Effects Of Material Degradation Research Articles

Application of a delamination model to laminated composite structures

A model for progressive interlaminar delamination is presented for laminated composite structures. Instead of a cumbersome 3D description, a computationally efficient 2D technique is adopted which models the laminated structure as an assembly of sublaminates connected through their interfaces. Constraints between sublaminates are removed to represent the presence of delaminations. The use of laminate theory results in jumps in stress resultants across the delamination tip and this helps to avoid dealing with the singular stress field at the delamination front. A stress-based failure criterion is used to predict delamination initiation. Delamination propagation is analysed by adopting a fracture mechanics approach. The major intralaminar damage mode, matrix cracking, is also included in the present analysis. This is detected by a stress-based failure criterion and a ply discount model is used to account for the effects of material degradation. Finite element analysis has been carried out to assess the deformation and the delamination development in a range of typical structures: a double cantilever beam, a cross-ply laminate and some filament-wound composite pipes. Good agreement has been achieved between the predictions and available experimental data. A study of the effect of mesh size shows that a relatively coarse mesh gives sufficiently accurate results. These examples give a useful indication of the versatility and feasibility of the present approach for real structural applications.

The Effect of Material Degradation on Bond Properties of Fiber Reinforced Plastic Reinforcing Bars in Concrete

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A simple, inexpensive, hyperthermal atomic oxygen sensor

With the increasing duration of space flights, the development of a permanent facility in space, and the increasing use of ground-based hyperthermal atomic oxygen sources there is a need for a simple instrument to provide long-term monitoring of the beam flux. Such an instrument can also be used as a diagnostic tool to investigate the material degradation process. Reliance on models of the upper atmosphere to determine the fluence of atomic oxygen is not only necessarily complex but also imprecise due to the strong dependence of oxygen concentration on day/night, latitude, and solar activity. Mass spectroscopy, the traditional method for determining the gas phase species densities at low pressure, is not only expensive but is limited in the area that it can monitor as well as subject to effects of material degradation. Our group has developed a simple and inexpensive dosimeter to measure the atomic oxygen fluence via the change in resistance as the sensor element is gradually oxidized. The sensors consist of thin-film circuit elements of selected material deposited on a suitable substrate. Four-point resistance measurements are used to monitor the change in sensor resistance with respect to time. Results obtained from silver and carbon dosimeters flown on STS-46 (CONCAP-II-01) indicate that such sensors are sensitive enough to monitor the diurnal variations in atomic oxygen distribution and sufficiently durable to last the lifetime of a mission.

Shear Band Initiation of Brittle Damage Materials

Some effects of material degradation on failure mechanisms of brittle damage materials are investigated. Conditions for the localization of deformation within a shear band are established for rate independent damage materials. In contrast with previous work on shear band initiation that relied on plasticity and flow theory formulation, the present study finds it sufficient for the shear band to emerge in the regime of infinitesimal strain for brittle damage materials. Bifurcations from the homogeneous deformation mode in the form of shear bands are captured for loading conditions of plane strain compression and uniaxial compression using conventional continuum damage mechanics formulations. The inclination angle and critical strain for shear band initiation are calculated using well-established mathematical theory. A finite element simulation of shear band initiation for a rectangular mesh deformed in plane strain compression is also presented.

Robust methods of life assessment during creep

The paper presents a simplified, yet robust analysis procedure for structural problems occurring in the high temperature regime where the combined effects of permanent creep deformation and progressive material degradation must be taken into account. The method described here is based on the continuum damage concepts proposed by Kachanov and Rabotnov in the late fifties and early sixties, and on the reference stress notions that originated in the late sixties. These ingredients are combined with well-tried numerical techniques such as finite element procedures to form a basis for rational creep/damage analyses. The excessive computer capabilities required for this type of analysis, together with the scarcity and uncertainty of material data, have led to the need to de-couple the creep stress analysis from the material degradation. This approach gives the designer adequate results with accuracies commensurate with the inherent uncertainties in material data and service loadings, and within acceptable computing time limits. At the same time the robust approach gives the designer the ability to establish which effects are most important whereas the coupled analysis does not. The development of the method is described in detail in the paper and is illustrated with an example of a pressure vessel nozzle. Monte Carlo simulations methods for dealing with randomness of material and geometrical parameters are illustrated in other examples.

