WO2002003059A1

WO2002003059A1 - Testing of leather

Info

Publication number: WO2002003059A1
Application number: PCT/GB2001/002905
Authority: WO
Inventors: Amanda Long; Peter Jardine; Terry Deadman
Original assignee: Blc Leather Technology Centre Limited
Priority date: 2000-06-30
Filing date: 2001-06-29
Publication date: 2002-01-10
Also published as: GB2370356A; AU2001267699A1; GB0203930D0; GB0016063D0

Abstract

The present invention relates to the testing of leather, in particular the testing of leather for grain surface cracking. Leather is tested for its strength and grain distension in order to determine its suitability for a particular use, such as shoe manufacture. There is described a method for the determination of physical characteristics of a leather sample (10), the method including the steps of applying a physical procedure to the sample of leather; and monitoring and analysing audible acoustic emissions from the sample. There is also described an apparatus adapted for use in such method comprising means for applying a physical procedure to a sample of leather (10) and an acoustic sensor (12). Typically, the acoustic sensor comprises at least one transducer, such as a microphone, and monitors acoustic emissions in the range of 200 Hz to 22kHz. For certain analyses, the acoustic sensor comprises an annular array comprising a plurality of transducers each mounted equidistantly from the means applying the physical procedure.

Description

TESTING OF LEATHER

The present invention relates to the testing of leather, in particular the testing of leather for grain surface cracking.

Leather is tested for its strength and grain distension in order to determine its suitability for a particular use, such as shoe manufacture. The official lastometer test for the leather industry is detailed in method SLP9 'Measurement of Distension and Strength of Grain by the Ball Burst Test' (IUP/9; BS 3144: Method 8). During the test a disk of leather is clamped around its circumference, and a probe pushed perpendicular to the flesh side of the leather until cracking of the grain surface is observed by a skilled technician. Testing can then continue if required until the probe punctures the leather. Measurements are taken of the distension required to cause grain crack and ball burst.

Research has been published by RG Stosic, 'Evaluation of the Stable Micro Systems TA.XT2 Texture Analyser andMT-RO Material Tester. ' (BLC Labor atoiy Report LR- 221), describing the use of a materials tester to automate the technique and this was used during research into the present invention. Stosic also mentions that it is possible to determine the point of grain crack via deflections in the force / displacement curves obtained during testing. Whilst this may be the case, in a large majority of samples, the deflection can be quite subtle and difficult to distinguish from other events on the curve. It is therefore necessary to observe the test and determine the point of grain crack visually. This is labour intensive and may result in inaccuracies due to human reaction speeds, and the inherent subjectiveness of the test.

There is therefore a need to provide a means for reliably determining physical properties of a sample of leather which overcomes these disadvantages in the art. It is with these problems in mind that the present invention has been devised.

In its broadest sense, the present invention provides, in one aspect, a method for the determination of physical characteristics of a leather sample, the method including the steps of applying a physical procedure to the sample of leather; and monitoring and analysing audible acoustic emissions from the sample. The present invention also provides, in a second aspect, apparatus for the determination of physical characteristics of a leather sample, the apparatus comprising application means to apply a physical procedure to the sample; an acoustic sensor providing an output in response to detected audible acoustic emissions from the sample; and analysis means to analyse the output and provide a indication of the physical characteristic to be determined.

Typically, the acoustic sensor detects emissions up to around 22 kHz.

In one embodiment, the acoustic sensor is a microphone.

In another embodiment, the acoustic sensor comprises a plurality of transducers equidistantly positioned from the sample of leather.

Preferably, the apparatus is a lastometer and the physical procedure application means comprises a clamp to retain a circular sample of leather with a uniform tension and a probe to apply a force of known magnitude to the centre of the sample.

Preferably, the analysis means comprises a suitably programmed computer.

