Utilization Of Metal Foams In Structural Functions And Energy Absorption Capacity

1. what are the fundamental science, math, engineering concepts related to your research (scope),
2. what part of your research work has ever been investigated before and what has not, (some of this may have been included in the introduction)
3. how does your research work relate to that done by others,
4. How have others defined/measured/identified the key concepts of your research,
5. what data sources have you used or have other researchers used in developing general explanations for observed variations in a behavior or phenomenon in a concept in your thesis etc.

Mechanical Performance of Metal Foams

Metal forms are a unique type of materials with special physical, thermalcellular, mechanical, acoustic, and electrical properties  and  a porous structure. They demonstrate a structure with low values of density and high levels of stiffness, strength, sound absorption, high energy of impact capacity, heat expulsion, and fire dissipation. These metal forms can be utilized as foundation for sandwich plates, as firmer in shell configurations to  buckle the blocks, for the sake of the energy sucking parts in vehicle crumple zones and as effective exchangers of heat to reduce heat generated by high power electronics (Adler, Standke, and Stephani, 2004). These materials can be utilized in structural functions due to their high strength to density ratio, excellent structural stability and sturdiness when compared to that of other foams or in absorbing of energy and explosion safety purposes due to their energy absorption capacity in any path and direction at low through modest levels of stress contrasted against bulk metals.  Upon collision, metal foams lose form plastically at a comparatively level of stress over an expansive area of strain, while absorbing all the kinetic force prior to reaching densification.

The metal foams’ mechanical performance was determined to be greatly affected by the characteristics of the foams like cell shape, cell size, cell connectivity, and cell wall width (thickness). The present metal foam development methods can solely control the size of the cell to a certain degree (Angel, Bleck, and Scholz, 2004). The curvy deformations of the cell wall width, cell walls, and the non-homogeneous size and shape of the cells lead to tainted and degraded mechanical performance making it hard to forecast the material’s failure and performance. These distortions can be surmounted by integrating executed void balls in metal foams. The executed spheres of the metal void have a homogenous cell shape, size, and wall width and can be crammed into a solid collection of structure of foam. The metal void spheres are developed by covering EPSs (expanded polystyrene spheres) using a technique that uses suspended metal powder. This takes place as the perfectly round objects move violently through a surface that has been fluidized. This type of movement gives a homogenous outside layer on the round objects (spheres) and permits almost immediate liquid drying.

The spheres that have been coated are exposed to heat to pyrolyze the binding agent and the expanded polystyrene spheres, pursued by coalescing of the metal dust particles to produce a shell that is dense. These rounded substances (spheres) can be coalesced to produce a void ball structure or they can alternatively be left as single spheres to be utilized in other manufacturing functions. This compound metal foam has gains in isotropy, mechanical characteristics, and uniformity that allow for homogeny in the designs developed in technical applications (Ikeda, Nakajima, and Aoki, 2005). 316L stainless steel and LC (low carbon) steel void perfect balls were used in the development of CMFs (composite metal foams). CMF testing delves into the physical, mechanical, and microstructural characteristics of the Aluminum stainless steel and Aluminum-Low Carbon steel composite metal foams through the use of different methods including compression and micro hardness testing, and through the use of ANSYS and suggests a relation among the microstructure of the metal foam,  the development temperature, and the material’s mechanical performance during loading.

Development of Composite Metal Foams

Compression tests of the Quasi-static type indicates an even deformation activity without the occurrence of centralized collapse bands, which results in a great plateau rigidity in the range of 50 to 150 MPa which depends on the substance properties and processing methods. CMFs (Composite metal foams) have also been subjected to testing widely in compression-compression forces, unloading-loading compression forces, and four-point bending forces. Nevertheless, their performance at greater loading speeds has never been subjected to testing. Exclusive of a lone exception, many of the studies done on loading of great strain speed of metal foams in the literature account for an increase in the yield and strength of the plateau as the rate of loading rises for metal foams made from steel, aluminum, and void spherical structures. The effect of rate of strain on foams of metal is discussed as a merger of air that has been compressed and trapped within the cells for the duration of impact, rate of strain sensitivity of the material of the cell wall, and micro-inertial impacts of the walls of the cells, though a few dissimilarities occur in the write up as to which of the mentioned impacts is principal. A number of studies have examined Aluminum foams at greater speeds of up to 210 meters/s, and account for an added phenomena characterized by shock that dictates the metal foam’s performance over a vital velocity. This vital velocity seems to rely on properties of foam, such as mass and density, and for a few metal foams it is detailed to be as low as 50 to 55 meters/s (Bao, and Han, 2009). In this study, composite foams of structural steel, developed through the use of powder metallurgy method with a number of sphere sizes, are subjected to testing through the use of software simulation called ANSYS to determine benefits obtained from the development and use of composite metal foam.

