Composite Material Testing and Analysis

A composite material is made by combining two or more distinct materials to create a new material with enhanced properties. The main idea behind composites is that the combined material can exhibit characteristics superior to those of its individual constituents.

Key components of composite materials:

1. Matrix: The matrix binds the reinforcements together, transferring the load among them. It can be made of polymers (like epoxy), metals (like aluminum), or ceramics.
2. Reinforcement: The reinforcement provides strength and stiffness to the composite. Common types include fibers (like carbon or glass fibers) and particles (like ceramic or metallic particles).

Examples of composite materials:

• Fiberglass: Made of glass fibers embedded in a polymer matrix. It’s known for its high strength-to-weight ratio and is used in boat hulls, automotive bodies, and sports equipment.
• Carbon Fiber Reinforced Polymer (CFRP): Consists of carbon fibers in a polymer matrix. It offers excellent stiffness, strength, and lightweight properties, making it ideal for aerospace, automotive, and sporting goods applications.
• Concrete: Made from cement (matrix) and aggregates like sand and gravel (reinforcement). It’s widely used in construction for its compressive strength.
• Plywood: Consists of thin wood veneers (reinforcement) bonded together with adhesives (matrix). It’s stronger and more durable than regular wood.

Advantages of composite materials:

• High strength-to-weight ratio: Composites can be stronger and lighter than traditional materials like metals.
• Tailorable properties: The properties of composites can be customized by varying the type, amount, and orientation of the reinforcement.
• Corrosion resistance: Many composites are resistant to corrosion, making them suitable for harsh environments.
• Design flexibility: Composites can be molded into complex shapes and sizes, providing greater design freedom.

Composite materials are used in a wide range of industries, including aerospace, automotive, construction, and sports, due to their superior performance and versatility. They play a crucial role in developing advanced engineering solutions and pushing the boundaries of modern technology.

What is the Need for Testing of Composite Materials?

Testing composite materials is crucial for several reasons, as it ensures the reliability, safety, and performance of these materials in various applications. Here’s a detailed look at the need for testing composite materials:

1. Understanding Material Properties

• Mechanical Properties: Testing helps determine properties like tensile strength, compressive strength, flexural strength, and fatigue life. This information is vital for designing and engineering applications.
• Thermal Properties: It’s essential to know how composites behave under different temperatures, including their thermal conductivity, expansion, and degradation.

2. Quality Control

• Consistency: Ensuring that composite materials meet specific standards and specifications is crucial for maintaining quality and consistency in production.
• Defect Detection: Testing can identify manufacturing defects such as voids, delaminations, and inclusions that could compromise the material’s integrity.

3. Performance Evaluation

• Load-Bearing Capacity: Understanding the load-bearing capacity of composite materials is essential for designing safe and efficient structures.
• Durability: Testing helps assess the long-term performance of composites under various environmental conditions, including moisture, UV exposure, and chemical exposure.

4. Safety Assurance

• Failure Analysis: Identifying the modes and mechanisms of failure helps engineers design safer and more reliable composite structures.
• Regulatory Compliance: Testing ensures that composite materials comply with industry standards and regulations, which is critical for applications in aerospace, automotive, and construction industries.

5. Optimization and Innovation

• Material Optimization: By understanding the properties and behavior of composite materials, engineers can optimize their use, improving performance while reducing weight and cost.
• New Developments: Testing facilitates the development of new composite materials with tailored properties for specific applications.

Mechanical Testing & Performance Assessment

Uniaxial Tension Test (Directional) (ASTM D638, ISO 527, ASTM D3039):

The stress (ζ) in a uniaxial tension testis calculated from;

ζ = Load / Area of the material sample ……………………………………..(1)

The strain(ε) is calculated from; ε = δl (change in length) / l (Initial length) ……………..(2)

The slope of the initial linear portion of the curve (E) is the Young’s modulus and given by; E = (ζ2- ζ1) / (ε2- ε1) ……………………………………..(3)

4 Point Bend Flexure Test (ASTM D6272): 

The four-point flexural test provides values for the modulus of elasticity in bending, flexural stress, flexural. This test is very similar to the three-point bending flexural test. The major difference being that with the addition of a fourth nose for load application the portion of the beam between the two loading points is put under maximum stress. In the 3 point bend test only the portion of beam under the loading nose is under stress.

