Thermal, Physical, and Mechanical Properties of Composite Materials
By Annelie Eveborn, Choothum Jeenjitkaew, and Marianne Kilbride
Companies that manufacture final products or parts using composite raw materials must understand each of the individual properties of the different constituent materials before use. This is in addition to understanding the properties of the final part or product prior to installation and is necessary for several reasons, not least to ensure that each material is fit for purpose and to guarantee batch-to-batch consistency.
There is a range of test methods that can help to qualify and quantify the different properties of composite materials, including thermal, mechanical, and physical tests. These tests can also be used to predict the lifetime of a material that might be subjected to various environmental conditions. An unknown material can also often be identified by applying a combination of the different analysis techniques.
Thermal properties of composites
Some raw materials used in the manufacture of composite materials can be negatively impacted by the effects of temperature and humidity. It is therefore essential to ensure they are stored within highly controlled environmental conditions prior to use.
When the raw material is delivered from the raw material manufacturer to the product or parts manufacturer, mandatory checks must be carried out to ensure that the batch properties of each material have not been negatively affected by the transportation process. This checking process can be especially important in warmer, more humid climates or during a long courier transportation trip. Materials being delivered from the UK to Indonesia, for example, have the potential to be exposed to outside temperatures that could advance the material cure or allow the material to absorb moisture.
Differential Scanning Calorimetry (DSC) is one of the common tests used to check the cure and confirm the material’s thermal properties. DSC is one of the most important test types when discussing quality control, especially where the supplier has built up a sufficiently large database of previous results with which to compare against. DSC can provide information on the physical structure of the material via important thermal transitions such as glass transition temperature (Tg) [link], melting temperature (Tm), crystallization temperatures, percent crystallinity, enthalpy of melting and crystallization, specific heat capacity (Cp) and oxidation induction time (OIT).
Oxidation Induction Time (OIT) is an assessment of the time for which an antioxidant system present in the polymer inhibits oxidation while the material is held at a specified temperature or heated at a constant rate in an oxygen atmosphere. Antioxidants are often added during the formulation to hinder the aging caused by oxygen and to increase the material’s lifespan. OIT can be used to study the degree of polymer aging throughout the material’s lifetime due to exposure to heat, oxygen, light, and radiation.
Dynamic Mechanical Analysis (DMA) is a technique that can be used to provide information on the material’s physical structure via its viscoelastic mechanical properties. The test allows the material’s response to a sinusoidal force during a temperature or frequency sweep to be obtained. DMA can be used to determine the mechanical properties (mechanical modulus or stiffness and damping) of the composite and important thermal transitions of the adhesive, such as the glass transition temperature and the degree of cure of polymer and composite materials. DMA and DSC tests can also be used together to provide an easy, quick test program to compare a proposed alternative new raw material.
Thermogravimetric Analysis (TGA) can be used to provide information on the material’s chemical and physical structure via thermal decompositions. TGA provides information on the temperature and rate of decomposition of materials and the number of volatiles and fillers they contain. With advanced analysis software, characteristic temperatures such as melting points and decomposition temperatures can also be evaluated.
Thermomechanical Analysis (TMA) can be used to look at the material’s coefficient of thermal expansion (CTE). This technique enables the deformation of a material under a constant load to be measured while the material is subjected to a controlled temperature program. This is important to quantify because the expansion or contraction of a material under various conditions can cause issues during the manufacture of a new part or when the material is in final use. Quantification at an early stage can help to resolve any possible future-fit and assembly issues.
Mechanical properties of composites
A basic tensile strength test will provide an indication of the fundamental mechanical properties of the composite material. These results can then be used as a quick test to check the material integrity and ensure the manufacture conforms to a ‘fit for purpose’ standard. The tensile strength can also be used to provide an easily comparable indication of batch-to-batch material quality.
Climbing drum peel tests can be used to provide information on the suitability of potential raw material used in composite sandwich materials. In this test, thin monolithic facing sheets are bonded to a central honeycomb core. The facing sheet is peeled from the core to determine the peel resistance of the adhesive bond. This test is ideally looking for failure in the honeycomb and to demonstrate that the adhesive is the strongest material part and therefore fit for purpose to be accepted for further use.
Most thermoset resins, adhesives, and prepregs have a limited shelf life under cold storage conditions, which becomes even shorter when the temperature is increased to room temperature. For this reason, the times and temperatures of material exposure are carefully monitored and recorded. However, even with careful management, it is important to quantify the suitability of adhesive batches upon purchase from the supplier. This can be achieved through lap shear testing, which involves bonding two composite or aluminum plates together using the adhesive and pulling in tension.
As adhesive batches can be accepted or rejected for use based on the lap shear strength results, it is crucial that the samples are carefully prepared, and the test alignment is correct. A lap shear test can also be used to check the properties of an expired material, potentially providing data to support an extension of the adhesive’s shelf life.
Physical properties of composites
Density, volatile material percentage, resin percentage, fiber percentage, and percentage of porosity or void content are important basic material physical properties of composites that need to be quantified through constituent content testing before the raw material reaches the next stage of production.
There are extremely strict limits, usually set by the end-user, with regard to the required results, which have been calculated to provide optimized properties. The quantification of the material density is of particular importance as this can provide information on the level of crystallinity present. Molecules in the crystalline phases pack together more tightly than those in the amorphous phases, providing the material with a higher density.
Fiber areal weight and prepreg areal weight are often required to be checked when signing off on the quality of newly purchased raw material. This is to ensure that the mix ratio between the fiber and resin in the prepreg is optimized to provide the desired end properties and to avoid the addition of any unnecessary weight to the end component when used in production.
Fourier transform infrared spectroscopy (FTIR) provides information on the chemical structure of a material. This technique is employed to obtain an infrared spectrum of a solid, liquid or gas, which can be used to characterize the component. The different chemical bonds between atoms give different signals, with each material having a unique FTIR spectrum. FTIR can be used as a material quality control test to ensure that the chemical composition is right. It can also be used to investigate an unknown material by comparing the unknown sample spectrum with stored library spectra. Any suspected heat damage can be detected using this technique as the chemistry of the material may change after exposure to elevated heat.
Gel Permeation Chromatography (GPC), also known as Size Exclusion Chromatography (SEC), is a technique that can be used to separate the polymer molecules in the sample according to chain length. The polymeric molecular weight distribution (MWD) can be determined using GPC. It can be important to quantify this particular material property because it relates to the physical characteristics of the final material, such as tensile strength and crack propagation characteristics. The polymer manufacturer will have an optimized weight range where the idealized properties are balanced. Examples of such properties include viscosity which is important for processability, sample strength, toughness, and crack resistance.
The Element advantage
Element provides the full suite of tests to help material suppliers and manufacturers understand the raw material properties of composite materials. Our experts will prepare and test samples for routine tests as well as tests requiring specialized analysis equipment, such as DSC, TGA, TMA, DMA, FTIR, and GPC. Element’s extensive experience advising, preparing, testing, and certifying raw materials allows our customers to ensure the delivered material batch is the same as expected.
For more information on how to qualify and quantify the thermal, chemical, and mechanical properties of composite materials, contact an expert here.
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