Tension fatigue

Figure 6-4 Typical Tension-Tension Fatigue Data (After Pinckney [6-3])...

Several criticisms of these parameters have recently been pointed out. First, they have no specific association with a material plane (i.e., they are scalar parameters), despite the fact that cracks are known to nucleate on specific material planes. With traditional parameters it is difficult to account for the effects of crack closure under compressive loading. Traditional parameters have not been successful at unifying experimental results for simple tension and equibiaxial tension fatigue tests. Finally, a nonproportional loading history can always be constmcted for a given scalar equivalence parameter that holds constant the value of the scalar parameter, but which results in cyclic loading of material planes. For such histories, scalar parameters incorrectly predict infinite fatigue life. 

ISO 15850 2002 Plastics - Determination of tension-tension fatigue crack propagation -Linear elastic fracture mechanics (LEFM) approach... 

A coincidence between the proportional limit, fatigue limit, o-fi, has also been observed for other composite systems fatigued at loading frequencies of 10 Hz or lower. For example, as shown in Fig. 6.6, under tension-tension fatigue at 1000°C, 0° SCS-6 SiQ/HPSN43 composites exhibit a fatigue limit (5 x 106 cycles) at the monotonic proportional limit stress of 200 MPa. 

Fig. 6.6 Fatigue life diagram for the tension-tension fatigue of unidirectional SiCf/Si3N4 at 1000°C and a stress ratio (o /cr, ) of 0.1. Fatigue run-out (5 x 106 cycles) was observed when the maximum stress was below apt. After Holmes et al.43...

Fig. 6.7 Fatigue life diagram for the tension-tension fatigue of unidirectional SiQ/1723 at room temperature (10 Hz, VmJ max — 0.1). The 106 cycle fatigue limit of 440 MPa is higher than cr. and

Summarizing, from numerous experimental investigations of fatigue behavior, it has been established that a fatigue limit exists for the tension-tension fatigue loading of fiber-reinforced ceramics, at least to 108 cycles. In... 

Fig. 6.12 Influence of loading frequency on the surface temperature rise measured during the tension-tension fatigue of a woven 0790° CVI Q/SiC composite. The fatigue experiments were conducted at 20°C between fixed stress limits of 10 MPa and 250 MPa. After Holmes and Shuler.51...

Pruitt L, Suresh S (1994) Cyclic stress fields ahead of tension fatigue cracks in amorphous polymers. Polymer 35(15) 3221-3229... 

Nevertheless, if materials arc to be evaluated in more than a simple comparative manner, then tests are needed as a function of at least the amplitude of deformation. Perhaps because they are newer materials, it is more common to find this being allowed for in procedures for plastics and composites. Amplitude effects can also be studied in the more recently introduced tension fatigue test for rubbers. This allows the application of fracture mechanics concepts to the results, as will be discussed in a later section. 

Cyclic tension-tension fatigue tests were conducted on a computer controlled MTS servo hydraulic machine. The load ratio R (the ratio of minimum load to the maximum... 

Fatigue hves of impact-damaged laminates in compression—compression fatigue ate significantly worse than lives under tension—tension fatigue. 

Figure 16.38 Variation of longitudinal electrical resistance during tension—tension fatigue testing of a unidirectional CFRP composite specimen. The cyclic change of resistance is due to reversible cyclic strain the permanent increased in resistance around cycle 218,281 may he due to permanent damage, e.g., some fiber fracture [89].

Because of the virtually unlimited variations in fibre arrangements (short and continuous fibres, aligned and randomly distributed, laminated, woven, knitted or braided) there are too many materials available to discuss fully all the details of their fatigue behaviour. This chapter will therefore concentrate on some selected composites for demonstration purposes. We shall discuss both tension fatigue, which is the most important load situation in metals, and compression fatigue. The influence of harsh environments will be mentioned as well. 

The fatigue behaviour of the woven fabric shows that this textile structure is sensitive to fatigue loading. This sensitivity is confirmed by testing thermoplastic matrix systems (polyamide 12, PA 12) reinforced with a glass fabric. A strong stiffness reduction, with a pronounced creep deformation was observed under tension-tension fatigue [24]. 

Standard test method for tension-tension fatigue of polymer matrix composite materials... 

FIGURE 9. Cyclic tension-tension fatigue behavior of Piepieg HiPeiComp composites tested in air using sinusoidal loading. Black points ate for 815°C, gray points for 1093°C and white points for 1204°C. (Arrows indicate test run-outs.)... 

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