Quantified metrics from nondestructive evaluation (NDE) data were used to predict residual fatigue life of corroded AA2024-T3 specimens. The specimens were corroded under a wide range of conditions to achieve a range of corrosion morphologies, including a mix of pitting, intergranular, and general corrosion. Ultrasonic testing (UT), white light interference microscopy (WLIM), and microradiography images were then analyzed to extract damage metrics. The corroded specimens were then fatigued to failure. The resulting data were used to obtain an excellent correlation between the NDE metrics and the residual fatigue life using multiple linear regression (MLR). While there are several important metrics that can be extracted from UT and WLIM, the correlation with microradiography metrics was weak in this damage range. Residual life models including WLIM surface roughness measurements performed better for lightly damaged specimens than those using only UT measurements. However, the addition of microradiography metrics resulted in no significant improvement to the correlation models.

In many applications, the onset of damage occurs at the component surface due to plastic deformation, fretting, wear, corrosion, or crack nucleation. One result of these processes is a change in the surface topography. Therefore, the first step in nondestructive evaluation of a component is a thorough evaluation of the surface conditions. Inspection techniques that characterize the damage extent on the surface often present the results as a fraction of the component surface area. For damage originating from environmental or mechanical conditions, the severity of the defect or damage can also be described quantitatively by the three-dimensional surface topography. When using many common inspection methods, information regarding the depth of penetration into the surface layers, or protrusion above the surface is difficult to obtain. In addition, when the defects are of submicron or nanometer scale, high resolution three-dimensional inspection techniques are required.

Acoustic (ultrasonic) inspection is among the most widely used NDE techniques. Ultrasonic techniques have proven to foster early detection of defects and enhance the understanding of failure processes. Acoustic imaging techniques and for instance Scanning Acoustic Microscopy (SAM) provides very detailed information about material defects with high spatial and depth resolution. It enables studying elastic properties, characterization of interfaces and detecting small defects, for instance, cracks in metals, polymers and ceramics. This chapter will consider some special applications of acoustic imaging techniques concerning recent aging aircraft problems. Characterization of corrosion, corrosion protective coatings, as well as fatigue damage are in the scope of the research reported here.

Maintenance and reliability of aircraft is a major safety concern and economical factor. Many civilian and military aircraft have been in service for 35 years or more. Aircrafts that were originally designed for a service life of 20 years are currently considered for life extensions of up to 80 years. The cost of corrosion and fatigue related maintenance on these aging structures has increased dramatically. A study conducted in 1998 showed that the direct costs of corrosion maintenance to the United States Air Force were $775 million/year [1] . These costs continue to rise in spite of Air Force structure changes resulting in a 20% reduction in the overall fleet. In an effort to reduce these maintenance costs, several programs have been initiated for the development of methods to manage and control corrosion and fatigue damage in aging aircraft.

The fretting fatigue tests of Ti-6Al-4V were, carried out using Cu-Ni plasma coated and as-received pads (un-coated pads). Fretting fatigue tests of Ti-6Al-4V, where Cu-Ni plasma coated and as-received pads were used, repeatedly were also carried out. The characterization of pads before and after testing were carried out in order to investigate the mechanism of fretting fatigue of Ti-6Al-4V in contact with the Cu-Ni plasma coated pad used. Cu-Ni plasma coating on the pads prolonged the fretting fatigue life of Ti-6Al-4V. The fretting fatigue life of Ti-6Al-4V decreased with repeated use of test when the Cu-Ni plasma coated pads. On the other hand, the opposite trend was observed when the as-received pads were repeatedly used. The surfaces of Cu-Ni plasma coated pads were smoother after testing than before testing due to the plastic deformation. On the other hand, the surface of as-received pads were rougher after testing than before testing.

Current requirements for structural integrity, product safety, process feedback control and smart structures require methods for monitoring material properties and component performance to improve quality control and decrease maintenance costs. In most applications the onset of damage occurs at the component surface due to plastic deformation, fretting, corrosion, or crack nucleation. Therefore, the first step in nondestructive evaluation of a component is a thorough evaluation of the surface conditions. White light interferometry provides a fast, non- contact method of characterizing the 3D surface topography with a 3 nm vertical resolution. Thus far, white light interferometry has not been widely utilized as a nondestructive inspection tool. In this paper, the applicability of white light interferometry for monitoring damage progression for applications that are surface condition sensitive will be examined. These applications include corrosion initiation and progression, fretting damage characterization, combustion byproduct characterization and surface crack detection. For each application, a method for obtaining a quantitative measure of damage and a metric that can be used to determine remaining component life will be discussed.

A method to characterize the deformation zone at a crack tip and predict upcoming fracture under load using white light interference microscopy was developed and studied. Cracks were initiated in notched Ti-6A1-4V specimens through fatigue loading. Following crack initiation, specimens were subjected to static loading during in-situ observation of the deformation area ahead of the crack. Nondestructive in-situ observations were performed using white light interference microscopy. Profilometer measurements quantified the area, volume, and shape of the deformation ahead of the crack front. Results showed an exponential relationship between the area and volume of deformation and the stress intensity factor of the cracked alloy. These findings also indicate that it is possible to determine a critical rate of change in deformation versus the stress intensity factor that can predict oncoming catastrophic failure. In addition, crack front deformation zones were measured as a function of time under sustained load, and crack tip deformation zone enlargement over time was observed.

