Fatigue static

Static fatigue, or creep rupture, is the phenomenon of fracture which occurs some time after the application of a constant load. The applied stress is usually lower than that required to cause fracture under monotonic loading conditions but, as shown later, there is no clear distinction between static fatigue and monotonic loading since in the latter the stress is continuously applied and a finite time must elapse between application of the stress and fracture. Again such experiments have been conducted in the presence of a hostile environment, but these will be considered later in Section 8.3.5. 

Only a few detailed studies have been reported on the static fatigue of adhesive joints and, even fewer, on modelling the data in order to predict the long-term behaviour of stressed joints from short-term data. Some workers [16-19] have concentrated on studying the single lap-shear joint whilst others [20,21] have used precracked specimens and adopted a fracture mechanics approach. 

Lewis et al. [16,17] and Wake et al. [18,19] have examined in some detail the concept of an endurance limit for static fatigue tests, i.e. a value of the applied stress below which joint failure will not occur. Such a concept is obviously of considerable benefit to the design engineer and is analogous to that discussed above for dynamic fatigue failure. Indeed, Wake et al. have considered this concept both for the case of simple static fatigue and for the case when the joint is first subjected to a number of load cycles and then the residual tensile fracture stress measured. 

In the case of silicate glasses, which break in a brittle manner, the fracture is time dependent. That is, for a given load, the fracture takes place after a period of time. This phenomenon is called static fatigue. The hme to failure is related to the fracture stress by the following equation  

Fracture path in a sample of thermally cracked polycrystalline material. 

Static fatigue depends on the environment. If moisture is present, the time of fracture decreases. The phenomenon of static fatigue is also temperature dependent. At room temperature, the strength increases. This is attributed to the decreasing atomic mobility as temperature is lowered. It is also found that, at temperatures above 450 K, the strength increases with temperature. The reasons given for this occurrence are the following  

In the temperature range between -323 K and 423 K, it is found that the time to fracture and temperature follow an Arrhenius relation. The activation energy for soda-lime-silica glass was found to be equal to 18.1 kcal/mole. This kind of behavior shows that the static fatigue is a temperature-dependent activated process, similar to any chemical reaction or diffusion. 

In room temperature tests carried out on soda-lime-silica glasses that were given different abrasion treatments, it was found that each treatment gave a different static fatigue curve [9]. Static fatigue strength is about 20% of the low-temperature strength. 

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Figure 19.6. Acelal copolymer static fatigue failure vs time at 20°C (R.H. ca 65%) (ICI Publicity...

Creep Rupture. When a plastic is subjected to a constant tensile stress its strain increases until a point is reached where the material fractures. This is called creep rupture or, occasionally, static fatigue. It is important for designers... 

Charles, RJ. (1958). Static fatigue of glass I. Journal of Applied Physics 29 1549-1553. 

Creep leads ultimately to rupture, referred to as creep-rupture, stress-rupture or static fatigue. Creep-rupture of thermoplastics can take three different forms brittle failure at low temperatures and high strain rates ductile failure at intermediate loads and temperatures and slow, low energy brittle failure at long lifetimes. It is this transition back to brittle failure that is critical in the prediction of lifetime, and it is always prudent to assume that such a transition will occur [1], A notch or stress concentration will help to initiate failure. 

Fatigue or dynamic fatigue can be defined as the decrease in load bearing capacity with time under cyclic or intermittent load, the term static fatigue being sometimes used to describe creep-rupture (see Sections 4.9.2 and 6.10). 

Methods G and H are obviously not directly comparable with the other adhesion methods. They could be called static fatigue tests, or perhaps creep... 

Equation (6.1.6) gives the rate of displacement of the crack tip under the stress and chemical reaction, and can also be used to calculate the static fatigue limit. If we assume after Wiederhorn (1968) that the rate of displacement of the crack tip is roughly equal to the rate of surface penetration, then... 

Glass that has been under stress for a period of time may fracture suddenly. Such delayed fracture is not common in metals (except in cases of hydrogen embrittlement of steels) but sometimes does occur in polymers. It is often called static fatigue. The phenomenon is sensitive to temperature and prior abrasion of the surface. Most important, it is very sensitive to environment. Cracking is much more rapid with exposure to water than if the glass is kept dry (Figure 15.11) because water breaks the Si-O-Si bonds by the reaction — Si-O-Si—H H2O -> Si-OH + HO-Si. 

McGovern, M. High resiliency polymethane foams with improved static fatigue properties, PCT US Patent 5157056, assigned to ARCO Chemical Technology, 1992. 

The word fatigue denotes the mechanical decay of a component subjected to variable cyclic or random forces. It is important to dinstinguish between dynamic fatigue and static fatigue. The constraint types can be very different they provoke local deformations whose intensity and orientation can vary. This type of excitation generates heat and favors the mobility and sometimes... 

Figure 14.39 Applied stress versus failure time (static fatigue) for a sample of high density polyethylene at various temperatures. The inflection shows the point of change from brittle failure to ductile failure. (From Ref. 49.)...

It may be of interest to note here that the failure of glass exhibits a dependence on time. For example when a constant stress of the order of magnitude of K/c is applied, the time-to-failure varies inversely as the applied stress. On the contrary if the applied stress is continuously increased such as to keep a constant strain rate, stress-to-failure varies directly as strain rate. These are manifestations of delayed failure which is due to the phenomenon of fatigue. Observations made under constant stress measures the static fatigue while the observations under constant strain rate conditions measure dynamic fatigue. The difference in... 

The strengths of crystalline and glassy oxides decrease with time under a constant applied load. This static fatigue is usually modeled with a power law equation between times to failure I when a sample is subjected to an applied stress, v ... 

Yet another variation of this test, which is also used to obtain creep information, is to simply attach a load to a specimen and measure its time to failure. The results are then plotted in a format identical to the one shown in Fig. 12.1 la for cyclic fatigue, and are referred to as static fatigue or stress/life curves. 

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