In the last decade, numerous new processes designed for creating metal foams, mostly steel and aluminum or their alloys have been crafted. Studies have led to the monetization and commercialization of some products. Closed-cell aluminum form provides a special mixture of properties like high stiffness, low density, energy absorption capacity, and strength. Among the different possible applications, a few are common to other kinds of foams. Key field of application is the energy dissipation and absorption with regard to polymeric foams, even considering the higher weight / volume unit (Brown, Vendra, and Rabiei, 2010).

Testing of Composite Metal Foams

One of the benefits of using metal forms, like steel foams, is the higher range of permittable temperatures; it is around 100 °C for polymers and around five times that for steel foams. A key dissimilarity in the mechanical performance of materials pertaining to conventional homogenous materials is that the fracture or bending of a foam material is dependent on the material’s hydrostatic stress state. Hence, it is impossible to explain the failure from a sole machine test and failure standard.

The highest energy of distortion (Von Mises criterion) depends on the examination of the deviatoric constituent of stress, a result that strength energy in compression is varied from strength in tension. Thus, it is crucial to conduct analyses using different mixtures of hydrostatic and deviatoric states of stress. As widely as it is understood, the hydrostatic element of stress σ_hyd is the initial stress invariant described as

σ_{hyd = (}σ₁+σ₂+σ₃)/3 = (σ_x+σ_y+σ_z)/3

Equally understood is the model of the deviatoric stress constituent σ_dev, which is also taken to be the stress of the octahedral tangent:

σ_dev = √ (σ₁ – σ₂)² + (σ₂ – σ₃)² + (σ₃ – σ₁)²

Shear test is one of the examinations that provide untainted deviatoric stress (σ_hyd = 0_, σ’_{dev =} τ / √3). The triaxial or hydrostatic test, on the other hand, is the assessment gives untainted hydrostatic stress in the substance (σ_hyd = -p / 3_, σ’_dev = 0). A tensile examination provides a mixture of hydrostatic and deviatoric stress constituents σ’_dev / σ_hyd = -√2 (Banhart, and Seeliger, 2008). Nonetheless, for simplicity in analysis, the highest energy of distortion effective stress is utilized in this task as the parameter of the deviatoric stress, varying from the actual devairtoric stress by a factor of (√2/3):

σ_{dev =}  √(σ₁ – σ₂)² + (σ₂ – σ₃)² + (σ₃ – σ₁)² ≡ σ_VM

Therefore, a tensile test analysis matches to a ratio of the hydrostatic and deviatoric stress element: σ_dev / σ_hyd = 3. The key purpose of this test was to achieve a full description of the mechanical behavior of composite metal foam for the structural steel. To obtain results, a series of tests were conducted to typify the material plus assembling the necessary testing apparatus to carry out the necessary tests. The assessments that were described for the examination were the compression and tensile tests. These were needed to determine the mechanical performance of structural stel composite metal foam. Lastly, it was crucial to show the efficiency of the use of this metal foam for the absorption of energy. A chain of tests on a composite makeup made of the foam were introduced inside of a non-complex tubular structure (Coquard, Rochais, and Baillis, 2012). The tests conducted on this substance were easy bending and compression, which are the most pertinent modes of loading during bumping and crashing of vehicle outer components.

Application of Metal Foams

The capacity to foam steel provides impending non-structural and structural benefits over firm steel that have been widely utilized in the applicable design purposes: 

Non-structural benefits	Structural merits
• Reduce the conductivity of heat	• Reduce weight
• Develop acoustical performance	• Maximize stiffness (particularly bending)
• Allow the movement of air and fluids within material	• Enhance energy dissipation
• Providing protection against Electromagnetism and radiation	• Amplify mechanical damping
• Combination of thermally dissimilar materials	• Tune vibration frequency

Table 1: Steel foam characteristics

When contrasted against aluminum foams, for instance, structural steel foam uses are at their infant stage. Whereas primary benefits of utilizing steel over aluminum as a foundation metal are clear (for instance greater preliminary E and F_y), processing has been more difficult (Cardoso, and Oliveira, 2010). However, steel foam rods, bars, foam filled tubes, and foam foundation sandwich plates have been developed and tested at order 50 mm diameter and 300 mm length which is the lab scale.

Current uses are mostly in the aerospace, mechanical, and automotive fields. Foams of steel illustrate high ration of the material’s stiffness to weight when a load is paced in flexure. Specifically, panels of foam have greater bending rigidity than firm steel sheets of similar weight. Thus, most of the current structural uses aim to either reduce weight given rigidity limitations, or increase on stiffness with the provision of weight limitations. For instance, a processed 16 mm sandwich board (1 mm faces of steel with 14 mm core of the metal foam) has equivalent bending rigidity to a firm and solid plate of steel which is 10 mm in thickness, but at merely 35% of the weight (Friedl, et.al, 2008).

