D. Knauss

Depth measurement of short cracks with an acoustic microscope

The depth of short cracks (70–200 μm surface length) has been measured with an acoustic microscope by utilizing the nondestructive time-of-flight diffraction technique (TOFD). The depth measurements were first carried out in the transparent polymer polystyrene, thus allowing a comparison between the acoustical values and direct optical measurements: the agreement in the results was better than 95%. The depth of a 70 μm long crack in an aluminium alloy was then measured, demonstrating the application of the technique to metals.

Evaluation of diffusion bonds formed between superplastic sheet materials

Diffusion bonds produced in microduplex titanium and stainless steel sheet materials for various bonding conditions have been evaluated using a range of techniques. These include light and scanning electron microscopy (SEM), scanning acoustic microscopy (SAM) and compressive lap shear testing. The potential of other procedures such as ultrasonic inspection and resistivity measurement are also discussed. For imperfect bonds, the bond line in titanium alloys consists of clearly defined interfacial voids separated by metallurgically sound bonded regions, while the unbonded regions in stainless steel often consist of long flat voids in which the opposing surfaces have contacted but not bonded. It was observed that light microscopy and SEM observations provide a convenient and reliable method for the assessment of the bond quality, and in the case of titanium alloys it is possible to obtain quantitative data on the extent of bonding. High frequency SAM also proved to be an effective procedure for qualitative assessment. A linear relationship between the fraction of parent metal strength achieved and bonded area fraction as determined by metallography was observed for titanium alloys.

Depth measurements of short cracks in perspex with the scanning acoustic microscope

The geometry, such as surface length, depth, shape and orientation, is one of the key factors that dominate the propagation behaviour of short fatigue cracks. Thus, for a quantitative understanding of short crack growth behaviour, it is necessary to monitor the crack geometry throughout the fatigue test. However, a technique for the direct measurement of short crack geometry has been lacking, which, together with the difficulty of short crack detection, is one of the major reasons for the slow progress taking place in the study of short fatigue cracks. The growth of short fatigue cracks is commonly monitored only in its surface length with an optical or scanning electron microscope. The depth determination for the crack is then usually made on the basis of an empirical estimation from the crack surface length. For instance, the penny-shape or semi-elliptical estimation by assuming the profile of the crack as a semi-circle infers the depth of a short crack to be half of its surface length1,2. Obviously, such an estimation cannot usually provide reliable data on the depth of short cracks in many materials, because of the strong dependence of the short fatigue crack shape on local microstructural characteristics and environment.

Measuring Short Cracks by Time-Resolved Acoustic Microscopy

Detecting defects, for example cracks, (1) is important in predicting the lifetime of a material. The growth behavior of short cracks(2) plays an essential role in the lifetime of a component, since the lifetime is mainly controlled by the time required for a crack to grow from a certain initial size to about 1 millimeter. Cracks are defined as short when for example the crack length is small compared with the microstructure of the specimen or when the crack is simply shorter than ≈0.5 mm.(3) The growth of short surface breaking cracks can be measured by light microscopy (LM) or scanning electron microscopy (SEM). A common method of studying short cracks is a replica technique based on taking several plastic replicas at various stages of crack growth and subsequently examining these replicas with LM or SEM.(4,5) The disadvantage of these techniques however, is that they give information about crack development only on the surface of the specimen, so that the depth of the crack has to be determined indirectly by assuming the shape of the crack. For long cracks this may be appropriate because a local change in propagation direction does not alter the overall crack geometry on which the driving force of the crack depends.(6–8) However for short cracks, a change in propagation direction can alter the crack geometry significantly and thus change the driving force for the crack propagation. If its size is comparable with the microstructure of the material, a deflection of the crack at a grain boundary can alter the overall crack geometry. To understand the behavior of short cracks, it is therefore necessary to measure their three-dimensional growth. This can be achieved by using acoustic waves, which can penetrate into the material. In this way the crack depth can be measured directly.

