Literature Review

Discuss About The Composite Defects In Materials Literature?

Composites refer to materials more than one material, so as to take advantage of the structural and mechanical properties of the characteristic components to form a material with desirable properties. The structure of a composite entails the different component materials transferring the forces it experiences to the adjacent structural members of the composite structure. The commercial application of these elements continues to grow in the contemporary world, as the demand for materials with challenging properties continues to increase. For this reason, regular material are continually being replaced by these composite materials especially in the aviation and space, as well as the automobile industry, as this new breed of composite materials exhibit higher strength than that experienced by the component materials of a composite. In addition, these materials can be custom combined to come up with thousands of different formulations in the mechanical engineering fields, with varying flexural and tensile strengths higher than those of the composite materials (Persson, et al., 2007), p. 142).  These materials also couple their high strength levels with low weight and low densities, making them appropriate for high performance and design uses in these fields of mechanical engineering.

These materials maintain their durability by having a weight that is less than that of other materials in their caliber meaning that they are better materials than all the once used in the aviation and automation industry due to lower fuel cost economy. This achieved through the use of carbon composite materials in specific parts of the body of the plane or the vehicle, reducing the weight of the structure by about half of its original weight (Landel & Nielsen, 2013)  p. 96). In this regard, failure of composite materials is dependent on the mechanical and thermodynamic properties of the constituent materials, as well as the geometry and loading types felt by the individual constituent materials. These factors determine the distribution and states of stress in the material and this in turn determines the mode and location of failure in the material (Huang & Talreja, 2005) p. 1967). This paper thus delved into the effect of defects in the composition of composite materials and how they affect the material’s ability to transfer loads and stresses to the adjacent members of the composite structure.

Composite materials are made up of two or more component materials whose general categories are the reinforcement particles, fibers, or flakes in a matrix of another material. The combination of the matrix and the reinforcement forms up a discontinuous material phase that is stronger and even of a higher mechanical thickness than the continuous form of the matrix material. The strength of these materials is greatly determined by the size of the reinforcements and their distribution within the matrix (Hutchinson & Suo, 2011)p. 67). This is because the strength of these materials is improved by the presence of the reinforcement particles (Hocheng & Tsao, 2006)p. 1407). In the process of manufacturing these materials however, several defects may occur in the distribution of the materials and this greatly impacts the strength and hardness of the composite material. In addition, there are some necessary defects that have to be placed on the material for it to be applicable such as the poring of screwing holes. These activities are the major defects that affect the distribution and concentration of these reinforcement particles and thus affecting their strength other chemical properties. The defects caused as a result of drilling holes on the material are also known as circular defects and they could either be filled or left as they are. The are other defects that exist within the material, such that the distribution and concentration of the reinforcement particles and fibers is not equal due to irregularities in the lamination process of making the material.  Defects can also be brought about as a result of dents in the composite materials, as this affects the concentration and distribution of particles (Wisnom, 2009)p. 1937). This could also be done by the inclusion of laminates in the matrix component and thus affecting the distribution of the reinforcing particles.

(Fu, et al., 2008) p. 934)

Fig 1. The classification of composite materials and the reinforcement types

The addition of flakes or particles into a continuous matrix improves the specific modulus and the specific strength of the material, and thus improving the mechanical properties of the materials.

Where E= Young’s modulus

Material Density

 = ultimate strength of the material

These measures may sometimes differ within the same material as a result of defects caused by an uneven distribution and concentration of the reinforcement particles. The mechanical characteristics of may greatly differ within the particles since the irregularities in distribution and concentration of the particles may contribute to differences in the kinetics of material reactions as well as thermal expansion within the material. Other basic properties of the material that are significantly affected by these differences include the fiber volume fraction, specific strength, shear stresses and strains, density, thermal expansion and the possibility of voids (Almeida & Neto, 2004)p. 141). These properties can be easily investigated in the laboratories using the tests of tensile loading and bending loading.

It is also important to note that composite materials do not exhibit ductility due to the nature of the relationship of particle reinforced composite materials whose tensile loading does not allow for ductility. Composite materials snap soon after the matrix and the reinforcements are separated in tension, unlike ductile materials like steels and aluminum that exhibit ductility and take some plasticity before fracturing. Failure in these composite materials is cumulative in that damages ought to have taken place within the material under load until when all the reinforcements have been completely disassociated from the matrix and from each other. The disassociation takes place in three different mechanisms, which entail the breaking of the reinforcement particles or fibers, followed by the matrix material cracking, and then finally the de-bonding of the interface, which is the interface between the fiber and the matrix. In a situation where the distribution and concentration of these fibers of the reinforcement material is uneven or irregular, the fracturing of the material becomes way easier as the process of the reinforcement being broken could either require more loading or less, and thus the fracturing of composites is greatly impacted by the distribution and concentration of the reinforcement particles.

