Static strength

Static Strength. Foi a straight pipe the internal design pressure, P, is given by... 

A strength value associated with a Hugoniot elastic limit can be compared to quasi-static strengths or dynamic strengths observed values at various loading strain rates by the relation of the longitudinal stress component under the shock compression uniaxial strain tensor to the one-dimensional stress tensor. As shown in Sec. 2.3, the longitudinal components of a stress measured in the uniaxial strain condition of shock compression can be expressed in terms of a combination of an isotropic (hydrostatic) component of pressure and its deviatoric or shear stress component. 

In plastics, these correlative properties, together with those that can be used in design equations, generally are called engineering properties. They encompass a variety of situations over and above the basic static strength and rigidity requirements, such as impact, fatigue, flammability, chemical resistance, and temperature. 

A strength increase is also produced at ultimate strength (F ) for steels however, the ratio f dynamic to static strength is less than at yield. A typical stress-strain curve describing dynamic and static response of steel is shown in Figure 5.5. Elongation at failure is relatively unaffected by the dynamic response of the material. 

It is possible to determine the actual strain rate of a material during calculation of dynamic response using an iterative procedure. A rate must be assumed and a DIF selected. The dynamic strength is determined by multiplying the static strength (increased by the strength increase factor) by the DIF. The time required to reach maximum response can be used to determine a revised strain rate and a revised DIF. This process is repealed until the computed strain rate matches the assumed value. There are uncertainties in many of the variables used to calculate this response and determination of strain rates with great accuracy is not warranted. 

Dynamic Increase Factor - The ratio of dynamic to static strength which is used to compute the effect of a rapidly applied load to the strength of a structural element. 

To incorporate the effect of material strength increase with strain rate, a dynamic increase factor (DIF) is applied to static strength values. DIFs are simply ratios of dynamic material strength to static strength and are a function of material type as well as strain rate as described above. DIFs are also dependent on the type of stress (i.e. flexural, direct shear) because peak values for these stresses occur at different times. Flexural stresses occur very quickly while peak shears may occur relatively late in time resulting in a lower strain rate for shear. 

Preliminary results of work at the University of Calgary confirm the high fatigue life with unmodified sulphur as the binder. In 1/3 point bending tests at a load nominally 95% of static strength, the fatigue life was on average 1670 x 103 cycles. 

PETN, LX-04-1, PBX 9404, PBX 9011 and RX-08-AZ were tested using the Hopkinson split-bar technique. This technique allows data acquisition up to strain rates of ca 5 x 103 in/in/sec. The " dynamic" tensile strength of these expls is ca four times greater than their "static" strength. 

Refs 1 and 3 give dynamic tensile strength compared to static strength (Table 2)... 

J.C. Halpin et al Time dependent static strength and reliability for composites. Compo. Mater. 4,462-474 (1970)... 

From the safety point of view, cyclic loading means that fatigue can occur at stress levels far below the limits for static strength. 

Although the Finite Element Method is still not very common for calculating the static strength of a vessel, it became a standard tool for computation of peak stresses (Figure 1) for fatigue analysis. 

The observational effect of dynamic weakening includes also a strength drop from the static strength to a lower dynamic strength ... 

The conclusion based on the data obtained was that for FSSW of A6111-T4, there was no significant change in the static strength in both coach peel and lap shear of specimens that were as-welded and those subjected to a paint bake cycle. 

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