冲击强度.doc

材料的韧性是断裂时所需能量的度量。描述材料韧性的指标通常有两种： （1）冲击韧性aK (单位为MJ/m2)冲击韧性是在冲击载荷作用下，抵抗冲击力的作用而不被破坏的能力。 通常用冲击韧性指标aK来度量。aK是试件在一次冲击实验时，单位横截面积（m2）上所消耗的冲击功（MJ），其单位为MJ/m2。 aK值越大，表示材料的冲击韧性越好。 标准冲击试样有两种，一种是常用的梅氏试样（试样缺口为U型）；另一种是夏氏试样（试样缺口为V型）。 同一条件下同一材料制作的两种试样，其梅氏试样的aK值显著大于夏氏试样的aK值，所以两种试样的aK值不能互相比较。 夏氏试样必须注明aK（夏）。 实际工作中承受冲击载荷的机械零件，很少因一次大能量冲击而遭破坏，绝大多数是因小能量多次冲击使损伤积累，导致裂纹产生和扩展的结果。所以需采用小能量多冲击作为衡量这些零件承受冲击抗力的指标。 实践证明，在小能量多次冲击下，冲击抗力主要取决于材料的强度和塑性。 （2）断裂韧性K1 (单位为J/m)在实际生产中，有的大型传动零件、高压容器、船舶、桥梁等，常在其工作应力远低于S的情况下，突然发生低应力脆断。 通过大量研究认为，这种破坏与制件本身存在裂纹和裂纹扩展有关。 实际使用的材料，不可避免地存在一定的冶金和加工缺陷，如气孔、夹杂物、机械缺陷等，它们破坏了材料的连续性，实际上成为材料内部的微裂纹。 在服役过程中，裂纹扩展的结果，造成零件在较低应力状态下，即低于材料的屈服强度，而材料本身的塑性和冲击韧性又不低于传统的经验值的情况下，发生低应力脆断。 材料中存在的微裂纹，在外加应力的作用下，裂纹尖端处存在有较大的应力集中和应力场。 断裂力学分析指出，这一应力场的强弱程度可用应力强度因子K1来描述。 随应力的增大，K1也随之增大，当K1增大到一定值时，就可使裂纹前端某一区域内的内应力大到足以使裂纹失去稳定而迅速扩展，发生脆断。 这个K1的临界值称为临界应力强度因子或断裂韧性，用K1C表示，单位为J/m。它反映了材料抵抗裂纹扩展和抗脆断的能力。 材料的断裂韧性K1C与裂纹的形状、大小无关，也和外加应力无关，只决定于材料本身的特性（成分、热处理条件、加工工艺等），是一个反映材料性能的常数Izod Impact Testing (Notched Izod)ASTM D256 and ISO 1801.Scope:Notched Izod Impact is a single point test that measures a materials resistance to impact from a swinging pendulum. Izod impact is defined as the kinetic energy needed to initiate fracture and continue the fracture until the specimen is broken. Izod specimens are notched to prevent deformation of the specimen upon impact. This test can be used as a quick and easy quality control check to determine if a material meets specific impact properties or to compare materials for general toughness.2.Test procedure:The specimen is clamped into the pendulum impact test fixture with the notched side facing the striking edge of the pendulum. The pendulum is released and allowed to strike through the specimen. If breakage does not occur, a heavier hammer is used until failure occurs. Since many materials (especially thermoplastics) exhibit lower impact strength at reduced temperatures, it is sometimes appropriate to test materials at temperatures that simulate the intended end use environment.3.Reduced Temperature Test procedure:The specimens are conditioned at the specified temperature in a freezer until they reach equilibrium. The specimens are quickly removed, one at a time, from the freezer and impacted. Neither ASTM nor ISO specify a conditioning time or elapsed time from freezer to impact - typical values from other specifications are 6 hours of conditioning and 5 seconds from freezer to impact.4.Specimen size:The standard specimen for ASTM is 64 x 12.7 x 3.2 mm (2 x x 1/8 inch). The most common specimen thickness is 3.2 mm (0.125 inch), but the preferred thickness is 6.4 mm (0.25 inch) because it is not as likely to bend or crush. The depth under the notch of the specimen is 10.2 mm (0.4 inches).The standard specimen for ISO is a Type 1A multipurpose specimen with the end tabs cut off. The resulting test sample measures 80 x 10 x 4 mm. The depth under the notch of the specimen is 8mm.5.Data:ASTM impact energy is expressed in J/m or ft-lb/in. Impact strength is calculated by dividing impact energy in J (or ft-lb) by the thickness of the specimen. The test result is typically the average of 5 specimens.ISO impact strength is expressed in kJ/m2. Impact strength is calculated by dividing impact energy in J by the area under the notch. The test result is typically the average of 10 specimens.The higher the r