Influence of Inertial Load in Instrumented Impact Tests

Abstract

The effect of inertial loading in instrumented impact tests has been studied, and it is shown that during such tests large errors can be made in calculating the bending stress of a specimen at fracture if inertial loading is not taken into account. By instrumenting the tup, anvil, and specimen it is shown that in the period over which inertial effects occur, there is no simple relationship between the load measured on the tup, anvil, and specimen. To assist in interpretation and evaluation of impact load-time curves a model has been used which relates the spring constants and masses of the tup and anvil of the test machine and specimen to the frequency, phase, and relative amplitude of the oscillations occurring in the impact test. The importance of inertial loading is emphasized, in particular, with regard to dynamic measurement of plane strain fracture toughness KIc.

Interface Forces in Laterally Impacted Steel Tubes

“…The difference between the impact load measured on the striker and that applied to the specimen was attributed to specimen inertial effects. There were also some attempts to differentiate the recorded impact load from the inertia effects, assuming that part of the impact load from the striker was absorbed by inertia forces in the impacted body (Venzi [1]; Gopalaratnam, et al [2]; Banthia, et al [3]; Jerome and Ross [4]). They evaluated the equivalent mass of the specimen contributing in the response and multiplied it by the recorded striker acceleration, to produce the time history of the inertia force in the impacted member.…”

“…There has been a trend to express the interaction of the collided bodies in the form of equivalent interface forces (for instance Venzi, et al, [1]; Gopalaratnam, et al, [2]; Banthia, et al, [3]; Jerome and Ross, [4]; Huang, et al, [5]; Laird and Schrems, [6]; Langseth et al, [7]; and Zeinoddini et al, [8]). It was assumed that during an impact a level of load is transferred from the striker to the struck body.…”

Rapid Inexpensive Tests for Determining Fracture Toughness

“…Tbe most common arrangement uses strain gauges on the striking tup to sense the load versus time . An example of such a record in idealized form is shown in The details of these problem areas , their anal ysis, and procedures for minimizing them have been described (Venzi et al, 1970;Turner , 1970;Saxton et al, 1973;an~ Irelan d, 1973). Conducting the test at several impact velocities was recommended as one means of separating some of these effects , especiall y the inertial effects , in the early part of the record.…”

Characterising the fracture toughness of polymers at moderately high rates of loading with the use of instrumented tensile impact testing

“…At impact the force fluctuations are most severe due to the contact stiffness and inertia effects (causing for example loss of contact). The force measured at the striker, can therefore differ significantly from the resulting stress wave which travels through the specimen [7,8]. Additional analyses using for example a lumped (mass/spring/dashpot) model [9,10] might therefore be required to translate the measured signal to the stress at the notch.…”

“…For a Charpy-wise or three-point bending setup downstream force measurement would either require force cells at the supports (out of the notch plane) or a construction commonly found in Hopkinson bar type setups [11]. Moreover, loss of contact can occur at the striker and at the supports [7,8,12], complicating the analysis to extract the stress state at the notch for Charpy or three point bending setups. Thirdly, impact tests at increasing impact rates can give rise to a considerable amount of dynamic effects caused by contact and inertia effects, wave propagation etc.…”