It is also important to point out that each disassociation step in the composite fracturing process caused by different types of loading. This means that the differences in mechanical properties affect the mode of occurrence of each of these consecutive processes in during fracturing. The even distribution and concentration of reinforcement particles and fiber within the matrix material leads to a uniform distribution of the damages caused by the load within the material, which then coalesce to make even larger cracks within the material and thus fracturing or failure. The interface between the reinforcement and the matrix is mainly contributed to by shear stresses, which cause failure in the interface due to transferring that shear stress between the reinforcement particles and the matrix. For this reason, the interfaces that have high strength to shear stress thus exhibit high levels of strength and stiffness, although the toughness of the materials is compromised by this aspect. Consequently, having weak interfaces regions allows the matrix to deflect the cracks on the matrix in the same direction with the interface, and thus allowing the stiffness and strength of the material to be low and the failure toughness of the material to be low. The shear strength in the interface can thus be used to determine the disassociation procedure stages. Finally, the strength of the particle or fiber reinforcements determines the ultimate strength of the composite material. For this reason, the distribution and concentration of the reinforcement particles is of key importance to the strength, stiffness, and toughness of the material (Nik, et al., 2014) p. 164). Defects have an impact on the distribution and concentration of these materials as they reduce the concentration of the reinforcement particles and fibers within the matrix material and thus make the material weaker than if they had not existed.

In the longitudinal tensile loading conditions, the average stress (s₁) is equally distributed across the entire composite material. Considering that the equilibrium loading is a total of the loading forces felt through the interface, the matrix and the reinforcement:

s₁ A =s_f A_f + s_m A_m + s_i A_{i  …………} (1)

Where f is for area of the fiber

m for area of the matrix

i is for area of the interface and

A = A_f + A_m+A_i

Therefore, dividing equation 1 by A on both sides, we get:

s₁ V =s_f V_f + s_m V_m + s_i V_{i  …………} (2)

where f is for volume ratio of the fiber

m for volume ratio of the matrix

i is for volume ratio of the interface and

A = A_f + A_m+A_i

(Adams & Cawley , 2008) p. 211)

Finally since composite materials are only affected by defects if the defects affect the distribution and concentration of the reinforcement particles and fibers. This gives composite materials an advantage of not being affected by physical damages that do not interfere with the internal structure of the material. Thus, defects do not lead to the damage of the material when it is exposed to zones of high temperatures, corrosive environments and even through scratches. The material experiences a difference in behavior when undergoing thermal expansion encouraging the reinforcement material to undergo superficial destruction of the reinforcement material which then eases when the heat conditions are removed. When the fibers and particles undergo this partial and superficial destruction, the matrix panel warps as a result of this heat treatment (Cantwell & Moton , 2012) p. 31). This explains why composite materials have commendable thermal properties.

The role played by defects in materials mainly compromises the strength and applicability of those materials in different uses and purposes. The use of composite materials has been greatly contributed to by the fact that the materials have presented immense opportunities in the manufacture of automotive and aviation systems, and thus improving the costs and efficiency of the material for its intended use. The comprehension of what comprise of defects and how they are manufactures allows the manufacturers and users of the composite material to aim at keeping the functionality and mechanical properties of this material in the best possible conditions. It also helps to highlight the different defects that compromise the functional quality of the material in different conditions (Chamis, 2009) p.7). Knowing how the material respond in the event of defects allows the material uses to be evaluated for the intended purpose and corrected in the event that the defects in the particular material compromises the stability, stiffness, and structural strength of the material for its intended purpose.

When analyzing the composite materials in a laboratory during an experimental analysis of the effect of defects in these composite materials, the researchers will know how to predict and analyze the precision of the results as is expected from the theoretical analysis due to the literature review (Kessler, et al., 2012) p. 269). In addition, the insights collected from this project will also be helpful in explaining the results that will be obtained in the mechanical and thermal analysis of properties that are being investigated in the study. Having a theoretical background is important because it helps to come up with the correct deductions and inferences concerning the impact of these defects to the structural and mechanical intelligence of composite materials for their application in their intended purpose.

