Impact test


Published on

Published in: Technology, Business
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Impact test

  1. 1. Impact test report Objective To determine ductile to brittle transition temperature using V notch Charpy impact test. Apparatus Charpy impact testing machines Steel samples Thermometer Boiling water for high temperature Ice + salt for sub-zero temperature Theory and Background Background During World War II a great deal of attention was directed to the brittle failureof welded Liberty ships and T-2 tankers Some ofthese ships broke completely intwo, while, in other instances, the fracture did not completely disable the ship. Most of the failure occurred during the winter months, Failures occurred bothwhen the ships were in heavy seas and Figure 1: titanic ship when they were anchored atdock Thesecalamities focused attention on the fact that normally ductile mild steel canbecome brittle under certain conditions1. The Titanic began its maiden voyage to New York just before noon on April 10, 1912, from Southampton, England. Two days later at 11:40 p.m., Greenland time, it struck an iceberg that was three to six times larger than its own mass, damaging the hull so that the six forward compartments were ruptured. The flooding of these compartments was sufficient to cause the ship to sink within two hours and 40 minutes, with a loss of more than 1,500 lives. The scope of the tragedy, coupled with a detailed historical record, have fuelled endless fascination with the ship and debate over the reasons as to why it did in fact sink. A frequently cited culprit is the quality of the steel used in the ship's construction. A metallurgical analysis of hull steel recovered from the ship's wreckage provides a clearer view of the issue. Scientist observed several fractures in Second World War and predicted that 1. The fractures are of brittle type. 2. These occur below yield strength. Inspection and Testing Lab report Page | 1
  2. 2. Impact test report Analysis of these fractures showed that the reason behind is the notches. These may be due to 1. Design feature 2. Fabrication process (e.g. welding) 3. Flaws in the materials (e.g. porosity) Theory A broad research program was under-taken to find the causes of these failures and to prescribe the remedies for their future prevention, In addition to research designed to find answers to a pressing problem, other research was aimed at gaining a better understanding of the mechanism of brittle fracture and fracture in general1. Fracture Fracture may be defined as the mechanical separation of a solid owing to the application of stress. Fracture toughness Fracture toughness is an indication of the amount of stress required to propagate a pre-existing flaw. It is a very important material property since the occurrence of flaws is not completely avoidable in the processing, fabrication, or service of a material/component. Flaws may appear as cracks, voids, metallurgical inclusions, weld defects, design discontinuities, or some combination thereof (http://www.ndt- Toughness Toughness is defined as the ability of a material to absorb energy. It is usually characterized by the area under a stress-strain curve for a smooth (unnotched) tension specimen loaded slowly to fracture (3). Notch toughness Notch toughness represents the ability of a material to absorb energy usually determined underimpact loading in the presence of a notch. Notch toughness is measured by using a variety of specimens such as the Charpy V-notch impact specimen, the dynamic-tear specimen, and planestrain fracture-toughness specimens under static loading (KId) and under impact loading (KIc) (3asm). Types of Fracture There are various types of fracture which are encountered. 1. Ductile fracture Inspection and Testing Lab report Page | 2
  3. 3. Impact test report 2. Brittle fracture 3. Inter-crystalline 4. Fatigue fracture Apart from these there fracture may be both Brittle and ductile Fracture dependence Ductile-to-Brittle Fracture Transition. Traditionally, the notch-toughness characteristics of low- and intermediate-strength steels have been described in terms of the transition from ductile to brittle behavior as test temperature increases. Most structural steels can fail in either a ductile or a brittle manner depending on several conditions such 1. Temperature 2. Strain rate (ϵ o) 3. State of stress 4. Section size 5. Notch acuteness These variables may change a ductile fracture to a brittle fracture in service leading. Notched-Bar Impact Tests Various types of notched-bar impact tests are used to determine the tendency of a material to behave in a brittle manner. This type of test will detect differencesbetween materials which are not observable in a tension test. The results obtained from notched-bar tests are not readily expressed in terms of design requirements, since it is not possible to measure the components of the triaxial stress condition at the notch. Furthermore, there is no general agreement on the interpretation or significance of results obtained with this type of test. A large number of notched-bar test specimens of different design have beenused by investigators of the brittle fracture of metals. Two classes of specimenshave been standardizedfor notchedimpact testing. Charpay V-notch impact test (in USA) Izod test (in UK) Charpy V-notch impact test The most widely used specimen for characterizing the ductile-to-brittle transition behavior of steels. These specimens may be tested at different temperatures and the impact notch toughness at each test temperature may be determined from the energy absorbed during fracture, the Inspection and Testing Lab report Page | 3
  4. 4. Impact test report percent shear (fibrous) fracture on the fracture surface, or the change in the width of the specimen (lateral expansion). Figure 2: Charpy v-impact test The notched-bar impact test is most meaningful when conducted over a range of temperature so that the temperature at which the ductile-to-brittle transition takes place can be determined. Note that the energy absorbed decreases with decreasing temperature but that for most cases the decrease does not occur sharply at a certain temperature. The principal advantage of the Charpy V-notch impact test is that it is a relatively simple test that utilizes a relatively cheap, small test specimen. Tests can readily be carried out over a range of sub ambient temperatures. Moreover, the design of the test specimen is well suited for measuring differences· in notch toughness in low-strength materials such as structural steels. The test is used for comparing the influence of alloy studies and heat treatment on notch toughness. Inspection and Testing Lab report Page | 4
