Testing of impact strength with a pendulum impact tester
The notched bar impact test is used to determine the toughness or notched bar impact strength of a material. The test is relevant for production control and quality monitoring, e.g. in the manufacture of tanks, pipes, steel bottles, etc. Liquid media and gases are usually stored and transported in these products. The toughness of the steel walls must be high enough that no leaks occur even in the event of blows or impacts, as otherwise the gases, oils, acids, alkalis, etc. could escape and endanger people or contaminate the environment.
Example: A tanker truck has an accident and tips over. The tank falls onto the post of a guardrail. This impact load must not result in the tank wall being penetrated and the tank becoming leaky so that the load (petrol, diesel, gas) does not escape.
Notched bar impact test ISO 148
In the impact test, a notched bar impact test specimen is subjected to an impact test. The standardized specimen is placed on the abutments of a pendulum impact tester. A pendulum hammer, which is attached to the axis of the pendulum impact tester, is dropped from a defined height and bends or breaks the specimen. The hammer cutting edge (impact fin) hits the specimen and bends (ductile materials) or breaks it. The impact energy is measured in joules (energy) and is used to characterize the material. The higher the value, the tougher or more impact-resistant the material is.
This test is often a safety-related test that is required for tanks and pipelines. Therefore, a pendulum impact tester that is to be used to carry out these mandatory tests must undergo an official DAkkS calibration according to ISO 148-2 by a company accredited according to ISO 17025.
The impact speed of the pendulum hammer or the impact fin is specified in the ISO 148 (metal) standard and is approximately 5,5 m/s. This speed is only achieved if the pendulum is of a corresponding length and the fall height (fall angle approximately 160 degrees) is designed accordingly (acceleration due to falling speed / acceleration due to gravity).
The geometry of the fin is also precisely specified (radius at the point of impact) as well as a narrowly limited angle of impact with respect to any twisting or inclination of the fin.
The potential (stored or kinetic) impact energy is determined from the weight of the hammer and the height of the fall. There are different pendulum impact testers with different impact energies for different tough metallic materials, e.g. 50 / 150 / 300 / 450 / 600 / 750 joules. The development of ever better materials has meant that the 300 joules pendulum impact testers that were most commonly used up until the turn of the millennium have increasingly had to be replaced by pendulum impact testers with an impact energy of 450 / 600 or even 750 joules.
The pendulum impact testers determine the energy consumed from the potential impact energy and the angle of inclination. In older pendulum impact testers, this is indicated by a main and a trailing pointer and does not need to be calculated. The scale of these analogue displays is non-linear and takes into account the acceleration and deceleration of the hammer due to gravity. Modern pendulum impact testers have an electronic display instead of a mechanical display with a scale and pointers. An electronic angle sensor determines the angle of inclination and calculates the energy consumed using the following formula:
Formula for impact test: The impact energy or impact work is calculated in the impact test according to ISO 148 (and other standards) from the difference between the potential energy of the pendulum hammer and the angle of rise:
W = mg(h1 – h)
W: impact energy in J (Joule)
m: Mass (weight) of the pendulum impact tester in kg
g: Earth's gravitational acceleration 9,81 m/s2
h1: fall height (hammer release)
h: Height of the hammer (slowed down by the resistance of the sample and the force of gravity
The fall height is not measured directly but results from the pendulum length (usually approx. 800mm for an impact speed of approx. 5,5m/s) and:
- the angle at which the pendulum is released (approx. 160 degrees)
- the angle at the reversal point of the pendulum motion (angle of ascent)
The angle of climb / angle of ascent is:
- in an analogue display by the trailing pointer (the trailing pointer is dragged along by the main pointer and stops at the end point)
- in a digital evaluation by recording the last angle of an electronic angle sensor and counter
determined
Angle of ascent / angle of climb: The pendulum hammer passes the lowest point of the falling movement (at 18:00) and rises again on the opposite side until it stops at the angle of rise and pendulums backwards again.
Sample dimensions of impact bending specimens according to ISO148
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