criteria with material properties degradation rules were proposed to model the joint failure behaviour. The technique of progressive failure analysis was used to simulate the ultimate failure load of mechanical joints in laminated composite plate. Numerical results were compared with experimental data. They showed excellent agreement.

Multiaxial fatigue behaviour of unidirectional plies based on uniaxial fatigue experiments—II. Experimental evaluation

To validate the generalized residual material property degradation model, developed in the first part of this paper, an experimental program was conducted using graphite/epoxy AS4/3501-6 material. The main advantage of the model is that it can simulate the fatigue behaviour of a unidirectional ply under multiaxial state of stress and arbitrary stress ratio by using the results of uniaxial fatigue experiments. The fatigue behaviour of the unidirectional ply under combined tensile matrix stress and in-plane shear stress (biaxial conditions) is considered. To prepare input for the model the material must be fully characterized in the matrix direction and in-plane shear condition under static and fatigue loading. For this purpose the static strength, residual strength and fatigue life of the material in the transverse direction and under in-plane shear conditions are measured. The results obtained by these series of experiments are used as input data for the model and the behaviour of a 30° off-axis specimen under uniaxial fatigue load, which in fact is a unidirectional ply under biaxial stress conditions, is simulated by the model. To evaluate the results calculated by the model, a series of uniaxial tension-tension fatigue tests are performed on unidirectional 30° off-axis laminates. A comparison between S- N curves (curve fitted mathematically and simulated by the model) shows the one simulated by the model slightly overestimates the fatigue life of the 30° off-axis specimen. However, the S- N curve simulated by the model is located reasonably inside the range of experimental scatter. It should be noted that the model proposed in this study is a deterministic model and further research is needed to couple a probabilistic for a more realistic simulation.