What Is Impact Strength?

An index for measuring the toughness of a material, which is usually defined as the energy absorbed per unit cross-sectional area when a specimen is broken or fractured under the impact of an impact load.

An index to measure the toughness of a material, which is usually defined as the energy absorbed per unit cross-sectional area when a specimen is broken or fractured under the impact of an impact load.

Chinese name

Impact strength

Foreign name

Impact Strength,

Category

An indicator of material toughness

nickname

Impact toughness

Gap number

Four

Standard

ISO international standards and American materials ASTM standards

Unit

GB: MJ / m2ASTM: J / m

Basic overview of impact strength

(1) Impact strength is used to evaluate the impact resistance of materials or to judge the brittleness and toughness of materials. Therefore, impact strength is also called impact toughness.

(2) Impact strength is the ratio of the energy absorbed by the sample during the impact failure to the original cross-sectional area.

(3) The measurement standards for impact strength are mainly ISO international standards (GB refers to ISO) and American materials ASTM standards.

(4) The most common impact strength test is the impact strength of plastic products.

According to the different test equipment, it can be divided into simple supported beam impact strength and cantilever beam impact strength.

GB / T 1043.1-2008 Plastics. Determination of impact properties of simply supported beams. Part 1: Non-instrumentated impact test ^[1] ASTM D6110-2010 Standard Test Method for Determining the Charpy Impact Resistance of Notched Specimens of Plastics ^[2] A method for determining the impact strength of a simply supported beam under specified conditions is described.

GB / T 1843-2008 Determination of the impact strength of plastic cantilever beams ^{[3] [3]} and ASTM D256-2010 Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics ^[4] stipulates that plastics are used to determine simply supported beams under specified conditions Method of impact strength.

The specific differences are as follows:

GB: It is the impact energy (J) consumed per unit cross-sectional area (m2) of the specimen during an impact test. Its unit is MJ / m2.

ASTM: It reflects the material's ability to resist crack propagation and brittle fracture, the work consumed per unit width, the unit is J / m.

Equipment differentiation:

There is a striker in the middle of the impact direction of the cantilever beam, a concave block is perpendicular to the impact direction of the simply supported beam, and the front shape is a concave pendulum.

Gap distinction:

The gaps are generally divided into four types, there are two types of V-shaped and U-shaped, each of which is divided into two according to the short arc radius.

Spline differentiation:

GB: Generally 80 * 10mm splines and 63.5 * 10mm splines have a gap of 2mm, and there are also 63.8 * 12.7mm splines.

ATSM: generally 63.5 * 12.7mm, the remaining width of the gap is 10.16mm (useful domestic 80 * 10 splines)

Test formula:

GB: a = W / (h * d) Unit KJ / m2 ATSM: a = W / d Unit: J / m

a: Impact strength

W: impact loss energy

h: remaining gap width

d: spline thickness

Therefore, GB and ASTM cannot be equivalently measured, but the empirical formula can be summarized from the measurement formula: GB value * 10.16 or 8 (wrong spline) = ASTM value, and the ratio can also be summarized by actual measurement.

(5) In addition to plastic impact strength, the commonly used impact strength are:

GB / T 229-2007 Charpy pendulum impact test method for metallic materials

GB / T 13465.4-2014 Test methods for impervious graphite materials-Part 4: Impact strength

SJ / T 11041-1996 Electronic glass impact strength test method

HG / T 3845-2008 hard rubber. Determination of impact strength

JB / T 7609-2006 Carbon graphite material impact strength test method

Routine calculation of impact strength

The impact stress generated by the impact load in the part is related to the shape, volume, and local elastoplastic deformation of the part, as well as the objects connected to it. If the object connected to the part is an absolute rigid body, all the impact energy will be borne by the part; if the stiffness of the object connected to the part is a certain value, the impact energy will be borne by the entire system, and the part only bears portion. In addition, the magnitude of impact stress also depends on the magnitude of impact energy. Therefore, the strength calculation under impact load is much more complicated than that under static load. When designing parts that are subjected to impact loads, a dynamic load factor (see load factor) must be introduced and the design shall be based on static strength. The dynamic load coefficient can also be determined by the response method in vibration theory.

When studying the impact strength of a part, it is necessary to consider the change in the mechanical properties of the material under the impact load and the magnitude of the impact effect on the part. For structural steel, when the strain rate is 10-6 10-21 / second, the mechanical properties of the steel have no obvious change. However, at higher strain rates, the strength limit and yield limit of structural steel increase with increasing impact velocity. And the yield limit increases faster than the strength limit. Therefore, it is safer to treat impact load as static load for general structural steel. On the other hand, the impact load is more sensitive to material notches than the static load is more sensitive to material notches. At this time, if the impact load is treated as a static load, the safety factor must be improved.

Shock intensity

When a part is impacted, its impact stress and strain cannot be transmitted to the entire part immediately, but are transmitted in the form of stress waves or strain waves. According to the different parts and loading conditions, the stress wave appears as plane, cylindrical, spherical, etc., and has components of longitudinal wave (normal stress wave) and transverse wave (shear stress wave). When a stress wave (incident wave) propagates through a part, it encounters a free surface and causes reflection, resulting in a reflected wave. If the longitudinal wave is a compression wave perpendicular to the surface, the reflected wave is a tensile wave. When two or more stress waves meet, complex interference phenomena will occur. According to the superposition principle of incident wave and reflected wave, the peak stress of a certain section at a certain moment is calculated. When the peak stress exceeds the strength limit of the material, the part will have impact damage. The impact strength is calculated based on the principle of stress wave propagation and is limited to parts with simple shapes. For parts with complex shapes or the entire machine subjected to impact loads, experimental methods can be used to determine the impact strength.

Impact strength

Most of the mechanical parts and components in actual work are small energy multiple impact loads with less impact energy and more impact times. Their failure is the result of the formation and propagation of cracks caused by the accumulation of multiple impact damages. The damage resistance of a material under one impact is mainly determined by the impact toughness; but the resistance with more impact times is mainly determined by the fatigue strength of the material. Between these two, when the number of impacts to reach failure increases, the effect of impact toughness decreases and the effect of fatigue strength increases. Based on the test results of multiple impacts on steel samples, it can be concluded that the impact toughness range is less than 100 to 1000 times.

Applying the data of multiple impact tests to the actual design of parts requires the simulation of multiple impact strengths of samples and objects, such as size, shape change, and material property changes. In the approximate calculation, the following methods can be used: when the number of impacts is less than 1,000 times, the strength is calculated by one impact; when the number of impacts is more than 1,000 times, the strength is calculated by a method similar to fatigue.