Moreover, the design satisfied set serviceability and strength needs, with the inclusion of strength and deflection under centralized loading, and the utilization of the steel foam sandwich boards decreased the weight of the floors by 75% in contrast to conventional reinforced concrete floors and decks. Decreasing the weight may have astounding advantages. The firm body dynamics of the arm of a crane determines that the weight of the crane arm controls the highest lift. The arm of a crane with equal rigidity but less mass can lift much more with the same ballast.

With this primary code, a metal foam lifting arm weighing about half than the solid steel was developed. The machine is being manufactured between 60 to 100 units annually and effectively underwent great sequence fatigue tests; therefore showing that greatly loaded beams in fatigue loading are probable with metal foams. Additionally, mechanical instances include enhancements in fabrication equipment and the tip of an experimental rocket which examines the structural advantages of enhancing mechanical damping, and altering the vibration rate of constituent elements. 

During compression, foams of steel show a stress-strain curve, with elastic area, a flat region where the hollows start deforming the plastics and a region of densification where cell walls make contact with each other and compressive resistance quickly increases. The probability for noteworthy dissipation of energy in compression is a goal for numerous current applications.

Conclusion

Dissipation of energy through huge compressive deformations at constant and low levels of stress has been utilized in the vehicle manufacturing industry for protection against crashes. The stress at yield of the foam is crafted so that it cannot significantly alter the weight carrying properties of the main vehicle frame (Kremer, Liszkiewicz, and Adkins, 2004). Automobiles filled with foamed metals tend to decelerate over a great period and distance thus decreasing accelerations seen by the occupants of the vehicle. The capacity to absorb impact or blast energy at the same time as limiting levels of stress is important to the design of vigorous solidifying systems for civil structures and systems.

A crucial structural benefit for metal forms that has not been shown to this date is the alleviation of buckling both for plates and rods, and the alteration of states of limit from unbalanced modes of buckling with small dissipation of energy to balanced crushing modes and/or post-buckling. Moreover, uses with great rate of strain low-cycle fatigue have not been studied. Current structural benefits show the promise for steel foams in civil and automotive uses. 

Current uses are hugely in the mechanical engineering discipline, thus, for every use the possible for multifunctional steel foam elements (roof and/or wall boards that have greater energy and structural behavior) is clear.

The uniqueness of steel foam as and technical material, and the strange, greatly porous, microstructure make data of the processing essential. Important studies have been conducted with regard to optimal processing techniques for metal foams, like titanium, copper, and aluminum but steel give odd challenges which includes steel exceptionally high melting point, that needs new technology (Lefebvre, Banhart, and Dunand, 2008). Existing techniques of steel foam processing can create permeable voids (open-celled) or sealed hollows (closed-celled) foams with changing isotropy, regularity, and density. Three specific processing methods are used, namely: powder metallurgy as it has already been followed successfully to make structural scale steel foam model, void spheres as this technique is in current commercial scale production, and the Lotus type as this technique has great possibilities for constant casting processes essential for little cost steel foam manufacturing.

Process	Primary variables	Min density	Max density	Cell morph.	Morphology notes	Major advantages	Major disadvantages
Powder metallurgical	Foaming agents cooling patterns	0.04	0.65	Closed	Anisotropic if not annealed for long enough, or with some combining methods	High relative densities possible
Injection molding with glass balls	Types of glass (e.g. IM30K, S60HS)	0.48	0.66	Closed	Glass holds shape of voids, and increases brittleness of material	High relative densities possible	Potential chemical reactions with glass; some glass can break in forming process
Oxide ceramic foam precursor	Ceramic/ cement precursor materials	0.13	0.23	Open	Polygonal shapes on small scales, residues of reactions remain	Foaming at room temperatures; complex shapes possible; standard equipment
Consolidation of hollow spheres	Sphere manufacture, sphere connections	0.04	0.21	Either	Two different cell voids: interior of the spheres, and spaces between spheres	Very low relative densities possible; highly predictable and consistent behavior	High relative densities not possible
Working and sintering of bimaterial rods	Types of working before sintering, filler materials	0.05	0.95	Open	Anisotropy is controllable	Wide range of relative densities possible; anisotropies are controllable
Composite PM/hollow spheres	Matrix material used, casting may be done instead of PM	0.32	0.43	Closed	Powder metallurgical region may be foamed or a semi-solid matrix	Behavior is both predictable and strong; no collapse bands until densification
Slip reaction foam sintering	Dispersant, bubbling agent, and relative quantities	0.12	0.41	Open	Highly variable cell diameters are produced	Many optimizable manufacturing parameters; foaming at room temp.
Polymer foam precursor	Polymer material used	0.04	0.11	Open	Cells take on whatever characteristics the polymer foam had	Low density open-cell structure for filter and sound absorption applications	Too weak for most structural applications
Powder space holder	Filler material used, shapes and gradation of material	0.35	0.95	Closed	Porosity may be graded across material	Porosity may be graded across a wide range across the material
Lotus-type/gasar	Partial pressure of gas, which gas to use	0.36	1.00	Closed	Highly anisotropic but aligned cell shapes are unavoidable	Manufacturing by continuous production techniques; high relative densities are possible	Isotropic cell morphologies are not possible

Table 2: Steel foam manufacturing processes

Initially meant for Al foams, the powder metallurgy technique was among the original procedures to be used on foams of steel and is still among the most popular. It is used to chiefly produce foams of closed-cell and is able to develop greatly anisotropic cell morphologies. The comparable densities probable with this technique are among the utmost, with readings of up to 0.65, which makes it a strong contender for application of structural engineering that require that the foam preserves a comparatively great portion of the strength of the foundation material. The powder metallurgy technique entails mixing metal powders with an agent of foaming, compacting condensing the resulting combination, and then sintering the compressed void at pressures of between 90 and 1000 MPa (Lee, Jeon, and Kang, 2007). The metal reaches the melting point and kept there for a certain amount of time depending on the agent of foaming and wanted cell morphology, typically about 15 minutes. The finished product can also be treated with heat to maximize the crystal structure of the foundation metal. A deviation, called the powder space holder procedure, involves the use of a simple packing material rather than the agent of foaming and permits for rated porosity across the substance. 

Providing greatly expected mechanical behavior and needing only negligible heat treatment, the void spheres technique comes in second in the most currently popular methods for processing steel foams. Depending on the geometry of the sphere closed or mixed open and closed cell morphologies are probable, with comparable densities form about 4 to 20%.

The technique creates highly expected material behaviors as cell (hollow) size is highly restricted. Void sphere techniques include taking void metal spheres before manufacturing and combining them with the use of an adhesive substance, condensed through powder metallurgy methods, casting in a metal medium, or through sphere sintering. One unique variation entails processing the spheres using a blowing agent inside and then permitting the spheres to expand and sinter into the consequent shapes and sizes. 

The lotus-type technique, also called the gasar technique, is able to produce high density foams, varying from around 35% to 100% comparative density with highly anisotropic closed cell structure. The lotus-type features the huge benefit of being readily adaptable to a constant casting procedure. This technique also permits for great tensile ductility and strength of up to 190 MPa at above 30% strain for metal foam of half relative density. Lotus-type foams of steel take advantage of the fact that numerous gases are highly soluble in metals whilst they are still in state of liquid than when they are in their state of solid. In steel’s case, either hydrogen–helium mixture or hydrogen is added into molten steel. As the metal hardens and solidifies, the gas escapes the solution, producing pores inside the solid steel frame.

Two like procedures of performing the lotus-type method have constantly been created. In continuous zone melting, one section of a foundational metal rod is melted in the occurrence of the diffused gas, and then permitted to solidify again soon thereafter (Levine, 2008). In continuous casting, the foundation metal is maintained in a molten state in a container in the occurrence of the gas, and thereafter steadily drains and solidifies.

It has been indicated in Table B where many other methods have been explained for the manufacture of steel foams. For example, polymer or ceramic forerunners can be utilized in the establishment of the voids. The ultimate steel foam will adopt similar morphology as the forerunner material and comparative densities that range from 4 to 23%, depending hugely in the forerunner, have been determined. Reaction of slip form sintering, a procedure particular for the iron-based metal foams, has also been utilized effectively in production and makes foams of temperate densities, varying from 12 to 41% (Lu, and Chen 1999).

There are numerous additional techniques of steel foam production that have been subjected to numerous preliminary studies by material scientists, including biomaterial rods, injection molding, and fibrous foams which include sintered fibers and truss cores. Truss cores entail welding or twisting these fibers together into mesoscale trusses of different shapes while sintering of fibers entails laying fibers out together. These fibrous foams possess low strength, but may work effectively as foundation material in sandwich boards.

For technicians and engineers, the substance development process is in fact minor to the material behaviors that are attained by that development process. For the processes described, limited tests of the machine behaviors of the resultant steel foams have been completed and summarized as in table 3. The degree of accessible results varies highly between manufacturing techniques. Void sphere foams and powder metallurgy have the most results and values in the published literature field.

