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An Introduction to Hydrogen Embrittlement. An Intro to Pipeline Corrosion in Seawater. Follow Connect with us. Sign up. Thank you for subscribing to our newsletter! Connect with us. Fracture Toughness. What Does Fracture Toughness Mean? Figure 1 Fracture toughness as a function of material thickness.
Retrieved from Ref. Fracture toughness is not to be confused with fracture strength. Fracture strength — also known as tensile strength — describes the maximum stress a material can withstand before experiencing fracture.
Fracture toughness, on the other hand, represents the energy required to fracture a material containing a pre-existing flaw or crack [5]. The fracture process consists of two stages: Crack initiation and crack propagation. The propagation of a crack that would result in fracture provides information about what mode that fracture is in.
According to Irwin, fracture exists in three modes [6]. Mode I fracture is also referred to as the Opening mode , within which a tensile stress acts perpendicularly to the crack plane. Mode II fracture is also called the Sliding mode , where an in-plane shear stress acts normal to the crack front. Mode III fracture , or the Tearing mode , is when a torsional out-of-plane shear stress exists parallel not only to the crack plane, but also to the crack front.
In other words, depending on the amount of plastic deformation that a material can undertake, the characterization of fracture changes.
If significant plastic deformation takes place before and during the propagation of the crack, the fracture is considered a ductile fracture. Conversely, if only deformation at the microscale takes place, the fracture would be a brittle fracture. Plastic deformation is a caution signal to an impending fracture. Yet, it is not easy to find the boundary between brittle and ductile fracture since there are several factors that can affect material deformation, including the stress state, loading rate, ambient temperature and crystal structure.
Fracture toughness spans over a broad number of materials, showing a variation up to four orders of magnitudes. Engineering ceramics have a relatively lower fracture toughness despite their higher strength. Engineering polymers are also less tough when it comes to resisting cracking, yet engineering composites of ceramics and polymers show an enhancement in fracture toughness than both components.
The materials with the lowest fracture toughness are types of foams and polymers. Figure 2 shows different materials on a graph relating the fracture toughness to the material strength [7].
Figure 2: Fracture Toughness vs Strength: Distribution of different materials. Engineering Fracture Mechanics, 1 , NDT Resource Center. Industrial Metallurgists, LLC. Woburn, MA: Butterworth-Heinemann. Metals and engineering alloys have high fracture toughness values due to their high resistance to cracks. We connect engineers, product designers and procurement teams with the best materials and suppliers for their job.
Thus it is clear that continuum mechanics provides the theoretical basis for designing against fracture in castings, but a thorough understanding of the microstructure and its effects is essential to fine-tune the final design against fracture. As noted by Ashby [ 4 ],. Schey, Introduction to Manufacturing Processes [ 6 ]. Such knowledge can often fall under the category of "too little, too late".
In what follows, the process variables of the different members of the family of castings will be briefly considered with a view to differentiate the type of microstructure that is developed in the castings.
The principles and evaluation methods of fracture toughness will then be briefly described. Selected papers from the literature will next be analyzed with a view to highlight the role of the microstructure in determining the fracture toughness of the castings. The effect of common castings defects on fracture toughness will then be very briefly considered.
The use of fracture toughness - yield strength bubble charts for design against fracture [ 5 ], based on continuum mechanics, will be indicated. It is clear from Figure 2 that there are many avenues for making a casting, depending on the type of pattern, the type of mold and whether pressure is used for assistance in filling the mold. Not all processes are suitable for all the casting alloys. Investment casting ceramic slurry, lost wax is perhaps the most accommodative process for most alloys and others have limitations based on resistance to high temperature, chemical reaction and other factors.
It is therefore customary to choose the casting process with due regard to the casting alloy. A recent addition to the umbrella is the squeeze casting process which is somewhat analogous to transfer molding of polymers. The microstructure of the casting is strongly affected by the process used for making it.
The explanation is given below. As stated earlier, casting is the product of solidification, which consists of nucleation and growth of solid from the liquid metal alloy. The final microstructure is decided by the composition of the alloy, the solidification rate and any melt treatment used. The alloys, based on their phase diagram may be of long-freezing range or short-freezing range type.
The solidification rate is governed by the rate at which the mold is able dissipate the latent heat and superheat of the metal poured into the mold.
Permanent molds like metal and graphite molds have higher thermal conductivity than disposable molds like sand and ceramic shells and therefore provide higher solidification rates.
