The effects of the biaxial stretching of leather on fibre orientation and tensile modulus

Leather has been subjected to different degrees of equal biaxial strain (up to 20%) during drying and its tensile modulus has been measured when dry. The collagen fibre orientation distribution in the dried leather has been assessed using wide angle X-ray diffraction. It was found that drying under biaxial strain caused the tensile modulus to increase markedly (by up to 400% at 20% biaxial strains) but with a dependence on the angle of test axis in relation to the principal axes of biaxial strain. The fibre orientation distribution in planes parallel to the surface was affected less by biaxial strain than in planes perpendicular to the surface and it is concluded that the latter type of fibre reorientation is the main factor responsible for the observed increases in tensile modulus.     

The anatomy of leather

Leather is prepared from vertebrate skin by the chemical stabilization of the fibrous protein collagen, the main solid constituent of skin. The natural three-dimensional fibrous weave of the collagen fibrils is retained intact in the leather, and this paper describes in detail the relation between the fibrous weave as seen under the light and scanning electron microscopes and the physical properties of the leather. In the main, skins of cattle, sheep and goats are used. These differ in total thickness, fibre bundle size and weave pattern, but offer a variety of raw material from which the tanner can select the skin type best suited for a particular end use. The tanning process can also modify the natural weave in order to achieve the required physical properties. For example the processing can allow fine spaces to remain between the fibrils, so that they are free to move over each other within the fibre bundle. Such a bundle will be highly flexible as will be the leather as a whole. Conversely, if fibrils adhere to each other and are not free to move the leather will be firm. Such fine spaces can be recognized under the microscope as longitudinal striations. Another important feature which is influenced by the tanning process is the angle at which the fibres interweave in relation to the grain surface. A low angle of weave is required for high tensile strength as this allows the pull to be transferred along the fibre axis. A low angle of weave also allows more frequent interweaving of the fibres within a given thickness of leather, than if the angle is high. As frequent interweaving of the fibres is a prerequisite for strength the tear strength of leather is highly dependent on the angle of weave. This is particularly so in certain types of leather prepared from cattle skin, which is cut into layers to obtain the required leather thickness. For leather to accommodate stretching, such as that occurring when lasting shoe uppers, or compression and creasing, the individual fibrils need to be free to move within the bundle, and the bundle within the weave as a whole. The changes in the fibre structure that occur under such conditions can be followed microscopically and are described in detail in this paper.     

The set and mechanical behaviour of partially processed leather dried under strain

The stress-strain behaviour of partially processed leather which has been dried under a range of uni-axial strains has been investigated. It has been found that the dependence of the tensile modulus on the strain applied during drying is non-linear, increasing slowly at first then more rapidly later on. A two-dimensional microstructural model based on an idealised fibre network can describe this non-linear relationship and account for differences between samples. High values of set are produced by drying under strain and some of this set is retained even after soaking in water. It is suggested that this is due to the formation of stable crosslinks between the chemically modified collagen fibres that comprise leather.     

The micro-structural strain response of tendon

Tendons are multi-level fibre-reinforced composites, designed to transmit muscle forces to the skeleton. During physiological loading, tendons experience tensile loads, which are transmitted through the structure to the cells, where they may initiate mechanotransduction pathways. The current study examines the structural reorganisation and resulting local strain fields within the tendon matrix under tensile load. It uses confocal microscopy to photobleached a grid onto the collagen and image its deformation under the application of incremental tensile strain. Six parameters are used to quantify fibril and fibre movement and examine the mechanisms of extension employed by fascicles. Results demonstrated an inhomogeneous strain response throughout the matrix and large variability between samples. Local strains in the loading axis were significantly smaller than the applied values. However, large compressive strains, perpendicular to the loading axis, were recorded. The average Poisson’s ratio (0.8) suggested cells may experience significant compression during loading. Deflection of the grid lines, indicating sliding between collagen fibres, and rotation of the grid were also recorded. These data highlight the non-homogenous strain environment of fascicles and provide further evidence for fibre sliding under tensile load. They also suggested a rotary component to tendon response, which may indicate a helical organisation to the tendon matrix.     

