Abstract
Compression of biological materials facilitates their transport and storage. The compressive rheological properties of silage sweet sorghum were studied via stress relaxation test with an electronic creep universal testing machine and a self-made adjustable compression device. The moisture content, compression density, cutting length, and compression speed were analyzed. Relaxation characteristics of sweet sorghum stalk were evaluated in terms of the stress decay time and the equilibrium elastic modulus. The stress relaxation characteristics of sweet sorghum stalks were consistent with the two Maxwell models, and the relevant parameters of the model at different levels were obtained. The rapid stress decay time first increased and then decreased with the increase of compression density, and the length of the smashed sorghum. The moisture content and the compression speed had greater fluctuations. The equilibrium elastic modulus increased with increasing straw compression density and the length of the shredding segment, and the equilibrium elastic modulus gradually decreased with increasing moisture content. The compression speed had little effect on the equilibrium elastic modulus. The research results lay a theoretical foundation and a basis for further study on the stress relaxation characteristics of sweet sorghum.
Download PDF
Full Article
Stress Relaxation Characteristics and Influencing Factors of Sweet Sorghum: Experimental Study
Xiaoxue Du, Chunguang Wang,* Wenbin Guo, Hongbo Wang, Min Jin, Xiaodong Liu, and Jian Li
Compression of biological materials facilitates their transport and storage. The compressive rheological properties of silage sweet sorghum were studied via stress relaxation test with an electronic creep universal testing machine and a self-made adjustable compression device. The moisture content, compression density, cutting length, and compression speed were analyzed. Relaxation characteristics of sweet sorghum stalk were evaluated in terms of the stress decay time and the equilibrium elastic modulus. The stress relaxation characteristics of sweet sorghum stalks were consistent with the two Maxwell models, and the relevant parameters of the model at different levels were obtained. The rapid stress decay time first increased and then decreased with the increase of compression density, and the length of the smashed sorghum. The moisture content and the compression speed had greater fluctuations. The equilibrium elastic modulus increased with increasing straw compression density and the length of the shredding segment, and the equilibrium elastic modulus gradually decreased with increasing moisture content. The compression speed had little effect on the equilibrium elastic modulus. The research results lay a theoretical foundation and a basis for further study on the stress relaxation characteristics of sweet sorghum.
Keywords: Sweet sorghum; Silage; Stress relaxation; Models; Experiment
Contact information: College of Mechanical and Electrical Engineering, Inner Mongolia Agricultural University Hohhot 010018, China; *Corresponding author: jdwcg@imau.edu.cn
INTRODUCTION
Sweet sorghum is a crop with high economic value. Its biological output and sugar content are very high (Han et al. 2012; Thomas et al. 2013; Zhang et al. 2014; Harper et al. 2017). Sweet sorghum has higher bio-productivity than green-fed maize (Zea mays), and can save arable land, reduce production costs, and increase agricultural production efficiency (Li et al. 2016; Shao et al. 2016). When sweet sorghum is used as silage, it can be stored for a long time without reduction of its nutrient content and palatability. It can also promote the secretion of digestive glands of livestock, enhance animal immunity, improve digestibility, prevent livestock constipation, etc. (He et al. 2017; Huang et al. 2017). However, sweet sorghum stalks are too loose and inconvenient to collect and transport. It is necessary to increase the density of the sorghum stalks by compressing them prior to mechanized collection and silage. At present, research on sweet sorghum silage is limited to the research of fermentation and other aspects. Research on silage compression and bale coating has been rarely reported.
Sweet sorghum is an agricultural fiber material, and the compression rheological properties of sweet sorghum will inevitably affect the working performance of compression equipment (Du et al. 2006; Zhu and Niu 2014; Lei et al. 2015). While there have been few studies on the compressive rheological properties of silage sweet sorghum, similar research on corn, alfalfa, and rice straw is common. Nona et al. (2014) used the compression times and moisture content as test factors, and they used straw and hay as the test objects. Through the fitting of the data it was found that the Peleg model had better relaxation characteristics than the Maxwell model. However, the generalized Maxwell model could be replaced with the fractional model without losing accuracy, which was useful for the reduction of viscoelastic parameters in the data processing of stress relaxation tests (Guo et al. 2017). There are some studies on the compressive and relaxing properties of compressed materials. The compression density, moisture content, material type, initial density (feeding amount), and compression speed have a significant influence on the rheological properties of the compressed material (Wang et al. 1997; Li et al. 2014; Yan et al. 2015; Guo et al. 2016; Ma et al. 2016).
In this study the stress relaxation test of sweet sorghum was conducted using a universal computer-controlled electronic tester and a self-made adjustable compression discharge device to obtain the stress relaxation characteristics, model, and corresponding rheological parameters of sweet sorghum stalks. The influence of moisture content, compressive density, length of chopping section, and compression speed of sweet sorghum stalk on its stress relaxation law was analyzed. This data provides the necessary theoretical basis and technical basis for the design of sweet sorghum baled silage and harvesting machinery in practical production.
EXPERIMENTAL
Material
Sweet sorghum was obtained from the suburbs of Hohhot, China. The average height was between 2000 and 4000 mm, and the average diameter was between 10 and 15 mm. Sweet sorghum straw is rich in cellulose, lignin, hemicellulose, pectin and crude protein, and also contains traces of tannins, crude fats and minerals. Compared with corn straw, the nitrogen-free extract of sweet sorghum straw was 40% to 50%, which was 64.2% higher than corn; the crude ash was 82.5% higher than corn, the crude protein was 3% to 5%, and the crude fat content was about 1% (Shao et al. 2016). It has been determined that the density of sweet sorghum straw was 20-40 kg/m3 in natural loose state.
The straw of the stem was cut using a 9Z-6A silage mower (China Agricultural Machinery North China Group Co., Ltd., Henan, China), and the length of the shredding segment was controlled by changing the number of gears of the silage mower and the number of moving blades. The straw was classified by the standard sieve to ensure that the length of the shredding section of the material was less than 40 mm, as shown in Fig. 1. The moisture content of the straw was tested by moisture meters (Hebi Electronic Research Institute Co., Ltd., DYSF-8000W automatic moisture analyzer, Henan, China). The initial average moisture content of the straw was 22.6%. According to the test requirements, each of the ground straws was conditioned to moisture contents of 50%, 57%, 64%, 71%, and 78% (w.b) by adding appropriate amounts of distilled water to the samples contained in Ziploc bags and stored in a cool room at 4 °C for 24 h. The calculation formula of the moisture content of the material was as shown in Eq. 1. The stress relaxation test was performed after the moisture of the straw was uniform.
(1)
In Eq. 1, Mw is wet basis moisture content (%), m1 is the quality of fresh materials (g), and m2 is the quality of dry materials (g).
Fig. 1. Test samples
Device
The experimental device was a DDL-200 universal computer-controlled electronic tester produced by the Changchun Research Institute for Mechanical Science Co., LTD, China and a self-made adjustable compression discharge device. The device is hydraulically powered, as shown in Fig. 2. Its force sensor range is 0.4% to 100% FS, and its displacement rate adjustment range is 0.005 to 500 mm/min. The error is controlled at ± 0.5%, and the test process can be controlled by the computer to complete the force, displacement, time, and other data collection. The self-made adjustable compression discharge device is mainly composed of a pressure cylinder and a discharge plate. The pressure cylinder has an internal diameter of 98 mm and a length of 300 mm. The movable beam is moved up and down by the beam of the universal computer-controlled electronic tester to complete the compression process.