Manufacturing process	Relative density	Base metal	Comp yield stress (MPa)	Elastic mod. (MPa)	U. tensile stress (MPa)	Min energy abs (MJ/m3 at 50% strain)
Casting HS — Al-steel composite	0.42	A356 + 316 L	52–58	10000–12000		51 (at 57%)
Ceramic precursor — CaHPO4*2H2O	0.23	Fe-based mixture	29 +/− 7
Ceramic precursor — MgO–NH4H2PO4, LD	0.13	Fe-based mixture	11 +/− 1
Ceramic precursor — MgO–NH4H2PO4, HD	0.21	Fe-based mixture	19 +/− 4
Injection molding — S60HS	0.49–0.64	Fe 99.7%	200
Injection molding — I30MK	0.47–0.65	Fe 99.7%	200
Lotus type — 50%	0.5	304L steel	95		190
Lotus type – 62%	0.62	304L steel	115		280
Lotus type — 70%	0.7	304L steel	130		330
Polymer precursor — 4.3%	0.04	316L steel	1.2	83
Polymer precursor — 6.5%	0.065	316L steel	3	196
Polymer precursor — 7.6%	0.076	316L steel	4.8	268
Polymer precursor — 9.9%	0.099	316L steel	6.1	300
PM — MgCO3 foaming	0.4–0.65	Fe-2.5C powder	30 (par)-300 (perp)
PM — MgCO3 and CaCO3 foaming	0.53–0.54	Fe-2.5C powder	40 (5e-5 s-1)–95 (16 s-1)			50 (4.5E-5 s-1)

Table 3: Table of material properties as extracted from selected publications

Experimentally measured structural properties are summed up in table 3. It is wholly accepted throughout the write up, that foams of metal steel of a certain foundational material and relative density will perform the same. However, the material characteristics rely on the manufacturing technique; specimen size tested, and cell structure and size (Daxner, Tomas, and Bitsche, 2008). For instance, the lotus-type foams of steel have anisotropic hollows, which results in compressive and tensile strengths that differ by up to a factor of 2 depending on direction. 

The most widespread calculated mechanical characteristic is the plateau strength or the compressive yield strength. The compressive yield strength is usually around 5% larger than the measured and calculated yield strength. At around 50% density, steel foam’s strength of compression ranges from 100 MPa for ordinary specimens, to over 300 MPa for greatly anisotropic or specially heat-treated specimens (Losito, Barletta, and Dimiccoli, 2010). Compressive yield strength (σ_c) regularized by the solid steel compressive yield (σ_c,s) is drawn against elastic modulus (E_c) regularized by the solid steel elastic modulus (E_c,s) in Fig B and shows that a broad range of rigidity of stiffness to strength ratios have been attained with foams of steel, again demonstrating the huge material collection accessible to engineering designers.  

Other mechanical characteristics include: poisson’s ratio, elastic modulus, densification strain, tensile strength, and absorption of energy. Calculated elastic modulus values vary from 200 to 12000 MPa, though considered values for the lotus-type foams as well as other foams of steel anticipated to have great modulus are unavailable (Neugebauer, 2004). Poisson’s ratio for foams of steel is usually assumed to be the elastic foundational metal value of 0.3; nevertheless, for void sphere foams of steel, trials reports vary from 0 (or even sometimes slightly negative) to 0.4 and 0.09 to 0.2 depending on the processing technique and density of the material.

Assessment of the strain of densification and absorption of energy is achievable in most tests, but only a few values are able to be published. Densification mostly occurs at between 55 and 70% strain and energy absorption that is measured up to 50% strain, vary from 40 to 100 MJ/m³ for near 50% densities. In the few tensional trials conducted, tensile strengths vary from 1 MPa for low density sintered void metal foams, up to and above 300 MPa for the anisotropic lotus-type foam equivalent to the orientation of the pore.

Experimentally determined steel foam material properties display important changeability across processing methods. This happens within limited identical samples, and in research teams. There is bias in the data due to strong connection between research team and processing type. Variability is also because of lack of standardization during the testing phase. 

Property	Minimum	@ Density	Maximum	@ Density
Thermal conductivity (W/mK)	0.2	0.05	1.2	0.1
Acoustic absorption coeff @ 500 Hz	0.05	0.12	0.6	0.2
Acoustic absorption coeff @ 5000 Hz	0.6	0.27	0.99	0.12
Permeability (m2 ∗ 10− 9)	2	0.14	28	0.1
Drag coefficient (s2/m ∗ 103)	0.3	0.9	2.2	0.14

Table 4: Properties of steel

Explicit designing of the foam of steel microstructure has been finished by a diversity of researchers. Whereas almost all of the researches comprise plasticity in the software simulation, only five consist of contact, and none of them consist of materiel rupture; meaning that simulation of the tensile ductility and the strain of densification is quite underdeveloped. Continuum constitutive structures of metal foams have also been created, enhanced, and authenticated for aluminum foams, and are accessible in commercially available software like ANSYS (Muriel, 2009). Main features of the created structures are pressure reliance in the plastic phase, tensile fracture, and hardening nonlinear strain.