  5. 5. Impact test report It frequently is used for quality control and material acceptance purposes. The chief difficulty is that the results of the Charpy test are difficult to use in design. Since there is no measurement in terms of stress level, it is difficult to correlate Cv data with service performance. Moreover, there is no correlation of Charpy data with flaw size. In addition, the large scatter inherent in the test may make it difficult to determine well-defined transition-temperature curves. Specimen in Charpay v-notch impact test The Charpy specimen has a square cross section (10 X 10 mm) and contains a 45° V notch, 2 mm deep with a 0.25-mm root radius. The specimen is supported as a beam in a horizontal position and loaded behind the notch by the impact of a heavy swinging pendulum (the impact velocity is approximately 5 ms - 1 ). The specimen is forced to bend and fracture at a high 3 Figure 3: sample for charpy v-notch impact test -1 strain rate on the order of 10 s . Inspection and Testing Lab report Page | 5
  6. 6. Impact test report Figure 4: micrograph of charpy tested samples INSTRUMENTED CHARPY TEST The ordinary Charpy test measures the total energy absorbed in fracturing thespecimen. Additional information can be obtained if the impact tester is instrumented to provide a load-time 'history of the specimen during the test.In figure an idealized loadtime curve foran instrumented Charpy test. Withthis kindof record it is possible to determine the energy required for initiatingfractureand the energy required for propagating fracture. It also yields information on the load for general yielding, the maximum load, and Inspection and Testing Lab report Page | 6
  7. 7. Impact test report the fracture load. Transition temperature As mentioned previously, the absorbed energy of BCC metals changes drastically within the transition region, we therefore have to identify a transition temperature, which can be used todetermine the suitable service temperature of particular materials in order to avoid metal failure in acatastrophic manner. There are several criteria for the identification of the transition temperature. Transition temperature is the temperature at which the test sample absorbs the mostfracture energy and possesses 100% fibrous fracture surfaces. This means brittle fracture is neglected in this case and is considered to be the safest among other criteria. The transition temperature is also called the fracture transition plastic or FTP. Transition temperature is the temperature at which the percentage of cleavage and ductilefractures are equal. This transition is also called fracture appearance transition temperature orFATT because the fracture surface area is used as an indicator to determine the transition temperature. Transition temperature is the temperature correlating to an average absorbed energy valueof upper and lower shelf energy absorption. At or above this temperature, there is acorrelation that less than 70% of the brittle cleavage fracture that indicates a high probability at which failure will not occur if the stress does not exceed about one-half of the yield stress. Transition temperature is the temperature at which the absorbed energy (C) equals 20J. This criterion was introduced to determine toughness value of steels used during the World War II. It is based on the idea that brittle fracture will not occur if the sample has the absorbed energy above 20J. However this criterion might show no significant meanings forother materials. Transition temperature is the temperature at which there is none of the ductile dimplesappearing on the fracture surfaces. This temperature is also called nil ductility temperature orNDT since there is no plastic deformation during fracture. Inspection and Testing Lab report Page | 7
  8. 8. Impact test report Metallurgical Factors Affecting Transition Temperature The factors which affect the transition temperature are Composition of steel Grain size Ageing phenomena Notched size and direction Tempering time Presence of martensite Lowest possible finish temperature of rolling Procedure Room temperature test is first carried out by placing the Charpy impact specimen on the anvil and positioning it in the middle location using a positioning pin where the opposite site of the notch is destined for the pendulum impact. Raise the pendulum to a height corresponding to the maximum stored energy of 300J. Release the pendulum to allow specimen impact. Safely stop the movement of the pendulum after swinging back from the opposite side of the machine. Inspection and Testing Lab report Page | 8
  9. 9. Impact test report When the pendulum is still, safely retrieve the broken specimen without damaging fracture surfaces. Repeat the test at the same test condition using another specimen to average out the obtained values. Charpy impact testing at temperatures other than room temperature is carried out following Prior to specimen impact, specimen is submerged in the medium for at least 5 minutes to ensure uniform temperature across the specimens. Specimen impact must be within 5 seconds after removing from the medium. Repeat the test at the same test condition using another specimen to average out the obtained values. Data Sample Mild Steel Angle during free falling Condition Temperature oC 135o Room temperature 10 Ice + salt 0 Boil water 98 Calculation Hammer lift angle α = 139.5o Distance from axis to centre of gravity D = 0.694m Weight of hammer P = 40.98kg E = PD (cosβ-cosα) Case I Room temperature β E= = 96o energy calculated = 18.77 Kgfm Inspection and Testing Lab report Page | 9
  10. 10. Impact test report Case II Boiling water = 95o E β = 19.135 kgfm Case III Ice + salt β = 112 E = 10.96 kgfm Result As we decrease temperature material fails with brittle manner. And absorbed energy also decrease. Because material is much prone to brittle behaviour. Reference Mechanical metallurgy 'g.e. dieter' 3rd edition Mechanical Testing and Evaluation was published in 2000 as Volume 8 of the ASM Handbook. The Volume was prepared under the direction of the ASM Handbook Committee. Hand out lecture ASTM standards e 2 Inspection and Testing Lab report Page | 10