This arrangement helps when testing high stiffness materials like ceramics infused polymers, where the number and severity of flaws under maximum stress is directly related to the flexural strength and crack initiation in the material. Compared to the three-point bending flexural test, there are no shear forces in the four-point bending flexural test in the area between the two loading pins.

Poisson’s Ratio Test as per ASTM D3039: 

Poisson’s ratio is one of the most important parameter used for structure design where all dimensional changes resulting from application of force need to be taken into account, specially for 3d printed materials. For this test method, Poisson’s ratio is obtained from strains resulting from uniaxial stress only. ASTM D3039 is primarily used to evaluate the Poison’s ratio. Testing is performed by applying a tensile force to a specimen and measuring various properties of the specimen under stress. Two strain gauges are bonded to the specimen at 0 and 90 degrees to measure the lateral and linear strains. The ratio of the lateral and linear strain provides us with the Poisson’s ratio. 

Flatwise Compression Test as per ASTM D695: 

The compressive properties of 3d printed materials are important when the product performs under compressive loading conditions. The testing is carried out in the direction normal to the plane of facings as the core would be placed in a structural sandwich construction. The test procedures pertain to compression call for test conditions where the deformation is applied under quasi-static conditions negating the mass and inertia effects.

The test procedures pertaining to compression call for test conditions where the deformation is applied under quasi-static conditions negating the mass and inertia effects.

Modified Compression Test as per Boeing BSS 7260:

Modified ASTM D695 and Boeing BSS 7260 is the testing specification that determines compressive strength and stiffness of polymer matrix composite materials using a loading compression test fixture. This test procedure introduces the compressive force into the specimen through end loading.

Axial Fatigue Test as per ASTM D7791 & D3479:

ASTM D7791 describes the determination of dynamic fatigueproperties of plastics in uniaxial loading conditions. Rigid or semi-rigid plastic samples are loaded intension (Procedure A) and rigid plastic samples are loaded incompression (Procedure B) to determine the effect of processing, surface condition, stress, and such,on the fatigue resistance of plastic and reinforced composite materials subjected to uniaxial stress for a large number of cycles.The results are suitable for study of high load carrying capability of candidate materials. ASTM recommends a test frequency of 5hz or lower.The tests can be carried out under load/stress or displacement/strain control. The test method allows generation of stress or strain as a function of cycles, with the fatigue limit characterized by failure of the specimen or reaching 10E+07 cycles.The maximum and minimum stress or strain levels are defined throughan R ratio.The Composites testing laboratory at AdvanSES is fully accredited for static, dynamic and fatigue tests.

3 Point Bend Flexure Test (ASTM D790):

Three point bending testing is carried out to understand the bending stress, flexural stress and strain of composite and thermoplastic 3d printed materials. The specimen is loaded in a horizontal position, and in such a way that the compressive stress occurs in the upper portion and the tensile stress occurs in the lower portion of the cross section.This is done by having round bars or curved surfaces supporting the specimen from underneath. Round bars or supports with suitable radii are provided so as to have a single point or line of contact with the specimen. The load is applied by the rounded nose on the top surface of the specimen. If the specimen is symmetrical about its cross section the maximum tensile and compressive stresses will be equal. This test fixture and geometry provides loading conditions so that specimen fails in tension or compression.

For most composite materials,the compressive strength is lower than the tensile strength, and the specimen will fail at the compression surface. This compressive failure is associated with the local buckling (micro buckling) of individual fibres.

Advanses has the setup for full service composites testing laboratory under one roof and we are accredited for static, dynamic and fatigue test

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