Current research shows the feasibility of steel foam and gives information on the huge potential of the structural steel. Nevertheless, crucial openings subsist in current research, spaces that are limitations to enlarged acceptance and demand for steel foams.

Analytical, experimental, and computational examination of the phase of foams of high relative density, a phase that gives improved non-structural and structural characteristics with the smallest decrease in effectual strength and modulus is required. A few of the foams of steel mechanical properties have not been thoroughly studied, including low-cycle fatigue and tensile reaction, as well as Poisson’s ratio and densification strain stress (Nishiyabu, Matsuzaki, and Tanaka, 2005). Testing of materials standards is required to aid sharing of the information and to improve the industry. Joining of steel foams, through welding and by use of tool fasteners, needs deeper research. Computational microstructural models indicate high probability for bettering understanding of the properties and for lengthening test results for calibration of continuum plasticity structures and homogenization.

Microstructural structures including higher realistic hollow models, uncertainty, fracture, and contact are required. Continuum plasticity calibration structures for steel foam that can be manufactured are required as well. Model based illustration projects that measure the advantage of foams of steel in particular engineering design circumstances are required. Fundamentally, illustration candidates for every of the nonstructural and structural benefits, from thermal and acoustic to stiffness and weight require completion (Neville, and Rabiei, 2008). Exception candidates include lightweight multi-functional (stiffness, thermal, energy, acoustic) wall and floor boards, amplified energy absorption through reducing buckling in members, vibration modifying girders for high-speed rail, energy expulsion fuses for seismic design, as well as other uses. Structural and solid illustration models of foams of steel are required. Outside of lab scale plates and bars, structural elements, floors, and walls require manufacturing, testing and application in actual engineering purposes. In agreement with this effort structure standards are required for foams of steel so that the substance material can be taken up and utilized in engineering design.

Stainless steel void spheres and LC (Low carbon) steel as well as AL (aluminum) alloy solid medium were employed in the development of composite metal foam model. The void spheres are manufactured from a powder metallurgy procedure at the Fraunhofer Institute situated in Germany. Al (Aluminum) alloy was selected as the main solid medium due to its very high strength, low density, decreased shrinkage and ease of casting during the process of solidification. The choice of Aluminum and steel with specifically different melting points was to ensure that the walls of the spheres would not liquefy in the process of casting. The contents of the 316L stainless steel balls, LC (Low carbon) steel balls, and Al (Aluminum) alloy (A356) are illustrated in Table 5 (Neugebauer, Hipke, Hohlfeld, and Thümmler, 2004).

Foam of compost steel samples with varying sizes of spheres of 5.2-, 4-, and 2.2-millimeters outer diameter was developed through powder metallurgy method. The void spherical objects were manufactured at Hollomet, Dresden situated in Germany and the specimens’ thickness of the wall and chemical composition are illustrated in table 1 alongside the chemical characteristics and properties of the medium powder. The material of the matrix for all the dynamic and static compression maples is 316L stainless steel powder manufactured by North American Hoganas High Alloys LLC with the size of the particles separated and sieved to -325 mesh (95%) and 5% ( mesh) (Park, and Nutt, 2001).

Composition of different sphere and matrix materials used in the processing of Al–steel CMF by casting
Sphere and matrix materials	Composition of alloying elements (wt%)
316L stainless steel spheres	0.03% C, 0.3% O, 17% Cr, 13% Ni, 0.9% Si, 0.2% Mn, 2.2% Mo, bal-Fe
Low carbon steel spheres	<0.007% C, 0.002% O and balance iron
Aluminum 356 alloy	7.01% Si, 0.5% Fe, 0.39% Mg, 0.28% Mn, 0.11% Cu, 0.09% Ti, 0.06% Zn, 0.02% Cr, balance-Al

 Table 5: Contents of the 316L stainless steel balls, LC (Low carbon) steel balls, and Al (Aluminum) alloy (A356) 

Composite Foam Component	Chemical Composition Weight-Percent							Sphere Properties
	Fe	C	Mn	Si	Cr	Ni	Mo	Wall thickness (μm)
5.2 mm spheres	balance	0.87	0.07	0.34	17.09	12.60	2.12	275
4.0 mm spheres	balance	0.58–0.69	0.15–0.07	1.14–0.32	17.34–16.48	12.28–12.42	2.28–2.11	225
2.2 mm spheres	balance	0.68	0.13	0.82	16.11	11.53	2.34	110
Matrix material	balance	0.030	2.00	1.00	16.00–18.00	10.00-14.00	2.00–3.00	NA

