Moreover, in vitro studies have noted that microdamage can increase resistance to crack growth ( 14), particularly if the damage is in the form of linear microcracks ahead of a larger crack ( 15).įracture of cortical bone can occur from repetitive, subcritical loads (fatigue failure) or from applied loads that cause local stresses exceeding the strength of the tissue. While these collective results might suggest a prominent role for microdamage in increasing fracture risk, this mechanistic link has not been established. Microcrack accumulation increases exponentially with age in cortical bone and is significantly higher in the bones of women versus men ( 12, 13). In vitro mechanical tests have found that microdamage is associated with a decrease in modulus ( 9, 10), and a weak inverse relationship between fracture toughness and microdamage density has been reported ( 11). Microdamage is a possible contributor to bone fragility but also a mechanism of toughening. Both of these microstructural events may give rise to the residual strains that are observed upon unloading after the specimen has been loaded past the yield point. Microdamage may appear as debonding of the proteinaceous–hydroxyapatite composite (such as debonding of hydroxyapatite aggregates and noncollagenous proteins) or as slippage of the lamellae along one another or along cement lines ( 6– 8). Modified from Reference 111 with permission. Panels a and c were acquired using bright-field microscopy panels b and d were acquired using laser scanning confocal microscopy (stain is xylenol orange). We have chosen to emphasize what is known about the effects of aging and common diseases on the mechanical properties of bone, anticipating that this review can serve the needs of researchers from many disciplines who seek to understand the age- and disease-related bases of bone fragility. Recognizing that this article is not the first to review the mechanics of bone, we intend to present the basic tenets and to connect them to some of the arcs of research that have brought the field to recent advances. This review provides a foundation for these areas of study by summarizing the current state of knowledge on the basic mechanical behavior of bone at length scales ranging from hundreds of nanometers to tens of centimeters. Yet, setting aside the biological functions of bone as an organ system or of the bone tissues that are the main constituents of whole bones, the mechanical behavior of bone is a multifaceted, broad subject relevant to studies of clinical fractures, development, adaptation, and healing and regeneration. The test fixture for these materials usually has self-aligning anvils.The bones in the human skeleton must meet a diverse set of functional demands, not all of which are mechanical in nature. Alignment of the support and loading anvils is critical with brittle materials. The 4-point test can also be used on brittle materials. This test provides flex strength data only, not stiffness (modulus). When a 3-point flexure test is done on a brittle material like ceramic or concrete it is often called modulus of rupture (MOR). Test results include flexural strength and flexural modulus. The 4-point test requires a deflectometer to accurately measure specimen deflection at the center of the support span. The 4-point flexure test is common for wood and composites. Specimen deflection is usually measured by the crosshead position. The 3-point flexure test is the most common for polymers. In a 4-point test, the area of uniform stress exists between the inner span loading points (typically half the outer span length). In a 3-point test the area of uniform stress is quite small and concentrated under the center loading point. There are two test types 3-point flex and 4-point flex. Flexure testing is often done on relatively flexible materials such as polymers, wood and composites.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |