Longitudinal versus Transverse Impact testing
The term “longitudinal” or “transverse” refers to whether the grain in the steel in the test piece runs the length of the rectangle (“longitudinal”) or across the width of the rectangle (“transverse”).
In an impact test, the test piece is shaped into a rectangle, with the “notch” being made in the middle of the length of the rectangle on one side. The purpose of the impact test is to measure the force needed to break the test piece at the notch when hit with a pendulum against the long side of the test piece in the middle of the notch.
In a longitudinal test direction, the force of the impact goes across the grain of the test piece. In a transverse test direction, the force of the impact from the hit goes parallel to the grain of the specimen (or effectively, between the strands of the grain).
Differences obviously arise between the longitudinal test results and the transverse test results:
Longitudinal (the directional force is across the grain): it is harder to fracture the specimen and therefore takes more energy to fracture the steel across the grain. Longitudinal test pieces therefore have higher notch toughness’ (i.e. it will take more joules (or foot pounds) of absorbed energy to break a longitudinal specimen).
Transverse: the transverse impact energy required will be less to break the steel across the notch.
Which to use?
It depends upon the application and where the customer expects directional force to be applied during use.
One other factor is the ability to obtain a reliable transverse test piece. This is normally only possible for above a 3“or 76.2 mm diameter bar- on smaller diameters there is not enough metal to get a test specimen.
In terms of expected results, obviously it is harder to produce impact (or “notch”) toughness longitudinally because the grain direction of the steel provides a natural resistance to fracture. Therefore, we should expect to see higher impact energy requirements. On transverse testing the grain of the steel will not be a factor in resisting the fracture, therefore we should expect to see lower energy requirements.
It is therefore somewhat of an eyebrow raiser when we see specifications calling for the same minimum impact strength in both the longitudinal and transverse direction. The implication is that we will need a longitudinal energy used much higher than the minimum requested to get an acceptable result in a transverse direction.