長期使用劣化材の信頼性工学的疲労寿命予測

The authers have been attempting to establish a proper method for predicting fatigue lives of structures which have been subjected to material degradation.For this purpose, the present paper describes a statistical investigation on the effects of material degradation on the crack initiation life and on the crack growth rate in a 2¼ Cr-1Mo (SCMV4) steel. The steel studied in this paper has been used in a plant having operated for over 60000 hours and subjected to material degradation due to temper embrittlement. The results obtained for the degraded steel were discussed in comparison with those for the recovered SCMV4 steel.The crack initiation life and crack growth rate in the degraded SCMV4 steel and in the recovered SCMV4 steel followed the log-normal distribution. The crack initiation life was shorter in the degraded steel than in the recovered one at a high probability. The crack growth rate da/dN was higher in the degraded steel at higher stress-intensity-factor-range ΔK levels at a high probability. At lower ΔK levels, on the other hand, the mean of da/dN was a little lower in the degraded steel; nevertheless, a larger variation in da/dN implies that da/dN can become higher in the degraded steel.A Monte Carlo simulation was made on the basis of the data obtained in the present experiments in order to determine the P-S-N diagrams for the degraded and the recovered SCMV4 steels. The obtained P-S-N diagrams statistically revealed that the fatigue life of the degraded steel was shortened at higher stress levels and that its variation became larger at lower stress levels. The larger variation in the life of the degraded steel is attributed to shorter fatigue life in the degraded steel than in the recovered steel at a considerably high probability, although the difference of the mean fatigue lives of the two steels became smaller at lower stress levels.

Fatigue crack growth life prediction of a Cr-Mo steel degraded in long service (1st report, Fatigue crack growth properties in a long-service-degraded material)

To investigate the effect of material degradation on life prediction, fatigue crack propagation in a degraded steel was studied. Fatigue crack growth rate, da/dN, vs stress intensity factor range, ΔK, relationships were obtained for 72 crack tips in degraded 2 1/4 Cr-1 Mo steel due presumably to temper embrittlement during long service, a recovered one made from the above degraded steel and an unused carbon steel S50C. The main results are : (1) High intergranular fracture surface ratio and preceding cracks ahead of main cracks are remarkable characteristics of the degraded steel. (2) The da/dN vs ΔK relationship was not well represented by a single line for the degraded steel. Above 13 to 15 MPa √(m), da/dN in the degraded steel was higher than that in the other two steels. (3) The m and C in the da/dN = C(ΔK)m for each steel tested followed normal and log normal distributions, respectively. (4) The coefficients of variation in the values of m and C were larger in the degraded steel than in the other two steels. (5) Almost the same linear relationship was obtained between log C and m for the three steels tested. (6) The predicted fatigue crack growth life given by Monte Carlo simulation was shorter at higher stress levels, and the scatter of lives was larger in the degraded steel than in the recovered steel.

Use of Integrated Day-Long Stagnation Temperature for Measuring Changes in Solar Collector Materials Properties

Measurements of the maximum temperatures reached by solar-energy-absorbing surfaces provide a useful method for detecting possible degradation in the optical and heat transfer properties of materials used in collectors. This investigation shows that a method based on integrating the absorber temperature rise over ambient and solar irradiation on a daily basis has advantages over alternative test methods based on steady-state measurements of either absorber stagnation temperature or collector thermal efficiency. A nonsteady-state analytical model is developed and used to evaluate the sensitivity of the proposed test method to material degradation and environmental effects. Outdoor data are presented to validate the analytical model and depict typical results that can be expected using the test procedure.

Status of risk assessment analysis for mechanical systems in nuclear plants

In the frame of a short review of the status of risk assessment analysis for mechanical systems in nuclear power plants the following concepts are discussed: confidence limits; Bayesian statistical approach; material degradation effects; Fracture mechanics; and fault tree analysis.