If there is no melt treatment, finer scale microstructure can be expected when these higher conductivity molds are used. Melt treatment however, can change this picture. The object of this treatment is to refine the microstructure and the treatment is variously termed as grain refinement in the case of single phase alloys , modification or inoculation in the case of second phase alteration of binary alloys.
The application of continuous pressure as in squeeze casting may also substantially affect the microstructure. Long-freezing range alloys cooled at a relatively slow rate, as for instance in a sand mold, tend to solidify in a "mushy" or "pasty" manner. If this zone has large width, the final microstructure will consist of large amount of distributed interdendritic shrinkage areas, as any feed metal from the riser will find it difficult to access many of these areas due to tortuous path involved.
The width of the mushy zone is reduced as the cooling rate increases, as in metal mold castings, with consequent reduction in distributed shrinkage. When the mushy zone is absent or too small, the solidification is termed "skin-forming" and the feed metal from a properly designed riser will have good access to the solidifying areas, thus minimizing distributed shrinkage.
The shrinkage under these conditions can be totally eliminated that the feed metal has access to the final solidifying area. The application of Chroninov's rule, which states that the solidification time is proportional to the square of the volume-to-surface area of the casting and the riser or its modifications to account for the shape, will be helpful in this regard.
The basic idea is to design the riser such that its solidification is more than that of the casting and its feeding distance is appropriate to reach the last solidifying zone of the casting. In long freezing range alloys solidifying in a mushy fashion, hot tear or hot crack can develop near above the solidus temperature when the network of solid crystals is unable to sustain any thermal stress gradients, particularly when the feed metal is unable to reach these locations.
These cracks are usually sharp, capable of rapid propagation. Another important consideration in castings is the porosity caused by gas liberation during solidification. Gases like hydrogen are easily soluble in the liquid state but the solubility is substantially reduced in the solid state.
This may result in pores of various sizes in the solid or even microcracks when there is significant resistance to the escape of the gases. It is therefore desirable to degas the liquid metal prior to pouring in the mold. A useful law in this context is Sievert's law which states that the solubility of a dissolved gas is proportional to the square root of its partial pressure.
Using this law, degassing in the liquid state can be achieved by applying vacuum difficult and expensive or purging with an inert gas which serves the dual purpose of lowering the partial pressure of the dissolved gas and acting as a carrier for the escape of the dissolved gas, thus reducing the harmful effect of gas porosity in the solid. As microstructure is the key to fine-tuning of the fracture toughness of castings, the influence of casting process factors on the microstructure must be well understood, if such fine-tuning is attempted.
Needless to say, metallurgical knowledge such as phase diagram and the effect of non equilibrium cooling rate on it, nucleation and growth of the different phases in the microstructure, evolution of defects through impurities and interaction of the molten metal with melting atmosphere, the furnace lining, the mold, etc.
Heat treatment can substantially affect the microstructure and therefore, knowledge of kinetics of solid state transformations is also important to understand the effect of the particular heat treatment on the microstructure.
Linear Elastic fraction mechanics approach may be defined as a method of analysis of fracture that can determine the stress required to unstable fracture in a component. Standardized fracture mechanics test specimens: a compact tension CT specimen, b disk-shaped compact tension specimen, c single-edge-notched bend SEB specimen, d middle tension MT specimen and e arc-shaped tension specimen.
Source: T. Anderson [ 9 ]. A sharp crack or flaw of similar nature already exists; the analysis deals with the propagation of the crack from the early stages. The size of the plastic zone near the crack tip is small compared to the dimensions of the crack. Figure 3 below shows standardized test specimens recommended for LEFM testing. Each specimen has three important characteristic dimensions: the crack length a , the specimen thickness B and the specimen width W. A typical view of the test set up for fracture toughness testing Source: Seetharamu [ 10 ].
The CT specimen is pin-loaded using special clevises. The standard span for SEB specimen is 4W maximum; the span can be reduced by moving the supporting rollers symmetrically inwards. It is to be noted that the tip of the machined notch will be too smooth to conform to an "infinitely sharp" tip.
As such, it is customary to introduce a sharp crack at the tip of the machined notch. Fatigue precracking is the most efficient method of introducing a sharp crack. Care must be taken to see that the following two conditions are met by the precracking procedure: the crack-tip radius at failure must be much larger than the initial radius of the precrack and, the plastic zone produced after precracking must be small compared to the plastic zone at fracture. This is particularly necessary for metal castings as many exhibit plasticity; a notable exception is flake graphite cast iron castings made in sand molds.
