Composite Structural Analysis Services

Composite Strength Analysis

Modern high-performance composite materials can be used to make light-weight, stiff, fatigue-resistant structures that resist corrosion. However, an inexperienced structural analyst may look at material properties for unidirectional carbon-fiber composite and assume structures made from this material will be much lighter than those of more conventional materials such as aluminum and steel. While a well-designed composite structure can be considerable lighter than a comparable metallic structure, the weight savings is usually less than most people will assume. There are two main reasons for this. First, laminates made of composite materials only have stiffness and strength in the direction of the fibers. So, for a laminate to have stiffness and strength, fibers must run multiple directions. For example, if a carbon fiber composite part is required to have the same axial stiffness in all directions in the plane of the laminate (quasi-isotropic), the effective stiffness ends up being slightly more than half the stiffness of aluminum. It just so happens, that its density is also slightly greater than half that of aluminum. Therefore, the only strength advantage, when designing with quasi-isotropic laminates, is the increase bending stiffness caused by the thicker section of the composite. This allows for stiffeners to be space farther apart than those of a similar metallic part. The second reason that the weight saving of composites structures may be disappointing is that most published material allowables are for pristine lamina or laminates. Regulatory agencies will require knockdowns on these allowables generated from undamaged coupons. In aerospace, composite laminates are testing with notches, or holes, to account for actual holes required to fastener joints, as well as to mimic damage caused by impact. These notched allowables end up being around half those of un-notched.

So why use composite material at all? It is true that in many applications a metallic part will be superior to one made from composite. However, there are many applications when composite materials are not only a better choice, but may be the only material suitable for the job. High performance composite materials have the biggest structural advantage over metals in parts with consistent load paths, loaded in-plane, in a high cyclic environment. One example of this would be a propeller operating in unsteady inflow, such as behind a wing. An aluminum propeller may begin to crack in little number of flight hours, whereas the composite propeller may show no signs of damage. There are obviously many other application where composite materials have a structural advanage over metals, we just have to be realistic about the amount of weight we can save.

Composite Fatigue, Durability and Damage Tolerance Analysis

Properly designed composite laminates generally do not have limited durability from fatigue when working strains are kept below that which is required to meet a damage tolerance requirement. In aerospace, fracture mechanics solutions are used to predict the damage tolerance of some metallic parts. Durability is either predicted using fatigue or fracture mechanics approaches. For aerospace composite parts, notched allowables are usually shown to be more conservative than coupons impacted with an appropriate amount of energy. Fatigue is generally not a design limiting factor for laminates made with modern, toughened epoxies that are loaded in-plane because of the strain limits imposed from the damage tolerance requirements.

The durability and damage tolerance capability of a composite laminate, loaded out-of-plane, is matrix dominated. Therefore, structures with high out-of-plane loads may have limited fatigue life even when undamaged. Laminates with flaws between plies may have limited life if interlaminar stress are too high.

If a structure is to be used in an application where failure will not result in an injury, death or property damage, less stringent damage tolerance methodologies can be used. Strain cutoffs closer to those obtained from un-notched coupons can be used.

Common Design Mistakes Using Composite Materials

There have been many aerospace projects built from composite materials that have failed to meet durability or damage tolerance criteria because the analysts, either, did not check the interlaminar stress levels, or allowed them to be too high to achieve a desired life. These situations are usually cause by high interlaminar shear stresses cause from short laminates with high out-of-plane loads, or high interlaminar tension stresses caused from tight radiuses in laminates under high bending loads.

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