EP0413696A1

EP0413696A1 - Impact testing apparatus

Info

Publication number: EP0413696A1
Application number: EP19890903195
Authority: EP
Inventors: Michael Ian Cobbe; Leonard Threadgold
Original assignee: GEOTECHNICS Ltd
Current assignee: GEOTECHNICS Ltd
Priority date: 1988-02-05
Filing date: 1989-02-03
Publication date: 1991-02-27
Also published as: GB8802626D0; WO1989007176A1; AU3975889A

Abstract

Appareil utilisé pour effectuer des essais au choc sur le sol comprenant un poids (15, 16, 17) monté sur un bras pivotant (12) et disposé de manière à osciller, par l'effet de la gravité, autour d'une pointe de pivot (14) du bras et à frapper le sol dans une direction sensiblement perpendiculaire à celui-ci. Cet appareil peut être manipulé par une seule personne; celle-ci peut soulever le bras (12) à l'aide d'un câble jusqu'à une hauteur prédéterminée indiquée par un capteur rotatif placé sur la pointe de pivot (14). Le poids est pourvu d'un accéléromètre et le freinage maximum à l'impact est mesuré, de même que le temps nécessaire pour atteindre ce freinage maximum, à compter du début du freinage; ces mesures permettent de calculer l'amplitude apparente de l'impact pour le sol faisant l'objet des essais.Apparatus used for carrying out impact tests on the ground comprising a weight (15, 16, 17) mounted on a pivoting arm (12) and disposed so as to oscillate, by the effect of gravity, around a point of pivot (14) of the arm and to strike the ground in a direction substantially perpendicular thereto. This device can be operated by one person; the latter can raise the arm (12) using a cable to a predetermined height indicated by a rotary sensor placed on the pivot point (14). The weight is provided with an accelerometer and the maximum braking at impact is measured, as well as the time necessary to reach this maximum braking, from the start of braking; these measurements make it possible to calculate the apparent amplitude of the impact for the soil under test.

Description

IMPACT TESTING APPAEATϋS

This invention relates to an impact testing apparatus for use in carrying out impact tests on ground, soil or similar materials.

In the course of construction work, e.g. of motorways, it is necessary to test the ground, for example to check for satisfactory compaction of layers of stone, clay, flyash, gravel or other materials that have been laid down. Similar tests are also necessary when back-filling trenches which have been cut into an existing road or other ground surface, e.g. when laying or gaining access to utility pipes or cables.

One known form of impact tester comprises a cylindrical weight which is free to slide within an upright tube placed on the ground which is to be tested. In use, the weight is raised within the tube and then dropped from a fixed height so as to impact onto the ground. The weight is provided with an accelerometer connected to a readout so as to give a measure of the deceleration of the weight at the instant of impact. Whilst this form of impact tester is useful it has certain recognised drawbacks, including the following. Firstly, the weight is of relatively small diameter so that the reliability of the measurement given by the read-out will depend upon the average size of the stones etc. which make up the ground surface. Secondly, the test tends to cause bearing failure in the material being tested rather than testing within the range of stress up to failure in which the material would be required to function in practice. Thirdly, it is difficult to ensure that the weight will drop cleanly within the tube and not be retarded by rubbing on the wall of the tube. Fourthly, the tester requires the weight to be pulled up the vertical tube manually prior to each drop: this places strain on the user's back and the size of the weight and drop height which can be used is limited. Fifthly, it is necessary to ensure that the tube is vertical and the surface to be tested horizontal.

We have now devised an impact testing apparatus which overcomes these drawbacks and which is of simple construction, easy to use and yet provides for greater reliability and uniformity of measurement and flexibility of application.

In accordance with this invention, there is provided an impact testing apparatus, comprising a weight mounted to a pivoted arm and arranged so that the weight will swing under gravity about the pivot and impact with the ground perpendicular to the latter.

