During the past several years a number of design-centered “global” approaches have been offered that deal both with materials synthesis and with the manufacture of materials into finished products. Although the structure and constituent components of these approaches differ because of their different end purposes, they share major dependencies upon design, theoretical modeling, extensive computations, and confirming measurements using various NDE techniques. Materials-by-Design (MBD) is an example of these approaches that is focused on the synthesis of materials. As noted by Eberhard[l], a principal purpose of Materials-by-Design is to produce materials with prescribed macroscopic material properties by designing and controlling material structures at the atomic and molecular levels. At the other end of the spectrum, Unified Life Cycle Engineering (ULCE)[2] is an example of a “global” model for manufacturing. In this case, emphasis is placed upon the development of ways to predict the total set of important properties of a product–performance, quality, reliability, maintainability, and life cycle costs–at the designer's board. Taken together, these approaches offer the opportunity for designercontrolled materials with specified material properties to be fabricated into components of specified performance, quality, reliability, and cost. Even though this combination represents an idealistic vision which may never be perfectly realized, the potential payoff is so large that even imperfect realization may be worth a significant investment. Efforts in these directions can now be made because of the convergence of theoretical, instrumental, and computational techniques.

In recent years classical nondestructive testing techniques for detecting macroscopic defects have been augmented by more sophisticated nondestructive evaluation methods for characterizing the microstructure and associated physical and chemical properties of materials. This paper will briefly describe several such nondestructive evaluation methods developed in the Center for Nondestructive Evaluation (CNDE) at The Johns Hopkins University.

Several material properties and microstructural features can be determined or monitored by measuring ultrasonic velocity and/or ultrasonic attenuation. Conventional techniques which use piezoelectric transducers for generation and reception have several limitations, in particular in the case of materials at elevated temperature, of samples of complex shapes, and in regard to the detection bandwidth. These limitations are eliminated by laser-ultrasonics, a technique which uses lasers for generation and detection of ultrasound. Following a review of the various principles and methods used for generation and detection, we discuss the use of laserultrasonics for velocity and attenuation measurement. Examples of application to various materials are presented.

We describe a technique by which ultrasonic measurements can be made in the picosecond time domain. A light pulse (duration of the order of 0.1 psec) is absorbed at a surface, thereby setting up an elastic stress. This stress launches an elastic pulse into the interior. The propagation of this strain, including its reflection at interfaces within a microstructure, is monitored through measurements of the time-dependent changes of the optical reflectivity. These measurements are made using a time-delayed probe pulse. In these experiments the spatial length of the elastic pulses can be as short as 50 Å. We can therefore use this technique to perform a nondestructive ultrasonic evaluation of thin-film microstructures. We describe here results we have obtained which demonstrate the application of the method to the study of the mechanical properties of thin films, the geometry of microstructures, and the quality of bonding at interfaces.

An Inconel Alloy 600 C-ring was subjected to various strain levels and the deformation process was monitored by a Computer Aided Rocking Curve Analyzer (CARCA). A large grain population was sampled, and the calibration curve of average rocking curve halfwidth of the individual grains relating to the nominal strain was established. The strain distribution as a function of the angular position along the peripheral surface layer and layers at different depth distance was obtained. Up to C-ring closure at the nominal strain of 3.3% at the apex, the induced plastic strains were confined to a surface layer of 30–40 gm in depth.The largest strain and strain gradients below the surface occurred at the apex and near apex region. The extent and the spread of microdeformation inhomogeneity increased with applied strain. At ring closure some grains exhibited large plastic strains while others exhibited only small plastic strains or were not affected by the deformation process at all. These experimental results were not in agreement with the current theoretical understanding of the deformation of C-ring since these theories did not take shape changes into account. When such changes were included, good agreement on the angular strain dependence for the apex and near apex region were achieved between experiment and theory. It was concluded that the CARCA X-ray method can be a useful research tool in aiding and guiding mathematical modeling of non-linear inelastic behavior of solids by disclosing important microstructural and micromechanical aspects.

The axial and tangential components of residual strain have been measured through the region of an electron beam weld in Zr 2.5 wt.% Nb tube. Marked changes in grain orientation in the tube occur as far away as 10 mm from the weld-centre. In particular a large fraction of the grains which had an [0002] direction aligned tangentially in the original tube were reoriented with this direction aligned axially. These grains showed high (5 x 10-4) tensile strain. Compressive strains (-5 to - 10 x I0-4) were measured in the region 5 - 10 mm from the weld-centre in those grains with [1010] directioný aligned axially. The hoop straint vary from tensile (maximum 5 x 10-4) to compressive (maximum -5 x 10-4) as a function of distance from the weld centre.

