材料的热稳定性.ppt

材料的热稳定性( Thermal Stability) 热稳定性的定义热稳定性(抗热震性): 材料承受温度的急剧变化(热冲击)而不致破坏的能力。热冲击损坏的类型: 抗热冲击断裂性------ 材料发生瞬时断裂; 抗热冲击损伤性------ 在热冲击循环作用下, 材料的表面开裂、剥落、并不断发展,最终碎裂或变质。 Thermal Shock Stress leading to the fracture of brittle materials can be introduced thermally as well as mechanically. When a piece of material is cooled quickly, a temperature gradient is produced. This gradient can lead to different amounts contraction in different areas. If residual tensile stress es high enough, flaws may propagate and cause failure. Similar to behavior can occur if a material is heat rapidly. This failure of a material caused by stresses introduced i by sudden changes in temperature is known as thermal shock . 热稳定性的表示方法( 1)一定规格的试样,加热到一定温度,然后立即置于室温的流动水中急冷,并逐次提高温度和重复急冷,直至观察到试样发生龟裂,则以产生龟裂的前一次加热温度 0C表示。(日用瓷) ( 2)试样的一端加热到某一温度,并保温一定时间,然后置于一定温度的流动水中或在空气中一定时间,重复这样的操作,直至试样失重 20% 为止,以其操作次数 n表示。耐火材料: 1123 K; 40min ; 283 - 293K ; 3(5 - !0)min ?材料在未改变外力作用状态时,仅因热冲击而在材料内部产生的内应力成为热应力。? Stresses introduced into a material due to differences in the amount of expansion or contraction that occur because of a temperature. ( 3)试样加热到一定温度后,在水中急冷,然后测其抗折强度的损失率,作为热稳定性的指标。(高温结构材料)。热应力( Thermal Stress) 热应力产生原因?(1) 构件的热膨胀或收缩受到约束时造成热应力; ?(2) 相连接的构件存在温度差,构件间相互约束造成热应力; ?(3) 构件本身存在温度梯度,其间各部分相互约束造成热应力; ?(4) 不问构料的组合和约束造成热应力 Thermal Stress & Strain ? Thermal stress and strain are caused by temperature change. ? Materials expand at temperature increase and contract at temperature decrease. ? Restricting thermal strain cause thermal stress. Thermal Strain ? Bar of initial length L ? Thermal strain Δ L due to heat, computed as: Δ L= αΔ t L ? where α=Coefficient of thermal expansion Δ t=temperature increase/decrease L=initial length Thermal Stress ? Bar of initial length L ? Elongation Δ L due to heat ? Hot bar reduced to initial length by load P ? Thermal stress in restrained bar Δ L= αΔ t L ε= αΔ t E=f/ ε f= αΔ t E ? where f=thermal stress α=Coefficient of thermal expansion Δ t=temp