Constitutive modelling of arteries considering fibre recruitment and three-dimensional fibre distribution

Structurally motivated material models may provide increased insights into the underlying mechanics and physics of arteries under physiological loading conditions. We propose a multiscale model for arterial tissue capturing three different scales (i) a single collagen fibre; (ii) bundle of collagen fibres; and (iii) collagen network within the tissue. The waviness of collagen fibres is introduced by a probability density function for the recruitment stretch at which the fibre starts to bear load. The three-dimensional distribution of the collagen fibres is described by an orientation distribution function using the bivariate von Mises distribution, and fitted to experimental data. The strain energy for the tissue is decomposed additively into a part related to the matrix material and a part for the collagen fibres. Volume fractions account for the matrix/fibre constituents. The proposed model only uses two parameters namely a shear modulus of the matrix material and a (stiffness) parameter related to a single collagen fibre. A fit of the multiscale model to representative experimental data obtained from the individual layers of a human thoracic aorta shows that the proposed model is able to adequately capture the nonlinear and anisotropic behaviour of the aortic layers.     

Preparation and evaluation of biocomposites as wound dressing material

Collagen was isolated from the chrome containing leather waste (CCLW) which is a major solid waste in leather industry. Composite films were made using sago starch (SG), soya protein (SY), and collagen (C) and were cross linked with glutaraldehyde (G).The films prepared were characterized for their physico chemical properties like tensile strength, infrared spectra, thermogravimetric analysis, surface morphology, and water absorption studies. Better mechanical properties and surface morphology were observed for SG–SY–G–C films compared to other films prepared using collagen. The composite films prepared were used as wound dressing material on the experimental wounds of rats and healing pattern was evaluated using planimetric, biochemical, and histopathological studies. These studies have revealed better wound healing capacity of SG–SY–G–C film and utilization of CCLW in the preparation of value added product like wound dressing material.     

Acoustic emission studies on the lubrication of a fibrous collagen material-leather

For thousands of years, collagen materials, such as leather, have been among the most dominant natural fibrous materials used by humans. Fatliquoring is one of the critical steps in the leather-making process, wherein oil or a lubricant is added to the leather to prevent the leather fibers from sticking together, thereby providing sufficient pliability to the leather. We have examined the feasibility of using the acoustic emission (AE) technique to characterize the degree of lubrication of leather produced with various fatliquor concentrations. In a tensile test, an acoustic transducer was contacted with the leather samples to collect their AE quantities and properties. The samples lubricated with a fatliquor concentration less than 10% showed twin peaks on the plot of hits rate versus time. This implied that a non-uniform fracture occurred in a leather structure that was not sufficiently lubricated. In contrast, a sufficiently lubricated leather structure showed a steady increase in hits rate with time until it fractured. Traditional stress-strain tests did not reflect these behaviors. Observations also showed a direct correlation between the cumulative hits and fatliquor concentration. The results of this work may provide a route to identify an adequate degree of lubrication in the leather.     

Statistics of single‐fibre tensile strength incorporating spatial distribution of microcracks

The random distribution of single‐fibre tensile strength has been commonly characterized by the two‐parameter Weibull statistics. However, the calibrated Weibull model from one set of strength data at a given gauge length cannot accurately predicts the strength variation of the fibre at different gauge lengths. Instead of presuming the two‐parameter Weibull distribution or any other specific statistical distribution for the single‐fibre strength to begin with, this work proposes an approach to incorporating the appropriate spatial flaw distribution within a fibre and synchronizing multiple sets of tensile strength data to evaluate the single‐fibre strength distribution. The approach is examined and validated by published single‐fibre strength data sets of glass, ceramic and synthetic and natural carbon fibres. It is shown that the single‐fibre strength statistics does not necessarily always follow the two‐parameter Weibull distribution.     

Recycling of solid waste rich in organic nitrogen from leather industry: mineral nutrition of rice plants

The leather industry produces a large quantity of solid waste (wet blue leather), which contains a high amount of chromium. After its removal from wet blue leather, a solid collagenic material is recovered, containing high nitrogen levels, which can be used as a nitrogen source in agriculture. In order to take more advantage of the collagen, it was enriched with mineral P and K in order to produce NPK formulations. The objective was also to evaluate the efficiency of such formulations as a nutrient supply for rice plants in an Oxisoil, under greenhouse conditions. The application of PK enriched-collagen formulations resulted in N contents in the vegetative parts and grains of rice plants which were equivalent or superior to those obtained with urea and commercial NPK formulations.     