In this study the progression of thermally and mechanically induced internal strains (stresses) in metal-matrix composites was investigated by X-ray methods. The materials studied were whisker-reinforced 2124 Al-SiC(w) and 6061 Al- SiC(w) composites. X-ray diffractometry was used to measure thermally induced stresses on samples cycled from ambient to 280°C. Significant variations in residual stress values were observed in the matrix depending on the location and direction of the measurements with respect to the whisker orientation. The determined stress states of the as-processed and the thermally cycled samples were evaluated with continuum models. The microstrains in composites induced during processing and tensile loading were also investigated by nondestructive means. Individual grains of the matrix were analyzed by rocking-curve measurements using a modified X-ray doublecrystal diffractometer. The relationship between the plastic deformation induced by applied loads and the progression of the microstrain/excess-dislocation values was determined.

X-ray diffraction has long been the mainstay for materials characterization in the laboratory. This characterization includes the determination of phase composition, residual stress, microstrain, grain size, and crystallographic texture of polycrystalline metals, ceramics, and minerals. The analytical capabilities of XRD techniques have been expanded recently by the application of computer control to data collection and processing. These capabilities include the identification of irregularities in metals and ceramics that are caused by processing and fatigue damage, as well as the apriori prediction of processing anomolies. While the above applications have been largely restricted to the laboratory, the possibility for exploitation of the nondestructive nature of x-ray diffraction for inprocess evaluation of materials is now being realized. The availability of computer-controlled position-sensitive x-ray detectors can now provide rapid, non-contacting, in-process interrogation of materials. The examples of nondestructive characterization illustrated in this paper will be those that can be used for process control and/or damage assessment.

The fourth order orientation distribution function (ODF) coefficients of textured low carbon steel sheets were determined nondestructively from the anisotropy of the velocity of Lamb (So) and SHO plate waves measured using electromagnetic acoustic transducers (EMATs). The three coefficients (C411, C412, C413) are calculated from five velocity measurements made in three directions in the rolling plane of the sheet using the Hill approximation by an iterative numerical method. The coefficients were also determined from Young's modulus measurements based on a resonance technique and are compared to those obtained ultrasonically. The comparison with coefficients determined from X-ray diffraction pole figures permits adjustment of the C411 coefficient and then very good agreement is obtained. The plastic strain ratios (R-values) of the steel samples are predicted from the adjusted coefficients using a series expansion method based on the Taylor theory of crystal plasticity. These are compared with experimental measurements and again good agreement is displayed.

This paper will propose a new technique to characterize texture of rolled plates of cubic crystallites. This technique uses information from ultrasonic velocities of high order plate mode to improve the estimation of orientation distribution coefficients (ODC's), especially W400. Also discussed will be the generalization of this technique to the case of hexagonal crystallites.

Significant changes in microstructure occur in many fossil plant components that operate for long times at elevated temperatures. Some examples include hydrogen damage in steam-generating boiler tubes, carbide coarsening in superheater tubes, creep cavitation in the heat-affected zones of thick-section weldments, and temper embrittlement in turbine rotors and disks. Quantitative estimates of the remaining useful lives of components affected by these damage mechanisms require that the degree of damage be determined and related to the life-limiting mechanisms.

The paper discusses recent progress in the development and field demonstration of nondestructive techniques for characterizing the damage types listed above. This research has produced several products that have achieved or are close to commercial status. The research has also helped to define some of the limitations related to access, sensitivity, and signal interpretation of the nondestructive techniques. The paper discusses how these limitations can often be mitigated by using several nondestructive techniques in concert to obtain a more complete picture of material condition or by removing small specimens for detailed laboratory analysis.

The early detection of material degradation by NDT is of basic importance for safe and economic operation of many industrial components. Depending on the type of component as well as on service exposure conditions, NDT-techniques, which are sensitive to deformation, erosion, wear, porosity, microcracks and so on, are required. At present, only very few techniques are available for monitoring early stages of degradation. An overview of NDT-techniques already in use as well as under development in Germany for this problem is given. Special emphasis will be laid on the presentation of recent advances towards better detectability and objectivity.

Hydrogen attack and creep damage may reduce the fracture toughness as well as the strength of steels. This reduction is caused partially by the presence of cavities and microcracks at the grain boundaries. A large reduction in fracture toughness can make some components unsafe for operation. Ultrasonic tests performed on damaged samples showed a decrease in wave velocity and an increase in backscatter. The longitudinal-wave velocity decreased more than shear-wave velocity. Where the fracture toughness was reduced by 43 percent, samples with hydrogen attack showed a tenfold increase in backscatter. Such results demonstrate the potential for ultrasonic nondestructive testing to quantify damage and assess the component's remaining life.

This paper reviews the use of positron annihilation methods for technological applications, particularly the use of positron annihilation gamma ray lineshape analysis for the non-destructive assessment of static deformation, machining processes, high cycle fatigue and creep in metal and alloy componen:s. The paper includes description of a transportable lineshape analysis system recently developed for field applications.