Because the weight is exposed rather than being contained within an upright tube, it may be of larger cross-section. Moreover, the weight may be made removable and replaceable by others of different size and weight to suit the application. There is no risk of the weight being retarded by rubbing aganst any guide channel or tube. Any convenient arrangement may be provided for raising the weight prior to each drop, such as a length of cable which the user can pull on. This places less strain on the user than if he were required to pull a weight vertically upwards. The weight may be raised to any required angle of the pivoted arm and preferably a transducer is provided to give a read-out of the angular position of the arm.

The impact testing apparatus is preferably provided with wheels or the like so that it may be pushed or pulled by hand between positions at which impact tests are to be made. For example the apparatus may be provided with a pair of wheels or a roller at one end, arranged to come into contact with the ground and support the apparatus when this is lifted at the other end, in the manner of a wheelbarrow.

The weight may be provided with an accelerometer coupled to a read-out for giving a measure of the deceleration of the weight at impact. The apparatus may alternatively or instead be provided with arrangements for carrying out one or more other impact tests, such as measuring the depth to which the weight penetrates the ground upon impact, or the distance through which it rebounds.

An embodiment of an impact testing apparatus will now be described by way of example only and with reference to the accompanying drawings, in which:

FIGURE 1 is a plan view of an impact testing apparatus in accordance with the invention;

FIGURE 2 is a side view of the apparatus; and FIGURE 3 is a section through a loading plate set for use with the apparatus.

Referring to the drawings, there is shown an impact testing apparatus comprising a frame 10 mounting an arm 12 which is pivoted at one end to the frame by means of a shaft 14 and carries a cylindrical boss 15 at its other end, to which weights 16, 17 are attached.

In the example shown the weights 16, 17 are circular but are removable from the top and bottom of the boss 15 and can be replaced by weights of different shapes and sizes to suit the particular application of use. The weights 16, 17 may each be in the range of 5 to 8 kg with diameters in the range 100-300 mm although weights in excess of 10kg may be used. The arm 12 might typically have a length of 750 mm. The frame 10 comprises two inclined side members 10a, 10b, a cross-member 10c at the wider end and a foot plate 11 across the narrower end.

Wheels 18 are mounted to projections 20 of the side members 10a, 10b at the wider or forward end of the frame. The frame 10 rests flat on the ground G via two feet 24 on the side members adjacent the forward end of the frame and a foot 25 at the narrower or rearward end.

The arm 12 is fixed to the shaft 14 which is journalled at its opposite ends in blocks 26 which are slidably mounted to vertical guideways 27. The shaft 14 also passes through an adjustment block 28 through which a vertical adjustment shaft 29 is threaded. Adjustment shaft 29 can be turned by means of a wheel 30 at its upper end, in order to adjust the height of the pivot shaft 14 above the footplate and so bring the arm 12 into a line parallel with the ground when the weight 16 is resting on the ground.

A shaft encoder 31 is coupled to the pivot shaft 14 and provides a signal to an electronic unit 32 to display the angular position of the arm 12 relative to the horizontal.

Any convenient arrangement may be provided for lifting the arm 12 and the weights 16, 17 to the required position before release. In the example shown, a pull- cable 33 is connected to any eye 34 on a bolt which fastens the weight 17 to the top of the boss 15. The user may pull the arm up using the pull cable 33 whilst standing on the footplate 11.

Preferably the lower end of the boss 15 is provided with an accelerometer 13 which relays a signal over a communication cable extending along the arm 12 to the unit 32 which determines and displays a measure of the peak or maximum deceleration of the weight at impact, and also the time delay between the onset of deceleration and the peak.

Once the apparatus has been used at one location, it can be moved manually by lifting at the rearward end of the frame so that the wheels 18 come into contact with the ground and the frame itself is lifted out of contact with the ground. The arm 12 now rests on an underslung cross- piece 19 of the frame. The apparatus can then be pushed or pulled in the manner of a wheelbarrow. In a modified apparatus, three or four running wheels may be provided for the apparatus to run on, these wheels being raised off the ground to place the frame flat on the ground when the apparatus is to be used.

It will be appreciated that when the arm 12 is released, it will swing downwards under gravity until the weight 16 strikes the ground. The apparatus may be used on ground which is level or sloping, but in any event the weight will impact against the ground in a direction (at the instant of impact) which is perpendicular to the ground surface.