Anderson [ 12 ]. A typical test set up is shown in Figure 4. All except the MT specimen noted in Figure are permitted to be used as per this standard. The ratio of 'a' as defined in each figure to the width W should be between 0.
The load-displacement behavior that can be obtained in a LEFM test, depending on the material, can be one of three types as shown in the Figure 5 below. First a conditional stress intensity factor K Q is determined from the particular curve obtained using.
If this is not the case, the result is invalid, most likely because of significant crack tip plasticity. This would imply that triaxial state of stress required to ensure plane strain condition at the crack tip is not achieved and any determined stress intensity factor at fracture as per ASTM E would be an overestimate of the resistance to crack growth.
Use of such values in design would be dangerous. In such cases, an elastic-plastic fracture mechanics EPFM method must be employed to determine the specimen's resistance to the propagation of a sharp crack. Anderson [ 14 ]. Among the different methods available to determine the sharp crack growth resistance in specimens with significant plasticity at the crack tip much less than what is required to cause total plastic collapse the J-integral method and the Crack-tip Opening Displacement CTOD have been more widely adopted.
The recent trend however, is to use the J-integral approach and only this method will be briefly described here. ASTM E [ 13 ] gives two alternative methods: the basic procedure and the resistance curve procedure. The basic procedure normally requires multiple specimens, while the resistance curve test method requires that crack growth be monitored throughout the test.
The main disadvantage of this method is the additional instrumentation and skill are required. Though this method has the advantage of using a single specimen, making of multiple specimens as nearly externally identical-looking castings is not a major problem; any inconsistent results among the different specimens will give an opportunity to see if the casting microstructure is properly controlled.
Therefore only the basic test procedure will be considered here. The first step is to generate a J resistance curve. To ensure that the crack front is straight the use of a side grooved specimen as shown in Figure 6 , is recommended. A series of nominally identical specimens are loaded to various level and then unloaded The crack growth in each sample, which will be different is carefully marked by heat tinting or fatigue cracking after the test.
The load-displacement curve for each sample is recorded. Each specimen broken open and the crack growth in each specimen is measured.
In equations 6 and 7 b 0 is the initial ligament length. Plastic energy absorbed during J-integral test Source: T. Anderson [ 15 ]. Anderson [ 16 ]. M value in the figure is related to crack blunting and the default value is 2.
The provisional J Q is taken as the critical value J I c if the condition:. If it is assumed that a steel sample has a yield strength of MPa, tensile strength of MPa and Young's modulus E of GPa and fracture toughness of MPa- m , it can be shown that E thickness requirement for validity is 0.
The advantage of E approach over E approach in regard to valid specimen thickness requirement is thus obvious. In what follows reported fracture values of various castings will be presented and discussed. In recent times, the most widely studied nonferrous casting alloys for fracture behavior are aluminum casting alloys. Among them, aluminum silicon alloys have attracted the most attention as they are widely used because of good castability and high strength-to-weight ratio. The figure shows the variation of microstructure with cooling rate.
Figure 9 a refers to a sand casting where the cooling rate is the lowest among the three, sand cast, permanent mold cast and die cast. The dendrite cells are large, the silicon flakes dark are coarse and iron-silicon-aluminum intermetallics light grey are seen.
The resistance to crack propagation will be the lowest with this type of microstructure. Figure 9 b refers to a permanent mold casting where the cooling rate is higher than in a sand castings.
It is seen that there is refinement in both primary aluminum and eutectic silicon as well as the intermetallics. The resistance to crack growth will be higher than in sand castings. Figure 9 c shows the microstructure of a die casting of the alloy where high degree of refining of dendrite cells and eutectic silicon are seen. Other things being equal, the resistance to crack growth will be maximum in this type of microstructure. However other things will not be equal in general, the main factor being the yield strength of the casting.
Thus crack tip plasticity will be high in the sand casting, intermediate in the permanent mold casting and lowest in die casting. Thus the fracture toughness increase in the casting will not be in direction proportion to the reduction in cooling rate. The factors favoring increase in fracture toughness would be decreased dendritic cell size and refinement of the covalent bonded silicon and mixed bonded intermetallics.
The opposing factor would be reduced plasticity due to increase in yield strength, both due to primary cell and eutectic refinement.
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