部分材料的断裂韧度包含AISI 4340.docx

1、部分材料的断裂韧度包含AISI 4340ADVANCED MATERIALS LABORATORYFracture Toughness MeasurementObjectives 1. To make a valid measurement of the plane strain fracture toughness of a given material. 2. To observe and understand a typical brittle fracture process. Background The phenomenon of brittle fracture deals wi

2、th sudden failure of structural components without warning. This is attributed to presence of cracks or crack-like defects in the material that appear during processing or during fabrication and assembly of the component. The theory behind this phenomenon, as it applies to many engineering structure

3、s, is referred to as Linear Elastic Fracture Mechanics (or LEFM). According to this theory, the condition for brittle failure can be expressed as where KI is called the stress intensity factor and is dependent on loading conditions and the flaw size in the material, and KIC is a material property kn

4、own as the plane strain fracture toughness. The stress intensity factor is usually expressed as where Q is a geometry correction factor depending on the geometry of the structural component and the crack geometry, is the applied stress, and a denotes the crack size. Definitions of these quantities f

5、or many typical situations are presented in an Appendix at the end of this handout for your convienience. Finally note that KI and KIC have dimensions of stress (i.e. Mpa or ksi). In order to use the above criterion for fracture two conditions have to be met. These are (i) small scale yielding condi

6、tion. All in-plane dimensions of the component as well as the crack size should be larger than fifteen times the critical plastic zone size (rIC), which is defined as where is the yield strength of the material. (ii) plane strain condition. The thickness of the sample should also be larger than fift

7、een times the critical plastic zone size (rIC). Experimental Procedure The ASTM standard (E399) for plane strain fracture toughness testing provides a procedure for calculating values of KIC for metallic materials. The test permits three different specimen shapes: a bend specimen, a C-shaped specime

8、n, and a compact test specimen (CTS). The CTS will be used in this laboratory. The procedure for measuring KIC with a CTS is as follows: 1. Make a guess of the expected value of KIC. This enables you to calculate an estimated critical plastic zone size. 2. To ensure that only small-scale yielding oc

9、curs at the crack tip, the length, a, of the crack and the remaining ligament, (W - a), should be greater than or equal to 15rIC. a, (W - a ) 15rIC. 3. To ensure plane strain, the thickness, B, of the CTS should be greater than or equal to 15rIC. B 15rIC. 4. Once a CTS is machined, according to the

10、dimensions calculated above, a sharp crack is introduced at the root of the machined notch. This is accomplished by fatigue pre-cracking the specimen. This procedures involves imposing a time-varying tensile load on the CTS to cause a sharp crack to initiate and slowly grow at the root of the machin

11、ed notch. The maximum fatigue load should be less than 0.6 times the value of the estimated final fracture load: Pfmax 0.6 PQ. 5. The fatigue-generated protion of the crack should be at least 1.2 mm long. 6. Once a sharp crack exists, the actual KIC test can be performed. The test consists of increa

12、sing the tensile load, P, on the specimen slowly while measuring the crack opening displacement, . Plotting the P versus produces a curve similar to the one shown in Figure 1. Fast fracture is indicated by a gross nonlinearity in the load-displacement record. 7. To calculate the KIc, first calculate

13、 a conditional KQ using The geometric variables a, W, and B are defined in the sample schematic in the datasheet. Determine, a, by measuring the initial crack length (notch plus fatigue pre-crack). PQ is determined by projecting a line whose slope is five percent less than the original slope of the

14、P - curve. PQ is the load corresponding to the intersection of this line with the P - curve. See Figure 1. 8. The ratio Pmax/PQ should be less than 1.10, where Pmax is the maximum load encountered in the test. Pmax / PQ 1.10. 9. If condition 8 holds, then calculate (KQ / y)2. If this quantity is les

15、s than the specimen thickness, B, the crack length, a, and the remaining ligament (W - a), then KQ is equal to KIc. Otherwise the test is not a valid KIc test.Figure 1. Schematic of the typical load-COD plot obtained in a fracture toughness experiment. References W.T. Matthews, Plane Strain Fractue

16、Toughness (KIc) Data Handbook for Metals, AMMRC MS73-6, U.S. Army Materials and Mechanics Research Center, Watertown, MA, 1973. Damage Tolerant Design handbook, Metals and Ceramics Information Center, Battele Columbus Laboratories, Columbus, Ohio, 1975. C.M. Hudson and S.K. Seward, Compendium of Sou

17、rces of Fracture Toughness and Fatigue Crack Growth Data for Metallic Alloys, International journal of Fracture, V. 14, 1978, pp. R151-R184. C. Hudson, S.K. Seward, A Compendium of Sources of Fracture Toughness and Fatigue Crack Growth Data for Metallic Materials, Part II, Int. J. Frac., V. 20, 1982

18、, pp. R59-R117. DATA SHEETMaterial:B =mmPmax = Na =mmPmax/PQ (KQ / y)2 ? y= MPaInvalid Test ?KQ=MPaValid Test KIC = KQ =MPa (KQ / y)2 mmPfmax =NRepresentative data for KIc for several metals are given in Table 1, along with the values of corresponding critical plastic zone sizes. Also include in Tab

19、le I is the crack length L* = 2a which in a Griffith crack configuration would cause fracture initiation at an applied stress of (1/2)y. Note that L* is essentially the characteristic length dimension which specimen crack length, remaining ligament and thickness must exceed in order to obtain a vali

20、d KIc value for material. The combination of high KIc and low y leads to relatively large values of critical plastic zone size, and rather long cracks are required before initiation will occur at stress levels which are some fraction of the general yield stress.Table 1. Typical values of plane strai

21、n fracture toughness, KIC, at room temperature (for illustration purposes only)MATERIALS E (GPa)y (MPa)KIC(MPa)rIC(mm)L*(mm)SteelsMedium carbon (AISI-1045)210269505588.0Pressure Vessel (ASTM-A5330-B)21048315316.0256.0High Strength Alloy (AISI-4340)2101,593750.46.4Maraging Steel (250-Grade)2101,78674

22、0.34.8Aluminum Alloys2024-T472330341.727.27075-T65172503270.58.0 7039-T65172338321.422.4Titanium AlloysTi-6AL-4V1081,020500.46.4Ti-4Al-4Mo-2Sn-05 Si108945720.914.4Ti-6Al-2Sn-4Zr-6Mo1081,150230.11.6PolymersPS3.250.6 - 2.3PMMA3. - 4.1.2 - 1.7PC2.352.5 - 3.8PVC2.5 - 3.1.9 - 2.5PETP33.8 - 6.1CeramicsSi3N4 3104. - 5.SiC4103.4Al203 3503. - 5.Soda-Lime Glass730.7WC - 15 wt% Co (cermet)57016. - 18.Electrical Porcelain-1.