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KEPITAL

Physical and mechanical properties

The physical and mechanical properties measured by standard test methods should be the principle guideline to select material from an engineer’s stand point. It is also important to review both short-term (strength, modulus, elongation and impact strength etc) and long-term properties (creep, stress-relaxation and fatigue etc) in determining which material to use. This is because of the fact that plastic materials are affected by varied factors such as temperature, stress and time.

Behavior under short-term stress

The short-term properties of plastics under a stress are determined by means of stress measurements in tensile/ flexural or sudden blow impact strength, depending on the materials’ characteristics, level of the stress, loading speed, temperature and chemical environment. However, long term properties show time dependent behavior. From the tensile test (ISO 527), such data like elastic Young’s modulus, strength, and elongation at yield and break point can be obtained. Those properties are determined from the stress and strain curve (S-S curves) that shows elastic and plastic behavior of a material under a dynamic load. When the stresses are removed within the elastic limit, a thermoplastic is capable to recover its original shape. But on the other hand, if the stress is greater than the elastic limit, the material is deformed permanently after reduction of the stress.

KEPITAL stress-strain curve

S-S curves of KEPITAL in tensile and maximum stress, representing tensile strength are shown in Figure 2-2 and in the following table.

(ISO 527, Temp. 23 °C)
Grade Tensile strength(MPa) Testing speed(mm/min)
F20-03 65 50
FG2025 160 50
TE-24 41 50
Temperature dependency on mechanical properties

KEPITAL maintains balanced physical and mechanical property characteristics over a wide range of temperatures. Figure 2-4 shows stress-strain curves of tensile tests at various temperatures, and Figure 2-5 shows dependence of the tensile strength on temperatures.

Impact strength

The impact strength is the energy to withstand a dynamic impact rather than static stress. There are several ways to evaluate impact strength, and the Charpy(ISO 179) and Izod (ASTM D256) tests are mostly used to determine the toughness of plastic materials. Impact strength can be measured in either notched or un-notched sample; however, it is generally evaluated after notch-processed on a specimen so that the stress of an impact load may be concentrated.

KEPITAL has good impact resistance at low temperatures (-30 to -20 °C) as it has a very low glass transition temperature below -40 °C.

Table 2-1. Notched Charpy impact strength of KEPITAL (ISO 179, 23 °C)
Grade F20-03 FG2025 TE-24
Impact strength 6.5 8 18
Shear strength

The maximum shear stress at which a material can be maintained prior to shearing (punching) is referred to as shear strength. Shear strength represents the maximum load required to completely shear a sample by the maximum strength of a material that is influenced by shear stress. The shear strength is determined by dividing the force required to shear the specimen by the area of the sheared edge. (ASTM D732)

Specific volume

As shown in Figure 2-7, the molding shrinkage of KEPITAL results from both its high crystalline alignment in solidification and its thermal shrinkage from the molten state to the solid state as a function of temperature and pressure. Furthermore, higher cooling rates or cooling under higher pressures causes less volume shrinkage. Figure 2-7 notes a steep volume shrinkage of KEPITAL around 160 °C in the specific volume curves.

Hardness

Hardness of a plastic material is usually indicated in terms of Rockwell Hardness that measures surface pen-etration with a steel ball under specific conditions. The Rockwell Hardness scale is dependent on ball diameter and load (ASTM D785). Rockwell Hardness of a plastic is divided into a M scale or a R scale, and the higher number the higher hardness.

The following figure shows the hardness differences as a function of the viscosity of the standard unfilled grades.

Poisson’s ratio

Poisson’s ratio (υ) is defined as the ratio of the transverse strain to longitudinal strain of plastic materials and it is useful to calculate this physical property in perpendicular direction to loading. Poisson’s ratio is dependent on time, temperature, stress etc. The ratio of KEPITAL F20-03 is approximately 0.35.

The property values of a material used for structure analysis are tensile modulus (E) and Poisson’s ratio (υ). With Poisson’s ratio (υ) and the tensile modulus (E), the material’s shear modulus (G) can be simply calculated. It is because a material deforms not only in the tensile direction but also in its perpendicular direction.

Behavior under long-term static stress

When static stresses are loaded to thermoplastics constantly, not only does the initial strain occur but also an incremental strain is followed as time goes by due to its viscoelastic property. Creep is the total strain of initial elastic deformation and plastic flow for loading time. The creep behavior of KEPITAL is time, temperature and load dependent. Therefore a good resilient material like KEPITAL recovers its original shape entirely or partially when the loaded stress is removed.

  1. (1) Stress, environmental factors; temperature, high humidity, chemicals etc.
  2. (2) Molecular weight and filler content
  3. (3) Part design

The following figures show the tensile creep characteristics and flexural creep characteristics of KEPITAL.

The creep failure is a phenomenon in which a part strained and then eventually fractured under a constant stress for a long period. Because plastics have viscoelastic properties, creep strain is more readily exhibited than in metallic materials. In particular, when designing parts such as pressure resistant containers, screw fasteners, insert formations and insertion parts for a posttreatment process, the creep property of material must be considered in advance.

Property under cyclic stress

Designing based on a dynamic structure analysis, obtained where a part is subjected to loading once, can only provide information if a part can be used without fracture under the single loading environment. Engineering parts are often subjected to the fatigue by stress or strain which is applied repeatedly and periodically over a long period. Fracture or failure that results from this phenomenon is called fatigue failure. Therefore, when designed, the fatigue properties of a material should be considered. Fatigue strength of plastics is generally determined without failure and it is provided through a S-N curve (Wöhler curve). The fatigue property is dependant on the frequency of increasing temperature and various stresses ranges as shown in Figure 2-16. In general, there are methods for evaluating the fatigue characteristic of plastics

  1. (1) Load control method (Load control)
  2. (2) Strain control method (Strain control)
  3. (3) Strain control between grips method(Position control)
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