The effects of material constitutive behavior, tooling design, and friction conditions on metal flow, stress fields, and the tendency for tensile fracture during equal channel angular extrusion (ECAE) were established using a finite element modeling (FEM) technique. Three different material behaviors, typical of those encountered during cold and hot working, were investigated; these comprised (i) strain hardening, (ii) rigid, perfectly plastic, and (iii) flow softening types of behavior. The tooling geometries consisted of a so-called “simple” design with no moving channel members and a “complex” design with a sliding bottom floor. The FEM results indicated that the most uniform flow was obtained during ECAE of a strain-hardening material having a low strain-rate sensitivity in tooling with a sharp inner corner (“front leg”) radius. The ECAE of materials with other constitutive behaviors or in tooling with a radiused front leg showed some degree of flow nonuniformity, even away from the head and tail of the extrusion. Tooling design and material properties were also predicted to have an important influence on the tensile stresses and hence tensile damage developed during ECAE. The FEM results were validated using visioplasticity and fracture observations for AISI 4340 steel and a near-gamma titanium aluminide alloy.

The objective of this work is to develop a methodology for predicting material failure by evaluating changes in material characteristics directly prior to unstable crack growth. In an effort to establish and document these changes, several Ti-6Al-4V flat, notched samples have been subjected to fatigue loading to partial life. After a fatigue crack was initiated characterization was performed during in-situ application of an incrementally increased static load. White light interference microscopy was found to be a successful nondestructive tool for characterizing changes in the deformation zone in front of the crack tip. A relationship between the applied load and the surface area of the deformation zone was obtained. This relationship was exponential directly prior to failure of the specimen. Surface observations during in-situ testing allowed samples to be brought near to failure without complete fracture. This result can have important applications for optimizing the service life of ah-frame structural components.

Plastic flow behavior and globularization kinetics during subtransus hot working were determined for Ti-6Al-4V with three different transformed beta microstructures. These conditions consisted of fine lamellar colonies, a mixture of coarse colonies and acicular alpha, and acicular alpha. Isothermal hot compression tests were performed on cylindrical samples at subtransus temperatures and strain rates typical of ingot breakdown (i.e., T∼815 °C to 955 °C, \(\bar \varepsilon \)∼0.1 s−1). For all three material conditions, true stress-true strain curves exhibited a peak stress followed by noticeable flow softening; the values of peak stress and flow softening rate showed little dependence on starting microstructure. On the other hand, the kinetics of dynamic globularization varied noticeably with microstructure. By and large, the globularization rate under a given set of deformation conditions was most rapid for the fine acicular microstructure and least rapid for the mixed coarse-colony/acicular structure. At temperatures close to the beta transus, however, the difference in globularization rates for the three microstructures was less, an effect attributed to the rapid (continuous) coarsening of the laths in the acicular microstructure during preheating prior to hot working. The absence of a correlation between the globularization kinetics and the observed flow softening at low strains suggested platelet/lath bending and kinking as the primary deformation mechanism that controls the shape of the flow curves.

The occurrence of cavity initiation and gross, free-surface fracture during subtransus hot pancake forging of Ti-6Al-4V with a transformed beta (colony) microstructure was established. Cavity initiation mechanisms were one of two distinct types. At temperatures approximately 75 °C or more below the beta transus temperature ( T β ), cavity initiation occurred at relatively low strains in the beta phase lying between the grain-boundary alpha phase and the lamellar colonies. By contrast, at temperatures near the transus (i.e., T≈ T β −25 °C), cavity initiation occurred at much larger strains as a result of microfracture of partially-to-fully globularized alpha phase. Finite element method (FEM) modeling of the pancake forging process revealed that secondary tensile stresses had been developed in the regions that had exhibited cavitation/fracture. The FEM analyses were used to correlate both the cavity initiation and the gross free-surface fracture results to previous observations from uniaxial hot tension tests in which identical damage mechanisms had been observed. The tensile work criterion of Cockcroft and Latham (C+L) gave moderately good (quantitative) correlation between the forging and uniaxial tension behaviors. An alternate comparison based on the Rice and Tracey cavity growth model gave reasonable predictions of free-surface fracture but tended to overestimate the incidence of subsurface cavity initiation.