Tensile Behaviour and Strength Distribution of Polyvinyl-Alcohol Fibre at High Strain Rates

Mechanical properties of Polyvinyl-alcohol (PVA) fibre bundles were studied at three high strain rates (270/s, 600/s and 1500/s). It was found that, except for the elastic modulus, which remains unchanged, both the maximum stress and the failure strain show an apparent increase with strain rate. Two failure modes of PVA fibre under tensile impact were observed. A four-parameter Weibull function was adopted to describe the strength distribution of PVA fibre and the Weibull parameters were obtained by a fibre bundle testing method. Consistency between simulated and experimental results indicates that such a function and the method are valid and reliable.     

Differences in strength between the grain and corium layers of leather

The tearing resistance of the two principal strata of leather (the grain and the corium layers) has been assessed via the measurement of tearing energy and notch sensitivity. Observations of the distribution of strain around notches and the form of the tear tips are also reported. It was found that the grain layer had only 20% of the tearing energy possessed by the corium. In addition, the strength of the grain was considerably more sensitive to the presence of sharp notches. It is suggested that these differences in strength between the two strata of leather are associated with the greater ability of the corium layer's fibre structure to impede propagating tears by means of tear tip blunting and fibre pull out.     

Experimental and theoretical study on the strain rate and temperature dependence of mechanical behaviour of Kevlar fibre

The tensile impact experiments on Kevlar 49 fibre bundles were carried out at strain rates 140, 440, 1350 s⁻¹ and at temperatures −60, −20, 15, 50 and 90°C. It was found that the tensile mechanical properties of Kevlar 49 fibre bundles depend both upon the strain rate and the temperature. A bimodal Weibull distribution statistical model of the strain rate and temperature dependence of fibres, and a test method of determining mechanical properties and Weibull parameters of fibres from the fibre bundle test were adopted to characterize the combined effects of strain rate and temperature on Kevlar 49 fibre strength distribution. The simulated results are in good agreement with the experimental data, which proves that the bimodal Weibull distribution function is suitable to represent the statistical strength distribution of Kevlar 49 fibre which has peculiar ‘skin–core’ physical structure, and that the test method is valid and reliable.     

Tensile strength of axially loaded unidirectional Nextel 610™ reinforced aluminium: A case study in local load sharing between randomly distributed fibres

The tensile failure of unidirectional alumina fibre reinforced aluminium is studied in uniaxial loading along the fibre axis. The tensile strength is measured as a function of matrix yield strength, which is varied by varying the testing temperature, from RT to 600 °C. Over the range of matrix yield strength (i.e., of temperature) examined, the fracture mode remains brittle. Batdorf’s (J Reinforced Plastics Compos 1982;1:153-164) simple ideal local load-sharing model describes well the observed behaviour, under the condition that it be adapted to account for the actual number of nearest neighbours characteristic of the fibre distribution in the composite. This is shown to be close to three, i.e., at variance with the usually assumed idealized hexagonal or square fibre arrangement patterns.     

Deterministic method for predicting the strength distribution of a fibre bundle

Owing to the non-strain hardening plastic behaviour of the aluminium matrix and the weak fibre/matrix interface, it has been shown that the strength of a carbon fibre-reinforced aluminium matrix composite made by diffusion bonding of prepreg layers can be derived from the corresponding fibre bundle strength. Application of Coleman's model to predict bundle strength leads to the conclusion that the composite must break when 15% of the fibres are broken. This greatly overestimates the experimental composite strength. Overestimations made by using the Coleman model are due to some implicit assumptions which are not valid in the case under consideration and which may consequently not describe our material. A new approach is proposed for the calculation of the strength distribution of a fibre bundle, based on the same fracture mechanism (fibres fracture progressively until the catastrophic fracture) but without restrictive assumptions. The real interpolated experimental fibre strength distribution (and not the Weibull distribution) is taken into account to predict bundle strength. The proposed method clearly shows the limit of strength prediction, in term of bundle size (number of fibres and gauge length). The risk of making predictions following the Weibull distribution out of the range of the observations (through single-fibre tensile tests) is demonstrated.     

