Specimen deflection

Constant deflection amplitude fatigue testing is probably the less demanding of the two techniques, because any decay in the modulus of elasticity of the material due to hysteretic heating would lead to lower material stress at the fixed maximum specimen deflection. In the constant load amplitude tests, maximum material stress is fixed, regardless of any decay in the modulus of elasticity of the material. 

The specimen deflection was recorded during the test using a displacement transducer connected to the load-points. 

ASTM D 790 also describes determination of the modulus of elasticity (MOE), or flexural modulus, which is the ratio of stress to corresponding strain. It is calculated by drawing a tangent to the steepest initial straight-line portion of the load-deflection curve, which is essentially a load at which the specimen deflects by 1 in. 

Figure 10 Double cantUever beam specimen showing nomenclature and specimen deflection. Copyright 2003 by Taylor Francis Group, LLC...

Heat Deflection Temperature The heat deflection temperature is defined as the temperature at which a specimen deflects a specified amount under specific heat and load conditions. 

The three environments investigated were unconditioned room-temperature air (295 K, 30% relative humidity), liquid nitrogen (76 K), and liquid helium (4 K). A specimen deflection gauge satisfying ASTM requirements [ ] was attached directly to the specimens for the duration of testing. The gauge showed a sensitivity... 

Fracture tests were conducted at a controlled ram displacement rate of 0.008 mm/sec. The load (P) vs. specimen deflection (5) curves were nonlinear, owing to plasticity and stable crack extension. Therefore, Landes and Begley s [ ] /-integral test procedure was applied to generate / resistance curves and //c data. 

The apparatus for this test resembles somewhat that of the flexural test in that the specimen is suspended between two supports 4 in apart with a downward load at the midpoint. However, in this case, the entire structure is immersed in a liquid whose temperature is increased at the rate of 2°C/min. Two loadings are used, 66 and 264 Ib/in. Consequently, the plastics engineer must be careful to compare values for the same loading. The heat deflection temperature is the temperature at which the specimen deflects 0.010 in. The specimen for this test is 0.50 in wide by 5.00 in long. Thicknesses vary from 0.125 to 0.50 in. The values for this resin are fairly high for an unfilled thermoplastic. 

A calibrated micrometer dial-gauge or other suitable measuring instrument capable of measuring to an acctiracy of 0.01 mm deflection at the midpoint of the test specimen shall be used. The deflection measuring device shall be capable of measuring specimen deflection to at least 0.26 mm and is readable to 0.01 mm or better. 

The compliance gives an indication of how much a specimen deflects with an applied force. For linear elastic materials, the compliance is the reciprocal of the slope of the load-displacement curve. Rearranging Eq. (58) to give Eq. (59) and taking partial differentials gives Eq. (60). 

In principle, a crack in the TBC leads to a different specimen deflection at a particular ratio of crack length compared to isotropic material behaviour in a bending test. However, the analysis simplifies for short cracks since then Y is essentially independent of specimen thickness and loading configuration. In the case of the thermal barrier coatings analysed in this investigation the ratio of crack length to total specimen thickness was 0.2. An additional factor that has to be... 

For higher number of thermal shock cycles two typical features arise on the load deflection traces. The first one is a marked pop-in behaviour at loads of about 10 N (marked by circles in Figure 10). The second phenomenon observed is increasing specimen deflection at fracture (maximum) load, as evident when comparing the curves for 10 and 24 cycles in Figure 10. This can be caused by the change of crack tip behaviour affected by damage development at the matrix/fibre interfaces. 

In testing the specimens, deflection can be limited such that the stress at failure does not exceed the yield value. Thus, a direct correlation is established between the NDT temperature and )deld stress. Such information is used in constructing the fracture analysis diagram (FAD). 

HOT Heat Distortion Temperature, or heat deflection under load (HDUL). A three-point flexural test is performed on a bar by applying a load to create a stress of either 1.80 MPa or 0.45 MPa at the midpoint. The temperature is raised by a predetermined rate and when the specimen deflects by a set amount, the temperature is quoted, in °C. 

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