The apparatus which has been described is a portable, low-cost test apparatus which allows one-man operation for applying repeated impacts from variable masses. The total weight (weights 16 plus weight 17) may exceed 20 kg and the weights may be of selected diameters and dropped through selected heights e.g. up to 750 mm. The falling mass strikes the surface of the material under test orthogonally and a measure is made of an elastic response over a depth of similar order to the depth of a compacted layer of soil or stabilised or granular material as used in civil engineering construction.

At each test location, several impact tests can be repeated using the same weights and drop heights. Before commencing, the height of the pivot shaft 14 is adjusted to ensure that the arm 12 is parallel to the plane of the frame 10, the display giving a read-out of zero. At each drop, the electronic unit 32 giving read-outs of the peak deceleration (orthogonal dynamic impact number - ODIN) and of the time delay TP to reach the peak from the onset of the detected deceleration. The deceleration can be recorded over different drop heights H and the relationship between ODIN and H can be used to indicate the range of conditions under which the response of the material under test will be elastic, and also the points at which the response becomes plastic (with largely non-recoverable deformation of the impacted material) can be determined. This provides a range of operating conditions to enable the apparatus to be adjusted (i.e. the magnitude and diameters of the weights and the drop height to be selected) in order to suit varying materials and site conditions.

Within the elastic range, the measured values ODIN and TP can be used to calculate an apparent modulus, on impact (AMI) using a simple formula which permits of rapid hand calculation. Alternatively, rapid testing within a particular situation can be carried out using measurement of peak deceleration (ODIN) alone. Anomalous conditions can be investigated non-destructively by measuring the impact response using different weight diameters whilst continuing to ensure that the loading is within the elastic response range of the material under test.

Instead of impacting the ground material directly with the weight 16, a loading plate set may be laid on the ground and itself impacted by the weight 16. The set (Figure 3) includes top and bottom plates 40, 41 with a resilient layer 42 (e.g a polymeric layer or an assembly of springs) in between. The effect under impact onto the top plate 40 is that the underlying ground material is subjected to a load pulse of extended duration simulating that applied by the wheel of a vehicle moving over the surface. Transient and permanent deflections of the lower plate 40 are measured by an array of contact-less displacement transducers 43 mounted on a ring which is supported on an arm 46 cantilevered from a counterbalance block 48 at a remote location and adjustable for height and inclination. The maximum applied stress may be determined from the deceleration of the dropped weight or from the mean of the displacements of the spring system measured by the further displacement transducers which record the relative displacement between the top and bottom plates.

Claims

1) A ground-impact testing apparatus, comprising a weight mounted to a pivoted arm and arranged so that the weight will swing under gravity about the pivot of the arm and impact with the ground substantially perpendicular to the latter.

2) An impact testing apparatus as claimed in claim 1, in which the weight is removable and replaceable by other weights of selected mass and size and in which the arm can be released from selected heights.

3) An impact testing apparatus as claimed in claim 1 or 2, comprising a transducer for providing a signal representing the angle of the arm relative to the apparatus.

4) An impact testing apparatus as claimed in claim 3, comprising means for displaying either the angle of the arm or the height of the weight about the ground.

5) An impact testing apparatus as claimed in any preceding claim, further comprising an accelerometer for providing a signal representing the deceleration of the weight upon impact with the ground.

6) An impact testing apparatus as claimed in claim 5, comprising means for providing a measure of the peak value of deceleration.

7) An impact testing apparatus as claimed in claim 5 or 6, comprising means for providing a measure of the time delay from the onset of detected deceleration to the peak deceleration.

8) An impact testing apparatus as claimed in claim 7, further arranged to calculate an apparent modulus on impact using the measured values of the peak deceleration and of said time delay.

9) An impacct testing apparatus as claimed in any preceding claim in which a loading plate is provided for laying on the ground to receive the impact from the falling weight, together with transducers for determining the displacement of the loading plate upon impact and for determining the applied stress.

10) A method of impact testing the ground, comprising allowing a weight which is mounted to a pivoted arm to swing under gravity about the pivot of the arm and impact with the ground substantially perpendicular to the latter.