Impact tests are performed to measure the response of a materialto dynamic loading. When two objects collide, damage is often doneto one or other of them. How well something resists damage iscalled its impact resistance. An impact test measures how muchenergy is absorbed when an object fractures or breaks under a highspeed collision.

It’s an important property. The safety of many consumerproducts depends on their resistance to breaking. Therefore,Engineers need to know about how materials and products behaveunder impact and the magnitude of forces they can resist.

Part or product tested no longer maintains its original shapeor surface texture.

Mechanical test methods are all destructive.

Other destructive tests include

• speed testing of grinding wheels to determine their bursting speed

• high pressure testing of pressure vessels to determine their burstingpressure.

Toughness is a measure of the amount of energy required to cause an item to

fracture and fail. The more energy that is required then the tougher thematerial.

Impact is a high force or shock applied over a short time period when two

or more bodies collide.

Impact resistance or Impact energy How well something resists damage is a measure of the work done to fracture a test specimen.

NotchesThe notch serves as a stress concentration zone and some materials

are more sensitive towards notches than others.

The impact test is a method for evaluating the toughness andnotch sensitivity of engineering materials. It is usually used totest the toughness of metals, but similar tests are used forpolymers, ceramics and composites. Metal industry sectorsinclude Oil and Gas, Aerospace, Power Generation, Automotive,and Nuclear.

The notched test specimen is broken by the impact of a heavypendulum or hammer, falling at a predetermined velocitythrough a fixed distance. The test measures the energy absorbedby the fractured specimen.

There are primarily 2 types of impact test -Charpy impact test and Izod impact test.

Both tests involves fracturing a notchedspecimen of standard dimension andmeasuring the amount of energy absorbedto fracture the specimen. Both involvestriking a standard specimen with acontrolled weight pendulum traveling at aset speed.

Measure in J (n-m) of lb-ft

These tests show that metals can beclassified as being either 'brittle' or 'ductile'.A brittle metal will absorb a small amountof energy when impact tested, a toughductile metal a large amount of energy.

The test was developed around 1900 by S. B. Russell (1898, American)and G. Charpy (1901, French). The test became known as the Charpytest in the early 1900s due to the technical contributions andstandardization efforts by Georges Charpy. The test was pivotal inunderstanding the fracture problems of ships during WWII.

A test specimen is machined to a 10mm x 10mm (full size) cross-section, with either a "V" or "U" notch. Sub-size specimens are usedwhere the material thickness is restricted. Specimens can be testeddown to cryogenic temperatures. -150

PRINCIPLE

The Charpy impact test is a dynamictest in which a test piece U-notchedor V-notched in the middle andsupported at each end, is broken by asingle blow of a freely swingingpendulum.

The test is named after the English engineer Edwin Gilbert Izod(1876–1946), who described it in his 1903 address to the BritishAssociation, subsequently published in Engineering.

The test specimen is machined to a square or round section,with either one , two or three notches. The specimen is clampedvertically on the anvil with the notch facing the Hammer.

PRINCIPLE

The Izod impact test is a dynamictest in which a test piece V-notched test piece, grippedvertically, is broken by a singleblow of a freely swingingpendulum (Fig.). The blow isstruck on the same face as thenotch and at the fixed heightabove it. The energy absorbed ismeasured. This absorbed energy isa measure of the impact strengthof material.

1) The test notches for the impact specimens for the tests have differentdimensions. The Izod test is a V-notch; the Charpy test has three differentspecimen types: U-notchand V-notch. However, other specimen types maybe specified as required for both tests.

2) The specimens are held differently. The Izod specimen is held in acantilevered manner; the Charpy test is held such that the specimen restsagainst two supports on either side of the test notch.

3) The impact location is different. The Izod test impact is against the endof the exposed cantilever; the Charpy test is struck directly behind the testnotch such that the specimen undergoes three point bending.

4) The test specimens have different dimensions. The basic Izod testspecimen is 75 x 10 x 10mm (2.95" x 0.394" x 0.394"); the basic Charpy testspecimen is 55 x 10 x 10mm (2.165" x 0.394" x 0.394")

Drop weight impact testers

A mass is dropped vertically on to a testspecimen. A tube or rails are used to guidethe falling mass. Since the mass either stopsdead on the specimen or breaks it, the testwas essentially pass/fail.

However, the energy absorbed by aspecimen when it breaks can be estimated:

the mass is dropped from increasingheights until the specimen fractures orbreaks

further tests are carried out on othersamples to get more accurate value. This caninvolve anything up to 100 test samples.

Keyhole Impact Test

The steel casting industry uses this type of specimen morefrequently. The notch is machined to look like a keyhole. It is testedin the same manner as the "V" and "U" notch.