The fatigue behaviour of Kevlar-29 fibres

Single Kevlar-29 fibres have been subjected to a variety of tensile cyclic and steady loading conditions. The dispersion of tensile strengths of the samples tested was found to be inherent to the fibre due to the distribution of defects in it and not due to variations of diameter between samples. Cyclic loading was found to produce both longer and shorter lifetimes than those recorded under steady loads equal to the maximum cyclic load. Longer lifetimes indicated failure due to creep mechanisms whereas shorter lifetimes, seen with greater load amplitudes, suggest a fatigue mechanism. No difference was seen in the fracture morphologies of Kevlar-29 fibres broken under simple tensile, fatigue and creep conditions because of the complex splitting which occurs in all cases.     

Testing E-glass fibre bundles using acoustic emission

In tensile tests on lubricated bundles of a few hundred parallel E-glass fibres it is shown that individual fibre breaks, to the last fibre in the bundle, can be detected using acoustic emission (AE). By this means the single-fibre strength distribution is deduced. Relationships are obtained between some AE signal parameters and the fibre fracture stress which are consistent with theoretical expectations. Studies are made of the distribution of fibre break locations, the occurrences of multiple (stimulated) fibre breaks and the attenuation of the AE signals.     

Longitudinal tensile failure of unidirectional fibrous composites

This paper presents a theoretical treatment of the tensile strength of a unidirectional fibrous composite, subjected to a tensile load in the fibre direction. The fibres are treated as having a statistical strength distribution which results in fibre failure prior to composite failure. The failure geometry of the model is similar to the observed geometry of fractured glass/epoxy and glass/polyester composites. Failure criterion is established and the strength is shown to decrease as the length of the specimen is increased. This size effect is very small.     

Experimental evaluation of the strength distribution of E-glass fibres at high strain rates

A bimodal Weibull distribution function was applied to analyse the strength distribution of glass fibre bundles under tensile impact. The simulation was performed using a one-dimensional damage constitutive model. The results show that there were two concurrent flaw populations in the fracture process. The regression analysis using the bimodal Weibull distribution function was in good agreement with experiment.     

The tensile and fatigue behaviour of Kevlar-49 (PRD-49) fibre

The tensile, creep and tension-tension fatigue properties of Kevlar-49 fibre (formerly known as PRD-49) have been determined. The fracture morphology of the fibre has been examined and is shown to be complex due to considerable splitting. The fibre quickly stabilizes under a steady load but failure due to creep can occur when it is loaded very near to its simple tensile breaking load. Kevlar-49 has been found to fail by fatigue, and its fatigue lifetime is dependent on the amplitude of the applied oscillatory load as well as the maximum load to which the fibre is cycled.     

Gewebeelastizität und biomechanische Interaktion vaskulärer Komponenten

Tissue Elasticity and Biomechanical Interaction of Vascular Components. Many contributions have been made on the mechanical behaviour of soft biological tissues. Relatively small, however, is the knowledge of the biomechanical interaction of their components. Vascular tissues show a highly nonlinear load-deformation response which in their passive state depends on the three following, major wall components: elastic and collagen fibres and groundsubstance, a nonfibrous matrix. With increasing pressure, namely, blood vessels lose their distensibility, collagen and elastic fibres becoming straight and form a strong wall reinforcement which keeps the vessel from being blown out at higher intraluminal pressures. In order to better understand the material properties of the vessel wall and the interaction of these vascular components a continuum mechanical model is developed. Based on thermodynamical considerations reasonably approaching the physiological situation, constitutive equations are derived by which the amounts of stresses contributed by these components as well as the deformation energy stored by them, can be determined. Of particular significance, thereby, are the distribution densities and distribution functions of the fibres becoming and having become aligned during the vascular dilatation. These functions were determined from uniaxial tensile tests performed with electronic tensile testing machines using quasi pure fibre structures histologically corresponding. As specimen human tendons were taken for the evaluation of the collagen and ligaments from the neck of cattle for the evaluation of the elastic fibres' mechanisms. When simulating the pressure-radii response of an aortic segment on a computer using these data the results definitely show that the contribution of elastic and collagen fibres to the vessel mechanics is considerable already at mean physiological pressures and by far prevailing at higher ones. The present investigations are a necessary step forward in the biomechanics and materials research of vessels and, hence finally, in the research of the circulatory system.