Fatigue testing modeled

In the following sections, we describe the ability to ran tests sensitive to values of Q that are characteristic of processes confined to the interfacial region. First, the sample preparation and experimental procedure are described for two types of tests a straight pull-off test and a cyclic interfacial fatigue test. Then, the results of these tests are presented with data for the forces required for fracture of the samples, as well as the calculations for Q related to the cycHc fatigue test We conclude with a discussion of the significance of the results that have been obtained for our model system. 

The lap shear fatigue test for the joints of the front cab module of the Maglev Transrapid TR08 has been analyzed by a fine detail analysis with 3D solid elements using the real cross-section geometry. The adherend materials are aluminum on one side and GRP on the other see also Section 32.5.1. The PU adhesive Sikaflex -254 Booster is modeled with the Ogden strain energy equation (Eq. (1), with N=2). The computed stiffness correlates well with the test results. The local stress distribution is visualized in Fig. 32.11. 

Figure 18.1 The results of modeling of the crack growth in comparison to the experimental data. B - the fatigue test for a low-carbon steel, a stress amplitude of 170 MPa, a frequency of 23 Hz [90] D - data for solution...

A more common method for medical devices is to run the life test until failure occurs. Then an exponential model can be used to calculate the percentage survivability. Using a chi-square distribution, limits of confidence on this calculation can be established. These calculations assume that a failure is equally likely to occur at any time. If this assumption is unreasonable (e.g., if there are a number of early failures), it may be necessary to use a Weibull model to calculate the mean time to failure. This statistical model requires the determination of two parameters and is much more difficult to apply to a test that some devices survived. In the heart-valve industry, lifetime prediction based on S-N (stress versus number of cycles) or damage-tolerant approaches is required. These methods require fatigue testing and ability to predict crack growth. " ... 

A thermal creep-fatigue test, whose specimen was a new test model "a cylindrical shell with cross-section gradually step-changing" is continued with use of the test facility (STST). 

Nanoindentation is a technique gaining increasing popularity [74-76]. Actually, the technique is sometimes abused by attempts to calculate the elastic modulus E on the basis of a model valid for fully elastic materials only [74]. While such attempts fail, a connection has been found by Fujisawa and Swain between E and the unloading strain rate [75]. As shown by Tweedie and Van Vliet [76], spherical indentation provides lower contact strains and more reliable results than conical indentation. A modification providing repetitive indenter hits perpendicular to the specimen surface at the same spot and thus nanoindentation fatigue testing (NIFT) exists also [77]. 

Significant scatter is often evident in time to failure data obtained from stress rupture tests conducted on either neat materials or on bonded joints. This scatter may obscure trends and frustrate the user. Results are typically plotted as load level versus the time to failure, a form that is analogous to S-N plots used in fatigue tests (see Durability Fatigue). In keeping with the principles of polymer physics, the time to failure axis should be plotted on a log scale, as illustrated in Fig. 1. Many creep-rupture models for homogeneous materials are based on forms like... 

This equation corresponds to Archard s law with = l/ crit- The wear coefficient in this case represents the inverse of the critical number of asperity contacts that lead to fracture by fatigue. At each passage of an asperity the subsurface crack grows by an increment until it reaches a critical size at which fracture occurs. The quantity Merit can be associated to the number of cycles leading to fracture in a common fatigue test. Many parameters affect the value of Mj in a wear test and it is not possible to calculate it from first principles. For the model structure shown in Figure 10.19, the critical number of cycles can be brought in relation with the number of inclusions or precipitates that act as crack initiation sites [10]. 

Fatigue-life model Constants Test conditions Reference... 

A number of fatigue life models are available for the lead-tin solders and have been summarized in Ref 39. For the case of lead-free solders, however, only recently have such models been developed (see Table 6). Among them, for one particular model, fatigue tests were performed at two temperatures and frequency conditions (25 °C at 1 Hz and 125 X at 10 Hz) on bulk SAC solder specimens (Ref 40). For each of the test... 

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