Instrumentation for measuring plant fracture mechanics under tension

Tensile (mode I fracture) tests have arguably received less attention than out-of-plane shear (mode III) tests in the literature. While this reflects the greater attention that has been given to the importance of chewing, it also, in part, reflects the fact that tensile tests are more awkward to perform successfully. Nevertheless, the increased reporting of tensile tests with reference to ruminants is recognition that forages place different food procurement constraints on ruminants, which feed by grasping and tearing herbage with their muscle mass, as opposed to invertebrates that chew between the fibers of leaves.

Tensile tests are commonly conducted by securing the test piece between two clamps and breaking the specimen by longitudinal pull. Examination of the literature, however, shows profound deficiencies in the reporting of instrumentation and procedures for assessment of tensile strength in grassland studies [15,21,22,26,29,31,34-36]. The instrumentation of Sun and Liddle [22] was a modification of that used by Evans [31]. The apparatus consisted of a pivoting beam, with a clamp setup on one side and a bucket hung on the other side into which sand was poured until fracture of the specimen occurred. Spring-tensioned instruments used by Diaz et al. [21] and Adler et al. [37], modeled on that described by Hendry and Grime [38], are of similar design to a manually operated fiber-testing machine. Plant material is clamped between screw-type clamps, and tension is applied to the plant material by winding up a spring-operated crank until fracture results. These two forms of instrumentation provide a subjective measure of fracture force and can fulfill the objectives of an experiment designed to compare tensile strength across a range of plant species or genera under a prescribed set of environmental conditions. Translation to understanding grazing mechanics is, however, limited. The Instron testing instruments reported by Henry et al. [39] and Wright and Illius [15] offer tighter control over acceleration and greater precision in recording fracture force. Additionally, the machines are compatible with computers and/or plotters that plot the force-displacement curve for each test specimen, which provides visual reinforcement of the timing selection of fracture.

Following the choice of apparatus, a clamp that minimizes slippage while simultaneously minimizing damage from the compression force applied to the test specimen at the site of the clamp has been a serious obstacle in acquiring reliable and repeatable estimates of tensile strength. Samples that fracture at the vicinity of the clamp should be discarded, with good reason, because their inclusion will lead to erroneous data. Not all studies detail the clamp type used, but square clamps are often surfaced with rubber and/or emery paper [31]. Griffiths [40] used one clamp surfaced with emery paper, and a second clamp with one side surfaced with rubber that closed against a solid square cross bar, displaced 10 mm from the top of the clamp, to simulate the incisor grip. Henry et al. [39] devised cylindrical clamps and argued that the clamp method eliminated stress concentration by allowing a gradual increase in the transmitted force to the specimen around the periphery of the cylinder, avoiding fracture at the clamp. However, cylindrical clamps necessitate long specimen lengths and restrict the opportunities for assessment of the fracture mechanics of short vegetative forage material. Vincent [41] recommended that specimens be glued to tabs of aluminum, which could then be held by clamps, and a more recent study [34] described a "glue and screw" technique where the test specimen was glued into the slotted heads of screws.

Notching has been used to control the site of fracture, involving the creation of a small notch at the edge of the test piece using a needle or razor blade. Many monocotyledons with their parallel venation do not transmit shear and are considered notch-insensitive [16], although there are exceptions, and notch insensitivity should not be assumed to apply to all genera. Notch insensitivity implies that a single fiber can be broken without affecting the strength of the test specimen since the stress is distributed evenly among the remaining fibers. Given the ease with which notching can be carried out and the advantages that it offers in minimizing the number of samples fracturing in the region of the clamp, it is perhaps rather surprising the procedure has not been more widely utilized. Wright and Illius [15] assessed the fracture properties of leaf and pseudostem in the same manner, although the pseudostem samples could not be notched. By contrast, to assess the tensile strength of culms, Hongo and Akimoto [25] used the chuck of an electric drill as the clamp rather than jaw clamps (specifications not given) that had been used for leaves. Culms were wrapped with emery paper and enclosed within a thin rubber tube with one end inserted into the chuck. A further concern over the use of clamps is the need to standardize clamp compression force between specimen tests. Screw-type clamps [21,42] lead to inconsistency in the compression force between tests whereas pneumatic clamps, often found on floor-positioned or bench-top Instron testing machines [15], eliminate this problem.

Implicit in materials science is the principle that when any load is applied to an object, strain energy will be stored in that object. Atkins and Mai [43] suggested that it was critical that the test specimen be unloaded prior to specimen failure, and the most appropriate method to ensure that that this has occurred is to conduct mechanical tests at a slow and constant speed. Wide ranges of extension rates have been reported from as low as 5 mm/min [15] to 10 to 15 mm/min [25,26,39] through to 50 mm/min [34,44]. However, it must be noted that the removal of elastic strain energy from the test specimen is only a prerequisite where the stress-strain relation is to be assessed, and, therefore, the work to fracture is calculated using the work-area method. For estimates of fracture force, the use of faster rates of extension would more closely mimic the fast rates of head acceleration used by ruminants during bite severance [45].

The majority of studies utilize the youngest fully expanded leaf, which is usually the first leaf to make contact with the animal's mouth. However, some studies have involved measurements on older leaves, which require greater force to fracture [15,39], so caution must be exercised when comparing studies. Although an intact, whole leaf is usually tested, there have been reports of tests performed on an excised strip of leaf, running parallel with the midrib [34], reinforcing the point that it is critical to assess what the tensile strength estimate is related to. Moreover, the site of fracture can hamper comparisons across studies. Leaves are not homogeneous along their length, and thus the position of measurement can influence the estimate of tensile strength. This was the reason why Evans [31] assessed tensile strength as being equal to the breaking load divided by the dry weight of a 5 cm length, despite having correctly defined tensile strength in the introduction to the study. MacAdam and Mayland [35] showed that the position of maximum leaf width does not equate with the midpoint of the leaf and that there is a region, approximately 50 to 80 mm long, of constant maximal width in fully expanded tall fescue (Festuca arundincea) leaves. This approach contrasts with that of Zhang et al. [26] who used 20 cm lengths of the central portion of orchard grass (Dactylis glomerata) leaves and Sun and Liddle [22] who clamped leaves one-third of the distance from each end. It is interesting to note that Wright and Illius [15] did not assess tensile strength; rather, they quantified the energy required to fracture the specimen standardized for cross section, avoiding any confounding variation due to contrasting sites of fracture between plant species.

The instrumentation described has all involved plant material being cut from the field or from pots in chamber-grown complexes. A portable instrument consisting of modified pliers with a strain gauge to assess tensile strength of plant specimens growing in situ was developed by Westfall et al. [46]. However, it was not clear how clamp compression and acceleration of the longitudinal pull were controlled, factors that have been discussed previously, and the apparatus probably offers little advantage over the other forms of instrumentation other than the fact that plants are naturally anchored.

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