The benefits of different mechanical testing systems

Alan Thomas, marketing at ZwickRoell, explores the benefits of different testing systems.

Over the last 20 years, the use of plastic materials in healthcare has resulted in radical change and complete transformation of this industry. The versatile material properties, excellent biocompatibility and wear resistance are some of the key reasons why plastics are dominating the medical device and prosthetic sectors. 

The increased use of plastics in the healthcare industry also means that new quality control standards and systems must be developed and applied across the whole development and manufacturing chain and strictly monitored. 

During the manufacture of a finished product or a semi-finished product, the processed polymer is subjected to a variety of mechanical, thermal, and chemical loadings and normally forms orientations for the polymer chains and fillers. Therefore, the mechanical properties of a polymer differ at various points of the component. 

To compare material properties of a product produced by injection moulding, extruding, or compression moulding, a sampling plan for the specimen is defined. This sampling plan is based on the forming process of the product and defines the number, form, size, and direction of sampling of the specimen. 

After appropriate conditioning, the specimens taken are tested to the relevant test standards or to specific information found in the product specification. 

Regular specimen sampling from ongoing production ensures that specific material properties remain constant in one product within predefined tolerances and over longer production periods. 

Uniaxial tensile testing 

Uniaxial tensile testing is one of the most common types of mechanical test performed on engineering polymers, but other common tests include flexure, impact, and melt flow. The popularity of the tensile test is due to its ability to not only provide quantitative data on a range of mechanical properties, but also gives insight into the structural changes that occur during testing for example, there is an intimate relationship between molecular structure and performance in the test as exemplified by the significantly different responses obtained when testing thermoplastic, thermoset, and elastomer engineering polymer materials. It is essentially used for determining the mechanical characteristics of isotropic materials that is, materials whose properties are the same regardless of measurement direction.  

To determine the strength of the material, loading of the sample is carried out relatively slowly and increased continuously until the sample fails by either ductile or brittle fracture mechanisms; failure mode is dependent on the nature of the material. The maximum test force applied to produce the first local maxima in a stress/strain plot is a measurement of the material strength or the so-called yield strength.  

Though the basic dimensions of uniaxial tensile test samples can vary widely in size and geometry, they all have the same basic characteristics. Each sample has a well-defined gauge length of constant cross-sectional area, and a shoulder with a well-defined fillet radius which defines the end of the usable gauge length. The shoulder is the region of the sample that is gripped in a tensile testing machine and hence the available area and its dimensions will depend on the type of gripping system being employed. Secure gripping of samples and proper alignment are vital as poor gripping leads to slippage and misalignment imparts a bending moment on the sample, leading to non-uniform stress distribution over the sample cross section, resulting in errors in data collection and premature sample failure.  

Flexural testing 

The flexural test measures the force needed to bend a beam under three-point loading conditions. The measured data is often used to select materials for parts that need to support loads without significant flexing. Flexural modulus is used as an indicator of a material’s stiffness when flexed. The physical properties of many materials can vary depending on the ambient temperature, and it is sometimes relevant to test materials at temperatures which replicate the intended ‘in-service’ environment. 

In the standard flexural test, the specimen rests on two lower supports and the load is applied to the centre by the upper loading nose producing three-point bending. The parameters for this test are the support span, the rate of load application, and the maximum deflection. These parameters are based on the test specimen thickness and are defined in the ASTM and ISO international standards.  

Charpy V-notch impact testing 

The most common measure of toughness is the resistance to impact as measured in a Charpy V-notch impact test. A vee notch is machined in the centre of a test specimen and the specimen is supported horizontally on an anvil. The specimen is subjected to an impact from a pendulum of a specific weight, released from a fixed height such that the pendulum strikes the sample on the opposite side of the vee notch. The specimen fails in flexure under impact. The energy absorbed by the specimen when it is subjected to the impact from the pendulum is equal to the difference between the potential energies of the pendulum before and after impact. The result obtained from this test is resilience and a fracture type. The resilience is expressed as the energy required to break the specimen; this energy is measured via a rotary encoder on the pendulum axis.  

Extrusion plastometers 

Extrusion plastometers, also known as melt flow index testers, are used to determine the melt mass flow rate (MFR) and melt volume flow rate (MVR) of a plastic melted mass. Measurement of the flow behaviour of the melted material is used to compare different grades of the same polymer and provides an important parameter for quality assurance and incoming goods inspections, for material selection and the set-up of processing machines by plastics processors. Demands on melt flow indexers vary according to which plastics processing stage is involved. 

MFR or melt mass flow rate, formerly melt flow index or MFI measures the rate of extrusion of thermoplastics through a standardised orifice in a die at a specified temperature and load. While MFR is measured in mass per unit time, such as grams/10 minutes, MVR is measured in volume per unit time, such as cm³/10 minutes. The ratio of the two values, MFR divided by MVR, provides a measure of the melt density of a material in g/cm³. 

Because the melt density of a plastic material is not the same as its solid density, knowing these two values is important for plastics processors. 

The properties of a component or structural element cannot always be predicted exactly from the material properties. Components are therefore tested in typical ‘in-service’ loading situations. This enables strength and deformation characteristics to be determined and calculation methods validated.