WO2010150109A1

WO2010150109A1 - Impact device for materials analysis

Info

Publication number: WO2010150109A1
Application number: PCT/IB2010/050969
Authority: WO
Inventors: A Gibson
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-06-24
Filing date: 2010-03-05
Publication date: 2010-12-29
Anticipated expiration: 2011-12-24

Abstract

The present invention relates to a mechanical impact device for use in the field of nondestructive testing and evaluation of materials. The device is designed to deliver a consistently repeatable and appropriately tuned impact, to generate stress waves for the purposes of nondestructive testing. Potential applications include the evaluation of concrete and shotcrete structural members, when used as a source for impact-echo testing, pulse velocity measurement, or the analysis of the dispersion or attenuation characteristics of waveforms. A secondary application is the direct evaluation of surface hardness characteristics of the test material, by analysis of a recorded impact signal. The impactor is driven by a mechanism comprising a spring and an electromagnetic coil, and further comprises an integrated piezoelectric sensor element directly responsive to impact duration and intensity.

Description

Title of Invention: IMPACT DEVICE FOR MATERIALS

ANALYSIS

Technical Field

The present invention relates to the determination of the characteristics of a material through non-destructive testing . The characteristic being determined may be of any appropriate type; for example, the characteristic may be the thickness of the material, the, integrity of the material, the adequacy of bonding of the material to a substrate, the surface hardness of the material, early-age strength, compression and dispersive guided wave velocities, or any combination thereof. The material may be a homogeneous material, a heterogeneous material (such as concrete) or a composite material comprising layers.

The invention relates specifically to an instrumented, harmonically tuned impacting device that has been devised to generate transient vibrations via a mechanical impact, and the measure said impact characteristics. Testing procedures may include, but are not limited to impact-echo testing, pulse velocity measurement, analysis of waveform dispersion and attenuation. The invention may be concurrently used to infer dynamic hardness for the material surface by analysis of recorded impact characteristics.

Background Art

The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

In certain construction operations, it is advantageous to test in-situ properties of concrete or shotcrete material, to assist in condition evaluation or construction quality control functions. These properties include but are not limited to in-situ thickness, integrity and the material properties of elements such as rigid pavements, bridge decks, structural concrete members, reinforced concrete pipes, and concrete or shotctete tunnel linings.

There have been various proposals for determining the characteristics of materials, including thickness. Proposals involving non-destructive testing are advantageous and impact-echo testing, as well as waveform dispersion and attenuation analyis are useful approaches in this regard.

Impact-echo testing involves use of impact-generated stress waves that propagate through the material undergoing testing and are reflected by internal flaws or boundary surfaces, affecting the resonance characteristics of the member.

Impact-echo testing uses a mechanical impactor capable of imparting short duration impacts against a surface to produce stress waves and transient vibrations of appropriate frequency bandwidth that propagate into the test material. A separate receiver is used to measure the surface response, and a data acquisition and analysis system to capture, process and store the waveforms of the surface motion.

Typically, the impactor comprises an impactor head such as a steel ball supported on a spring arm configured as a spring steel rod. The arrangement is such that a user activates the spring arm manually (such as by flicking it with a finger) to cause it to deflect and cause the ball to undergo a short duration impact with the surface of the material undergoing the test.

Typically, a separate receiving sensor detects movement of the test surface and also serves as a trigger to initiate data acquisition, rather than directly detecting the impact event. As such no data pertaining to the initiation time, duration and contact force of the impact are recorded, which limits the opportunities for data analysis, as well as making the instrument susceptible to falsely trigger under certain testing conditions.

Further, there is likely to be some variability in the impact force generated by the impactor owing to the fact that the impactor is activated manually by a user. Specifically, because the impactor is activated manually it is likely that there is a lack of reliability and repeatability in the test results.

It is against this background that the present invention has been developed.

Disclosure of Invention Disclosure

According to a first aspect of the invention, there is provided an impact test device comprising a mechanical impactor for producing an impact against a surface of a test material, and means associated with the mechanical impactor responsive to the impact event.

The impactor comprises an impactor head for impacting the surface and said means associated with the mechanical impactor comprises means mounted on the impactor head. Because said means is mounted on the impactor head, it is directly coupled thereto for optimizing the response to the impact event.