Table 6: Chemical composition and physical properties of hollow spheres and the matrix material

The two spheres and metal powder are inserted into a mold and subjected to vibrations at a frequency rate of 20 Hz for ~50 minutes to attain a dense stuffing between the matrix and spheres preceded by sintering in a vacuum heated press at 1200 °C. Additional details of the metal powder metallurgy processing are accessible elsewhere. Every sintered sample was divided into blocks with a square cross section (36 × 36 millimeters or 24 × 24 millimeters) through the use of a Buehler Isomet 4000 linear precision saw (Park, and Nutt, 2000). Specimen sizes for machine testing were picked so that at least six (6) spheres are there within the cross-section to prevent any edge impacts and all specimens have an aspect ratio of width to length of 1.75. Foam specimen thin slices were cut as well for the SEM (scanning electron microscopy imaging).

The thin slice specimen surfaces were thoroughly ground with the use of progressive sand paper then followed by polishing through the use of a progression of slurries of diamond. The digital imagery of the surface of the foam macro-structure as well as the single void spheres from every batch was taken. A high-tech Environmental SEM equipped with EDS (Energy Dispersive Spectroscopy) was utilized to capture greater magnified images of the microstructure of the composite foam by use of accelerating voltage of between 20 to 30 keV, and Energy Dispersive Spectroscopy was executed at different locations to verify the chemical composition of different components inside the micro-structure (Rabiei, and Vendra, 2009). ANSYS was used to calculate the matrix area percentage taken up by porosity.

ANSYS simulation is employed in the simulation of engineering problems. The software used develops simulated computer forms of electronics, structures, or mechanical components to reproduce toughness, strength, temperature supply, elasticity, fluid flow, electromagnetism, as well as other attributes. ANSYS composite PrepPost is an added component devoted to the modeling of composite and layered structures. A shell component model was created in mechanical meshing casing geometry, through the application of boundary situations and loads. The model can be in ANSYS mechanical or APDL mechanical (Reisgen, Olschok, and Longerich, 2010).

Materials were defined through Engineering Data. The models were then imported in ANSYS Composite PrepPost. Composite fabrics are defined as any material which consists of an assembly of fabrics with orientations. The model is then solved through the computation of the standard ANSYS solver in batch mode. The results, which are the deformations, failure criteria, and thickness, are post processed in ACP.

Both the low velocity dynamic and quasi-static tests of compression were done on a servo-hydraulic high frequency test structure with 300 mm crush deepness at speeds of up to 8 meters/s and capacity of loading from 60,000 to 100,000 pounds depending on the velocity. The displacement of the crosshead is documented by a LVDT (Linear Variable Differential Transformer) that shows the actuator location (Shirzadi, Zhu, and Bhadeshia, 2008).

Tests were carried out on several composite foam samples at four varied crosshead displacement velocities: 8, 1, 0.1, and 0.01 meters/s. One test at every speed was also carried out on a steel void sphere foam (HSF) made by hollomet with an average diameter of the cells of 2.45 millimeters and dimensions of the specimens 23 × 23 × 40 millimeters for contrast. A high speed video clip was documented from each test at 500 to 10000 frames / second dependent on the speed of impact.

An additional set of stainless steel composite foam specimens was subjected to testing through the use of SHPB (Split Hopkinson Pressure Bar method). Cylindrical samples were developed using the powder metallurgy method illustrated using the 2.2- millimeter diameter stainless steel perfect balls and stainless steel (316L) powder, alike to those described in materials and processing.

The length of the diameter of the Split Hopkinson Pressure Bar samples was 19millimeters and 9.53 millimeters length for a length/diameter ratio of 0.5 (Smith, Szyniszewski, Hajjar,Schafer, and Arwade, 2012). The Split Hopkinson Pressure Bar used 19 millimeters diameter aluminum occurrence and conveyed bars and a 177.8 millimeters long striker bar. The gas gun air pressure ranged from 50 to 100 psi to attain rates of strain up to 26.4 meters/s (2770 s-1). The data was decreased to compute the stress-strain material reaction using the 2-wave analysis technique and through the recording of the high speed at 96,000 frames / second.

Figure C demonstrates digital and SEM images of the metal composite samples utilized in this study.  As shown, there is a homogenous circulation of spheres in the medium in every specimen with some the excess pores in the medium that is the nature of metal composite foams developed using powder metallurgy method. Through the use of ANSYS analysis, the level of porosity was calculated to be in the 10 to 13% for every specimen (Tuchinskiy, 2005).