The above and other aspects of the present invention will now be described in further detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of an embodiment of the apparatus of the present invention;

Figure 2 is a schematic perspective view of a transducer array for use as an acoustic sensor in the embodiment of Figure 1;

Figure 3 is a cross-section of the transducer array of Figure 2;

Figure 4 represents schematically a summing amplifier for the transducer array of Figure 2; Figure 5 is a graph showing the output of an acoustic sensor in a lastometer test using the apparatus of Figure 1;

Figure 6 is a graph showing the force applied to the sample of leather in the same test;

Figure 7 is a graph showing the correlation between results of lastometer tests on a range of specially prepared leathers using conventional methodology and the apparatus of the present invention;

Figure 8 is a graph showing the correlation for a range of commercial leathers.

Figures 9 to 12 are waveforms obtained from a leather tearing tests of a range of samples of leather; and

Figure 12 is a plot illustrating the effect of fat Uquor offer on the number of acoustic events.

By way of background discussion, when a force is applied to any material, resulting movements within the structure form sound waves. These sound waves can be monitored as acoustic emissions. Acoustic emission can be defined as being the transient elastic wave generated by rapid release of energy within a material (AA Pollock, (1988) 'Practical guide to Acoustic Emission Testing', LOCAN AT (3140) Users Manual, Physical Acoustics Corp., Princeton, New Jersey). Put another way, when a load is applied to a specimen, energy is stored as strain energy. When an inherent critical point is reached in the sample, there is a sudden redistribution of the energy. At this point some of the strain energy is converted into acoustic energy (Y Nakamura, CL Veach, BO McCauley, (1972), 'Amplitude Distribution of Acoustic Emission Signals' from 'Acoustic Emission ' ASTM, STP 505, American Society for Testing and Materials).

Various properties of sound waves can be measured, the most common being the amplitude and frequency The amplitude is a measure of loudness, the frequency is the number of cycles per second and is perceived as the pitch of the sound. Generally in industry the frequency range that is studied is the ultrasound region. In fewer cases is the audible range evaluated. Acoustic testing is an area that has been incorporated into many industries. Typically it is used for the non-destructive testing of materials such as pressure vessels, but it has been used in a wide range of other applications (K Ono, R Stem, M Long Jr, (1972), 'Application of Correlation Analysis to Acoustic Emission ' f -rom Acoustic Emissio ' ASTM, STP 505, American Society for Testing and Materials, pp 152 - 163). Results obtained have been correlated with properties such as the applied stress and the stress intensity at a crack tip.

Another application is the evaluation of fundamental deformation and failure behaviour of geological materials (HR Hardy Jr, (1972), 'Application of Acoustic Emission Techniques to Rock Mechanics Research 'from 'Acoustic Emission ' ASTM, STP 505, American Society for Testing and Materials, pp 41 - 83). In this case both the acoustic activity and the frequency spectra were evaluated. It should be noted that there are difficulties in interpreting frequency spectra due to issues such as inherent resonance frequencies in the test materials and loading system, the frequency dependent attenuation characteristic of the test material and the need to ensure that the test equipment has a flat / linear response in the region of interest.

It is worth discussing here the issue of attenuation of sound waves (AA Pollock, (1988) 'Practical guide to Acoustic Emission Testing', LOCAN AT (3140) Users Manual, Physical Acoustics Corp., Princeton, New Jersey). As a wave travels through any material, the peak amplitude diminishes. It is harder to detect sources at a greater distance from the sensor. Some causes of this phenomenon are the geometric spreading of the wave front, absorption or damping of the propagating medium and the 'leaking' of the wave form into adjacent media.

