Stress-strain impact

Creep rupture strength experiments have been conducted in the ZEMAK I - IV laboratory facilities in Julich. Fig. 2-15 shows some measurement results. Also fatigue tests with periodic stress - strain impact have been made to simulate load changes. 

Licea-Claverie and co-workers [57] studied mechanical stress-strain, impact properties and also thermal properties of PA 6,6 (including some recycled PA) with mixed glass fibre and carbon fibre reinforcements and compared these properties with those of the virgin polymers. No dependence on mechanical properties because of increasing amounts of scrap in the composites was found up to 10.4 wt%. The recycled composites generally showed lower mechanical properties when compared with the virgin composites because of a poor matrix-fibre adhesion. 

Some of the more important methods of failure studies include stress-strain, impact loading, and fatigue. Creep and stress relaxation (Chapter 10) may cause serious damage to engineering materials, but they normally do not result in fracture per se except for creep rupture. Emphasis in this chapter will be on the study of fracture energy, kinetics of crack growth, and molecular mechanisms. The reader is directed to Chapter 13 for a fuller discussion of plastic toughening. 

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

P.S. Follansbee, The Rate Dependence of Structure Evolution in Copper and its Influence on the Stress-Strain Behavior at Very High Strain Rates, in Impact Loading and Dynamic Behavior of Materials (edited by C.Y. Chiem, H.-D. Kunze, and L.W. Meyer), Springer-Verlag, New York, 1988, pp. 315-322, Vol. 1. 

If Cm -I- 3Cii > 0, a centered simple wave will be produced by impact loading, and a record of this waveform suffices to determine the entire uniaxial stress-strain relation over the range of strains encountered. Vitreous silica is a material responding in this manner, and its coefficients have been determined by Barker and Hollenbach [70B01] (see also [72G02]) on the basis of a simple-wave analysis. 

Fig. 18.8 Typical stress-strain curve of amorphous thermoplastics below their glass transition temperature. Area under the curve is small compared with many crystalline plastics and hence the impact strength is usually low...

Agglomeration also has a strong impact on the stress/strain characteristics of composites. This may be exemplified by the data of Table 6 borrowed from [215]. 

Long time dynamic load involves behaviors such as creep, fatigue, and impact. T vo of the most important types of long-term material behavior are more specifically viscoelastic creep and stress relaxation. Whereas stress-strain behavior usually occurs in less than one or two hours, creep and stress relaxation may continue over the entire life of the structure such as 100,000 hours or more. 

As practiced by the UL, the procedure for selecting an RTI from Arrhenius plots usually involves making comparisons to a control standard material and other such steps to correct for random variations, oven temperature variations, condition of the specimens, and others. The stress-strain and impact and electrical properties frequently do not degrade at the same rate, each having their own separate RTIs. Also, since thicker specimens usually take longer to fail, each thickness will require a separate RTI. 

As previously described (Chapter 2), the area under short-term stress-strain curves provides a guide to a material s toughness and impact performance (Fig. 7-6). The ability of a TP to absorb energy is a function of its strength and its ductility that tends to be inversely related. The total absorbable energy is proportional to the area within the lines drawn to the appropriate point on the curve from the axis. The material in area A is... 

The determination of tensile stress-strain properties is conducted in accordance with ISO 527 [4] and the values that can be obtained are illustrated in Figure 7.1. For weathering tests where cabinet space is restricted some workers have used a tensile impact dumbbell from ISO 8256 [5] with a square central section which allows test pieces to be exposed edge on. The considerable disadvantage is that modulus cannot be measured as there is no parallel gauge length. 

Mills and Gilchrist (270) analysed the heat transfer that occurs when closed cell foams are subjected to impact, to predict the effect on the uniaxial compression stress-strain curve. Transient heat conduction from the hot compressed gas to the cell walls occurs on the 10 ms... 

PP bead foams of a range of densities were compressed using impact and creep loading in an Instron test machine. The stress-strain curves were analysed to determine the effective cell gas pressure as a function of time under load. Creep was controlled by the polymer linear viscoelastic response if the applied stress was low but, at stresses above the foam yield stress, the creep was more rapid until compressed cell gas took the majority of the load. Air was lost from the cells by diffusion through the cell faces, this creep mechanism being more rapid than in extruded foams, because of the small bead size and the open channels at the bead bonndaries. The foam permeability to air conld be related to the PP permeability and the foam density. 15 refs. 

A method for the prediction of cushion curves for polymer packaging foams from a single impact stress-strain curve is proposed. The method is valid if there is a master curve for the increasing stress part of the stress-strain curve. For closed-cell polymer foams that deform by yielding... 

A study was made of the impact and recovery behaviour of three HDPE closed-cell foams with varying densities. Impact stress-strain curves were measured using a falling striker impact rig and the recovery monitored from 10s after the impact. Cell deformation was observed during compression and recovery using SEM. Recovery was found to occur by the viscoelastic straightening of the buckled faces and to be incomplete due to plastic deformation in the structure. 6 refs. 

Details are given of the change in cell structure of a closecell foam due to multiple impacts and the range of application of cushion curves as they relate to reusable packaging. Cushion curves and stress-strain curves for up to 15 compressions were generated for three different moulded closed-cell PE foams. 

Impact strength is a measure of the energy needed to break a sample—it is not a measure of the stress needed to break a sample. The term toughness is typically used in describing the impact strength of a material but does not have a universally accepted definition although it is often described as the area under stress-strain curves. 

---