On the interaction of ultrasound with cracks: Applications to fatigue crack growth

Partial contact of two rough fatigue crack surfaces leads to transmission and diffraction of an acoustic signal at those contacts. Recent experimental and theoretical efforts to understand and quantify such contact in greater detail are discussed. The objective is to develop an understanding of the closure phenomenon and its application to the interpretation of fatigue data, in particular the R-ratio, spike overload/underload and threshold effects on crack propagation

Ultrasonic NDE of Tubing Pinch Welds

Resistance tube closure welds, referred to as pinch welds, are used to reliably seal small stainless steel tubing. The pinch welding process, discussed in detail by Thomas et al [1], involves applying a load to force electrodes together to collapse the tube, sometimes in the presence of confining dies which prevent lateral expansion. Electric current is injected through the electrodes to soften the tube through Joule heating. The resulting hot plastic deformation causes contact and welding of the inner surfaces of the tube

Material Modelling of the Photopolymers for Additive Manufacturing Processes

Ultraviolet (UV) curing of polymers is a key phenomenon for several additive manufacturing technologies. This contribution presents a model relating the process parameters of UV light intensity and temperature to the thermal and mechanical properties of the polymer and the experimental results used to calibrate the model. Moreover, photo-differential scanning calorimetry (photo-DSC) measurements are performed to investigate the crosslinking reaction and to model the degree of cure as a function of the light intensity and temperature. The viscoelastic properties are measured by UV rheometry and it is shown that the classical time-cure superposition principle can equally be applied to the experimental results. Complete curing and mechanical model equations are provided to describe the material behavior as a result of our experimental findings.Mechanical Engineerin

Ultrasonic Investigation of Tensile Fatigue Overloads in Fatigue Cracks

The growth of a fatigue crack is generally modeled in terms of empirical rules such as the Paris law [1] which states that da/dN = A(ΔK)m where a is the crack length, N is the number of fatigue cycles at a particular stress intensity range ΔK and A and m are material constants. Recently, evidence has accumulated that establishes that the full excursion of the applied load or the full stress intensity range does not drive the crack tip forward due to a variety of phenomena often referred to as crack tip shielding [2,3]. This shielding arises as a consequence of the formation of contacting asperities between the surfaces of the fatigue crack. Since these contacting asperities bear a portion of the load, they modify the stress intensity at the tip, thereby altering the growth rate

Strong reduction of laser power noise by means of a Kerr nonlinear cavity

We demonstrated the power noise reduction of a continuous-wave laser field by means of an effective third-order Kerr nonlinear cavity. In contrast to conventional noise reduction schemes relying on linear cavities, a strong noise suppression at Fourier frequencies below the linewidth of the nonlinear cavity was possible. The laser light was reflected off a Kerr nonlinear cavity that had a half width half maximum linewidth of 4.5 MHz. The cavity was operated slightly off-resonance at approximately half of the maximum power buildup, close to its so-called critical state; a power noise reduction of up to 32 dB at Fourier frequencies below 1 MHz was observed after reflection. The effective third-order nonlinearity was a so-called cascaded second-order nonlinearity of MgO:LiNbO3. The laser had a power of 0.75 W at the wavelength of 1064 nm

Strength and Ultrasonic Characterization of Metallic Interfaces

In recent years, the process of diffusion bonding has found considerable usage in both the nuclear power and aerospace industries. This process requires the compression of mating surfaces at an elevated temperature for a given time. If optimum conditions of time, temperature, pressure and surface cleanliness are achieved, diffusion of material across the interface will occur, yielding interfacial mechanical properties identical to those of the bulk material. The use of insufficient bonding conditions may result in void formation, precipitation of undesired phases or lack of grain growth across the interface. The consequence will be an interface that is less than fully bonded, which will result in severe degradation of the mechanical properties. Applications of diffusion bonding to nuclear reactor fuel elements, helicopter rotor hubs, jet engine turbine blades, etc., thus make the ability to characterize the strength of these interfaces highly desirable

Anisotropy of the Fracture Toughness in Aged Inconel 718

Inconel 718 is a widely used material for applications at temperatures up to 650°C. Many research groups have studied the properties of Inconel 718 at different conditions [1, 2]. It is known that, after several thousand hours at 650 °C, Inconel 718 shows roughly a 75% decrease in the Charpy V-notch impact energy [3]. However, any anisotropic characteristics of this steel have not been investigated in adequate detail. Consequently, in order to maintain the integrity of the components, it is important to clarify the orientation dependence of the mechanical properties of INCONEL 718

Tensile Overload and Stress Intensity Shielding Investigations by Ultrasound

Growth of a fatigue crack is modified according to the development of contacts between the crack faces [1,2] creating shielding, thus canceling a portion of the crack driving force. These contacts develop through a number of mechanisms, including plastic deformation, sliding of the faces with respect to each other and the collection of debris such as oxide particles [3]. Compressive stresses are created on either side of the partially contacting crack faces resulting in opening loads that must be overcome in order to apply a driving force at the crack tip. In this way, the crack tip is shielded from a portion of the applied load, thus creating the need for modification [1] of the applied stress intensity range from ΔK = KImax − KImin to ΔKeff = KImax − KIsh. Determination of the contact size and density in the region of closure from ultrasonic transmission and diffraction experiments [4] has allowed estimation of the magnitude of Kish on a crack grown under constant ΔK conditions. The calculation has since [5] been extended to fatigue cracks grown with a tensile overload block. The calculation was also successful in predicting the growth rate of the crack after reinitiation had occurred. This paper reports the further extension to the effects of a variable ΔK on fatigue crack growth. In addition, this paper presents preliminary results on detection of the tightly closed crack extension present during the growth retardation period after application of a tensile overload as well as an observation of the crack surface during reinitiation of growth that presents some interesting questions