Preferably, said means comprises an impact sensor, which may comprise a miniature piezoelectric ceramic element soldered to the impactor head. Said means associated with the mechanical impactor is adapted to respond to the impact event, generating an electrical pulse by virtue of its piezoelectric properties.

Said means associated with the mechanical impactor is also adapted to respond to the impact event by recording characteristics of the impact event. The characteristics recorded may include, for example, the impact duration and/or the impact peak force.

In certain configurations, the test device is intended for use in conjunction with a suitable receiver or receiver array to measure the response of the surface to the impact. Receivers may comprise, for example, of vibration sensors of appropriate type, ac- celerometers, non-contact microphones or ultrasonic transducers. The testing configuration may comprise a plurality of transducers, depending to the nature of the dynamic response to be analyzed. In one arrangement, the plurality of transducers may be configured to operate in concert, thereby enhancing the quality of the measured data (as a result of an enhanced signal to noise ratio). In another arrangement, the plurality of transducers may be configured to operate independently of each other, thus allowing characteristics of the test material to be determined at multiple locations, or for the analysis of dispersive vibration modes.

Said means associated with the mechanical impactor is adapted to activate a data acquisition process. With this arrangement, surface movements detected by the receiver(s) can be recorded with a consistent time-domain frame of reference. This is capability is beneficial for applications related to the measurement of constant and dispersive wave velocities, travel-time tomography and crack depth evaluation.

The impact test device further comprising activation means operably connected to the mechanical impactor for selectively moving the impactor in a first direction against the influence of a biasing force and releasing the impactor for movement thereof in a second direction, under the influence of the biasing force for impacting upon the surface of the test material.

Preferably, the activation means comprises an electromagnetic coil, activated by a high- voltage pulse. Preferably, the impactor comprises a spring-mounted hardened steel mass, guided through the coil centre within a non-magnetic guiding tube. The inherent nature of the spring provides the biasing force for biasing the impactor head.

Preferably, the activation mechanism comprises actuation of the electromagnetic coil, causing a spontaneous, transitory retraction of the impactor head, inducing compressive force on the spring. Subsequent deactivation of the electromagnetic coil releases the impactor element to move under the influence of the spring force, in the direction of the test surface, and create an impact.

Such a mechanism is conducive to the excitation of the harmonic ranges necessary for various stress wave based methods for non-destructive testing of materials. The design of the impactor instrument may be tailored to excite a specific frequency bandwidth by selecting an appropriate impactor head mass. Therefore the instrument configuration may be adjusted according to testing requirements.

The impacting mechanism incorporates a guiding tube designed to rest against the test surface, with the function of providing a constant offset distance between the pre- launch head position and the surface of the test material. In such an arrangement, the impactor head's trajectory distance and velocity at impact remain constant, in order to enhance the repeatability of measurements. The designed trajectory and velocity may be modified according to specific testing requirements, by the provision of tube extensions or modification of the characteristics of the high- voltage driver pulse.

According to a second aspect of the invention, there is provided an impact test device comprising a mechanical impactor having an impactor head for producing an impact against a surface of a test material, and means associated with the mechanical impactor adapted to detect the impact event, for the purposes of triggering a data acquisition system with a consistent time-domain frame of reference.

According to a third aspect of the invention, there is provided a device for determining a characteristic of a test material (such as the elastic modulus, early-age strength, or delineation of damaged or degraded areas thereof) comprising a mechanical impactor having an instrumented head for producing an impact against the surface of the test material, whereby the first arrival time of propagating wave modes may be detected at the location of an offset sensor, in relation to the recorded time- domain characteristics of the impact event.

According to a fourth aspect of the invention, there is provided a method of conducting an impact-echo test on a test material comprising striking a surface of the test material with a tuned, instrumented mechanical impactor, to deliver an impact thereto to produce stress waves and transient vibrations that propagate into the test material. Using this method, the interpretation of transient vibrations measured near the impact location is enhanced by the availability of impact data, including contact duration and peak impact force, and consistent time-domain triggering with respect to the measured impact signal.

According to a fifth aspect of the invention, there is provided a method of conducting awaveform travel-time and attenuation analysis on a test material comprising striking a surface of the test material with a tuned, instrumented mechanical impactor, to deliver an impact thereto to produce stress waves and transient vibrations that propagate into the test material. Using this method, the interpretation of transient vibrations measured near the impact location is enhanced by the availability of impact data, including contact duration and peak impact force, and consistent time-domain triggering with respect to the measured impact signal. According to a sixth aspect of the invention, there is provided a method of conducting a test on a test material for determining a characteristic of a test material, such as material quality or integrity, comprising striking surface of the test material with an impactor head to deliver an impact thereto and detecting characteristics of the impact event, namely duration and time-dependant impact force, using a sensor mounted on the impactor head.