Acoustic emission studies have been applied to areas where it is difficult to observe the sample under test. An example of this is the testing of a fibre reinforced composite material. Acoustic techniques enabled assessment of the breakage of the fibres whilst within the matrix. Also of importance is that the onset of failure can be detected before actual catastrophic sample failure occurs. This allows action to be taken to prevent the destruction of the test sample. Work has been carried out (mostly in the USA) to evaluate the potential for acoustic emission studies in leather. The work that has been reported in the literature investigated the emissions within the ultrasound range (1. PL Kronick, AR Page, (1996), 'Recoveiγ of Properties of Staked Leather on Storage ' J. Am. Leather Chem. Assoc, vol. 91, pp39 - 46; 2. PL Kronick, (1989), 'Fibre Adhesion in Solvent Dried Calf Skin Studies by Acoustic Emission Spectroscopy ' J. Am. Leather Chem. Assoc, vol. 84, no 9, pp257 - 265; 3. PL Kronick, B Malee (1992). on-destructive Failure Testing of Bovine Leather by Acoustic Emission. ' J. Am. Leather Chem. Assoc, vol. 87, no 7, pp 259; and 4. F Scholnick, et al, (1994), 'Use of an Elastomer Formed in Situ for Softening Leather ' J. Am. Leather Chem. Assoc, vol. 89, no 8, pp 260 - 268). Various parameters were investigated, such as the effect of mechanical softening, and the possibility for non destructive tensile testing. Some information was gained concerning the mechanisms of leather failure and deformation under strain, and the effects of applying polymers to soften leather.

Whilst the work that has been published has mostly concerned the evaluation of the ultrasound region of acoustic emission, there are some drawbacks to this approach. As the frequency of sound increases its attenuation also increases i.e. higher frequency sound does not travel as far as low frequency sound. In the ultrasound region, the sound waves do not propagate through air, and a coupling material is required. It is for the same reasons that gels are used when carrying out medical ultrasound examinations. However the presence of any gel or grease to improve propagation of sound between leather and the sensor could interfere with the results, by, for example, softening the leather.

The use of the apparatus and method of the present invention as a replacement for the standard lastometer test will now be described. An embodiment of the apparatus of the present invention is illustrated schematically in Figure 1. The apparatus comprises a jig into which can be secured a sample disc 10 of leather in such a way that tensioning forces 11 can be applied to the sample disc, typically by means of a probe (not shown) pressing into the centre of the sample in a direction perpendicular to the plane of the sample. Such an arrangement is conventional in the field of materials testing (for example, the Stable Micro Systems MT-RQ Material Tester) and will not be described in further detail.

The apparatus also includes an acoustic sensor 12. In its most basic form, the acoustic sensor is simply a microphone, such as a standard tie-clip microphone, typically monitoring frequencies from about 200Hz up to 22 kHz. A simple microphone has been found to be the most suitable acoustic sensor for certain tests such as tear testing. Indeed, in preliminary tests, a simple Altai tie-clip was found to perform substantially as well as an Audio Technica AT30M Studio microphone. However, for the lastometer test, a transducer array has been developed and provides advantageous results.

A transducer array 20 is illustrated schematically in Figures 2 and 3. A plurality of transducers 21, typically four are mounted in a ring 22 typically formed of a resilient foamed material. The ring 22 is positioned adjacent the sample under test, such that the transducers are all equidistant from the probe of the testing jig. The use of an array of transducers allows summing of the outputs of all transducers to provide additional gain to the desired signal against unwanted background signals. Sound emanating from the centre of the jig will have a wave front 15 which reaches all the transducers simultaneously.

In contrast, when background noise falls upon the transducer array, it is unlikely to emanate from the centre of the transducer array. The wave fronts will therefore reach different microphones at different times. The summation of all the transducer outputs causes the output from some transducers to cancel the output from some others, such that the overall gain given to these signals is less than that experienced by signals emanating from the centre of the array. Figure 4 shows a block diagram of a suitable Summing Amplifier which was constructed to allow combination of the signals from the separate transducers.

This technique for removing coherent signals from noise has been used widely in many technologies for many years. Statistical theory states that if the noise signal is random then the ratio between the coherent signal and the noise should improve by the square root of the number of transducers. So for four transducers an improvement in signal-to-noise ratio of V4, namely 2, is expected. For eight transducers the improvement should be ^8 i.e. 2.82. This is of course an idealised description of the system operation.