An analysis of the defects will contribute to the literature review through conducting tests that will prove that the insights highlighted by the theoretical analysis in the literature review are indeed accurate and precise. The literature review was able to expound on the reasons why defects have the said effects on the composite materials and thus the research will experimentally confirm these allegations. Thus conducting an experimental research about the impact of defects on the structural and mechanical intelligence of composite materials for the purpose the material is intended for allows for better design and more caution in the manufacture of these materials (Croft, et al., 2011) p. 489). Properly understanding the impact of defects in these materials and how it varies from one material to the next will discourage the use of defective materials and give experts reasons for ensuring that the mechanical and structural components are measured before the materials is applied in any items.

Conclusion

Composite materials continue to improve in their applicability in different fields as a result their versatility and mechanical and structural capabilities. In situations where the composite materials are subjected to loading, the materials have exhibited enhanced structural and mechanical properties promoting their demand to be applied for different functions. However, the impact of defects on this materials greatly reduce their strength, stability, and stiffness, and thus affecting their ability to be suitably used for different purposes. The weight of these defective composite materials and their toughness are also improved by the defects but this happens at the cost of the mechanical and structural properties.

References

Adams, R. & Cawley , P. R., 2008. A review of defect types and non-destructive testing techniques for composites and bonded joints. NDT International, 21(4), pp. 208-222.

Almeida, S. d. & Neto, Z. S., 2004. Effect of void content on the strength of the composite laminates. Composite Structures, 28(2), pp. 139-148.

Cantwell, W. & Moton , J., 2012. The significance of damage and defects and their detection in compoiste material: Economics. The Journal of Strain analysis for Engineering Design, 27(1), pp. 29-42.

Chamis, C., 2009. Mechanics of composite materials: Past, present, and Future. Journal of the Composites, 11(1), pp. 3-14.

Croft, K. et al., 2011. Experimental study of the effect of of automated fiber placement induced defects on performance of composite laminates. Composites Part A: Applied Science and Manufacturing, 42(5), pp. 484-491.

Fu, S., Feng, X. Q., Lauke, B. & Mai, Y. W., 2008. Effects of particles size, particle/matrix interface adhesion and particle loadingon mechanical propertis of particulate-polymer composites. Composite Part B: Engineering, 28(2), pp. 933-961.

Hocheng, H. & Tsao, C. C., 2006. Effects of special drill bits on drill nits in drilling- included delamination of composite materials. International Journal of Machine Tools and MAnufacture, 46(13), pp. 1403-1416.

Huang, H. & Talreja, R., 2005. Effects of void geometryon elasti propertiesof unidirectionalfiber reinforced composites. Composites Science and Technology, 65(13), pp. 1964-1981.

Hutchinson, J. W. & Suo, Z., 2011. Mixed mode cracking in layered materials. Advances in Applied Mechanics, 29(3), pp. 63-191.

Kessler, S., Spearing, S. M. & Soutis, C., 2012. Damage detection in composite materials using Lamb wave methods. Smart Materials and Structures, 11(2), p. 269.

Landel, R. & Nielsen, L. E., 2013. Mechanical properties of polymers and composites. 2nd ed. London: accounting.

Makeev, A., Nikishkov, Y., Carpentier, P. & Noel, J., 2015. Manufacturing issues and measurement techniques for assessment of the effects on structural performance of composite partes: Computed Tomography (CT( offers accurate 3D measurement and characterization of defects in composite structure. Quality, 54(11), pp. S12-S12.

Nik, M., Fayazbakhsh, K., Pasini, D. & Lessard, L., 2014. Optimization of variable stiffness composites with management defects induced by automated fiber placement. Composite structures, 107(4), pp. 160-166.

Persson, E., Eriksson, I. & Zackrisson, L., 2007. Effects of hole machining defects on strength and fatugue life of composite laminates. Composite Part A: Applied Science and Manufacturing, 28(2), pp. 141-151.

Tajvidi, M., Falk, R. H. & Hermanson, J. C., 2008. Effect of natural fibeers on thermal and mechanical properties of natural fiber polypropylene composite studied by dynamic mechanical analysis. Journal of Applied Polymer Science, 101(6), pp. 4341-4349.

Wisnom, M., 2009. Size effects in the testing of fiber- composite materials. Composite Science and Technology, 59(13), pp. 1937-1957