Description Of Drawings

The invention will be better understood by reference to the following description of one specific embodiment thereof as shown in the accompanying drawings in which:

Figure 1 is a schematic view of an instrumented impact device according to the embodiment;

Figure 2 is a sectional view of the impact device, detailing the disposition of internal elements;

Figure 3 is a detailed view of the impact head, sensor element and mounting bracket;

Figure 4 is a schematic view of the internal disposition of the internal elements at different stages of the firing sequence;

Figure 5 is a graph showing an example impact sensor output signal and related processing steps.

Best Mode

Throughout the specification and claims, unless the context requires otherwise, the word 'comprise' or variations such as 'comprises' or 'comprising', will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The embodiment shown in the drawings is directed to an instrumented impact testing device 10 which has been designed particularly, although not necessarily solely, for creating transient vibrations for the purpose of measuring thickness, integrity, bonding conditions and material properties of concrete and shorcrete structural elements. Device 10 according to the embodiment may be used in various applications, including (but not limited to) measurement of the thickness of materials, measuring early-age strength of materials, testing the adequacy of bonding of the material to a substrate, and also determining the integrity of materials such as, for example, detection of flaws (such as voids and delaminations) in a test material.

The impact device 10 according the embodiment comprises guiding tube 13, electro- magnetic coil 14, and associated peripherals 15 including a trigger switch 16, charging circuit 17 and a battery pack 18. The impactor device also comprises an electrical connection to an external data acquisition system 19.

The guiding tube 13 extends beyond the normal resting position of the internal impactor head, and defines a contact plane (not shown) which, in use, contacts the outer surface of the test material.

The mechanism further comprises a mechanical impactor head 20, positioned within the guiding tube 13. The mechanical impactor mechanism comprises an impactor head 20 and a spring 21. The impactor head 20 is mounted on one end of the spring by means of a steel wire bracket 22, at position 23. The spring is attached at its other extremity to the upper edge of the guiding tube, at position 24.

An activation means is provided for deflecting the impactor head 20 inwardly, away from the operating end to induce a spring force therein. The activation means comprises an electromechanical mechanism having an electromagnetic coil 14. With this arrangement, actuation of the coil 14 by means of a high-voltage pulse causes retraction of the impactor head 20 in a first direction which in operation is away from the contact plane. With this arrangement, the impactor head 20 and bracket mounting 22 are moved against the influence of the spring force induced in the spring 21.

A trigger switching mechanism 16 is provided to allow the user to discharge a short- duration pulse to the electromagnetic coil 14. The induced magnetic field causes the impact mechanism to temporarily retract. Subsequently, upon dissipation of the biasing electromagnetic force, the induced spring force therein carries the impactor head 20 outwardly in a second (reverse) direction, which in use is towards the test surface.

In this way, the spring force within the spring 21 propels the impactor head 20 towards the contact plane (and thus the test surface), thereby causing the impactor head 20 to deliver a short duration impact thereto.

The impact generates stress waves in the test material which propagate through the material and cause modal resonances within the cross-section. Where the test material comprises a layer of material, the impact event causes guided wave modes to propagate within the layer, thereby providing a way of determining the thickness of the layer based on analysis of resonant frequencies measured adjacent to the impact position.

In other arrangements, flaws within the test material, such as voids, density variations or de-bonding affect the section stiffness and therefore cause variations in the resonance characteristics.

An impact sensor 25 is mounted on the impactor head 20 for detecting the impact event. The impact sensor 25 is bonded to the impactor head and responds to strain variations caused by the impact event, producing a voltage pulse that is proportional to the time-dependent impact force by virtue of the sensor's piezoelectric properties. The impact sensor's output is a function of the intensity and duration of the impact event, and is also arranged to activate the data acquisition process, thereby enabling the surface movements generated to be recorded with a consistent time-domain frame of reference.

In the arrangement illustrated, the impactor head 20 comprises an elongated hardened steel element 26 having one end of rounded profile to define a strike surface 27. The sensor 25 is mounted on the impactor head 20 at the end thereof opposed to the strike surface 27.