Remembering that destructive interference will occur if the positional error is in the order of half a wavelength of the sound signal, it is possible to estimate the frequency at which a positional error will compromise the system performance. Assuming a worst case positional error of two millimetres. This would cause destructive interference of signals with a wavelength of 4mm.

where N - fλ

N is the velocity of sound; f is the frequency; and λ is the wavelength

N is approximately 344 metres per second at 20 degrees centigrade. So as λ is 0.004 metres, f is calculated to be 344/0.004 which is 86kHz. The array will therefore be vulnerable to operational difficulties due to assembly tolerances at around 86kHz. This is a good result as the transducers have little output signal above 20kHz due to their internal design and so the principle should hold across the frequency range of interested.

The acoustic sensor 12 is connected to a sound input device such as a conventional sound card, of a conventional computer 13. The data obtained by the acoustic sensor is processed using conventional software to provide a useful graphical output of the acoustic data obtained. One particularly suitable program is 'Texture Expert Exceed' from Stable Micro Systems Limited of Vienna Court, Lammas Road, Godalming, Surrey. This program is well known in the field of materials testing and, for our purposes, allows control of the jig and allows multi-channel data to be displayed graphically.

An example output from the apparatus of the present invention acting as a lastometer is shown in Figures 5 and 6. Figure 5 shows the relative amplitude of the acoustic data 30 obtained as a function of time. Figure 6 shows the corresponding force applied by the probe, again plotted as a function of time. A sharp peak 31 is shown in the acoustic data at approximately 18.3 seconds, corresponding, by reference to Figure 6, to an applied mass of around 19.4 kg (equivalent to a force of 190.3N)

To determine the accuracy of the apparatus and method of the present invention, a large number of samples were tested for grain crack both in accordance with the present invention and by conventional observational techniques. The results of each technique were plotted against each other (Figure 1, units are seconds). As will be seen, there is considerable grouping of the results about a straight line, illustrating a clear correlation between the two sets of results showing that the present invention provides a useful and reliable technique for determining the strength and grain distension properties of leather.

The samples tested to produce Figure 7 were samples of five different leathers produced by us for the purposes of these trials. The leathers were prepared to be quite hard and would therefore be likely to produce an audible gain crack.

It could thus be argued that these leathers tested were not totally representative of real leather samples. Accordingly, several commercial leather samples were selected for testing according to the lastomer test, to confirm its suitability and sensitivity for routine testing.

Testing of commercial leather samples illustrated that it is possible to correlate the observed gain crack with the acoustic data and this is seen in Figure 8. During the sting there were just two leathers that did not produce an acoustic event at gain crack. These however were leathers not designed^' for use in shop uppers and so may not typically be tested using this method. This is further proof of the success of the acoustic lastometer test and its ability to reliably detect gain crack.

Further experiments have been made to determine the general applicability of these techniques to the measurement of other properties of leather. In particular, the present invention has been found to be useful in the conduct of tear tests. In this case, a different software package was used - 'Cool Edit Pro' by Syntrillium Software of Phoenix, Arizona, USA. This program was designed for use in the music industry and allows recording, editing and mixing of audio tracks. This program was supplemented with additional programming to allow individual acoustic events to be counted, based on a user-specified amplitude threshold. This program also introduced a 'dead time' following an identified event during which time further acoustic data was ignored in order to allow the exponential decay of the event to fall to a level below the detection threshold before continuing with the detection. This enabled multiple triggers from single events to be filtered out. A typical waveform from a tear test using a single tie-clip directional microphone mounted close to the sample with clamping of the jig is shown in Figure 9.

