Ultimate shear strength, yield stresses

Other estimates of the ultimate shear strength of amorphous polymers have been made by a number of authors and generally all fall within a factor of 2 of each other (38,77,78). Stachurski (79) has expressed doubt as to the validity of the concept of an intrinsic shear strength based on the value of the shear modulus, G, for an amorphous solid. He questions which modulus is the correct value to use— the initial small strain value or the value at higher strain (the yield point or the ultimate extension). Further, the temperature and strain-rate dependence of both the yield strength and modulus (however defined) suggests that perhaps the ratio of yield strength to modulus is not a true intrinsic material property. We remark however that the temperature and strain-rate dependence of both the yield stress and the shear modulus are often similar. 

Sy = yield strength Su = ultimate tensile strength L = loading stress Ty = shear yield strength. 

ASME Code, Section VIII, Division 2, and Section III use the term stress intensity, which is defined as twice the maximum shear stress. Since the shear stress is compared to one-half the yield stress only, stress intensity is used for comparison to allowable stresses or ultimate stresses. To define it another way, yielding begins when the stress intensity exceeds the yield strength of tlie material. 

J Mechanical testing parameters (a) A representative strain-stress curve in tensile testing. Yield stress (o-yg) and yield point strain (eyp) can be obtained by recording values at the point where the curve transitions from a linear relationship between stress and strain (elastic deformation) to a non-linear relationship (plastic deformation). Ultimate tensile strength (fr ,s) is the maximum stress in the curve, and the corresponding strain is called uniform strain (cp). The strain at fracture (eO can also be obtained from the curve, (b) When the transition point between elastic and plastic deformation is difficult to identify, a 0.2% strain offset line parallel to the elastic portion is drawn to obtain the <7ys or 0.2% offset o-ys. (c) Schematic of the deformation that occurs when shear force is applied to a viscoelastic polymer. 

In all cases the composite films were found to be more resistant in the shear direction and consequently less deformable. The high anisotropy of the mechanical properties is evident from the results of Table 15.3, where it can be seen that Young s modulus, yield stress and ultimate tensile strength are higher in the shear direction. 

So what are we looking for All the theoretical methods for predicting joint strength need the elastic moduli such as E (Young s) and G (shear). In addition, those theories which allow for adhesive non-linear behaviour will need data such as the yield stress (strain) and the ultimate stress (strain). More sophisticated analyses, e.g. Adams et al. 

Ultimate strength Yielding Toughness Weibull distribution Stress concentration Trousers tear Simple shear test Energy to cause rupture Impact tester Izod test Charpy test... 

In many cases, a product fails when the material begins to yield plastically. In a few cases, one may tolerate a small dimensional change and permit a static load that exceeds the yield strength. Actual fracture at the ultimate strength of the material would then constitute failure. The criterion for failure may be based on normal or shear stress in either case. Impact, creep and fatigue failures are the most common mode of failures. Other modes of failure include excessive elastic deflection or buckling. The actual failure mechanism may be quite complicated each failure theory is only an attempt to explain the failure mechanism for a given class of materials. In each case a safety factor is employed to eliminate failure. 

Ultimate strength n. The maximum nominal stress a material can withstand when subjected to an applied tensile, compressive, or shear load. If the mode of loading is not specified, it is assumed to be tensile. In materials that exhibit a definite yield strength, ultimate strength will usually mean the nominal stress at break, which can be less than the maximum. Shah V (1998) Handbook of plastics testing technology. John Wiley and Sons, New York. 

Upon re-heating, regions of tensile stress may develop in the TGO. With an increase in the number of thermal cycles, the shear stresses induced in the bond coat upon cooling exceed the bond coat yield strength, so that it flows plastically from valleys to peaks at the undulation sites, allowing the undulation amplitude to increase (Fig. 11). As a result, the normal tensile stresses increase and ultimately lead to separation of the TGO [69]. 

Mechanical testing is the determination of the behavior of a material caused by some applied loading. The material is loaded in its bulk form via a mechanical testing machine (i.e., MTS, Instron, etc.) and its properties are evaluated. Typically these include the elastic modulus or stiffness, the yield strength, the fracture stress or ultimate strength, the elongation, and Poisson s ratio. These properties depend on the mode of loading, such as tension, compression, shear, or flexure. 

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