The ANSYS of the sphere wall and the medium demonstrated the diffusion of the various elements in the matrix such that steep compositional gradients are avoided. Also, though not directly measured and determined, the steep slope in sphere carbon content (smallest of 0.6 weight-percent) relative to the medium (0.03 weight-percent) sustains the external dispersion of carbon from the spheres into the medium. This mixed componential diffusion aided in the equilibrium of the compositional variances between the matrix and the spheres and reduced the carbon content of the spheres (Verdooren, Chan, Grenestedt, Harmer, and Caram, 2006). Resulting from this, the   stiffness of the composite metal foam was decreased while their absorption of energy and ductility was enhanced.

The structural steel sample is subjected to tests through the use of simulation software called ANSYS to determine the properties and behaviors of structural steel composite foam. The following results were obtained after running a simulation of the ANSYS software.

Figure 10 presents a graph image of the equivalent stress undergone by the structural steel spheres. As can be seen, both the yield and plateau strength of all of the composite foams is reduced by escalating the spheres diameter while a rise in the loading speed raises the yield and plateau strength of the material (Xu, Bourham, and Rabiei, 2010).

As the result, the highest strength belonged to the composite foams made out of the smaller diameters. Additionally, a distinctive increase is also noted in the plateau stress when the reaction of the composite foams is contrasted against a sintered sphere-only behave of like material and sphere proportions. This shows the significance of the medium in creating a bond between the spheres together and strengthening the sphere walls under loading.

The figures above present the ansys simulation depiction of composite foams. As can be seen, spheres have an extremely smooth surface while some features can be seen on the surface of the other spheres. In spite of the enlarged contact area between the medium and spheres due to the larger surface irregularity, these spheres have not joined well to the medium (Xu, Bourham, and Rabiei, 2010). This effect made some spheres to be unbounded from the matrix through mechanical loading. Moreover, studies are ongoing to modify the medium powder size for improved filling of the exterior features on sphere walls, and to give optimal bonding at the border of the medium with the sphere wall.

The entire multiaxial description of a cellular metal material, which needed a chain of tests in tension, torsion, compression, and hydrostatic loading, proved to be a daunting task. Particularly in tests with a hydrostatic element, the separation from the fluid used to load it was especially hard. In numerous tests, the lid that as being used to shield the sample from seepage failed and the matrix penetrated into the sample which caused a premature termination of the test. On this basis, large amount of lab work was carried out inorder to setup the experiment apparatus and methodology (Smith, Szyniszewski, Hajjar,Schafer, and Arwade, 2012)

To confirm whether the strain frequency affected the material performance, analysis of the composite metal foam was carried out using a software package called ANSYS. Finally, as the substance is intended to be utilized in various engineering applications especially where there may be need for great energy absorption, some tests of a simple compound structure made of structural steel foam were undertaken (Withers, 2001). Both in compression and tensional stresses, the contribution of the metal foam are quite crucial, more than the unpolluted addition of the assistance of the two substances to the immersed energy.

Conclusion

Existing uses of metal foams shows that panels, shells, and members, may all be developed and utilized in the field of engineering. Instances of panels (balcony, parking garage slab) and members like the crane arm illustrate the structural needs, including serviceability, fatigue, and strength, can be satisfied by composite metal forms; and these needs may be met as the weight is reduced, and potentially adding the absorption of energy, as well as a multitude of non-structural characteristics (Shirzadi, Zhu, and Bhadeshia, 2008).

The processing of foams of steel is currently mature in the lab set up and is in anticipation of proper momentum for commercialization. A number of procedures of manufacture have been created, with void sphere, lotus-type, and powder metallurgy, showing without a doubt that it is the most preferred method for its unique benefits. Comparative densities ranging from 4 to 100% have been attained in closed cell and open cell foams, a number with unique or anisotropic cell structures. A number of techniques have even shown industrial scale manufacturing abilities via continuous manufacturing processes.

Material properties of foams of steel have demonstrated special anticipation in their great capacity to absorb energy, huge variety of likely strength, high ductility, and elastic modulus ratios, in particular kinds of steel foam (Tuchinskiy, 2005). These structural characteristics may possibly be mixed with non-structural benefits such as the low heat conductivity of steel foam, or high absorption of vibrations capacity in the creation of new uses for the material.

Current analytical representations for the homogenization of foams of steel give fundamental bounds on the performance, and microstructural analysis representations give an indication of increase understanding and experimental characterization of materials. Additionally, plasticity models at the continuum level suitable for the metal foams have been created.

Steel foam gives a special chance for structural steel to fill up a larger section of the potential design space. Introducing density as a variable of design enhances freedom of the designer extensively (Xu, Bourham, and Rabiei, 2010). Additional studies involving more comprehensive material testing, foam models created by more processing methods, and structural based illustrations of foams of steel will fill up current gaps in knowledge and permit taking up of its use as a structural material. 

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