The waveform in Figure 9 is typical of those seen for tearing tests and illustrates some interesting features. The waveform is comprised of a series of acoustic events or sound bursts. These have a characteristic shape. It should be noted that initially the shape was typical of a "ringing" effect, and was an artefact of the test jig vibrating after each pulse from the leather. Damping of the test jig jaws removed this problem and Figure 9 illustrates the results after this. These pulses are typical of damped harmonic motion seen for example, in a musical string being plucked. It was considered initially that the breakage of individual fibres or fibre bundles was being monitored using this technique.

To investigate the technique further, preliminary samples were evaluated that had been prepared using different post tanning conditions. This was to determine the sensitivity of the technique to different leather types and determine the potential for any further analysis. Figure 10 and Figure 11 show the waveforms obtained from leathers treated with different offers of mimosa. It is clear that there are more acoustic events occurring in the sample treated with a higher offer of mimosa. It is also possible that the amplitude distribution may be different, however it is not possible to quantify this currently.

Comparison of these samples with a leather treated with a waterproofing fatliquour illustrates the extremes of acoustic behaviour that can be obtained (Figure 12). This is almost certainly indicative of variations in the failure mechanisms of the leather types. The waterproof leather is showing few events, however the fibres must be breaking for failure to occur. It is therefore possible that the breakage of bonds between fibres rather than the breakage of the fibres themselves is being observed. This is in agreement with the conclusions of ERRC (HR Hardy Jr, (1972), "Application of Acoustic Emission Techniques to Rock Mechanics Research" fi-om "Acoustic Emission" ASTM, STP 505, American Society for Testing and Materials, pp 41-83) in their study of mechanical softening of leather.

It is quite possible that the data obtained in the audible region is related to the subjective parameters of leather (such as softness). This is indicated by the extreme differences observed in the acoustic emissions from the post tanned samples (Figures 10 to 12). Figure 10 shows the waveform obtained by tear testing leather containing 4% w/w Mimosa, Figure 11 that obtained with leather having an 8% content of mimosa and Figure 12 shows that obtained from leather containing a waterproofing fatliquor.

During the previous experiments detailed above qualitative observations on many acoustic emission waveforms were carried out. One easily identifiable common factor was the way that many of the sounds were constructed from individual bursts or pulses of sound. These features an initial sound event followed by an exponential decay. The events varied greatly in amplitude but occurred with a greater regularity during times of greater martial deformation. A software program developed by Stable Micro Systems Ltd in Visual Basic allows the identification and tagging of these acoustic pulses, (referred to henceforth as "events").

The program applied a user specified amplitude threshold to the acoustic emissions waveform to allow the detection and time stamping of individual events. The program also applied a "dead time" after identified events during which any further acoustic information was ignored. This was to allow the exponential decay of the event to have fallen to a level below the detection threshold before continuing with the detection process. Hence multiple triggers from single events could be avoided, provided the threshold and dead time were set to suitable levels. Output from the program was in the form of an ASCII text file consisting of a list of times that corresponded to detected events. This file was then imported to analysis software in order that the event density against time could be calculated.

To evaluate the technique, the tear test was investigated further. Leather samples were prepared using identical tanning conditions. The only variation between the samples was the offer of fatliquor. Observation of the waveforms indicated that there were differences in the number of events occurring. This was therefore quantified using the event counter software. Figure 13 illustrates the results obtained.

It is clear that the level of fatliquor applied significantly influences the acoustic emissions observed. At offers in excess of 5% the pattern observed is as expected. Increasing the offer of fatliquor results in increased lubrication of the leather fibres. As the lubrication is increased, there is a decrease in the number of acoustic events recorded. This indicates that the pulses are due to the bonds between the fibres being broken, rather than recording the breakage of the fibres themselves.

Between 0 and 5% offer there is an increase in acoustic events as the offer of fatliquor is increased which is unexpected. At 0% offer of fat there is no lubrication of the fibres and therefore failure is most likely due to fibre breakage. At low fatliquor offers, there will be an increasing contribution to the sound from the fibres being able to move past each other as they are lubricated slightly (consider the analogy of a bow on a violin string).

