1.1 Mechanical Strength
DUROPLASTS: the addition of mineral fillers, natural textile fibres and the optimum selection of the basic resin give this material an excellent mechanical strength, a high superficial hardness and a good impact strength.
TECHNOPOLYMERS: the rich selection of basic polymers available and the possibility of combining these with reinforcing fillers or additives of various kinds make a wide range of performance levels possible in terms of mechanical strength, impact strength, creep and fatigue.
The mechanical properties of a moulded plastic component may vary significantly according to its shape and the technological level of the manufacturing process.
For this reason, instead of providing tables containing specific data on the mechanical strength of test pieces of various types of material, ELESA has decided to inform designers of the forces which, in the most significant cases, may cause the component breakage. For most products, the mechanical strength values indicated in the catalogue are therefore loads at breakage.
The deformation under a load is not negligible for some products and may therefore jeopardise their performance, even before their breakage. Thus for these products, two load values are provided:
- “maximum working load” below which deformation DOES NOT jeopardise the component performance;
- “load at breakage” in accordance with the concepts outlined above.
In these cases, the “maximum working load” will be used as maximum design data to guarantee the correct performance, while the “load at breakage” will be used for safety tests. Obviously, in both cases suitable safety coefficients must be applied.
Working stress has been taken into account (e.g. the transmission of torque in the case of a handwheel, the tensile strength in the case of a handle) as well as accidental stress (e.g. an impact with the component), in order to provide designers with a reference for determining suitable safety coefficients, according to the type and importance of the application.
All the strength values supplied were obtained from tests carried out in ELESA Laboratories, under controlled temperature and humidity (23°C - Relative Humidity of 50%), under specific working conditions, and by applying a static load for a necessarily limited period of time.
The designer must therefore take into account adequate safety coefficients according to the application and specific operating conditions (vibrations, dynamic loads, working temperatures at the limits of the allowed temperature range). In the end, however, the designer is responsible for checking that the product is suitable for its intended purpose.
For some thermoplastics, for which the mechanical properties vary significantly in relation to the percentage of moisture absorbed (see chapter 1.5), the resistance tests on the component are carried out in compliance with ASTM D570, so that the moisture absorbed is in equilibrium with respect to ambient conditions of 23°C and a RH of 50%.
• Compressive strength for levelling elements (working stress)
The levelling element is assembled on its threaded metal stud and placed on special testing equipment. The element is then subjected to compressive stress with repeated and incremental loads until it breaks or undergoes a permanent plastic deformation of the plastic element.
• Resistance to transmission of torque (working stress)
Use is made of an electronic dynamometer that applies increasing torque values as shown in the chart hereunder. The dynamometric system in the torque is shown in a traditional way to make the comprehension easier. The mean values of the torque C, obtained in the breaking tests, are shown in the tables for the various components and expressed in [Nm].
• Impact strength (accidental stress)
The special equipment is used as shown in the chart.
The mean values obtained in the breaking test, shown in the tables for the various models and expressed in [J], correspond to the breaking work L of the element subjected to repeated impacts, with the falling height (h) of the percussion weight (P) being increased by 0.1 m each time. Percussion weight (P): metal cylinder with a rounded ogival shaped end and weighing 0.680 kg (6.7N).
• Tensile strength of U-shaped handles (working stress)
This test entails fitting the handle to be tested on an electronic dynamometer, with two types of stress:
- perpendicular to the mounting screws (F1): here the stress on the handle is a mixed combination;
- parallel to the mounting screws (F2).
The load applied by the electronic dynamometer increases gradually in order to obtain a deformation of the tested element within a limit of 20 mm/min.