To aid in predicting and preventing failures, composites are tested before and after construction.
Pre-construction testing may use finite element analysis (FEA) for ply-by-ply analysis of curved surfaces and predicting wrinkling, crimping and dimpling of composites. Materials may be tested after construction through several nondestructive methods including ultrasonics, thermography, shearography and X-ray radiography.
11.1 Composite FEA
Composite Finite Element Analysis - can help creating Robust Composite Designs using"Composite Design Principles" - which is by far the most critical competency that a
composite manufacturer can have. Only a panoramic understanding of the various design aspects of composites can lead to a realistic Finite Element Analysis result and an Optimal Composite Design - that performs as intended.
Ironically these design aspects are numerous and many of them defy conventional engineering design logic. For example, as design engineers we are taught to "ignore" shear forces, in say a
"beam" made in a Structural Steel Rectangular Box Section - because as compared to the flexural / bending forces, the shear forces are negligible (Actually the shear forces are not negligible - it‘s just that the shear modulus and shear strength of isotropic materials like steel etc. is relatively high - thus the shear forces pale in comparison).
If one were to follow this logic and design a composite rectangular box section - then one is setting oneself up for a rude awakening. In Advanced Composite Structures shear forces cause significant deflections and in most poorly designed cases - it is the primary cause for failure.
Over the past few years we have developed sufficient expertise in using "Composite Design Principles" or "Composite Science" to design optimal composite components - relying heavily on the Finite Element Analysis Method.
Composite Finite Element Analysis (FEA) Capabilities
Our Composite Finite Element Analysis Capabilities can lead to the following tailor-made
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• Composite Mechanical Properties - Tensile, Compression, Deflection etc.
• Composite Thermal Properties - Thermal Expansion, Thermal Conductivity etc.
• Composite Fatigue Properties
• Composite Electric Properties - Electric Insulation, Electric Conductivity etc.
• Composite Radio Transparency - Electromagnetic Radiation, Diagnostic and Treatment Radiation like X-Rays, Gamma Radiation etc.
We use special software, tailor-made for composite finite element analysis and optimization for designing composite products. Dedicated high-end workstations for product modeling as well as for finite element analysis support the composite design process.
11.2 Ultrasonic Testing
Nondestructive testing (NDT) methods play an important role in physical characterization of new composite materials and in assessment of their quality and serviceability in structures. Ultrasonic NDT methods are an effective instrument for evaluation of elastic modules, strength, stiffness, and other essential parameters which are vital for analysis and design of structures. Elastic modules are very important for characterization of materials, but ultrasonic testing of composites gives a considerable dispersion of data. An attempt is made to approach composites as stochastic materials and to develop a new concept of ultrasonic data analysis. It is proposed to use effective dynamic elastic modules for ultrasonic characterization of composites
11.3 Thermography
New rotorcraft structural composite designs incorporate lower structural weight, reduced manufacturing complexity, and improved threat protection. These new structural concepts require nondestructive evaluation inspection technologies that can potentially be field-portable and able to inspect complex geometries for damage or structural defects. Two candidate technologies were considered: Thermography and Laser-Based Ultrasound (Laser UT).
Thermography and Laser UT have the advantage of being non-contact inspection methods, with Thermography being a full-field imaging method and Laser UT a point scanning technique.
These techniques were used to inspect composite samples that contained bothembedded flaws
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and impact damage of various size and shape. Results showed that the inspection techniques were able to detect both embedded and impact damage with varying degrees of success.
11.4 Shearography
Shearography is a variation of laser technology ESPI (holography), specifically designed for NDT applications. Shearography provides full-field, non-contact testing for rapid wide-field inspection of composites, bonded structures and other advanced materials. Shearography is an optical video strain gauge and an appropriately applied stress is used to locate strain
concentrations caused by internal defects. As an example, a composite helicopter blade can be inspected in production with vacuum excitation, while it can be rapidly inspected in the field with thermal excitation from a heat source, such as a heat gun or even a hairdryer. Typically light vacuum, thermal, acoustic or mechanical loading is used. This technology is ideal for
components with complex geometries and material compositions.
Common uses include:
- complex geometries, such as co-cured composites structures - complex material composition, such as engine inlets and blades - core structures (e.g.: honeycomb & foam)
- composite repair evaluation - corrosion inspection - production inspection
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11.5 X ray Radiography
Radiographic inspection of the laminate
This inspection method will detect the foreign material if material density is different from the graphite. Radiography will not detect delaminations because there is no difference in density between a delam area and clean (defect free) area. Radiography may be able to detect a crack in a composite material, but this is difficult due to the orientation of the crack.
Radiographic inspection of bonded part
Radiographic inspection of the bonded part will detect an unbond if there is a lack of adhesive condition (adhesive missing) because this would cause a density change. Radiographic
inspection can also detect foreign material, and core crush if the damage to the core is extensive.
Radiographic inspection is commonly used in conjunction with ultrasonic inspection for bonded components.
Radiography of composite materials is generally done at lower energy levels to obtain the required contrast and definition. Lower KV and smaller portable systems such as 160 KV units are very practical for performing radiographic tests on aircraft.
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