These results provide further evidence to indicate that the acoustic data is related to the aesthetic properties of the leather rather than the physical properties as the offer of fat will have a direct effect on the leather softness.

The results presented above illustrate that the data obtained from the acoustic emission of leather tear tests may be related to the subjective properties of leather.

Within this research the subjective evaluation of leather handle was based on characteristics such as grain firmness (looseness), stiffness (softness) and fullness (emptiness). Twenty-five commercial leather samples were assessed for handle by a panel of eleven experts and the subjective parameters listed previously were assessed on a five point scale. The results from these were then correlated with the data obtained from analysing the acoustic emission data obtained from tear tests carried out on these samples.

In order to carry out the statistical analysis, it was necessary to determine whether the data obtained fitted a normal distribution. Using the Saphiro-Wilks test, the variables were tested for normality and transformations through LOG were made to reach normality where required. The following variables were considered in this analysis:

• log (counts per second across Tear Propagation)

• The average RMS power across Tear Propagation

• Maximum Amplitude across Tear Propagation

The RMS power is a parameter that was calculated through software and is a measure of the perceived "loudness" of the noise or effective energy of the signal.

It was found that several significant correlations occur between the acoustic data and the subjective parameters. Log(₀₀unts per second across tear ropagation) was strongly related with Stiffness, Fullness and Emptiness. RMS power was strongly correlated to softness and emptiness.

The acoustic emission data shows good correlation with the subjective parameters of leather, allowing the development of a mathematical model to allow these parameters to be quantified. It is therefore of interest to compare the results from the two techniques and indeed combine them to determine if improvements can be made in the prediction of handle.

It has been found that the data obtained from the tear test has increased the accuracy of the model. The acoustic emission data alone can be used as an indication of properties such as the stiffness/softness and fullness of the leather. The data is illustrated in Table 1 below.

Table 1. Correlation Of Objective And Subjective Evaluation Of Leather

It has thus been demonstrated that analysis of acoustic emissions in the audible frequencies provides a useful new procedure in the determination of quantitative properties of samples of leather and that there is a relationship with the aesthetic properties of leather. Furthermore, the present invention allows the development of methods for the quantitative determination of parameters such as grain firmness/looseness, stiffness/softness and fullness hitherto only determinable in a subjective or qualitative manner by highly experienced analysts.

Claims

1. A method for the determination of physical characteristics of a leather sample, the method comprising the steps of applying a physical procedure to the sample of leather; and monitoring and analysing audible acoustic emissions from the sample.

2. A method as claimed in claim 1 wherein the monitored acoustic emissions are in the range of 200Hz to 22kHz,

3. A method as claimed in claim 1 or claim 2 wherein the physical procedure comprises clamping a sample of disc of leather and applying a tensioning force thereto.

4. An apparatus adapted for use in the method of any one of claims 1 to 3 comprising means for applying a physical procedure to a sample of leather and an acoustic sensor.

5. An apparatus as claimed in claim 4 wherein the acoustic sensor comprises at least one transducer, such as a microphone.

6. An apparatus as claimed in claim 5 wherein the acoustic sensor comprises an annular array comprising a plurality of transducers each mounted equidistantly from the means applying the physical procedure.

7. An apparatus as claimed in claim 5 or claim 6 further comprising a computer having a sound input device and wherein the output of each transducer is connected, through a summing amplifier as appropriate, to the sound input device.

8. An apparatus as claimed in claim 7 wherein the transducer output is processed by means of software to produce a graph plotting relative amplitude of acoustic data against time or force applied by the physical procedure.

9. An apparatus as claimed in any one of claims 4 to 8 wherein the acoustic sensor operates in the range of 200Hz to 22kHz,

10. An apparatus as claimed in any one of claims 4 to 9 comprising a clamp adapted to retain a sample of disc of leather and means for applying a tensioning force to the sample of leather.