The data acquisition module 19 forms part of a system to capture, process and store the impact sensor output, as well as wave forms representing surface movements as detected by any adjacent receivers that may be used. This data acquisition and analysis can be performed in a conventional manner well known to persons skilled in the art of non-destructive testing.

The typical impact sensor output 31 is presented in the form of a discrete waveform whereby sensor Voltage output 32 is plotted against time 33. Impact characteristics such as maximum impact intensity 34 and impact duration 35, can be obtained via the analysis of this data. It may also be beneficial, for the purposes of analysis to obtain a best fit to an analytical impact force-function 36. The data acquisition and analysis can be performed in a conventional manner well known to persons skilled in the art of nondestructive testing.

From the foregoing, it is evident that the instrumented impactor device 10 offers particular benefits. Because the impact sensor 25 is coupled directly to the impactor element 26 by being mounted thereon, the detection of impact events is optimized.

Furthermore, the activation means 14, by virtue of its electromechanical nature, facilitates application of a repeatable force to the impactor head 20 for delivery of a correspondingly reliable and repeatable short duration impact to the test surface.

The impact device according to the embodiment can be utilized for various measurements. By way of example, direct measurements from the impactor can be used in the determination of impact duration and peak impact force. Additionally, measurements of adjacent surface vibrations can be used in the determination of acoustic resonance frequency, travel times (i.e. wave velocity), impact-echo attenuation and guided wave dispersion and attenuation, the interpretation thereof is enriched by the avaialbility of consistent mechanical impacts, consistent data-acqusition triggering, and time-domain information pertinent to the characteristics of the impact.

Claims

[Claim 1] An impacting device comprising an appropriately tuned mechanical impactor for producing an impact against a surface of a test material, and means associated with the mechanical impactor responsive to the impact event.

[Claim 2] An impact test device comprising an appropriately tuned mechanical impactor having an impactor head for producing an impact against a surface of a test material, and means associated with the mechanical impactor adapted to detect the impact event, for the purposes of triggering a data acquisition system with a consistent time-domain frame of reference.

[Claim 3] An impact test device comprising an appropriately tuned mechanical impactor having an impactor head for producing an impact against a surface of a test material, and means associated with the mechanical impactor adapted to provide repeatable impact velocity.

[Claim 4] A device for determining a characteristic of a test material (such as the elastic modulus, early-age strength, or delineation of damaged or degraded areas thereof) comprising a mechanical impactor having an instrumented head for producing an impact against the surface of the test material, whereby the arrival time of propagating wave modes may be detected in relation to the recorded time-domain characteristics of the impact event, using an offset transducer or transducer array. Means associated with the mechanical impactor enable the detection the impact event and activation a data acquisition process, whereby characteristics of the impact event and surface movements detected by the receiver are recorded with a consistent time-domain frame of reference.

[Claim 5] A device for determining the integrity of a test material, including the evaluation of crack depth, comprising a mechanical impactor having an instrumented head for producing an impact against a surface of the test material, to be used in conjunction with an offset transducer or transducer array for detecting the first arrival time and attenuation characteristics of a propagating wave mode.

[Claim 6] A method of conducting an impact-echo test on a test material comprising striking a surface of the test material with an impactor to deliver an impact thereto, producing stress waves that propagate into the test material. The impact event is detected using a sensor mounted on the impactor, and the surface response measured independently at an adjacent location. The interpretation of transient vibrations measured near the impact location is thereby enhanced by the availability of impact data, including contact duration and peak impact force, and the provision of consistent time-domain triggering with respect to the measured impact signal.

[Claim 7] A method of conducting travel-time and attenuation analysis on a test material comprising striking a surface of the test material with a tuned, instrumented mechanical impactor, to deliver an impact thereto to produce stress waves and transient vibrations that propagate into the test material. The interpretation of transient vibrations measured near the impact location is enhanced by the availability of impact data, including contact duration and peak impact force, and consistent time- domnain triggering with respect to the measured impact signal.

[Claim 8] A method of conducting a dynamic hardness test on a test material for the purpose of determining a characteristic of a test material, such as quality or integrity, comprising striking a surface of the test material with an impactor head to deliver an impact thereto, with controlled projectile mass and velocity, and detecting characteristics of the impact event, namely duration and time-dependant impact force, using a sensor mounted directly to impactor head.