Ti-6Al-4V alloy specimens cut from a forged plate with a duplex microstructure, similar to the microstructure used in fan blades (i.e. consisting of similar to 50 pet, equiaxed primary alpha phase and similar to 50 pet, of fine lamellar transformed alpha plates) were tested under conditions of high-cycle fretting fatigue. The contact geometry (flat pad on a flat specimen), the normal stress, as well as the cyclic stress were selected such that the mixed, slip-stick regime prevails during the experiments. Following testing, the specimens as well as the fretting pads were characterized by a variety of techniques including white light interference profilometry, scanning electron microscopy, ultrasonic force microscopy, microhardness testing, and electron dispersive spectroscopy (EDS). The results revealed that the surface roughness (mainly expressed by the height and spacing of the asperities) of the slip region increases compared to the roughness of the stick, and non-contact ones. In addition, at the higher spatial frequencies, the power spectral density (PSD) of the slip region increases compared to the PSD of the stick and non-contact regions, thus revealing that an increase of the population of the smaller size asperities occurs. The microstructure of the material below the slip zone was found to be transformed to a finer one; and the percentage of the transformed beta phase has been decreased substantially. The elastic property variation (e.g. Young's modulus) of this region was determined by ultrasonic force microscopy; the results revealed that in contrast to what found for the bulk of the material, there are significant local differences of the elastic properties inside the fretting-affected zone. In addition, the changes in the plastic behavior of the region below the slip zone, was determined using microhardness measurements. It was found that this transformed microstructure area, has also a higher hardness compared to the hardness of the bulk structure. Both elastic and plastic property variations were attributed to the increased percent of alpha phase and the decreased amount of beta in the transformed zone, since the former phase exhibits higher elastic moduli as well as flow stresses. In addition, changes in the concentration of the oxygen at the specimen's surface as well as inside the transformed zone were examined by energy dispersive spectroscopy (EDS). The EDS analysis revealed a high concentration of oxygen on the specimen's surface only at the slip region of the two contacting materials. This finding indicates that elevated temperatures were developed during the fretting fatigue testing, which enable the diffusion of oxygen from the atmosphere to the alloy. However, within the transformed zone, no detectable differences (with respect to the bulk) in the oxygen concentration were revealed. This finding allowed us to assumed that stress induced transformation is the most probable mechanism.

The parameters that govern the life of metallic materials under conditions of fretting fatigue may be divided into two broad categories. The first category deals with the materials properties and characteristics while the second includes the externally imposed loading conditions and contact geometry. The two materials in contact may either stick, slip or stick-clip with each other. It has been shown that the life reduction is highest under partial slip. The objective of the present research effort is focused to the prediction of the particular fretting fatigue regime and hence get an estimate of the life reduction of a particular materials/component. The above may be accomplished by using as input to a specific model, a series of data obtained from non-destructive and other characterization techniques. To this end, the bodies in contact is developed and hence the different materials as well as external parameters which influence the process are identified. Although the external are implied by the should be determined quantitatively. This is accomplished by white light interference profilometry, which was used to characterization were used as input into the model to predict the actual contact conditions. Experimental results concerned with the fatigue life were plotted on the fretting maps; the fretting fatigue regimes indicated by the latter enabled the interpretation of the experimental data.

Ti–6Al–4V alloy specimens were tested under conditions of fretting fatigue, with the contact geometry, the normal stress, as well as the cyclic stress selected such that the mixed, slip-stick regime prevails during the experiments. Following testing, the specimens were characterized using white light interference profilometry, scanning electron microscopy, microhardness, and electron dispersive spectroscopy (EDS). The results revealed that the surface roughness of the slip region increases compared to the roughness of the stick, and non-contact ones. In addition, at the higher spatial frequencies, the power spectral density (PSD) of the slip region increases compared to the PSD of the stick and non-contact regions, thus revealing that an increase of the population of the smaller size asperities occurs. The microstructure of the material below the slip zone was found to be transformed to a finer one; and the percentage of the transformed β phase has been decreased substantially. This area of the transformed microstructure, has also a higher hardness compared to the hardness of the bulk structure. EDS analysis revealed a high concentration of oxygen on the specimen’s surface at the slip region of the two contacting bodies. This finding indicates that elevated temperatures are developed during fretting fatigue and enable the diffusion of oxygen from the atmosphere to the alloy.

Cavity initiation, growth, and coalescence phenomena during the hot tension testing of Ti–6Al–4V with several different transformed β-microstructures were established to obtain an understanding of the failure process during subtransus hot working. Samples with either a colony-type microstructure (containing grain-boundary α) or a martensitic, acicular α-microstructure were pulled to failure at strain rates between 0.01 and 3 s−1 and various subtransus temperatures ranging from 540 to 955°C. Metallographic analysis of sectioned tension samples with colony microstructures revealed that cavity initiation occurred at very low strains (≾0.2) irrespective of strain rate at temperatures below 900°C due to an apparent incompatibility of deformation between the grain-boundary α-layer and the Widmanstatten side plates. Initiation strains were somewhat higher (∼0.2–0.5) for samples with the acicular α-microstructure tested at temperatures below 900°C; however, failure was still intergranular. Subsequent cavity growth was more rapid for colony microstructures than that for the martensitic α-microstructure at these temperatures. Above 900°C, however, substantially higher cavity initiation strains and lower cavity growth rates were obtained for both microstructures. These latter behaviors were ascribed to the presence of a large volume fraction of ductile β-phase and moderate values of the strain-rate sensitivity of the flow stress, respectively.