sign in sign up
<<
Home , Shock (mechanics), Thermal shock

MAY 2018 I SS U E #113 TECHNICALTIDBITS MATERION PERFORMANCE ALLOYS

THERMAL AND THERMAL Due to , objects will expand has poor diffusivity, there will be a large gradient when heated and contract when cooled. If such an in temperature between the surface and the core, object were completely unconstrained, the change and thus a large gradient in thermal strain and in dimensions can be computed from the thermal as well. Heat check yourself expansion coefficient, as discussed last month. and you’ll wreck However, objects in systems are rarely com- The surface will want to expand greatly, but it will yourself! – A brief pletely free to expand and contract without limit. If be constrained by the core material below, which effectively puts compressive stress on the surface. discussion on a number of some kind of constraint prevents the material from expanding or contracting by the required amount, At a high enough temperature, the stress will other thermal properties. then the material experiences thermal strain and exceed the strength of the material, and it will stay thermal stress. in that state. When the thermal load is removed and the metal is allowed to cool, the surface will be Heat checking, or thermal fatigue, can happen unable to shrink as much as the core, since it has Thermal Strain when a surface is repeatedly heated and cooled, been permanently deformed. This places the sur- commonly found in molds used for die casting or face into a state of tensile stress. Over many such Thermal Stress injection molding of liquid metal. When the metal cycles of alternating compressive and tensile stress, Thermal Fatigue / surface comes into contact with liquid that is several fatigue cracks can develop on the surface. Usually, hundred degrees hotter, the surface will quickly the surface takes on a crazed appearance like old Heat Checking heat up and expand. Meanwhile, the core of the pottery (Figure 1). Thermal Fatigue metal is still at the initial temperature. If the material Resistance Parameter Figure 1. Crazing on Ceramic Surface. The inside of this coffee mug has a crazed appearance due to thermal cycling in an automatic dishwasher. Thermal Shock This is why it is important to teach your children Resistance what the phrase “Hand wash only” means. Parameter

Metallurgical phase changes can make heat checking initiate a crack. A high elastic modulus (E) increases worse, if the metal transforms from one solid phase the stress at a given amount of thermal strain, so to another upon heating, and vice versa on cool- the lower the modulus, the better. A high thermal ing. This is particularly true when two phases have expansion coefficient κ( ) also increases the thermal different crystalline structures or densities. strain. The (1 – ν) term accounts for a biaxial stress state. This assumes that an entire surface is heated, If heat checking is a potential issue in your applica- so the material expands in two directions instead tion, you should consider using a material with a of one. Most metals have a Poisson’s ratio (ν) of high thermal fatigue resistance parameter around 0.3, so when comparing different materi- (Q). It is calculated as: Q=(κ∙σ_y)/(E∙α) (1-ν), and has als, this term can usually be safely ignored. Note the units of W/m or BTU/ft hr. A high thermal con- The next issue of Technical Tidbits also that the equation for Q has the term (σ_y/E) will discuss thermal emissivity. ductivity (κ) minimizes the thermal gradient within in it, which is the elastic resilience of the material. the material, decreasing the differential thermal Therefore, thermal fatigue is yet another failure expansion. A high strength (σ_y) minimizes mode that you can protect against by using a highly the amount of plastic that occurs resilient material. on heating and cooling, reducing the tendency to ©2018 Materion Brush Inc. MATERION PERFORMANCE ALLOYS THERMAL FATIGUE AND THERMAL SHOCK (CONTINUED) Alternatively, when you know what the thermal cycle will be, you can use the fatigue strength at the Written by Mike Gedeon of Materion mean temperature in place of the yield strength Performance Alloys Marketing for the thermal fatigue resistance parameter. This Department. Mr. Gedeon’s primary would require you to also have the appropriate focus is on electronic strip for the elevated temperature fatigue data for the material automotive, telecom, and computer of interest. However, when you are simply making a markets with emphasis on first pass through multiple materials for a thermally application development. cycled application (such as a water cooled plunger References: for die casting of aluminum alloys), the yield strength provides a perfectly reasonable comparison. Donald J. Wulpi Understanding How Components Fail ©1985 ASM Thermal shock is similar to thermal fatigue, International except that the failure occurs during a single, rapidly applied thermal load. (While thermal fatigue is a J.C. Benedyk, D.J. Moracz & J.F. Wallace “Thermal Fatigue Behavior cumulative process over many cycles, thermal shock In defining thermal shock resistance, some engi- of Die Materials for Aluminum Die failures are immediate.) This is usually more of a neers prefer to replace the tensile strength with Casting” Proceedings of the 6th concern in ceramics or other brittle materials that the plane strain K1C, as fol- SDCE International Die Casting Congress ©1970 The Society of lack ductility, where the initiation of a crack would lows: RT=(κ∙K_1C)/(E∙α) (1-ν). In this case, the units likely lead to catastrophic failure. Thermal shock become W/√m or BTU/hr√ft. The second equation Die Casting Engineers, Inc. failures are familiar to anyone who has ever acciden- would be appropriate if there are pre-existing George E. Dieter, Jr. Mechanical tally set a piece of cold ceramic dinnerware onto a cracks in the material, so the formula then describes Metallurgy ©1961 McGraw-Hill hot stovetop, or mistaken plain cookware for theoretical resistance to catastrophic propagation Book Company, Inc. tempered . of an existing crack. If you have ever had a small crack in your windshield grow to a large crack over T. J. Lu and N. A. Fleck “The The Thermal shock resistance parameter a period of a day or two, due to swings in the out- Thermal Shock Resistance of ” Acta mater. Vol. 46, No. 13, pp. (RT) RT=(κ∙σ_T)/(E∙α) (1-ν) has the same units as the door air temperature, then you are familiar with this 4755-4768, 1998 ©1998 Acta thermal fatigue resistance (W/m or BTU/ft hr), and particular failure mode the formula is virtually identical, with tensile strength Metallurgica Inc. replacing yield strength. (Since tensile strength is higher than the yield strength, if a material has a high thermal fatigue resistance parameter, it will also Please contact your local sales have a high thermal shock resistance parameter.) representative for further RT describes how well a material can theoretically information on material hardness resist cracking (initiating a new crack), which is why or other questions pertaining to the tensile strength is used in the formula instead of Materion or our products. the yield strength. Health and Safety Handling copper beryllium in solid form poses no special health risk. Like many industrial materials, beryllium-containing materials may pose a health risk if recommended safe handling practices are not followed. Inhalation of airborne beryllium may cause a serious lung disorder in susceptible individuals. The Occupational Safety and Health Administration (OSHA) has set mandatory limits on occupational respiratory exposures. Read and follow TECHNICALTIDBITS the guidance in the Safety Data Sheet (SDS) before working Materion Performance Alloys Sales with this material. For additional 6070 Parkland Blvd. +1.216.383.6800 information on safe handling Mayfield Heights, OH 44124 800.321.2076 practices or technical data [email protected] on copper beryllium, contact Technical Service Materion Performance Alloys +1.216.692.3108 or your local representative. 800.375.4205 [email protected]

©2018 Materion Brush Inc.