Mechanical Properties of Feed Pellets: Weibull Analysis

Physical properties of feed pellets were investigated in the context of fracture mechanics of brittle materials. Such materials exhibit high scatter in strength due to variation in flaw sizes in the material, known as Griffith cracks. Weibull analysis assumes a relation between the size distribution of Griffith cracks and the scatter in fracture strength. This relation is outlined, and used relative to feed pellets for livestock. Four feeds were produced at FôrTek Inc., Agricultural University of Norway, from roller-milled barley. Three feeds were expanded at different temperatures (96, 115 and 155°C) prior to pelleting, whereas the fourth feed was pelleted without prior expansion. The stress at rupture was measured in diametral compression on 55 specimens of each feed. It was found that the strengths were Weibull distributed. The Weibull modulus varied between 4·1 and 9·4, and the scale parameter varied between 1·6 and 2 MPa. Lowest modulus was estimated for the feed processed without expander treatment. It is concluded that Weibull analysis can be used to assess physical pellet quality and that expander treatment yield a more homogeneous product.