Mixed mode crack mouth reflection in time-resolved acoustic microscopy of short fatigue cracks in single crystal aluminium

An Al single crystal with the axial direction of (452) was fatigued in air, at a constant resolved shear stress amplitude (4 MPa), a frequency of 20 Hz and at room temperature. Time-resolved measurements were then carried out on short cracks in persistent slip bands (PSBS) on the top surface, which has the largest slip steps, using the acoustic signal from the crack, when the acoustic lens was scanned over the crack sending convergent acoustic beams down the specimen, was detected and interpreted quantitatively. The signal results from the reflections of a mixed mode surface wave (Rayleigh longitudinal lateral wave) from the crack mouth. At 1.2*106 cycles, two short cracks in the PSB were measured to be 17 and 27 mu m in depth. The angle with the specimen surface was observed to be about 54 degrees , compared with a value of 51 degrees determined by an X-ray method.

Subsurface crack signals in time-resolved acoustic microscopy

Measurements of subsurface cracks using time-resolved acoustic microscopy are presented. By focusing on the signals reflected from the inclined faces of kinked cracks, which contain information about crack closure, a model was developed for the calculation of the amplitudes of these signals. The model is based on a two-dimensional diffraction theory of monochromatic waves in an isotropic material including the transmission of ultrasonic waves through water/solid and solid/water boundaries. Experimental and theoretical results are presented for longitudinal and vertically polarized shear waves reflected from a wedge and a crack. It is shown how the size of the crack influences the detected amplitudes of the longitudinal and the shear wave signals reflected from the crack face.

Image Processing for the Measurement of Crack Depth using the Scanning Acoustic Microscope

The ability to measure the depth of surface breaking fatigue cracks, especially while they are still in the short crack stage, is of great importance for the study of crack growth behaviour.1–3 Time-resolved scanning acoustic microscopy offers the possibility to accomplish such depth measurements in a nondestructive manner,4–6 and thus the ability to actually monitor crack growth throughout the fatigue process. In time-resolved acoustic microscopy pulses of very short temporal extent are used to excite the SAM lens, so that echoes from the individual scattering sites within a specimen can be separately identified in the received signal.7–9 The measurement of crack depth therefore relies on the behaviour of the crack tip as a scattering site, with the depth being determined by measuring the arrival time of the signals caused by diffraction of the acoustic waves at the tip. Although the crack tip diffraction signal is much weaker in strength than specular scattering, it has been successfully used to measure longer cracks (>1 mm) at lower frequencies (< 10 MHz), where the technique is referred to as Time-of-Flight Diffraction (TOFD).10,11

The effect of anisotropy in time-resolved acoustic microscopy

A model for evaluating time-resolved acoustic microscopy measurements of cracks in anisotropic solids is presented. Calculations of the time-of-flight traces of acoustic waves reflected from the face of an inclined crack (FR) and the diffracted waves from the crack tip were carried out. Furthermore, the theoretical results for the FR signals are compared with measurements in a Ni superalloy. The most significant effect of anisotropy is the possible appearance of a signal due to shear waves with horizontal polarization.

Sonography and quantitative measurements

A preliminary study of coronary artery wall topography and mechanical properties is presented. The aim of this study was to give a brief demonstration of scanning acoustic microscopy (SAM) as a sonographic technique, and to apply the time-resolved SAM (TR-SAM) technique for quantification of coronary artery wall mechanical properties under passive conditions ex-vivo, and compare the data for the tunica externa and tunica media of the wall. The authors chose the diagonal branches of the left anterior descending coronary artery (LADCA) of young healthy pigs for measurements. It is concluded that SAM is well suited for sonography at the micrometer level, and TR-SAM provides a refined tool for biorheological quantification ex-vivo, provided that a number of physical factors influencing measurements and tissue properties are considered and dealt with. With time and effort, SAM may also become a valuable tool for recognizing important relations of composition and structure to function. For future SAM studies of arteries, more detailed analyses of layer interfaces and better models of biorheology should be applied to describe the anisotropy and nonlinear viscoelasticity of the wall.

Detection of crack closure in time-resolved acoustic microscopy

Acoustic microscopy with time-of-flight measurements of short pulses has been used to detect the closure of short cracks. Operating the microscope in the pulse mode, the reflections from inclined cracks have been measured and a diffraction model for calculating the amplitude of these reflections as a function of the lens position was developed. The detected signal amplitudes depend on the size of the crack and on the contact between the crack faces