Deformation under tensile load

The explanation of this aging effect can be based on the common concepts of aging. The initial material is ductile and tough enough that the plastic deformation under tensile load proceeds relatively slowly, yielding weak acoustical signals which do not surpass the noise background. 

Because of the difference in form between Eqs. (2) and (3), the mechanisms of deformation and fracture change with the state of stress. For example, polystyrene yields by shear band formation under ccm ression, but crazes and frachues in a brittle matmer under tensile loading. Chants in failure nwchanian with state of stress are e cially important in particulate conqx tes, since the second phase can alter the local state of stress in the surrounding matrix. 

Reiterer A, Lichtenegger H, Fratzl P and Stanzl-Tschegg SE (2001) Deformation and energy absorption of wood cell walls with different nanostructure under tensile loading. Journal of Materials Science, 35 4681-6... 

Figure 3.14 Total deformation vs time under tensile load at 100°C.h i...

As we saw in the preceding discussion, several mechanical parameters can be derived from stress-strain tests. Two of these parameters are of particular significance from a design viewpoint. These are strength and stiffness. For some applications, the ultimate tensile strength is the useful parameter, but most polymer products are loaded well below their breaking points. Indeed, some polymers deform excessively before rupture and this makes them unsuitable for use. Therefore, for most polymer applications, stiffness (resistance to deformation under applied load) is the parameter of prime importance. Modulus is a measure of stiffness. We will now consider how various structural and environmental factors affect modulus in particular and other mechanical properties in general. 

Ductile properties such as crack pattern and deformations prefiguring the nearing failure are important characteristics regarding the fracture behavior of structural concrete members. The tests demonstrated that in general TRC members have a distinctive ductile behavior although the stress-strain-behavior of the fabrics is linear-elastic until a brittle tensile failure. While the deformations under service loads (SLS) are rather small, the load-bearing behavior of the specimens is characterized by a distinctive stabilized crack pattern as well as high deformations in ultimate limit state (ULS) of L/30 - L/20. 

Figure 5.13 shows the results for a cross-ply laminate, containing 0° and 90° plies. The fibres in the 90° plies hinder transverse deformation. They are under compressive load and their temperature change is negative. In addition, the matrix in the 90° plies is under tensile loading. It has a positive thermal expansion coefficient and cools down. 

Boniface L, Ogin SL, Smith PA. Damage development in woven glass/epoxy laminates under tensile load. In Proceedings second international conference on deformation and fracture of composites, Manchester, UK. London Plastics and Rubber Institute 1993. 

Fig. 9.4. Elastic deformation of the matrix near a fibre under tensile loads. Young s modulus of the fibre has been assumed to be 100 times larger than that of the matrix the fibre-matrix interface is perfectly bonded and cannot fail. Poisson s ratio of fibre and matrix has been assumed to be the same...

This shape of the creep curve occurs only in materials that do not change their microstructure during the creep process. This is the case in simple alloys, but not in many technical alloys (see section 11.2.1 for more about this). A constant strain rate is also only observed if the stress in the component is kept constant. Since the cross section of the component decreases under tensile load during the deformation, the force on the component has to be reduced over time. In service, this is usually not the case so that no region of constant strain... 

Figure 5.3. Sample under tensile load showing the deformation in the direction of applied force and the corresponding definitions of stress and strain.

Tertiary creep represents the final stage of creep deformation and involves an acceleration of the creep rate followed by failure of the component. This stage does not occur in all ceramics, and as previously noted certain ceramics exhibit only primary creep. Tertiary creep involves the formation of cavities that lead to crack formation, often along grain boundaries. The cracks can propagate rapidly, particularly under tensile loading. 

The tensile behaviour of concrete is determined by the complete stress-deformation relation under tensile loading. Previously, it has been assumed that the 0-6 relation obtained from a deformation controlled tensile test directly yields the material property. Recently, however, it has been demonstrated by experiments [1] and by finite element analyses [2] that the post-peak behaviour can be affected by a (macro-)structural behaviour of such a specimen. In this paper it will be demonstrated that this phenomenon can also be studied by means of a simple numerical model. 

On the basis of Fig. 4, it is possible to explain a fundamental correlation between tensile and compressive curves. This Figure shows the relationship between technical compressive stress and technical tensile stress as a function of the deformation. Apart from the result that considerably higher stresses can be achieved under compressive load than under tensile load, it can also be said that the quotients established display virtually no dependence on the temperature within the scope of the measuring inaccuracy and the error due to the approximation. Further investigations showed that the influence of the strain rate can be neglected by way of an initial approximation [16]. 

It should also be noted that in this case the material was loaded in compre-sion whereas the tensile creep curves were used. The vast majority of creep data which is available is for tensile loading mainly because this is the simplest and most convenient test method. However, it should not be forgotten that the material will behave differently under other modes of deformation. In compression the material deforms less than in tension although the efrect is small for strains up to 0.5%. If no compression data is available then the use of tensile data is permissible because the lower modulus in the latter case will provide a conservative design. 

Laminated composite plates under in-plane tensile loading exhibit deformation response that is both like a ductile metal plate under tension and iike a metai plate that buckles. That is, a composite plate exhibits progressive faiiure on a layer-by-layer basis as in Figure 4-34. Of course, a composite plate in compression buckles in a manner similar to that of a metal plate except that the various failures in the compressive loading version of Figure 4-34 could be lamina failures or the various plate buckling events (more than one buckling load occurs). 

---