Wave-propagation testing techniques

Whorlow (1992) published a book on rheological techniques that inctudes dynamic tests and wave propagation tests. In the appendix, he listed a number of rheologicat inves-tigahon equipment manufacturers. Some of the techniques appiy more to polymers and are not relevant to our discussion. Dynamic vibration tests have been extended to fresh concrete (Teixera et at., 1998). Concord and Tassin (1998) described a method to use rheo-ophcs for the study of thixotropy in synthetic clay suspensions. A rheometer optical analyzer was used on laponite, a synthetic hectorite clay. Laponite was mixed with water and tests were conducted at various intervals for up to 100 days. Rheo-ophcs seems to be... 

The common civil engineering seismic testing techniques work on the principles of ultrasonic through transmission (UPV), transient stress wave propagation and reflection (Impact Echo), Ultrasonic Pulse Echo (UPE) and Spectral Analysis of Surface Waves (SASW). 

In addition to the mechanical tests cited above, ultrasonic wave propagation techniques have been used to measure the anisotropic elastic properties of bone [Lang, 1969 Yoon and Katz, 1976a, b Van Buskirk and Ashman, 1981]. This is possible, since combining Hooke s law with Newton s second law results in a wave... 

However these techniques fall short in detecting soft layers below stiff layers (Kramer and Stewart, 2004). A relatively new technique that can be used to determine subsurface thickness and wave propagation velocities without necessity of borings is the Multi Channel Analysis of Surface Waves (MASW) test (Nazarian and Stokoe, 1983 Suto, 2007). In this test series of vertical receivers are place on the ground in line with an impact source. The output of the receivers are recorded and transformed into the frequency domain. The phase angles between the recoded responses are used to compute an apparent travel time of the surface waves. The surface wave phase velocity is... 

The line-source technique is a transient method capable of very fast measurements. A line source of heat is located at the center of the sample being tested as shown in Fig. 4. The whole is at a constant initial temperature. During the course of the measurement, a known amount of heat is produced by the line source, resulting in a heat wave propagating radially into the sample. The rate of heat propagation is related to the thermal diffusivity of the polymer. The temperature rise of the line somce varies linearly with the logarithm of time. Starting with the Fourier equation, it is possible to develop a relationship which can be used directly to calculate the thermal conductivity of the sample from the slope of the linear portion of the curve ... 

The simplicity of this plastic model is that all the parameter that is required isp , which can be determined through routine static indentation testing, as also demonstrated for various ductile projectiles (steels and brass) impacted on silicon nitride targets [10]. However, pertinent experimental techniques should be sought to determine more accurately the related dynamic patameters such as impact force, stresses, deformation, duration of impact, coefficient of restitution, and stress wave propagations, etc. Frictional constraint by property mismatch between projectiles and targets of dissimilar materials needs to be taken into account in some cases. 

In stress wave adhesion tests, a stress wave is propagated through the system and the reflection of the stress wave at the interface results in a tensile stress at the interface. The stress wave can be injected into the solid from a flyer plate, a flyer foil, or a laser pulse.f ° ° Conceptually, the stress wave technique could be used to initiate and then stop an interfacial fracture in order to study the fracture initiation. The onset of the fracture might be detected by acoustic emission. A small area, low thickness flyer plate can be generated by depositing a film on the end of an optical fiber, a then spalhng the film off with a laser pulse. 

The magnitudes of stress and strain generated by ultrasonic or seismic wave propagation are extremely small compared to stress and strain associated with the static testing technique. 

The effect of buffer materials is to soften shock waves. The safety wrapping technique for paper percussion cups which are commonly used to start sports events was tested by exposure to a weak shock wave. The small gap test (II) (Sec.5.2.3 in this book) was used. A propagated explosion test using a strong shock wave was also tried in sand (Sec.3.8.3). As a result of these tests, it was found that river sand was a practical materia] that could be used to soften shock waves. 

Waves reflected by test structures can propagate back to the paddle and then get re-reflected, corrupting the quality of data that prevails in the experimental setup. To overcome this difficulty active absorption techniques have been developed, and a description of these techniques is given in the following section. 

---