Many structural components are exposed to thermal environments that promote aging-induced changes in material microstructure and properties. The extent of these changes may lead to a reduction in the design safety factors and, as a worst case, premature failure of the component. In order to illustrate how nondestructive measurements can be used to indicate these changes, ultrasonic measurements were conducted on stainless steel. Specimens from a high-carbon, Type 304 stainless steel were subjected to various thermal treatments to introduce differing precipitation morphologies. Specimens were examined using transmission electron microscopy to quantify the size, density, and distribution of the precipitates formed. Ultrasonic measurements were performed to assess sensitivity to the change in precipitation levels. Shear-wave birefringence provided the most consistent measurements for tracking precipitate morphology changes. Birefringence velocity differences increased as precipitate size and density increased on grain boundaries and as additional precipitates formed in the matrix. The ability to nondestructively monitor the precipitation process is important since susceptibility to environmentally assisted cracking and low-temperature embrittlement correlated with this microstructural change.

Cathodic charging of hydrogen into 4340 steel produces drastic deterioration of mechanical properties such as the notch tensile strength, depending on current density, charging time, and the poison used as an inhibitor of hydrogen recombination and bubble formation at the specimen surface. Fusion of charged samples permitted hydrogen content to be measured and revealed a linear increase in the latter with charging current density. The change in the Doppler spectrum sharpness parameter Δ(P/W) deviates negatively with cathodic charging time but inversely with the charging current density (fugacity). Thus a low-fugacity hydrogen charging appears merely to screen existing positron trap sites, whereas a high-fugacity charge also creates new defect traps. This would result in a lesser net screeninig effect which would be seen as a smaller negative deviation in Δ(P/W) as observed. For cumulative charging, however, one finds oscillatory behavior in (P/W) as in earlier cumulative charging experiments with Ni. This behavior indicates that hydrogen screening of positron traps was followed by new positron trap site creation and that this sequence of events occurred repeatedly and in that order.

This paper deals with the relationship between the plastic deformation damage and microstructure by means of Acoustic Emmission. The plastic deformation behavior of AISI 4340 steel of various microstructures was investigated in both the tensile and creep-fatigue testings with a view to providing new insights into properties of high performance steel. Based on Theory of Damage Mechanics, a creep-fatigue law was derived and formulated. The reason of early failure and the service life prophecy of high strength steels was studied.

The damage micromechanism of four stages was studied by the optical microscope, scanning electron microscope, and microhardness tester. It seems that the nucleation and the growth of the voids at the martensiteferrite interface is the dominant mechanism of damage. The monitoring of Acoustic Emmission indicated that the plastic deformation did not appear in the circulating hardening stage. But in the circulating softening stage, the accumulation of the plastic deformation and the creep-fatigue damage become more and more severe. The total energy of Acoustic Emmission was successfully applied to measure the degree of the damage caused by the plastic deformation.

Residual lifetime analysis of power plant components requires information on the degree of degradation of the material. In case of hightemperature creep, material damage can be related to cavity formation or to the accumulated creep strain. At present, only metallographic replication technique is widely used for in-service inspection to detect creep cavitation. Other NDT-techniques like ultrasonic velocity and electrical resistivity, which have potential for detection of low pore concentrations, are being developed for practical application. The use of capacitive strain gages yields encouraging results for the long-time monitoring of creep deformation. Potential and limits of these NDT-techniques together with industrial needs for creep damage assessment are reviewed.

Studies have been ongoing at Defence Research Establishment Atlantic on the evaluation of non-destructive techniques for residual stress determination in structures. These techniques have included neutron diffraction, x-ray diffraction and blind-hole drilling. In conjunction with these studies, the applicability of these procedures to aid in metallurgical and failure analysis investigations has been explored. The x-ray diffraction technique was applied to investigate the failure mechanism in several bent turbo blower rotor shafts. All examinations had to be non-destructive in nature as the shafts were considered repairable. It was determined that residual stress profiles existed in the distorted shafts which strongly indicated the presence of martensitic microstuctures. These microstructures are considered unacceptable for these shafts due to the potential for cracking or in-service residual stress relaxation which could lead to future shaft distortion.

The pulse height distribution of a magnetoacoustic emission (MAE) spectrum is expected to be generally Gaussian due to its random nature. The functional form of distribution depends on the microstructure of a ferromagnet since the domain wall-lattice defect interaction produces MAE. The present study investigated the effects of grain boundary characteristics on the properties of MAE spectra obtained by external AC magnetic field-driven domain wall motions. The results show the enhancement of domain wall-defect interaction as more grain boundary disorder is introduced to HY80 steel samples. This was confirmed by the growth of a non-Gaussian-like distribution in the tail section of histograms with increased population of impurities trapped at the grain boundaries causing embrittlement. It is found that the enhancement of domain wall-defect interaction, which is responsible for generation of high amplitude MAE pulses, also tends to reduce the rate of such MAE events by limiting domain wall motions. Application of a stronger AC magnetic field should increase the count rate of high amplitude MAE signals in more embrittled samples.