8.9 Weldability

Weldability is the ability of materials (or structures) to form a strong defect-free weld. Poor weldability results in hot cracking, cold cracking or lack of fusion. Preliminary heating of base parts (preheating) decreases hot cracking. The best weldability can be obtained in the gravity position. Weldability depends on materials. This list is in order of decreasing weldability: Steel 0.2%C, cold-rolled Stainless steel Aluminum alloy 7075-T6 Ductile iron The melting points of aluminum alloy and steel differ by more than 500 degrees Celsius. The metals cannot be melted together. To form a defect-free tee-joint it is better to use similar thickness of welded parts. An all-around welding of two massive (rigid) parts results in hot cracking. The more rigid a welded structure, the more susceptible to hot cracking. The specimens are shown in increasing order for susceptibility to hot cracking and decreasing order of weldability. There are special tests to estimate weldability of materials: 1. Y-groove restrain cracking test for heavy plates with a new weld in the center. Hot cracking in the new weld is under investigation. This test helps to estimate susceptibility to hot cracking. 2.The implant method for studying weldability and determining susceptibility to cold cracking. 3.Tension of a machined specimen for studying susceptibility to lamellar cracking.

8.1

There are some simple rules how to design a reliable welded structure. We mention a few of them here: A. Keep welds away from zones of high stress concentration. B. Keep welds away from surfaces to be machined. C. Don't make butt-weld intersections. D. Place vertical walls where force is applied. E. Choose proper weld size. F. Avoid gaps. G. Don't use sharp rigidity transition in tensile flange.

References

Cary, H.B. Modern Welding Technology, Englewood Cliffs, N.J.: Prentice-Hall, 1979.

Gray, T.G.F., J. Spence, and T.H. North Rational Welding Design, New York : Butterworths, 1975.

Kalpakjian S. Manufacturing Engineering And Technology, Addison-Wesley Publishing Company, 1989.

Metals Handbook, 9th ed., Vol. 6: Welding, Brazing, and Soldering, Metals Park, Ohio: American Society for Metals, 1983.

Welding Handbook 8th ed., 3 vols, Maiami: American Welding Society, 1987.

THEMES

Theme 1. Stress Concentration Theme 2. Fracure Mechanics Theme 3. Mechanical Properties Theme 4. Strength of Materials Theme 5. Theory of Elasticity Theme 6. Structural Safety Theme 7. Material Science Theme 8. Welds Theme 9.Composite Materials Theme 10. Finite Element Analysis

9 Composites

Igor Kokcharov

9.1 Structure of composites

A composite material consists of two or more components. The components have different mechanical properties. There are the following types of composites: A. composites reinforced by particles; B. composites reinforced by chopped strands; C. unidirectional composites; D. laminates; E. fabric reinforced plastics; F. honeycomb composite structure; Composite materials are widely used in aerospace structures, passenger airplanes, cars, and sporting goods. Use of aramid fiber reinforced plastics 1, carbon fiber reinforced plastics 2, hybrid fiber (aramid + carbon) reinforced plastics 3, and glass fiber reinforced plastics 4 decreases the weight of passenger airplanes. Particle content is defined by studying the cross section of a specimen. The parameter is equal to the ratio of the total area of the particles (fibers) to the cross sectional area of the specimen. Regarding particle and fiber reinforced matrices, the modulus of elasticity will increase with larger hard particle content. Broken rigid fibers in a flexible matrix causes a stress concentration in the neighboring fibers. The stress concentration factor increases with the difference between the modulus of elasticity of the matrix and fiber. It can range from 1.2 to 1.5. There are shear stress concentrations at the bond surface between components. Stress concentration in laminate materials is higher than in isotropic materials. Hence, the strength of a notched composite specimen is rather high. The notation [0^o/90^o]_2S means the laminate is assembled with two layers oriented at 0^o and 90^o. Tensile and shear stresses cause different failure scenarios for composite structures. Honeycomb composite structure has high flexural strength. Mechanisms of stability loss under compression depends on many factors such as adhesive quality, size of honeycomb, fiber filament, etc. There is a residual technological microcracking in composites reinforced by particles. If thermal expansion is high for particles, then the particles are under compression after cooling. Weak particles contain internal microcracks. If the matrix or bond border is weaker than particles, there are tangential microcracks in the matrix and at the border.